aSotS'KJiSJl V-
CASSELL'S REINFORCED CONCRETE
Cassell's
•
Reinforced Concrete
A Complete Treatise on the Practice and
Theory of Modern Construction
in Concrete-Steel
Edited by
Bernard E. Jones
Editor of " Building World "
Assisted by
Albert Lakeman
M.C.I., Honours Medallist in Building Construction
and by a Staff of Specialist Writers
Illustrated by 171 Photographs and about Five Hundred
Diagrams and Working Drawings
Cassell and Company, Limited
London, New York, Toronto and Melbourne
1913
List of Chief Contributors and Revisers
FRANK B. GATEHOUSE, F.C.S Portland Cement
THOMAS POTTER, M.G.I Concrete
A. B. SEARLE Concrete, Cement, Waterproofing, etc.
E. L. RHEAD, M.Sc., Tech. F.I.G Steel
ALBERT LAKEMAN, M.G.I Theory, Examples, etc.
Honours Medallist in Construction
V. SUSSEX HYDE, M.G.I. . Practical Erection, Forms and Centerings
F. CHARLES BARKER, M.S.A Examples of Forms
Assist. Dangerous Structure Surveyor, City of London
T. ELSON HARDY Architectural Treatment
A. SEYMOUR JENNINGS, F.I.B.D. . . ... Painting Concrete
W. H. BROWN, F.S.I Estimating, Quantities, etc.
THE EDITOR .... Introduction, History, Examples, etc.
267323
Preface
THE purpose of this book is to provide a practical, simply-worded guide to construc-
tion in reinforced concrete, as distinct from the theoretical and often obscure treatises
that have appeared in large numbers during the past few years. The theoretical side
of the subject has not, however, been overlooked, as any treatise on reinforced con-
crete that ignored methods of calculating the various members would be obviously
incomplete, especially as this method of construction offers great possibilities of
scientific design. As a matter of fact, the two theoretical chapters are a strong feature
of the book, having been written, at my request, by Mr. Albert Lakeman, who has
brought a trained mind to bear upon his subject, and, in addition to explaining impor-
tant principles in a simple and concise manner and clearly showing the practical
application of the theories he expounds, has imparted to his text the unusual element
of freshness. Mr. Lakeman has wisely begun at the beginning, and, whilst he could
not finish at the end — for one thing, the end is not yet — he will be found to take an
apt and diligent student far enough on the road for him to be able to find the rest
of the way for himself ; and, after all, no book can do more than that for anyone.
The book is largely concerned with the practical side of reinforced concrete —
with what has been done and can be done in the compound material, and how to do
it. The introductory chapter having explained the advantages of reinforced con-
crete, an historical chapter (every definite date in which I have verified from official
records) puts the reader in possession of a variety of facts, many of which will throw
light on the subject as his knowledge of it grows. Concrete and steel, as materials,
are discussed in the two following chapters, in the earlier of which special attention
is devoted to portland cement and the modern methods of mixing and handling
concrete. Then come the explanations of theory, followed by two chapters respec-
tively devoted to the methods employed in erecting a building and the forms and
centerings necessitated in general reinforced concrete work ; these two chapters are
noteworthy, I think, from the practical character of the information they give and
the very large number of explanatory drawings and photographs which they include.
Concise descriptions of the chief commercial systems follow, and later chapters deal
respectively with architectural and surface treatment, durability, waterproofing,
arches and bridges, and quantity surveying, estimating, measuring and pricing. The
concluding chapter describes and illustrates a number of works carried out in re-
inforced concrete, varying from a palatial club-house and commercial " sky-scraper "
to railway sleepers and sewer pipes.
Every endeavour has been made to render the volume as complete as possible, in
the hope that it will be useful to all classes of readers, both as a text-book and as a
work of reference.
I have been happy in having the co-operation of a number of experts (a list of
viii PREFACE
whom is given elsewhere). Each of these writers has contributed or revised the section
relating to the particular phase of the subject he is most competent to deal with ; and
from many of them I have received valuable suggestions which I have been able to
apply. I wish to acknowledge, also, the kindly treatment accorded me by a large
number of firms identified with special systems, materials, machines, etc., for, in a
number of cases they have supplied me with information, drawings, and photographs.
Additional thanks are due to Messrs. Geo. West and Co. for permission to make brief
extracts from the historical notes published in a " Lock-woven Mesh " handbook.
I owe a special debt of gratitude to the premier concrete journal, Concrete and Con-
structional Engineering, which has placed many facilities at my disposal and has
been good enough to lend me a large number of blocks. To the files of that journal,
too, I have gone for help in many instances when other sources of information
proved vain.
The illustrations to this work include no less than 171 photographs and about
five hundred diagrams and working drawings ; and the large number of " progress "
photographs, showing the actual practice of reinforced concrete work, constitute, I
hope, a specially valuable feature of the book, and considerably enhance its teaching
value.
B. E. J.
La Belle Sauvage,
London, E.G.
Contents
PAGE
Introduction : What Reinforced Concrete Is 1
The Limitations of Concrete — Reinforcement — Terminology — A Compound
Material — Comparisons.
Historical Notes . . 5
Early in the Nineteenth Century — A Concrete Boat — Wilkinson — Frangois
Coignet — Dennett, Allen, Ransome, and Scott — Joseph Monier Phillip
Brannon — Thaddeus Hyatt — Progress between 1870 and 1892 — Edmond
Coignet and Frangois Hennebique — Progress since 1892 — Notable American
Inventions.
Concrete : Materials, Proportions and Mixing . . . 12
What Concrete Is — Natural Aggregates- — Artificial Aggregates — Size of Aggregate
—Washing Coarse Aggregate — Sand — Portland Cement — Natural Cements
— Slag Cement — Lime Concrete — Water for Concrete Mixing — Proportioning
Concrete — Gauge Boxes and Measures — Mixing Concrete by Hand — Machine
Mixing and Mixers — Batch Mixers Described — Continuous Mixers Described
—Testing Efficiency of Mixing — Conveying Concrete — Laying Concrete.
SteeJ . . 50
Composition — Malleability — Ductility — Elasticity — Limit of Elasticity — Testing
Steel — Specification — Modulus of Elasticity — Certificate of Tests — Fatigue
—Toughness — Hardness — Bending Tests — Shearing Strength — Resistance to
Alternating and Repeated Stresses — Effects of Heat — Co-efficient of Expan-
sion— Other Elements Present in Steel — Methods of Manufacture.
Stress Simply Explained . . 61
Formulae — Principle of Moments — Levers — Reactions — Bending Moments —
Moment of Inertia — Section Modulus and Moment of Resistance — Calculating
Safe Load on Beam — Columns and Struts — Shearing Stress.
The Theory of Reinforced Concrete • • 83
Introductory — Data for Calculations — Beams with Single Reinforcement — Calcu-
lations for Slabs — Beams and Slabs with Double Reinforcement — Tee Beams
— Tee Beams Continuous or Fixed at Ends — Tee Beams with Double Rein-
forcement-— Shearing Stress and Adhesion — Columns — Columns Eccentrically
Loaded — Retaining Walls.
The Erection of a Reinforced Concrete Building . • 126
Building a Factory — Tools and Appliances — Driven Piles — " Compressol " Con-
crete Piles — The Foundation Slabs — The Retaining Wall — Columns — Walls —
Staircases — Introducing the Reinforcements into Beam Moulds— Forming
Skeleton Reinforcements — Making a Floor.
Forms and Centerings
Sizes of Timber — Striking the Forms — Setting Out — Forms for Piles — Foundation
Forms — Column, Beam, and Slab Forms — Column Forms — Centerings for
x CONTENTS
PAGE
Concrete Floors Carried by Steel Joists — Wall and Partition Forms — Imitating
Masonry Walls — Staircase Forms — Metal Forms for Walls, Beams, etc. —
Forms for Cornices, Mouldings, etc. — Forms for Ornamental Parapet — Forms
for Retaining Walls — Forms for Silos — Forms for Domes — Forms for Tall
Chimneys — Forms and Centering for Arches and Bridges — Suspended Center-
ing for Bridges — Forms for Pipes and Sewers — Forms for Tanks — Form for
Rectangular Reservoir — Forms for Fence Posts.
Systems Described . 215
Armoured Tubular Flooring — Clinton System — Coignet System — Considere System
— Corr Bar — Dentile System — Expanded Metal System — Hennebique System
— Indented Bar System — Johnson's Lattice System — Kahn System — Keedon
System — Lock-woven Mesh System — Mushroom System — Paragon System —
Piketty System — Other Systems.
Architectural and Surface Treatment of Reinforced Concrete . . 229
Architectural Treatment — Surface Treatment — The Untouched Surface — Brush
Finish — Carborundum Finish — Sand-blast Finish — Bush-hammered Finish —
Facing Concrete in the Form — P.ebble Dashing — Sand Finish — Glazed Finish —
Plastered Surfaces — Tiles, Mosaic, Sgraffito, etc — Stuc Work — Colouring Con-
crete— Body Colours — Table of Pigments — Stains — Distempering — Painting.
Durability of Reinforced Concrete • • 25t>
Mechanical Destructive Influences — Blows and Shocks — Earthquakes — Settle-
ments— Frost — Fire — Chemical Destructive Influences — Atmosphere — Water
— Sea Water — Acids — Alkalies — Urine.
Waterproofing Concrete • 266
Superficial Waterproofing — Mass Waterproofing — Making a Non-porous Concrete.
Specifications, Quantities, Measuring, Estimating, and Pricing
The Specialist System — A General Specification — Quantities — General Outline
of Bill or Schedule — Designs Independent of the Specialist — Measuring Con-
crete— Small Holes — Measuring Steel Reinforcement — Measuring Centering
— " Use and Waste " — First-class Centering — " Centering " Clauses in
Preamble — Specially-designed Centering — Openings — Centering to Walls —
Moulds for Architectural Features — Centering to Inclined Slabs, Beams,
Columns, etc. — Abstracting — Billing — The Preamble — Billing Measured Items
— Pricing — Prices for Concrete, Steel and Centering.
Arches and Bridges
Design of Arches and Bridges — Examples of Arches and Bridges.
Examples • 311
Royal Automobile Club's Building — Royal Liver Building — Cathedrals at Manila
and Poti — Wesleyan Hall, Westminster — Free Church, Hampstead Garden
City — St. Barnabas' Church, Jamaica — The Majestic Theatre, Los Angeles
— National Gallery Extension — Aluminium Works at Kinlochleven — Royal
Insurance Building, London — Wall with Post Buttresses — Retaining Wall —
Stairways — Chimney Shafts — Pipes, Sewers and Conduits — Wharf at Denne-
mont — Jetty Head at Thames Haven — Lifeboat Slipway- — Roofed Reservoir
— Spectators' Stands, etc. — Railway Sleepers — Grain Silos at Silvertown
— Water Tower at Singen.
List of Illustrations
FIG. PAGE
1 to 3. — Columns of Plain Concrete, Steel, and Reinforced Concrete respectively : Comparative
Diagrams . . . . . . . . . . . . 3
4 to 6. — Beams of Plain Concrete, Steel, and Reinforced Concrete respectively : Comparative
Diagrams .............. 3
7. — Gauge Box with Fixed Handles . . . . . . . . . . .28
8. — Cement Gauge .............. 28
9. — Sectional Diagram showing Principle of Chicago Cube Mixer . . . . . .31
10. — Chicago Cube Mixer, with Engine and Boiler ......... 31
11. — " Cut-away " View of Eclipse Mixing Drum ......... 32
12. — Eclipse Mixer, with Petrol Motor . ... . . . . . . . .32
13. — " Cut-away " View of Koehring Mixing Drum . . . . . . . .33
14. — Koehring Mixer, with Engine and Boiler ......... 33
15. — Marsh-Capron Non-tilting Mixing Drum . . . . . . . . .33
16. — " Cut-away " View of Marsh-Capron Tilting Mixing Drum . . . . . .34
17. — Marsh-Capron Mixer, with Drum in Tilting Position ....... 34
18. — McKelvey " Gravity Shovel " 35
19 to 24. — Ransome Mixers . . . . . . . . . . . . .36
25. — Scoops in Ransome Mixing Drum (English) . . . . . . ... .36
26. — Scoops in Ransome Mixing Drum (American) . . . . . . . .36
27. — Elevations and Plan of Ransome Belt-driven Mixer. ....... 37
28. — Roll Mixer, with Elevator raised . . . 37
29. — Smith Hand-driven Mixer . . . . . . . . . . . .38
30. — Smith Power-driven Mixer ............ 39
31. — Victoria Mixer, with Skip Elevated .......... 40
32. — Victoria Mixer, with Skip Lowered .......... 40
33. — " Cut-away " View of Victoria Mixing Drum . . . . . . . . . 41
34. — Express Mixer .............. 4i
35. — Fawcett Mixer . ............. 41
36. — Gaspary Hand-driven Tilting Trough Mixer ........ 42
37. — Pansy Mixer ............... 42
38. — Whalley Mixer, with Engine and Boiler .......... 43
39. — Gaspary Drum-type Continuous Mixer .......... 43
40. — Mason Mixer .............. 44
41.— Bolte Mixer ^ .... 44
41a. — Hoppers of Bolte Mixer ............ 44
42.— Goltrin Mixer 45
43.— Coltrin Mixing Blades 45
44. — Kent Mixer ............... 45
45. — Perfect Mixer .............. 45
46. — " Cut-away " View, showing Principle of Trump Mixer ....... 40
47. — Ransome Concrete Cart ............. 46
48. — Ransome Concrete Skip or Bucket .......... 46
49. — Ground Plan, Top Plan, and Two Elevations of Ransome Tower. ..... 47
50.— Graph showing Influence of Carbon on Tenacity of Steel .... .51
51. — Graph showing Influence of Carbon on Ductility of Steel ...... 52
52. — Test Piece before and after Stretching 53
53 to 55. — Photo-micrographs of Steel . . . . . . • • • • 58, 59
56. — Lever of First Order 62
57. — Lever of Second Order ............. 62
58.— Lever of Third Order 63
59. — Diagram Showing how a Cantilever Tends to Move. ....... 63
60. — Triangular Portion of Wall Lifted by Movement of Cantilever. ..... 63
xi
LIST OF ILLUSTRATIONS
^G- PAGE
61. — Beam Loaded Eccentrically ........ .64
62. — Diagram Showing how Eccentrically Loaded Beam Tends to Rotate .... 64
63. — Beam Rotating on the other Abutment. ..... .64
64. — Beam Carrying Three Concentrated Loads ... 65
65. — Diagram Illustrating Clear and Effective Spans ..... 65
66. — Centrally-loaded Beam ...... 65
67. — Bending Tendency on Beam ........ 66
68. — Finding Bending Moment of Beam . . . . . . . . . .66
69. — Bending Moment Set Up to Scale ....... .67
70. — Finding Bending Moment of Beam .... 67
71. — Lever Arms of Reaction and Load .......... 67
72. — Beam with Two Loads Concentrated at Different Points ....... 67
73. — Cantilever, with Concentrated Load at Outer End ........ 68
74. — Failure of Cantilever due to Tension ...... 68
75. — Failure of Cantilever due to Compression ......... 68
76. — Bending Moment in Cantilever ....... .68
77. — Bending Moment of Cantilever Set Out to Scale ........ 69
78. — How Centrally-loaded Cantilever Tends to Bend ........ 69
79. — Cantilever with Two Concentrated Loads ......... 69
80. — Bending Tendency in Cantilever with Two Concentrated Loads ..... 70
81. — Beam with Uniformly Distributed Load .......... 70
82. — Distributed Load acting through Centre of Gravity ....... 70
83. — Bending Moment of Beam at Intermediate Point ........ 70
84. — Bending Moment on Beam Set up to Scale . . . . . . . . .71
85. — Beam with Concentrated and Distributed Loading . . . . . . . .71
86. — Cantilever Carrying Distributed Load .......... 71
87. — Finding Bending Moment at Intermediate Point in Cantilever ...... 72
88. — Bending Moment in Cantilever Set Out to Scale ........ 72
89. — Cantilever, with Combined Distributed and Concentrated Loading ..... 72
90. — Calculating Moment of Inertia of Simple Rectangular Section ...... 78
91 to 97. — Common Sections and their Moments of Inertia ....... 74
98. — Calculating Moment of Inertia of Rolled Steel Joist ....... 74
99. — Calculating Least Moment of Inertia of Rolled Steel Joist ...... 75
100. — Rotating Tendency of Beam, caused by Reaction and Weight ..... 75
.101. — Forces Acting upon One-half of Beam .......... 75
102. — Calculating Section Modulus : Beam Centrally Loaded ....... 75
103. — How Centrally Loaded Beam Tends to Bend 76
104 to 106. — Calculating Section Modulus of Beam ......... 76
107. — Strut under Load 78
108. — Diagram Indicating Stress in Part of Strut ......... 78
109. — Beam, under Vertical Shear, assumed to Consist of Separate Blocks .... 79
110. — Beam, under Horizontal Shear, assumed to Consist of Separate Planks .... 79
111. — Shearing Stress in Cantilever which Carries Concentrated Load at Outer End ... 80
112. — Shearing Stress in Cantilever which Carries Uniformly Distributed Load .... 80
113. — Shearing Stress in Cantilever which Carries Uniformly distributed Load and Two Concen-
trated Loads 80
114. — Shearing Stress in Beam which Carries a Central Concentrated Load .... 81
115. — Shearing Stress in Beam which Carries Uniformly Distributed Load. .... 82
116. — Shearing Stress in Beam which Carries Uniformly Distributed Load and Three Concentrated
Loads 82
117 and 118. — How a Single Reinforced Beam Resists Compression and Tension ... 86
119. — Diagram showing Proportionate Stresses above and below Neutral Axis to Produce Deforma-
tion 87
120. — Finding Position of Neutral Axis . . 91
121. — Section through Cross Rods . . 99
122. — Section through Longitudinal Rods ... 99
123 and 124. — Double Reinforcement in Beams . . 100
125. — Designing Beam having Double Reinforcement . . 101
126. — Section of Tee Beam and Method of Finding Total Compression ... . 104
127.— Section at Centre of Span of Tee Beam fixed at Ends 110
LIST OF ILLUSTRATIONS xiii
FIG. PAGE
128. — Section, at Ends of Span, of Tee Beam fixed at Ends . . . . . . .111
129. — Determining Stress in Ordinary and in Reinforced Concrete Beams . . . . .113
130. — Vertical Shear in Reinforced Concrete Beam carrying Uniformly Distributed Load . .115
131. — Horizontal Shear in Reinforced Concrete Beam carrying Uniformly Distributed Load . 116
132. — Diagrams showing Need for Lateral Reinforcement in Columns ..... 117
133 and 134. — Rectangular and Circular Columns with respectively Rectilinear and Curvilinear
Laterals . . . . . . . . . . . . . . .118
135. — Designing Column Eccentrically Loaded .......... 122
136. — Stress in Column Eccentrically Loaded .......... 122
137. — •Influence of Angle of Repose and Line of Rupture and Design of Retaining Walls . . 123
138. — Three General Types of Retaining Walls. 124
139. — Retaining Wall with Cantilevers under Footpath ........ 125
140. — Retaining Wall to Resist Water Pressure . . . . . . . . .125
141. — Vertical Cross-section of Typical Factory Building in Reinforced Concrete . . 126
142.— Mixing Stage 128
143 to 146. — Four Patterns of Iron Tamper 128
147. — Perforated Spade . . .128
148.— Ross Spade 129
149. — Tamper for producing Fine Surface .......... 129
150. — Special Spade for Facing 129
151. — Andrews Tamper 129
152.— Wooden Tamper 129
153. — Cutting Anvil and Hammer 129
154 and 155. — Wrenches for Bending Ends of Bars ......... 130
156 and 157. — Wrenches for Bending Ends of Stirrups 130
158. — Key, or Twister, for Bending Ends of Stirrups, . . . . . . . .130
159.— Curry Tyer 130
160. — Elevation and Plan of Bench Bending Machine . . . . . . . . 130
161. — Kennedy Bar-bending Machine, No. 1 . . . . . . . . • . 130
162. — Kennedy Bar-bending Machine, Geared Pattern . . . . . . . .130
163. — Making Bend to Given Inside Measurement ......... 131
164. — Making Bend to Given Outside Measurement . . . . . . . . .131
165.— Making a Double Set 131
166.— Making Sharp Bend in Thin Bar . . . . .131
167. — Photographic View of Right and Left Double Bar-bender ...... 131
168.— Method of Bending Small Bars 132
169. — Hennebique Square Pile ............ 133
170.— Coignet Round Pile 133
171. — Considere Octagonal Pile 133
172. — Lidgerwood Pile-driving Engine and Reinforced Concrete Pile ...... 134
173.— Simplex Pile . 136
174. — "Compressol" Borer, Rammer, and Tester 137
175. — " Compressol " Frame and Borer in Use . . . . . . ... . 138
176.—" Compressol " Pile 138
177. — Plan of Typical Pile Caps and Connecting Beams ........ 139
178. — Section showing Pile Caps, Filling, Beam, and Slab 139
179. — Foundation Slab Reinforcement, held in Notched Templates ...... 140
180. — Ra^t Foundation Beam Reinforcement on Wooden Supports ...... 140
181. — Raft Reinforcement, Kingsway Church, London ........ 141
182. — Completed Raft, Kingsway Church, London ......... 142
183. — Plan of Retaining Wall, with Tapering Counterforts . . . . . . . 142
184. — Reinforcement of Retaining Wall, Royal Insurance Building ...... 143
185 and 186. — Reinforced Concrete Retaining Wall for Royal Insurance Building . . . 144
187. — Diagram showing the above Retaining Wall if built with Brickwork . . . . 144
188. — A further View of Retaining Wall, Royal Insurance Building, in Course of Erection . . 145
189. — Cross Sections of Columns showing Right and Wrong Methods of Placing Reinforcements . 146
190 and 191. — Horizontal and Vertical Sections through Typical Factory Staircase . . . 147
192. — Hollowed-out Concrete Block to Facilitate Spacing of Bars 147
193.— Money Order Office, Holloway, showing the Centering for Pillars, Floors, and Walls. . 148
194. — Money Order Office, Holloway, showing Two of the Wings in Course of Construction . 149
xiv LIST OF ILLUSTRATIONS
FIG. PAGE
195. — Method of Supporting Reinforcement in Beam Mould . . . . . . .150
196. — Typical Floor with Continuous Mesh Reinforcement. ....... 151
197. — Typical Floor with Sheet Mesh Reinforcement ........ 152
198.— Craig Screw-bore 154
199.— The Wainwright Steel Kerb .154
200.— Steps with Steel Kerbs 154
201. — Section through Column with Steel Corner Bars . . . . . . . . 154
202.— Ebco Corner Bar 154
203.— Setting Out Piers of Rectangular Building . .157
204. — Horse used in Setting Out 157
205. — Setting Out Acute Angle of Building 157
206.— Setting Out Obtuse Angle of Building 157
207.— Form for Square Pile 158
208.— Bolted Form for Bound Pile 158
209.— Stayed Form for Round Pile . 158
210 and 211. — Elevation and Enlarged Cross Section of Pile-making Platform .... 159
212. — Plan and Elevation of American Pile-making Forms and Platform . . . . .159
213. — Enlarged Cross Section through Form shown in Fig. 212 ...... 159
214. — Concrete Piles and Forms Dissociated .......... 159
215.— Foundation Form Built after Slab is Hard 159
216. — Forms for Pile Caps, Foundations, etc. . . . . . . . . . .160
217. — Typical Form for Foundation 160
218. — Cheaper Type of Form for Foundations . . . ... . . . . .161
219.— Form Strengthened with Wire 161
220.— Typical Column Form, with Slid-in Front Boards 162
221. — Typical Column Form, with Spiked-on Front Boards ....... 163
222.— Clamped Form for Short Columns 164
223.— Cheap Type of Column Form 165
224. — Column Form with Two Sides held between Fillets and Battens ..... 166
225. — Column Form with Two Sides held between Fillets and Battens ..... 167
226.— Typical Beam Form .168
227. — Boards with Splayed Edges to allow for Expansion . . . . . . 168
228. — Beam Form strutted from Extended Base Battens ........ 169
229.— Folding Wedges under Dead Shore 169
230. — Form for Two Intersecting Beams ........... 170
231. — Elevation and Cross Section of Forms for Pillar and Beam with Splayed Angles . . 171
232. — Two Cross Sections through Hennebique Floor and Plan of One Bay .... 172
233.— Beam Form held by Clamp .... 172
234. — Column Form with Bolts and Thumbscrews ......... 172
235 and 236. — Beam Forms used in Messrs. Sainsbury's Premises, London .... 172
237. — Beam and Column Forms used at a Bermondsey Warehouse . . . . . . 173
238.— Plan of Column Form • 173
239. — Plan showing Position of Clamps for Reduced Column Form ...... 173
240. — Isometric Sketch of Forms for a Complete Bay of a Warehouse Floor .... 173
241. — Early Stage of Forms and Centerings at H.M.'s Stationery Office ..... 174
242. — Beam and Column Forms used at H.M.'e Stationery Office ...... 175
243 to 245. — Adjustable Beam Forms designed by H. Kempton Dyson . . . . .175
246. — Centering for Beams and Floors in Storied Building ....... 176
247. — Form for Twelve-sided Column 177
248.— Form for Fluted Column 177
249. — Form for Diminished Column ............ 177
250 and 251. — Columns at Wesleyan Hall, Westminster ........ 178
252. — Beam Forms, etc., at Wesleyan Hall, Westminster . . . . . . . . 179
253. — Form for Column Base at Wesleyan Hall, Westminster ....... 180
254. — Centering for Floor at Wesleyan Hall, Westminster ....... 180
255. — Centering Resting on Flanges of Steel Joists . . . . . . . . .181
256. — Centering Suspended from Flanges of Steel Joists . . . . . . . .181
257. — Centering for Concrete Floor having Steel Main Beams . . . . . . .181
258. — Centering for Arch Ceiling between Joist Flanges . . . . . . . .181
259 and 260. — Floor Centering supported by Hangers ........ 181
LIST OF ILLUSTRATIONS xv
FI°- PAGE
261. — Form for Wall : Part raised to Second Position . . . . . . . . 182
262.— Section of Wall Form 182
263.— Panel for Wall Form .182
264 and 265. — Plan and Cross Section of Shored-up Form for Wall ... . 183
266. — Form for Simple Foundation Wall .......... 184
267. — Ransome's Wall Form . . . . . . . .' . 184
268.— Panel Shutter .184
269.— Collar and Set-screw at x (Fig. 267) . . 184
270. — Side of Form for Imitating Masonry Wall 185
271. — Form and Centering for Staircase ........ . 185
272. — Stringers and Carriage of Stairs ......... 185
273.— Form for Steps .185
274. — Metal Panel Form for Walls ........ 186
275. — Method of fastening Panel Flanges together . . . . . . . . . 186
276. — Detail of Metal Beam Form ............ 186
277. — Ransome Form for Cornice . ... . . . . . . . . . 186
278. — Form for Spandrel Wall to Bridge . . . . 186
279. — Form for Curtain Wall with Moulded Cornice ........ 186
280. — Two Sections of Form for Cornice to Hollow Coping ....... 187
281. — Form for Ornamental Parapet ........... 187
282 and 283. — Vertical and Horizontal Sections of Form for Battered Retaining Wall . . 187
284 and 285. — Plan and End Elevation of Form and Centering for Retaining Wall at Bridlington 188
286. — Part End Elevation and Section of Centering for Retaining Wall at Local Government Offices 189
287. — Diagram, Plan, and Details of Centering for Retaining Wall at Local Government Offices . 189
288 and 289. — Plan and Section of Centering, etc., for Retaining Wall in Deep Basement . . 190
290 and 291. — Elevation and Plan of Form and Centering for Silo . . . . . . 191
292 and 293. — Vertical Section and Plan of Centering for Dome of Wesleyan Hall, Westminster 192
294. — Centering for Dome at Annapolis New Academy 193
295 to 297. — Centering for Octagonal Dome, Circular in Section ...... 193
298 and 299.— Reinforcing the Large and Small Half Cupolas of the Poti Cathedral . . .194
300 and 301.— Centerings for Belfry and Arches of the Poti Cathedral 195
302 to 305. — Centerings and Reinforcement for Cupolas and Vaulting at the Poti Cathedral . 196
306.— Centering, etc., for Chimney at Northfleet 197
307.— Centering for Flat Bridge 198
308. — Centering for Melan Arched Bridge. . . . ... . . . . 198
309. — Elevation of Centering for Arched Principal, Hammersmith Baths ..... 199
310. — View of Centering for Arched Principal, Hammersmith Baths. .... 199
311 and 312.— Centering for Arch Ring Bridge 200
313. — Part Elevation and Detail of Centering for Bridge at Teufen ...... 200
314.— Detail of Bridge Centering (see A, Fig. 313) . . 201
315.— View of Part of False-work for Bridge at Teufen 201
316 and 317. — Centering for Almandares Bridge, Havana . . . . . . . . 201
318.— Centering for Meadow Street Bridge, Pittsburg 202
319 and 320.— Centering for Bridge of 233-ft. Span 202
321. — Centering for Bridge over River Werra in Thuringia . . . . . . . 203
322.— Centering for Bridge of 229-ft. 6-in. Span 203
323.— Centering for Small Bridge •...''. . . .203
324.— Centering for Walnut Lane Bridge, Philadelphia . 204
325.— Centering for Bridge of 126-ft. Span 204
326.— Centering for Bridge of 138-ft. Span 204
327.— Centering for Bridge of 80-ft. Span . . 205
328.— Centering for Bridge of 110-ft. Span . . . .205
329 and 330.— Centering for Bridge at Deer Park, U.S.A. . 205
331.— Centering for Flat Bridge of 42-ft. 7-in. Span 206
332.— Centering for Flat Bridge of 88-ft. 6-in. Span . . 206
333.— One of the Three Spans of the Chickahominy River Bridge 206
334. — Elevation of Panel of Suspended Centering ......... 206
335. — Section of Panel of Suspended Centering. ......... 206
336. — Cables supporting Concrete Sections, showing Key Spaces to be filled in . . . . 207
337.— Arch Ribs before Striking Suspended Centering . . 207
xvi LIST OF ILLUSTRATIONS
FIG. PAGE
338. — Arch Bibs formed in Suspended Centering ......... 208
339.— Centering for 6-ft. Sewer 208
340.— Centering for 8-ft. 6-in. Sewer 208
341. — Centering for Conduit at Jersey City 209
342.— Centering for 5-ft. Arched Culvert 209
343.— Centering for 8-ft. Arched Culvert 209
344.— Form for Small Box Culvert . . 209
345.— Collapsible Steel Centering for Sewer 210
346. — Conduit at Woolwich, showing Collapsible Steel Centering in Use ..... 210
347 and 348.— Form for Square Tank 211
349 to 352.— Form for Circular Tank 211
353 and 354.— Form and Centering for Gasholder Tank 212
355.— Form for Gasholder Tank at San Sebastian 213
356.— Gasholder Tank at San Sebastian 213
357. — Elevation, Plan, and End View of Form for Tapered Square Posts ... .214
358. — Cross Section (enlarged) through Form for Tapered Square Posts . . . . .214
359. — Plan and Elevation of Multiple Form for Tapered Square Posts ... . 214
360. — Section of Form for Triangular Posts 214
361 and 362.— Armoured Tubular Floor 215
363.— Coignet Beam and Slab 216
364. — Coignet Beam Reinforcement Consisting of Group of Small Bars . . . . .216
365. — Beam Supporting Floor Slab Centering .......... 216
366. — Section through Coignet Column ' t . 216
367 and 368.— Base of Coignet Column 216
369 and 370.— Coignet Pipe or Conduit 216
371.— Corr Bar Beam " Unit " .217
372.— Types of Corr Bars 217
373.— Dentile Floor with Mitre Tiles .218
374.— Dentile Floor with Bridge Tiles .218
375.— Dentile Floor with " L " Tiles .218
376.— Diamond Mesh Expanded Metal .218
377.— Bib Mesh Expanded Metal .218
378.— Section of Expanded Steel Bar . 219
379.— Four Types of Expanded Metal Floors . . 219
380. — Hennebique Stirrup round Tension Bar . 220
381. — Hennebique Tension Bars and Stirrups .... . 220
382. — Simple Hennebique Beam • 220
383 and 384. — Hennebique Beams Continuous over Intermittent Supports .... 220
385. — Beam Beinforcement with Compression Bar and Double Stirrups ..... 220
386. — Hennebique Column 220
387 and 388. — Base of Hennebique Column ....... . 220
389.— Sheet Piles .220
390.— Hollow Diaphragm Pile . .220
391. — Square Section Indented Bar 221
392.— Bound Section Indented Bar . .221
393.— Johnson's Steel Wire Lattice .... .222
394 to 397. — Four Methods of Supporting Mesh-reinforced Floors 222
398.— View of Kahn Bar 223
399. — Section and Elevation of Kahn Trussed Bar 223
400 and 401.— Lintel Beinforced with Kahn Bar ... 223
402. — Keedon Beam Beinforcement ... . 223
403. — Keedon Column Beinforcement . . 22&
404. — Column and^Beam Beinforcements, Keedon System 223
405. — Lock-woven Mesh ... . 224
406. — Lock- woven Mesh Floor . . . 224
407. — Fireproof Construction with Lock- woven Mesh. . 224
408. — Floor Slab Supported by Four Columns, Mushroom System 224
409. — Head of Column, Mushroom System . . 224
410.— Paragon Stirrups . .225
411.— Paragon Column, Beam, and Floor Slab . 225
LIST OF ILLUSTRATIONS xvii
FIG. PAGE
412. — Paragon Column Hoopings ............ £25
413 and 414. — Paragon Helical Column Wrappings ......... 225
415. — Piketty Beam .............. 226
416 and 417. — Piketty Beams with Two and Three Tension Bars ...... 226
418 and 419. — Cross Sections of Piketty Beam with Four Rows of Bars ..... 226
420 and 421. — Square Piketty Column ........... 226
422 and 423.— Round Piketty Column 226
424. — The Upper Storeys of a Reinforced Concrete Warehouse at Cologne, Front View . . 230
425. — End View of Reinforced Concrete Warehouse at Cologne 230
426 to 428. — Vertical and Horizontal Sections of Small Hall with Arched Ribs . . . 231
429. — Interior View of Hall with Arched Ribs . . . . . . . . .231
430 to 432. — Vertical and Horizontal Sections of Small Hall with Barrel Roof . . . 232
433. — Interior View of Hall with Barrel Roof .......... 232
434 to 436. — Vertical and Horizontal Sections of Small Hall with Modified Barrel Roof . . 233
437 to 439. — Vertical and Horizontal Sections of Small Hall with Pierced Arched Ribs . . 234
440. — Interior View of Hall with Pierced Arched Roof ........ 234
441. — Interior View of Hall with Arched Ribs and Vertical Columns . . • . . . 235
442. — Part Longitudinal Section of Hall with Arched Ribs and Vertical Columns . . . 235
443. — Cross Section of Hall with Arched Ribs and Vertical Columns ..... 236
444. — Part Longitudinal Section of Exhibition Hall with Three Flat Saucer Domes . . . 237
445.— Part Plan of Exhibition Hall . 237
446.— Interior View of Exhibition Hall 238
447. — Cross Section of Exhibition Hall through one of the Domes 238
448.— Part Plan of Ceiling of Exhibition Hall 238
449 and 450. — Elevation and Horizontal Section of Reinforced Concrete Fa9ade suitable for Club 239
451 and 452. — Detail of Reinforced Concrete Fa9ade in Elevation and Vertical Section . . 240
453. — Small Hall with Mask Walls of Brick and Principals and Roof of Reinforced Concrete . 241
454 to 457. — Longitudinal Section, Plan, Front Elevation and Cross Section of Hall with Reinforced
Concrete Principals and Roof ........... 242
458. — T-piece for Sand-blast Apparatus ........... 244
459.— A Type of Bush Hammer . 244
460 to 462. — Front and End Elevations and Plan of " Granolithic Plate " .... 245
463.— Sand Concrete, 1 : 2. Full Size 246
464.— Crushed Stone Concrete (Cement 1, Yellow Bank Sand 2, and f-in. Screen Stone 3). Full Size 246
465. — Pebble Concrete with Scrubbed Surface (Cement 1, Bar Sand 2, and iVin. White Pebbles 3).
Full Size 247
466.— Granite Grit Concrete (Cement 1, Bar Sand 2, and J-in. Granite Grit 3). Full Size . . 247
467._Pebble Concrete (Cement 1, Bar Sand 2, and Screened Yellow Pebbles 3). Full Size . . 249
468.— Sand Concrete, 1:3 249
469. — •Californian State Normal School at San Jose ......... 251
470. — Single-air-space Mortar Blocks (Sand and Portland Cement) Laid in Fireclay, after Firing
and Quenching .............. 260
471. — Double-air-space Mortar Blocks, after Firing and Quenching 260
472. — Terra-cotta Three-air-space Partition Tiles Laid in Cement Mortar, after Firing and Quenching 260
473. — Various Building Stones Laid in Cement Mortar, after Firing and Quenching . . . 260
474. — Concrete Blocks Laid in Fireclay, after Firing and Quenching 261
475. — Granite Blocks Laid in Cement Mortar, after Firing and Quenching . . . .261
476. — Graph Showing Relative Water Absorption of Concrete 267
477. — Section through Floor Slab supported by Two Beams ....... 273
478. — Cross Section through Beam Centering .......... 274
479.— Horizontal Thrust at Crown of Arch 288
480. — Three Forces Acting on Half of Arch .......... 288
481.— Finding Line of Thrust in Arch • 289
482. — Forces acting on Arch to Produce Equilibrium ........ 289
483.— Three-hinged Arch . . .290
484.— Two-hinged Arch . . 290
485. — Foot-bridges at Railway Terminals, Vera Cruz ...... . • • • 290
486. — Foot-bridge at Railway Terminals, Vera Cruz .... ... 291
487. — Foot-bridge over Weisseritz, at Cotta-Dresden 292
488. — Moortown Bridge. Wimborne, Dorset 293
xviii LIST OF ILLUSTRATIONS
FIO. PAGE
489. — Decking and Parapets of Bridge in Cowley County, Kansas ...... 293
490. — Underneath View of Main Floor Span of Bridge in Cowley County, Kansas . . . 294
491. — Elevation and Details of Bridge in Cowley County, Kansas ...... 294
492. — Bridge over Merrimac River, New Hampshire ........ 295
493. — Foot-bridge at Mizen Head 296
494. — Erecting the Eibs for Foot-bridge at Mizen Head ........ 296
495. — Suspended Centering for Bridge ........... 297
496. — Bridge Crossing the Bremba, Italy . . 297
497.— Bridge at Kiel Dock Works 298
498. — Oceanside Bridge over San Luis Key River, California ....... 300
499.— Scenery Hill Viaduct, Philadelphia . 301
500.— Meadow-Street Bridge, Pittsburg 302
501. — Sectional Plan, Elevation and Details of Meadow Street Bridge, Pittsburg . . . 303
502.— Grafton Bridge, Auckland 304
503. — Bridge over River Werra, Thuringia . . . . . . . . . . 306
504. — Elevation and Plan of Bridge over River Werra ........ 307
505. — Details of Beams for Bridge over River Werra ........ 307
506. — Plan and Elevation of Walnut Lane Bridge, Philadelphia 308
507. — Bridge near Teufen, Switzerland 309
508. — Retaining Wall with Asphalt Damp-proof Course .311
509. — Timbering to Support Earth, before Building Retaining Wall (R. A. C.'s Building) . . 312
510. — Shuttering and Reinforcements for Retaining Wall (R. A. C.'s Building). . . . 312
511.— Retaining Wall Complete (R. A. C.'s Building) .313
512. — Inside Face of Retaining Wall, and Methods of Supporting Mains (R. A. C.'s Building) . 314
513.— Section Showing Front Vaults (R. A. C.'s Building) 315
514.— Plan of Grillage Foundation (R. A. C.'s Building) 315
515. — Section through Top Layer Beams of Grillage ........ 315
516. — Section through Bottom Layer Beams of Grillage ........ 315
517. — Trench for Grillages under Stanchions (R. A. C.'s Building) ...... 316
518. — Asphalt Damp-proof Course under Base of Back Retaining Wall (R. A. C.'s Building) . 317
519.— Section of Steel Pile 317
520.— Top of Finished Steel Piling 318
521.— Cast-iron Cap to Head of Steel Pile 318
522. — Reinforcement and Centering for Floor (R. A. C.'s Building) 319
523. — Elliptical Reinforced Concrete Lintels 320
524. — Shuttering to Curb of Skylight over Elliptical Vestibule (R. A. C.'s Building) . . .321
525. — Shuttering and Reinforcements for Sloping Roof (R. A. C.'s Building) .... 322
526. — Shuttering and Reinforcements for Sloping Roof (R. A. C.'s Building) .... 323
527. — Reinforcement around Elliptical Openings in Sloping Roof (R. A. C.'s Building) . . 324
528.— Shuttering for Swimming Bath 325
529. — Reinforcements for Swimming Bath (R. A. C.'s Building) 325
530. — Reinforcements for Swimmng Bath (R. A. C.'s Building) 326
531. — Reinforcing Sides of Bath and Concreting First Layer of Bars in the Bottom . . . 327
532. — System of Handling Concrete at Royal Automobile Club's Building .... 329
533. — Columns and Arched Ribs in the Royal Liver Building ....... 330
534. — Arched Beam (before Concreting) in Royal Liver Building . . . . . .331
535.—" Skeleton " of Royal Liver Building . 331
536.— Cathedral at Manila, Philippine Islands 332
537. — Various Sections through Nave of Manila Cathedral ....... 333
538. — Longitudinal Section (looking north) and Details of Manila Cathedral .... 334
539 and 540.— West and East Elevations of Manila Cathedral 334
541. — Interior View of Manila Cathedral (Looking West) ........ 335
542. — Interior View of Manila Cathedral (Looking East) 335
543. — North Elevation of Manila Cathedral with Plan of Organ Room (A) and Section of Window
Jambs (B) 336
544. — Details of Nave, Manila Cathedral 336
545.— Details of Chancel 336
546. — Longitudinal Section through Poti Cathedral 337
547 and 548. — Elevation and Horizontal Section of Dome of Poti Cathedral 338
549. —Ground Plan of Poti Cathedral . 338
LIST OF ILLUSTRATIONS xix
FIG. PAGE
550 and 551. — Cross Section and Plan of Wesleyan Hall ....... 339
552. — Key Plan showing Positions of Beams over Tea Room, Wesleyan Hall .... 340
553. — Section and Plan of Arch over Tea Room, Wesleyan Hall . . . . . . 340
554.— A Typical Pillar, Wesleyan Hall 340
555 and 556. — Details of Ceiling and Floor over Tea Room, Wesleyan Hall .... 341
557 and 558. — Details of Arched Ceiling to Basement Hall, Wesleyan Hall .... 342
559. — Cross Section, through Part of Arched Ceiling to Basement Hall, Wesleyan Hall . . . 343
560.— A Beam in the Floor of Conference Hall, Wesleyan Hall 343
561 to 564. — Sections through Floor of Conference Hall, Wesleyan Hall ..... 344
565. — Section through and Details of East Gallery, Wesleyan Hall ...... 345
566. — Free Church, Hampstead Garden Suburb, in Course of Construction .... 346
567. — South, West, and East Elevations of Free Church, Hampstead Garden Suburb . . 347
568 and 569. — South Elevation and Ground Plan of St. Barnabas' Church, Jamaica . . . 348
570. — Longitudinal Section through St. Barnabas' Church, Jamaica ...... 349
571 and 572. — East and West Elevations of St. Barnabas' Church, Jamaica • . . . 349
573.— North Elevation of Chancel 349
574. — Details of the Reinforcement, St. Barnabas' Church, Jamaica ...... 350
575. — Sections through Nave and Chancel, St. Barnabas' Church, Jamaica . . . • 351
576. — Balcony and Cantilevers in Majestic Theatre, Los Angeles ... . . . 351
577. — Cantilevers at Majestic Theatre, Los Angeles ......... 352
578. — Trussed Girder at Majestic Theatre, Los Angeles ........ 352
579 and 580. — Trussed Girder in Majestic Theatre, Los Angeles ...... 353
581. — Roof Plan and Details, National Gallery Extension. ....... 354
582. — Details of Intermediate and End Ribs of Dome Roof to National Gallery Extension . . 355
583 and 584. — Dome Roof to National Gallery Extension ....... 355
585. — Detail of Ribs to Dome, National Gallery Extension . . . . . . . 356
586. — Detail of Intermediate Ribs, Northern Gallery ........ 356
587.— Detail of Intermediate Ribs, West Gallery 356
588.— Detail of Pendentive . . .356
589.— Detail of End Ribs, Northern Gallery . . . 356
590.— Detail of End Ribs, West Gallery 356
591. — Bunker at Kinlochleven Aluminium Works ......... 357
592. — Section through Bunker at Kinlochleven Aluminium Works . . 357
593.— Detail of Beam of Bunker (A, Fig. 592) ... .357
594 and 595.— Plan and Section of Foundation Slab, Royal Insurance Building . . 358
596. — Details of Strong Room in Sub-basement, Royal Insurance Building . 359
597 and 598. — Vertical and Horizontal Sections of Dome over Main Entrance, Royal Insurance
Building .360
599 and 600.— Walls with Post Buttresses ... .361
601 to 604.— Moulding Box for Fence Posts . . .362
605.— Fence Posts .362
606. — Reinforcement for Fence Post .... • 363
607. — Iron Straining Post • • 363
608 to 610.— Reinforcement and Shuttering to Wall . . 364
611.— Retaining Wall, Dilworth Street, Pittsburg . .364
612.— Reinforcement of Heel of Retaining Wall • 364
613 to 616.— Photographs, Elevation, Plan and Sections of Spiral Staircase . . 365
617. — Stairway in Mclntyre Building, Salt Lake City ... .367
618.— Staircases in Ritz-Carlton Hotel, New York .
619.— 247-ft. Chimney at Northfleet
620. — 144-ft. 9-in. Chimney near Drury Lane, London • • 369
^70
621. — Sewer Pipe Reinforcements, Paris
622.— Sewer Construction at St. Louis, U.S.A.
623 and 624. — Sewer Construction at Acton, London
625.— Cross Section of Culvert at Kilton .
626. — End Elevation of Culvert at Kilton
627. — Cross Section of Conduit at Woolwich .
628.— Wharf at Dennemont ....
629. — Details of Wharf at Dennemont .
630.— Details of Jetty Head at Thames Haven .... ... 375
LIST OF ILLUSTRATIONS
FIO. PAGE
631 and 632. — Jetty Head at Thames Haven, in Course of Construction ..... 376
633 and 634. — Piles and Bracing of Thames Haven Jetty Head ...... 377
635. — Lifeboat Slipway, Ackergill ............ 378
636 to 639.— Lifeboat Slipway, Ackergill 379
640 and 641. — Columns and Arched Roofing of Kloof Nek Reservoir ..... 380
642. — Bridge forming part of Brooklands Motor Track ........ 381
643.— Stadium at Shepherd's Bush, London 382
644. — Platform Construction, Stadium 382
645. — Bradford Football Stand in Course of Construction ....... 383
646. — Floors, Columns, and Beams, Bradford Football Stand 383
647. — Details of Foundations and Columns, Bradford Football Stand ..... 384
648.— Plans of Bradford Football Stand 384
649. — Details of Beams and Brackets, Bradford Football Stand ...... 385
650. — Various Sections through Bradford Football Stand ........ 385
651. — Grand Stand, St. Paul, in Course of Construction ... . . . . • • 386
652. — Another View of Grand Stand, St. Paul, in Course of Construction ..... 387
653.— East and West Elevations of Grand Stand, St. Paul 388
654.— Half-elevation (to street) of Grand Stand, St. Paul . . . . % . . .388
655. — Railway Sleepers .............. 389
656.— Details of Sleepers 389
657 and 658.— Two Sections through Silos 390
659.— Section through Silo Walls 390
660.— Horizontal Section through Silos 390
661. — Horizontal Section through Silo Roof .......... 390
662. — Horizontal Section through Silos (looking upwards) ....... 390
663. — Horizontal Section through, showing Tunnel Arrangement ...... 390
664 and 666.— Water Tower at Singen 391
666.— Vertical Section of Water Tower 392
667 and 668.— Three Horizontal Half-sections of Water Tower . . 392
Introduction; What Reinforced Concrete Is
The Limitations of Concrete.— For thou-
sands of years concrete has been known
and used as a building material possessing
many valuable properties — universality and
consequent cheapness ; ease of handling,
placing, and shaping ; ability to resist fire,
water, and other destructive influences ; and
great strength under compression. (A
body, itself evenly and rigidly supported, is
under compression when a load is placed on
it tending to squeeze it.) Until the twenties
of the nineteenth century, concrete was made
by mixing together stones, gravel, etc., and
lime, but such was the weakness of the
material in tension (that is, when subjected
to a force that tended to stretch it or bend
it) that its employment had to be restricted
to such applications as foundations, rela-
tively thick walls, and the like. On the
introduction of portland cement in 1824,
the quality of concrete, particularly as
regards the tensile strength, very greatly
improved ; and modern cement manufacture
— a highly specialised industry — has pro-
duced a material with an appreciable ability
to resist a stretching force. But good as
the best cement is, it is still not good enough
for use in a structure by itself except under
compression.
Reinforcement. — Fortunately, it early-
occurred to some ingenious workers that if
they could use with it a second substance
which would supply the tensional strength
lacking in the concrete, they would then be
in possession of a well-nigh ideal material
for structural purposes. Tile, wood, bronze,
iron, and steel have all been tried, and the
preference goes to the last-named. Modern
reinforced concrete, then, is simply a com-
bination of a material (concrete) strong in
compression but weak in tension with one
(steel) that is itself strong in compression,
but so much more strong relatively in
tension.
It may occur to some readers to ask why
. steel should be used to bolster up a deficiency
in another material. Why, it might be
asked, is not the steel used alone, and full
advantage taken of its valuable qualities ?
Steel-frame construction was earlier in the
commercial field than reinforced concrete,
but it has not prevented the latter system
from forging ahead and making a truly
notable progress. There must be a good
reason for that, and it is to be found in a
number of considerations. Steel is relatively
expensive ; the minimum quantity is used
in reinforced concrete construction. A steel
structure involves the making of thousands
of riveted joints, and calls for a large amount
of skilled work ; reinforced concrete con-
struction is monolithic (literally, "as one
stone "), and there is practically no jointing.
Steel is corroded by atmospheric action
unless immediately protected, and the pro-
tection needs periodic renewal ; concrete is
scarcely affected by atmosphere, requires
no protection and no maintenance, and it
preserves the steel even brighter than when
it was inserted ; indeed, steel soon loses a
coating of rust when embedded in concrete.
Most important of all, an unprotected steel
structure is the worst possible for resisting
fire, by reason of the metal being such a
good conductor of heat ; the steel rapidly
expands as the temperature rises and con-
tracts again as the cold water from the hose
reaches it, thus pushing and pulling apart
the elements of the structure and often
causing complete ruin ; then its softness,
should an extreme temperature be reached,
robs it of its strength, and the steel girders
become bent and twisted into a mere
entanglement. On the other hand, concrete
is a poor conductor of heat, and therefore well
protects the embedded steel and localises
any ill effects. Keinforced concrete possibly
brings the goal of an absolutely fireproof
structure within the attainment of our own
time.
Terminology. — Keinforced concrete, in
the commercial acceptance of the term, is
REINFORCED CONCRETE
concrete in which steel rods have been em-
bedded to increase its strength. The term is
somewhat vague, but no completely satis-
factory substitute for it has been suggested.
The material was (and often is) known on
the Continent as " Monier concrete," but
this does not explain itself, and may there-
fore be dismissed. " Ferro-concrete " — one
of the best terms yet introduced — has been
monopolised as a trade name, although it is
used as a generic term in technical literature ;
even that designation, though, is not strictly
correct, since " ferro " of course, is formed
from " ferrum " or " fer " (respectively Latin
and French for iron) ; but iron, as a struc-
tural material, has long given place to
steel, made by " alloying " iron with carbon.
The term " reinforced concrete " does not,
unfortunately, suggest the material employed
with the concrete ; for example, at an early
date concrete was reinforced with wood ;
indeed, wood-reinforced concrete, known as
" ligno-concrete," is now attracting some
attention, but the term under discussion
does not differentiate between " ferro-
concrete," " ligno-concrete," or "sesso-con-
crete " (bronze-concrete), the earliest type of
reinforced concrete known. The term is
open to another objection ; in a sense, the
steel is itself just as much " reinforced "
as is the concrete, and therefore " rein-
forced steel " might seem as logical a de-
signation. The term " concrete-steel " has
its advocates, amongst whom we beg to
take our place ; but it has not caught
the imagination to the extent that some of
the other terms have. Much the same
can be said of " armoured concrete." It
appears that the one term — other than
trade names — likely to become universal is
" reinforced concrete," and for that reason
it has been adopted as the title of this book,
the more precise " concrete-steel " finding
a place in the sub-title.
A Compound Material. — Concrete-steel
may be regarded as a compound material,
and not as two distinct materials. The steel
is wholly embedded in the concrete, and the
adhesion between the two, after the maturing
of the concrete, is such that a force sufficient
to pull the bars from the concrete would re-
quire to be 500 Ib. to 600 Ib. per square inch
of surface contact, although it is regarded
as safe to allow only 100 Ib. per square inch.
Further, the two materials may be con-
sidered as expanding and contracting at
the same rates. The coefficient of expansion
of concrete is -000006 (per 1° F. change of
temperature), and that of steel averages
•0000066 ; the difference is therefore only
1 in about a million and three-quarters.
In spite of all this, there exists a division
in the ranks of the concrete-steel engineers
and specialists. On the one hand are those
who believe that the natural adhesion of the
concrete to the steel is sufficient to answer
all ordinary requirements. On the other are
those who say that experience teaches that
this adhesion ought to be assisted by a
mechanical bond, and they therefore " de-
form " the bar to remove the least likelihood
of its slipping when subjected to severe ten-
sion, this being a time at which the sectional
area of the bar is liable to be reduced, thus
lessening or destroying the adhesion. De-
formed bars are also used to allow of the
employment of a steel having a greater
tensile strength than mild steel. The chief
Continental systems use plain bars, while
the chief American systems use bars of
special shape. This subject suggests other
considerations, which would, however, lead
us too far into the theory of the matter,
and which will therefore be relegated to a
later chapter.
Comparisons. — Stated in approximate
terms, the compressive strengths of concrete
and steel are as 1 : 28 ; the tensile strengths
as 1 : 280 ; and the weights, bulk for bulk,
1:4. To these ratios may be added those
of cost, bulk for bulk ; let us assume the
approximate costs of good concrete (mate-
rials only) and steel to be 1 : 80, although
this of necessity will vary. Working on
these data, some interesting comparisons
may be made. Figs. 1 to 3 show sections
of circular columns, all assumed to be of
the same height and capable theoretically of
supporting the same weight. The concrete
column must be 28 times the area in cross-
section of that of the steel column, and
nearly 1| times that of the concrete-steel
one ; roughly, the relative areas would be
about : steel, 4 ; plain concrete, 112 ; rein-
forced concrete, 80. This comparison does
not adequately show the true advantage of
reinforced concrete, since the reinforcement
in a tall concrete column is absolutely neces-
sary, the rods and hooping preventing the
concrete from bursting under a heavy load.
Again, in the case of eccentric loading, one
side of the column may be in tension, and
steel is then required to take this stress.
The relative sizes will depend upon the
INTRODUCTION
percentage of reinforcement, and the fore-
going is based upon the minimum usually
employed. In the case given, the area of
steel in the steel column is twice that in the
reinforced column, and it is this material
that is the more expensive of the two.
Again, the weight of the concrete column
the relative costs (material only) working
out as (roughly) 6-2, 1-7, and 1. This
economy of steel is possible because all the
steel in the reinforced concrete beam is
placed as far as possible from the " neutral
axis," and can therefore be stressed and
utilised to its full value ; whereas in the
AEEA-200
Fig. 2
AEEA- 5600
Fig. 1
S>[EEL- 100 £<3. INO.
TOJAL M?E.A- 4000 SQ. IH5
Fig. 3
Figs. 1 to 3. — Columns of Plain Concrete, Steel, and Reinforced Concrete, respectively :
Comparative Diagrams
is 7 times that of the steel one, and about
1*3 times that of the concrete-steel ; but
the costs (for materials alone) will be approxi-
mately in the proportions of 7, 20, and
(nearly) 15.
Now consider three beams, respectively of
concrete, steel, and reinforced concrete, of
the same depth and of the same value of
resistance — that is, capable of supporting
the same load. Figs. 4, 5, and 6 are cross
sections of these beams, d indicating depth
steel beam, much of the metal is adjacent
to the neutral axis, and part of its value is
lost. (When a beam is loaded reasonably,
it tends to bend, and its length is altered,
but there is a layer of fibres or a plane, the
length of which remains unaltered ; in a
cross-section this " neutral surface " is, of
course, a line, which is known as the " neutral
axis.")
It will be understood by the practical
reader that the diagrams to this chapter are
A
X
i
PLAIN CONCRETE:
Fig. 4
ARtA= 3-3625 1 AietA or ajttL = '67^ i
Fig. 6
(Hi.
Fig. 5
Figs. 4 to 6. — Beams of Plain Concrete, Steel, and Reinforced Concrete, respectively :
Comparative Diagrams
(the same in all three cases) and 6 breadth.
Expressed in inches, the concrete beam has
a breadth of 76 in., the steel beam -285 in.,
and the reinforced concrete beam 8 in. Their
relative weights are (roughly) 70, 1, and 7£.
this might seem to show an advantage for
the steel beam, but it will be noted that the
steel beam contains nearly five times as
much steel as the reinforced concrete beam,
purely comparative, as steel is not commonly
used in the form shown. If for the rect-
angular steel beam were substituted a rolled
steel joist having an equivalent moment of
resistance, its area would exceed 3£ sq. in.,
that is, more than five times that of the
steel in the reinforced concrete beam.
These comparisons show at a glance that
it is cheaper to use concrete than steel to
REINFORCED CONCRETE
withstand compression ; but that to with-
stand tensional stress concrete theoretically
would cost two and a half times as much as
the steel, but, practically, would be im-
possible. Keinforced concrete effects the
compromise between the two ; where it
costs more than either steel or concrete
alone, it offers advantages not possessed
by the material with which it is compared.
The parallel columns below put the case for
the employment of reinforced concrete at
a glance.
CONCRETE
STEEL (MILD)
REINFORCED CONCRETE
Weak in tension
Strong in compression
Cannot resist a strong shearing
force
A relatively cheap material
Very heavy, strength for strength
Fire-resisting
Not attacked by weather and
atmosphere ; low mainten-
ance cost
Easy to place and shape
Very strong in tension
Very strong in compression
Can resist a fairly strong
shearing force
A relatively expensive material
Very light, strength for strength
Destroyed by fire
Rapidly oxidised ; high main-
tenance cost
Difficult to place and shape
Strong in tension
Strong in compression
Can be made to resist a strong
shearing force
Price competitive with that of
any other system
Lighter than plain concrete,
strength for strength
Fire-resisting
Not attacked by weather and
atmosphere ; low mainten-
ance cost
Easy to place and shape
Historical Notes
THE history of a practical science is seldom
complete and proper without an early refer-
ence to the ancient Eomans. The history
of reinforced concrete is not one of the
exceptions. Readers will be familiar with the
oft-repeated statement that the Romans
commonly used this system in the construc-
tion of their public buildings ; but while this
is an exaggeration of the truth, there are
certain unassailable facts that stand out
prominently. The Romans made good con-
crete which can be seen to-day ; they antici-
pated ligno-concrete (wood-reinforced con-
crete) ; they combined tiles and concrete ;
and they roofed the Frigidarium of the Baths
of Caracalla with a coarse concrete, made
with lime hydra ulicised with trass or volcanic
scoria, and reinforced with bronze and iron
rods. It is fairly certain, however, that
there was no general knowledge at the time
of the principle of reinforcement as it is
understood to-day. There is no reinforce-
ment, be it noted, in the Pont du Gard, a
bridge in the south of France erected about
56 B.C., and still in existence — a worthy
memorial to honesty of construction ; in this
bridge the coarse material (stones) and
cementing material (lime) were not mixed to-
gether, but were apparently placed in alter-
nate layers. It is difficult to believe that
for eighteen centuries the idea applied by
the old Roman bath builder could have lain
dormant ; but while we know that concrete
of a sort was commonly used, there is no
evidence of the principle of reinforcing it, and
so extending its usefulness, having been
commonly applied at any time during that
long period, unless exception is made in the
case of the dome of St. Paul's Cathedral
(1675-1710), in which, as is well known, Sir
Christopher Wren caused chains to be
embedded in concrete to help in resisting the
lateral thrust. Certain of these chains were
examined some years ago, and were found not
to have corroded, although the concrete had
been continually in a damp state.
Early in the Nineteenth Century.—
It is not until 1830 that we find any definite
mention of the idea of reinforced concrete,
in which year was published J. C. London's
" Encyclopaedia of Cottage, Farm, and
Village Architecture," containing the sug-
gestion that roofs might be constructed of
cement in which were embedded iron tie-
rods in the form of a lattice work, the whole
being cased with flat tiles. Six years pre-
viously (in 1824) Joseph Aspdin patented
his method of making (portland) cement,
and it had already (in 1828) been used
extensively in the construction of the Thames
Tunnel ; but for many years it had to fight
opposition and was not in general use. For
example, in the year 1840, when, as the
result of a strike of carpenters, some fire-
proof floors were constructed in Paris, they
failed to become popular because a local
material, gypsum (calcined to form what is
now known as " plaster-of- Paris "), was used
as the cementing material in the concrete,
the result being that the embedded iron soon
rusted. These floors were described in a
paper read before the Royal Institute of
British Architects in 1849 by G. R. Burnell,
and more fully described five years later
before that body by H. H. Burnell. Two
systems were known ; in one — the Vaux —
round rods, close together, were hooked at
each end on to a flat wrought-iron bar lying
on its edge ; in the other — the Thuasne —
iron joists were employed, stirrups hanging
from these and containing holes through
which the round reinforcing rods passed.
In each case the ironwork was embedded
in plaster concrete.
About the same time (July 9, 1840) a
reinforced ceiling slab was patented by a
Frenchman, Louis Leconte ; he proposed
" the use of trusses of iron plates for floors,
from which iron rods were suspended to carry
a wire meshwork for sustaining plaster."
There is evidence of an indefinite nature that
shortly before this period English architects
had constructed fireproof floors of concrete
or brickwork in which flat iron bars were
embedded.
A Concrete Boat. — A fertile period of
invention, as relating to the subject under
consideration, was the fifties of the nine-
teenth century. Lambot, a French con-
tractor, had proposed to build the hulls of
REINFORCED CONCRETE
boats with concrete, and at the first Paris
International Exhibition (1855) he actually
showed a flat-shaped boat, with sides 2 in.
thick, made of hydraulic lime concrete
reinforced with a skeleton of iron rods ; and
in the same year he patented his invention
in Great Britain. At the present time,
boats are often built of reinforced concrete.
Wilkinson. — In spite of the systems that
had been suggested, and of those that had
already been practically tried, the chief
credit for the invention of reinforced con-
crete is commonly ascribed to William
Boutland Wilkinson, who, on October 27,
1854, patented a method of constructing a
fire-resisting floor of concrete slabs rein-
forced with a network of flat iron rods
placed on edge or with secondhand wire
ropes. Wilkinson was a Newcastle-on-Tyne
plasterer, and as by 1852 there was estab-
lished at Gateshead-on-Tyne, by William
Aspdin, a cement factory, it may be fairly
assumed that Wilkinson was thoroughly
familiar with the use of cement concrete.
We cannot do better than quote from the
admirable " History " printed in the " Lock
Woven Mesh Handbook." Wilkinson it was
" who first suggested, in his 1854 patent,
the use of a layer of sand kept wet upon the
surface of a freshly made concrete floor, for
the purpose of allowing the concrete to gain
the maximum hardness. His patent also
included the construction of hollow partition
blocks very similar in detail to the inter-
locking plaster partitions of to-day. The
chief object, however, of the patent was the
construction of fire-proof floors ; these
Wilkinson proposed constructing both in arch
form and flat, and he suggested reinforcing
them with either flat iron bars placed on
edge, or with secondhand wire rope. Wil-
kinson states that the reinforcement was to
be placed in the concrete to take the tension,
and his drawings clearly show that he
thoroughly understood the construction of
such floors, for the reinforcement is placed
in positions in the arch construction where
it could give maximum service, while in
floor slabs the rods or wire ropes were bent
down at the centre of the span where the
maximum bending moment occurs, but in
continuous spans, or where built-in, were
carried up to the top over the supports, so
as to resist the reverse bending moment.
The ends of the rope were also directed to
be frayed out so as to bond in with the
concrete at the ends of the span ; the advis-
ability of this was evidently fully appre-
ciated by Wilkinson, namely, to give anchor*
age or mechanical bond and serve the same
purpose as the fishtail or bend at the end
of a rod in other systems of a later date.
He also described and illustrated independent
beams of reinforced concrete. His patent
showed a grasp of most of the principles of
modern reinforced concrete construction, and
it is obvious that it was drawn up from
thorough practical experience and perhaps
some theoretical grasp of the subject."
In course of time, Wilkinson erected a
number of reinforced concrete buildings in
Newcastle and elsewhere in the North of
England, of which, however, no records are
now known to exist.
Francois Coignet. — Less of the builder
and more of the engineer was the French
contractor, Francois Coignet, who in 1855
patented, both in France and England, a
system of forming floors by laying iron
" planks," or rods, crossing each other, from
wall to wall of a building ; iron beams could
be placed to support the " planks " or rods.
A false flooring is supported underneath the
reinforcement and the concrete applied.
Coignet had at an earlier date invented
" Beton Coignet " (concrete for which the
hydraulic lime and aggregates were mechan-
ically mixed in certain proportions), and the
advantage of reinforcement possibly occurred
to him of itself, or he may have remembered
the reinforced plaster-concrete experimented
with some years before. In the years follow-
ing the grant of his patent he carried out a
variety of works, including a lighthouse at
Port Said (Egypt), retaining walls in Paris,
and thirty-three miles of aqueduct for the
Paris water supply. He undoubtedly com-
monly applied his system of reinforcement
in the construction of these works, but very
little is known as to the details.
Dennett, Allen, Ransome, and Scott. —
In 1857 (patent dated March 9), a Notting-
ham builder, C. C. Dennett, took advantage
of the introduction of rolled iron joists and
constructed a floor with reinforced concrete
arches resting on _L-beams. The arch rein-
forcement was of wood or iron. This floor
was the prototype of the modern fireproof
floor of steel joists and concrete.
The 'sixties also saw a number of not-
able introductions. In 1862 (January 30)
Matthew Allen, a London builder, patented
a system of building staircases, floors, etc.,
with which he afterwards did much business ;
HISTORICAL NOTES
iron bars 3 in. by J in., on edge, were
embedded in concrete (cement 1, cinders,
etc., 3) 2 ft. apart near the under-side of the
slab, the bars crossing to form a network.
The method was sufficiently practicable to
be adopted in the construction of the
Columbia Market, Shoreditch. Frederick
Kansome, in 1865, suggested the construc-
tion of girders, etc., by moulding a kind of
cement around a hoop-iron skeleton.
The year 1867 saw two important patents.
One, by an Englishman, H. Y. D. Scott,
related to a floor of concrete with inter-
laced rods and hoop -iron or wire, supported,
not on joists, but on wrought-iron tie-rods
embedded in the concrete, and is of interest
because the specification alludes to the use
of " the tie-rods and hoop -iron taking the
tensile strain, and the concrete the com-
pressible," conclusive evidence that the
basic principle of reinforced concrete was at
that time understood.
Joseph Monier. — The other, a French
patent dated 1867, by a Frenchman, Joseph
Monier, was for the construction of plant tubs
and the like by means of concrete reinforced
with a meshwork of rods or wires. Monier
started as a gardener, and developed into a
manufacturer of gardening tools and appli-
ances. He thought of substituting for the
ordinary plant tub a vessel of concrete, but,
finding this was brittle, he hit upon the idea
of making a skeleton of iron network or
trellis and then enveloping it with mortar
or concrete. The second Paris International
Exhibition (1867) contained examples of
construction in the Monier and the Coignet
styles. Monier followed up his 1867 patent
with further inventions, patented in France
in 1873, and, after the Antwerp Exhibition
of 1879, at which he exhibited, he sold his
inventions to G. A. Wayss (of the firm of
Wayss and Co., Germany), who was soon
instrumental in introducing the new system
of building into a number of European
countries. So closely was reinforced con-
crete identified with Monier on the Continent
outside France that it was commonly known
as the " Monier system," a term replaced in
course of time, and as other systems appeared,
by the German -French " eisenbetonbau "
and " betoneisenbau " (eisen [G.], iron ;
beton [F.], concrete ; and bau [G.], build-
ing or construction — that is, " ferro-concrete
construction ").
A patent granted to T. Lythgoe and H.
Thornton, in February, 1868, is of some
interest. The inventors illustrate a floor
consisting of _L-shaped iron bars suitably
spaced and having a concrete filling between
them ; a hoop-iron reinforcement laces the
bars together, and passes alternately over
and under them ; the bars and the reinforce-
ment are wholly embedded in the concrete.
It cannot help striking the student that
the principle and almost the details of
reinforcing were re-invented quite a number
of times in the middle of the nineteenth
century. Under the present laws — British,
at any rate — it is doubtful whether any sys-
tem already mentioned as being invented sub-
sequent to 1840 — when, as has been shown,
reinforced floors were constructed in Paris —
could have been granted a valid patent.
The Monier floor, introduced in 1873, is
much the same as Allen's, patented in 1862 ;
and Allen's was almost certainly anticipated
by Wilkinson's, patented in 1854.
Phillip Brannon. — Brannon's provisional
patent in 1870, completed the next year
(October 12, 1871), shows that the inventor
had grasped the principle of reinforcing con-
crete. The specification of 1871 shows a
concrete floor reinforced with an iron mesh-
work, as well as the application of reinforced
concrete to the construction of sea defences.
Brannon was the first to suggest a reinforced
concrete pile, the longitudinal reinforce-
ments being of angle-irons united by bars
riveted latticewise across them, the whole
being wound spirally with wire. Brannon's
patents were worked by a company formed
by him and known as " The Monolithic
Fireproof and Sanitary Construction Works,
Ltd.," which in the ensuing years erected
several reinforced concrete buildings, some
at Walton-on-the-Naze, Essex, and two
(houses) in Islington.
Thaddeus Hyatt. — Between the years
1870 and 1877 much interest in the " new "
system of construction was exhibited by
Thaddeus Hyatt, an American who had
become known from his invention of glass
pavement lights. It is doubtful whether the
published accounts of Monier's and Coignet's
work in Europe had attracted his atten-
tion. Between 1870 and 1877 he had a series
of beams tested by Kirkaldy, and in these
beams both single and double reinforcements
were tried. He indulged in an interesting
series of experiments, and these led him to
believe in the necessity of anchoring the
ends of the reinforcing rods, for which 'pur-
pose he fitted them with nuts and washers
REINFORCED CONCRETE
to prevent their pulling through the con-
crete. Between the years 1873 and 1881
he obtained between thirty and forty differ-
ent patents, most relating to reinforced con-
crete ; and although he did much valuable
work in assisting * engineers to a proper
understanding of the theory of the rein-
forced concrete beam, it does not appear
that commercially Hyatt made any success
with concrete construction. Hyatt's book
showed that he regarded a reinforced con-
crete beam as corresponding to a rolled steel
joist, the steel rods being considered as
equivalent to the bottom flange and the
concrete as the top flange, the neutral axis
being assumed to lie half-way up the beam.
Later, a number of engineers made a similar
mistake.
From the 'Seventies to the 'Nineties.—
About the year 1870, a number of reinforced
concrete sewer pipes were laid in Germany,
and these are still in service. In 1877 an
experiment was tried at Croydon by W. H.
Lascelles, who applied a system based on
patents obtained earlier, and built a number
of cottages having a timber framework, the
slabs being of concrete reinforced with
diagonal rods.
We now arrive at a period in which interest
was being taken in reinforced concrete con-
struction by engineers in all the progressive
countries of Europe. Angelo Lanzoni, of
the firm of Lanzoni, Galli and Co., made
some applications of the system in Italy in
1878, the first Italian patent being taken out
by him five years later, anticipating Monier's
Italian patent by four months. In Switzer-
land, Monier's system was applied to the
erection of small vaultings as far back as
1880. In the same year, Rudolph Schuster
came to terms with Monier, and in the suc-
ceeding years carried out a series of works
in Austria, these including large numbers
of reservoirs and vats, vaulted flooring, fire-
resisting doors, etc. Deep interest had been
aroused in Great Britain, but the Monier
system, for which a British patent was taken
out, dated July 7, 1883, failed to become
successful there. The activity was not con-
fined to Europe, for in the United States —
where, in 1875, W. E. Wood had built a
reinforced concrete house — E. L. Ransome
was building reinforced concrete warehouses
about 1884, following later with a factory
building, the Californian Academy of Science
(architect, G. W. Percy), about 1888, and the
Museum Building of Leland Stanford, Junior,
University (architect, G. W. Percy), in 1892,
the last-named building containing spans of
45 ft. and being reinforced throughout. It
is stated that the Academy building with-
stood remarkably well the San Francisco
earthquake of 1906. In 1884 Ernest L.
Ransome patented in the United States a
twisted square bar reinforcement, and in the
preceding year John F. Golding obtained
an American patent for " slashed metallic
screening," otherwise expanded metal, which
was employed as a lathing for plaster,
its use as a reinforcement for concrete
dating from about 1890. Benjamin Scarles
took out American and British patents as
far back as 1884 for the use of wire cloth as
lathing, but even he had been anticipated.
In the year 1885 W. H. Lindsay patented
a system of reinforcement by which steel
rods were passed through holes made in the
webs of I- joists, one at the top and one at
the bottom ; the vertical pairs crossed, and
at the point of intersection they were made
to form a loop through which was threaded
a rod parallel with the iron joists. About
the same time, too, a practical experiment
was tried in Lincoln's Inn Fields, London,
a block of offices with plain concrete walls
and reinforced concrete floors being erected
to the designs of William Simmons.
From this point onward, we must omit the
names of many patentees and inventors,
as during the next twenty years or so there
were hundreds of inventions relating to
reinforced concrete, and scores of dif-
ferent systems were exploited in Europe
and the United States.
In Hungary reinforced concrete was first
applied in the erection of stables at the
Artillery Barracks, Kassa, in 1886, by
Robert Wiinsch, of Budapest. To take up
the thrust of the vault, the separate metallic
ribs were anchored, and the resulting system
•was at a later date widely applied in Austria
and Hungary. Into Hungary the Monier
system was introduced in 1887 by Wayss,
being first adopted for a series of barrel-
arched bridges. Merely mentioning by the
way that on March 27, 1890, J. Mayoh
patented in Great Britain the use of corru-
gated plates on edge for strengthening con-
crete floors ; that on July 7, 1891, C. A.
Day patented in Great Britain the use of
wire lattice suspended over steel joists for
reinforcing concrete slabs ; and that in the
same year Franz P. Meyenberg patented in
the United States a floor constructed of
HISTORICAL NOTES
hollow terra-cotta tubes supported on steel
rods embedded in concrete above and below,
there being loose stirrups hooked round the
bottom rods, we may pass to the period
in which, in spite of all that had been done
before it, reinforced concrete first compelled
the serious attention of all progressive
engineers.
Edmond Coignet and Frangois Henne-
bique. — The two great names are those
of Edmond Coignet (son of Francois Coignet)
and Francois Hennebique, the former of
whom, by applying the known principles
of mechanics, evolved a system of calcula-
tion that has proved remarkably truthful,
and the latter of whom, basing his methods
of calculation upon results obtained in prac-
tice, has also made extremely important
contributions to the technical consideration
of the subject. Coignet as the scientific
investigator, and Hennebique as com-
mercial organiser, are properly regarded as
" the pioneers of the modern evolution in
the art of building." The story has often
been told of the opposition which Coignet
had to fight in getting the masonry of the
proposed new system of main drainage in
Paris in 1892 replaced by reinforced con-
crete. He promised a large saving of money
and of time required for construction, and
his system, which was finally adopted, was
carried out with complete success. Henne-
bique, having organised a technical staff and
licensed a large number of the most influ-
ential contractors to work his system, was
able to secure between the years 1892 and
1899 work to the total value of two million
sterling, representing three thousand con-
structions, among the most remarkable of
these being the bridge of Chatellerault,
460 ft. long, comprising three arches, two
of 133 ft. span and one of 167 ft.
Hennebique's first patent dates from 1892
(British patent, No. 14,530), and in this he
demonstrates the utility of stirrups to rein-
force beams against shear, in which matters
he had to an extent been anticipated by
Hyatt in 1877 and Meyenberg in 1891. In
1897 Hennebique introduced cranked-up
rods, and placed these one above the other,
so as to reduce the width of the beam,
following (to some extent) the lines laid
down by Hyatt in 1877 and F. G. Edwards
in 1892, in which latter year M. Koenen and
G. A. Wayss, of Germany, patented in
England a method of floor construction with
rods cranked up at the point of contra-
flexure, " the parts in tension being strength-
ened by roughened or serrated metal rods
or strips embedded in the structure."
Progress since 1890. — Speaking of the
period following the years 1890 to 1892,
N. de Tedesco, Ingenieur des Arts et
Manufactures, and editor of Lz Citnent, has
remarked that the modern method of build-
ing remained for a long time in very few
hands, and nothing was to be found in
literature concerning the calculation of
structures constituted • by two different
materials. Contractors were obliged to ex-
periment in order to ascertain the most
suitable proportions and arrangements of
these materials for resisting the determinate
load on a determinate span, but they were
scarcely able to undertake with advantage
investigations of that kind. Such work was
rather the role of "ingenieurs des ponts et
chaussees," but many of these attempted to
co-ordinate the results of the tests carried
out by Coignet and Hennebique, only to
find that it was impossible to deduce mechan-
ical laws from them, inasmuch as the
mechanical properties of concrete are vari-
able with the quality of sand or gravel, the
cement used, the degree of fluidity of the
mixture, the climatic influences during the
hardening process, the age, and with many
other surrounding conditions. Considere
came to the rescue, and by a series of tests
undertaken upon small specimens manu-
factured under uniform conditions he was
able to discover the laws governing the
deformation of concrete submitted to stress.
In 1890 Paul Neumann, in a published
memoir, described his mode of calculation,
which recognised the relation between the
coefficients of elasticity of the two materials,
steel and concrete ; and about the same
time T. Melan published a treatise in which
he came to practically the same conclusion
as Neumann — namely, that the elastic be-
haviour of concrete was not the same under
compression as it was under tension. Melan,
by the way, invented in 1890 the system of
constructing floors and vaulting named
after him. The first important contribu-
tion to a rational theory was the memoir
written by Edmond Coignet and N. de
Tedesco, and published in the Bulletin of the
Societe des Ingenieurs Civils of France for
March, 1894. Into Holland and Denmark,
reinforced concrete was introduced about
1890, the floors of the National Gallery
at Copenhagen being built of reinforced
IO
REINFORCED CONCRETE
concrete in 1891, the same system being
used in the next year or so for har-
bour works, and in 1894 for a 19-metre
span bridge for foot passengers, just out-
side Copenhagen. In Switzerland, where
reinforced concrete had previously been used
for vaulting, 1892 saw the introduction
of Hennebique beams subjected to bending
stress. Prof. F. Schule, of Zurich, has
pointed out that two conditions favour
the development of the new mode of con-
struction in Switzerland : first, Switzerland
has to import all its steel, and reinforced
concrete involves a saving in this material
not to be undervalued ; and, secondly, the
excellent position occupied by Switzerland
with regard to the manufacture of port-
land cement. Both in Hungary and Italy,
reinforced concrete was now making rapid
progress, and by 1896 a bridge having a
span of more than 25 metres was being
erected at Sarajevo, in Bosnia. In 1897 the
Hennebique system was introduced into
Great Britain by (the late) L. G. Mouchel,
Francois Hennebique's partner, and since
then its progress in this country has been
truly remarkable.
Coming to the Paris International Exhibi-
tion of 1900, the graceful palaces built by
Edmond Coignet and Francois Hennebique
attracted the favourable notice of architects
and engineers the wide world over, and
thousands of illustrated papers throughout
the Continents published photographs of the
Chateau d'Eau, erected by Coignet. If there
was anything necessary to stimulate the
adoption of reinforced concrete design by
the engineers and architects of the day, it
was afforded by this advertisement, from
which dates an interest in the subject out
of all proportion to that formerly existing.
Of the few further inventions which it is
worth specially to note may be mentioned
the Siegwart beam, introduced in Switzer-
land in the year 1902. The inventor, a
Lucerne architect, desired to avoid con-
struction on the building site other than
the actual laying of the prepared beam, and
his solution of the problem is satisfactory
where the span does not exceed about
5 metres. The Visintini system, invented
in Austria and patented in Great Britain in
1902, introduces framed beams into the
reinforced concrete construction, and it came
into practical employment in 1903, later
coming into use for the vaulting of the
Evangelical Church at Aussig.
A specification of particular interest is
No. 24,371 of 1904, the first British patent
granted to Edmond Coignet. It describes
and illustrates beams, floors, domes, roofs,
columns, and walls, and has a high educa-
tional value. The drawings of beams show
the tension bars in the lower part of the beam
to be connected to the compression bars in
the upper part by means of ties. These ties
also hook over the tension bars, which extend
through the floor slabs and which are bent
up to pass over the compression rods.
Notable American Inventions. — As a
result of the rare commercial ability behind
them, certain American systems have be-
come extremely well known in Great Britain,
and perhaps in consequence of this fact
there is a common tendency to look upon the
United States as the real " home " of
reinforced concrete. While there is no
doubt that the temperament of the Americans
led them to welcome the " new " method
of building (as, indeed, they welcome any
promising invention), as a matter of fact,
there is not, prior to 1892, much that is of
truly American origin and of real note in
the United States records with regard to
reinforced concrete excepting the Thaddeus
Hyatt inventions and the Ransome twisted
bar, to both of which reference has already
been made. In 1892, August C. Storck, of
St. Louis, patented a method of strengthen-
ing a concrete block with an embedded
" brace " of slashed (expanded) metal ; and,
in the following year, Thomas A. Lee, of
Kansas City, filed a specification for a " non-
shearing rod for strengthening concrete,"
two or more rods to be laid together spirally ;
and E. L. Ransome took out what is appar-
ently his first patent for a definite method of
reinforcing concrete, his specification describ-
ing and illustrating a slab supported at its
ends by walls and reinforced by two bars, a
shorter one above a longer one, the bars
tapering to nothing at each end.
Some years previous to this, Ransome had
erected some important buildings in rein-
forced concrete.
In 1896, Edwin Thacher, of Detroit, filed
an interesting specification showing a con-
crete arch between abutments, the arch being
reinforced with two series of flat metal bars
in pairs, one bar of each pair above the
other. At intervals, the bars had pegs which
projected on each side, apparently to aid
adhesion. Ira A. Shaler, of New York,
patented in 1900 a method of reinforcing
HISTORICAL NOTES
ii
slabs by means of a metal skeleton comprising
longitudinal and cross members, the one
welded to the other. Then came, in 1901,
two important E. L. Eansome patents ; in
the earlier, webs of hardened reinforced con-
crete are placed in position, the spaces be-
tween them spanned with falsework, and
the top compression member of the floor
then made with concrete ; in the later, a
reinforced concrete floor extends to the
exterior face of a building, and there becomes
a belt course which caps the piers, a down-
ward extension forming heads or lintels to
the windows below. From this date the
inventions become too numerous even to
mention, but space must be found for a
reference to the first Kahn patent, filed on
December 11, 1902, and bearing date of
August 18, 1903. It describes the now
well-known Kahn bar, which has members
projecting obliquely so as to form the
diagonal members of a part of a truss. An
improvement followed in a patent filed on
May 4, 1903, and bearing date November
3, 1903. These were the forerunners of
a large number of patents by the same
inventor, the assignees of the patents being
the Trussed Concrete Steel Company, now
well known both in the United States and
in England.
Concrete : Materials, Proportions
and Mixing
What Concrete is. — Concrete is an arti-
ficial stone made by cementing together
fragments of hard material. The hard
material, such as brick, stone, etc., is known
as the " aggregate " (literally, something
that has been gathered together) ; the
cementing material, such as lime or cement,
is termed the " matrix " (literally, but, in the
case of concrete, not actually, a mould in
which something is embedded). The active
agents in the process of solidifying are the
cementing material and the water ; the
aggregate is inactive.
Making a Strong Concrete. — The
strength and quality of concrete depend
chiefly upon (1) the nature of the aggregate ;
(2) the nature of the matrix ; (3) the pro-
portions in which the materials (including
water) are mixed together ; and (4) the
efficiency of the mixing process.
The compressive strength of concrete is
limited by that of the aggregate, and the
tensile strength cannot exceed that of the
cement bond. The ideal concrete can be
pictured as a composite block of stone,
brick, etc., in which all the pieces fit together
perfectly, being attached one to the other
by a film of cement. Such concrete is not
attainable in practice, the shapes of the
fragments preventing such extremely close
fitting, and thus causing the formation of
pockets, which become filled with a mixture
of cement and sand. Better concrete than
is generally made would result from the use
of graded aggregates ; first, a very coarse
aggregate would be selected, then a much
finer aggregate to occupy the larger of the
voids, then a still finer one to occupy the
lesser voids in the aggregate, and finally
the cement. According to much of the
present practice, there are but two grades
in the aggregate, the coarse (stones, gravel,
etc.) and the fine (sand).
A little thought will show that the strength
of concrete may be affected unfavourably
by any one of a number of conditions : Any
pait of the surface of the aggregate not
covered with cement ; any grain of sand
without a cement coat ; any coarse particle
of cement which may in the future undergo
chemical change ; any defective particle of
cement that was incapable of doing its duty
when brought into contact with moisture
in course of mixing ; any very fine dust
from the aggregate, inert and useless ; any
crack or fissure in the broken stone ; any
hollow place resulting from insufficient tamp-
ing, etc. Any one of these will constitute
a source of weakness.
Concrete-making materials will now be
considered in detail. In preparing the
information here presented on aggregates,
care has been taken to make it conform to
a schedule issued with the " Interim Keport "
of the Special Commission on Concrete
Aggregates appointed by the British Fire
Prevention Committee.
NATURAL AGGREGATES
Gravel. — This usually consists of smooth,
round pebbles with adherent sand, clay,
and carbonaceous matter. The large stones
need to be broken, and the sand to be sifted.
Clayey gravel must be washed, as raw clay
seriously affects the strength of the concrete
and retards the setting of the cement. It is
generally wise, as a matter of fact, to wagh
all gravel. Soft or shaly should be avoided.
1 cub. ft. of coarse gravel usually weighs
about 97 lb., and clean shingle about 93 Ib.
Shingle. — This is a kind of gravel found
on the seashore, its particles being of a
rounded shape. It may contain a small but
significant proportion of absorbed free salts,
not readily removed by washing. If coarse
material, free from sand, is required, it is
best to specify, " clean washed and broken
shingle."
Sandstone. — This consists of grains of
sand, usually quartz, held together by a
natural cement, the nature of which may
vary in different examples. It is far from
being the best of the stone aggregates, owing
to its absorbent nature, and its weakness
when wetted. It cannot withstand abrasion,
and, as a rule, is of doubtful value as a
CONCRETE: MATERIALS AND MIXING
fire resistant. When it must be used, care
should be taken that it is dense, uniform,
and as homogeneous as possible in struc-
ture. The Interim Report before alluded
to states that the weights of sandstones,
limestones, quartzites, and rocks of similar
character should not be less than 130 Ib.
per cubic foot, the crushing strength not less
than about 3,080 Ib. per square inch, and the
water absorption not more than 8 per cent,
of the weight of the stone after a 24-hour
immersion. The aggregate after preparation
should be free from all dirt, decomposed rock,
clay and organic material.
Limestone. — As in the case of sandstone,
the quality varies. The oolites, when of
fine grain, answer very well as a concrete
aggregate, especially certain crystalline lime-
stones of the magnesian or dolomite series.
Limestone consists mainly of calcium car-
bonate (with or without magnesium car-
bonate), deposited originally from water, or,
in any case, formed through the agency of
water. It should have the qualifications
mentioned in the " sandstone " paragraph
as regards weight and strength. An average
weight is about 168 Ib. per cubic foot, and
the average strength under compression about
4,700 Ib. per square inch. Limestones are
dangerous in case of fire, becoming converted
into lime.
Limestone, sandstone, etc., are sometimes
used in a crushed form unscreened so that
the very fine pieces and the dust take the
place of sand, which can therefore be omitted.
There is, however, a strong objection to the
use of this material. No flour-like dust,
except that of cement, ought to be incor-
porated into concrete. Stone dust is quite
inert, and should it collect, as is likely, in
the form of small lumps throughout the
mass, it will constitute a source of weakness.
Argillaceous or clayey limestones and the
softer and more shaly limestones are un-
desirable aggregates.
Chalk. — This is a variety of limestone, and
varies in hardness, the upper strata being
soft, while the middle and lower strata are
much harder, and make better concrete than
some other materials. Even the top stratum
forms a fairly good concrete for walls where
the height is inconsiderable. It has been used
for two-storey cottages having suspended
concrete floors with satisfactory results, being
found to resist frost effectually and to
possess a considerable crushing stress. The
fine sandy portions should be eliminated by
screening and coarse, sharp pit or river sand
or fine brick sand or shingle substituted. In
view of the large chalk area in the south of
England, and where better materials are
often not available, it might be used to
advantage for walls of buildings where
economy in construction is desirable.
Quartzite. — This in many respects re-
sembles sandstone, but is harder and denser,
resists abrasion better, is not so absorbent,
and is therefore a better aggregate. It
actually is a sandstone which has become
hardened by natural forces and heat.
Flint. — This very hard but brittle stone
consists of grey or black siliceous matter, and
its probable source was a tiny sponge around
which colloidal silica has collected and coagu-
lated from the surrounding water, thus form-
ing the peculiar shapes in which flint occurs.
Flints are generally found in bands in one
of the chalk series, but they also occur on
the seashores, where they have been deposited
after the erosion of the chalk beds.
Flints obtained from the surface of
arable land are somewhat dirty and re-
quire washing, which is more easily per-
formed after they have been broken or
crushed. Flint cannot be broken by hand
to make a suitable aggregate, but should be
crushed by a stone breaker.
Flint weighs about 162 Ib. per cubic
foot, and the compressive strength is high,
about 7,800 Ib. per square inch. Flint is
liable to fly when subjected to heat, but the
tendency is reduced by crushing to small
particles less than f in. in size. It is not
absorbent, and is not durable under shock.
Shingle is largely water-worn flint.
Granite, — This is an igneous rock of
greater strength under compression (exceed-
ing 16,000 Ib. per sq. in.) and of varying
specific gravity, an average specimen weigh-
ing about 170 Ib. per cubic foot.
Granite is an excellent natural aggregate
providing that it is free from appreciable
quantities of weathered or partially decom-
posed material. The chief objection to its
use is its high specific gravity, which tends
to make it settle out of the concrete when
the latter is being placed in position, but
with a fairly stiff concrete the danger of
settlement is small. The granite must be
crushed to pieces of suitable size and sifted
free from sandy and dust-like particles by
screening over a J-in. mesh. The granite
should be well watered immediately before
use, as some varieties are highly porous and,
REINFORCED CONCRETE
if used dry, greatly reduce the strength of
the concrete. If the watering is effected by
means of a hose it will incidentally wash
off any loosely adherent material, the pre-
sence of which is detrimental to the concrete.
The value of granite depends on the sharp-
ness and angularity of the fragments as well
as on the roughness of the surface. Those
granites should, therefore, be chosen that
are naturally brittle, and those which pro-
duce rounded fragments in a crusher should
be avoided as far as possible. The maximum
size of granite particles is usually taken at
| in., but for very large blocks fragments up
to 2 \ in. in diameter are used.
There is little difference in the quality of
granites from various sources, so far as their
value as aggregates is concerned, so that
the chief attention should be paid to select-
ing a granite of clean appearance, with as
rough a surface as possible (to give a good
key to the cement), and one of which the
fragments are highly angular. Naturally,
where a granite can be obtained locally, that
will be the granite selected for a particular
case.
Other Volcanic Rocks. — These include
basalts, lavas, pumice, etc. They should be
free from soft portions, and should be tested
by immersion in water for 72 hours, at
the end of which time there should be no
signs of expansion, disintegration, or solu-
tion. Basalts, traps, dense lavas, etc.,
should have a thoroughly vitrified structure,
be homogeneous, and show a clean fracture.
The basalts make really good aggregate,
being strong (compressive strength, about
19,000 Ib. per square inch) ; they are durable
under abrasion, and they are of low water
absorption ; but they have two disadvantages
— they are rather heavy (180 Ib. per cubic
foot), and they have no marked resistance
to fire unless care is taken to break them
up small. Not so hard as the basalts, traps,
etc., are the lavas, including pumice, which,
however, for use as aggregates, should be
moderately hard, show a bright, silky frac-
ture and be free from dust and impurities.
ARTIFICIAL AGGREGATES
Burnt Clay. — Under this sub -heading
may be included broken bricks, clay ballast,
broken terra-cotta, broken pottery, etc.
The broken bricks should be such as, if
whole, would be suitable for building pur-
poses ; all soft and under-burned portions,
should be discarded. Broken bricks should
be free from old mortar, and from dust
that will pass through a J-in. mesh. Most
broken bricks are very absorbent, and for
this reason should be watered before mixing
with the other ingredients, as otherwise the
cement is robbed of moisture and the setting
is injuriously affected. Burnt clay ballast
always needs to be inspected carefully for
unburnt particles, and it is best to test a
sample by soaking in water for a few days
to see whether it disintegrates or not ; it
needs watering in the same way as the
broken brick. Porous terra-cotta and un-
glazed earthenware make good aggregate,
and these also should be watered before use ;
the material should be clean, hard, and well
burnt, but any large flat pieces or pieces
of distinctly curved shape should be regarded
with suspicion, since they do not work in well
with the other pieces, and, by "bridging,"
may easily form cavities.
Residues. — These include blast-furnace
slag, clinker, coke breeze, and cinders, which
have been proved valuable for special appli-
cations, being light and fire-resisting ; but
it is of the utmost importance that sulphur,
if present in anything more than a mere
trace, be eliminated, as otherwise it tends to
combine with the cement during the process
of setting and seriously affects its strength.
Then, too, any dense combustible material
present (as coal) is a defect which militates
against the successful use of residues.
If materials containing sulphur are exposed
to the atmosphere for a time and moved
occasionally, the sulphur tends to disperse.
The smell of the materials when moved is
the simplest evidence of the presence of
sulphur.
Blast-furnace Slag. — This should come
from pig-iron smelting furnaces, and basic
slag should be rejected. If there is any
doubt as to the sulphur content, and in
cases where the source of the slag is known,
the works chemist should be asked for the
analysis, which in many cases he should
be able to supply without much trouble.
Only a mere trace of sulphur is permissible,
and the slag should be washed to remove
dust and possibly some of the sulphur.
Washing will also remove any free lime.
Coke Breeze. — The Interim Keport, of
which mention has already been made,
recommends that coke breeze for use as
aggregate shall be entirely coke taken from
gas retorts, coke ovens or special furnaces,
and be absolutely free from clinker, coal,
CONCRETE: MATERIALS AND MIXING
and all substances that will not float in
water, and from any admixture of material
taken from the retort furnace or water pan
below it, and from cinder, ash, or other
adulterant. On no other aggregate is
expert opinion divided to such an extent as
on coke breeze. All agree that its light-
ness makes it particularly adapted to sus-
pended floors, and it has another advantage
— nails can be driven into it ; but it lacks
strength. Its weight is about 35 Ib. per
cubic foot. Being so very porous, it might
be thought highly absorbent, but such is
not the case, as many of the cavities formed
by the bubbles of gas evolved when the
coal is heated are sealed and therefore have
no communication with the outside, and
it is therefore only the surface (and
broken) cavities into which the water can
penetrate. Some coke breeze has so high
a content of sulphur that its use is a posi-
tive source of danger, but where this is not
the case, the material is a valuable one.
Much depends upon the kind of coal of
which the breeze is the residue. Thomas
Potter, in a paper read before the Concrete
Institute, stated that he had used thousands
of tons of coke breeze for floors, and did not
remember a failure of any kind, and that he
had proved it to be one of the best materials
for floors and roofs. This, however, is not
the experience of all users of the material, but
this may be due to sifted ashes (" builders'
breeze") having been substituted for true
coke ashes.
Clinker, Cinders, etc. — Clinker used for
concrete should be the thoroughly burnt
and hard waste product of furnaces, free
from dust, shale, or free lime. The quality
of clinker, cinders, etc., varies greatly, but
no material containing ashes, dust, rubbish,
and more than a trace of sulphur is suit-
able as an aggregate. In a later chapter
(p. 261), the results of some fire tests of
cinder concrete are given, and their some-
what contradictory nature will be noted.
Broken Concrete. — Old and very hard
concrete may be broken up so as to pass a
H-in. ring, well screened in order to remove
all dust, and then used as the aggregate in
concrete for foundation work or rough
walling ; but the material would not be
sufficiently reliable for floors or fine work.
On no account should old concrete in
which calcium sulphate or plaster-of-paris
was the matrix, as was the general practice
before portland cement was used, be utilised
as an aggregate, because when water is
added it expands and ruptures the concrete,
no matter how long since it was origin-
ally employed. Failures of the kind have
occurred where the plaster concrete was
forty years old.
SIZE OF AGGREGATE
In no work ought the stones to be greater
than 2J in. or 3 in. across, this size being
used for large retaining walls, foundations,
etc. For walls, the stones should pass a
2-in. mesh and be retained on a 1-in. sieve ;
and for floors they should pass a f-in. mesh
and be retained on a ^-in. one. For the
surface of concrete floors the stones should
be broken so as to pass a f in. or \ in. mesh.
The larger the stones, the stronger will
be the concrete, the quantity of cement
remaining constant. According to experi-
ments undertaken by Messrs. Fuller and
Thompson, concretes with stones 2£ in., 1 in.,
and \ in. in diameter respectively required
cement in the relative proportions of 6, 7,
and 8 to be of the same strengths. The
experiments proved, too, that round, water-
worn material, such as gravel, gave a denser
mass than broken stone, but that the latter
gave the stronger concrete. The size of the
aggregate particles greatly affects the pro-
portioning of the ingredients.
WASHING COARSE AGGREGATE
It is nearly always desirable to wash the
aggregate, and the work should be done on
a wooden platform, inclined somewhat to
allow water to run off freely. In preparing
fairly large quantities for use, two men
should shovel the material backwards and
forwards, while a third keeps a constant
supply of water playing on the aggregate,
this being continued till the water runs off
clear. The use of a mechanical washer —
some form of tower or tumbling drum — gives
better results. For good concrete the im-
portance of washing the aggregate cannot
be over-estimated. The water leaves the
material in the proper condition for causing
the cement to adhere closely to it, and the
dampness undoubtedly assists the setting.
SAND
There appears to be a difference of opinion
as to the function of sand in concrete.
Some regard it as a cheapener to be added
to the cement to increase its bulk and form
a mortar in which the coarse aggregate is
i6
REINFORCED CONCRETE
embedded. The writer does not share that
view. The sand is essentially the fine part
of the aggregate, and the proportion of it
to be used depends not on the amount of
cement thought to be necessary, but on the
percentage of voids in the coarser aggregate.
Then, in turn, the proportion of cement
will depend on the percentage of voids in
the sand and between the sand and the
coarser aggregate, there being in addition
enough to coat the whole of the aggregate.
But for the trouble, sand of two sizes could
be used, the finer occupying the voids in
the larger. It is significant that experi-
ments carried out by Messrs. Fuller and
Thompson seem to prove that the strength
and density of concrete are affected by
variation in the size of sand grains, and
that an excess of fine or medium sand
decreases both strength and density.
Sand must be sharp and gritty, and
washed thoroughly free from everything
that does not consist of small particles of
stone, quartz, or non-plastic material. Dust,
clay, earth, vegetable matter, and all other
plastic substances, are sources of weakness,
and the necessity of eliminating them invests
with the greatest importance the washing
and screening of pit sand. Kiver sand may
not need washing, but is improved by screen-
ing. Sea sand should have the salt washed
out. Pit sand, when of glacial origin, is
unsuitable ; so is sand containing more than
10 per cent, of very fine grains.
1 cubic foot of pit sand weighs about
102 Ib. ; river sand, 106 Ib. ; coarse gravel,
97 Ib. ; and clean shingle, 93 Ib.
Substitutes for Natural Sand. — Where
sand is difficult to obtain, a substitute may
be used. Indeed, the crushed stone sub-
stitutes, washed perfectly free from dust,
give a stronger concrete than do some sands,
because the artificial crushing, when properly
carried out, produces very sharp and angular
particles. In addition to crushed stone,
sand substitutes include ground burnt clay
(pottery, bricks, etc.), slag sand (made by
running molten slag into water whereby it
becomes granulated), etc. All these need to
be washed or screened to remove flour-like
dust, because any material that passes
through a sieve of 50 meshes to the linear
inch is too fine to be satisfactorily used.
Washing Sand. — The best way to wash
sand is in a stream of water, the rate of
flow of which is adjusted to remove the par-
ticles of clav, silt and rock-flour without
carrying of! the sand. Some mechanical
arrangement for keeping the sand in motion
and to prevent the formation of eddies is
necessary, or some portions will not be well
washed. One of the most effective sand-
washers consists of a large inclined pipe
about 15 ft. in length, in which revolves a
screw conveyor. A hopper is fitted to the
lower end of the pipe, and through this the
dirty sand is supplied, together with the
necessary water. The water flows out at
the upper end of the pipe carrying the clay,
silt and rock-flour with it, and leaving the
washed sand behind. As soon as the water
flows away in a clear stream the water supply
is cut off and the sand removed. This
arrangement has been made continuous in
action, and greatly improved by the addition
of a second pipe about half-way along the
larger one ; this second pipe carries off the
washed sand, and delivers it in a very wet
state into wagons or on to a receiving bed.
Such an arrangement has been proved to use
much less water in proportion to sand than
the ordinary cylindrical washers generally
employed.
The use of sieves for washing sand is con-
venient for quantities of not more than
3 to 5 cwt., but is too slow and costly for
larger amounts. The sand to be washed is
laid on the sieve so as to cover to a thickness
of not more than an inch, and the sieve is
then partly immersed in a tub of water and
shaken to and fro. The idea is that the
finer particles will pass through the sieve
and fall into the water beneath, but an
examination of sand treated in this manner
will show that it contains a considerable
quantity of fine mud, which is too adhesive
and remains suspended too readily in water
to be removed by this treatment. Another
method of sieve-washing consists in choosing
a sieve with a rather coarser mesh than
corresponds with the sand, and then washing
the sand through the sieve into a tub of
water. The coarse material remains on the
sieve, and it is supposed that the clay and
silt will be removed when the water is poured
slowly out of the tub. An examination of
the sand left will show, however, that it
contains a considerable proportion of adhe-
rent clay and rock-flour, and is not really
satisfactory for use in concrete.
Where no mechanical means of washing
the sand is available, a tank about 6 ft. by
6 ft. by 18 in. should be half-filled with sand
and a stream of water turned in. The sand
CONCRETE: MATERIALS AND MIXING
is stirred continuously with a wooden pole,
the stream of water being continued and
allowed to overflow until it runs off perfectly
clear. The remaining water is then run off,
and the sand in the washer removed.
Circular wash-mills, such as those used in
the manufacture of cement, are not satis-
factory for sand, the best sand washer being,
as already mentioned, a stream of water
flowing directly forward at a carefully regu-
lated rate, in combination with some
mechanical means to keep the sand in
vigorous movement whilst it is immersed in
the water.
PORTLAND CEMENT
From the times of the Romans until
Smeaton's experiments in 1750 there was
no material addition to our knowledge of
mortars and cements. This intrepid experi-
menter, urged by the necessity of finding a
reliable mortar for use in the construction
of the celebrated Eddystone lighthouse,
upset all traditions by formulating the
theory that the hydraulic or setting property
of a lime-mortar did not depend on the
hardness or whiteness of the stone from which
the lime was made, but on the presence of a
quantity of clayey or argillaceous matter
interstratified with the stone. Aspdin in
1824 patented portland cement, which was
so-called from a fancied resemblance to the
well-known Portland building stone. Chiefly
owing to a lack of scientific knowledge
amongst the earlier makers of portland
cement, the first years of its history were full
of failures, which tended to bring discredit
on the industry and all concerned with it.
Principles of Cement Manufacture. —
An endeavour will now be made to trace
briefly the various stages in the experiments
that led to the discovery of the principles
underlying the manufacture of portland
cement. The practice of burning limestone
for the production of lime to be used as a
binding agent or mortar is of great antiquity.
The properties that distinguish the burnt
lime from its original form of linestone are
well known. Briefly, what occurs is that
the calcium carbonate (CaC08) in limestone,
when brought to a sufficiently high temper-
ature, loses carbon dioxide (CO.,), and true
lime (CaO) remains. Perfectly pure lime
when treated with water forms calcium
hydrate (Ca(OH)2) or slaked lime, much heat
is evolved, and the lime falls to powder.
As a rule, the purer the lime the more
2
energetic is the reaction, and conversely
the more impure it is the less intense does
the action become and the less is the inclina-
tion to fall to powder evinced. The chief of
the impurities in a poor or slow slaking lime
are sand or silica (SiO.,) and alumina (Al.,03) ;
when combined these two substances form
the basis of all clays or claylike materials.
Smeaton first pointed out that the pres-
ence of these clayey materials renders lime
hydraulic (gives it the property of setting).
James Parker was the first to use naturally
occurring nodules of clayey matter which
were sufficiently rich in lime to render them
hydraulic when burnt and ground, and
Aspdin was the first to use fairly pure lime-
stone artificially mixed with clay.
The table shows (roughly) the relative
proportions of the principal constituents
that are present in (a) a pure or " fat " lime,
(b) a lean, partly hydraulic lime, (c) a natural
cement, and (d) a modern portland cement :- —
Silica
Alumina .
Lime
(a)
1
•5
98
After burning, these constituents are no
longer merely mixed together, but are
chemically combined. That is to say, the
lime does not exist as such, but with the
silica and alumina it forms compounds
known as silicates and aluminates and
aluminosilicates, just as lime when slaked
with water forms a hydrate ; thus, without
chemical analysis, it is impossible to obtain
as such either the lime or the silica and
alumina.
Portland cement may, therefore, be re-
garded as containing compounds of lime
with silica or alumina or both, its precise
composition depending on many factors and
varying slightly with each sample tested.
Composition of Portland Cement. —
In order to follow more clearly the various
steps in the manufacture of cement, it is
desirable that the principles underlying the
various processes should be understood.
Portland cement is a combination of three
bodies, and in order to bring about such a
combination the interacting substances must
be in such a condition that absolutely
intimate union between the various particles
may take place. When the reacting sub-
stances are liquids, or solids that are soluble
in a convenient medium, it is a compara-
tively easy matter to bring about the desired
18
REINFORCED CONCRETE
union (and this also is the case when union
by fusion is employed) ; but although there
is no reason why cement should not be made
by mixing substances in solution (a method
much used in the production of dental
cements), it has been customary hitherto
to heat the mixed materials to a state of
incipient fusion or vitrification in the com-
mercial manufacture of portland cement.
Hence, to produce a satisfactory portland
cement, the raw material must be brought
to the finest possible state ; also, for physical
and chemical reasons, the finished article
must consist mainly of flour or inconceivably
minute particles. Both the initial and final
operations in portland cement making, there-
fore, entail the employment of methods and
machinery that are calculated to bring about
a state of intimate union of the materials
and uniform fineness of the product.
The intermediate stage, in which the mix-
ture is subjected to a temperature sufficiently
high or prolonged to bring about the desired
combinations, is where the greatest altera-
tions and improvements of recent years have
been effected. In earlier days the kilns,
after being loaded by hand, required a week
or more of firing, and then needed time to
cool ; now, the mechanically fed and fired
rotary kiln, electrically driven and con-
trolled, turns out in a day more cement
clinker than its almost obsolete forerunners
produced in a week. For the successful
production of a uniform and high- class
cement, raw materials of as regular and
unvarying a composition as possible are
essential.
Cement Works in England.— Owing
to the readily available deposits of chalk
and clay in the Thames and Medway valleys,
numerous works were started and still con-
tinue in those neighbourhoods, and it was
thought at one time that a good cement
could not be produced save from these
materials ; but whilst London cement still
holds its own, the growth of knowledge and
extended experience has shown that in
many parts of Great and Greater Britain
other materials exist from which some of
the best cement may be manufactured, par-
ticularly in the Tyne Valley, in Warwick-
shire and Cambridgeshire. Probably, as
landowners avail themselves of the know-
ledge and experience now obtainable, an
increasingly large number of factories will
spring up in the neighbourhocd of the large
centres of population where suitable raw
materials and fuel may be found, together
with a steady demand for a sound cement.
Preparing Materials for the Kilns.
— Two chief methods are employed in pre-
paring the materials for the kilns. They
are known as the " wet " and " dry " pro-
cesses respectively. The former is used
chiefly for chalk, clay, and soft materials,
and the latter for argillaceous limestones and
harder rocks. In some works, a modifica-
tion or combination of both methods is
employed.
The Wet Process.— This is used for most
of the cement made in England at the pre-
sent time, and is chiefly applied to the vast
deposits of chalk found in Kent and Essex,
in which districts the chalk is obtained by
quarrying, and the men employed in this
work are known as " chalkies." The material
is sometimes loosened by blasting, but this
is a practice not often necessary. After
removal of the bull-head or gravelly soil,
the chalk is loosened by crowbar or pick
and slides down the face of the cliff, and is
directed by temporary wooden shoots into
the wagons awaiting its descent. Large
quantities of flints occur in layers in the
chalk deposits, and these are removed either
at the quarry or when the wagons are un-
loaded at the washmill. The chalk wagons
are conveyed to their destination by small
locomotives. These locomotives also bring
the clay in trucks from the pit, or, more
generally, from the wharf whither it has been
brought by barges from the dredging grounds
on the Medway. In one of the most up-to-
date works in Great Britain the clay is now
dumped into the washmills at the pit. The
object of the washmill is to separate the
coarse impurities and to mix the fine ones
together intimately.
The washmill consists essentially of one or
more circular tanks, usually about 15 ft. in
diameter and 5 ft. deep. In the centre is a
brickwork pier carrying a vertical shaft bear-
ing radial arms from which is suspended a
series of harrows. The central shaft is
rotated by suitable gearing, and the steel
teeth of the harrows run in the annular
space between the central pier and the mill
walls. Into this mill the chalk and clay are
introduced in the proper proportions, to-
gether with a sufficient amount of water to
form with the disintegrated chalk and clay
a liquid of creamy consistency known as
slurry. Often the chalk and clay are treated
separately in washmills, and the mixed
CONCRETE: MATERIALS AND MIXING
slurries are then treated in another mill to
secure intimate contact. As the slurry
leaves the mill it passes through screens and
fine sieves, so that any coarse particles are
retained in the mill. From the washmills
the slurry is led through the grinding mills,
which formerly consisted of French burr
stones, but these are being gradually replaced
by tube mills and more effective grinding
machines.
Both the materials led into the mills
and the resulting slurry should be under the
immediate control of the works chemist or
his assistants. The rule-of-thumb methods
of former times are now superseded by very
•careful sampling and speedy analysis which
permit the composition of the mixture to
be accurately adjusted, if necessary. As a
general rule, the amount of chalk is about
three times that of clay ; in some cases the
mater als are weighed, and in other cases
measured. In either case, the chemist
should be able readily to alter the relative
proportions of the ingredients, and this is
generally accomplished by varying the
amount of chalk, the clay remaining constant
in weight or volume. As the slurry passes
through the mill samples are taken at short,
regular periods ; these samples are carefully
mixed, and an average sample is then sent
to the laboratory for analysis. This analysis
•consists generally of a speedy and reasonably
accurate estimation of the calcium carbonate
present, together with a test of the fineness
-and moisture in order to ascertain whether
the grinding apparatus is working satisfac-
torily.
Formerly the slurry was pumped from the
washmills into huge settling tanks or backs,
where it was allowed to dry, and it was then
•dug out and loaded into upright bottle
kilns. After the slurry had been pumped
into the backs there was little possibility of
altering its composition, and so, right or
wrong, it went into the kilns. At a later
date, the slurry was pumped from the wash-
mills into chambers or tunnels through or
over which passed the waste heat from the
kilns, and this was a decided economy,
saving both time and labour. But this
practice is now giving way to the method of
burning in rotary kilns with previous mix-
ing in tanks, the slurry being constantly
mechanically stirred in order to obtain a
material of more uniform composition.
This method is also of great advantage to
the chemist, as it affords another opportunity
of testing the composition of the mixture
and correcting it if necessary.
The Dry Process. — As already men-
tioned, the dry process is often used for lime-
stones, shales, etc., in the manufacture of
portland cement, which cannot be reduced
to powder or slurry merely by stirring them
with water in a washmill. The chief
materials for which it is used are the lime-
stones and shales of the lias formations in
Warwickshire, and the limestones and harder
clays of Wales. The materials are dried
and reduced to a fine powder by means of
crushers similar to those used for grinding
clinker. The powder is mixed with water
into a stiff paste in a pugmill or mixer, con-
sisting of an open trough fitted with revolv-
ing blades on a horizontal shaft. The paste
is broken into irregular lumps or is com-
pressed into bricks, which are dried and then
taken to the kilns. If a tube mill is used for
crushing the material, an open pan mixer
may be used. Where a rotary kiln is em-
ployed, there is no need to dry the lumps or
bricks of " compo," and some firms prefer
to use a very soft paste and to load this
direct into the rotary kiln.
Opinions are divided as to the relative
values of the cement produced by the wet
and dry processes respectively. The choice
of one or other method depends chiefly on
the nature of the raw materials, so that a
detailed comparison is of little value, and
concerns the materials rather than the
method employed. For this reason such
comparisons are best avoided as being often
misleading and based on insufficient grounds.
It does, however, appear to be a fact that
the clinker produced from materials treated
by the dry method is somewhat denser and
more difficult to grind than that obtained
when the wet process is used.
Producing the Cement Clinker. — The
mixed materials, in the form of liquid slurry
or of solid lumps or bricks, must be heated
sufficiently to cause it to fuse partially or to
vitrify, in order to effect the necessary
chemical combinations and to produce
cement clinker. Until recently, this heating
was effected in kilns of two types — (a) vertical
shaft or bottle kilns, and (b) continuous
Hoffmann kilns. The former consist of an
upright shaft into which fuel and lumps of
cement mixture are fed so as to form alter-
nating layers, in a manner resembling the
burning of lime. When the kiln has been
filled the fuel is lighted, and when it has all
20
REINFORCED CONCRETE
been burned the clinker is withdrawn from
the bottom of the shaft. This intermittent
working produces a cement of variable
quality, but, with care, satisfactory results
are obtained. The Hoffmann kiln consists
of a ring-shaped space in which the lumps
of cement mixture are stacked, and the fuel
is burned in hollow pillars, about 6 ft. apart,
left in filling the kiln. Its action is quite
continuous, one part of the kiln being filled
and another emptied whilst others are being
burned. Owing to the regenerative use of
the air and fuel gases, this kiln is highly
economical in fuel, but the labour of stack-
ing the lumps of material and of withdrawing
the clinker by hand makes it more expensive
than the rotary kiln, which is rapidly re-
placing all other types, one of its advantages
being that it can burn liquid slurry.
Although rotary kilns were introduced at
the beginning of the eighties of the nine-
teenth century, their extensive and success-
ful use is a matter of comparatively recent
history. The rotary kiln as now generally
used may be briefly described as a steel or
iron cylinder from 5 ft. to 9 ft. in diameter
and from 75 ft. to 250 ft. in length ; many
kilns originally of the former length have
been extended, and now 150 ft. is the more
general length. This cylinder is lined with
a material that is calculated to withstand
both the high temperature and also the
action of the highly basic, almost molten,
clinker. The cylinder is erected at an in-
clination varying with the materials that are
to be used and the intended speed of rotation,
the modern tendency being to decrease the
angle and increase the speed. The kiln is
supported on roller bearings and set in
brickwork hoods at each end, several kilns
being often connected to one chimney shaft
by means of flues. It is not necessary to
enter into the details of driving and control-
ling the machines ; electric power is usually
employed, and by means of an ingenious
device the speed of rotation and the fuel or
raw material feed may be varied at will.
Very finely powdered coal, which has been
passed through a rotary drier before grind-
ing, is the usual fuel ; the coal is brought
from the grinding mills by screw or other
conveyers to hoppers at the lower end of
the kilns, and is blown into the latter by
steam or air supplied by a fan. The raw
mixture, or slurry, is pumped into the upper
end of the kilns, down which it is carried by
the rotary motion. During its passage
through the kiln the moisture is first driven
off, then the carbon dioxide is removed,
and finally the lime thus produced combines-
in the hottest part of the kiln with the silica,
and alumina of the clay ; the semi-molten
globules that are thus formed drop out at
the lower end of the kiln, and then pas»
through cooling cylinders. Cold air passes-
up through these cylinders, which in con-
struction are very similar to the kilns ; and
in this way the clinker is cooled and the now
hot air is supplied to the kilns. The clinker
thus produced is of a dark greenish colour, in
pieces about the size of hazel nuts, and is-
received into hopper trucks, which, as soon
as they are filled, are conveyed to the weigh-
ing bridge and thence on to the storage-
hopper or dry grinding mill.
Grinding the Clinker. — Dry grinding
with burr stones has now become quite
obsolete, the work being done largely by ball
and tube mills, pendulum mills, or other
modern contrivances, no preliminary crush-
ing of the rotary clinker being necessary.
The form of pendulum mill chiefly used
in England is the Griffin mill, consisting
essentially of a horizontal driving pulley,
from which the shaft is suspended by a
universal bearing. To the lower end of
the shaft a crushing roll is attached, which
is thus free to swing in any direction within
the pan containing the ring against which
the roll works, the pulverising being done
between the roll and the edge of the pan.
Just above the roll is a fan, and on the
under side are a number of ploughs. The
roll is within the ring, and centrifugal
action carries it against the edge of the
pan. When the clinker is fed into the
mill it is thrown up by the ploughs between
the roll and the edge of the pan. When
ground sufficiently fine the cement passes-
out through a screen surrounding the pan,
which for that purpose has a number of
openings downwards thrdugh it. The whole
grinding part of the mill is covered with a
conical sheet-iron case, and the revolving
fan draws in air through the top and forces
the cement out through the screens.
The ball mill, briefly described, consists-
of a cylinder revolving round a horizontal
axle. The circumference is formed of per-
forated steel plates arranged to overlap each
other. Outside the plates are coarse screens,
beyond which again finer screens may be
placed. Sometimes an air-separator replaces
the screens. The clinker is fed into the mill
CONCRETE: MATERIALS AND MIXING
21
through a hollow trunnion, and in the mill
are placed a number of steel balls which are
•carried round and dropped from plate to
plate as the mill revolves, thus grinding the
material that is fed into the mill.
As a rule, the ball mill is only used for
•coarsely grinding the clinker, which is then
passed into a tube mill, which consists of an
iron shell from 16 ft. to 22 ft. long and
about 5 ft. in diameter. This shell is about
half-filled with flint pebbles, and the mill
is fed and discharged by means of hollow
.shafts at each end ; as the mill rotates the
pebbles are carried partly round up the side
of the mill until a point is reached where they
•drop back in a cascade upon the material
below, thereby reducing it, in time, to finest
possible powder. The material to be ground
•enters at one end of the tube and gradually
works its way to the other end, where it is
•discharged on to screens or into an air-
separator, the coarse material being returned
to the mill to be re-ground. The pebbles
are fed into the mill through a manhole, and
are prevented from leaving by the smallness
of the apertures in the mill. The tube mill
is only used as a finishing mill, and usually
it must be worked in conjunction with a
coarse grinding mill, a ball mill being
admirable for this purpose.
From the mills the now finely ground
cement is carried by belt, screw, or other
conveyers to the storehouse, and there
packed in sacks or casks or stored in bins
until required.
Regulating the Setting of Cement. —
€ement made from clinker burnt in shaft or
Hoffmann kilns is relatively slow in setting,
but that made under the best modern con-
ditions and burnt in rotary kilns sets with
inconvenient rapidity and requires the addi-
tion of some retarding agent. Several sub-
stances are available, the one most gener-
ally employed being gypsum (plaster-of-
paris) or other forms of calcium sulphate
(CaS04). Most specifications of portland
cement recognise the necessity for this
retardation of the setting, and they permit
the addition of not more than 2 per cent,
of one of the substances above mentioned,
the addition of the substance being made
during the operation of grinding. An-
other plan now frequently followed is the
blowing of steam into the tube mill ; in this
way any traces of free lime are hydrated.
The cement thus manufactured should be fit
for use immediately after grinding, which
was not the case in former days ; it used
to be inadvisable to use cement freshly
made. On no account should the con-
tractor keep cement long on hand before
using.
Testing Cement. — In the early days of
the cement industry the methods of testing
employed were on all fours with those
obtaining in the majority of the cement
works of that period, namely, if existent at
all, entirely unscientific, rough, and crude.
The apparent antipathy of the earlier cement
makers towards chemists or other trained
scientific workers and methods was only
overcome when, as a result of numerous
disasters, their aid was perforce invoked.
This probably accounts for the many useless
tests introduced at a later period of the
industry, every engineer, architect, or sur-
veyor, in their anxiety to avoid disaster and
disappointment, devising tests that pleased
their individual fancy. At this time, also,
every works of note had its own system of
mechanical and physical tests, according to
the ideas of those in control. This diversity
of opinion and specifications continued until
the opening years of the present century,
when, in 1903, a Committee on Cement was
appointed by the Engineering Standards
Committee, which is supported by the prin-
cipal engineering institutions and societies.
The sectional committee on cement which
drafted and revised what is now the recog-
nised British Standard Specification (obtain-
able through any bookseller for 5s.), is com-
posed of representatives of various institu-
tions, public bodies, cement manufacturers,
experts, and large users. Every buyer or
large user of cement is recommended to
obtain a copy of this specification, and to use
only cement that is guaranteed to comply
with its requirements. It will at once be
recognised that it is impossible for any in-
experienced person to carry out many of the
physical and mechanical tests therein de-
scribed, whilst the question of ascertaining
the chemical composition is one for the
trained analytical chemist. The users of
cement, therefore, must of necessity depend
largely upon the guarantee of the manu-
facturer or employ the service of an ex-
perienced tester or expert. If, however, the
quantity purchased, or the importance of the
work, does not warrant the expense thus
involved, the user, having taken the pre-
liminary precaution of obtaining a well-
known brand of cement, may for his own
REINFORCED CONCRETE
gratification carry out a few simple tests
that will at once tell him whether the cement
is suitable for the object in view.
Before describing these " unofficial " tests,
it may be well if we briefly enumerate the
requirements of the British Standard Speci-
fication. The following is abstracted, by
permission of the Engineering Standards
Committee, from Report No. 12 (revised
August, 1910), " British Standard Specifica-
tion for Portland Cement," which specifies
the limits of the following properties : (a)
fineness, (b) specific gravity, (c) chemical com-
position, (d) tensile strength (neat cement),
(e) tensile strength (cement and sand), (f)
setting time, (g) soundness.
For tests (d) and (e) special apparatus is
required, whilst (b) and (c) can only be pro-
perly performed in a chemical laboratory or
specially fitted testing department. In order
properly to determine (a), a good balance
and set of weights and sieves are required,
and for the official method of ascertaining the
" setting time " (/) a Vicat needle is neces-
sary. A little piece of apparatus must be
obtained to carry out the Le Chatelier test
for " soundness " (g). Each one of these tests
requires some amount of experience before
reliable and concordant results can be
expected.
It will at once be realised that cement
testing is not work that can rightly be left
to the " odd job " man, but requires care
and a fair amount of intelligence and common
sense. The tests formulated above are such
as any normal, unadulterated, and well-made
cement will, as a rule, satisfactorily pass.
At the same time, it is possible that
cements not absolutely in accordance with
the requirements of this specification may
prove satisfactory and reliable in practice,
though the employment of cement that is
guaranteed to pass the test enumerated is
strongly advocated.
Fineness Test for Cement. — The object
of determining the degree of fineness of
cement is to ascertain that its particles are
in such a condition as to practically con-
stitute an impalpable powder. It may
be definitely stated that it is only the
" flour " in a cement that possesses any real
cementitious properties. All the mcderately
coarse particles are not only useless, but
may prove to be a source of real danger.
The determination of the fineness is a test
that may be easily carried out by any
intelligent man. In order to make the test
quantitative, a balance and weights, and
two " Standard Wire " sieves of 180 by 180,
that is 32,400 meshes, and 76 by 76 (5,776)
meshes, per square inch are required. One
hundred grams (metric system) or 4 oz. of
cement is carefully and continuously sifted
for fifteen minutes on each sieve. The
residue on the 180 sieve must not exceed
18 per cent., that is 18 grams or '72 oz.
(nearly f oz.), and 3 per cent, on the " 76."
The latter constitutes practically an unweigh-
able amount except a delicate balance and
weights be used.
Quantitatively, a rough test of the fineness
of the cement may be made by sifting a
handful of cement through the " 76 " sieve ;
there should be practically nothing left on the
sieve.
Specific Gravity Test. — This takes the
place of the now obsolete " weight per
bushel " test. It cannot be carried out
except in a properly equipped testing-room.
The object of this test is to eliminate
adulterated or lightly burnt cement.
The British Standard Specification requires
the minimum specific gravity of portland
cement to be 3-15 at works, and 3-10 after
delivery ; taking the higher figure, 1 cub. ft.
of solid cement would weigh 196 Ib. It is
not easy to get uniform results in determin-
ing the specific gravity of a fine powder, and
the specific gravity will vary with the extent
to which the cement is packed into the
weighing vessel, and also with the conditions
of manufacture. The accepted weight of
1 cub. ft. is 90 Ib., although actually it may
vary between 75 Ib. and 110 Ib. A cement
ton varies slightly. It may be 2,200 Ib.,
made up of 10 two-cental sacks, or it may
be 2,244 Ib., consisting of either 11 sacks,
each of 204 Ib., or 12, each of 187 Ib. All
these weights are net.
Analytical Test. — The services of an
experienced analytical chemist should be
employed if there is any doubt as to the
genuineness of the cement. The addition
of ground slag, bricks, or other adulterant
can only be proved by accurate analysis.
Certain definite ratios between the " basic "
and " acid " components of the cement are
stated in the Standard Specification, as well
as limits for the percentages of magnesia,
sulphuric anhydride, and water.
Tensile Strength Test.— The cement
both " neat " and " with sand," is made up
into briquettes under certain stated condi-
tions, and the tensile stress of the briquettes
CONCRETE: MATERIALS AND MIXING
ascertained by pulling each piece until it
breaks in a special machine after the bri-
quettes have been stored for stated periods
of time. A properly equipped testing-room
and the services of an experienced gauger
are required.
"Setting Time" Test.— The study of
the " setting time " of a sample of cement
when gauged with water is, although appar-
ently simple, one of great complexity. In
order to obtain concordant results it is
necessary to strictly adhere to the conditions
expressed in the Specification. A Vicat
needle and gauging tools are necessary. The
setting time may be roughly ascertained by
gauging up a small portion of the cement
with sufficient clean water to form a thick
paste. A pat is then made up on a small
sheet of glass or piece of slate, and the surface
tested with the thumb-nail until it is im-
possible to make any impression thereon.
It is possible in this way to distinguish
between a " quick " and " slow " setting
cement, but not with that accuracy obtain-
able with the standard needle. Care must
be taken in gauging not to " work " the
cement after it shows any signs of " setting."
Soundness Tests. — The object of all the
suggested tests for " soundness " is to ascer-
tain rapidly the probable effect of time upon
the set cement. It is a practical endeavour
to prove that the physical state and chemical
composition of the cement are as nearly as
possible correct. At present there are no
fully satisfactory methods of attaining this
object. The Le Chatelier test has been
adopted by the Engineering Standards
Committee because it is the only simple
quantitative method that has yet been
devised. Many other tests have been sug-
gested, but they are either not quantitative
or else require expensive and complicated
or delicate apparatus. For a full description
of the test, the British Standard Specifica-
tion should be consulted. Briefly, it con-
sists in filling a small mould, consisting of a
split brass ring fitted with indicating needles,
with the gauged cement. The mould and
contents is then immersed in water for
twenty-four hours, removed, and the distance
between the indicating needles measured in
millimetres. The apparatus is then placed
in water again, which is caused to boil con-
tinuously for six hours. When cool, the
distance between the indicating points is
again measured, and any increase noted.
A figure is thus obtained which should not
exceed the prescribed limits of the Specifica-
tion. As before mentioned, this is the only
simple quantitative test for " soundness "
that can be performed without the aid of
expensive and delicate apparatus.
It may not be out of place, however, to
briefly describe one or two quantitative tests
that require practically no apparatus. Some
people, before testing for setting time and
soundness, spread out the cement to be
examined in a layer 3 in. deep for twenty-
four hours at a normal temperature; but
the practice is not recommended. It
should be understood that no reliable results
can be expected unless the operator knows
how to gauge cement properly and make it
up into a pat on a piece of clean glass. It
is not a simple matter to do this, and ex-
perience is the only teacher.
An excellent test is to plunge a pat, before
the cement sets, into cold water. A good
cement will easily stand this test, a faulty one
will either fall to pieces almost immediately,
or show signs of cracking at the end of
twelve or twenty-four hours. A quantitative
" hot " or accelerated test is to place a pat
after setting into cold water, which is then
gently caused to boil and kept boiling for
six hours. The pat should remain on the
glass, and not show any signs of cracking
or disintegration. It is not serious if the pat
leaves the glass, but is otherwise sound and
not buckled or cracked.
It may be remarked that the British
Standard Specification does not require a
cement to undergo or stand either of the
tests last mentioned, but in most cases any
cement that complies with the requirements
of the Specification will also pass these
qualitative tests.
NATURAL CEMENTS
" Natural " cement manufacture differs
from that of portland cement, which is
always regarded as an " artificial " cement,
in one important particular : for natural
cement, the material — highly calcareous marl
or septarian nodules — is dug, calcined, and
ground, thus producing a cement of varying
composition and quality ; whereas, for port-
land cement, additions are made to the raw
material in accordance with the results of
skilled tests, so as to obtain a cement of per-
fectly uniform composition. The cement is
also tested at various points in the manu-
facturing process, and the resultant product
is therefore a reliable and uniform article,
REINFORCED CONCRETE
whereas the natural cements vary greatly,
even at the same works.
A natural cement very inferior to
portland cement is produced in Belgium,
and a few years ago large quantities of it
were used in Great Britain, the material
being introduced into this country by the
most unscrupulous methods. The packages
were put up in close imitation of those con-
taining genuine portland cement of British
origin and often bore labels of well-known
British brands. The importation of this
fictitious portland cement is now practically
dead, but as recently as 1904 it is computed
that nearly 234,000 tons entered Great
Britain.
SLAG CEMENT
Ordinary blast-furnace slag without any
addition does not, as a rule, possess cementi-
tious properties ; it requires the addition of
lime powder to make it hydraulic. Cement
made from blast-furnace slag, and known
as " slag cement," has, therefore, chiefly
consisted in the past of a mixture of granu-
lated blast-furnace slag and finely powdered
lime. In certain patented processes, a
cement is made by causing the molten slag
to become impregnated with alkaline salts
in solution.
A small proportion of granulated blast-
furnace slag when finely ground with good
cement clinker is stated to make the con-
crete of which it forms part less pervious to
water, and substantially to increase its
mechanical strength ; burnt clay, trass, or
other puzzolanic material, is said to have
the same effect. There are, however, reasons
for supposing that these statements have
been originally made by those interested in
the utilisation of slag cements or adulterated
portland cements, and that they are subject
to the suspicion of being biassed.
Slag cements are now made by a variety
of processes, and in some cases they resem-
ble true portland cements so closely in com-
position and properties as to be indistin-
guishable from them. Most slag cements,
on the contrary, are quite distinct from port-
land cements, and are nothing more than
artificial puzzolanas. Such cements should
be rigidly excluded from admixture with
portland cements or from sale under this
title ; their nature should be clearly stated,
as puzzolanic cements are very inferior to
portland cements. Apart from the careful
adjustment of the composition of the slags
by the addition of limestone, followed by a
recalcination of the mixture in the kiln
(whereby a true portland cement is formed), it
does not appear probable that there will be
any improvement in the manufacture of
cement from blast furnace slag that will
render it a keen competitor of true portland
cement. It must be remembered that slag
is a waste product, and that in the manu-
facture of iron from its various ores the com-
position of the slag must vary according to
the character of the ore smelted, without any
consideration relative to its ultimate employ-
ment as a cement.
Iron Portland Cement. — For some years
a mixture of portland cement and granulated
blast furnace slag has been used in Germany
under the term " Iron Portland Cement,"
and has been the subject of much discussion.
The portland cement is made by mixing the
granulated slag with lime and heating to
sintering, and the clinker so produced is
mixed with about three-sevenths of its
weight of slag. The mixture is then ground
and forms the iron portland cement of com-
merce. A commission appointed by the
Prussian Minister of Public Works reported
in 1908 that iron portland cement and port-
land cement may be considered of equal
value if in air-hardening tests under standard
conditions the iron portland cement gives
satisfactory results ; but this has not been
borne out by the iron portland cements
placed upon the British markets, these having
a variable composition and often containing
deleterious substances.
LIME CONCRETE
Concrete made with lime is weaker and
more porous than that made with portland
cement. Lime concrete will not always set
properly in a damp soil, and it has therefore
little value beyond that of a simple hard core.
Chalk lime is quite unsuitable for damp
situations, only the best hydraulic limes
being permissible. The best lime that can
be used for concrete is that known as blue
lias lime, which is made from a limestone
containing approximately 79 per cent, of
carbojiate of lime and 17 per cent, of clay ;
and it is the clay content that makes the
lime hydraulic, that is, capable of setting
under water. The less the proportion of
uncombined lime present, the greater is the
hydraulicity. Lime concrete is not used
for reinforced work where portland cement
is obtainable.
CONCRETE: MATERIALS AND MIXING
WATER FOR CONCRETE MIXING
The water used in mixing the concrete
should be clean and preferably fresh. The
use of hard water causes a white efflorescence
to come out on the walls after a time, similar
to the well-known patches that appear on
red-brick fronts. On no account should
dirty water, or water containing organic
matter, as that from stagnant pools, be used.
Warm water causes the cement to set a little
quicker. An excess or insufficiency of water
in mixing is to be guarded against — 20 gals,
of water per cubic yard is a fair amount, i)ut
a larger or smaller quantity may be necessary,
according to the nature of the aggregate.
Whether the concrete is wet enough can be
judged after it is deposited and rammed ;
there should be just a slight wetness of the
top surface. When there is not enough
water, the cement does not set properly ;
where there is too much, the cement may
be washed off the aggregate, and, if the
forms are roughly made, some of the thin
stuff will run away. Another point is that
a moderately wet mixture prevents the form-
ation of voids and secures sufficient plasticity
to ensure a complete filling of the space round
and below the steel reinforcement ; but an
excessively wet concrete contains numerous
globules of water, which, when absorbed,
leave the concrete porous, and tend to accu-
mulate on the surface of the reinforcement,
particularly on the under side. According
to the Proceedings of the American Society
of Civil Engineers, the weakening of the
bond from this cause was evident in certain
beams in which the adhesion was noticeably
weak, the water cavities being apparent at the
bottom and sides of the steel bars.
Sea-water should be avoided, as it con-
tains many impurities.
PROPORTIONING CONCRETE
Correct practice in proportioning concrete
is based upon a proper understanding of the
purpose of the ingredients, which has
already been explained. The cement, in
addition to its general binding power, fills
the interstices in the sand, and the sand the
interstices in the aggregate. Thus, the
coarser the aggregate, the more the sand
required ; and the coarser the sand, the
greater the proportion of the cement that
must be used, though the addition of a small
amount of a much finer sand is a more
economical proceeding. The coarser the
cement, the greater the porosity of the
resultant concrete ; and by filling the fine
interstices in the cement itself with a still
finer substance (hydraulic lime, or extremely
finely ground cement-flour), it is possible
to produce a non-porous concrete.
Round pebbles do not interlock with one
another so well as do materials of an angular
shape, and as a result more sand and cement
are necessary to fill the spaces (in concrete
parlance termed " voids ").
The essential condition is the production
of a solid body without voids, all the pieces
of aggregate and particles of sand being
united together by the cement matrix.
Considered in the light of the above,
concrete consists of but two component
parts, aggregate and matrix ; and the sand
may be regarded as the finer part of the
aggregate, its purpose being merely to
occupy the spaces between the coarser
pieces. Thus, if the sand were omitted,
cement to nearly the same bulk as that of
the sand would have to be added.
It follows, then, that although success in
concrete making is often thought to depend
entirely upon strict adherence to a formula,
there is no one formula of any value for
general adoption ; and the correct propor-
tions ought always to be settled after
experimenting with the particular coarse
aggregate and sand which is to be used
on the job.
Unfortunately the composition of concrete
is frequently settled in a very haphazard
manner. It is not uncommon to find
specifications in which one part of cement
is assigned to so many parts of gravel, sand,
and broken stone, without apparently any
systematic determination as to whether the
sand and cement combined together will
entirely fill the interstices in the larger
materials. The aggregate should by no
means be uniform in size, as already ex-
plained. The average percentage of voids
in a well-graded aggregate of crushed stone
is from 36 per cent, to 48 per cent. ; in gravel
the percentage is a little less, varying from
30 per cent, to 40 per cent. Sand usually
contains 40 per cent, to 50 per cent, of
voids — the actual proportion depends on the
degree of uniformity in the size of the grains.
From what has been said, the reader will
know better than to fall into the error of
supposing that, for example, 1 measure of
cement, 2£ of sand, and 5 of stone will make
8£ measures of concrete. Mixed dry, the
8f measures would make theoretically only
26
REINFORCED CONCRETE
5 measures, or slightly more, because the
cement and sand should just fill the voids
between the stones.
Determining Percentage of Voids in
an Aggregate. — A method of determining
the percentage of voids in an aggregate
is by means of two watertight vessels of
known capacity ; for the larger vessel, if
the concrete is for reinforced work, a con-
venient capacity would be 1 cubic foot ; and
for the smaller vessel an imperial pint
measure, there being 49-82 pints in 1 cubic
foot, which is near enough to take as 50 ; or
the correct capacity could be arrived at by
experiment based on the fact that a pint
of water weighs 20 oz. Fill the larger vessel
with the aggregate, slightly shake down, and
strike off level with the top. Should the
aggregate be of a porous material, it is first
saturated with water and drained before
placing in the vessel, as otherwise it will
absorb water into itself during the measuring
of the voids, thereby giving a greater value
for the voids than is actually the case. Now
pour water into the vessel containing the
aggregate by means of the pint measure,
until no more water can be introduced ; note
the amount required, and take care to do
the work accurately and without spilling
any of the water. Frcm the number of
pints required to fill the vessel in addition to
the aggregate, the percentage of the voids
is found by the following rule : —
Number of pints ,T
— — - x 100, or N X 2,
50
N being the number of pints to fill the vessel
containing the aggregate, and 50 (49'82)
the number of pints in 1 cubic foot.
The following is a more accurate method,
and it involves the use of a galvanised iron
bucket and a weighing machine. First
weigh the empty bucket denoting weight
by W; secondly, weigh the bucket full
of water, denoting weight by Wj ; thirdly,
weigh bucket full of aggregate or sand,
denoting weight by W2 ; fourthly, weigh
the bucket full of aggregate and water,
denoting weight by W$. Then the cubic
contents C of the bucket in cubic feet will be
W — W
C = - — , since a cubic foot of water
oz'4
weighs 62-4 Ib. The volume of the voids
W — Wo
V = — ^fiiTT^ • The percentage of the voids
Vx 100
in the sand or aggregate will be — •
With sand, it is difficult to drive out with
water all the air contained in the voids, and
therefore an error of from 8 per cent, to 10 per
cent, may be easily made. This can be
avoided by pouring a measured volume of
dry sand slowly into a graduated glass vessel
containing water ; the sand sinks to the
bottom free from air bubbles, and the
volume of displaced water may be measured
and deducted from the volume of the sand,
the difference being the voids.
As an illustration of the first method,
assume that it requires 24 pints of water to
fill the foot cube (50-pint) vessel of aggregate ;
24
then the percentage of voids will be r ,- x 100
= 48 per cent. By the second method,
assuming the bucket to weigh 4'12 Ib. = W ;
when full of water, 41-5 Ib. = Wa ; full of
dry aggregate, 61 Ib. = Wo ; and when full
of aggregate and water, 79 Ib. = W3.
The volume C of the bucket will be
41-5 - 4-12
= (approx.) 06 cubic toot.
62-4
The volume of the voids is
79 - 61
"62-4
0-29 cubic foot.
The percentage of voids will be
•29 X 100
— =48 (approx.).
Next determine the voids in the sand
by the same method as for the coarser
aggregate. Then proportion the cement
and sand so that the cement paste will be
10 per cent, in excess of the voids.
Now, cement when mixed with water
reduces very much in volume, and it has
been found that it takes approximately
100 Ib. of cement to make 1 cubic foot of
cement paste. It is a simple matter to
provide 100 Ib. of cement to each cubic foot
of voids in the sand, allowing an extra 10
per cent, in excess of the voids, and to pro^
portion the volume of the aggregate so that
all the voids are filled with the cement and
sand mortar with an excess of 10 per cent.
One bag of cement (eleven bags to the
ton) weighs 204 Ib. net, and will make
2 cubic feet of cement paste. This is a
convenient basis on which to proportion the
volume of sand and aggregate for the voids
to be filled ; the volume of the cement being
thus fixed as one bag, no measuring or weigh-
ing of cement will be required, and the result-
ing concrete will not be too large in volume
to handle before the first set begins.
CONCRETE: MATERIALS AND MIXING
27
The volume of sand in which the voids
will be filled with a given volume of cement
paste is found by the following rule, which
also answers for finding the volume of
aggregate to be filled by a given volume of
C
cement and sand : ^— - — ^-j -- ^. , where C
v + (ITT X V)
is the volume of cement paste (or cement
paste plus sand), and V the percentage of
voids in the sand (or aggregate).
Applying this rule to a particular case,
assume that the voids in the sand are 38 per
cent, and in the aggregate 48 per cent.
(That is, taking the bulk of the sand and
aggregate as 1 each, the voids are re-
presented by 0-38 and O48 respectively,
these corresponding to the percentages
named.)
The volume of sand in which the voids
will be quite filled with 2 cubic feet of
cement paste (made with one bag of cement
2
weighing 204 Ib. net) will equal .
0 = 4-785 cubic feet = 8,268 cubic
('41o
inches. The internal dimensions of a cube
having this capacity is arrived at by finding
the cube root. Thus ^8268 = 20'22 in.
cube, say 20^ in. = the internal dimensions
of the sides of the gauge box for measuring
the sand.
The volume of cement and sand mortar
produced is found by Vc + (Vs X (1 — P)) :
where Vc is the volume of cement paste
in cubic feet, Vs the volume of sand in cubic
feet, and P the percentage of voids in sand.
Therefore the volume of mortar produced
will be 2 + (4-785 X (1--38)) = 4-966
cubic feet.
The volume of aggregate for the voids to
be filled with this quantity of cement and
4-966
sand mortar will be
4-966
•528
•48 + (TV X -48)
= 9-405 cubic feet, or 9'405 X 1728 =
16,251 cubic inches ; for this there will be
required a gauge box fyl&251 = 25 -33
inches cube, say, 25T5g- in. in width, depth,
and length, internal dimensions, for measur-
ing the aggregate.
The resulting concrete would be of
maximum density, after which increase of
strength would be directly attained by
increasing the volume of cement paste. As-
suming that the aggregate had been more
perfectly graded and the voids had been
40 per cent., the volume of aggregate re-
quired for one bag of cement would then have
4966 4-966
= : u 28
.40 + (Tv x -40)
cubic feet for the same volume of cement
paste, thus increasing the volume of concrete
1-885 cubic feet for 2 cubic feet of cement
paste, and yet maintaining practically the
same strength.
The weights of the bags of cement should
be checked to obviate risk of error.
Specifying Proportions. — In days gone
by, concrete was specified as 4 to 1, 5 to 1,
etc., the sand and coarser aggregate being
measured together, and leading to very
irregular mixtures and results that could
not be relied upon. All experienced archi-
tects and engineers now specify the sand and
coarser aggregate separately. Thus, a mix-
ture of 1 : 2 : 4 means 1 part of cement to 2
parts sand and 4 parts of coarse material
of various sizes.
GAUGE BOXES AND MEASURES
Aggregate Gauges. — Gauge boxes con-
sist merely of four sides fastened together.
They have no bottom, so that when they
are filled and levelled off at the top, the
gauge can be lifted off, leaving the meas-
ured materials in a heap. They should
not be made too high, and their capacity
should not be so great as to make them
unwieldy. Some form of handle should
be attached. A good arrangement is shown
at Fig. 7, where the handles are placed
midway at the sides. This arrangement
admits of the gauge box being used either
end up. Projecting handles at the four
corners are objectionable as being in the way
of the workmen, and therefore liable to get
broken off. The top and bottom edges
should have strips of hoop-iron nailed along
them to resist wear. It is convenient to
make the capacity some definite measure,
so that it can be used for measuring a cubic
foot or multiples of a cubic foot, or so that it
can be used in connection with the scientific
system of proportioning already described.
A gauge of J cubic yard capacity is a
convenient size in general concrete work,
and if the height is fixed at 1 ft., allows a
fairly large surface for spreading the cement
after the gauge has been slightly lifted ; the
final removal of the gauge is almost the
equivalent to one turn over. The follow-
ing example shows how to arrive at the
28
REINFORCED CONCRETE
dimensions of J cubic yard gauge 1 ft. in
depth : —
i—ff- ' = 13J ft. super., then v/13^ =
3 ft. SyV in. nearly.
So that a gauge 3 ft. 8^ in. by 3 ft. 8rV in.
by 1 ft. deep = £ cubic yard, and a gauge of
Fig. 7. — Gauge Box with Fixed Handles
the same dimensions top or bottom area, but
2 ft. deep = 1 cubic yard. Care should be
taken to make the gauge accurately square,
and to provide that it shall maintain this
form, or its capacity may become seriously
diminished. The joints in the sides should
be matched or grooved and tongued together,
and the inside should be rough-planed.
Cement Gauges. — A wooden gauge for
cement may resemble Fig. 8. It should be
made as light as possible consistently with
strength, and furnished with some means of
transporting it when charged, the cement
store being often unavoidably some distance
from the mixing platform. As illustrated, the
arrangement for conveyance takes the form
Fig. 8. — Cement Gauge
of two loose carrying bars, fitting under
wrought-iron clips or brackets, fixed to the
cleats at the sides with stout ironwood
screws. This gauge should be accurately
square, and have a bottom to it.
Metal gauges for cement are lighter and
handier than wooden ones ; but cement
gauges are not necessary when the whole
system of proportioning is based on the
weight of cement contained in one bag.
The best method of apportioning cement is
by weight, and not by cubic measure.
Water Gauges. — If the water is not
measured accurately, defective concrete will
be produced. Odd buckets and cans should
be avoided as tending to errors, which,
occasionally, become very serious, and only
those measuring appliances should be used
that have been carefully examined and certi-
fied by some responsible person. For small
quantities of water a conveniently-sized can
or bucket, as tall and narrow as possible, may
be used, providing that it has had its exact
capacity or some distinguishing mark legibly
painted on it. For larger quantities, a cistern
holding just the amount o£ water required
for one batch of concrete should be placed
above the mixing board or machine. This
cistern should be provided with an outlet pipe
and tap as near to the bottom as can be
arranged. A short piece of rubber hose
attached to the outlet pipe is often a great
convenience. All buckets, cans or cisterns
should have, legibly painted inside them, a
mark to which they are to be filled. This
mark should be as near as possible to the top
of the vessel, as the risk of adding too much
water is thereby reduced. If the quantity
of concrete is very large, it is more con-
venient and accurate to have a cistern fitted
with an overflow pipe, and to run water
into the cistern until it begins to overflow ;
the disadvantage of this arrangement is that
it is troublesome to adjust the overflow pipe
in the first instance. Some firms have found
tanks operated by siphons to be convenient
and accurate ; a cistern controlled by a ball-
cock is used to fill the measuring tank, and
the latter is emptied siphonically with such
suddenness that the amount of water intro-
duced by the opening of the ball-cock is too
small to cause any appreciable error. Where
variable quantities of concrete are to be
mixed, a cistern with its capacity at different
levels painted on it should be used. All
the water to be used for one batch should
be held at one time in the water gauge.
MIXING CONCRETE BY HAND
There are but few hard-and-fast rules
with regard to the mixing of concrete.
Most foremen and clerks of works favour a
particular system of their own, and as long
as the result in all cases is a properly mixed
material the quickest method is the best.
On no account must the concrete be worked
after setting has begun. In a method that
was common until a few years ago, the
materials were measured out (on a 10 ft.
CONCRETE: MATERIALS AND MIXING
29
square platform) in a heap on the top of
one another, the cement being added last
from a wooden box. The heap was then
turned over with shovels, one man having
an iron prong ; the water was applied gently,
so as not to wash the cement away. The
concrete was turned often enough to obtain
a thorough incorporation of the ingredients.
In another method, the aggregate was
placed in a layer from 8 in. to 12 in. thick
over a platform, the smaller pebbles being
at the bottom. The cement was then spread
as uniformly as possible over the whole, the
materials being then mixed by four men,
two with shovels and two with hoes, the
former facing each other, and always working
from the outside to the centre, then stepping
back and going over it again in the same
way ; the operation was continued until the
whole mass was turned. The heap was then
turned over again in an opposite direction,
the surface of every pebble thus being
covered with cement. Two turnings usually
sufficed to make the mixture complete.
A method that once had the approval
of engineers is to spread out the sand over
the platform to the depth of a few inches,
and over this to spread the cement. A hole
is made in the middle by means of a hoe,
and the water poured in, the whole being
then mixed by means of shovels and hoes
to form a thin paste. The aggregate straight
from the washing, or purposely wetted, is
added, and the whole mixed. This is a bad
method ; unless the mixing board has a
fillet all round it, there is a risk of losing
much of the fine stuff, and the method is
slow, giving rise to the temptation to
scamping.
A better method is first to spread out the
aggregate, then the sand over it, and lastly
the cement over the sand. Two shovellings
will then mix the material fairly well. It
is usual to measure the aggregate in a gauge
box, smooth out the aggregate so that the
sand gauge may go on top of it, and then fill
the gauge with sand. In some cases a slight
allowance is made for the sand which will
fall into the voids of the aggregate, but this
is scarcely necessary if the top of the heap
of aggregate is smoothed fairly well. Lastly,
the cement is added from a wooden or metal
measure, or possibly straight from the bag.
Preferably, the water should be supplied
through a rose, as the more gentle the
application the better. The water is applied
during a third shovelling, following which
may be a fourth, or the concrete may be
shovelled direct into the barrows or other
conveyers.
The Association of American Portland
Cement Manufacturers recommends the
following method : the sand is spread over
the board in a layer 3 in. or 4 in. thick, and
over it is spread the cement. Two men start
mixing the sand and cement, each man
turning over the half on his side, starting at
his feet and shovelling away from him. In
turning the shovel, the materials must fall
off the end and sides so that the materials
are mixed as they fall. The mixed sand and
cement are spread out carefully, and the
gravel or stone measuring box is placed
beside it, filled, lifted off, and the gravel is
shovelled on top of the sand and cement,
spreading it evenly. With some experience
equally good results can be obtained by
placing the gravel measuring box on top of
the carefully levelled sand and cement
mixture, and filling it, thus placing the gravel
on top without an extra shovelling. Add
about three-fourths the required amount
of water, using a bucket and dashing the
water over the gravel on top of the pile as
evenly as possible. The men turn over the
materials in much the same way as they
did the cement and sand, except that, instead
of shaking the materials off the end of the
shovel, the whole shovel load is dumped and
dragged back towards the mixer, so that
the wet gravel picks up the finer material.
Water is added to the dry spots as the mixing
proceeds until the allowed quantity has been
used. The mass is turned over again, and
if it is streaky or shows dry spots it must be
turned again. After the final turning it is
shovelled into a compact pile.
"Dry" Mixing.— The so-called "dry"
method of mixing is of interest to the concrete
block maker who desires to remove the
shaped block from the block-making machine
with as little delay as possible. In the
United States, too, it is used in cases where
the contractor is working on a time limit,
and wishes to remove the centering, etc., at
the earliest possible moment. By this
method, no more water is used than will hold
the ingredients together when some of the
material is taken in the hand and squeezed.
Even such concrete as this will show moist
on the suiface after tamping. The concrete
attains its strength more quickly by this
method than when it is made really wet, but
there appears to be no difference in results as
REINFORCED CONCRETE
regards strength when samples of the two
kinds are tested after an interval of a couple
of years.
MACHINE MIXING AND MIXERS
The popularity of reinforced concrete has
been the underlying cause of the attention
now being paid to the design and construc-
tion of concrete mixers. The necessity of
producing a concrete of high quality, and
in which the whole of the surface of the
aggregate is coated with cement, has had
the effect of directing inventive faculty
towards the provision of a machine to do
the work more efficiently than is possible
when shovels are used. A certain knack is
necessary in hand mixing, and it is not all
labourers that possess it ; then, too, the
mixing is heavy work, and is likely to be
scamped when opportunities for doing so
arise. The introduction of the mechanical
mixer was the occasion for much con-
troversy, which has long since been settled
in its favour ; only now and then, as in the
case of an important building at West-
minster erected in 1910-12, do these re-
sponsible prefer hand mixing.
In selecting one of the forty mixers obtain-
able, the question is not so much nowadays
whether it mixes concrete efficiently, but
how long the operation takes. To the time
employed in the actual mixing must be
added that for charging and discharging,
but there is no doubt that the employment
of any of the well-known machines now
obtainable is sure to save time and money.
Machine mixing undoubtedly exercises
economy in the most costly of the three
materials — namely, cement — inasmuch as a
given amount can be incorporated efficiently
with a larger amount of sand and aggre-
gate than is possible with hand mixing.
Choosing a Mixer. — The concrete mixer
has to be designed and constructed to work
under very severe conditions, the mention
of some of which will suggest to the practical
man the points to be borne in mind when
choosing a machine. It must be simple, and
have as little mechanism as possible, as it
will be tended, not by mechanical engineers,
but chiefly by labourers, and any delicate
mechanism would soon be clogged by the
cement. It must be strong, because it has
to accommcdate heavy batches, and the
metal in contact with the concrete must
be thick and hard, since the aggregate has
great abrasive properties. All parts sub-
ject to wear by the concrete should be easily
renewable, and the makers should be asked
whether or not they can replace from stock
the parts in question. It must be compact
and eminently portable, because it may have
to be used on a number of floors in the same
building, and thence may require to be carted
miles away to another job. In its design
there must be a complete avoidance of
arrangements which theoretically are good
but practically are bad ; in other words,
the machine needs to have been designed
by someone with practical experience in
mixing concrete, as the problem is a different
one from that which the designer of mixing
machines for soft goods is required to solve.
Not only the actual mixing, but all the
accessory processes must be executed
speedily ; it must be easy to charge the
machine, and a matter of a few seconds to
empty it. It must do its work thoroughly,
by which is meant that the cement must
be uniformly distributed throughout the
sand and aggregate, every particle of which
must be coated.
Concrete is not an easy material to mix,
each of the three ingredients presenting
difficulties of its own. The cement is liable
to bridge in a tapered hopper ; its dust flies
about and clogs any mechanism present ; if
the machine is not cleaned out regularly the
cement will be retained in angles and
corners ; and a moist cement mixture does
not work freely, the trouble increasing as
the proportion of cement increases. It
might be thought that if cement and sand
were run into a revolving drum and this
speeded up to a high velocity, the maximum
of efficiency would be obtained, but it is
not so, as at a certain velocity the cement
flies to the side and clings there. Sand, if
at all damp, refuses to run unless the shoot
or hopper is steeply inclined (experience
shows that the inclination must be at least
30 degrees to the vertical). The difficulty
in dealing with the aggregate is, first, its
abrasive action, which rapidly wears out
soft metal ; and, secondly, the trouble caused
by stones becoming wedged between two
rotating arms or between an arm and the
side of the mixing vessel ; the latter trouble
is not apparent in most of the tumbling
barrel mixers.
Types of Mixers. — Coming now to
details, two broad types of mixer are at
present in use, respectively (1) the batch or
intermittent mixer, which mixes at one
CONCRETE: MATERIALS AND MIXING
time any quantity up to the full capacity
of the mixing vessel ; generally it is a
rotating drum, to the inner sides of which
are riveted blades which assist the mixing
action ; a lesser-used type has a pan, either
revolving or stationary, with paddles or
Fig. 9. — Sectional Diagram showing Principal
of Chicago Cube Mixer
stirrers to agitate the material. (2) The
continuous mixer, which delivers a con-
tinuous stream of mixed concrete ; the most
popular type consists essentially of an
inclined cylinder with suitable mixing or
agitating appliances.
The method of measuring or gauging the
materials is of the greatest importance in
connection with machine mixing. In the
case of the batch mixer the measures are
boxes, hoppers, or even the wheelbarrows
in which the material is conveyed to the
machine ; but the most convenient form
of measurer is a slap which may be lowered
to the ground to receive the charge, and
then in a few seconds elevated and tilted
so that its contents pour into the mixing
drum.
As regards continuous mixers, owing to
the nature of the mixing process the
materials have generally to be measured
and roughly incorporated in their dry state
before they are introduced into the machine ;
or, as an alternative, there is, as a part of
the machine, the necessary mechanism for
proportioning the ingredients. Examples of
machines so equipped are illustrated later.
There is a growing consensus of opinion
in favour of the batch mixer for reinforced
concrete work, while the other type is found
to have advantages in connection with large
works, such as harbours, coast defences, etc.,
where the concrete requires to be deposited
in bulk, and where, possibly, immense
quantities are required in one place. The
preference given to the batch mixer is due
to a number of reasons : (1) It lends itself to
the intermittent nature of the work. (2) It
is thought that the clerk of works can exer-
cise a more careful supervision over the pro-
portions of the ingredients, but whether as
a matter of fact he can do so in practice is
a moot point, as obviously in a building of
any size he has other things to do than
watch a mixer at work for several consecu-
tive hours. (3) Both the proportions of
the ingredients and the duration of the
mixing can be easily varied, whereas it is
a matter of difficulty in some continuous
mixers to make any appreciable alteration
in the duration of the mixing without inter-
fering with the efficiency of the product, as
any slowing down or speeding up of the
machine often has the effect of necessarily
altering the proportion of water admitted
to the mixture. The continuous mixer has
a different mixing efficiency with different
classes of aggregate, and, theoretically, with
any increase in the proportion of sand the
inclination of the cylinder should be in-
creased, this not often being possible. (4)
The batch mixer enables the ingredients to
be mixed dry, if required.
Fig. 10.— Chicago Cube Mixer with Engine
and Boiler
The power for driving a mixer may be
either hand or engine, and in the majority
of cases the larger mixers are self-contained,
a boiler and engine, an oil engine or petrol
motor, or an electric motor to be connected
up to the supply mains, being mounted on
the same base as the mixer. Naturally, the
REINFORCED CONCRETE
power-driven machine has a much greater
capacity than the hand-driven one. A
warning with regard to advertised capacities
may here be given. Makers often state most
definitely the weight or the number of cubic
feet of concrete which their machines can
mix in the course of an hour, and they
Fig. 11. — "Cut-away" View of Eclipse
Mixing Drum
advertise the results of perfectly honest
tests ; but inasmuch as the capacity of a
machine must vary according to individual
conditions — the disposition on the site of
the cement, aggregate, etc., the nature of
the job, and the amount of experience of
the operatives — a contractor who is choosing
a machine should not place too much reliance
upon any comparison he may draw between
two sets of figures.
BATCH MIXERS DESCRIBED
Revolving Drum Machines. — There is
any number up to a score of these from
which to choose.
Chicago Cube Mixer. — This is based essen-
tially on the old-style tumbling drum — a
cubical box journalled at diagonally opposite
corners — but the shaft in the old-style mixer
is replaced with hollow trunnions riding on
rollers and made large enough to serve as
openings for charging and discharging the
mixer (see Fig. 9). To rotate the cube
there is a circumferential rack fastened
around it on a drum at right angles to, and
midway between, the trunnions ; this rack,
geared with a pinion shaft, is operated by
the engine shaft in such a way that there
is no gearing in the way of the materials
either in charging or discharging. The
sharp corners and edges of the cube are
rounded off to obviate the possibility of the
fine material effecting a lodgment. The
manufacturers of this mixer have found that
the batch is folded over on itself, and pressed
into a contracting space shaped alternately
like a wedge and like a pyramid ; and on
this fact is based their statement that the
mixing is done by kneading and not by
stirring. Fig. 10 shows the cube mixer,
with steam engine and boiler ; another type
has a petrol motor.
Barker and Hunter. — The special feature of
this machine is that it cannot be used as a
continuous mixer. It is fed while revolving
in one direction, and must be reversed before
the mixture can be discharged. The actual
mixer consists of two truncated cones, whose
bases merge into a cylinder, the ends of the
cones being mounted with roller path bear-
ings and rotating on rollers mounted on a
suitable frame. Two hoppers are fitted, and
the mixer can be charged from either end or
from both ends, which are always open, with
the cone either stationary or in motion.
The makers say that it takes twenty seconds
to charge the material into the cone, sixty
seconds to mix them completely, and ten
seconds to discharge.
Eclipse. — This machine is built shallow
so as not to necessitate mechanical charging
devices. The drum has a shell of rolled
steel plate, and has flanged steel heads ; it
is supported by trunnion rollers, and is
rotated by gears meshing into two gear
rings. At the charging end is a large cir-
cular opening in the flanged head, and
Fig. 12.— Eclipse Mixer with Petrol^Motor
diagonal overlapping charging blades extend
into the drum. In the interior of the drum
are diagonal blades firmly attached to the
sides. One blade extends from the charging
end diagonally to the discharging pocket
shown at the rear of the drum in Fig. 11.
The aggregates are carried into the mix-
ing chamber by conveying blades rigidly
CONCRETE: MATERIALS AND MIXING
33
attached to the sides and ends, so designed
that the material cannot slop out when
mixing. The discharging shoot extends
into the drum through the head at the dis-
charging end. A shaft extends through the
drum near the shoot, and on it is mounted
a door held during either mixing or dis-
charging by a strong spring under tension.
When the operator throws the door past the
centre of tension in either direction, the
spring completes the movement and holds
the door until it is again held by the operator.
Fig. 12 is a special design mounted on a
truck and equipped with a petrol motor
housed in the casing.
Gauhe. — In this mixer the drum (supported
by friction rollers) revolves, while paddles
Fig. 13.—" Cut-away " View of Koehring
Mixing Drum
and a scraper are fixed and serve to mix
the material. The material enters the drum
through an opening at one end, and the
discharge is through a sliding door controlled
by a lever. The elevating device is on the
lines of the Fawcett, described later, and
therefore, like that machine, it requires con-
siderable head-room. The road wheels are
of large diameter, so as to allow trolleys to
pass beneath the mixing drum to receive the
concrete. The equipment includes an auto-
matic water-measuring tank.
Koehring. — In this the drum is of boiler-
plate steel with cast heads, surrounding each
of which is a gear ring. The double drive
has the advantage that, should a tooth
break, the machine can continue on a single
drive. An interior view of the drum is
given in Fig. 13, whilst Fig. 14 is a photo-
graph of the mixer with steam boiler and
3
engine, side loader, and water measuring
tank (shown above the mixing drum) ; the
Fig. 14. — Koehring Mixer with Engine
and Boiler
operating levers and hoisting mechanism are
clearly shown. The discharge shoot is made
in two pieces, the outer end being stationary,
and the inner end pivotal, so affording ample
clearance for wheelbarrows in the discharge,
and permitting the inner end of the shoot
to assist in the mixing when tilted inwards.
The side loader is a short, wide, and low
bucket with round corners and sufficiently
low pivotal point to require but little power
Fig. 15. — Marsh-Capron Non-tilting
Mixing Drum
to raise it ; it can be elevated to almost a
vertical position, so that the material readily
flows into the drum.
34
REINFORCED CONCRETE
MarsJi-C apron. — Both tilting and non-
tilting machines of this manufacture are
Fig. 16. — " Cut-away " View of Marsh-Capron
Tilting Mixing Drum
known, and in each the drum is the special
feature. In the non-tilting style, the drum
(see Fig. 15) is cast in two parts, the con-
necting flanges fitting one within the other
and centering the two halves, which are
securely bolted together. The steel mixing
blades are bolted to the drum, and both
these and the steel mixing buckets are so
placed as to give a thorough mixing action
to the material, providing an end-to-end as
well as a lifting and pouring action. The
driving gear surrounding the drum consists
of five segments absolutely interchangeable,
so that should a tooth be broken a segment
can be renewed at the labour of removing
and repl icing five bolts,
the drum being kept in
place 'and the rest of the
machine remaining undis-
turbed. In the tilting
machine the drum (see
Fig. 16) has its blades
arranged in serpentine
form and attached with
brackets, there being
sufficient space between
blades and drum to allow
of thorough washing out.
Fig. 17 shows the machine
with the drum in the
tilting position.
McKelvey. — In this ma-
chine the drum consists Fig. 17. — Marsh
of a short cylinder with a conical hopper
on the feed side and a trumpet-shaped
discharge funnel on the other side. It re-
volves on friction rollers, and is operated
by a chain and sprocket ring. Its chief
feature is the means adopted for ensuring
the proper mixing of the material. The
special device used (see Fig. 18) is known
as a " gravity shovel," and is pivoted to the
inside of the drum ; the top edge lies against
the rising side, collects the materials in that
part of the drum, and, after it is filled, the
overflow slides rapidly of? the lower edge
and the balance is carried upward to a point
where gravity causes the shovel to cast the
material outward and downward, turning
it over in the fall.
Messent. — This is based on the old tum-
bling barrel, the design being a closed vessel
of irregular shape revolving on an axle.
As used on the Dover Harbour works, it
was mounted on a steel-framed carriage,
there being one electric motor to rotate
the mixing vessel and another to give a
travelling motion to the carriage, the
materials being thus mixed in course of
transit between place of charging and the
work.
-Capron Mixer, with Drum in tilting position "
CONCRETE: MATERIALS AND MIXING
35
Pioneer. — This is a small hand mixer,
capable of handling between 250 Ib. and
350 Ib. per batch. It can easily be worked
by one man, and is too simple to require
detailed explanation, being simply a mixing
drum supported on a horizontal spindle.
The materials are introduced, the lid closed,
Fig. 18.— McKelvey "Gravity Shovel"
the drum rotated three or four times, then
the water contained in the tank at the side
is allowed to enter the drum, through the
perforated shaft, and the drum is finally
rotated again a few times, the mixture then
being complete.
Ransome. — Machines of this make largely
owe their conception to E. L. Eansome,
whose name is of importance in the history
of reinforced concrete in the United States.
A variety of Ransome machines is made,
all incorporating the vital feature — namely,
a drum with mixing blades of a special type.
Figs. 19 to 24 are photographs of six
arrangements of the Ransome mixer, there
being an explanatory inscription to each
figure. The drum is not of the tilting type,
the discharge being made possible merely
by opening a shoot. Each type of machine
has a hopper which measures the exact
amount of aggregate for each batch, there
being in addition a water tank which
automatically gauges the exact amount of
water required. Within the drum is a series
of steel scoops (see Fig. 25) which plough
through the material repeatedly, pick it up,
and carry it upwards till it slides out. The
aggregate having been delivered from the
hopper into the drum, the hopper gate is
closed and the hopper re-charged in readi-
ness for the next batch. The elevating
hopper is so arranged that it can be lowered
until its bottom rests on the ground. When
required, a lever is pushed, and the elevator
tips the material into the hopper. Fig. 27
shows two elevations and a plan of a
Ransome belt-driven mixer.
In Great Britain the Ransome mixers are
built by Ransomes and Rapier, Ltd., of
Ipswich, and in the United States by the
Ransome Concrete Machinery Co., Dunellin.
The English and American machines con-
form to the same general patterns, but there
are minor differences of detail ; for example,
in the American mixing machine the interior
of the drum resembles Fig. 26.
Roll. — This has distinct points of novelty
(see Fig. 28). The drum consists of two
cup-shaped halves mounted on a spindle
in such a way that they can be drawn apart
to discharge the concrete. The drum con-
sists of two castings, to one of which is
bolted a sleeve carrying a worm which
serves to move one half of the drum along
its spindle when discharging is necessary.
The mixer is equipped with an elevator
consisting of a measuring skip hauled up
by a wire rope and returning by gravity.
Smith Hand Mixer. — As shown by Fig.
29, this machine is driven by means of
crank handles — one man to each side — but
it is easily adapted for power driving. The
ends of the drum are sloped inwards until
they almost meet, forming two wedge-like
chambers united by a 4|-in. slot, extending
diametrically across the drum. The drum
itself is suspended on chains, four guide
rollers being provided to keep it in align-
ment and prevent it from swinging during
the mixing process. The principle of the
mixing can be understood by bearing in
mind the old-fashioned hour-glass, the
material from one chamber having to pass
through a restricted opening into the other
one. Only two or three turns of the drum
are required. The charging platform sup-
plied is only 15J in. high, thus enabling a
wheelbarrow to charge directly into the
machine, and the mixer is so constructed
that, if it were desired, it could discharge
directly into a trench.
Smith Power Mixer. — The drum of this
machine consists of two cones united at
their bases, as shown in Fig. 30. The
blades which assist the mixing are arranged
spirally ; and at the discharge end are drip
rings which prevent the wet material from
running down the cone and over the roller
tracks. The drums are partially lined ; for
instance, the large ends of the cones are
made of double thickness, because that is
where the principal wear comes. This
REINFORCED CONCRETE
lining is a regular repair part and is easily
fitted. With certain of the sizes a power
having a double conical drum which is tilted
to discharge the concrete.
Fig.
19. — Belt-driven Ransome
Mixer
Fig. 20. — Ransome Mixer
with Hoist
Fig. 21. — Ransome Mixer
with Elevating Hopper
Fig. 22. — Ransome Mixer
with Electric Motor
Fig. 23. — Ransome Mixer
with Boiler and Engine
Fig. 24. — Ransome Mixer
with Oil Engine
tilting apparatus is supplied, the makers
believing that the tilted drum provides the
fastest method of discharging. It is only
the smaller sizes of the Smith power mixer
Victoria. — The drum of this mixer is a
cylinder with four deflecting surfaces or
inclined planes (see Fig. 33). Lifting
blades are riveted to the drum and extend
Fig. 25. — Scoops in Ransome Mixing Drum
(English)
that are recommended for reinforced con-
crete work.
Taylor. — This resembles the Smith in
Fig. 26. — Scoops in Ransome Mixing Drum
(American)
from the discharge end well towards the
feed end. As the drum rotates, the lifting
blades elevate the material, which drops
CONCRETE: MATERIALS AND MIXING
37
upon the inclined planes and is thus thrown
across the drum and returned again by the
opposing deflectors. The process is repeated
the spout is inserted into the drum the mass
falls from the blades upon it, and the con-
crete flows into the receptacle prepared for
twice for each revolution, and submits the
mass to twelve distinct mixing actions per
turn of the drum. The complete machine,
on a truck, with engine and boiler, side
loader, and automatic water tank, is shown
in Fig. 31. There is a low feed level, which
is of especial advantage in a portable plant
charged by wheelbarrows. For the purpose
of emptying the mixture, a swinging spout,
pivoted below the discharge opening, is pro-
jected into the drum. It will be understood,
on referring to Fig. 31, that the spout is
pivoted to the vertical frame shown, and
easily swings forwards or backwards. When
c _ _
Fig. 27, — Elevations and Plan of Ransome
Belt-driven Mixer
Fig. 28.— Roll Mixer, with Elevator raised
REINFORCED CONCRETE
it. The flow is under easy control, as the
shoot may be removed from the drum at
any moment by a mere turn of the wrist.
Any quantity, from a quart to the full mixing
capacity, can be withdrawn. The charging
skip is shown in the elevated position in
Fig. 31, and in its bottom position in Fig.
32. This skip will hold a complete batch,
and, as it progresses upwards on the curved
guides shown, its nose is thrust into the
feed opening of the drum. Care has been
of concrete ; for this purpose the gauge is
set for the maximum capacity, the tank is
allowed to fill, and the valve is then opened.
The pointer on the outside will then fall, and
a glance at the gauge will show at any instant
how much water has run out. The necessary
water for a batch in a large-size mixer is
best obtained by measuring the water into
the mixing drum in two lots ; this is safer,
as a rule, than introducing all the water at
one time. When this automatic water tank
Fig. 29.— Smith Hand-driven Mixer
taken to make the feed angle steep to pre-
vent clogging, and to increase the rapidity
of the feed. The makers provide extensions
for the loader frame when required, so that
the skip can be lowered to receive the batch
on a level below the mixer. In the two com-
plete views a rectangular tank will be noticed
at the top. This is an automatic water
tank of 18-gal. capacity, with a gauge which
can be set so that the tank will supply
uniform quantities of water varying from a
few drops up to the full capacity of 18 gal.
The tank serves as an indicator to show the
amount of water introduced into any batch
is a part of the equipment, it is necessary
to supply the water to it under pressure, or
from a reservoir placed at a higher level
than the tank. The makers fit either steam
engine, petrol engine, or electric motor to
their mixers, and they particularly recom-
mend the slow-speed electric motor which
may be geared direct to the mixer.
Other Batch Mixers. — The drum mixer
is not the only type of batch machine. There
are a few examples of mixers having pans
and stirrers.
Express. — This has a stationary pan to
hold the material (see Fig. 34), revolving
CONCRETE: MATERIALS AND MIXING
39
in it being a series of paddles and rakes
attached to arms which project from a central
power-driven capstan head. By means of
two discharge doors in the bottom of the
pan the smaller size mixer can be emptied
in fifteen seconds, both feeding and dis-
charging taking place while running.
Faivcett. — The material is contained in a
fixed drum in which a number of arms or
filled, ascends along a pair of inclined guides
by means of a wire rope which is wound on
a drum actuated through a friction wheel.
When the skip arrives at the top, it tips
automatically into the mixing vessel and is
then lowered for another charge.
Gaspary. — The hand-driven mixer (Fig. 36)
has a trough capable of treating up to 45
cub. ft. an hour, according to size. Above
Fig. 30. — Smith Power-driven Mixer
paddles rotate in different directions, the
arrangement being such that when the dis-
charge door at the bottom of the drum is
opened, the revolving paddles scrape the
concrete out (see Fig. 35). The mixing is of
an efficient character, and the open pan
enables the attendant to see when the con-
crete has attained an even colour through-
out. The machine has an elevator for
measuring and feeding the charge, and an
automatic water tank for measuring the
water. The elevator is a skip which, when
the tilting trough is a small water tank, the
water reaching the materials through a per-
forated pipe. In the power-driven mixing
machine, there is a drum with blades
which rotate in either direction, this change
in the direction being of advantage for
cleaning and emptying the mixing drum,
and allowing of any stones stuck fast be-
tween the drum and the blades being in-
stantly removed. Continuous machines are
also made by the Gaspary firm, as noted
on a later page.
4o
REINFORCED CONCRETE
Fig. 32. — Victoria Mixer, with Skip Lowered
Oehler. — This is a Swiss machine -
resembling the Fawcett, but the
paddles rotate in one direction only.
Open - Drum. — This mixer has
a cylindrical drum, normally up-
right, with conical top which
facilitates discharging. The sides
of the drum are of steel plate,
but the bottom is a casting having
a circumferential rack, by means
of which the drum is rotated
through gearing. The drum is
carried by a supporting frame, in
which is a ball-bearing for a
short vertical shaft which pro-
jects into the mixing drum. The
mixing is efficient, and is much
facilitated by parallel blades
riveted to the sides of the. drum.
The drum, being so near the ground,
can easily be charged from wheel-
barrows on a loading platform,
while for discharging there is a
hand or power arrangement for tilting the Pansy. — As shown in Fig. 37, there is
drum. The capacity varies from 2J to 18 an annular vessel which runs on a ball-
cub, ft., according to the size and style. bearing cast-steel base, and is driven through
toothed gearing. Through the
centre of the machine is a per-
pendicular shaft, to which are
hung angle-iron frames, and to
these again adjustable ploughs
are fixed. The pan revolves at
the rate of ten or twelve revolu-
tions per minute, and as the
ploughs remain stationary, the
materials are thoroughly mixed
in a short time. The materials
are gauged in the ordinary way
on a platform above the ma-
chine, and tipped directly into
it, being there levelled by a
scraper, the ploughs lowered and
the material mixed dry. Next,
water is added, and the materials
further mixed. When mixing
is complete, the ploughs are
raised, the scraper lowered, and,
by means of the trapdoors in
the bottom, the mixed concrete
passes into a receptacle prepared
for it. The doors close auto-
matically by means of a special
catch, and as soon as the
material is all discharged the
pan is ready for another batch.
Whalley. — In this machine the
Fig. 31.-Victoria Mixer, with Skip Elevated materials are discharged from a
CONCRETE: MATERIALS AND MIXING
hopper (see Fig. 38) into a stationary pan
and the mixing is done by stirrers
mounted on arms projecting from a verti-
for hand use, and which may be either
movable or stationary according to require-
ments. The materials have to be fed in
Fig. 33. — " Gut-away " View of Victoria
Mixing Drum
cal shaft. Each stirrer comprises a vertical
cutting part and a scraping part, the latter
being almost in contact with the bottom
of the pan. The stirrers
tend to force the mate-
rial towards the sides,
and when the mixing
has been completed a
door is opened and the
mixture discharged sim-
ply by the continued
rotation. The parts of
the machine liable to
wear or replacement are
the bottom portions of
the stirrers, and they can
be readily renewed.
CONTINUOUS MIXERS
N o n - p r o portioning
Machines. — These are
of much simpler con-
struction than the mixers
fitted with proportioning
or measuring arrange-
ments, and a few exam-
ples will now be briefly
described.
Gaspary. — Only the
smallest of the Gaspary
continuous mixers need
be mentioned here. Fig.
39 shows a power-driven
machine, which, how-
ever, can be adapted
Fig. 34. — Express Mixer
measured quantities, the water being
obtained from a supply tank shown above
the mixing cylinder, there being a perfor-
ated pipe extending from the tank right into
the cylinder. The dry mixture enters the
cylinder, is wetted and mixed, the mixing
action being assisted by shovels which are
turned by the rotation of the cylinder.
Mason. — The feature of this machine is
a cylinder slightly larger in diameter at one
Fig. 35. — Fawcett Mixer
REINFORCED CONCRETE
end (the discharge end) than at the other
where the feed hopper is situated (see
Fig. 40). The cylinder is mounted on a
Fig. 36. — Gaspary Hand-driven Tilting Trough
Mixer
central spindle and has ribs to assist the
mixing. Machines for hand or power are
made.
Gravity Mixers. — No power is required in
the operation of a gravity mixer, the force
of gravity alone being relied upon to do the
work. The materials are shovelled into the
number of baffle plates. The rows of pins
are staggered with respect to the rows
immediately above and below them, and
it follows that when the materials fall from
the hopper on and off the pins and deflectors,
they become incorporated. The water is
added (by means of a flexible hose) half-way
down, this giving the materials a chance to
get partly mixed before being wetted.
The Owens is a gravity mixer in which,
there is a steel shoot about 7 ft. long con-
taining three sinuous mild steel bars extend-
ing in one length from the top to the bottom.
The bars are fixed rigidly at the top, but at
the bottom are held loosely by means of a
|-in. pin, which passes through them. In
addition, there are eight large and six small
baffle plates fixed to the sides of the shoot.
To facilitate cleaning, there is an inspection
door. The water supply pipe is fixed behind
the lower edge of the top baffle plate, and
is perforated both back and front, the
whole of the baffles and bars receiving a
spray of water. The materials, having been
introduced in any convenient manner, fall
through the shoot and are alternately split
up into two columns and then united,
this action being repeated eight times.
Proportioning or Measuring Machines.
— Brief particulars will now be given of
eight examples of continuous mixers with
provision for proportioning the concrete
ingredients.
Bolte. — This is an American machine (see
Fig. 41) adapted for hand or power, accord-
Fig. 37. — Pansy Mixer
top end of a vertical or inclined shoot, a
common pattern of which has a large num-
ber of round pins penetrating the shoot from
front to back, there being in addition a
ing to size. The principle of this machine
is the conveying of proportioned quantities
of cement and aggregate, by means of a
rubber belt, into a mixing cylinder, where
CONCRETE: MATERIALS AND MIXING 43
the material is agitated by six blades. The may be taken as representative. It has
hoppers are shown in Fig. 41a. The belt three hoppers or pockets for automatic-
forms the bottom to them and draws off ally proportioning aggregates, sand, and
cement, and the proportioning
device works without the use of
gears, sprockets, or chains. The
pockets .on each side are oper-
ated by one sliding belt, which
moves forward and backward on
rolls and acts as a plunger for
delivering the material. The
sliding part is moved by two
crank arms attached to the square
shaft, which also passes through
the cement hopper, where its
reciprocating action prevents any
bridging of the cement. Each
material can be proportioned to
a nicety, and the materials can
be fed in from one or both sides.
The mixing device is a special
feature, and unlike any other
mechanism described in these
pages. It consists of seven steel
blades twisted spirally and bolted
Fig. 38. — Whalley Mixer, with Engine and Boiler
at each end to a solid casting,
as shown in Fig. 43. This con-
struction cuts through, lifts, and
the materials through adjustable gates, pours the material, the result being a
discharging them into the cylinder, where, most efficient mixing. Between the engine
15 in. from the lower end, the water is intro-
duced by means of a perforated pipe con-
nected to the water tank above. As it is
well known that cement and sand do
not readily flow at
all times, the hop-
pers are fitted with
agitators, the cement
agitator being
worked by a cam
on the rear belt
shaft and the sand
agitator consisting
of a worm on a con-
tinuation of the
cylinder shaft, this
being clearly shown
in Fig. 41a. These *
mixers are essen- ;
tially portable, and V,
can be taken from
floor to floor as the
work proceeds.
Coltrin. — Machines of this make are made and the mixer is a friction clutch, which,
in eight sizes, between which there are besides being generally useful, has one
various differences of detail, but for the pre- special advantage : should a large stone
sent purpose the No. 14 machine (Fig. 42) get into the mixer accidentally and bind
Fig. 39. — Gaspary Drum-type Continuous Mixer
44
REINFORCED CONCRETE
between the blades and the vessel, the
clutch will slip, so avoiding breakage of the
mixer. The water tank with special pro-
Fig. 40. — Mason Mixer
portioning arrangement is furnished with all
machines for use where regular direct pres-
sure is not available.
Carey-Latham. — In this machine, chain
and bucket elevators are employed, one for
the sand and another for the aggregate, to
feed the mixing cylinder, which is inclined
towards the discharge end at an angle of
8 degrees, revolves on friction rollers, and
has projections on its inner surface to assist
the mixing. The cement is fed to the
cylinder by means of a worm. Other types
of machine bearing this name have been
used, but the above is the most generally
useful.
Gaspary. — The funnel-dish mixer made
by this firm has a number of funnel-shaped
reservoirs open at the bottom, below them
being rotating blades. The materials are fed
into the reservoirs, passed through the
adjustable openings, dropped upon the pro-
jecting rims of the blades, and are swept
down into a mixing trough, in which a screw
is working and in which the moistening is
effected.
of bridging of the sand and other materials,,
without employing agitators. The appear-
ance of a typical machine is shown in
Fig. 4i. The three hoppers with their feed
mechanism deliver the materials to the
mixing trough at the rate of from 35 to
40 discharges per minute for each material.
The sand and cement drop in the same
place, and the dry mixing begins imme-
diately and continues for more than half
the entire length of the trough. Two water
sprays are provided, one spraying directly
into the trough, and the other into the
aggregate before it is discharged into the
dry mixture of sand and cement, the object
being to cause the fine stuff to adhere to
the aggregate instantly. Each spray has a
separate regulating valve. Passing through
the cylinder is a square shaft to which
numerous paddles are bolted.
Nims. — In this machine, the mixing vessel
is of cast iron in one piece, shaped some-
thing like a pair of cubes interpenetrating
diagonally and revolving on the long axis.
A belt delivers the materials, which have been
proportioned in a special form of measurer,
to the mixer. The mechanism for measuring
consists of a wheel 15 in. in diameter and
11 ft. long, revolving under a hopper which
is 12 ft. long. In the wheel are receptacles
which receive the materials as they fall from
the hopper, and, as the wheel continues to
rotate, deliver them to the belt conveyer
below. In the hopper are partitions which
can be moved to vary the proportions. As
above described, the mixer acts on the con-
tinuous principle, but by fitting an auto-
GATE FOR
AGGREGATE
Fig. 41.— Bolte Mixer
Kent. — A variety of machines are made
by this firm, the principal being a three-
hopper machine in which very carefully
designed arrangements are employed to pro-
portion the materials accurately. Special
devices have for their purpose the prevention
Fig. 41a. — Hoppers of Bolte Mixer
matic gate to the discharge end, it can be
used as a batch mixer.
Perfect. — As shown by Fig. 45, there are
in this mixer three distinct hoppers, in con-
nection with each of these being a revolving
feed drum with deep flanges at each end,
CONCRETE: MATERIALS AND MIXING
and a regulating gate for proportioning the
material set by a lever moving over a gradu-
ated quadrant. The feed drums make only
one or two revolutions a minute, and a
regulator strikes of? the cement and allows
a fixed quantity to pass as a layer of uniform
width and thickness. The stream of cement
combines with a stream of
sand, and then falls on a
stream of aggregate, passing cc
thence to the mixing cylin-
der, in which is an adjust-
able water spray.
Trump. — This type of
machine requires but little
power, as gravity to some
slight extent is utilised.
The measuring device consists of three
cylinders for the sand, aggregate, and
45
the knife can be altered to vary the pro-
portion. The principle of the measuring
device is clearly illustrated by Fig. 46.
Fig. 43.— Coltrin Mixing Blades
The materials, having been proportioned,
flow together in the form of one stream
Fig. 42.— Coltrin Mixer
cement respectively, and from this the
materials pass to three rotating tables,
Fig. 44.— Kent Mixer
where they meet fixed knife edges that
scrape the materials off into a trough
below in required proportions. The set of
Fig. 45.— Perfect Mixer
(see Fig. 46), which falls into a cylinder,
where the materials are moistened by
water sprayed from a perforated pipe. In
the cylinder, steel paddles rotate on a shaft
and complete the mixing.
TESTING EFFICIENCY OF MIXING
Of the many methods that have been
proposed for testing whether concrete has
been properly mixed, the most practical
and the one best suited for adoption in
everyday work is that due to Dr. J. S. Owens,
and described by him before the Society of
Engineers. It is based on the principle of
sampling a heap of concrete at different
places, and then seeing whether all the
REINFORCED CONCRETE
samples contain the same relative propor-
tions of stone, sand, and cement. Each
sample is put into a tall glass cylinder
nearly filled with water, shaken up, and
allowed to settle. The rate of settlement
of cement in water is about thirty times
as slow as that of the sand, which, in turn,
is slower than that of the larger particles of
stone ; as a result of the variable rates of
deposition the stones, sand and cement
settle in distinct layers, whose depth is pro-
portional to the amount of the material
present in the sample. Thus it is possible
to tell, with a fair degree of accuracy,
whether each sample was compounded with
the same proportions of cement, sand, and
coarse aggregate ; obviously, the test re-
quires reasonable care in its execution.
barrows of the usual type, or they may
have an iron framework supporting a steel
Fig. 47. — Ransome Concrete Cart
CONVEYING CONCRETE
It always happens that either the
raw materials have to be conveyed
from their storage place to the stage
or mixer or that the concrete is made
close to where the materials are stored
and then has to be carried to the
place where it is to be used. The
means of conveyance employed em-
brace barrows, handcarts, derrick
hoists, bucket elevators, and special
hoisting systems, such as the Ransome,
that have been particularly designed
for use in connection with concrete
buildings.
The barrows may be deep wooden
Fig. 46.—
" Cut-away "
View show-
ing Principle
of Trump
Mixer
box in which the concrete is
carried, the design being such
that the load is carried as much
over the wheel as possible.
A convenient type of iron
barrow is arranged in such a
way that the box is pivoted on
the axle at each side of the
wheel, so that it can be tilted
forward to discharge its load. In
Fig. 48. — Ransome Concrete Skip or Bucket
Fig. 49. — Ground Plan, Top Plan, and Two Elevations of Ransome Tower
48
REINFORCED CONCRETE
engineering work, but not often in general
building work, it is economical to have bar-
rows or tray dumpers running on fixed rails ;
the boxes need to be made of substantial steel
plate strengthened round the top edge to
avoid bending and buckling, and there must
be an arrangement for tilting the buckets.
One of the best forms of hand conveyers is
the Ransome cart (Fig. 47), which has a work-
ing capacity of from 5 to 5J cub. ft. of wet
concrete. The wheels are 42 in. in diameter
with eighteen staggered spokes, the axle,
1 J in. in diameter, passing right through the
cart body ; the tread of the tyre is flat, and
it is essential to have a good runway. The
handle is reversible, and the whole cart is
designed to discharge its contents in a
second or so. For elevator work, in con-
junction with a special form of tower
described in the next paragraph, the cart
can be fitted with legs.
Hoisting appliances are frequently neces-
sary, and their nature will depend some-
what upon the design of the building. Some-
times it is convenient for the concrete to
pass straight from the machine mixer into
cylindrical steel buckets, which are then
hoisted by a steam crab or other suitable
means as required. Another arrangement
is to instal a bucket elevator of the endless-
chain type, there being sprockets at the
lowest and highest points which drive the
chains to which the buckets are attached ;
these buckets need to be very strongly made
and have strengthened rims. Other hoist-
ing arrangements as used in general building
work will frequently be found useful, and
these do not need description here. Space,
however, must be found for mention of the
Ransome tower, which has been specially
designed for concrete work. The bucket
used is a modification of the one previously
illustrated, and is shown by Fig. 48. It
has trunnions which are carried in journals
at the bottom of a steel frame, which slides
up and down between wooden guides in a
well-braced tower (see Fig. 49). The
bucket is carefully adjusted for balance by
means of stops until it has a slight tendency
to tilt forward. When loaded, it is pulled
up inside the tower with its nosepiece press-
ing against a front guide, and when the
bucket arrives at a point in the tower where
the front guide has been cut away to leave
a space, as shown at B, Fig. 49, it auto-
matically tilts forward and discharges its
contents into a bin from which the concrete
is drawn off by means of gates into barrows
or handcarts, or in some other suitable way
is carried to the work. The hoist tower is
constructed of light timber, and, built accord-
ing to Fig. 49, it will accommodate the
smallest size of Ransome bucket, the
dimension A being 30£ in., whereas in
the three larger sizes this dimension is
respectively 37 in., 44 in., and 50| in., the
other dimensions of the tower varying
accordingly. It will be realised that the
arrangement is one of great ingenuity,
and, at the same time, simplicity. The
mixer discharges a batch into a bucket at
the lowest point of the system. The friction
crab hoist shown to the left of the mixer
operates a rope or chain, by means of which
the bucket is raised until it automatically
tilts forward and discharges. The bucket,
on descending automatically, rights itself
and comes to rest on a rubber cushion in
readiness for the next batch. The bucket
may take other forms, and may embrace an
arrangement for dumping through the
bottom when an attendant raises a handle
or lever.
Gravity System. — The " gravity sys-
tem " of placing concrete has lately come to
the fore. Briefly, it consists in hoisting the
concrete by means of a skip and pouring it
into a hopper supported at the top of a
skeleton tower. From this hopper the con-
crete passes as required through pipes which
discharge the concrete directly into the
forms. This system has been used success-
fully for all classes of work, including an
eight-story office building and long bridges,
in one case the concrete having been con-
veyed by gravity a distance of 502 ft.
LAYING AND POURING CONCRETE
It is desirable to deposit concrete in the
forms or moulds as gently as possible, and
in such a way as will allow of thorough
compacting, a result which will be facilitated
by working with a spade or punner, until
the surplus water appears on the surface.
Gentle punning consolidates the concrete
and is of the utmost value in producing
good work. Of course, punning must not
continue after the cement has begun to set.
The depth to which the concrete is
deposited obviously depends entirely upon
the nature of the job. To say that common
sense should be employed, and the concrete
never deposited so thickly that there is
doubt as to whether the reinforcement is in
CONCRETE: MATERIALS AND MIXING
49
contact with the concrete at every part, is
better than to specify layers of any par-
ticular thickness. Much has been said as
to whether concrete may properly be tipped
from a height into its final resting-place,
such a method having been thought at one
time to be necessary in order to consolidate
the material. The method is not harmful
where the vertical distance or " head " is
only a few feet, but it is not to be recom-
mended when the distance is any more than
that, as there is a tendency for the heavier
ingredients to separate from the bulk of the
concrete.
The very greatest care should naturally
be exercised to prevent any interference
with the wet concrete after it has been
deposited and rammed. Any disturbance
after setting begins is detrimental to the
strength.
It is usual to cover up the concrete at
night, one object of so doing being to prevent
any violent rain washing some of the cement
out of the top surface, and another, in winter r
to prevent frost reaching the work, since it
appears that frost occurring before the
cement has set exercises a disintegrating
action, and influences the setting power and
ultimate strength unfavourably.
When depositing concrete in layers, it is
important to secure a good bond between
the old and the new material. When there
has been any length of time between the two
layers, it is necessary to wet the old work,
hack it over, and sweep clean. Many
authorities take the precaution of treating
the first layer, after hacking and sweeping,
with cement grout, so as to ensure a strong
bond. When one layer follows the preceding
one at a close interval, all that is necessary
is to see that the surface of the concrete
already deposited is wet.
Steel
Composition of Steel. — Steel consists of
iron containing carbon in the form of iron
carbide, the proportion of which, and the
manner in which it is held, largely deter-
mining the character of the steel. Other
elements are also present. Steel always
contains manganese, and also small amounts
of silicon, sulphur and phosphorus. The
proportions of the two latter are important,
and only traces not exceeding 0-04 per cent,
are permitted in structural steel. The table
given below indicates the relative composi-
tion of the various forms of iron in use.
to the method by which they have been
produced. They are cast, not forged, into
shape, and the metal is not malleable in
the above sense. The bending and twisting
of wire used in reinforced concrete work
for tying the bars or making meshed
wire supports requires that the metal to be
used shall possess the property of malle-
ability.
Tenacity. — The resistance to rupture by
a stretching force is known as tenacity.
It is determined by subjecting a piece of
the metal of known sectional area to a
Silicon
Carbon
Sulphur
Phosphorus
Graphite
Combined
Cast iron (castings)*
Wrought iron
Mild steel
1—3-5
tr.— 0-1
tr.— 0-3
1—3
0-15—0-9 0—2
tr.— 0-25 tr.— 0-1
0-1—0-5 0-3—1-0
0—0-2
tr.— 0-1
tr.— 0-04+
0—1-5
tr.— 0-2
tr. — 0-04|
Harder steel
tr.— 0-3
—
0-6—0-8 1 ditto
ditto
ditto
Cutlery steel
—
—
0-5—1-5 tr.— 0-3
trace
trace
Not pig iron or malleable cast iron,
f For rails this may be 0-06-0-07.
Alloy steels contain, in addition to carbon,
other metals which give their name to the
steel — nickel, chromium, tungsten, molyb-
denum, vanadium, and manganese — and
each of these alloy steels has some special
character.
It will be seen that the tough, tenacious,
and malleable forms of iron are, with the
exception of alloy steels, those containing
only small amounts of foreign bodies other
than carbon, and that as that element
increases, the hardness and elasticity increase.
Cutlery and tool steels contain the larger
amounts of carbon.
Malleability. — No confusion should exist
as to the term " malleable " applied to iron
and steel. Metal that can be forged, rolled,
bent and worked at a red heat, and to a
less extent in the cold, is malleable. The
term is often applied without qualification
to certain kinds of castings which have had
their brittleness removed by treatment.
The composition of these varies according
gradually increasing stress in a testing
machine till rupture takes place. The force
applied is indicated by the machine. In
Great Britain, tenacity is usually stated in
pounds or tons per square inch of sectional
area. If a test-piece of 0-7979 in. diameter
broke when subjected to a load of 30,000
lb., the tensile strength of the material
would be 60,000 lb. or 26-78 tons, since the
bar is half a square inch in sectional area.
The tensile strength varies with the
composition, and the treatment of the
metal. In carbon steels that have been
heated above 900° C. and allowed to cool
in the air — normalised — the tenacity in-
creases as the percentage of carbon rises, till
it attains a maximum of 61 tons at about
1-2 per cent, carbon. Steel that has been
annealed by heating the metal to redness and
allowing to cool very slowly has a lower
tenacity. The maximum of about 36 tons
is reached with 0-9 per cent, carbon, above
which it falls away again. The graph
STEEL
presented by Fig. 50, and the figures on the
preceding page are given by Prof. Arnold.
Increasing the rapidity of cooling — by
quenching from redness — may raise the
tensile strength, but affects also other
properties. The smaller the amount of
carbon present, the less is the effect produced
by varying the rate of cooling. Very soft
steels, containing little carbon, are not
greatly affected, as will be observed from
the diagram just referred to.
The divergence in the indicated strengths
between the normalised and annealed speci-
mens becomes more marked as the MAX.
percentage of carbon increases. |NTToSNSs
The quenching effect also follows the PER so IN.
same course, and is much greater 60
with the higher carbon steels. They at
the same time become much harder and
more brittle. The hardening effect 50
becomes sensible above 0-35 per cent.,
and with 045 per cent, is quite distinct.
With 1 per cent, the metal becomes
brittle and very hard.
Under ordinary treatment a percen-
tage of carbon below 0-8 per cent.
'^n
raises the tensile strength, but does
not make the metal brittle. Steel
containing from 0-7 to 0-85 of carbon,
although its ductility has been greatly 20
reduced, is quite suitable for reinforced
concrete construction, as it is still suf-
ficiently ductile to be free from liability 10
to breaking under ordinary conditions ;
it is, however, much stiffer and more
rigid and elastic than metal having o
a lower carbon content. Do steels with
higher carbon contents, used in plain
bars, and not ridged, notched or other-
wise " deformed," secure a more per-
manent grip on the concrete than do
steels with lower carbon contents ? It is a
moot point.
Ductility. — The power of being drawn
out or extended is known as " ductility,"
and it is indicated by the extent by
which the test piece in a tensile test
elongates before fracture. This elongation
is expressed as a percentage of the original
length of the piece. Before testing, two
centre- punch or other marks are made on
the piece, at a carefully measured distance
apart. After fracture the two portions
are laid together and the distance again
measured. The increase is calculated in
percentage terms of the original length.
In stating elongation, the length of the test-
piece must be given, since, owing to the
extensive thinning of the bar at the point
of fracture, much of the elongation is con-
fined to that portion in which the fracture
occurs. This is the case with both short
and long test-pieces, but on the short pieces
the local elongation represents a higher per-
centage of the original length.
Ductility decreases as the carbon increases,
as the graph (see Fig. 51 on next page)
by Prof. Arnold shows. Here, again, the
effect of rapid cooling is plainly indicated.
Carefully annealed material shows a mini-
>A
/
/
^
/
7^
/
/
£L
^*^
^
«/
<y ^
^
/
\
A
*T
A
• 25% '5% -75% \'Q°l \'25% \'5%
CARBON
Fig. 50. — Graph Showing Influence of Carbon on
Tenacity of Steel : N, normalised specimens,
and A, annealed specimens
mum ductility at about 0-89 per cent, carbon,
after which the ductility rises. With normal-
ised metal the decrease continues to little more
than 2 per cent, at 147 per cent, carbon.
The difference in the behaviour of the
metal in the two states results from the
difference in the manner in which the car-
bide is held. The carbide dissolves and
diffuses through the metal when heated.
If slowly cooled it separates completely
from the iron during the cooling. More
rapid cooling prevents the separation from
being completed, while quenching may
prevent the separation altogether. Thus the
properties of the metal vary with the rate
of cooling and the amount of carbide.
REINFORCED CONCRETE
Prom the above it will be seen that for
material that must be subjected to severe
treatment in forging and use, such as boiler
plates, rivets, etc., only soft, low-carbon
steels can be applied. Steel containing
more carbon would be too hard and not
sufficiently malleable and ductile, so that
cracking or fracture might occur. With
materials which are not subjected to severe
treatment and where high tensile strength,
elasticity, and rigidity are more important,
higher proportions of carbon are allowable.
Thus, with 0-9 per cent, carbon, a tensile
strength of 51 tons per square inch and
ELONGATION
PER CENT
ON 2"
50
40
30
20
10
•25%
•5%
•75%
'25%
Fig. 51. — Graph Showing Influence of Carbon on
Ductility of Steel
an elongation of 14 per cent, are obtained,
the latter being somewhat low, but higher
than in the case of much wrought iron, the
tensile strength of which is less than that
of the steel.
Elasticity. — This is the power of the
steel to recover its original shape, and it
is influenced by the manner in which the
carbon is held in the metal.
Limit of Elasticity. — This term is
applied to the force which will produce a
permanent set or extension of the metal
under tensile test ; that is, the point at
which the metal ceases to be perfectly
elastic. It is usually about half the total
strength. The metal will continue to
stretch on the further application of force,
but the extension is not then proportional
to the force applied, and the metal does
not recover its original dimensions on the
removal of the stress, but is permanently
strained.
Testing Steel. — Fig. 52 shows test-
pieces. They are carefully machined to
shape, marked by means of a centre -punch
for determining elongation, and the ends,
firmly gripped by the jaws of the machine.
Force is gradually applied to pull the ends
apart and the piece carefully watched. At
a certain point, a change in the outer
skin may be observed. With a lever
machine the sudden elongation may cause
the lever to drop. This point is
the commercial elastic limit. Fur-
ther application of force produces
further extension, and ultimately
a point is reached when the in-
creasing load causes the piece,
which up to this point has been ex-
tending uniformly over its whole
length, to begin to contract locally,
and rapidly to diminish in sectional
area ; in this region the piece ulti-
mately breaks. The contraction in
area is generally stated as a percent-
age of the original area. The frac-
ture on examination should be
uniform, free from flaw or defect,
and silky or finely granular. Kound
pieces should show cup and cone
formation if soft. Fracture should
occur near the middle, and with
uniform material there should be
'*5* only one region of contraction. Tests
in which this does not occur or
which break very near one end are
not satisfactory. The effective
length — that is, the distance between the
marks — varies from 2 in. to 10 in. ; but
usually 2-in., 6-in., 8-in., or 10-in. test
pieces are employed.
SPECIFICATION FOR STEEL REINFORCE-
MENT
The following is a suitable specification
for steel reinforcement :
The steel used shall be manufactured
by the Open Hearth process, and shall
be of the very best description, free from
blisters, scales, laminations, and shall be
of the size and weight specified (a vari-
ation of 2£ per cent, either way is usually
allowed).
The chemical analysis shall show — per
cent, carbon, and not more than the fol-
STEEL
53
lowing percentages
sulphur :
Phosphorus
Sulphur
of phosphorus and
. 0-06 (maximum)
. 0-04 (maximum)
On this assumption a force of 1 Ib. would
extend or reduce the length of a test-piece
of the oriinal
No allowance above these figures must be
made. The steel shall have an ultimate
tensile strength of not
less than 28 to 32
tons per square inch,
and an elongation of
at least 15 per cent.
on an 8-in. length. No
welds shall be made in
the steel under any
•circumstances. When
forging is necessary
special attention shall
be paid to the smith's
work. No overheated or burnt steel to be
used. All bending (where possible) of
CERTIFICATE OF TESTS
The following is a sample certificate of
t ~
Fig. 52. — Test Piece Before and After Stretching
tests for steel used in reinforced concrete
construction : —
CERTIFICATE OF TEST
Messrs. X.
Tests of Mild Steel to your order.
For delivery to A. B.
No. of Sample
Original size
Contraction of Area
Elongation
on 8 inches
Ultimate
stress
Elastic limit
^ £
1
l-i
|i
•3 4
5 8
£ S 1
£8*
1*1
j
S3
Hj
S't
fi
§'".
1J Rounds .
1 X i Flats .
Ditto .
1 inch square
1-12
1-01 x 0-495
1-00 x 0-505
1-OOx 0-99
0-985
0-5
0-505
0-99
0-76 Diam.
0-79 x 0-295
0-75 X 29
0-7 X 0-69
0-454
0-232
0-217
0-482
54
53-5
57-0
51-3
10-22
10-24
10-16
10-25
27-7
28-0
27-0
28-2
28-6
14-6
14-4
29-8
29
29-2
28-5
29-9
18-64
9-75
10-10
20-8
18-9
19-5
19-9
21
1 X J Bend tests satisfactory.
Cold Bend tests satisfactory.
1 X J Flats cold bend tests satisfactory.
small metal to be done cold. Larger
metal to be heated to a dull cherry
red. The bending force must be applied
gradually and regularly. Before use, all
steel must be clean and free from scale and
rust. No oil or paint must be used on the
steel.
Modulus of Elasticity. — This is the
force in pounds that would be required to
extend a bar to double or compress it to
half its original length on the assumption
that it remained perfectly elastic throughout.
It is frequently called the coefficient of
elasticity ; thus for steel the modulus is
about 30,000,000 Ib., and the coefficient
*s ^a^v constant for open-
JO 000 000'
liearth steel with varying carbon contents.
Fatigue. — When subject to varying and
fluctuating stresses, metals are liable to
alteration. The tensile strength, ductility,
and toughness are reduced. In some cases
this is very pronounced, and the strength
may fall below the ordinary working load
and fracture take place. The alteration
under ordinary conditions takes place slowly
in steels that are free from injurious con-
stituents and of uniform texture. The
elasticity generally depends on the latter
condition, and metals of high elasticity are
less liable to fatigue. The elastic limit in
the case of steel is higher than for wrought
iron, and its resistance to internal change is
greater.
Toughness. — This may be defined as
the resistance to fracture by bending beyond
54
REINFORCED CONCRETE
the elastic limifci It is especially necessary
in steel that must be bent or twisted. Soft
malleable steel, of uniform texture and
character, is toughest ; but very much
depends on the heat treatment the steel
has received.
Hardness. — This is difficult to determine
with accuracy, and for reinforced concrete
work is only of secondary importance. It
is dependent on the heat treatment and rate
of cooling of the metal, and on the percentage
of carbon present in the steel. Reference
to the following table, and a consideration
of the uses of steels of varying composition,
will illustrate the latter point, and the study
of the effects of carbon to be dealt with
later will explain the cause of the increased
hardness produced by quenching from a
red heat.
Steel
Carbon contents
Soft malleable metal
plates and rivets .
for
For ordinary wire for ropes
Hard wire for guide ropes .
Rails ....
Rolled sections
Hard open-hearth
Tool steel for dies and steel-
ing purposes (axes and
plane irons, die temper) .
Setts, minting dies, smith's
tools (sett temper)
Cold chisels, miners' drills,
large punches (chisel
temper)
Circular cutters, taps,
rimers, large turning
tools and drills, screwing
dies (punch temper)
Turning tools .
For small tools, sawfiles, etc.
For razors and special
purposes
01 —0-3
0-2 —04 Ma.
0-15—0-3
045—0-75
0-3 —0-6
0-22—0-25
0-6 —0-8
0-75
0-825
1-00
1-125
1-25
1-375
1-5
matter included in the steel. Overworking
the steel may produce lamination.
Usually with increased diameter a smaller
demand is made for elongation. For an
increase of £ in. above f in. a deduction
of 1 per cent, is made, for decreases of j\ in.
below yV a deduction of 2| per cent, from
the specification.
Bending Tests. — These consist of bend-
ing the metal as per specification upon itself.
Rivet steel is tested at full size as rolled.
It must bend flat on itself — through 180°
— without fracture on the outside of the
bend. All material less than f in. is tested
at full thickness, but above that thickness the
specimen used is £ in. thick, and if possible
1£ in. wide. Soft steel must bend over on
itself — through 180° — without fracture at
any part of the bend. Medium steel must
bend through 180°, round a diameter equal
to the thickness of the metal, so that the
two limbs are separated by a distance equal
to the thickness of the test, without showing
sign of fracture on either side. For works
tests, the pieces are often doubled closely
together. Tests after quenching from a
red heat are somewhat less severe, the
distance between the limbs being equal to
1| times the thickness of the metal.
The importance of a high elastic limit
will be appreciated when the effect of
subjecting to tensile strain a smooth round
bar, one end of which is embedded in con-
crete, is considered. In such circumstances
the load may be increased till the elastic
limit is reached without in any way per-
manently lessening the grip of the concrete
on the metal. When the elastic limit is
TENSILE STRENGTH AND ELASTIC LIMIT
OF STEEL IN TENSION AND COMPRESSION
IN POUNDS PER SQUARE INCH.*
High-speed steels containing tungsten,
chromium, and other metals to render the
metal self-hardening contain only 0-5 — 0-7
per cent, carbon.
Referring to the specification already
given, freedom from blister may be secured
by using ingots free from blow holes. Scales
and laminations result from faulty treatment
in the forging or rolling, or to some foreign
Percentage
Ultimate
Elastic
Elastic
of tensile
limit in
limit in
Carbon
strength
tension
compression
0-14
62,944
41,955
39,536
0-19
68,096
47,084
43,008
046
75,712
49,056
48,944
0-55
80,416
46,995
49,772
0-66
89,600
53,244
53,648
0-78
92,064
53,312
53,648
0-82
102,816
57,003
63,168
0-87
104,608
61,017
56,000
0-96
118,048
69,216
71,120
* Quoted from Twelvetrees' " Concrete-Steel."
STEEL
55
reached, the permanent extension with the
accompanying thinning of the bar would
release it as far as contraction extends, and
this would probably progress along the bar
by the repetition of the stress.
Shearing Strength. — The value of this
factor is very largely influenced by the
structure of the steel developed in rolling
and the heat treatment. Usually it amounts
to 70 to 75 per cent, of the ultimate tensile
strength.
MODULI OF ELASTICITY OF STEEL WITH
DIFFERENT PERCENTAGES OF CARBON
IN 1,000,000-LB. UNITS.
Carbon
Tension
Compression
Bending
Mean
0-19
30-8
37-0
29-1
32-9
0-46
32-0
32-7
29-3
31-8
0-54
30-6
36-1
28-8
32-4
0-66
32-4
35-7
32-1
33-6
0-78
33-5
32-4
30-1
32-4
0-87
31-0
31-5
30-4
31-1
0-96
30-9
32-7
29-2
31-2
Resistance to Alternating and Re
peated Stresses. — For all structural pur-
poses the permanence of the steel is of the
highest importance. Repeated stresses in-
duce a brittle condition which is not in-
dicated by the testing machine. The piece
breaks as though it were absolutely brittle.
Tests of brittleness are made by bending
small prepared test-pieces in opposite direc-
tions through a small angle by means of a
suitable machine. The number of bendings
before fracture occurs varies with the com-
position and the treatment the steel has
received. Mr. Stead gives the following
figures for steel containing 044 per cent,
carbon :
Normal
Overheated
Reheated .
Annealed .
Sorbitic
1,432,500
844,950
2,080,440
1,971,000
3,517,200
The effect of subjecting bars to mechanical
treatment, and the consequent variation in
the results obtained, is evidenced in the
table given below. The bars tested had
been twisted at one end while cold.
The rate of alternation has a very con-
siderable effect. Prof. Arnold has shown
that the resistance to rupture is inversely
proportional to the rate of alternation.
Steel that has become brittle under such
conditions cannot be restored by annealing,
neither can it, indeed, by any other heat
treatment.
Description
of Bar
Original
size in
inches
Orioinal
area in sq.
inches
Elastic stress
•per sy. inch
Ultimate
strength -per
sq. inch
Ratio of
Elastic to
Ultimate
Contraction
in area at
fracture
Extension
in 8 inches
Extension
in 10 inches
5-88 twists per
foot
Plain end
| square
•377
X
•382
0-144
43,000 Ib.
= 19-2 tons
58,847 Ib.
=26-3 tons
73-1
71-5
27-5
26-2
Silky
Twisted end
do.
do.
59,700 Ib.
=26-7 tons
86,250 Ib.
=38-5 tons
69-2
59-7
6-7
6-0
do.
4 '21 twists per
foot
Plain end
£ square
0-5
X
0-5
0-25
44,000 Ib.
= 19-6 tons
62,400 Ib.
=27-9 tons
70-5
67-6
28-7
27-6
do.
Twisted end
do.
do.
78,400 Ib.
=35-0 tons
88,480 Ib.
=39-5 tons
88-6
56-4
7-2
6-3
do.
3- 15 twists per
foot
Plain end
f bar .
0-613
X
0-615
0-377
31,500 Ib.
= 14-1 tons
53,475 Ib.
=23-9 tons
58-9
69-8
35-5
33-9
do.
Twisted end
do.
do.
64,000 Ib.
=28-6 tons
73,820 Ib.
=33-0 tons
86-7
63-7
7-0
6-0
do.
REINFORCED CONCRETE
Effects of Heat on Physical Pro-
perties.— Sir W. Fairbairn showed that
the strength of rivet iron and boiler plates
(best Yorkshire iron) increased as the
temperature rose to 435° F., rising from
62,720 Ib. at 60° F. to 86,016 Ib. at 435°.
At 1290° F. the strength was only 35,840 Ib.
Mild steel does not so increase in strength.
Styffe showed that between 212° F. and
392° F. the strength remained practically
unchanged.
Other experimenters, including Howard,
Barnaby, and Martens, have shown that
between 60° F. and 150°— 250° there is a
fall in tenacity, followed by an increase up
to 450° or 600° F. Beyond this point the
tenacity falls, and at 1100° to 1200° F. is
only about half the original.
Although an actual increase in strength
may be registered, the ductility is diminished.
At about 550° to 570° F. the metal shows
signs of brittleness, and fracture occurs
suddenly without elongation. Eesistance
to shock at or above these temperatures
is therefore probably much diminished.
This temperature, be it noted, is much below
redness.
The minimum length of a bar to be em-
bedded in concrete in order to ensure its not
slipping under tension will depend on the
adhesion between the two substances, the
elasticity and the area of the metal surface
embedded. Until the bar stretches the
adhesion will remain unimpaired. It may,
however, be affected by unequal expansion
of the materials, when the composite
material is subjected to heat and also to
internal stress of a variable character such
as vibration and shock. Any advantages
derived from the shape of the bar is due
either to the increased area caused by the
shape given or to the irregularity of form,
securing a bond when the natural one may
have been reduced or destroyed.
The adhesion varies greatly, and is in-
fluenced by the nature of the cement, the
amount of water used and other factors.
Coefficient of Expansion. — The co-
efficient of expansion of steel is 0-000066
for mild steel, and 0-000069 for hard steel.
Other Elements Present in Steel.—
Beside carbon, steel also contains small
amounts of other elements.
Silicon. — Usually only traces of this ele-
ment are present, being the remains of what
was present in the pig iron used in making
the steel, and which was not eliminated
during the process. It is generally below
0-1 per cent. In such amounts its effects are
negligible. Small amounts of silicon in steel
have the effect of ensuring sounder metal
freer from blowholes, and it is often added
for this purpose to the metal before casting.
In larger amounts, silicon has the effect of
largely increasing the size of the crystal
grains, and with 2-5 per cent, the metal
presents on fracture an appearance more or
less like spiegel. Further increase produces
a fracture resembling silicon iron. Turner
found that up to 0-315 silicon had no effect
on the tensile strength or ductility. With
0-5 per cent, an increase of 7 tons in the
tensile strength and a diminution of 4 per
cent, on the elongation took place.
Larger amounts of silicon have the
effect of raising both the tensile strength
and the elastic limit of the metal.
With small proportions of silicon, no loss
of ductility occurs, but with amounts beyond
2 per cent, the loss in ductility is very
marked, and with 4 per cent, the ductility is
practically nil. The metal remains malleable
if the carbon content is low, until more than
6 per cent, of the element is present. Beyond
this it is brittle. With increased carbon less
silicon will produce brittleness. Silicon up
to 2 per cent, has no effect on the hardening
of steel by quenching in water even from
a welding heat. More than this renders the
steel somewhat stiffer, but not harder.
Silicon steels are being used for tram rails
and in work where high elasticity is required.
It has been applied to the making of springs
for motor-cars and other vehicles For rails
0-3 per cent, with 0-5 — 0-6 per cent, carbon
and 1 per cent, manganese, and for the latter
0-8 — 0-9 per cent, silicon and 0-7 per cent,
carbon and 04 per cent, manganese. With
the latter alloy, very careful treatment is
necessary to develop the mechanical pro-
perties. Carefully annealed, the tensile
strength is about 50 tons, with an elastic
limit of 30 tons, and 14 to 18 per cent,
elongation. After quenching at about 900°
C. and reheating to 500° C. the tensile
strength is 85 — 95 tons, elastic limit 63 — 67
tons, with an elongation of 5 to 12 per cent.
The table on p. 57 is quoted from Greenwood
and Sexton's " Steel," and shows the chief
mechanical properties of the metal.
Manganese is a constant constituent of
mild steel. Iron free from or containing
little carbon cannot be melted and cast
without becoming burnt. Manganese is
STEEL
57
Analysis
Unannealed
Annealed
Per cent.
Tons per sq. in.
Per cent.
Tons per sq. in.
Per cent.
C.
Si. T.S.
E.L.
E.
C.
T.S.
E.L.
E.
C.
0-14
0-24
33
22
30
54
25
15
37
61
0-18
0-73
34
25
29-5
54-5
29-5
19
34
52-5
0-19
1-6
37-5
28
31
50
33
25
35
54-5
0-20
2-18
39-5
31
18-5
28
34
25-5
36-5
60
0-20
2-7
42-5
32
17-5
24
32
24
6
6-5
0-21
3-4
47-5
35
11
14
39
30
9
9
0-25
4-2
49
45
0-004
0-2
38
none
0-64
0-98
026
5-5
48
none
0-3
0-7
25
25
0-37
0-5
added in the form of spiegeleisen or ferro-
manganese to deoxidise it and thus restore
malleability. Its tendency is to make the
metal harder and to raise the tensile strength
slightly, but to reduce the ductility. It
may perhaps correct to some extent the
effect of sulphur in producing red shortness.
For structural steel the amounts present
vary from 0-2 to 1 per cent. The latter is
allowed in rails and is occasionally exceeded.
Generally the amount is below 0-6 per cent.
Phosphorus. — This element is present in
steel as the phosphide, containing 15-57
per cent, of phosphorus ; that is, phosphorus
produces nearly 6J times its weight of the
compound, and the effects must be judged
in this light. Its general effect is to produce
cold shortness, brittleness, and sensitive-
ness to shock. It produces a crystalline
structure. The metal rolls well hot, and
its tensile strength and elasticity are raised,
but the ductility is reduced, and may be
completely destroyed. The effect on the
ductility and toughness is intensified by
the presence of carbon. With amounts
under 0-1 per cent, but little change can
be observed in the metal under test, but
its sensitiveness to shock may be very greatly
increased. Although its behaviour varies,
its reliability in this respect is destroyed.
It is necessary therefore to specify a maxi-
mum. For rails and similar purposes a
maximum of 0-08 is permissible, and 0-07 is
usually specified, but 0-04 is often required
in steels containing 0-4 to 0-6 carbon.
With higher-carbon steels less phosphorus
is allowable, and the lower figure should be
adhered to. For tool steels the phosphorus
should not exceed 0-02 per cent.
Sulphur. — This element also exercises,
even in small quantities, a serious influence
on the properties of steel. It induces red
shortness when the amounts present are
not sufficient to produce any effect on the
tenacity or ductility of the steel in the
cold. The steel cracks on rolling even when
the sulphur present is less than 0-1 per cent.,
and the steel is therefore very unreliable,
since, although externally satisfactory, in-
ternal cracks and flaws may be produced
which, under the conditions of application
— vibration, variable temperature, etc. —
may ultimately weaken the steel below its
working load and produce fracture. It is
most likely to produce fatigue, and its
presence destroys the welding power of the
steel. Not more than 0-05 per cent, should
be allowed in any circumstances.
Carbon. — As already noted most of the
changes in the mechanical characters of
steel are produced by the carbon present.
Not only does an increase in the element
produce a change in properties, but
different properties may be presented
by steels containing the same amount, if
they have received different heat treatment.
Steel heated to redness and quenched
is made harder to an extent depending
on the amount of carbon present.
The same steel, if reheated and allowed to
cool slowly, is softened. In the softened
state it is harder than iron free from carbon.
The tensile strength is raised, but the
ductility is lowered. The amount is not,
however, in proportion to the increase of
strength. This renders carbon the best
hardening agent, as it produces the greatest
increase in tenacity with the least diminution
of ductility. In hardening, the change
occurs at a definite temperature or at least
over a very limited range of temperature.
If the steel cools below this temperature
before quenching, it is not hardened, nor is
it softened unless this temperature is reached.
REINFORCED CONCRETE
Below this temperature it is immaterial
whether the cooling is rapid or slow. The
temperature is known as the " critical
temperature " of hardening, and also as
Fig. 53. — Photo-micrograph of Hard Steel
(3 per cent, carbon) Cooled Slowly
the " recalescence point." Steel of suitable
carbon content and volume actually becomes
visibly hotter as it cools from a red heat
when the critical temperature is reached.
Hence the term " recalescence." It marks
the temperature at which the carbon
contents of the steel change their condition,
this coinciding with the hardening effect.
Carbon exists in steel in chemical combina-
tion with iron. The carbide that has been
separated has the composition Fe3C, and is
insoluble in iron below the recalescence
point, but soluble above it. If quenched
from a higher temperature its separation is
prevented, and it remains in solid solution,
in which condition it produces its maximum
effect. The condition it assumes after
or during separation may seriously affect
the properties. A microscopic examination
of polished and etched steel shows that
when cooled in air the carbide has collected
into small areas, surrounded by uncar-
burised iron. The size of these areas in-
creases with the carbon contents, and with
0-9 per cent, carbon the whole surface is
uniform. The grey areas are seen under a
high magnification to consist of alternate
plates of light and dark material. One of the
components is the carbide — Fe3C (cementite),
and the other the iron (ferrite) from which it
has separated. If cooled very slowly the
structure is coarsely laminated. Cooled
more quickly it is finer, and the laminated
structure may be very fine or even granular.
The latter condition, which is stronger and
more elastic, is the sorbitic structure. The
laminated material under inclined illumina-
tion often shows a play of colour and was
named by its discoverer, Dr. Sorby, pearlite.
Heated beyond the recalescence point the
cementite first dissolves in the iron (ferrite)
of the pearlite, and then diffuses through the
mass. On quenching, its condition remains
what it was in the heated state, and the
hardening effect is produced.
If cooled slowly, steels containing more
than 0-89 carbon first separate the surplus
cementite, and this continues till the recales-
cence point is reached. The structure is
shown at Fig. 53. If less than 0-89 per
cent, carbon is present, the excess of iron
(ferrite) first separates, and the carbide
concentrates into what ultimately become
the pearlite areas (see Fig. 54), final separa-
tion and lamination being produced at the
recalescence point. The more complete
the separation, the softer the material will
become, and the less will be the effect of the
Fig. 54. — Photo-micrograph of Steel (Dark
Areas show Pearlite)
carbon in raising the tensile strength "and
elasticity. With largely laminated pearlite
the ductility is not increased, as must always
be the effect of producing planes that may
become planes of cleavage or separation.
Obviously the qualities of quenched steel
STEEL
59
will depend on the carbon (carbide) present
in solid solution, and at the recalescence
point it is only capable of retaining carbide
equal to 0-89 per cent, carbon. Above
that temperature it will retain more in
solution, but this, as shown, separates as
the temperature falls towards the critical
point. The solid solution of cementite in
quenched steel is known as martensite. It
is harder, stronger, more elastic, and less
ductile in proportion to the carbon it
contains. For structural purposes the
modification of the cooling rate to prevent
lamination of the pearlite by somewhat
hastening the cooling, so as to produce the
sorbitic structure, is sometimes carried out.
Fig. 55. — Photo-micrograph of Laminated
Pearlite in Steel after Prolonged
Annealing
Prolonged annealing and slow cooling will
produce the coarsest lamination and the
weakest metal.
Normal air-cooled metal holds a position
intermediate between the two.
With regard to the photo-micrographs
here shown, Fig. 53 shows hard steel (3 per
cent, carbon) cooled slowly; the borders
of cementite (Fe3C) surround the pearlite
areas. Fig. 54 is a low magnification in
which the dark areas represent pearlite.
Fig. 55 (a very high magnification) shows
laminated pearlite in steel after prolonged
annealing.
In carbon steels the recalescence tempera-
ture varies from 655° C. to 680° C. and coin-
cides with the temperature at which the
carbide areas assume the pearlitic structure.
Visible redness is about 580° C. In hardened
steels the metal is in a condition of internal
stress, due to the retention of the carbide
in a condition that is not normal in the cold.
The raising of the temperature in tempering
operations gives a certain amount of mole-
cular liberty, and to some extent the marten-
sitic condition is destroyed, the tendency
being for the carbide to assume the normal
pearlitic condition. Softening takes place
to some extent, and the brittleness is re-
duced. The nearer the temperature ap-
proaches the critical point and the greater
the latitude given, the greater is the change.
At the critical point it is completed.
With mild steels there are other tempera-
tures at which the rate of cooling is retarded.
The highest point is at 880° C. Another
occurs at 735°, and the third about 680°.
The latter coincides with the critical point
already noticed. In steels free or nearly free
from carbon this is insignificant. As the
carbon content increases, the upper retarda-
tions diminish and finally disappear, and
the whole retardation is concentrated at
the lower point. The higher critical points
are ascribed to molecular changes in the
iron itself. These cannot, however, be
made permanent by quenching, and in the
absence of carbon their effect is not notice-
able in the metal, whether cooled rapidly or
slowly.
In alloy steels such as tungsten, chrome,
chrome - tungsten, molybdenum, nickel,
manganese, etc., the added metals lower
the recalescence point and bring it below
ordinary temperature. Such metals in
consequence retain their hardness and are
known as self -hardening or air-harden-
ing steels. They can be heated without
becoming softened, and hence cutting
speeds which develop so much heat as to
destroy the temper of ordinary steel tools
can be employed without risk. Hence
the term rapid-cutting, high-duty, or high-
speed steels.
METHODS OF MANUFACTURE
Blister or cementation steel is manu-
factured by heating bars of nearly pure
iron embedded in carbon for a long period.
Such metal is covered with blisters, hence the
name. It is usually converted into shear
steel by welding together a number of such
bars to secure greater uniformity.
Cast crucible steel is made by melting
blister steel in crucibles and casting into
ingots. Different tempers are secured by
additions of pure soft iron, carbon, and other
ingredients to obtain the desired product.
The principal use of such steel is for cutting
REINFORCED CONCRETE
instruments. The process is costly and the
output limited.
Bessemer steel is made by blowing air
through molten pig iron contained in a
suitable vessel. The metal is introduced in
the molten state. The carbon, manganese,
and silicon in the metal are removed by
oxidation, and sufficient carbon is introduced
by adding the necessary amount of a
manganiferous pig iron — spiegeleisen — to the
metal. The operation is very rapid, a
charge of several tons being converted in
from 16 to 20 minutes. The metal is cast into
ingots and rolled. In the basic bessenier
process, the lining of the converter is made
of basic material — calcined dolomite — and
this permits of the removal of the phosphorus.
The operation is somewhat longer. In both
of the processes, exact control of the carbon
contents is difficult. Owing to the rapidity
with which the operation must be con-
ducted, there is no time for its determina-
tion, and it is impossible to hold the metal
in a sufficiently fluid state for the necessary
length of time to enable this to be done, as
all the heat required is generated in the
vessel by the oxidation going on during the
passage of the air through the metal.
Open hearth steel — usually specified for
reinforced concrete work — is made from
pig iron and steel scrap in a gas-fired furnace
of special design, known as a regenerative
or open hearth gas-fired furnace. In this
furnace the materials are melted, and the
carbon and other elements removed by the
action of air and by adding iron oxide to the
charge. The heat required to melt and
keep the metal molten is supplied by the
burning gas. The time occupied extends
over some hours, dependent on the weight
of the charge and other conditions. Facilities
are thus afforded for the careful control of
the contents of carbon and other elements,
and metal of a definite composition can be
obtained. In the basic open hearth, a basic
lining is used, and, as in the basic bessemer
process, pig-iron containing phosporus may
be used, as that element is eliminated during
the process.
Basic steel is the term applied to steel made
in converters or furnaces lined with basic
materials, such as calcined dolomite, magne-
site, and chromite.
Acid steel is steel made in converters or
furnaces lined with siliceous materials, such
as silica sand and bricks, or ganister.
Stress Simply Explained
EXPERIENCE shows that, in a large pro-
portion of cases, the failure of the practical
man, and even of the student, to grasp the
essential ideas underlying the theory of
construction in reinforced concrete is a
direct result of his ignorance of the simple
mechanics of construction, particularly his
lack of acquaintance with " stress " — a term
meaning the effect produced on a beam by
loading it. The present intention, therefore,
is first to deal with the fundamental principles
that must be understood by anyone desirous
of successfully studying the constructional
design of buildings, and then, in the suc-
ceeding chapter, to show how those princi-
ples are applied in reinforced concrete con-
struction.
It must here be emphasised that it is
absolutely essential that the reasoning upon
which the various formulae are based should
be understood if these formulae are to be
used intelligently. Students frequently fail
to grasp the theory of design owing to the
fact that they do not start with a sufficient
knowledge of the principles of force and
resistance. In many instances the very
expressions that are used are imperfectly
understood, and the students are endeavour-
ing to obtain the value for something of
which they do not know the meaning, and
often when they have found a result they
do not realise why it Was necessary to find
it. To the general reader this may appear
to be an exaggeration of the state of affairs,
but we are merely expressing views that
have been gathered during several years of
tuition to building students of all kinds.
FORMULA
There are two kinds of formulae which are
met with, namely " rational " and " em-
pirical."
" Rational " applies to formulas that have
been deduced by reasoning alone, such
reasoning being based upon definite axioms
known to be correct. It is not sufficient,
therefore, for the student to know a rational
formula ; he should also know and under-
stand the construction or reasoning through
which the formula was evolved.
" Empirical " formulae are based upon
actual experiments through which certain
values have been obtained, according to the
material, load, and disposition under the
test ; and, as such, their application is
usually limited, and not possible for cases
which do not agree with the conditions of
the test.
Again, it will be necessary to use certain
factors or constants derived by the person
responsible for the test, and these constants
must be accepted as being correct, and
become merely a matter of memory and not
of reasoning. There are some cases, how-
ever, where pure theory is liable to give
slightly inaccurate results, owing to the
peculiar behaviour of some materials under
a varying stress, and any inconsistency of this
kind would result in the theory not agreeing
with the practice. An excellent instance of
the difference between theory and practice
is that explained in connection with columns
and struts on a subsequent page. Wherever
possible, rational formulae are to be pre-
ferred, and it is with these that the following
notes will deal in the first instance.
PRINCIPLE OF MOMENTS
By far the most important of all the points
to be grasped is that of the " principle of
moments," and the definition of this princi-
ple is often given in flowing language and
repeated by the student without a proper
understanding.
A " moment " really means an amount,
and is a distinctive term, inasmuch as it
means not only the actual amount of a force
in pounds, hundredweights, or tons, but the
actual value of a force to cause stress at
any point, this value being dependent on its
amount, direction, and disposition in relation
to other forces or constructional members.
Thus, " taking moments round a certain
point " really means " taking values or
amounts acting at that point."
Now, the " principle of moments " is that
when several forces are acting at a point
and equilibrium is produced, then all those
forces that would tend to cause movement
in one direction are exactly equalled by all
61
62
REINFORCED CONCRETE
those forces that would tend to cause move-
ment in another direction.
This principle may seem somewhat obvious
to many readers, and they may not realise
its value ; but upon consideration it will
be seen that it is an axiom which is frequently
used. For example, in the case of a beam
that carries a load at any point, it is obvious
that the load is a force tending to cause
movement in a downward direction, and if
equilibrium is maintained, then there must
be some force or forces at work which are
causing or exerting an upward tendency ;
this force is supplied by the reaction or
reactions, which together must equal the
load.
LEVERS
Before investigating the method of ascer-
taining the value of the reactions with any
given load, it will be advisable to give a
few notes on the three orders of the levers.
If these levers are thoroughly understood,
matters will be greatly simplified, as most
problems, both in ordinary construction and
in reinforced concrete work, can be practically
reduced to a question of leverage. A lever
can be denned as a member which turns on
a point known as the fulcrum, and it is the
relative positions of the forces acting on the
lever, and the fulcrum, which give us the
order of the lever.
"First Order." — A common example of
a lever is that illustrated in Fig. 56, which
shows a crowbar being utilised to lift up a
weight, such as a block of stone. It will be
seen that the lever is tending to turn on
the fulcrum under the action of the two
forces which oppose one another through the
medium of the lever ; this is an example of
a lever of the first order, as the fulcrum is
between the weight and the power. Now
the value of the power and the weight to
cause the turning of the lever will depend
not only upon their actual amount in pounds
or hundredweights, but also upon their dis-
tances from the fulcrum. In other words,
the " moment " of the power at the fulcrum
is equal to its amount in pounds multiplied
by its distance to the fulcrum, the latter
being known as the long arm of the lever ;
this is, therefore, 20 Ib. x 30 in. = 600 in.-lb.
The weight is opposing the power, and if
equilibrium is produced, then the " moment "
of the weight at the fulcrum must also equal
600 in.-lb., and this moment is found by
multiplying the amount of the weight by its
distance to the fulcrum. Therefore weight
x 6 in. = 600 in.-lb.
Weight = f~ = 100 Ib.
To produce equilibrium, then, we must have
20 LB.
Fig. 56. — Lever of First Order
a weight of 100 Ib. and a power of 20 Ib.
if the arms of the lever are 6 in. and 30 in.
respectively. Any fraction over 20 Ib; in
power would raise the weight of 100 Ib., and
thus there is a mechanical advantage or a
gain in the value of the power by the intro-
duction of the lever. A rule can now be
deduced which will apply to all levers and
enable us to ascertain the weight or the
power required to produce equilibrium if
the value of one force is known, as follows :
The power multiplied by its distance to the
fulcrum is equal to the weight multiplied
by its distance to the fulcrum.
" Second Order."— In the first order it
was seen that the fulcrum was situated be-
tween the power and the weight, while in
Fig. 57 an illustration of the second order is
given, where it will be seen that the weight
POWER
Rf5l5JANCE.
OK. WEIGHT
Fig. 57. — Lever of Second Order
is situated between the fulcrum and the
power.
"Third Order."— In this (Fig. 58) the
power is between the weight and the fulcrum.
The power required will always be in excess
of the weight, which is not the case with
the other orders. The student is advised to
work out examples in each order by the rule
STRESS SIMPLY EXPLAINED
given above, in order to familiarise himself
with the principle of taking moments round
the fulcrum.
REACTIONS
A reaction, as its name implies, is a force
POWER
FULCRUM
RL5I6TANCE
OE. WE.IOHJ
Fig. 58.— Lever of Third Order
or resistance which is the outcome of an
action or force. It is obvious that, when
any force or action is .introduced, and
equilibrium is still maintained, there must
be a reaction or reactions which are equal
to the initial action, but opposite in tendency.
No difficulty should present itself in ascer-
taining the values of these reactions, but the
student should fully understand the method
of calculation under any condition of load-
ing before seeking to understand the theory
of bending moments.
The reactions in the case of a beam can
be denned as the passive resistance at the
supports offered by the strength of the
materials at these points to resist crushing.
/ "!
' A
rt-t.
-f- CCNTBE OP GRAVITY
/ Op WALL
CANJILCVER
-18'-
V
Fig. 59. — Diagram showing how a Cantilever
Tends to Move
The resistance or reaction at each support
Tvill depend on the amount of the loading
and its disposition on the beam.
Cantilever Reactions. — In the case of
a cantilever, as there is only one support,
the whole of the reaction must be supplied
by this ; at the same time there is a tendency
ior the cantilever to overturn, due to the
leverage of the weight, and lift up the work
above the tailing-down portion. To prevent
this overturning movement from occurring,
a downward reaction or resistance will be
required at the support, and its value must
be equal to or greater than the moment of
the load. This resistance will be supplied
by the weight of the brickwork or stonework
over the tail end of the cantilever ; or, if
this is insufficient, anchor bolts may be
carried down into the lower part of the wall.
In Tig. 59 the tendency to move is shown
diagrammatically by the dotted lines, and
the problem will be quite simple if the
arrangement is considered as that of a lever
of the first order. The overturning moment
at the point F, which is the fulcrum, will be
equal to the weight multiplied by its dis-
tance to this point, which equals 5 tons x
\ A /
Fig. 60.— Triangular Portion of Wall Lifted by
Movement of Cantilever
12 ft. = 60 ft.-tons. The weight of the wall
above is distributed over the length of
18 in., and for the purpose of calculating its
moment at F, it may be considered as acting
at its centre of gravity, namely, 9 in. from
the fulcrum. The weight of the wall acting
at A multiplied by 9 in. or f ft., must there-
fore equal 60 ft.-tons = expressed as an
equation :
A :
A :
| ft.
| ft.
| A
A
A
= W x I
=5 tons x 12 ft.
= 60 ft.-tons
= 60 ft.-tons -*- | ft.
= 80 tons
The weight of the wall over the end of the
cantilever must therefore exceed 80 tons if
the latter is to be safe against overturning ;
or some other form of anchorage must be
provided which will give an equivalent of
60 ft.-tons at F. Owing to the bonding of the
64
REINFORCED CONCRETE
material of which the wall is composed it
would be necessary to lift all the work inside
the dotted lines shown in Fig. 60, and, of
course, this amount would be considered in
the calculations.
Beam Reactions. — Certain variations
load) is situated between the resistance and
the fulcrum. Therefore :
W-
^
t
A
W
r8
if'f\"
Fig. 61. — Beam Loaded Eccentrically
occur in the case of beams, as there are two
reactions to consider, and these are not
necessarily equal to one another. When the
beam is uniformly loaded, whether the loads
are concentrated or distributed, the reaction
at each abutment will be equal, and their
combined total must be equal to the sum
of the loads on the beam. If the loading is
not uniform, then the reactions will vary,
and it is these cases that will be dealt with
here.
Let a beam be taken with a span of 15 ft.
loaded with a concentrated weight of 5 tons,
situated at a point 6 ft. from one support,
as shown in Fig. 61. A greater proportion
of the load will be carried by the abutment
A than by the abutment B, as the weight is
nearer the former, and the reaction will
require to be equal to the amount of the load
Fig. 62. — Diagram showing how Eccentrically
Loaded Beam Tends to Rotate
carried. The actual amount may be calcu-
lated by again referring to the principle of
the levers. In Fig. 62 it will be seen that
the load has a tendency to push the beam
downward and exert a pressure on abutment
A by rotating on the abutment B at point F.
This is an example of a lever of the third
order, where the power (supplied by the
W x 9 ft.
5 tons x 9 ft.
= resistance at A x 15 ft.
= R1 x 15 ft,
45 ft. -tons
15 ft.
R1 = 3 tons
The reaction at A, therefore, is 3 tons, and,
if the sum of the reactions is equal to the
load, the reaction at B = 5 tons — 3 tons =
2 tons. This can be proved by considering
the beam as rotating on abutment A to cause
a pressure on abutment B, as indicated in
Fig. 63. Then :
W x 6 ft. = resistance at B x 15 ft.
5 tons x 6 ft. = R2 x 15 ft.
30 ft.-tons
R2
15 ft.
2 tons,
which agrees with the amount above stated.
RESISTANCE'
Fig. 63. — Beam Rotating on the other Abutment
A rule can now be deduced as follows :
The reaction at an abutment is equal to
the load multiplied by its distance from
the opposite abutment, divided by the whole
span.
If more than one load is carried, then the
reaction is found by multiplying each load
by its respective distance from the abutment,
adding the amounts thus found, and divid-
ing the total by the whole span ; this will
be made clear by an example. The example
in Fig. 64 shows a beam which carries three
different concentrated loads ; then :
R1 = [ (3 tons x 15 ft.) + (4 tons x 10 ft.)
+ (5 tons x 6 ft.) ] -f- 18 ft.
45 +40+30
R = To
18
tons
R2 =
(Stons x 3ft.) +(4tons x 8ft.) +(5tons x 12ft.)
~
STRESS SIMPLY EXPLAINED
9+32+60
101
18
= R2 = 5^4 tons
R1 + R2 must equal total load 6T7¥ tons +
5 {* tons = 12 tons — total load.
Any number of loads may be dealt with
in this manner, and if a distributed load over
part of the length only has to be considered,
it can be taken as acting at its centre
of gravity for the purpose of calculating
the reaction, while if it is distributed
uniformly over the whole length it is ob-
vious that one half will be carried by each
support.
The weight of the beam itself has purposely
been neglected with the object of simplifying
the explanation, but when it is required to
allow for this it should be dealt with as a
distributed load as explained above.
Effective Span. — The beam diagrams in
4TOI«> S70N6
\
i
r i
^
, „
. ,
"
////>
A \
IQ' nf
l&
i t
r»2
Fig. 64. — Beam Carrying Three Concentrated
Loads
this and the following chapter do not show
any bearing on the support. What is
actually illustrated is the effective span, this
being the distance between the centres of
the bearing surface, where the pressure is
theoretically considered as acting. Fig. 65
shows how the portion of the support inside
the centre of pressure and the portion of
the beam outside the same point are neg-
lected, these portions being hatched. The
area of the bearing surface will be proportion-
ate to the amount of the reaction, and will
vary with the material of which the support
is composed ; this, of course, is merely a
matter of dividing the total load on the
support by the safe load per unit on the
material, when the required area will be
given.
BENDING MOMENTS
The term " bending " needs no explana-
tion ; and the term " moment " has already
been explained ; it will therefore be under-
5
stood that the expression " bending
moment " refers to the amount or value
of a force or forces to cause a tendency to
bend. This value will depend on the
actual amount of the force and on its
disposition.
NTEE.OF 5EARIN6
Fig. 65. — Diagram Illustrating Clear and
Effective Spans
Bending Moment at Centre of Beam.
— The bending moment (B M) in a beam or
cantilever is caused by the load and the
reaction opposing one another ; and, as the
beam or cantilever is the agent through
which this opposition is allowed to act, it is
called upon to resist a certain amount of
stress due to the bending tendency.
Assume a beam to carry a load of 10 tons
in the centre of its length, as in Fig. 66;
The load will set into force an action which
can be expressed as a downward action, due
to the force of gravity ; in order to produce
equilibrium, this must be counteracted by
a resistance at the abutments which is equal
in amount and opposite in direction. There-
fore, the reactions can be expressed as
upward forces, which are set into action by
the passive resistances of the abutments.
It will be seen, then, that these actions (the
load and the reactions) exert a bending
tendency in the beam through which they
act as shown (in an exaggerated manner) by
i
1
f'
;
J
|
B' R2
Fig. 66.— Centrally-loaded Beam
Fig. 67, in which the beam has a tendency
to turn on the point F.
Now, when two parallel forces are acting
in opposite directions and opposing one
another, they are said to form a couple, and
their greatest value to cause stress is the
amount of one force multiplied by the
66
REINFORCED CONCRETE
distance between them. In the instance just
W
given, the greatest value is -^ multiplied by
I
o, where W equals the total weight and I
equals the total span. It is obvious that
x „- which
Fig. 67. — Bending Tendency on Beam
half the weight is taken by each abutment,
and therefore the opposition from each end
of the beam can only be due to this amount.
This is explained by Fig. 67. Hence the
formula for finding the greatest bending
moment in the case of a beam with a central
W
concentrated load is B M = -~
WZ
equals B M = -r~>
W I
This formula -j- expresses the greatest
bending moment in the case of a supported
beam carrying a concentrated central load,
and this greatest bending moment will be
found to occur at the centre of the span,
where moments have just been taken.
Bending Moment at any Point in
Beam. — Now consider the method of ascer-
taining the bending tendency at any point
other than the centre. By taking any
particular example and calculating the B.M.
at various points, it will be seen how the
variation becomes uniform and diminishes
from the centre of the beam to the abut-
ment, where it becomes nil. This uniform
variation will only be found to occur in the
case of beams and cantilevers carrying con-
centrated loads, and it will be shown later
how the variation occurs in the case of dis-
tributed loads. Assume a beam to carry a
concentrated load of 10 tons at the centre
of the span which is 10 ft., as before, then,
Wl
according to the formula B M =
the
is due to the reaction multiplied by its dis-
tance to the point at which the moments are
taken ; and on applying this principle it will
be found quite easy to ascertain the B.M.
at any point. Assume the same beam with
the concentrated load of 10 tons at the
centre, and find the B.M. at a point 3 ft.
from one support (see Fig. 68). Now, the
reaction at each support will be equal to
5 tons, therefore, it is only necessary to
multiply the reaction at A by its distance to
the point at which the B.M. is required,
namely, 3 ft. Hence, the B.M. = 5 tons
x 3 ft. = 15 ft.-tons.
Again, the B.M. at a point 2 ft. from the
support = 5 tons x 2 ft. = 10 ft.-tons, thus
showing how the B.M. diminishes towards
the abutment, and as the reaction is a con-
stant figure in the calculations for the B.M.
at any point, and the only factor that varies
is the distance of the point from the support,
it can be said that the B.M. at any point
varies directly as its distance from the support.
If the B.M. at the centre is set up to scale
over the beam, as shown in Fig. 69, and the
span of the beam itself is drawn to scale,
then the B.M. at all points will be expressed
graphically by this diagram, and by scaling
the vertical line at any intermediate point
the B.M. at that point can be ascertained.
This explanation has now deduced the
following rule :
The bending moment at any point in a
beam with a concentrated load equals the
reaction at the abutment multiplied by its
distance to that point.
r~
*!
i
B1.
It!?
f. T ' s* " —
/
\
5 TO'
5"-0 >•
ir>'
n"
i i
5 TOto e*=
. ' 10 x 10
bending moment = j — = 25 ft.-tons.
As already explained, the bending tendency
Fig. 68. — Finding Bending Moment of
Beam
Weight between Reaction and Point
of Calculation. — If the weight is situated
between the reaction selected and the point
round which moments are taken, then allow-
ance must be made for the moment of
the weight at this point.
This can be explained in the following
manner : Again assume the same condition
of loading and span and also that the B.M.
STRESS SIMPLY EXPLAINED
67
at a point 3 ft. from abutment B is required
to be calculated from abutment A (see Fig.
70). It will be seen that W is situated
between the abutment selected and the
point at which the B.M. is to be found.
Then W is situated on the arm of the lever
TO6CALE
Fig. 69.— Bending Moment Set up to
Scale
with two loads concentrated at different
points, and we will assume that it is neces-
sary to ascertain the B.M. at the centre of
the beam. Then it will be necessary, which-
ever abutment is selected as the one to work
from, to calculate with a Weight between the
f
«. r'-o'1-
(OTOtli ./
UPWARD LEVC.RAGE-
op ts1
f ,
m
•f //I ' n"
*-2'o;-l
— S'O'^-,
LL
V
i
1
5 TOS5 DOWNWARD LtVtR
o^ w
Fig. 71. — Lever Arms of Reaction and
Load
working from A, and the downward effect
of W must be subtracted from the upward
effect of R1 at A at the point where the
B.M. is to be found.
Then BMatF — R1 x upward leverage
- W x downward leverage, as shown in
Fig. 71.
From this equation it will be seen that
the B.M. at F actually equals (5 tons x 7 ft.)
-(10 tons x 2ft.) = 35 ft.-tons -20 ft.-tons.
= 15 ft. -tons — the same result as that
obtained in the previous calculation for the
B.M. at a point 3 ft. from one support with
similar load and span.
The principle previously explained must
be thoroughly understood, because it will
be seen that in the case of a beam which
3-0'-
•S'C"
57010
Fig. 70. — Finding Bending Moment of
Beam
carries two or more concentrated loads at
different points it will be quite impossible
to determine the B.M. at any point between
two of the loads without considering the load
which is situated on the arm of the lever.
Take an actual example showing these con-
ditions.
The diagram (Fig. 72) illustrates a beam
reaction and the point at which moments
are to be taken.
Working from abutment A, then
2 x 12 + 4 x 6
~
Ri = 33 tons.
B M at centre = R1 x
- W1 x
distance to centre
its distance to
centre.
B M = 3f- tons x 7 ft. - 2 tons x 5 ft.
B M = 24 ft.-tons - 10 ft.-tons.
B M = 14 ft.-tons.
The following rule can now be deduced :
The bending moment at any point in a beam
which carries one or more concentrated
loads can be found by multiplying either
W - 2 T&N6
Z- 4 TON5
,
^
»2.'o"-4« — 6'-o »|« — 6 -o *•
1
K' n' >
'^
t
Fig. 72.-
-Beam with Two Loads Concentrated
at Different Points
reaction by its distance to the point at which
the B.M. is to be found, but if any weights
situated between the reaction selected
are
and the point at which the moment is to be
found, then a deduction must be made equal
to the sum of the weights multiplied by
their respective distances to this point.
This rule can be applied to any number
68
REINFORCED CONCRETE
of loads, and once this principle is thoroughly
understood no difficulty will be found with
a beam carrying any number of concentrated
loads, however the latter may be situated.
Bending Moment in Cantilever. — The
mathematical method of determining the
W
Fig. 73. — Cantilever with Concentrated Load
at Outer End
B.M. with cantilevers carrying concentrated
loads will now be considered.
Owing to the fact that only one support
has to be dealt with, the calculations are
much simplified, as it will be obvious that
the reaction is always equal to the load ;
and no method is required to ascertain this
as in the case of a beam.
The most simple case will be found to be
that of a cantilever that carries a single
concentrated load at its extreme outer end,
as in Fig. 73. The greatest bending tendency
due to this weight will be found to occur at
the support. This bending tendency is due
to the weight pressing down on the cantilever,
and tending to cause failure by turning on
the point F as shown in Figs. 74 and 75.
Now if the moments round this point F
Fig. 74. — Failure of Cantilever due to
Tension
are taken, the moment of the weight is equal
to its value in Ibs., cwts., or tons x by
its distance or leverage to this point. This
moment, then, is equal to W x I, where W
equals the weight and I the length of the
cantilever.
Hence, the formula for finding the greatest
bending moment in the case of a cantilever
which carries an end load is B M = W I.
The following is an example. A canti-
lever with a projection of 10 ft. carries an
end load of 5 tons. Then B M = W I or
B M = 5 x 10 - 50 ft.-tons.
To ascertain the B.M. at any intermediate
Fig. 75. — Failure of Cantilever due to
Compression
point it is only necessary to apply the same
principle — namely, multiply the weight by
its leverage to the point at which the moment
is to be found. Take the same \\eight and
projection as in the last example, and assume
that it is necessary to calculate the B.M. at
a point 5 ft. from the extreme end. Then
it will be seen that the bending tendency at
this point is less than that at the support,
as, although the weight remains the same,
the leverage has diminished, and is now only
5 ft. instead of 10 ft., as Fig. 76. The
B.M. will now equal 5 tons x 5 ft. = 25
ft.-tons.
A rule may now be deduced as follows :
The bending moment at any point in a
cantilever which carries a single load at its.
outer end can be found at any point by
- 5
I'iO-O
Fig. 76. — Bending Moment in Cantilever
multiplying the load by its distance to that
point.
It will be seen that one factor in the calu-
lations always remains the same — namely,
the weight, and it is only the distance or
leverage that varies. It has been shown
that the greatest B.M. occurs at the support,,
and it will be obvious that there is no bend-
STRESS SIMPLY EXPLAINED
69
ing tendency at the extreme outer end, as
the weight at this point will have no lever-
age. Therefore, the greatest B.M. will occur
at the support, and it will diminish from
this point to the outer end, where it is nil.
It is also known that the diminution will be
uniform, as it is dependent only on the dis-
tance from the support. Therefore, if the
B.M. at the support is set up to scale at
this point and a straight line is drawn
through to the outer end, a diagram will be
obtained which will represent the B.M. to
scale at all points in the cantilever due to
the end load, as Fig. 77.
Bending Moment in Cantilever carry-
ing more than one Concentrated Load.
— In the case of a cantilever which carries
two or more concentrated loads at different
Fig. 77. — Bending Moment of Cantilever Set
out to Scale
points, it is only necessary to apply the
same principle of multiplying the loads by
their respective leverages at any particular
point, and the sum of their values will give
the bending moment. There are one or
two points, however, to be borne in mind,
and the chief of these is the fact that a load
does not cause a bending tendency at any
point which is not situated between this
load and the support.
An illustration will render this quite clear.
In Fig. 78 the load represented by W will
only cause a bending tendency in that part
of the cantilever marked A, and no bending
tendency whatever in the part marked B,
which is not between the load and the sup-
port, and this is obvious, as the load will
travel inwards to the support over the por-
tion A to meet the reaction, and there will
be no tendency to travel outwards over B,
as no resistance is offered by the outer end.
Take an example with two concentrated
loads and calculate the B.M. at different
points. This will show how the moments
are to be found.
The diagram, Fig. 79, shows two loads of
2 tons and 4 tons respectively. The B.M.
at the support will be due to each weight
Fig. 78. — How Centrally-loaded Cantilever
Tends to Bend
multiplied . by its respective leverage, and
these must be added, as they are not opposing
forces, but forces acting together.
Then B M at support = (W1 x 6 ft.) +
(W2 x 9 ft.) = 2 tons x 6 ft. + 4 tons x
9 ft. = 12 + 36.
L B M = 48 ft.-tons.
As a weight will not cause any bending
tendency at a point which is not between
the weight and the support, W1 will not
cause any B.M. between W1 and W2, and
the only tendency will be due to W2, as
Fig. 80.
B M under W1 then equals W2 x 3 ft. =
4 tons x 3 ft. =12 ft.-tons.
Bending Moment in Beam carrying
Distributed Load. — Concentrated loads
only have been dealt with up to the present,
and although the same principles apply in
the case of distributed loads a few brief
- 2 TONS WMTON5
4—
6-0'-
Fig. 79. — Cantilever with Two Concentrated
Loads
notes should be of value, more especially as
distributed loads are more generally met
with in practice. A common case that has
to be calculated is that of a beam carrying
both concentrated and distributed loads, and
an example of this nature presents great
difficulty to one who is not well versed in
structural design.
REINFORCED CONCRETE
As a first example, consider a beam which
carries a uniformly distributed load over its
whole length, as shown in Fig. 81. Then the
total load will be the length multiplied by
the weight per foot run. This equals 10 ft.
x 1 ton per foot = 10 tons. The reactions
The greatest bending moment then equals
W I [ W I
"2" : 2 ' 2 I
Wj Vfl Wj
~T ~ IT = "8"
i V
W
2
1 1
f W
J
; 4
t J
1
?i
— hiq
7
'//"
P
\ z ^~ 1
Fig. 80. — Bending Tendency in Cantilever with
Two Concentrated Loads
will obviously be equal, and must together
equal the total weight, which may be
expressed as W; Then R1 and R2 each =
W
-o-. The bending moment at any point is
equal to the reaction multiplied by its dis-
tance to that point, minus any weight
situated between the reaction and the point
at which the bending moment is to be found
multiplied by its distance to the same
point. The greatest bending moment will
be at the centre of the span, and upon
reference to Fig. 82 it can be seen what
values have to be taken.
W
There R1 equals -^t and this acts upwards
I
with a leverage of ~ at the point F, which is
the centre of the span. There is, however,
/
LOAD - I ion pec POOJ
t-\o'-o-
p' I
Fig. 81. — Beam with Uniform Distributed
Load
Fig. 82.— Distributed Load Acting through
Centre of Gravity
This is the formula for the greatest bending
moment in the case of a beam which carries
a uniformly distributed load over its whole
length. To work out the example in Fig. 81
it is only necessary to apply the formula as
follows :
Cr 13 M = Q = ~ • = ~TT~ = 12'<>
o o o
ft.-tons.
Let it be now assumed that the bending
moment is required to be calculated at a
point 3 ft. from abutment A.
Then the length of the lever arm is reduced
to 3 ft., and in addition the load acting
downward is reduced to 3 tons with a lever-
age to the point F of 1 ft. 6 in., as in Fig. 83.
The bending moment equals :
3 JONS
Fig. 83. — Bending Moment at Intermediate
Point
a load on the lever arm which is acting
downward, and this load is equal to
-Q-, and being uniformly distributed, it can
be considered as acting at it 3 centre of
gravity, which will be situated at a distance
equal to -r from point F.
(5 tons x 3 ft.) - (3 tons x 1-5 ft.)
= 15 ft.-tons — 4'5 ft.-tons
= 10-5 ft.-tons.
Although the reaction, which is one of the
factors in the calculations for any inter-
mediate point, remains the same, the lever-
age diminishes towards the support, and in
addition to this, the weight acting down-
STRESS SIMPLY EXPLAINED
ward, together with its leverage, diminishes
as the lever arm of the reaction becomes
less. The diminution of the bending moment
from the centre of the span to the support
is therefore not regular, as in the case of a
central concentrated load, but actually
varies in a compound ratio. If the bending
moment at the centre be set up to scale
over the beam, then the parabolic curve,
as shown in Fig. 84, will give the bending
moment at all intermediate points. With
regard to a combination of distributed
and concentrated loading, an example is
given in Fig. 85, and, if this is explained,
no difficulty should be experienced with
other examples, as the principles are
Fig. 84. — Bending Moment on Beam Set up
to Scale
always the same, and the procedure is
quite simple.
F,irst calculate the reactions :
K1 =
6 tons (distributed load) x 9 ft. + 8 tons x 4 ft.
12 ft.
E1 =
54 +32
12
--S-
under the load of 8 tons is to be found by
working from abutment A, then :
B M = (7J tons x 8 ft.) - (6 tons x 5 ft.)
B M = 57J ft.-tons - 30 ft.-tons = 27£ ft.-
tons. This can be checked by working from
abutment B. Then :
B M = 6 tons x 4 ft. = 27 ft.-tons.
I TON PER fOOT
Fig. 85. — Beam with Concentrated and
Distributed Loading
The bending moment at any other point can
be found in a similar manner, and, if neces-
sary, a diagram set up showing the value at
all parts of the beam.
Bending Moment in Cantilever Carry-
ing Distributed Load. — In the case of a
cantilever carrying a uniformly distributed
load over its whole length, the greatest
bending moment occurs at the support, and
the method of finding its value is very simple.-
The portion of the weight at the extreme
outer end of the cantilever will have a lever-
age equal to I, while the portion of the weight
at the extreme inner end will have a leverage
equal to nil. The mean leverage may there-
Ws
§
i
i
/ 1 TON PE2 fOOJ
J >
i
m
~- 4-z'-6'-H
5-0" »•
Fig. 86. — Cantilever Carrying Distributed
Load
R2 =
R2 =
8 tons x 8 ft. + 6 tons x 3 ft. fore be assumed to be ^ which represents
12 ft. distance from the support to the centre of
64 + 18 gravity of the weight. The formula for the
~~12 greatest bending moment then becomes :
W x 2 = ~2~
In the exampls in Fig. 86, the greatest
82
= jTj = 6£ tons.
Assume that the bending moment at F
REINFORCED CONCRETE
bending moment is equal to the weight
I
of 5 tons multiplied by v>, which is 2 ft. 6 in. ;
this then gives a moment of 5 x 2 ft. 6 in.
= 12J ft.-tons.
Fig. 87. — Finding Bending Moment at Inter-
mediate Point in Cantilever
In the case of the bending moment at any
intermediate point, it is only necessary to
consider that portion of the weight situated
between the point and the extreme outer
end. As an example, assume that the bend-
ing moment is to be calculated at the point
F in the cantilever shown in Fig. 87. Then
the weight to be considered is 3 tons, and
Fig. 88. — Bending Moment in Cantilever Set
out to Scale
acting at its centre of gravity it will have a
leverage of 1 ft. 6 in. to the point F. The
bending moment = 3 tons x 1 ft. 6 in. =
4J ft.-tons. The moment varies in a com-
pound ratio, as in the case of a beam, owing
to the weight diminishing, because the point
at which the moment is found is moved out-
ward from the support, and at the same
time the lever arm also decreases.
If the bending moment at the support is
set up to scale as illustrated in Fig. 88, a
diagram can be drawn to show the bending
I TON RLE FOOT
3 TON5
TONi
IZ'-O"
Fig. 89. — Cantilever with Combined Distributed
and Concentrated Loading
moment at all points in the cantilever due
to a distributed load.
Combined Distributed and Concen-
trated Loading on Cantilever. — With re-
gard to a combination of distributed and
concentrated loading on a cantilever, an ex-
ample is given in Fig. 89, and the greatest
bending moment will be calculated as an
illustration of the method to be employed.
It will be seen that three loads are carried,
namely, a load of 3 tons 5 ft. from the sup-
port, a load of 5 tons at the extreme outer
end, which is 12 ft. from the support, and a
distributed load of 12 tons, which can be
considered as acting with a leverage of 6 ft.
The greatest bending moment will therefore
be:—
(3 tons x 5 ft.) + (5 tons x 12 ft.) + (12 tons
x 6 ft.)
= 15 ft.-tons + 60 ft.-tons + 72 ft.-tons
= 147 ft.-tons.
The bending moment at any other point
can be found by considering only that por-
tion of the loading which is situated between
the point and the outer end of the cantilever.
MOMENT OF INERTIA
The moment of inertia is extremely import-
ant in all cases of advanced calculations. The
expression is used to define the relative
values of sections of different shapes. From
the moment of inertia, the section modulus
and moment of resistance (two terms that
will be explained later) can be found.
Explanation of Moment of Inertia.—
The moment of inertia of a section is an
indication of the strength of that section.
It is a constant value, and depends on
the shape of the section. The area alone
STRESS SIMPLY EXPLAINED
73
is no indication of the strength of a section
from a structural point of view. Two
sections of the same area do not necessarily
have the same power to resist stress. For
example, place a piece of timber, 9 in. by
2 in. in section, as a beam over a certain
span — say 10 ft. — it will have a certain
amount of power to resist stress, or, in other
words, a certain amount of power to remain
" inert," which power can be expressed as its
*' moment of inertia." The timber will have
two values, according to the Way in which
it is placed. In the form of a beam 9 in.
deep and 2 in. wide, it \vill be much stronger
as a weight-carrying member than as a beam
2 in. deep and 9 in. wide. This can be
proved by a practical demonstration quite
easily, or by the theory of the moment of
resistance for a rectangular beam, to be
given later.
The reason for this difference in strength
with the same section is entirely due to the
disposition of the fibres in relation to the
neutral axis of the section, which is different
in each case. (See page 3 for an explana-
tion of what is meant by the term " neutral
axis.") In the first instance, the beam was
9 in. deep, and therefore some of the fibres
were situated at a distance of 4i in. from the
neutral axis, whereas in the second instance
the beam was only 2 in. deep, and therefore
the extreme fibres were only 1 in. from the
neutral axis. Now the stress in a beam is
nil at the neutral axis, and it increases in
intensity as it gets farther away from the
neutral axis. This fact allows the fibres that
are at a distance from the neutral axis to be
utilised to their fullest capacity, whereas
those adjoining the neutral axis cannot be
called upon to exert their greatest resistance.
Again, it will be shown later that the lever
arm of the tensional and compressional areas
is increased as the depth is increased, and
this greatly increases the value of the resist-
ance offered.
These remarks should suffice for the pre-
sent to show that the area of a section alone
is no criterion as to the strength of the
member, and that the same section will
possess two different capacities according to
the way in which it is placed.
It will now be understood why the two
values for the moment of inertia of a section
are known as the " least moment of inertia "
and the " greatest moment of inertia,"
according to the neutral axis around which
the " moment " has been taken. In the
case of a section which is square or circular
there will only be one value for the moment
of inertia, as the axis will be at the same
distance from the extreme fibres in both
cases, except in the case of the square
section being placed on one edge, which is
so unusual in structural work that it need
not be considered for the present. The
moment of inertia in value \vill be dependent,
in every case, on the area of the section, and
the disposition of that area in relation to the
neutral axis of the section.
Calculating Moment of Inertia. — The
method of finding the moment of inertia
of a plane figure is based on the following
principle. The section or plane surface is
Fig. 90. — Calculating Moment of Inertia of
Simple Rectangular Section
imagined to be divided into an infinite num-
ber of thin layers, and the area of each layer
is multiplied by the square of the distance
between the centre of gravity of the layer
and the axis ; the sum of all these products
is the moment of inertia of the plane surface
with respect to that axis.
An attempt should now be made to calcu-
late the moment of inertia of a simple rect-
angular section as given in Fig. 90. Assume
that the section is 12 in. deep and 6 in. wide,
and the moment of inertia is to be taken
about the axis x — x. Also assume that
there are three layers on each side of the
axis 2 in. thick. Then the moment of
inertia will equal
[(6 x 2 x 52) + (6 x 2 x 32) + (6 x 2 x I2)]
x 2 = [300 + 108 + 12] x 2 = 420
x 2 = 840.
The presence of fifty layers on each side of
74
REINFORCED CONCRETE
the axis, instead of three, could be assumed,
and thus a great amount of time could be
tediously spent in calculating the " moment."
A formula has, however, been devised for the
moment of inertia of all the simple sections,
and the formula will be given for each case.
about the axis x — x than about Y — y (Fig.
99). The formula will be I = & ^
6 x 123 - 2 x 2-75 x 10-53
12
= 333421.
To calculate the moment about the other
p— b-
— *• (« — -D f
r
d
1
I
L
/ \
3 W
Fig. 9
I - b
i
d8. ! _
12
b— >|
Fig. 92 Fi«' 93
6~d3 - 6' d'3 I = '7854 r
12
1
t *~r n^
'igs. 91-97 ^
mon Sections
^ ., . int.-u.TRAi AXIO -k
T7
t±
I F
Com
Ll \
Foments of cf
Fig. 96
j _ 6 d8 - 26' d'3
Inertia ^ J[c'
Fig. 97
12
i.=
Fig. 94
•7854 (r4 - r'4)
I = j | b d3 + b' d'3
- (6'- b)d"3\
I -I {&*- (b -i) (d -c)3
+ b' d'3 - (b' - t) (d' - c')3[
axis, place the joist so that it is 6 in. deep and
12 in. wide, as Fig. 99. The inertia will be
flanges web
•75 x 63 x 2 + io-5 x -53
-b'— J
Fig. 95
o a3
Now the formula for a rectangle is - ^-,
12
fe4
and for a squara j^» an(i so t^6 true value
of the inertia moment for the section given
in Fig. 90 will be I = ~~-
6 x 123
-12— - 864'
It will be seen that there is some difference
in the result obtained by the formula and
that obtained previously, this being due to
the fact that the number of layers in the
first instance was taken at three only. The
moments of inertia of the common sections
are as given by Figs. 91 to 97.
In (ach case these formulae give the
least moment of inertia in the positions
shown, and as an example a rolled steel
joist will be taken and the moment of
inertia calculated (see Fig. 98).
The moment of inertia will be more
1 n " — ^('JLUy.
L K - A*
» _ 1
^
7
r )
d
-"72"
x-
i
v
A
t ^>
/
i
f
— '
i
d
- lO'J
Fig. 98. — Calculating Moment of Inertia of
Rolled Steel Joist
STRESS SIMPLY EXPLAINED
75
SECTION MODULUS AND MOMENT OF
RESISTANCE
The terms " section modulus " and
" moment of resistance " are often used
indiscriminately, but although they are
are equal, opposite, and parallel forces form-
ing a " couple."
Take one-half of the beam and imagine
the forces to be acting upon it, as shown in
Fig. 101. Then the rotating tendency
Fig. 99. — Calculating Least Moment of Inertia
of Rolled Steel Joist
closely allied, they are distinct terms, and
have different values.
The section modulus can be found by
dividing the moment of inertia by half the
depth of the section.
The moment of resistance can be found by
multiplying the section modulus by the
modulus of rupture of the particular material
under consideration.
Thus, the section modulus is a fixed
quantity for any given section, whilst the
moment of resistance depends on the nature
of the material employed.
The bending moment or tendency to cause
failure in a member must be equalled or
exceeded by the moment of resistance of
that member ; and to illustrate this fact,
and also how the section modulus is obtained
— and from this the moment of resistance —
will be the next step.
Calculating Section Modulus.— It has
L
Fig. 100. — Rotating Tendency of Beam, caused
by Reaction and Weight
already been shown that the reaction and
the weight on a beam causes a certain
rotating tendency, as indicated in Fig. 100.
It has also been shown that the value of
these external forces on the beam is equal
to the amount of one force multiplied by
its distance from the other force, as they
Fig. 101. — Forces Acting upon One-half
of Beam
caused by the external forces must be
equalled by the internal forces having a
rotating tendency in the opposite direction.
This rotating tendency of the internal forces
is offered by the compressional and tensional
resistances of the beam, which are equal to
one another, opposite in direction, and
parallel, as shown in the diagram by c and T.
Then these two resistances form a " couple,"
and their value to resist the bending moment
will equal the amount of one resistance
multiplied by their distance apart. The
method of ascertaining this value will be
shown.
It will be seen that the rotating tendency
of the reaction and weight is in a " clock-
wise " direction, whilst the tendency of the
internal forces is " anti-clockwise." The
direction of the tension and compression
arrows, shown on the diagram, can be proved
in the following manner. The firm arrows
N -
w
a 4 b
1
-f? —
-N
1
'v\
C I d
W1
'/
Fig. 102. — Calculating Section Modulus :
Beam Centrally Loaded
indicate the fibres on the half of the beam
under consideration, and the dotted arrows
show those on the other half of the beam.
The top fibres are being compressed or pushed
together, and therefore the tendency of the
resistance is to pull the fibres apart as shown,
and vice versa in the lower part of the beam.
76
REINFORCED CONCRETE
The amount of the rotating tendency of
the internal forces can be reasoned as
follows. Assume that a beam carries a
central load, as Fig. 102, and mark a piece
of the beam abed as shown. The tendency
Fig. 103. — How Centrally Loaded Beam tends
to bend
of the load is to cause the beam to bend in
the middle, and this is shown in an exagger-
ated manner in Fig. 103. Now mark on the
beam the same portion abed as shown on
the previous diagram. In order to do this,
begin by marking the length n n on the
neutral axis, as it is known that no change
has taken place in the length of the fibres
at this point, there being no compression or
tension. Through these points n n draw the
lines a c and 6 d, which must, of course,
radiate to the same centre as that from
which the curved beam was struck. It will
D
-N
12."
VARYING INTENSITY
OF t)JKLt>$
Fig. 104
parallel to the line a c, as shown on the
diagram in Fig. 103, the distance 6 a; "will
equal the amount of compression that has
taken place in the extreme fibres at the top
and the distance x d will indicate the amount
of tension that has taken place in the extreme
bottom fibres. The stress in the extreme
fibres must not exceed the resistance of the
material, as otherwise failure would result.
This will demonstrate the fact that as the
extreme fibres only can be stressed to the
safe resistance, the full value of the other
fibres can never be realised, and the value
of each layer of fibres decreases as it
approaches the neutral axis.
This principle can now be applied to the
section of a beam and the value of the com-
pression or tensional areas ascertained. Take
the section of a beam 12 in. by 6 in., as shown
in Fig. 104, where the intensity of the stress
is indicated by the graduation of the lines,
which become fainter as the stress becomes
less. Assume also that the limiting stress
on the extreme fibres is 5 tons per square
inch. Then the stress at the neutral axis
is nil, and the mean stress on the fibres is
2^ tons per square inch. The resistance
must be equal to the stress if there is to be
equilibrium, and therefore the resistance of
the compressional area will be 6 in. x 6 in.
x 2£ tons per square inch = 36 x 2| =
90 tons.
Of 6RAVIJY
Fig. 105
Fig. 106
Figs. 104 to 106. — Calculating Section Modulus of Beam
then be seen that a c and b d are no longer
parallel to one another as in the original
beam, or, in other Words, the distance
between a and b has become shorter owing
to compression and the distance between
c and d has become longer owing to tension.
Now, if a line a; a; be drawn through n2
Next, instead of assuming that the stress
on the fibres varies in intensity from the
outer edge to the neutral axis, assume that
the intensity is the same, but the area over
which it acts varies from the outer edge to
the neutral axis as shown by the shaded
triangles in Fig. 105, and it is found that the
STRESS SIMPLY EXPLAINED
77
same result is given, with the same sectional
area of beam. Here the shaded triangle
which shows the compressional area is equal
to one- quarter the sectional area of the
beam, or, in other words, -j-. Afterwards
multiply this by the uniform maximum in-
tensity of 5 tons per square inch, then
6 x 12
— j — x 5 tons per square inch
72
360
= 90 tons
bd
as before. This area -^ is called the
equivalent inertia area, and it simplifies
the calculations to consider the beam as
offering its resistance in this way.
There are two resistance areas, namely, the
compressional and the tensional, and it will
be seen that they are equal. They repre-
sent the internal forces which are opposite
in direction, and their value to resist stress
is the amount of one multiplied by its dis-
tance from the other, as they form a
" couple." The distance apart of the two
forces will be the distance between their
centres of gravity as they are represented
by triangles, and the centre of gravity of a
triangle is situated at a distance of two-
thirds of its height from the apex, as shown
in Fig. 106. The centres of gravity are then
d
each f ~ from the neutral axis, and their Tnen M R =
The section modulus is usually expressed
in formulae by the letter Z.
Calculating Moment of Resistance.—
Having ascertained the " section modulus "
and followed the principle of the action of
the internal forces, the actual resistance of
these forces, which not only depends on the
area and leverage, but also upon the strength
of the material, has to be considered.
Up to the present, no account has been
taken of the strength of the material, and
therefore the section modulus for any shape
section will be constant, no matter what
material is employed, and this is a great
consideration.
b d2
Thus -JT~ is tne section modulus for a
rectangular section, and the moment of
resistance can be found from this, according
to the strength of the material employed,
by multiplying the section modulus by the
ultimate or safe strength of the material
per square inch, according to whether the
actual or safe resistance is wanted.
As an example, ascertain the safe resist-
ance of a rectangular steel beam 8 in. x 2 in.
b d2
Then M R = „- x 7£ tons, or M R =
j 7 79
— ^ — , where / equals the limiting stress on
the fibres, which is 7| tons per square inch.
2 in. x 8 in. x 8 in. x 7| tons
distance from one another is § ~ +f~9
which equals -f d.
The distance f d then equals the lever-
arm of the internal forces. The area of one
force = . and the area multiplied by the
lever-arm will give the value of the section.
This can be expressed as
bd 2bd2 bd2
~~ ~~
The value
b d2
then gives the value of a
rectangular section, or, in other words, its
" section , modulus." It was stated pre-
viously that the moment of inertia divided
by half the depth of the beam = the section
modulus. The moment of inertia for a
rectangular beam is y^-.
bd2
= -7T- as before.
12
bd* d
Then -J2- -*- 2
M R = 160 in.-tons. This, of course, must
equate with the bending moment to produce
equilibrium.
It has been seen that the moment of inertia
divided by half the depth of the section gives
the section modulus, and the section modulus
multiplied by the limiting stress on the fibres
gives the moment of resistance. This is
expressed in the following formula, which is
one greatly used in practice : MR = / -,
where / = limiting stress on the fibres, I =
moment of inertia, and y = half depth of
section. Upon working out the example
just given by this method, the same result
will be obtained.
CALCULATING SAFE LOAD ON BEAM
The following example should assist the
student to understand the application of
formulae already given. Find the central
load that can be safely carried over a
span of 16 ft, by a rolled-steel joist 12 in.
78
REINFORCED CONCRETE
x 6 in., with flanges and web 1 in. thick.
MB*/= ,/ = 7|tons, 1 =
y 1Z 1J
6 x 12 x 12 x 12 2 x 2| x 10 x 10 x 10
~~12~ 12~~
- 864 - 416-6 = 447-4
d 12
v = -R = -TT = 6 in.
then M R =
7-5 x 447-4
~6~
M R = 559-25 in.-tons.
w I
But B M - M R, and B M = -^ for central
load.
w I
Therefore -r- = 559-25 in.- tons
I = 16 x 12 = 192 in.
^^= 559-25
559-25 x 4
WEI6HT
w = 11-65 tons.
This method is absolutely accurate, and
is not approximate, as would be the case
with an empirical for-
mula. The student is
strongly advised to
work out several ex-
amples himself, which
will familiarise him
with the use of the
formula, and impress
it upon his memory.
A good method is to
select several rolled-
steel joists from a
manufacturer's list,
calculate the safe loads
for various spans, and
then check the results
by reference to the
table of s.afe loads
for the various joists
invariably given in
the list of sections.
REACTION
Fig. 107.— Strut
under Load
COLUMNS AND
STRUTS
The theory of long
columns and struts
unfortunately is in an unsatisfactory con-
dition, as there is a great difference of
opinion as to the best formula to use ; and
as the formulae in general practice give
very varying results, the student is often
confused and doubtful as to the best course
to adopt; Most of the formulae in use are
of a more or less complicated nature, and
practically all are empirical formulae, con-
taining some factor which has been derived
from experiment, and which must be taken
Fig. 108. — Diagram Indicating Stress in Part
of Strut
for granted. Unlike a tension bar, a com-
pression bar which is under a certain stress
is apt to bend, and this bending tendency
has to be resisted, in addition to the com-
pression proper, as shown in Fig. 107.
Theoretically, with the stress passing abso-
lutely through the centre line of the strut
there is no tendency to bend, but practically
this condition is never fulfilled, and the
slightest deviation from the exact centre
will cause a bending stress. The diagram
in Fig. 108 shows a piece of the strut at
A B enlarged, and the arrows indicate the
stress. The horizontal line represents the
compression proper, with an amount equal
to x, while the sloping line indicates the stress
due to the bending tendency, and this, of
course, will vary in intensity at different
points in the section ; the edge of the strut
at B will be under great compression, while
the fibres at A will be in tension. Now the
maximum intensity of compression will be
at B, where the stress will equal x x y,
while at A the stress will be at a minimum,
and will be equal to a; — y, and if y is greater
than x, then the fibres at this point will be
in tension. It is clear that the greatest stress
x + y must not be greater than the safe
resistance to compression of the material.
The principal difficulty occurs in the deter-
mination of the bending tendency.
Struts and columns are usually divided
into " long " and " short " compression bars,
and in the short bars the calculations are
based on the assumption that the failure is
STRESS SIMPLY EXPLAINED
79
by direct crushing only, while long bars are
those which fail by bending. A sudden
change has been assumed, although, of
course, this does not actually happen. The
following is the ratio between the least
diameter and the length when the bars cease
to be considered as short. Wood, 20 ;
wrought-iron and steel, 10 ; cast-iron, 5.
These ratios will be found to vary greatly
according to different authorities.
The formula chiefly used in this country
for long struts is that known as " Gordon's
Formula," which is as follows : —
R_ = Ar' -. where
a = a constant found by experiment.
d — the least diameter.
A = area of the cross section.
re = safe resistance to compression of the
material.
I = length in same units as d.
Re = total pressure in same units as rc.
Material
CVoss
Section
Values of a
rc
*«
"® 21
1^
ITS
14
!•»
23-
TIMBER . .
Rectangular 1
Circular . . j
4
250
i
250
i
100
13
cwt.
WROUGHT-
IRON
Rectangular
Circular . .
Solid
Hollow . .
4
1
1
2500
2500
1000
LT+ I
n J
4
900
1
900
1
360
-4 tons
I U j
STEEL
do.
do.
do.
do.
1\ tons
CAST-IRON
Circular
1
1
1
Solid
100
400
160
Circular
1
1
1
Hollow . .
Rectangular
200
3
800
3
320
3
640
: 8 tons
400
1600
X shaped . .
3
200
3
800
3
320
/
In re a factor of safety of 4 only is allowed,
and therefore, in the case of timber and cast-
iron, it will be advisable to use a higher
factor in important work, and this will
reduce the value of r^
SHEARING STRESS
Before studying calculations for the value
of the shearing stress under different con-
ditions of loading, it will be advisable to
explain what the expression " shearing
DOTTED
BEAM
1
¥
v\
'(/'
i
1
Fig. 109.— Beam, under Vertical Shear,
Assumed to Consist of Separate Blocks
stress " means. This stress is actually a
sliding tendency caused by the opposition
of the load and the reaction, and as the
beam or cantilever is the medium through
which these two can oppose one another,
the beam is subjected to this tendency both
vertically and horizontally. As an illus-
tration of this, let it be assumed that it is
possible to divide a beam up into a cer-
tain number of parts by cutting vertically
through it, so that each block is separate
from the adjoining block, and yet it is pos-
sible to put the beam in position and apply
the load. Then the load, in travelling along
the beam toward the abutments would tend
to push down each block and cause it to
slide past the adjoining block as illustrated
in Fig. 109. This does not actually take
place in practice, as the beam would be
homogeneous ; but the tendency is pre-
sent, and must be provided for. This is
known as vertical shear. There is also
DOJTtD LlNEt) 6HtW
Fig. 110. — Beam, under Horizontal Shear,
Assumed to consist of Separate Planks
a tendency to shear in a horizontal manner,
and this is illustrated in Fig. 110, where the
beam is divided into a number of horizontal
parts or planks ; upon the load being
applied, there is a tendency for the beam
to assume the form shown in an exaggerated
REINFORCED CONCRETE
manner. As the ends of the planks coin-
cided in the first instance, and after the
load has been applied they are no longer
coincident, it is obvious that they have
slid past one another, thus forming a hori-
W- 10 TOKS
Fig. 111. — Shearing Stress in Cantilever which
carries Concentrated Load at Outer End
zontal shearing action. At all points in
the beam the vertical and horizontal shear-
ing stress are equal, and they will be found
to be at the maximum at the support. The
distribution of the shearing stress over the
area of the cross section is explained in the
next chapter and need not be considered
here. The amount of shearing stress is not
due to any question of leverage, but is due
to the amount and nature of the loading,
and not to the span, as in the case of the
bending moment. The conditions under
which it is likely to become a serious con-
sideration are those where a heavy load
has to be carried over a short span, as in
this case the bending moment will be com-
paratively small the leverage being small.
Dl6TRi&ujED LOAD- (TON pte
FT ^UH
112. — Shearing Stress in Cantilever which
carries Uniformly Distributed Load
The possibility of failure by shear is fre-
quently ignored on account of the fact
that the amount of material required to
resist the bending moment is usually such
that ample provision for shear is given.
In the case of heavy loads over a short span
this does not always follow, and consequently
the shearing stress must be calculated for
as well as the bending moment. With
regard to the distribution of the shearing
stress over the length of the cantilever or
beam, it must be fully realised at the out-
set that this stress is due to the opposition
of the load and reaction, and consequently
the value of the shear at any point can only
be equal to the load that is travelling
through that point to reach the abutment.
As a first example, consider a cantilever
that is carrying a concentrated load at its
outer end. Then at the support the shear-
ing stress will be equal to the reaction, as at
this point the total load will be opposed by
the total reaction, and the opposition is
therefore at the maximum. If any inter-
mediate point in the cantilever is considered,
it will be seen that the shear is still equal to
5TON6 5 TONS
I DISTRIBUTED LOAD I
if I TON pEEf FT RUN. I
^__?TONi
JTONi
I
5TOITS
-Jr_ -i
i
Fig. 113. — Shearing Stress in Cantilever which
carries Uniformly Distributed Load and
Two Concentrated Loads
the total load, as in passing outward from
the support no load is passed over or left
behind, as it were, until the extreme outer
end is reached. The shearing stress at all
points will therefore be as indicated in
Fig. Ill, where the length of the vertical line
at any point equals the shear at that point.
If W T= 10 tons, then the shearing stress at
the support or any intermediate point equals
10 tons.
In the case of a cantilever carrying a
uniformly distributed load, the shearing
stress at all points is somewhat different, as
will be seen on reference to Fig. 112. In
this case the shear will uniformly diminish
from the support to the outer end, where it
will be nil. In passing out from the sup-
port, it will be obvious that the load will
be gradually passed over, as, for example,
STRESS SIMPLY EXPLAINED
81
at a point 1 ft. from the support 1 ton will
be left behind and the opposition of 10 tons
reduced by this amount, giving a shearing
stress equal to 10 - 1 = 9 tons. When
5 ft. has been passed over, then 5 tons will
be left between this point and the support,
and the shear will equal 10 tons - 5 tons =
5 tons, as shown in the diagram.
It has been stated that the shear at any
point can only equal the load that is passing
through that point on its way to the sup-
port, or, in other words, in a cantilever the
shear at any 'point is equal to the load between
that point and the other end. Apply this rule
to a cantilever carrying both distributed
and concentrated loads ; it will be shown
how the shear is obtained. The example in
Fig. 113 shows a cantilever carrying a
uniformly distributed load of 1 ton per foot
run over a length of 10 ft., together with
» 5
Fig. 114. — Shearing Stress in Beam which
carries a Central Concentrated Load
two concentrated loads of 3 and 5 tons res-
pectively. Beginning at the outer end and
passing inward, and applying the rule that
the shear at any point equals the load be-
tween that point and the outer end, we shall
have placed a distributed load of 2 tons
between us and the outer end by the time we
have passed over 2 ft., and consequently
there will be a gradually increasing shear
from nil to 2 tons ; and upon passing over
the 2 ft. mark the centre of gravity of the
concentrated load of 3 tons will be passed
and placed between us and the outer end,
and the shear is therefore suddenly increased
by 3 tons, giving a total of 5 tons. The
next 5 feet give a gradually increasing shear
of 5 tons, which must be set downwards on
the diagram, and immediately on passing
the line of the second concentrated load
another 5 tons is added, giving a shear of
15 tons. From this point to the support
we pass over a distance of 3 ft., and gradu-
6
ally leave behind the remaining portion of
the distributed load, equal to 3 tons, thus
giving a total shear at the support of 18
tons, which is equal to the reaction at the
support. This is clearly illustrated on the
diagram referred to. The same principles
and rule apply to any kind of loading,
whether partly distributed or partly con-
centrated, bearing in mind that a distri-
buted load causes a varying shear, repre-
sented by a sloping line, and a concentrated
load causes a sudden increase in the shear
equal to the amount of the load.
Beams require a little more consideration
on account of there being two supports,
and consequently a portion of the loading
travels one way and is opposed by one re-
action and the remainder travels in the
opposite direction and is opposed by the
other reaction. We have, therefore, a shear
towards the left-hand abutment, caused by
the opposition of R1, and a shear toward the
right-hand abutment caused by R2, and the
shear at either support will equal the re-
action at that support. In the case of a
beam carrying a central concentrated load
only, as illustrated in Fig. 114, the reac-
W
tions will each be equal to -^, which in this
case is -~- = 5 tons. To distinguish between
a
the shear towards the left-hand and right-
hand abutments, a horizontal line is drawn,
known as the separation line, and the value
of R1 is set up to scale over this line and
the value of R2 below as shown. The full
opposition will exist at all points between
the support and the centre of gravity of
the load, as the whole of the load which
travels to the support will be passing through
every point in the beam, and there is con-
sequently no diminution in the shear at
any point.
The rule for the shear at any point in a
beam can be expressed as follows : the
shearing stress at any point in a beam is
equal to the reaction at the support minus any
load situated between that point and the sup-
port selected. If the load between the point
and support selected is greater than the
reaction, then it indicates a shear toward
the other support. As an illustration of this,
supposing the shear at a point 7 ft. 6 in. from
the left-hand abutment shown in Fig. 114
is to be calculated from R1. Then (R1 -
the load of 10 tons) = (5 tons - 10 tons)
= - 5 tons, thus indicating a shear toward
REINFORCED CONCRETE
the right-hand abutment which agrees with
the diagram. In the case of a uniformly
distributed load on a beam, the shear at
either abutment will be equal to the reac-
tion at that abutment, and it will gradually
diminish to the centre, where it will be nil,
as illustrated in Fig. 115. An example of
combined concentrated and distributed load-
ing is given in Fig. 116, and if this is ex-
plained no difficulty should be encountered
by the student in working out other examples,
however complicated the loading may be.
As the reaction at the support is equal to
the shear, it will be necessary to calculate
this in the first instance. Therefore K1 will
equal half the distributed load =
B
(3 x 8) + (4 x 6) + (5 x 3)
5 tons + -
Loftb - | pn peg 17- KM
Fig. 115. — Shearing Stress in Beam which
carries Uniformly Distributed Load
24 + 24 + 15
5 tons + - - = 5 tons + 6^
tons = 11TS7,
10
tons.
Working out R2 = 5
tons +
tons +
(3 x 2) + (4 x 4) + (5 x 7)
10
= 5 tons +
= 5
6 + 16+35
10
10T7^ tons. E1 +R2 = total loading, therefore
llfV + lOy7^ = 22 tons, which is correct.
If we now draw a horizontal separation
line and set up R1 to scale at the left-hand
abutment, we shall have the shear at this
point, and we can proceed to find the shear
at all points. After passing over the first
2 feet we have reduced the shear by 2 tons,
giving 9-^ tons at this point, and directly
we pass this mark we reduce the stress by
3 tons, due to the concentrated load leaving
a shear of 6^ tons. We now gradually
leave behind a further 2 tons by the time
the next concentrated load is reached,
making a reduction to 4j%- tons, and upon
passing the concentrated load we only have
T3o ton left as shear toward the left-hand
abutment. As the distributed load equals
1 ton per foot run, it will be obvious that
when we have passed over a further distance
of y'k ft., we shall have no shear to this
abutment, and it is therefore at this point
that the separation line will be crossed
and the shearing stress will be below the
line, and thus towards the right-hand abut-
ment. This point of crossing the line will
III ION
3 JONS
2 TO
LT
4_
5UJED Lc
PEE- FT "
i*
Fig. 116. — Shearing Stress in Beam which
carries Uniformly Distributed Load
and Three Concentrated Loads
be 4^ ft. from the left-hand abutment,
and it is important to be able to calculate
this in the case of reinforced concrete beams
when special shear members are provided
as shown in the notes on this work. In
continuing along the beam it will be seen
that after passing the point of crossing the
separation line a distance of 2T7TT ft. will
be passed over before reaching the load of
5 tons, thus picking up 2T7Ty tons, and upon
adding the 5 tons the shear will equal 7TV
tons. The remaining 3 ft. will add a gradu-
ally increasing shear of 3 tons, making a
total of lOy7^ tons upon reaching the sup-
port, and this amount is equal to the re-
action, and therefore is correct. Fig. 116
shows the shear at all points in a graphic
manner, and should assist the student in
following the explanation.
The Theory of Reinforced Concrete
Introductory. — The theory of reinforced
concrete is, unfortunately, in a very unsatis-
factory state at the present time, and, in
fact, it may be said that no kind of universal
agreement has been arrived at. Students
have no definite course of study that can
be followed with the surety that diligent
application will enable them to design and
calculate the material in a manner consistent
with a universally recognised standard. The
desirability of standardising the methods of
design and calculation is fully recognised by
the various Institutes who are using every
endeavour to bring about this reform ; and,
in the meanwhile, it is incumbent upon all
writers on the subject to help matters forward
by adopting the recommendations of the lead-
ing Institutes as regards notation and general
methods.
For the purpose of this treatise, it has been
decided to adopt the notation recommended
by the Concrete Institute, this being regarded
by the writer as the most satisfactory that
has been compiled. The working stresses
employed are those contained in the second
xeport of the Royal Institute of British
Architects.
A knowledge of stresses and strains is
necessary to every student before an attempt
is made to take up the calculations for re-
inforced concrete. Therefore, although these
notes apply to simple theory only, it is
necessary to assume that the reader has
mastered the preceding chapter and possesses
the knowledge referred to ; and only those
explanations will be given which are essen-
tial to a proper understanding of the methods
of deduction. There is a great tendency to
Tender formulae and explanations very com-
plicated, with the result that many students
possessing only a limited knowledge of
mathematics are unable to follow them.
Bearing this in mind, the writer has endea-
voured to delete all unnecessary complications
a.nd to put the matter in a simple manner,
which is capable of being understood by all
readers. The many different symbols used
in the formulae are apt to look formidable
at first sight, but the reader is advised to
make every effort to become familiar with
them and to endeavour to realise the full
meaning of each one.
It is not sufficient to learn several formulae
and to be able to put them down from
memory, as this will not enable them to be
used intelligently ; each one must be under-
stood and its construction realised. Indeed,
the user should be able to construct his own
formulae when he has fully grasped the
principles of force and resistance.
The calculations can be divided into two
sections, namely, (1) the determination of
the value of the external force or forces,
and (2) the determination of the necessary
resistance values to overcome these forces.
The value of the force to cause stress will
depend upon the nature, amount, and dis-
position to the resisting member, and the
value of the resistance will depend upon the
shape and size of the member and the
material of which it is composed.
DATA FOR CALCULATIONS
Ascertaining Loads to be Carried. —
There must be some basis upon which to
found the calculations, and the following
methods of ascertaining the loads to be
carried must be taken into consideration :
(1) The weight of the structure, including
flooring, plaster, and any applied decoration
or similar loading.
(2) The superimposed or accidental load,
which will vary in amount according to the
class of building being designed.
(3) Vibration and shock wherever this is
likely to occur. This will vary with the class
of building, as obviously a greater allowance
must be made in the case of a building
containing machinery than in the case of
an ordinary dwelling house.
In the case of item No. 1 the weight of the
concrete and steel may be taken at 150 Ib.
per foot cube. To this must be added the
flooring material or any other weight, accord-
ing to circumstances.
The load per square foot to be allowed,
as in item 2, may be taken on the following
basis : —
84
REINFORCED CONCRETE
lb. 'per sq. ft.
Ordinary dwelling house, flats,
hotel bedrooms, and hospital
wards ..... 75
Offices, schoolrooms, etc. . . 100
Theatres, libraries, concert-rooms,
banks, and shops . . . 120
Ballrooms and drill halls . . 150
Warehouses and stores vary ac-
cording to use . . . 230-670
Factories and machine shops. These
will vary greatly, according to
the number and weight of
machines, and each case must
have special consideration.
Koofs generally .... 50
The allowance for shock mentioned in
item 3 will be necessary in the case of build-
ings which are subjected to greatly varying
loads, as, for instance, public buildings,
factories, or workshops, and such allowance
in these cases should be taken at one half the
superimposed load as given above for the
various buildings. In structures where the
vibration is caused by machinery or heavy
traffic, as in the case of vault roofs, the
allowance should be equal to the actual
superimposed load.
In the case of columns and piers which
carry several stories above, a certain reduc-
tion of the load picked up at the different
levels is permissible, as follows : — For the
part of the roof or top floor supported, the
actual superimposed load calculated is taken.
For the next floor below a reduction of 10
per cent, is made, and for the floor below
this a reduction of 20 per cent, is made, and
so on until the floor is reached at which
the reduction is equal to 50 per cent.
Below this level all the floors are taken
at 50 per cent, below the superimposed
loads in determining the reactions on the
columns.
This reduction must, of course, be used
with discretion, as it is based on the assump-
tion that all the floors would not be loaded
to their full capacity at the same time, and
therefore, in the case of warehouses and
some buildings where all the floors may be
heavily loaded together, it is wide to make
no reduction whatever.
Working Stresses. — The following work-
ing stresses are based on the assumption
that the concrete is of such a quality that
its crushing strength is at least 1,800 lb. per
sq. in. after twenty-eight days, and that the
steel has a tensile strength of not less than
60,000 lb. per sq. in. :—
lb. per sq. in.
Concrete in compression in beams
subjected to bending . . GOO
Concrete in columns under simple
compression .... GOO
Concrete in shear in beams . 60
Adhesion or grip of concrete to
metal 100
Steel in tension .... 16,000
Steel in compression — fifteen times
the stress in the surrounding
concrete ....
Steel in shear .... 12,000
The resistance of concrete to tension is
neglected, as its value is small, and, in
addition, it cannot be considered as a reli-
able material to take such a stress ; there-
fore, the steel is calculated to take all the
tensile stress. The figures given above for
concrete are based on tests made with con-
crete made in the proportions of 1 cement,
2 sand, and 4 hard stone, and should different
proportions be adopted the stress may be
taken at one-third the crushing strength
twenty-eight days after moulding. This may
be considered a low factor of safety by some
designers, but it must be borne in mind that
the maximum strength of the material is
not developed in twenty-eight days.
If the steel has a greater ultimate strength
in tension than 60,000 lb. per sq. in., the
safe stress may be taken at one half of the
stress required to reach the elastic limit of
the material ; but in no case is it advi?able
to calculate for a greater stress than 20,000
lb. per sq. in.
The stress is considered as acting on the
concrete in a uniformly varying manner,
while in the case of the steel it is considered
as uniform over the cross section.
Elasticity. — The question of the relative
elasticity of the two materials is highly
important, and although there is some
difference of opinion on this point, it is now
generally accepted that the coefficient of
elasticity for concrete in compression gauged
as above mentioned is constant and can be
taken at one-fifteenth that of steel. The
modulus for concrete is expressed by E,.,
and equals 2,000,000 lb. per sq. in. The
modulus for steel is expressed by Es, and
equals 30,000,000 lb. per sq. in. ; therefore
E
T/ = 15. It is highly important that the
THEORY OF REINFORCED CONCRETE
student should realise this point, as it means
that at any given distance from the neutral
axis the stress per square inch on the steel will
be fifteen times as great as that on the con-
crete. In other words, the stress can only
be proportional to the resistance where
equilibrium is produced, and as the steel is
capable of offering fifteen times as much
resistance to stretching as the concrete is to
compression, it will require fifteen times
more stress in the steel to produce the same
effect a^ that taking place in the concrete.
Spans. — The span to be considered in the
calculations will not, of course, be the clear
span, but the effective span, and this, in the
case of beams, may be taken as the distance
between the centres of bearings. For slabs
supported at the ends only, the distance will
be the clear span plus the thickness of the
slab ; while for continuous slabs it becomes
the distance from centre to centre of the
supporting beams.
Bending Moments. — The question of
bending moments has been fully dealt with
in the preceding chapter, but it must be
borne in mind that a slab can only be con-
sidered as continuous when it is continued
over three or more spans, and then the spans
must be equal and the supports at the same
level. With a uniformly distributed load
the maximum bending moment may be taken
as + -r~- at the centre and — ^r^ at the
intermediate supports, where W = total
weight and I equals span.
BEAMS WITH SINGLE REINFORCEMENT
Some little difficulty is experienced by the
student in determining the moment of re-
sistance of a reinforced beam, as there are
two materials to be considered, each having
a different modulus of elasticity and re-
sistance to stress. It is necessary, however,
to be able to determine their moment of
resistance at any point in order that it may
be shown that it equates with the bending
moment at the same point.
The bending moment represents the value
of the external forces to destroy the beam
by tension or compression, the former taking
place on the lower part of the beam and
the latter on the upper, where the beam is
supported at the ends only. If the beam is
fixed at the ends, then the beam will be
called upon to resist compression in the lower
part and tension in the upper part for a
certain distance from the fixed ends, but for
the purpose of this present explanation it will
be considered that the beam is supported
only.
There are several formulae for the moment
of resistance of a reinforced concrete beam,
but it is not sufficient to give these without
explaining the principle of their construction.
It is shown in the preceding chapter that
the compressional and tensional resistances
must be equal to one another, and that they
are acting in opposite directions and are
parallel, thus forming a " couple," and the
value of the moment of resistance of the
whole section is the amount of one resisting
power multiplied by its distance from the
other.
The resistance to compression is supplied
by the concrete above the neutral axis, while
the resistance to tension is supplied by the
steel in the bottom of the beam, the concrete
below the neutral axis being neglected, as
shown in Figs. 117 and 118. Here it will be
seen that the stress on the concrete is shown
as being of a uniform intensity acting over
an equivalent inertia area which, as pre-
viously explained, is equal to a varying
intensity over the whole of the compressional
area. Hence the equivalent compressional
area will equal -^-, where 6 = the breadth
of the beam in inches and n = the distance
of the neutral axis from the compressed edge.
The safe compressional stress per square inch
on the concrete is expressed by c, and the
value of the compressional resistance, there-
C t) YL
fore = — . The safe resistance of the
A
steel in tension per square inch is expressed
by the symbol t, and the sectional area in
square inches by At, therefore the value of
the tensional resistance equals At x t.
The total compression and total tension
are equal, and therefore — ~- = t At. These
two equal resistances or internal forces act
in opposite directions, and the amount of
one of them must be multiplied by the dis-
tance between them to find the moment of
resistance. The centre of gravity of the
compressional area, at which point the com-
pressional resistance can be considered as
being concentrated, will be situated at a
distance of « from the top edge, or at a
o
distance of f n from the neutral axis. The
tensional resistance will be acting at a
86
REINFORCED CONCRETE
point equal to d — n from the neutral axis
*i
or d — -x from the centre of gravity of the
compressional area. Therefore the moment
c b n
of resistance =
('-8
or t At x
d — , and this must be equal to the
c b n
(
bending moment, B M. Thus B M = ^
f n\ f n\
x [ d — o I or B M = Z A( x I « — « I •
V «>/ v «*/
Before any use can be made of these
formulae it will be necessary to establish some
relation between certain factors which are
variable and unknown, namely, c, t, and n.
The factors 6, d, and At can be settled by
b
'i AX 15
will require fifteen times as much stress on
the steel as that on the concrete to produce
this result if the two materials are situated
at the same distance from the 'neutral axis.
This is illustrated in Fig. 119, where it is
assumed for the moment that the neutral
axis is situated at the centre of the depth ;
then the permissible stress on the concrete
is 600 Ib. per sq. in., and the stress on the
steel will equal c x m, which equals 600 x 15
= 9,000 Ib., m being the ratio between
Es and Ec.
It must be clearly understood that
although the deformation does not actually
take place if the beam is properly designed,
there is a tendency for it to do so, and conse-
quently the resistance has to be exerted by
the materials to overcome the tendency.
CEMTRE OP
COMPEt55IONAt
Fig. 117 Fig. 118
Figs. 117 and 118.— How a Single Reinforced Beam Resists Compression and Tension
the designer himself, but the value of n, t,
and c will be affected by the relation or pro-
portion of At to 6 d, and as there are two
unknown factors in each formula it would not
be possible to proceed beyond a certain point.
Again, it will not be possible to stress both
the steel and the concrete to the permissible
limit unless they are in the correct proportion
to one another. The two materials have a
different coefficient of elasticity which in
the concrete is expressed by Ec. and in the
steel by Es, and the ratio between them =
E,
E..
= 15, and this is an important point in
the determination of the stresses. It signifies
that in the case of deformation taking place
where the concrete is compressed by a cer-
tain stated amount (say, 1 in.), while the
steel is stretched by the same amount, it
The permissible stress on the steel is 16,000
Ib., and therefore if the proportion between
the two materials is such that the neutral
axis is at the centre of the depth, the steel
cannot be used to its fullest capacity. It
will be necessary to have the proportion
such that the neutral axis is higher up the
beam, and thus farther from the steel, be-
cause the stress varies directly as the dis-
tance from the neutral axis, and thus, in
addition to the ratio m, it is necessary to
consider the relative distances n and d — n.
If the steel is 2 in. from the neutral axis
while the concrete is only 1 in. from the
same point, then the stress on the steel will
be equal to twice that in the concrete ; or,
in other words, t = c x 2. The same rule
will apply if it is said that the stress in the
steel multiplied by n will equal the stress
THEORY OF REINFORCED CONCRETE
in the concrete multiplied by d — n, or
I x n = c x d — n. Now taking into con-
sideration the ratio m and the distance from
the neutral axis, the result is t n = m c (d —n).
It is allowable to reduce this to give a
relative proportion for n as follows : —
in = m c (d — n)
tn = mcd — men
tn + men = mcd
n(t + me) = mcd
mcd n me
t + me
""
t + me
In the case of this formula there are cer-
tain factors known if c and t are considered
as representing the permissible stress on the
concrete and steel ; thus, m =15, c =600 Ib.,
AX 15
Fig. 119. — Diagram Showing Proportionate
Stresses above and below Neutral Axis to
Produce Deformation
and t = 16,000 Ib. With the aid of these it
is possible to deduce n as a definite propor-
tion of d, as follows : —
mcd
n = —
t + me
15 x 600 Ib. x d
n = rt
n =
16,000 Ib. + (15 x 600 Ib.)
9,000 d
25,000
n = -~~ d = n — -36 d.
Now this is of great value, as it shows that
in the case of a well-designed beam where
t and c are at the permissible limit, then n
= -36 d. It does not necessarily follow that
c and t will be equal to the values given
above, as it has previously been stated that
this will only occur when At and 6 d, the areas
of the steel and concrete, bear a certain pro-
portion to one another.
The next step, then, is to determine this
proportion, and see how the neutral axis will
vary if another proportion is employed.
The ratio of the area of the tensile rein-
forcement to the area b d is expressed by the
symbol r, therefore r = r-v
Now it has already been seen that t At
cbn cbn At
= -s— , so that At = - — . If r = j-j
2 ' 2t bd
cbn cbn
then r also equals 0 . , , . as -^rr is the
Alba At
equivalent of A< and can be substituted for
cbn
it. In the expression r = 0.77, the
symbol b can be eliminated, as it occurs in
c n
both cases, thus reducing it to r = ^~TJ-
An expression is now obtained which will
give the ratio of the steel to the concrete to
develop certain given values for c and t.
In the first place, consider the case where
c = 600 Ib. and t = 16,000 Ib. as above,
where it was seen that n = -36 d, then c, n,
t, and d are known, thus —
600 x -36
r =
T =
r •-
2 x 16,000 x 1
216
32,000
27
= -00675-
It will be seen by this that if the greatest
permissible stress is represented by c = 600
Ib. and t = 16,000 Ib., then n = -36 d and
r = -00675 or At = -00675 b d.
Applying the Formulae. — Befoie pro-
ceeding to the derivation of any more
formulae, it will be as well to apply those
obtained up to the present and become
more or less familiar with their use, to pre-
vent the subject from becoming too com-
plicated for the student who is dealing with
symbols and reasoning which are quite new
to him.
Briefly summarised, the following have
been deduced : (1) The total compression
cbn
and total tension are equal, expressed — ^~
= t At. (2) The moment of resistance
equals either of these multiplied by d - -,
which is the lever arm of the internal forces,
88
C u 71
expressed M R = « x
REINFORCED CONCRETE
- ^ ) , or M R
= Z At x ( d - £ J . (3) These must be equal
to the bending moment, expressed B M =
'•-. £) or BM tAt x
At = -00675 x 8 x 12-5
At = -00675 x 100
At = -675 sq. in.
The moment of resistance can now be cal-
culated according to formulae in deduction
No. 2, namely : —
cbn / _ n>
MR = -
( d - ! j. (4) The position of the neutral MR =
600 x 8 x 4-5
axis is dependent on the coefficients of elas-
ticity, m = 15, and the values of c and t,
expressed n = 7— — , where c = 600 lb.
t "T" 7/1 C
and t = 16,000 lb., then n = -36 d. (5)
The ratio of the steel reinforcement to the
area of the beam will depend on the values
c n
of c and t, and will be expressed r = 2~Td'
when c = 600 lb. and t = 16,000 lb. ;
then r = -00675 or At = -00675 b d.
Calculating Amount of Reinforce-
ments.— As a first example, assume a con-
crete beam 12^ in. deep and 8 in. wide. It is
required to find the amount of steel neces-
sary to reinforce the beam if the working
stresses are to be c = 600 lb. and t = 16,000
lb. per sq. in. Also calculate the moment
of resistance of the beam when so reinforced,
and find what uniformly distributed load it
will carry over a span of 15 ft. if the ends
are supported only.
The values of c and t being known, the
first step is to ascertain the position of the
neutral axis. By using the formula in
deduction No. 4, it will be found n = -36 d.
Therefore n = -36 x 12-5
n = 4-5 in.
Work this out by means of the formula
mcd . . .
n = - for the purpose 01 getting
t •}- m c
quite familiar with the use of same, then n =
15 x 600 x 12-5 112,500
16,000 + 15 x 600 '' 25,000 "
in., as before.
Having derived the value of n, the amount
C YL
of steel willjbe given by r = ^j, and this
should work out at -00675, as c = 600 lb.
and t = 16,000 lb.
600 x 4-5
= 2 x 16,000 x 12-5
2,700
400,000
r = -00675
At therefore equals -00675 b d.
I 4-5\
(12-5 -IT)
~
M R = — £- x (12-5 - 1-5)
MR = 10,800 x 11
M R = 118,800 in.-lb.
Again, working the moment of resistance
according to the tensile reinforcement,
M R = t At x ( d -
/ 4-J
MR = 16,000 x -675 x ( 12-5 - -j
MR = 16,000 x -675 x (12-5 -1-5)
MR = 16,000 x -675 x 11
MR = 10,800 x 11
M R = 118,800 in.-lb., as before.
As the moment of resistance equals 118,800
in.-lb., the bending moment must not
exceed this amount, and the next step is to
ascertain what weight would produce this
bending moment.
In the case of a uniformly distributed load
with supported ends B M = -g-, therefore
118,800 in.-lb. = -g-. w is the unknown
quantity, and I = the span in inches =
15 x 12 = 180.
118,800 in.-lb. = —
118,800 x 8
w "
w = 5,280 lb.
w = 47-14 cwt.
The beam will therefore require -675 sq. in.
of steel reinforcement, and safely carry a
load of 47-14 cwt. over a span of 15 ft.
Calculating Beam for Certain Load.
— In this example, the size of the beam was
given and the reinforcement and the load
were calculated, for the sake of applying
the deductions previously made. In actual
practice it is usually required to design a
beam to carry a certain load or to check
a given beam and ascertain if the safe
stresses on the concrete and steel have been
exceeded. As an example of the latter
THEORY OF REINFORCED CONCRETE
89
instance, the beam just dealt with can be
calculated, when the stresses should agree
with those arranged in the question, namely
c = 600 Ib. and t = 16,000 Ib. The bend-
ing moment to be dealt with is 118,000
in.-lb., and as shown in deduction No. 3,
BM = -o x
/ W\
( d ~~3J-
The value of c will
therefore be found by the formula c =
2BM
2 x 118,800
x I a — „
c =
8 x 4-5 x
237,600
396
= c = 600 Ib.
This amount is correct, and the stress in the
steel can next be calculated as follows : — •
BM — t At xid — ^ J , therefore
BM
t =
A,
t =
118,800
t =
118,800
7.4.25
_
— t — 16,000 Ib.
This, again, agrees with the stress allowed,
and thus the two formulae are obtained
which will give the stresses in the beam
when this has to be checked after it has
been designed.
In the actual designing of the beam, the
load will be a definite quantity according
to circumstances, and the bending moment
will therefore require to be calculated in the
first instance. It will be necessary, there-
fore, to have some equation which will give
a suitable size beam to resist such bending
moment, and also some method of obtain-
ing the position of the neutral axis when
the values of c and t are unknown, such as
would be the case in designing to resist a
given bending moment.
The next step is to establish some relation
between 6, d, and the bending moment.
The draft regulations of the London County
Council limit the minimum depth of a beam
to one-twenty-fourth of the span, but as the
compressional resistance depends on the area
of concrete above the neutral axis and the
resistance must be equal to the bending
moment, it is preferable to calculate from
the bending moment direct without regard
to the span. Referring to deduction No. 3,
begin again with the equation B M = — ~ — x
( d - |Y
Thus 2 B M =c x b x n x
(d - |
The value of c can be taken at 600 Ib., and n
should equal -36 d if the economical per-
centage of reinforcement is to be employed,
this leaving only two unknown factors b and
d. If b is settled as some definite proportion
of d, then this proportion can be substituted
and d expressed as a fraction of B M. A
well-proportioned beam is frequently that in
which b = -6 d, and this will be taken as the
value, hence —
2BM = c x & x n x [d — 7;
2 B M = 603 x -6 d x -36 d x (d-
2 B M = 600 x -6 d x -36 d x (d— 12 d)
2 B M = 600 x -6 d x -36 d x -88 d
2 B M = 600 x -19008 d3
2 B M = 114-048 d3
B M = 57-024 d3
BM
57-024
d
3/
3 V
BM
57-024
To simplify matters and save a great
number of figures in the calculations, it will
be sufficient to use d = ?/ , the dele-
V 57
tion of the decimal figures in this case making
so little difference that it is inappreciable.
The writer is aware that there are many
students who have forgotten the method of
finding the cube root of a number, but when
this is the case the student should use a
table and thus overcome any difficulty on
this point until such time as he shall again
become conversant with the method. In
the absence of a table, the cube root can be
found by trial, but, needless to mention,
this is somewhat of a tedious method.
To illustrate the application of the formula,
it will be advisable to work out an example,
as before, the load and span being given.
It is required to design a beam supported at
the ends to carry a load of 650 Ib. per foot
run over an effective span of 15 ft. It is
assumed that this load includes the weight
of the beam itself, and the safe stresses are
to be 600 Ib. and 16,000 Ib.
9o
REINFORCED CONCRETE
The total load = 650 Ib. x 15 = 9,750 Ib.
id 9750 x 180
Ihe bending moment = -Q- = - — ~ —
B M = 219,375 in.-lb.
3 A
Therefore d = //-
'219375
_57
d = ^3848
d = 15-7 in.
The breadth should be -6 d, therefore
b = -6 x 15-7 in.
b = 9-4 in.
The size of the concrete beam itself has
therefore been found, and the amount of
reinforcement is now required.
According to deduction No. 5, the ratio of
steel to concrete should be -00675. Therefore
At = -00675 b d, b d = 147-58 sq. in., and
At = -00675 x 147-58. At = -996 sq. in.
— say three y|-in. diameter round bars.
This will give the nearest area, always bear-
ing in mind that there should be an excess
rather than a deficiency. The area of one
y£-in. diameter bar = -3712 sq. in., and the
total area of the three bars = 1-1136 sq. in.
The beam as designed has an effective
depth of 15-7 in. To this must be added,
say, 2 in. of concrete to cover the bars,
giving a total depth of 17-7 in. The com-
pleted section should therefore be : depth,
17f in. ; breadth, 9| in. ; reinforcement,
No. 3 y£-in. diameter round rods.
Checking the Beam.— In the case of
beams which have to be checked for the
purpose of ascertaining the values of c and
t, the necessary formulae up to a certain
point are at hand, but some further ex-
planations and deductions are required to
make the data complete. As previously
shown, the stress in the concrete can be
2BM
found by the formula c = - — r>
»-*M)
and the stress in the steel can be ascertained
by t = - „,- The difficulty in
the case of these formulae will be that of
finding the value of n. Previously, the
position of n has been said to be dependent
on the coefficients of elasticity and the
values of c and t.
In the case of checking a beam already
designed c and t are unknown, and the
formula cannot be used, and if the ratio of
steel to concrete is not -00675, then n will
not equal -36 d. The position of the neutral
axis, being dependent upon the comparative
values of c and t, must also be dependent
upon the proportion of steel to concrete,
and as both of these will be given in the
beam as designed, a value for n must be
deduced accordingly.
It has already been shown that t n =
, . . , t m x (d — n)
cm (a — n), and therefore - = — — -.
cbn
Again, — 2
9 t A
= t At, and therefore n =
Now t and c are unknown, in which
C 0
case substitute the value for -, namely,
m (d — n)
Then n =
2 At m (d — n)
and
b n
bn2 = 2 At m (d — n). Here n is on both
sides of the equation, and it must be reduced
as follows : —
b n2 = 2 At m (d — n)
bn2 = 2 Atmd — 2 Atmn
b n2 + 2Atmn=2Atmd
Divide both sides by 6, then
2 Atmn 2 Atmd
^ + 1— — T-
Add
then
(A, wiV
^-)
to both sides of the equation,
2 At m n
~b '
At m\2 2 Atm d
n +
Therefore
A, my _
b I
At m
(Atm
\ *>
+1-
+
/2
•v
~Atmd At m
2Atmd
+
(A1 m\a
m
Finding Position of Neutral Axis.—
A formula is now to hand for finding the
position of the neutral axis in the case of a
beam which has been designed, where the
stresses may or may not be in accordance
with the permissible limits.
Assume, as a first example, that the beam
given in Fig. 120 has been designed to resist
a bending moment of 219,375 in.-lb., and it
is required to ascertain if the stress in the
concrete and steel is in accordance with the
values allowed, namely 600 Ib. and 16,000 Ib.
This is actually the same beam that was
calculated in the previous example, and it
should be noted that the area of the steel
THEORY OF REINFORCED CONCRETE
reinforcement required was only -996 sq. in. ;
but it was increased to 1-1136 to make it
possible to use rods of a practical size, and
it is for the latter area that it must now be
calculated. The sizes of the concrete are
also the practical sizes, such as would appear
on a drawing of the beam, when the theo-
retical sizes would not be available to anyone
checking the stresses in the ordinary way.
The first step is to ascertain the position
n
R
Fig. 120. — Finding Position of Neutral Axis
of the neutral axis, and this will be done by
means of the formula given above, namely : —
» = V
••V-
2 At md
V b )
2 x 1-1136 x 15 x 15-75
9-5
1-1136 x 15
9-5
•/l-1136xl5V
V 9-5 )
n =
526-176
16-704\2 16-704
9-5 '_ \ .9-5 / 9-5
n = N/55-386 + (1-758)2 - 1-758
»= v/58-476|-l-758
n = 7-64 - 1-758
n = 5-882 in.
It will be seen that this result differs
slightly from that which would have been
obtained by taking n = -36 d, as in the
latter case this equals -36 x 15-75 = 5-67 in.,
and this difference has arisen on account of
At and b d having been altered to give prac-
tical sizes. Having now obtained the value
of n, the values of c and t can be found as
follows : —
2BM
c = -
b n :
c =
2 x 219375
c =
9-5 x 5-882 x
438750
9-5 x 5-882 x
438750
13-70
A< x ( d - o
~ 770-57
c = 569 Ib.
This is below the permissible stress of 600 Ib.
per sq. in., and the value of t may now be
determined : —
BM
t = -
t =
t =
t =
t
219375
1-1136
219375
"•('
5-882
1-1136 x
219375
15-356
14285 Ib.
13-79
This value, again, is well below the per-
missible stress, and consequently the beam is
quite satisfactory for the load it has to carry.
It is interesting to note that the area of
steel required theoretically was increased by
about 12 per cent, to give a practical area,
and this -reduced the stress on the steel per
square inch by about 10 per cent., and the
stress on the concrete by about 5 per cent.
In the draft regulations of the London
County Council, it will be seen that the
formula for finding the position of the
neutral axis is given as :
n = [ v''m2 r2 + 2 m r) —mr]d
and a note in explanation of this should be
of interest to those who are not sufficiently
advanced to trace the construction and
evolution of the formula. The symbol r, as
previously stated, equals the ratio of steel to
concrete, and the formula is constructed to
use this ratio for finding the position of n
when c and t are unknown. Upon reference
to the reasoning employed in deducing the
previous formula for the position of n, it will
be seen that b nz = 2Atm(d — n). Therefore
2 At m (d —n)
n2 = - — r-
REINFORCED CONCRETE
Now in this case At is also unknown, and is
simply available as a proportion of the con-
crete, such proportion being expressed as r.
We can therefore substitute this equivalent
and obtain
2 _2rbdm(d — n)
Hence —
n2 = 2rd2m—2rdmn
n2 + 2r dmn = 2r d2 m
To both sides of the equation add (r d m)2.
Then
n2 + 2rdmn + (r d m)2 = (r d m)2 +
2rd2m.
The left-hand side of the equation can
now be simplified as before, because n +
r d m is the square root of same, and therefore
(n + r dm)2 can be substituted as its value,
giving (n +rdm)2 = (rdm)2 + 2rd2m.
Hence —
n+rdm=*J(rd m)2 +2rd2m
n = \/ (r d m)2 + 2rd2m —rdm
n =[v/(r m)2 + 2 r m — r m] d
This, of course, is the same as the formula
given by the London County Council,
namely :
n = [v/wi2 rz +2mr — mr\d
In the previous notes it was shown that
when r = -00675, which is the economical
proportion of steel to concrete, then n
would always equal -36 d. It will be advis-
able, therefore, to use the above formula and
find what proportion n will be of d if the
economical amount of steel is used as an
illustration of the use of the formula, as
follows : —
n = [v/wi2 r2 +2mr — mr]d
m — 15, and r = -00675, therefore
n = [v/152 x -006752 + 2 x 15 x -00675
- 15 x -00675] d
n = [^225 x -0000455625 +-2025 —10125] d
n = [x/ -0102515625 + -2025 - -10125J d
V/-2127515625 - -10125] d
n = -46125 - -10125]<2
n = -36 d
A great number of figures are retained
after the decimal point with the object of
showing that the result absolutely agreed
with that previously given. Formulae, there-
fore, are available for finding the value of
n in all cases, namely (1) when c and t only
are known, (2) where the size of the beam
and area of reinforcement are known and c
and t are unknown, and (3) when c and t are
unknown and the area of the steel is expressed
as a proportion of the area of concrete.
Designing Beam for Certain Condi-
tions.— As a final example, showing the
application of the various formulae, a case
will be taken where it is required to design
a beam to carry a uniformly distributed
load of 350 Ib. per foot run, in addition
to its own weight, over a span of 24 ft.
When designed the stresses in the steel and
concrete will be checked to ascertain whether
they are in accordance with the permissible
limits.
It will here be seen that the weight of the
beam itself has to be added to the external
load before the bending moment can be
found, and as the size is not yet known, an
assumption must be made in order to do
this. A little experience will enable the
designer to gauge the size sufficiently near
the actual size required to give a weight
for the beam. In this instance the depth will
be taken at 16 in., which is one eighteenth
of the span, and b can be taken at 10 in.,
which is just over -6 d. To the amount of
d 2 in. should be added as a covering for
the reinforcement, giving a total size of
18 in. by 10 in. The weight of the beam
will be its cubical contents multiplied by
150 Ib. per cub. ft. This equals 24 ft. x
1-5 ft. x -63 ft. 'X 150 Ib., or for 1-ft. run
= 1-5 ft. x -63 ft. x 150 Ib. = 141-75 Ib.,
say 142 Ib.
The total load to be carried per foot run
therefore equals 350 + 142 = 492 Ib. This,
multiplied by the span, will give the total
load on the beam, thus 492 x 24 = 11,808
Ib. The bending moment will equal -^-.
BM =
11808 x 24 x 12
B M = 425,088 in.-lb.
The moment of resistance of the beam must
equal 425,088 in.-lb., and the depth must first
3/
be calculated by the formula d = A/
BM
57 '
d =
425088
d = ^/7457
d = 19-5 in.
Take 6 at -6 d = 19-5 x -6 = 11-7 in. The
area of steel will be given by taking the
economical ratio, namely -00675.
At = -00675 b d
At = -00675 x 11-7 x 19-5
At = 1-54 sq. in.
THEORY OF REINFORCED CONCRETE
93
If three bars, |f in. in diameter, are taken,
this will give an area of 1-55 sq. in., which
will be quite suitable. The beam, as now
designed, has an effective depth of 19-5 in.,
with a breadth of 11-7 in. and steel reinforce-
ment of 1-55 sq. in. Add 2 in. to the depth
for covering the reinforcement, and make
the breadth a practical size, and the beam
becomes say 22 in. by 12 in.
Checking Beam after Designing.— It
is now required to check the stresses in this
beam, and as the actual size of the beam is
now available it will be advisable to work
out the bending moment again, because the
calculated size is in excess of that assumed
in the first instance.
Therefore the weight of the beam per foot
run - If ft. x 1 ft. x 150 Ib. = 275 Ib.
Total load per foot run = 350 + 275 = 625
Ib. The total load to be carried equals the
load per foot run multiplied by the span =
625 x 24 = 15,000 Ib. The bending moment
wl , „_, 15000x24x12
= -A-. Therefore B M = ^
o o
= 540,000 in.-lb.
The position of the neutral axis must next
be found by the formula
/2 At m d /At m\2 Atm
/2 x 1-55 x 15 x 19-5 /1-55 x 15\2
n=V ~W~ H 12 /
1'55 x lo
12
t =
BM
t =
540000
1-55
t =
t =
540000
1-55 x 17-18
540000
m.-lb.
/906-75 /23-25X2
= V 12 ' \~12~)
23-25
12 \ 12 / 12
n == V75-562 + 3-751 - 1-93
n = v/79-313 - 1-93
n = 8-9 - 1-93
n = 6-97 in.
The stress in the concrete can now be
found by the formula
2BM
c =
b n >
c =
c =
2 x 540000
12 x 6-97 x
1080000
12 x 6-97 x 17-18
1080000
= := c~-~- 7531b"
This is too high, as the permissible limit
is 600 Ib.
Now check the stress in the steel.
This, again, is much too high, as the per-
missible limit is 16,000 Ib. for ordinary mild
steel, and even for stronger steel the stress
should not exceed 20,000 Ib.
This example was given to show the value
of checking the stresses after the beam has.
been designed, when the size has to be
assumed, and consequently an error of judg-
ment may take place, especially in the case
of those inexperienced in design.
The next step is to see in what manner the
beam can be adjusted to bring the stresses
within the limits. The weight of the beam
allowed in the first instance was insufficient,
it being only 142 Ib. per foot run, whereas
the calculated beam was 275 Ib. per foot run.
Here, however, is some basis upon which to
assume the size, and if the depth is increased
to 22 in., and the breadth to -6 d = say
13-5 in., they cannot fail to be near the
required size.
This may seem a great deal of work and
calculation in connection with one beam,
but it must be borne in mind that the
example is merely given to illustrate princi-
ples and familiarise the reader with the
formulae employed.
Again, in a building where the plan is
symmetrical, it usually happens that there
are several beams which have the same span
and carry the same amount of load, and con-
sequently when the calculations for one beam
are complete, many beams can be considered
as designed, and it is worth some little
trouble to produce economical and efficient
design.
The reinforcement might be increased in
this example, and thus the strength of the
beam increased, but the economical per-
centage would not then be employed.
Of course, it is quite simple to design a
member which is perfect and suitable for
carrying a certain load as given in various
textbooks, but a great number of these
examples are done by selecting a beam with
a certain ratio of reinforcement, finding the
94
REINFORCED CONCRETE
moment of resistance, and working back-
wards, as it were, to find the safe load per
foot run over a certain span. This method,
of course, is not stated, but the examples
frequently given in the textbooks are based
on the particulars obtained, and thus no
difficulty is encountered.
The object of the above example is rather
to show how to deal with a case where the
initial calculation is found to be incorrect,
and to show in what manner these may be
utilised in the second calculation. It will
be seen that in the first instance the depth
was based on the span, as the bending
moment was unknown, and it goes to show
how essential it is to have some connection
between the depth and the bending moment
if economical design is to result. However,
the actual bending moment to be resisted is
known when the beam had an effective depth
of 19-5 in. and a breadth of 12 in., namely,
540,000 in.-lb. Calculate d according to
this as a guide by the formula d =
BM
57 '
then d =
540000
57
= d = y 9473, d =
21 in. nearly. It must be realised, however,
that the fact of increasing the size of the
beam will again increase the weight of the
beam, and therefore the bending moment,
and an allowance must be made for this ;
and for this reason it will be advisable to
take d at 22 in. as stated above, and b at
•6 d = say 13-5 in.
The beam can now be worked out and
again checked to see if the stresses are quite
satisfactory. The size of the beam = 22 in.
x 13-5 in., and the sectional area = 297
sq. in. At should equal -00675 b d. This
equals 297 x -00675 = 2-00475 sq. in.
The area of a round rod with a diameter
of || in. = -6903 sq. in., and 3 rods =
2-0709 in., which is more than sufficient.
Provision must be made for covering the
bars with concrete, and if 2 in. be given, the
total depth becomes 22 +2 = 24 in.
Having now designed the beam, the actual
stresses can be checked, and to do this the
actual bending moment must be found due
to the given load to be carried, and the
weight of the beam itself.
The latter per foot run = 2 x 1£ x 150 Ib.
= say 337 Ib. The total load per foot run
= 350 + 337 = 687 Ib. The total load =
load per foot run x span = 687 Ib. x 24
= 16,488 Ib. The bending moment =
Mel 16488 x 24 x 12
—r, therefore B M = — ~ —
= 593,568 in.-lb.
The position of the neutral axis must next
be found by the formula
2Atmd
/£ ±±t
n=\/—b— +
Lrw\2 _ At m
b ) b
/2 x 2-0709 x 15
x22^
C1
0709
x!5>
2
/ 13-5
2-0709 x
15
(
13-
5 ;
13-5
/ 1366-794 /31-0635\2 31-0635
ss » / . _ _1_ I _ I _ _
V 13-5 ^V 13-5 / 13-5
n = v/ 101-24 +5-294 - 2-301
n = 10-32 - 2-301 =n = 8-01 in.
The stress in the concrete can now be
found by the formula
2BM
b n x
(-9
2 x 593568
c —
13-5x8-01
1187136
c —
13-5 x 8-01 x 19-33
1187136
2090-2
c = 567-9 Ib.
It will be advisable to check the stress in
the steel as follows :
BM
t =
A1 x
t =
('-8
593568
2-0709 x
t =
t =
593568
2-0709 x 19-33
593568
40-03
t = 14,828 Ib.
This amount is well below the permissible
limit of 16,000 Ib. The beam as designed
will therefore have a total depth of 24 in.,
a breadth of 14 in., and be reinforced with
three -J-g--in. diameter rods.
THEORY OF REINFORCED CONCRETE
95
Numerous problems and examples should
be worked out with a view to becoming quite
proficient in the use of the formulae, and
conversant with the method of making
assumptions which are necessary" in some
cases to form a basis for the calculations,
since, although the principles never vary,
the method of procedure is likely to do so
under different circumstances.
The beam would also be required to be
calculated for shear and adhesion, but these
will be dealt with subsequently. In any
examples shown up to the present any
stress other than that due to simple bending
has been disregarded.
CALCULATIONS FOR SLABS
Slabs can be dealt with in a similar manner
to beams as regards the calculations of
strength, but the value of the bending
moment will vary according to the propor-
tion of the slab, if the slab is supported on
all four sides ; the formula for finding the
depth in relation to the bending moment
must be varied, as b will no longer be equal
to -6 d, but will always equal 12 in., as a
strip of this width is calculated. In the case
of a slab supported on all four sides and
reinforced in two directions, the weight will
be distributed on all four supporting edges,
and consequently if a strip of the slab 12 in.
wide is taken and the bending moment cal-
culated, such bending moment can only be
due to the reactions which occur at the ends
of the strip. If the total weight upon the
slab is distributed upon all four edges, it
will follow that the reaction at any given
point will be less than if the slab were sup-
ported on two sides only. A reduction in
the bending moment must therefore be
made to allow for this. If the slab is per-
fectly square, all four edges will carry an
equal amount, and the reaction at any one
side will be one-half the amount that would
occur if the slab were supported on two sides
only. If the reaction is only half the
amount, then the bending moment will be
reduced by a similar factor.
The bending moment at the centre of a
beam or slab supported at the ends is equal
w I
to -£-, and therefore the bending moment
at the centre of a square slab, supported on
wl
all four edges, is equal to -TTT. Again, the
bending moment at the centre of a beam or
slab which is fixed at the ends is equal to
-, -
, while with a square slab fixed on all four
.TIT W
edges, the bending moment is equal to -^
These formulae, of course, apply to cases
when the load is uniformly distributed.
This advantage may be considered as occur-
ring when the length of the slab does not ex-
ceed twice the breadth ; when the length
exceeds this proportion, the slab is considered
as one which is supported or fixed on two
edges only. The advantages will vary be-
tween these two cases, namely, a square
slab, and a slab where the length is twice
the breadth, and the variation will depend
upon the ratio of the length to breadth.
The draft regulations of the London County
Council give formulae showing the allowance
that may be made as follows : —
w = weight on slab (total distributed
weight, including its own weight).
b = breadth of slab.
I = length of slab.
Bending
Condi-
B.M.
B.M.
moment at any
tion
for
for
given
of
shorter
longer
cross section
supports
span
span
wb 1
wl 1
At centre of span
Free
8 l + (b/l)*
8 l + (l/b)*
At end and
centre of span
Par-
tially
fixed
wb 1
wl 1
12 l + (6/J)4
12 1 + (*/&*)
Bending Moment at Centre of Slab.
— It will be advisable to give an example to
illustrate the use of these formulae and show
the decrease in the bending moment which
is given by same. Assume a slab 8 ft. by
6 ft., which is supported on all four sides,
and carries a uniformly distributed load of
250 Ib. per sq. ft., and let it be required to
calculate the bending moment at the centre
of the span in both directions, namely, (1)
by considering a portion 1 ft. wide and
6 ft. long, and (2) by considering a portion
1 ft. wide and 8 ft. long.
By referring to the table above, it will be
seen that the formula for the shorter span
is X w = the total
and this will equal the area x by the weight
per ft. super. Therefore w = 6 ft. x 1 ft. x
250 Ib. ; w = 1,500 Ib. ; 6 = the breadth
96
REINFORCED CONCRETE
of the slab, which is 6 ft. ; I = the length
of the slab, which is 8 ft. Hence,
t0& ^L
8 X 1-
1500 Ib. x 72 in. v 1
; . X —
1
= 13500 x
= 10,258 in.-lb.
1-316
The allowance made in this instance is
sufficient to reduce the bending moment
from 13,500 in.-lb. to 10,258 in.-lb.
Now calculate the bending moment for
the longer span, according to the formula
wl 1
In this case w = 8 ft. x 1 ft. x 250 =
2,000 Ib. Hence, B M = —
O i
B M = 24000 x
1
- 24000 x
4-16
1 + 3-16
B M = 5,769 in.-lb.
The reduction is considerably more in this
instance, due, of course, to the fact that
the edges of the short span will carry a
greater portion of the load than the
edges of the long span, and as the reactions
will be less, so the bending moment will
be reduced.
With regard to the thickness of the slab,
this should be calculated according to the
bending moment, and the following formula,
with its construction, is given for this pur-
pose. It has already been seen by previous
(n\
d ~~ Q )'
./
and that it is possible to substitute definite
values for certain symbols as follows : b
= 12 in., c = 600 Ib., and n = -36 d, thus
reducing the formula to
2 B M = 12 x 600 x -36 d x (d -
2 B M = 12 x 600 x -36 d x -88 d
2 B M = 7200 x -3168 d2
2 B M = 2280-96 d2 = B M = 1140-48 d2
/ BM
= V 1140-'
M
1140-48
It will be quite near enough to use the
/B~M
lormula as a == /y/ VMA' ™e omission of
the decimal figures making no appreciable
difference in the result. In the case of a slab
which is supported on all four edges, it has
been shown that it is necessary to calculate
the bending moment for both spans, and as
the shorter span will give the greater bending
moment, as shown by the example above, the
depth should be calculated to suit this, and
as this will be in excess of that required for
the longer span, the longitudinal reinforce-
ment required will be diminished accord-
ingly. Again, it will obviously be an advan-
tage, theoretically, to place the reinforce-
ment across the slab below that parallel
with the length of the slab, in order to obtain
the maximum effective depth to resist the
greater bending moment.
If the depth is found by calculation to be
less than 3| in., it should be increased to this
amount for practical purposes, as any thick-
ness under this is likely to be unreliable.
The least diameter or thickness of the main
reinforcement should not be less than \ in.,
and any other reinforcement should not be
less than \ in. in diameter. The maximum
distance between the main tensile reinforce-
ment should not be greater than 12 in., or
less than 1 in., and the spacing of the rein-
forcement, when the load is uniformly dis-
tributed, may be gradually increased from
the middle third to the outer edges, if the
slabs of the bars are kept of a uniform size,
provided that the spacing at the outer edges
be not greater than three times the spacing
at the centre of the slab.
Calculating Slab Supported on Four
Sides. — As an example of slab design, let
it be required to calculate the thickness and
reinforcement for a slab 12 ft. by 8 ft., sup-
ported on all four sides to carry a load of
275 Ib. per square foot in addition to its
own weight. The stresses in the concrete and
steel not to exceed 600 Ib. and 16,000 Ib.
respectively.
The weight of the slab itself will require
to be found in the first instance, as this
must be added to the external load in order
to ascertain the bending moment. Assume
a total thickness of 6 in. for this purpose,
and the weight per square foot will then be
1 ft. x 1 ft. x -5 ft. x 150 Ib. = 75 Ib.
The total load per square foot is equal to
275 Ib. + 75 Ib. = 350 Ib. It is now re-
quired to calculate a portion of the slab,
8 ft. long and 1 ft. wide, and the area will
equal 8 sq. ft. The total load on this por-
tion will be the area multiplied by the weight
per square foot ; thus 8 x 350 = 2,800 Ib.
total load. The bending moment can now
be calculated, bearing in mind that the slab is
supported on all four sides, and the formula
i /A/\4 must be employed. There-
THEORY OF REINFORCED CONCRETE
97
fore the bending moment
2800 x 96 in. 1
will equal
1 +
BM = 33600 x
1-197
B M = 28,070 Ib.
Having now obtained the bending moment,
the depth can be calculated from it by the
formula previously given, namely :
/BM
= V i
d
1140
d = V2.4'8
d = 4-9 in.
To use the economical percentage of rein-
forcement, the sectional area must be made
equal to -00675 6 d, and this, therefore, will
be -00675 x 12 x 4-9 = -3969 sq. in. The
area of one £-in. round rod equals -1963
sq. in., and two rods will give a sectional
area of -3926 sq. in. This is such a small
fraction under the required amount that
it can be used, and as d was found to be
equal to 4-9 in., this may be increased to 5 in.
for practical purposes. As there will be
two ^-in. rods for every foot in width, they
will be spaced at 6-in. centres, and this will
be quite satisfactory. By adding 1 in. of
concrete to the effective depth of 5 in., in
order to afford covering for the reinforce-
ment, the total depth will be equal to 6 in.,
which exactly agrees with the assumption
made in the first instance.
It will now be necessary to check the
stresses in the steel and concrete to see that
they do not exceed the permissible limit.
The weight of the slab having been cor-
rectly assumed, it will not be necessary to
recalculate the bending moment, as in the
case of the beam designed in the previous
example, and the position of the neutral
axis is the only unknown factor. This will be
found by using the formula already given : —
/2 Atmd /At m\ 2 A< m
V ~b +\~b ~) ~~b~
/2 x -3926 x 15 x 5 /-3926 x 15\2
=v IT- -+(~^-)
•3926 x 15
= yx 4-9075 + (-49075)2 - -49075
= y/ 4-9075 +:2408 - -49075
- v/ 54483 - -49075
- 2-26 - -49075
= 1-769
7
It is interesting to compare this value for
n with that which would be given by taking
same as -36 d, such as would be the case if
the stresses were known to be 600 and
16,000 Ib. respectively ; then it would equal
•36 x 5 in. = 1-8, which is very near.
To check the stress in the concrete will
be the next step.
2BM
c =
bn
x I a — 7:
c =
2 x 28070
12 x 1-769 x
56140
C = , r~
_ 56140
* C 93-4
12x1-769x4-4
c = 601 Ib.
This can be considered as satisfactory, and
the stress on the steel be checked by the
formula :
BM
t = —
t =
At x Id —
28070
28070
1-727
•3926 x 4-4
t = 16,253 Ib.
This is slightly in excess of the permissible
limit, and to overcome this the spacing of
the bars may be slightly reduced, say from
6-in. to 5-in. centres. The student is advised
to do this, and again check the value of t,
as these examples are merely given for the
purposes of illustration, and constant prac-
tice and experience will be the best asset of the
designer when he has once mastered the prin-
ciples and formulae which govern his design.
It is necessary now to find the longitudinal
reinforcement that will be necessary, and for
this purpose the bending moment must be
worked out for the longer span. As shown
above, the total load per square foot =
350 Ib., and the load on a strip 1 ft. wide
and 12 ft. long will therefore equal 350 x 12
= 4,200 Ib. The bending moment will be
found by the formula for the longer span ,
i.- -u • wl l
which is -tr x —
BM =
4,200 x 144 in.
1 +1-=
BM = 75COO x
BM
1 + 5-06
75600
6-06
= 12,475 in.-lb.
98
REINFORCED CONCRETE
It will not be necessary in this instance to
calculate the depth, as this is given by the
calculations for the shorter span, when it
was found to be 5 in., and as the longitudinal
reinforcement will be placed on the top of
the cross-rods, d will be reduced by the dis-
tance between the centres of the two sets
of rods, which will be £ in. if the longitudinal
rods are of the same diameter. It will be
better to keep them the same size if pos-
sible and space them farther apart. The
depth will then be 5 in. — -5 in., which
equals 4-5 in. In order to make quite sure
that this is sufficient, d may be calculated
according to the bending moment given and
the results compared. As before :
:
1UO
12470
1140
d =
d = VlO = 3-1 in.
This is less than the value already obtained,
and the amount of 4-5 in. may be worked to,
which will have the effect of reducing the
amount of reinforcement required, as the
lever arm of the internal resistances will be
increased. This being so, seek for some
method of deducing the value of At when
d is given and is in excess of that required
by calculating same from the bending
moment. The bending moment is known,
the depth is known, and n should be equal
to -36 d if the stresses are to be satisfactory,
and the formula for finding t, which is
BM
t = - —/ — — r, can be used. If, how-
At x (d -
ever, a value is given to t of 16,000 Ib., then
there is only one unknown factor, namely
At, and this can be found as follows : —
16000 =
A, =
-36 d
12475
16000 x (d - -12 d)
12475
16000 x -88 d
12475
12475
63360
16000 x -88 x 4-5
At = -1968 sq. in.
The area of one £-in. diameter bar equals
•1963 sq. in., and therefore this size can be
employed and spaced at 12-in. centres,
which is the maximum permissible limit for
tensional bars.
Checking the Slab. — It will now be
necessary to check the stresses in the con-
crete and steel, and in order to do this the
position of the neutral axis must first be
found as follows : —
2 At m d
/A^mY-
I b~)
At m
/2x -1963x15x4-5 /-1963xl5
: V ~i2~~ ' \
12
•1963 x 15
12
n = v/2-2083 + (-245S)2 - -2453
n = x/2-2684 - -2453
n = 1-5 — -2453 = n = 1-254 in.
For checking the stress in the concrete :
2BM
C = - —T-
bn x (d —
c =
2 x 12475
12 x 1-254 x
24950
c =
24950
61425
12 x 1-254 x 4-082
c = 406 Ib.
This is well below the permissible limit,
but t should be checked, and this will pro-
bably be very near the amount of 16,000 Ib.
BM
t =
-(•-9
12475
t =
•1963 x
12475
•1963 x 4-082
_ 12475
:
t = 15,568 Ib.
This amount is not very much below the
permissible value, and it will not be advis-
able, therefore, to decrease the reinforce-
ment in any way, and the slab can be con-
sidered as satisfactorily designed. It will
be impossible to stress the concrete to its
full extent without overstressing the steel,
as the depth, which influences the amount,
of concrete is in excess of that theoretically
required, as previously explained. Figs. 121
and 122 show how the dimensions are taken.
It is interesting to note what the result
would be if the area of the reinforcement
had been based on the theoretical depth
instead of taking advantage of the depth
given, and for the guidance of the student
THEORY OF REINFORCED CONCRETE
99
this will be worked out. It lias been seen
that this depth was 3-1 in., and the theoretical
area of the concrete in section therefore
equalled 3-1 x 12 in. - 37-2 sq. in. If At
was made equal to -00675 b d, then it would
be 37-2 x -00675 = -2511 sq. in. This is
6'l d=S'
/LONGITUDINAL BARS
Fig. 121. — Section Through Gross Rods
greatly in excess of the amount actually
required, and as the stresses as shown above
are even now below the permissible limits,
it is obvious that they would be far more
so in this case, and the design would not be
economical.
In the case of slabs which are fixed, the
bending moment at the ends of the spans
must also be calculated, and reinforcement
provided on the upper surface to take the
tension which will occur. This is effected
by bending up some or all of the bars in
the lower surface when they are no longer
required in the latter position ; or short
bars may be placed on the upper surface
and carried for the necessary distance into
the slab. The turning up of the lower bars
will assist in the resistance to shear, and
this will be more fully explained and illus-
i>=/2"-
2, PODS
^CgO
jr- —
12'
Fig. 122. — Section Through Longitudinal Rods
trated in the notes dealing with this por-
tion of the subject. It must be borne in
mind, however, that it will not be sufficient
to calculate the bending moment in the
centre of the span only, and although the
same theory and principles will apply in
both cases, the student is advised to take
an example of a fixed slab and calculate
the reinforcement necessary at the various
points for the purposes of practice.
BEAMS WITH DOUBLE REINFORCE-
MENT
With regard to the beams and slabs
described up to the present, no restrictions
have been placed on the size of the members,
and consequently the depth in some in-
stances has been such as would cause great
inconvenience in buildings of limited height.
To allow of beams of less depth being em-
ployed, it is necessary to introduce double
reinforcement, which consists of two sets of
rods, one of these taking the tension as
before, and the other taking a certain pro-
portion of the compression which the reduced
quantity of concret3 is unable to resist.
Another consideration is the weight of the
beam. In the case of long spans, a beam
with single reinforcement may be so heavy
that its weight is greater than the external
load to be carried ; the introduction of com-
pressional reinforcement reduces the size of
the beam necessary and thus reduces the
total load to be carried. This is an impor-
tant consideration, as not only is the bending
moment on the beam reduced, but so also
is the load to be carried by the supports
and the foundations.
The procedure in the design and calcu-
lations is somewhat different from that
employed in the case of beams with single
reinforcement, as with the latter it is neces-
sary to begin with an assumption regarding
the size of the beam required, in order to
calculate the bending moment ; but now a
definite size can be fixed for the beam at the
outset and suitable reinforcement inserted.
This size should be made as near the econo-
mical section as the circumstances will per-
mit, if there is any choice in the matter.
As the size will thus be available at the out-
set, the weight of the beam can be found and
the bending moment calculated. This bend-
ing moment will be the one to be resisted,
and there will be no variation or re-calcula-
tion on this point.
The next step will be to consider the
strength of the beam according to the fixed
size by adding the economic proportion of
einforcement, and considering it as a single
reinforced beam, and then calculating the
bending moment that such a beam will
resist. It will necessarily be less than the
actual bending moment to be resisted, and
the difference between the two will give the
100
REINFORCED CONCRETE
excess that has to be taken by the com-
pressional reinforcement and the extra ten-
sional bars that will be required over and
above the economical percentage.
Figs. 123 and 124 will be some guide as
to the disposition of the reinforcement, and
the meaning of the symbols employed.
It is necessary to introduce a certain
number of new formulae, owing to the new
factor in the compressional resistance, and
these can be explained in the simplest
manner by taking an example and working
it out step by step with the necessary
explanation as it becomes due.
Designing Beam with Double Rein-
forcement.— A beam is required to carry
a uniformly distributed load of 10 tons in
covering the reinforcement, and this can be
taken at 2 in., thus giving an effective depth
of 20 in. and a breadth of 12 in.
The economical amount of reinforcement
will be found by multiplying the effective
sectional area of the concrete by -00675,
thus 20 x 12 x -00675 = 1-62 sq. in. The
moment of resistance of this beam will be
equal to the sectional area of the steel multi-
plied by the safe stress per square inch
multiplied by the lever arm of the internal
/ n
resistance expressed as At x t x (d —
and the bending moment will equal the
moment of resistance, therefore
BM = At
d
x
•- I
n
-A
d-ff
Fig. 123 Fig. 124
Figs. 123 and 124. — Double Reinforcement in Beams
addition to its own weight over a span of
16 ft., while the over-all size is limited to
22 in. by 12 in. The actual weight of the
beam must first be calculated in order to
find the total load. This will equal 1|- x 1
x 150 lb. x 16 ft. = 4,400 lb., and the total
load = 10 x 2240 + 4400 = 26,800 lb. The
w I
bending moment will equal -?r = B M =
26800 x 16 x 12
— g— - = B M = 643,200 in.-lb.
The actual bending moment to be resisted
has thus been obtained, and it is now re-
quired to ascertain the moment of resist-
ance of the specified beam if same is rein-
forced in the tensional area only, and see
how far it is deficient. As the over-all depth
is 22 in., an allowance must be made for
To find the value of n, take this as -36 d
from previous deductions, therefore n =
•36 x 20 = 7-2 in., and
BM = 1-62 x leOOO x
B M = 1-62 x 16000 x 17-6
B M = = 456192 in.-lb.
The actual bending moment, however,
equals 643,200, and the excess which has to
be provided for by extra tensional reinforce-
ment, and compressional reinforcement =
643200 - 456192 = 187008.
It may assist the student if the moment
of resistance is worked out with regard to
the concrete, in order to show that the same
result is obtained and that the compressional
reinforcement is necessary. Considering the
concrete as taking 600 lb. per sq. in., this
THEORY OF REINFORCED CONCRETE
101'
C X
x n
= 25920 x
gives MR =
600 x 12 x 7-2
~^T " \~" 3
17-6 = 456192 in.-lb. as before. It is
therefore obvious that the compressional
reinforcement will be required to take the
excess of 187008 in.-lb.
The position of this compressional rein-
forcement must now be settled in order to
determine the lever arm between the two
sets of reinforcement. It is advisable to
place this as far as possible from the neutral
axis while obtaining a covering of concrete,
and this will be given if it is situated at a
distance of 2 in. from the upper surface, as
= as A/ =
N
20'
12'
r*
12' 8
Fig. 125. — Designing Beam Having Double
Reinforcement
shown in Fig. 125. There are thus two sets
of reinforcement, one situated at a distance
of 5-2 in. from the neutral axis and the
other at a distance of 12-8 in. from the same
point.
The distance from the upper surface to
the centre of the compressional reinforce-
ment is expressed by the symbol dc, and
the distance apart or lever arm of the two
sets of bars will be d — dc. Considering,
first, the additional tensile reinforcement
required, the limiting stress, which is
16,000 Ib. per sq. in., is known, and also
the lever arm, which is d — dc. If, there-
fore, the excess bending moment is divided
by t, x (d — dc), the amount of steel neces-
sary will be found. This may be expressed
Excess B M
t x (d - d~Y
Working this out
gives —
A/ =
A/ _
187003
16000 x(20
187003
-2)
/
t —
288000 ~ A
•'
This additional reinforcement must be
added to the amount already calculated for
in the tensional area amounting to 1-62 sq. in.
making a total of 2-269 sq. in.
The value of the compressional resistance
must also equal the excess bending moment,
and if the area of the steel in compression
be expressed by the symbol Ac and the
intensity of the stress by the symbol c,,
then Ac x cs x (d — dc) = excess B M and
Excess B M
c ' '' c — (d — d Y value of c.,. will
require to be found before proceeding further,
as although the stress intensity could safely
be 16,000 Ib. per sq. in. in compression as
in tension, it will not be possible to develop
this amount on account of the steel being
so much nearer the neutral axis in the com-
pression area than in the tension area. The
stress varies directly as the distance from
the neutral axis, and therefore, if the com-
pression steel is placed twice as near to the
axis as the tension steel, then it can only
develop one-half the stress taken by the
latter. In other words, the stress on the
compression steel multiplied by the distance
of the tension steel from the axis must equal
the stress in the tension steel multiplied by
the distance of the compression steel from
the axis. Expressed by symbols, this reads
Cg x (d — n) =tx (n — dc), therefore o =
t x (n - dc)
— -j-^ . In this example —
16000 x (7-2 - 2)
c* =
20 - 7-2
832000
12-8
c, = 6,500 Ib.
Again, this may be shown in another
way, as when steel and concrete are acting
together in compression the stress in the
steel can only reach m times that in the
surrounding concrete, as it is only under
these conditions that they can act together.
The stress in the concrete at the extreme
upper edge is equal to 600 Ib., and conse-
quently at a distance of dc from the point
it will be less, as it is closer to the neutral
102
REINFORCED CONCRETE
cxn- d
axis. The stress will be equal to
which expresses proportion of stress at this
point. The value of cs as above will equal
m times this amount, and therefore —
m x c x (n — dt)
15 x 600 x (7-2 - 2)
7-2
cs = 6,500 lb., as before.
Excess B M
The formula Ar = - —TJ j-\ can now
cs x (d - dc)
be worked out and the compressional steel
found, thus :
187008
A' == 6500 x (20 - 2)
187008
A' ~'~~ llTOOO
Ac = 1-598 sq. in.
The beam as designed, therefore, has an
over-all size of 22 in. by 12 in., with an
effective depth of 20 in., and the reinforce-
ment consists of 2-269 sq. in. in the tensional
area and 1-598 sq. in. in the compressional
area.
BEAMS AND SLABS WITH DOUBLE
REINFORCEMENT
In checking the stresses in a beam already
designed and containing double reinforce-
ment, it is natural that the formulae should
be a little more complicated than in the
case of single reinforced beams, as there are
more factors to consider in the resistance.
In the case of the stress on the concrete
there is some portion of the compression
taken by the steel in the upper part of the
beam, and this, obviously, must be con-
sidered in arriving at the value of c. It will
be necessary to see how the formula for
finding c is constructed. As a basis for
the reasoning, there is the theory already
evolved, namely :
cbn I n\
— qr~ x ( d — Q I and Ac x cs x (d - - d.)
Zi \ O /
together equal B M. This can be stated as
follows : —
2 B M = c b n (d - * ) + 2 Ac. x c, x (d- df)
\ 6'
The factor cs will be unknown until the
compression stress on the steel is found, but
its equivalent value, as previously shown,
c x m x n — dc
is - — , and this can be sub-
n
stituted, giving :
2 B M = c 6 w x
M
+ 2 Ac c m x
Therefore :
2EM.
= clbn x (d - ^ + 2 Ac
«•'-«']
m x
and c will be found by the formula :
c =
_ 2BM _
b n x ( d - 1 j + 2 Ac m x ( - — CJ x (d- dc)
This formula appears to be a very large one,
but it is perfectly simple, and will very
quickly become familiar to the student,
who, however, will be unable to make use
of the equation until the value of n is known ;
and it will be necessary to deduce a formula
for this, based on the relative values of the
concrete, steel in compression, and steel in
tension.
It is known that the total compression and
cbn
tension are equal, and therefore -~ - + Ac
x cs = t At, or c b n + 2 Ac x cs =
2tAt.
We do not know the values of t, c, or cs,
but can replace them with equivalents as be-
t m x d — n
, as shown by
n
c m (n — dc)}
fore ; thus - =
previous reasoning, and cs =
cs m
OI J — _
c n
— d)
—ci
x cs = 2 t At, and if we divide both
sides of the equation by c we shall get
2 Ac x c, 2tAt
bn + - ~ , or 6 M + 2 AS x
cs.
t
t C-
By substituting the values of - and - as
C C
given above, we can remove the unknown
factors and obtain the following : —
2 Ac m (n — dc) 2Atm(d—n)
bn + ~Y~ ~^~
Multiply both sides of the equation by n and
we have
b n2 + 2 Ac m (n — dc) = 2 AC m (d — n).
We have now got n on both sides of the
equation, and this must be simplified as
follows : —
THEORY OF REINFORCED CONCRETE
103
b w2 + 2 AC m n — 2 Ac m dc — 2 At md
— 2 At mn
bn2 + 2 Ac mn + 2 At mn = 2 At md
+ 2 AC m dc
bnz +2mn(Ac + At) = 2m(Atd+Ac dc)
2 mn (Ac + At) 2 m (At d + Ac dc)
,,
If we now add
, T, « -A
to both sides
of the equation it will be possible to take the
square root of the left-hand side and further
simplify as follows : —
2mn(Ac + At) fm (Ac H
~~b~~ ~ L b
2 m (At d + Ac dc) , f m (Ac + I
J
The square root of the left-hand side equals
n + -%- — — , therefore this will equal
/2m(Atd+Acdc) ,
m (Ac + At)
2
, and
V 6
b J
2 m (Atd 4- Acdc)
b
m (Ac + At
f
m
Having now obtained a formula for the
position of the neutral axis, it will be possible
to ascertain this and then calculate the
stress on the concrete. When this has been
done the stress on the steel can be found by
considering the elasticity of the two mate-
rials and their relative distances from the
neutral axis. Thus t will equal m times c
multiplied by - — , or, expressed as a for-
d
and cs will equal
mula, t = c m
n — d,,
cm — — .
n
It will be advisable to give an example
illustrating the use of these various for-
mulae for the purpose of checking ; the case
taken will be that of the beam previously
designed to carry a load of 10 tons distri-
buted in addition to its own weight over
a span of 16 ft., when the effective depth
was limited to 20 in. and the width to 12 in.
The tensile reinforcement necessary was
calculated to be 2-269 in., and the com-
pressional reinforcement 1-598 in.
There will be no need to re-calculate the
bending moment, as the size, and conse-
quently the weight, of the beam was avail-
able at the outset. This bending moment
was found to amount to 643,200 in.-lb. The
first step will be that of finding the position
of the neutral axis.
2 m (At d + Ac de) fro (Ac +A)12
m(A,; +At)
[~m(Ac +A)"p
b
n =
X 15 (2'269_ XJ20j+_r6ij8j><_2)_ , [15 (1'598
12
15 (1-598 + 2-269)
12
,
30 x 48-576
+ (4-833)2 - 4-833
n = N/121-44 + 23-357 - 4-833
n = x/144W - 4-833
n = 12-03 - 4-833
n = 7-197 in.
Upon reference to the figures taken when
designing the beam, it will be seen that n =
7-2 in., and this will afford some evidence
to the student that the formula is satis-
factory. The stress in the concrete can now
be ascertained as follows : —
2BM
b n x (d - o) + 2 Ac m x ^— — ^ x (d- dc)
c =
2 x 643200
(7'197\ /7'197-2\
20--g-)+2x 1-598 x l*x(rriW )x (2° ~2
1286400
c = 12 x 7-197~xl7-6 +2 x 1-598 x 15 x -71 x 18
1286400
c =
1520-006 + 612-673
1286400
2132-679
c = 603 Ib.
This is quite satisfactory, as it is only 3 Ib.
over the permissible limit of 600 Ib. per
square inch, and even this would not occur
if all the decimal figures were retained in
the calculations, but such a method would
entail a great deal of unnecessary labour.
The stress in the tensional reinforcement
d — n ,
= c m - — ; therefore
20 - 7-197
t = 603 x 15 x -- 7497"
t = 603 x 15 x 1-77
t = 16,000 Ib.
This can be considered satisfactory, and the
stress in the compressional reinforcement
will now be calculated by :
n -
cs= cm
104
REINFORCED CONCRETE
cs = 603 x 15 x
7-197 - 2
7-197
ca = 603 x 15 x -72
c, = 6,512 Ib.
The beam then, as designed, can be con-
sidered as satisfactory. It will be noticed
that the exact areas of the steel as calcu-
lated were retained and not substituted by
actual practical size bars, as these would of
necessity vary slightly, and this would affect
the stresses. The object of this example has
been to avoid all complications, as the
theory is a little more difficult than with
single reinforced beams, and the student
should become quite clear on all the points
in the formulae and then work out various
examples and check them, in some cases
working out the bars required, and substi-
tuting these for the theoretical areas before
finding the stresses in the concrete and steel.
All the theory and formulae given will
apply equally to slabs and beams, bearing
in mind that the bending moments in the
case of the former must, if necessary, be
calculated in accordance with the rules pre-
viously given if supported or fixed on all
four edges.
TEE BEAMS
These beams are those in which the com-
pressional resistance is partly or wholly
supplied by the slab above the beam, form-
ing a large T-shaped section. Such a method
of calculation is permissible and economical
when the slab and beam are cast in one
operation with no plane of cleavage between
the two, and reinforcement is provided to
resist the shearing stress occurring along
the plane of junction.
When designing a reinforced concrete
floor over a very large area it is necessary
to divide the floor up into a number of
panels by means of beams which will of
necessity project down below the slabs
which form the panels, and sucli beams will
generally consist of main and secondary
beams, the former spanning the greater
distances and carrying the latter, which
intersect at right angles.
In smaller floors the design may consist of
one set of beams only, and in this case the
slabs are considered to assist in the resist-
ance of such beams, while in the case where
two sets are employed it is usual to con-
sider the secondary beams only as being
assisted by the slabs, and it is essential that
the slab reinforcement, which is at right
angles to the beam, must extend through the
full width of that portion of the slab which
forms the compressional flange of the beam.
The width of slab that can be taken as
acting with the beam will depend on cir-
cumstances, but it must not exceed either
of the following : (1) One-third of the effec-
tive span of the tee beam, (2) three-fourths
of the distance from centre to centre of the
ribs of the tee beams, (3) fifteen times the
thickness of the slab, (4) six times the width
• ' 1
I
V
i
. ' '
__^4
}
-A-
N5—
c
r
L
-©-••«
1
'
A
B
Fig. 126.— Section of Tee Beam and Method
of Finding Total Compression
of the rib of the tee beam. It will be neces-
sary, therefore, to determine which is the
least of these and calculate accordingly.
The diagram presented by A, Fig. 126,
shows the section of a tee beam, and will be
helpful in explaining the value and meaning
of the various symbols employed.
There are three cases that require to be
considered according to the position of the
neutral axis, and these are indicated on
this diagram. The first is that when the
neutral axis falls within the slab, the second
where it coincides with the bottom of the slab,
and the third where it falls below the slab.
THEORY OF REINFORCED CONCRETE
105
The two former cases offer no difficulty,
as all the formulae that have previously been
given for beams with single reinforcement
can be employed, with the exception that b
will now represent the width of that portion
of the slab acting as the flange of the beam,
and in the second case ds, which expresses
the thickness of the slab, will be equal to n.
In the third case, however, it will be seen
that a portion of the rib will be acting in
compression, and this will affect the centre
of the compressional resistance, and conse-
quently the lever arm of the internal forces.
In most cases the amount of concrete in
the rib above the neutral axis which assists
in compression is neglected in the actual
calculations, as the area over which the
stress is acting is comparatively small, and
the resistance of such a small area situated
so closely to the neutral axis is not such as
to materially affect the economy of the
design. The second report of the R.I.B.A.
recommends that this method of calcula-
tion should be adopted, and it is therefore
justifiable to consider the compression
strength in this way.
Upon reference to B, Fig. 126, it will be
seen that the total compression is represented
by the shaded area, which has a depth equal
to ds and width equal to c at the extreme
top edge and c1 at the bottom edge, which
coincides with the bottom of the slab. The
area of the beam over which the stress is
considered as acting is equal to b x ds,
where 6 equals the width of the flange and
d3 the thickness. Now. the total compres-
sion in the slab will be the area 6 x ds mul-
tiplied by the stress per square inch. As
before explained, the stress diminishes uni-
formly towards the neutral axis, and conse-
quently the value of c1 will be less than c in a
direct ratio with its comparative distance
from n. Therefore c1 = c x s, and
n
the mean stress acting over the whole area
n — ds
c x c1 c + c —
will equal
or
If the
area acting in compression be multiplied by
the mean stress per square inch, then the
total compression will be given. As before
stated, the area =b x ds, therefore the total
compression =
, / n - ds\
o x ds x (c + c • 1
The moment of the compression must equate
with the bending moment, and the total
compression will therefore be required to be
multiplied by the lever arm of the internal
forces. In the case of singly reinforced
1%
beams this was always equal to d — „, but
this will no longer apply, as the centre of
the compressional resistance will no longer be
situated at a distance of ^ from the upper
surface. The symbol a, however, is used to
express the value of the lever arm, and this
can be employed to complete the formula
for the moment of the compressional resist-
ance, which will be equal to
/ n — ds\
ox as x Ic + c -
\ f 11'
s x a, and this
must equal B M.
From this formula may be deduced a
formula for finding the value of c when the
bending moment has been calculated, as
follows : —
/ i
c 6 x ds x 1 2
4^')
x a
"R M
2
c [6 x ds x (2 n
- 4) x
al BM
2BMn
Therefore c = g x rfg x (2 n _ d.) xa
The value of the total compression has
now been obtained, neglecting the concrete
in the rib, and also the method of finding
the value of c when the beam is designed.
It is now necessary to show how the lever
arm of the internal forces can be ascertainel,
n
as it will not be d — .7, as m previous cases,
owing to the total compression, as shown at
B, Fig. 126, no longer being represented by a
triangle. As before, the whole of the com-
pression must be considered as acting at
the centre of gravity, and it is necessary,
therefore, to find the centre of gravity of the
shaded portion, shown on the diagram.
The draft regulations of the London County
Council state that the lever arm can be
taken approximately as d — ^. This is a
useful rule for preliminary calculations when
the steel is required to be found and the
position of the neutral axis is not avail-
able, but an accurate method must be
deduced for exact calculations. Now if
io6
REINFORCED CONCRETE
the shaded portion was a triangle having a
base equal to c and a height equal to ds,
then the centre of gravity would be situated
ds
at a distance of -^ from the top edge. It is
necessary to consider, however, a figure
having four sides, where c1 will bear a cer-
tain relation to c, and it is obvious that the
d,
centre will be nearer to the top than w>
and by the principle of moments it will be
found that the actual position of the point
ds 2 + 2 c1
from the top edge will equal ^ x — — — ^.
Express the distance from the top edge by
the symbol a0, as shown in Fig. 126. It has
previously been seen, however, that c1 =
n — dg
c— —, and by substitution the following
is obtained : —
c + 2 c x
n — ds
a, =
,
3 c n — - -
n
2cn —
c + c x
n —
= — x
™ f
Therefore ae =
3 n-2ds _ 3 dsn - 2 d,2
2 w — d« 6 n — 3ds
j «. i
> and the lever
'
6 n —
arm of the internal forces will equal d — ac
3dsn -2ds*
-
The total tension will equal t x At, and
the, tensional resistance will be this value
multiplied by the lever arm, namely t x At
x a, where a = d — ac. This tensional resist-
ance must also equal the bending moment,
and therefore t A x d — ac = B M, and the
stress in the steel can be found in a beam
that has already been designed by the formula
At x d — ac'
Having ascertained the methods of deal-
ing with both the compressional and ten-
sional resistances, the next step will be to
deduce some process for finding the position
of n in the case of a beam already designed,
and this is of great importance, as there are
three possible cases, as stated previously.
In order to be quite accurate, it will be
necessary to consider the concrete in the rib,
as it is obvious that any compression in this
portion would affect the position of n, and
although it is neglected in the actual cal-
culations for strength, it must be taken into
account in the neutral axis formula. The
compression in the slab has been shown to
be equal to
and to this must be added the compression
on the leg. This will amount to
n — ds
b >< C X — —
where br is equal to the width of the rib,
giving a total compression of
n — d.\ in — d.
and as the total compression and tension are
equal, thus
n — d A In — ds
This can be simplified and deduced to give
a value for n as follows : —
2cb dsn — cb d? + c b, n — cbr ds
2~ ^^ t A.f
n
c (2 b ds n — b dsz + br n — b, ds)
2n
2 t At n
2bdsn — 6 ds2 + b n — b ds = -
t m(d
but - = —
c n
get:
2 b ds n — bdsz
n)
, and by substitution we
z + brn — brds =
2m At n (-I — n)
n
2 b ds n — b ds
2m Atnd — 2m
bn — br ds =
2bds n — bds* + br n — brds = 2 m At d
— 2mAtn
2 b ds n + br n + 2 m At n = b ds2 + br d,
+ 2 m At d
n [2 (b ds + m A,) + br] = ds (b ds + bf)
+ 2 m At d
d, (b d, + br) + 2 m A, d
therefore n = — o~71Tj — i -- /T\ — i — iT~
2 (b ds + m At) + br
Thus all the formulae necessary is obtained
for the checking of beams which are already
designed, and it is only necessary to ascer-
tain the method to be followed when design-
ing the beams to carry a stated load, and
this will now be given.
THEORY OF REINFORCED CONCRETE
107
In the designing of tee beams, there are
certain factors which will be available from
the previous calculations, and it will be
advisable to note these in the first instance.
The width of the compression flange, together
with the thickness, will be given, as the slabs
will have been designed previously, and,
consequently, the values for b and ds will
be known. There remains for consideration
the method of finding the thickness of the
rib ; the value of d required to give econo-
mical design ; and the amount of reinforce-
ment. The first-mentioned, namely, the
width of the rib, will generally be fixed by
the designer according to experience, as it
will be influenced by the fact that the rods
in the tensional area must be properly spaced
and covered ; and as the value of the
compression in the rib is neglected — even
when the neutral axis falls below the slab
— the calculations for the design will not
be affected. There is a minimum limit, how-
ever, and that is, the width must not be
less than one-sixth of the width of the com-
pression flange, but it is seldom that this
minimum will be employed.
The method of finding the economical
depth is one that calls for a little more con-
sideration. In the first instance, it is not
known whether the neutral axis will fall
below the slab or not, and if it should be
found to do so after the design is complete,
%
then the lever arm will not be d — «> ^ut
d — ac, and, unfortunately, no definite rela-
tion between ac and d can be fixed unless
the amount of reinforcement, and conse-
quently the position of the neutral axis, is
known. To keep matters as simple as pos-
sible, however, it will be advisable still to
consider n as equal to -36 d, as will be the
case if it falls within the slab, and to con-
struct a formula for finding d accordingly ;
and if the value for d thus found is not
quite accurate, it will form the best guide,
and the necessary adjustment can be made
after.
In previous notes it has been shown that
cbn / , n\ .
BM = — o- x Id — o ) for a single rein-
forced beam, and if the neutral axis is
assumed to fall within the depth of the slab,
then this formula will hold good. In de-
ducing a formula for giving the value of d,
values may be substituted for c and n, but
6 will be unknown, as it will vary in practic-
ally every example, and will not necessarily
bear any definite relation to d. By putting
in the values already known, it can be
deduced as follows :
( ~\
-ex x n * \ - 3)
c = 600 and n = -36 d, then
2 B M = 600 x b x -36 d x I d -
2 B M = 600 x b x -36 d x -88 d
2 B M = 180-08 b d2
B M = 90-04 b d2
,72 _ BM , _ /"BIT
= 90-04 b ~ : V 90-04 b
It will be sufficiently accurate if the deci-
mal figures are deleted, giving
906
With regard to the method of calculating
the amount of reinforcement required, or
the value of A/, this again presents difficul-
ties, as it will depend upon certain factors,
one of which will be unavailable, namely, the
lever arm ; and although it may be taken as
being equal to -00675 6 d when the neutral
axis falls within the slab, this method will
not be absolutely accurate when the axis
falls below the slab. Another method will
be that of assuming the value of the lever
arm, and for the preliminary calculations
ds
this may be taken as equal to d — <r when
a safe approximation will result under
ordinary circumstances. If this method is
considered and accepted, then A can be
found as follows :
B M = At x t x a, where a = the lever
dst
arm, and this is assumed as equal to d — -5
/ ds\
therefore B M = A« x t x I d — % I,
BM
then AC = -
TEE BEAMS CONTINUOUS OR FIXED AT
ENDS
In the case of tee beams that are continu-
ous or fixed at the ends, it will be necessary
to calculate the strength at the supports ;
and as the flange at these points will be in
the tensional area, its value will be lost and
the calculations must be made for a rect-
angular beam having a width equal to the
width of the rib only. If an example is now
io8
REINFORCED CONCRETE
taken, and the beam is designed and checked
according to the theory and formulae given,
the student will be able to realise the applica-
tion quite clearly.
It is required to design a tee beam, fixed
at the ends, to carry a uniformly distributed
load of 2,000 Ib. per ft, run, including the
weight of the beam itself, over an effective
span of 20 ft.- The slab has been designed,
and has a total depth of 6 in., while the
width acting in compression with the beam
can be taken at ten times ds, which equals
60 in.
The total load will equal the load per foot
run multiplied by the effective span, thus
w = 2000 x 20 = 40000 Ib. As the beam
is fixed at the ends, the bending moment at
the centre can be considered as equal to
wl 40,000 x 20 x 12
-jg, then B M = — jg —
800000 in.-lb.
The first step in the actual design of the
beam will be to calculate the depth by the
formula i.
B~M
t
then i -
= d = x/148 = say 12 in.
Let us next find the value of At by con-
sidering same as -00675 6 d. Then At =
•00675 x 60 x 12 = A, = 4-86 in. If the
value of the steel required is also calculated
by the alternative approximate method of
considering the lever arm as equal to d — -e
a
then the use of the formula will be shown.
This formula was A/ = —
Therefore At =
A, =
800000
16000 x (12 - f
800000
16000 x 9
At = 5-55 in.
The first method, which gave 4-86 in., is
therefore the most economical, provided the
permissible stresses are not exceeded, and
this will be ascertained by checking the
beam. Should the steel provided by the first
method be found insufficient, then it may
be increased by any amount up to that given
by the second method, when it will always
be found to be sufficient.
In checking the beam the position of
the neutral axis must first be found,
and the formula for this has been given,
namelv :
(b d, + br) + 2mAtd
2(bd, + mAt) + br '
Assuming br to be 10 in., then
6 (CO x 6 + 10) + 2 x 15 x 4-86 x 12
_
2 (60 x 6 + 15 x 4-86) + 10
370 + 30 x 58-32
_
2 x (360 + 72-9) + 10
2220 + 1749-6 3969-6
•=n=-.l.. s-s
865-8 + 10
The thickness of the slab was given as
6 in., and the distance from the top edge
to the neutral axis is 4-58 in., therefore the
latter falls within the slab, and the beam
can be considered as a singly reinforced beam
Al
when the lever arm equals d - Q.
o
stresses can be checked as follows :
2BM
c = - — ^ —^ ; therefore
b n :
The
2 x 800000
c =
CO x 4-58 x
1600000_
60 x 4:58 x 1048
1600000
(» - 4
2879-9
c = 555 Ib.
This is quite satisfactory, and t can now
be calculated.
BM
t = - — -, — -^r ; therefore
At x ( d - ^
800000
t —
t =
4-86 x 112 - —
800000
4-86 x 10-48
800000
50-835
t — 15737 Ib., which is also quite
satisfactory.
The steel required was 4-86 in., and the
area of one bar having a diameter of 1| in.
= -994 in., and five bars of this size would
give an area = -994 x 5 = 4-97 in., which
will do very well. As five rods are to be used,
it will be necessary to put them in two rows
one over the other, and the depth of 12 in.
will be the distance from the top edge to
the centre between these rows. In order
to get sufficient covering for the lower rods,
it will be necessary to add 3 in. of concrete,
giving a total depth of 15 in.
THEORY OF REINFORCED CONCRETE
109
In this example the beam would require to
be calculated at the ends, as it was stated
to be fixed, and this would be done by using
the formulae given for doubly reinforced
beams. As this example, however, gave
an instance where the neutral axis fell within
the slab, it will possibly be more useful
to give another example where the axis falls
~3elow the slab and calculate the section at
the centre and the ends also, and the student
will then have an illustration of the procedure
in each case. It must be borne in mind
that these examples are taken at random,
and, as before stated, are given for the sole
purpose of showing the application of the
formulae and the methods adopted, and the
reader will be well advised if he studies the
working drawings of reinforced concrete
work, and endeavours to follow the prac-
tical application of the theory. It is some-
what difficult to select an example where the
ixis will fall below the slab unless the load
to be carried is made exceptionally heavy,
sr the compression flange is taken as being
rery thin and narrow ; and if the load is
sxcessively heavy and the beam is fixed at
the ends, then a large amount of reinforce-
ment will be required in the compressional
area at the ends where the flange is situated
in the tensional area, as the steel will not
be situated at any great distance from the
neutral axis and cannot be stressed to its
full limit.
As an example, however, let it be required
design a tee beam securely fixed at the
mds, to carry a uniformly distributed load
2,551 Ib. per ft. run, including the weight
)f the beam itself over an effective span of
14 ft. The section at the centre of the span
is to be calculated in the first instance, and
then the section at the ends, and the slab
in be taken as 4 in. thick, and the width,
icting with the beam, as ten times ds,
hich equals 40 in.
The total load will equal the load per ft.
m multiplied by the span = 2551 Ib. x
L4 ft. = 35714 Ib.
The bending moment at the centre will
wl 35714 x 14 x 12
jqual j---, therefore B M = - — jc> —
= 499,996 in.-lb., say 500,000 in.-lb.
First calculate the depth, namely :
BM
therefore d
P
= V of
500000
90 b " 'V 90 x 40
= -s/139 = 11-8 in., say 12 in.
The value of At must next be found by
taking same at -00375 b d ; therefore At =
•00875 x 40 x 12 = 3-24 in.
The section at the centre is now obtained,
and before proceeding with the calculations
at the ends of the span it will be advisable
to find the position of the neutral axis,
and see that this falls below the slab, and
also check the stresses on the steel and
concrete.
The position of n will be found by the
, d, (b d, + br) + 2 m At d
formula n — 0 ,, j — — 1-\ — rr —
2 (b ds + m At) + b,.
There is only one unknown factor that must
be settled before working out the formula,
and that is the value of &,.. Taking this, as
before, at 10 in., then
4 (40 x 4 + 10) + 2 x 15 x 3-24 x 12
n —
n =
n =
n =
2 (40 x 4 + 15 x 3-24) + 10
4 x 170 + 30 x 38-88
2 (160 + 48-6) + 10
680 + 1166-4
427-2
1846-4
427-2
n = 4-32 in.
As the thickness of the slab is only 4 in.,
it will be seen that the axis falls -32 in.
below same, and although this is a very
small distance, it will serve as an example.
The position of the neutral axis having
been obtained, the next thing will be to
check the stress in the concrete, and as the
axis falls below the slab it will be necessary
to use the formula
6 x ds x (2 n — ds) x a
The factor a is at present unknown, as this
represents the lever arm of the internal
forces, and this must be found in the first
instance. It is known that a — d — ac and
3 d, n - 2 42 .. ,
a,. = — - S-T— , therefore
6 n
3
x 4 x 4-32 - 2 x 4 x 4
a,, =
6 x 4-32 - 3
51-84 - 32
fit- — f\r.
x 4
19-84
W92
25-92 - 12 ~
ac =T42 in.
As a comparison, it is interesting to note
that if the neutral axis had coincided with
the under side of the slab, the centre of
compression would have been equal to ^
which would give a distance of 1-44 in. from
the top edge ; and as the axis falls below the
slab by a distance of -32 in., the centre of
no
REINFORCED CONCRETE
compression is -02 in. nearer the top edge,
and consequently the lever arm is increased
by this amount. As already stated, a =d — ac,
therefore a = 12 in. — 1'42 in. = 10-58 in.,
and this value being now available, c can be
found as follows :
_ 2 x 500000 x 4-32
~
c =
40 x 4 x (2 x 4-32
4320000
160 x 4-64 x 10-58
4320000
_
4) x 10-58
c =
7854-592
c = 549 Ibs.
This is well below the permissible limit,
L
b-J
n" =1
r
1 f
ds-4'
t
1
•1.32
fcl
_, *
'Z'COVIR.F
^br
. A
C/-/2"
.1
Fig. 127. — Section, at Centre of Span, of
Tee Beam Fixed at Ends
and the value of t can be next calculated by
BM
the formula t = -i — when a again
J\t X Ct
= d — ac, which = 10-58 in., therefore
500000 , _ 500000 _
= 3-24 x 10-58 ~ 34-2792' l
Ib. This also is well below the permissible
limit, and the section at the centre may be
considered as quite satisfactory. If two bars
having a diameter of \\ in. are used, then
the sectional area of steel will equal 1-7671
x 2 = 3-53 sq. in., which is more than
sufficient.
The section as designed is illustrated in
Fig. 127, and the calculations for the sec-
tion at the ends of the span may now be
considered. In this latter case, a different
bending moment will exist having a value
wl , __, 35714x14x12
equal to -TQ, therefore B M = - — j~ —
= 599995 in.-lb., say 600000 in.-lb. This
is a greater bending moment than that
required to be resisted in the centre of the
span, and as the tension will now be in the
upper surface, a comparatively large amount
of reinforcement will be required. The
section will be calculated as a beam with
double reinforcement where the size of the
concrete is fixed. The portions of the slab
on each side of the beam must be neglected,
leaving a section 14 in. deep and 10 in. wide.
Allowing 2 in. from the top edge to the
centre of the reinforcement, the effective
section becomes 12 in. by 10 in. = 120 sq. in.
The economical percentage if singly rein-
forced would be -00375 x 120 = -81 in. The
bending moment that such a beam would
/ n\
resist will equal At x t x Id — ^j and n
would = -36 d = 12 x -36 = 4-32 in., then
B M = -81 x 16000 x 112 3- j = B M =
•81 x 16000 x 10-56 = 136857 in.-lb. The
actual bending moment to be resisted, how-
ever, is 600000, and the excess which will
have to be taken by the compressional steel
and additional tensional steel equals 600000
- 136857 = 463143 in.-lb. If the com-
pressional steel is placed 2 in. from the
bottom edge, then dc will equal 2 in., and
the lever arm or distance between the two
sets of reinforcement will equal d — dc =
12 — 2 — 10 in. The additional area of
steel required in tension will be found by the
formula :
463143
_ Excess B M _ _
At = t x (d - dc) = At ~~
463143
16000 x (12 -2)
9 sq. in.
' 160000 ,
This amount, plus -81 in., the area found
above, will give a total amount of 3-7 in.
for the steel required in the upper or ten-
sional area. The steel required in the com-
pressional area will be found by the formula :
Excess B M
Ac == cTx~(3 - dc)'
The value of cs must be found, and, as before
stated, this will depend on the position of
AC in relation to the neutral axis, namely,
t x (n - dc)
; therefore
cx =
d — n
16000 x (4-32 - 2)
37120
~ Cs = 7-68
12 - 4-32
cs = 4833 Ib.
It will be seen that the steel will be
stressed at a very low figure. The formula
for the amount of steel can now be worked
out as follows :
463143
Ac =
A, =
4833 x (12 ^
463143
= Ac = 9-58 sq. in.
THEORY OF REINFORCED CONCRETE
in
Before actually giving the finished section,
it will be advisable to check the stresses, and
see that they do not exceed the permissible
limits, and in order to do this, the position
of the neutral axis must first be calculated
by the formula given for double reinforced
beams, namely :
/2 m(Atd + Ac dc)
m (Ac
rm(Ac + At)J
b
— ; therefore
/•J X 15
X 12 + i)-58 X 2)
10
15 (9\5S 4- 37)
15 (9'5» +
10
- v
30 (44-4 + 19-16) |"15 x 13-2812
10 10
15 x 13-28
10
/J905-8 I"199'2"!2 _ 199.2
= V 10 ~_ LjOj ~TO~
n = x/190-68 + 396-7064 - 19-92
n = v/587-38 - 19-92 = w = 24-23 - 19-92
w = 4-31 in.
This compares very well with the position
calculated according to the economical per-
centage when designing, namely 4-32 in. The
stress in the concrete can now be found as
follows :
2BM
c =
bnx
(n
d-^
c =
2 X 600000
10 X 4-31 X (l2 - Ap) + 2 X 9-58 X 15 X (4 ^ 2) X 12 - 2
1200000
c =
c =
43-1 x 10-57 + 287-4 x
1200000
2-31
x 10
1200000
1995-967
455-567 + 1540-4
c = 601 Ib.
This is quite satisfactory, and t and cs
can be proceeded with.
- 601 x 15 x 1-784 = t = 16082 Ib.
"i =; cm
cs = 601 x 15 x
4-3i
- c, - 4832 Ib.
All these figures are quite satisfactory, and
the beam can be considered as being calcu-
lated for the centre and end sections.
The diagram in Fig. 128 shows the section
at the ends of the span. As the amount of
reinforcement to take the compression at
the ends was 9-58 in., it will be necessary to
employ, say, six l|-in. diameter rods, which
will give a total area of 10-5 sq. in., and these
must be placed in two rows as shown. The
depth taken in the calculations will be to
the centre of the two rows, and conse-
quently, it will be necessary to allow 3 in.
of concrete below this point, giving a total
depth of 15 in. instead of 14 in., which was
theoretically necessary. This increase will
have the effect of making the resistance
slightly in excess of that calculated, and
therefore it may be adopted. The extra inch
Fig. 128. — Section, at Ends of Span, of Tee
Beam Fixed at Ends
of covering is not indicated on the section
illustrated in Fig. 127, as it is not necessary,
and the under side of the beam could be
formed with a very slight camber, which
would improve rather than detract from the
appearance of the beam. The question of
bending up certain bars from the lower sur-
face to the upper near the supports, and the
calculations for shearing stress, are not con-
sidered here.
TEE BEAMS WITH DOUBLE REINFORCE-
MENT
With regard to tee beams with double
reinforcement, it is not considered necessary
to give any particular examples for this class
of calculation, as the same formulae and
principles will apply as were given for beams
with double reinforcement when the neutral
axis falls within the slab or coincides with
the under side, and, as previously stated,
these are the most common cases. It must
be borne in mind, however, that b will repre-
sent the width of the flange acting with the
beam in compression. When the axis falls
below the slab, the beam can be designed
upon the assumption that the concrete in
the rib is to be neglected, and the various
112
REINFORCED CONCRETE
formulae previously given can be used, and
if necessary slight adjustments can be made
after the preliminary calculations have been
made. Where double reinforcement is em-
ployed, the two sets should always be
connected by binding.
SHEARING STRESS AND ADHESION
The question of the shearing stress and
adhesion has been omitted up to the present
in order to simplify matters and enable the
student to become quite familiar with the
principles of designing for tensional and
compressional stresses ; but the provision
of steel in practically all concrete beams to
resist shear is absolutely necessary, and this
question has, in the past, received far too
little attention. When the use of reinforced
concrete was more or less in its infancy,
this side of the design was so little dealt
with that in nearly all cases of tests applied
to beams where failure occurred, such failure
was due to insufficient provision against shear
or diagonal tension, and it was this fact that
caused the more careful designers to take up
this part of the subject more seriously.
The draft regulations of the London
County Council make provision for shear
members as follows : " All beams shall be
provided with adequate shear members,
and such shear members shall — (a) Be
spaced according to the distribution and
intensity of the shearing stresses ; but the
distance from centre to centre of the shear
reinforcement at any part of the beam shall
not exceed the effective depth of the beam.
(6) At least extend from the centre of the
tensile reinforcement to the centre of pres-
sure in the concrete under compression.
(c) Be passed under or round the tensile rein-
forcement, or be otherwise secured thereto.
(d) Have a mechanical anchorage at both
ends, or they shall have a mechanical bond
with the concrete throughout their length."
The first point for consideration will be
that dealing with the intensity and distri-
bution of the shearing stresses. The shear-
ing tendency is due to the opposition of the
weight and the reactions, and as the great-
est opposition will occur at the supports
where the total weight meets, as it were, the
total reaction, it is at this point that the
greatest shearing stress is found, and the
stress at any intermediate point will equal
the reaction at the support, minus any por-
tion of the load situated between this point
and the support under consideration. Thus
the amount of shearing stress is affected by
the amount and nature of the load, but not
by the span ; and, cons quently, it is in
the case of beams which carry heavy loads
over a short span that the shear becomes
the most important consideration. The
shearing stress acts both horizontally and
vertically, and at any point in the beam
these two actions will be equal, and as a
result the shear members are often placed
at an angle of 45 degrees to resist the
resultant of the two forces. The distribu-
tion of the stresses over the area of the
beam is somewhat curious, as the combined
action of the bending moment and shearing
force causes the lines of maximum stress to
assume a curved form ; and the shearing
stress will be greatest where the tension
and compression are nil, namely at the
neutral axis.
Diagonal tension is often expressed as
shear, whereas it is dependent on the shear
and the longitudinal tension for its inten-
sity. The actual moment of this intensity
cannot be ascertained with any certainty,
and, consequently, it is usual in practice to
calculate the vertical shear, and take this
as the measure of the diagonal tension, or,
at least, provide sufficient steel for this pur-
pose, when the beam should be efficiently
designed. The shearing stress, as previously
stated, is not uniformly distributed over the
area of the section, but is greatest where the
longitudinal stresses are least, namely, at
the neutral axis, and in an ordinary homo-
geneous beam the stress diminishes from
the axis to the outer edges, where it becomes
nil, assuming a parabolic curve, as shown in
diagram A (Fig. 129). If S equals the total
shear at a vertical section, then the maxi-
mum shear which occurs at N A, and is ex-
pressed by m s, can be found as follows : —
The area of the shaded portion represents
the total shear on the section = S, and the
area of this figure = § m s x d, therefore
S 3 S
| m s x d = S and § m s = -7, or m s = ~-j.
If a diagram is set up as given at A (Fig.
129), then the actual stress at any point in
the section can be determined.
It is unimportant in most cases to know
the exact distribution of the stress, but
this instance is given rather to make the
theory more explicit.
Now, in the case of a reinforced concrete
beam, the distribution of the shearing stress
is somewhat different, and the reason for
THEORY OF REINFORCED CONCRETE
this is the fact that the concrete above the
axis only is called upon to resist longitudinal
stress, that below the axis being neglected,
and all the tension is taken as coming upon
the steel. The result of this assumption is
that, although the shear above the neutral
axis will gradually diminish towards the
outer edge, below this point it will be uni-
formly distributed over the section as shown
in diagram B (Fig. 129). As the upper part
of this figure gives a parabolic curve, the
equivalent area over which the stress acts
above the neutral axis will equal two-
thirds bn, and below the axis 6 x (d - n),
where b = the width of the section, and s =
the maximum shear per square inch. In
dealing with the calculations for shearing
stress it is also necessary to consider that
of the adhesion between the steel and the
concrete, as the two materials can only act
together when this adhesion is not overcome.
The permissible allowance for concrete in
shear has been given in the table of working
stresses as 60 Ib. per square inch, while the
allowance for adhesion between steel and
concrete is given as 100 Ib. per square inch,
and the shearing stress across the section
divided by the total circumference of the
rods or shear members must not exceed this
amount. If the perimeter or circumference
of the bars be expressed by 0, then the
intensity of stress per square inch around the
S
bars will be equal to — -, — -rr~. In the
--A
o
('-9'
Fig. 129. — Determining Stress in Ordinary
and in Reinforced Concrete Beams
therefore the total equivalent area, taking
the shear, will equal 6 x ( d - ^}. Where S
equals the total shearing stress acting over
the section, the greatest stress per square
o
inch will be equal to
b X
and this
is expressed by the symbol s.
In designing the shear members, it is neces-
sary to consider the amount of stress that is
acting over a portion of the section equal to
1 in. in depth, and with a width equal to 6,
S
and this value will equal
or & x s,
designing of shear members, it is usual
and advisable to utilise stirrups or rods of
one section in the same beam and vary the
spacing according to the amount of stress
to be resisted. As the greatest shear will
always occur at the abutment, the stirrups
will be fairly close together here, and will
gradually increase in spacing towards the
A point at which they are not required. To
resist the vertical shear, there are the con-
crete and the horizontal reinforcement,
but in the case of the horizontal shear it
will in most cases be necessary to provide
vertical members, as mentioned above, or
crank up some of the tension rods at a suit-
able point, or provide both, and it will be
necessary presently to illustrate the method
of determining the positions of the shear
members to suit the varying shearing stress.
To illustrate the use of the formulae already
given, examples may be taken and their
application shown. In the case of ordin-
ary slabs and rectangular beams with single
reinforcement which are accurately calcu-
lated to resist the longitudinal stresses, it
will be found that sufficient concrete is pro-
vided in the section to withstand the shear-
ing stress, and, consequently, no special
shear members are theoretically necessary,
but for practical reasons they should be pro-
vided, as they assist in holding the con-
' crete together, and, furthermore, they will
be required under the regulations of the
authority under whose jurisdiction the
building will come.
As an illustration, consider a beam sup-
REINFORCED CONCRETE
ported at the ends, which is required to carry
a uniformly distributed load of 3,000 Ib. per
foot run, including the weight of the beam
itself, over an effective span of 16 ft. t The
total load will equal the load per foot run,
multiplied by the span = 3,000 Ib. x 16 ft.
w I
= 48000 Ib. The bending moment = -g- =
48000 x 16 x 12
= 1,152,000 in.-lb. Gal-
<j
culating the economical size of d by for-
, 3 /EM. , 3 /1 152000
mula, we have d =\/ -^=- = d —^/ — — —
' 0 I 0 I
= d = V7 20210 = say 28 in. The breadth
should equal -6 d, therefore b = -6 x 28 in.
= 16-8 in., say 17 in. A, = -00675, b d =
•00675 x 17 x 28 = 3-213 sq. in. The
greatest shear stress, as previously stated,
will occur at the supports, where it will be
equal to the reaction. This value will be
W 48000
- = —5— = 24000 Ib. The load being
a a
uniformly distributed, the shear will diminish
from the support to the centre of the span,
where it will be nil, and, consequently, if
calculations are made for the position where
the greatest shear occurs, and the beam is
found to be sufficient, it will be obvious that
it will be safe at all points. The greatest
shear is represented by S, and this equals
24,000 Ib., while the formula for finding the
maximum stress per square inch has been
a
, where s = the
gven as s =
b x
maximum stress per square inch, which must
not exceed 60 Ib., or shear members will be
theoretically required. Working this out, we
24000 24000
get s = -
17 x
24000
10-08
\
17 x 24-64
= 57-5 Ib., which is below the per-
o
missible limit. The value of n was taken at
•36 d, as the economical percentage of rein-
forcement was used, therefore n = -36 x 28
= 10-08 in. as given.
The adhesion or shearing stress intensity
round the reinforcing bars will now be con-
sidered ; it must not exceed 100 Ib. per square
inch. The amount of steel required was found
to be 3-213 sq. in., and if bars having a
diameter of 1 J in. are used, the area of which
equals -994 sq. in., four of these would give
an area of -994 x 4 = 3-976 sq. in., which
is more than" sufficient. To test the adhe-
S
sion, the formula -, — -^Y must be used.
0
- 9
The circumference of one IJ-in. bar = 3*5343
sq. in., and the total circumference of the
four bars = 3-5343 x 4 - 14-1372 sq. in.
Then the intensity per square inch =
24000 24000
W08\ "14-1372 x 24-64
14-1372 x
24000
— 68-8 Ib., which is well below
348-3406
the limit.
This example should be sufficient to show
the method of procedure for a simple case,
and indicate the truth of the statement that
theoretically no special shear members are
necessary with a single reinforced beam
designed with economical proportions. As
the draft regulations of the London County
Council state that shear members must be
provided, and the distance apart, from centre
to centre, must not exceed the effective
depth of the beam, we could adopt stirrups
1 in. wide and T*F in. thick, and place same
apart at some distance, not exceeding 28 in.
The span being 16 ft., this can be divided up
into the requisite number of spaces, as
192
follows : 16 x 12 = 192 in., therefore -^
= say, seven spaces. If the stirrups are
arranged to give a space at the centre of the
span, six stirrups only will be required. The
next example will be one in which the con-
crete is insufficient alone to resist the shear,
and special shear members are therefore
theoretically necessary.
As a further example, let it be required to
design a beam to carry a uniformly dis-
tributed load of 2 tons per foot run over an
effective span of 16 ft., such load to be
inclusive of the weight of the beam itself,
which is limited in size to 20 in. effective
depth and 12 in. wide. Total load =
w I .
2 x 2240 x 16 = 71680 Ib. B M = -g- if
beam is supported only, therefore B M =
71680 x 16 x 12
8
size of the beam = 20 in. x 12 in., and the
economical percentage, if singly reinforced,
would be 20 x 12 x -00675 = 1-62 sq. in.
Next find the moment of resistance of such
a beam, and if necessary provide additional
THEORY OF REINFORCED CONCRETE
reinforcement to take the excess bending
moment, as explained in doubly reinforced
beams. M R = B M = At x t x ( d - ^ ),
and n = -36 x 20 = 7-2 in., therefore B M
= 1-62 x 16000 x (20 - -^-] - 456192
in.-lb.
The actual B M = 1720320 in.-lb.,
therefore excess B M - 1720320 - 456192
in.-lb. = 1264128 in.-lb., and this must
be provided for by compressional and
additional tensional reinforcement. Then
x Excess B M _1264128
•"•« — 4~~~"/,j~~~.i\ ~ •".( = -• CAAn ~
A,1 -
1264128
- 4-38
1 6000 x (20-2)
sq. in. This
288000
amount must be added to the value given
above of 1-62 sq. in., making a total amount
for the tensional reinforcement of 6'00
sq. in. The compressive reinforcement will
be found by the formula :
Excess B M t x (n - dc)
- — , where c =
x (d - dey
, 16000 x (7-2 - 2)
equals c, = ^ „„ ' = c, =
d - n
83200
Ac =
20 - 7-2
= 6500 Ib. Therefore,
1264128
= Ac =
1264128
117000
6500 x (20 - 2)
AC = 10-8 sq. in.
There is no need to explain the method of
checking the longitudinal stresses, as that
has already been done in previous examples,
but the provision for shear may now be
investigated. The total load is equal to
71,680 Ib., and the greatest shearing stress
will occur at the supports, where it will
, W 71680
equal -y = — ^— = 35840 Ib. The allow-
ance for shear on the concrete must not ex-
ceed 60 Ib. per square inch, and therefore the
value of the section to resist the shear will
be equal to the equivalent shear area, which
(n\
d " ~o) x 60 Ib., therefore resistance
/
of concrete = 12
x (20 - 7J
x 60 =
12 x 17-6 x 60 = 12672 Ib. This value
being less than the actual shear coming upon
the section, it is obvious that there will be
an excess equal to 35,840 -- 12,672 Ib. =
23,168 Ib., which must be met by steel shear
members. The working stress for steel in
shear is given as 12,000 Ib. per square inch,
and the area of steel required will equal
23168
V2000 ~ ^ ^ s^' m* ^ ^e beam is SUP"
ported at the ends only, and the tensional
reinforcement at the centre of the span
equals 6-00 sq. in., it will be clear that as
the bending moment diminishes towards the
supports the tensional reinforcement may be
reduced, and consequently one or more of
the bars may be cranked at a point where
the shear on the concrete becomes over the
permissible limit and carried through the
shear area in such manner that it takes up
Fig. 130. — Vertical Shear in Reinforced
Concrete Beam carrying Uniformly
Distributed Load
the excess shear. Reference to Fig. 130 will
show how the shearing stress decreases from
the maximum value of 35,840 Ib. at the sup-
port to nil at the centre, the diagram illus-
trating one half of the beam only, and
consequently the total shear on one half the
beam is represented by the triangle ABC.
The shearing value of the concrete in section
is 12,672 Ib., and if this amount is set down
to scale from B, as indicated, and a hori-
zontal line is drawn through to the line A c,
then the position where it cuts this line, as
shown by the dotted vertical line, will be
the point at which the shearing stress
becomes greater than the concrete alone is
capable of taking.
This point may be found by calculation
as follows : The shear at any point is equal
n6
REINFORCED CONCRETE
to the load at the support minus the load
situated between that point and the sup-
port. Now the load at the support = 35,840
lb., and as the value of the sectional concrete
to take the shear is only 12,672 lb., it is
necessary to move outwards from the sup-
port along the beam until sufficient load has
been placed between us and the support to
reduce the load to this amount. It will be
necessary, therefore, to move outwards
until an amount of 35,840 12,672 =
23,168 lb. has been passed over. The load
is equal to 2 tons, or 4,480 lb. per ft. run, and
the distance from the support, therefore,
OQ1 £Q
equals TTOQ = 5 ft. 2 in. If the diagram
be set out to scale, as in Fig. 130, this can be
checked. It is from this point that the
necessary tensional bars can be cranked up,
and from this point to the support provision
for horizontal shear must be made.
On considering a horizontal plane taken
along the beam at any level below the
neutral axis, and finding the total stress
intensity existing along this plane, sufficient
stirrups can be calculated to take the total
excess of shear, and by keeping all the
stirrups of one size, they can be spaced at
varying distances apart and be thus equally
stressed. Taking, firstly, the total shear at
the supports, which equals 35,840 lb., it is
required to find the shear on a portion 1 in.
deep and with a width equal to b by
S 35840 35840
d
20 -
7-2
17-6
Of this amount the concrete is capable of
taking an amount equal to the width of the
section multiplied by 60 lb., therefore value
of concrete = 12 x 60 = 720 lb., leaving an
excess of 2,036-36 - 720 = 1,316-36 lb. Now
this excess, which has to be taken by the
steel, will gradually diminish from the sup-
port towards the centre of the beam until it
reaches a point 5 ft. 2 in. from the support,
when it will be nil, as the concrete alone is
capable of taking all the shear from this
point to the centre of the beam. This is
clearly shown in the diagram in Fig. 131.
Having now obtained the maximum excess
which requires to be taken by the steel,
and the length of beam over which this is
acting, the total excess may be determined
by taking the mean of the excess at the sup-
port and at a point 5 ft. 2 in. from the sup-
port, and multiplying same by the length
over which it is acting This amount will
obviously be equal to the area of the triangle
shown as DBF in Fig. 131, which equals
DE;DF _ 1316'3!! X 6g = 40807-16
Zi A
in.-lb. This is the total horizontal shear,
therefore, for which steel must be provided,
and as the allowance for steel in shear is given
as 12,000 lb. per square inch, the sectional
. , ... .,40807
area required will equal? 1 0Ann = 3-4 sq. in.
If it is decided to use stirrups 1 in. wide'and
J in. thick the sectional area would equal
•25 sq. in., and as each stirrup will have two
Fig. 131. — Horizontal Shear in Reinforced
Concrete Beam carrying Uniformly
Distributed Load
wings, the area provided by each stirrup will
equal -5 sq. in.
The total number of stirrups required
will therefore equal -^- = say 7, and
these must be spaced to give a uniform stress
on each one. This may be done graphically,
as illustrated in Fig. 131, where a semicircle
is set up over the length of beam in which
the stirrups are to be placed, and the length
is also divided up into fourteen equal divi-
sions, being twice the number of stirrups
to be employed. Perpendicular lines are
then erected to cut the semicircle as shown,,
beginning at one end and setting up a
line from the first division mark, and
then from every alternate mark until the
opposite end is reached, and the last line
erected over the point marking the end divi-
THEORY OF REINFORCED CONCRETE
117
sion. The points at which these perpendicu-
lar lines cut the semicircle are then trans-
ferred to the horizontal line, marking the
top of the beam with the varying radii as
shown, when the positions of the stirrups
will be obtained. The object of thus spacing
the stirrups is, of course, to divide up the
shearing stress represented by the triangle
DBF into equal portions, and if the lines
representing the stirrups are continued down
to cut the triangle as indicated, it will be
seen that it is divided up into divisions of
varying width which have more or less
equal areas.
To comply with the draft regulations of
the London County Council it would be
necessary to introduce the additional stirrups
shown by dotted lines. These stirrups are
spaced 20 in. apart, equal to the effective
depth of the beam, and should have the
ends turned out to afford a good bond with
the concrete. In the case of inclined stirrups
the best angle for these will be 45°, and it
is necessary that they should be firmly
attached to the horizontal rods if they do
not actually form a part of same. The
spacing for these can be found in a similar
manner to that employed for vertical
stirrups, except that the inclined line must
be drawn through the point at which the
vertical division cuts the neutral axis.
There is no doubt that the best method is
that where vertical stirrups are employed
together 'with a certain number of bent -up
bars, the method being effective in the
resistance of diagonal tension.
COLUMNS
The theory and calculations in the case
of columns must be divided into two dis-
tinct sections, namely, short columns and
long columns, the reasons for dealing with
compression members in this manner having
previously been given in the notes on columns
in the preceding chapter. In the case of
reinforced concrete work the column may
be considered as belonging to the first sec-
tion where the length does not exceed
eighteen times the least diameter, and as
such short columns are considered to fail
by direct crushing only, the full working
stress may be adopted, and the design is
comparatively simple. These will be con-
sidered in the first instance after a few
general remarks relating to reinforced con-
crete as a material for columns.
The steel is usually placed in the form of
vertical bars spaced equidistant on the cir-
cumference in the case of a circular column,
and at each corner in the case of a rect-
angular section, except in the case of rect-
angular sections where the sides are un-
equal as given below ; and these vertical
bars must be linked or bound together with
some form of lateral reinforcement to pre-
vent the possibility of the load causing the
bars to burst outwards through the com-
paratively thin covering of concrete coming
over them. The steel rods themselves assist
the concrete core when under the load, and
prevent the bursting tendency, and to do
this effectively they must be well connected,
and by links which are close together, thus
forming practically a steel cage which confines
A -i-
1 1
I I
p
• -1
', ,-L-4 jj
h
'l
Fig. 132. — Diagrams Showing need for Lateral
Reinforcement in Columns
the concrete and allows it to develop its full
strength. It will be readily understood that
in the case of a concrete column with ver-
tical rods only there will be a great ten-
dency for the rods to act as shown in an
exaggerated manner by dotted lines in dia-
gram A (Fig. 132), whereas, supposing two
intermediate links were introduced, this
tendency would be lessened, and the rods
could only bend as shown in diagram B.
If sufficient intermediate links are intro-
duced the tendency will be overcome alto-
gether, and it will be obvious, therefore,
that the lateral reinforcement is an impor-
tant factor in the design.
The draft regulations of the iondon
County Council state that the total cross
sectional area of the vertical reinforcement
shall not be less than 0-8 per cent, of the
nrea of the hooped core, and the volume of
lateral reinforcement shall not be less than
0-5 per cent, of the volume of the hooped
n8
REINFORCED CONCRETE
core. The effective diameter must be mea-
sured from the outside of the outermost ver-
tical reinforcement, and it must be measured
in the direction of the lateral supports
which determine the length of the pillar.
When the column is such that the laterals
are rectilinear, there must be at least four
lines of vertical reinforcement throughout
the entire length, and if the laterals are
curvilinear, there must be at least six lines
of vertical reinforcement, and the diameter
of the rods in any case must not be less than
£ in., and the pitch of the laterals must not
exceed six-tenths of the effective diameter
of the pier. In the case where rectangular
piers are used in which the ratio between
the greater and lesser diameter exceeds
EEC Jl LINEAR LAjER
C-f
A
(^
«&
\'\
~ "JTl
^
t
]HI5 ODTANCf
EXCEED "A"
J
b
t^
0
Figs. 133 and 134. — Rectangular and Circular
Columns with Respectively Rectilinear
and Curvilinear Laterals.
one and a half, the cross section of the
pillar must be sub-divided by cross ties,
and the number of vertical rods shall be
such that the distance between the rods
along the longer side of the rectangle shall
not exceed the distance between the rods
on the shorter side. This is illustrated in
Fig. 133, where the laterals are rectilinear,
and the diagram in Fig. 134 gives an illus-
tration of the use of curvilinear laterals. In
the case of concrete in columns in simple
compression the limiting stress is 600 Ib.
per square inch, and as the steel and con-
crete are considered as being shortened in
the same proportion, the stress on the steel
can never exceed m times the stress on
the concrete, or, in other words, m x c.
As m equals 15, then the permissible value
to allow for the steel equals 600 x 15 =
9,000 Ib. per square inch. In the case of
columns which are axially loaded, there-
fore, and when the height does not exceed
eighteen times the least diameter, the safe
load will equal the net sectional area of the
concrete in the core multiplied by c plus the
sectional area of the longitudinal reinforce-
ment multiplied by m x c. As an example,
let it be required to find the safe load that
can be carried by a rectangular column 15 ft.
high which has an effective size of 12 in.
by 12 in., and is reinforced with four verti-
cal bars, each having a diameter of 1 in.
The area of one rod will equal -7854 sq. in.,
and the total area of steel will equal 4 x -7854
= 3-1416 sq. in. The net area of the con-
crete will equal (12 x 12) -- 3-1416 =
144 - 3-1416 = 140-8584 sq. in. The safe
load on the column will equal A x c + Ar
x m c, where A equals the effective area of
the concrete, and Av equals the sectional
area of the steel. Working this out gives
140-8584 x 600 + 3-1416 x 15 x 600 =
84515-04 + 28274-4 - 112789-44 Ib., which
equals just over 50 tons. If preferred, the
total area of steel and concrete can be taken
and multiplied by c, and to this the extra
value of the steel can be added by taking
the sectional area and multiplying same by
fourteen times c. Having already con-
sidered the area of steel as taking 600 Ib.
per square inch, it will be clear that the
extra value will equal 9,000 - 600 Ib. =
8,400, which is the same as fourteen times c.
Let the previous example be worked out in
this manner. Then the safe load = 144 sq.
in. x 600 + 3-1416 x 14 x 600 = 86400 +
26389-44 = 112789-44 Ib. as before. To
state this as a formula, then W the weight
= A x c + Ay x 14 c, or Ar x 14 c = W
W - (A x c).
- (A x c), therefore Av = - , .
J.4: C
This last formula will give the method to
be adopted when designing a short column
to carry a certain given load, an example
of which is shown. Design a reinforced con-
crete circular column which is 12 ft. high to
carry an axial load of 40 tons. First settle
the diameter, which must be at least one-
eighteenth of the height. If we take 9 in.,
this will be one-sixteenth, and therefore well
within the limit. The sectional area will
equal 9 x 9 x -7854 = 63-6174 sq. in., and
the load equals 40 x 2240 = 89,600 Ib.
89600 - (63-6174 x 600)
14 x 600
51429-56
" = 8400™ = =
THEORY OF REINFORCED CONCRETE
119
rods are used, the area of each one will require
to be 1-02 sq. in. The area of a rod having
a diameter of lyV in- equals 1-1075 sq. in.,
and this size will therefore do very well, and
the total area of the steel will equal 1-1075 x 6
= 6-645 sq. in. For the purpose of practice,
check the column and see what safe load it
will carry, bearing in mind that the practical
area of the steel equals 6-645 sq. in. as
against the theoretical area of 6-12 sq. in., and
the safe load should therefore be slightly
over 40 tons.
The safe load equals A x c + Av x m c,
therefore W = { [(9 x 9 x -7854) - 6-645]
x 600} + (6-645 x 15 x 600) = W =
{[63-6174 -- 6-645] x 600 } + (6-645 x
9000)
W = 34183-44 + 59805
W = 93988-44 Ib. - 41-5 tons.
Which is correct. The column will then be
as shown in Fig. 134.
Having dealt with short columns, it now
remains to deal with those known as long
columns, namely, where the effective dia-
meter is less than one-eighteenth of the free
length. In such columns, as previously
stated, there is a tendency for the column to
fail by bending, although, theoretically, there
should be no such tendency if the column is
absolutely axially loaded ; but such loading
cannot be absolutely guaranteed in practice,
and experiments that have been conducted
from time to time with long columns go to
show that it is not advisable to load the
column to its full capacity as though failure
would occur through direct crushing. This
bending tendency cannot be measured or
calculated unless a certain definite amount
of eccentricity is known, and consequently
an empirical formula has to be used where
certain constants are given deduced by
experiment.
It is interesting to note that the London
County Council draft regulations contain
conditions which limit the loading on long
columns, and if the designer is content to
accept the rules laid down by such regula-
tions, he will have no need to employ a
complicated formula. The conditions are
given, as they will no doubt be of some
assistance, and particularly in the case of
checking the carrying capacity of a column
already designed. Briefly stated, the rules
are as follows : — A pillar shall be deemed
to have fixed ends when the ends of the
pillar are sufficiently secured to other parts
of the construction, having such rigidity
as will maintain the axis of the pillar at
the ends in its original vertical position
under all loads less than the crippling load.
When both ends are fixed, and when the
ratio of length to the effective diameter does
not exceed —
18 The full stress may be allowed on the
pillars
21 -8 of above may be allowed
24 -6
27 -4
30 -2
For other ratios the stress shall be pro-
portionate to the above. In the case of
compression members not having both ends
fixed, the loads shall be as follows : Let P =
the maximum pressure on pillars and com-
pression members having fixed ends.
Condition of Ends
One end fixed and one end hinged
Both ends hinged
One end fixed and the other end
free and not guided, stayed or
supported in all directions . .
Working
load
_P
2
P
4
P
16
Unfortunately, there is a great difference
between the various formulse that have
been deduced by various authors, and if the
student were calculating, say, a wooden
strut, and he adopted Gordon's formulse, and
checked the result by using Bitter's formula,
he would find that the latter would give a
load which would be about five times as
great as that found by Gordon's. Thus,
there is no real proof as to what safe load
can be adopted, or such a variation would
not be possible, and the most that can be
done is to recommend the student to adopt
a fairly safe method. Gordon's formula has
already been given in the preceding chapter ;
it is one of the best known in this country,
and is greatly used. In the absence of exten-
sive experiments with long, reinforced con-
crete columns, this formula may be adapted
for designing such columns, as hereafter ex-
plained. The formula is as follows : Rc =
Ar
c—r2, where Rc = the safe load in
1 + a^
the same units as rc, A = the area of the
120
REINFORCED CONCRETE
cross section, rc = the safe resistance of the
material in compression, a is a constant
derived by experiment, I is the length in the
same units as d, and d is the effective dia-
meter. It will be seen that the value Arc
would equal the safe resistance if the column
were a short one, and consequently the
I2
value 1 + a -ft is the reducing factor which
is dependent on the ratio of length to dia-
meter, and also upon a, which is governed
by the conditions of fixing. In the table
given on p. 79 it will be seen that for
wrought-iron and steel, where the section
was rectangular or circular, the values for
4
a were as follow : Ends hinged ^v
ends fixed
one hinged and one •&-„ —
fixed jTyyv an<^ these same constants may be
taken. As the value A rr in this formula
equals the safe resistance of a short column,
this can be substituted by the expression
which is already deduced for giving this
value, namely, A x c + Av x me, and if
the reducing factors given above are applied,
the complete formula becomes
A x c + Av x me
W = /2
l+(ld*
and this must be used when checking the
weight-carrying capacity of a column which
has a diameter less than one-eighteenth of
the free length.
In the case of designing a short column,
it was shown that the area of steel re-
quired was calculated by the formula Av =
W - (A x c)
— j, — , where A = the total area of
the cross section. It will now be obvious
Z2
that the value 1 x a ^ must again be in-
troduced, and more steel will be required
than would be the case with a short column.
The actual weight must therefore be multi-
plied by this value, giving a formula as
follows :
W
Av =
(12\
1 + a -p ] — (A x c)
/ •
14 c
Having now deduced formulae for finding the
area of steel required, and also for checking
the column to see the safe load which it
can carry, the application of these will be
shown in an example. A column is fixed at
the ends, and has a free length of 20 ft.,
while the load to be carried is 50 tons. The
column is to be rectangular, and it is limited
in size to 12 in. square. Calculate the
amount of vertical reinforcement required,
and afterwards check the load the column
as designed would carry. The size given is
the effective size, and the load is assumed
to include the weight of the column itself.
The load equals 50 tons, which is equivalent
to 112,000 Ib. The effective sectional area
will equal 12 x 12 = 144 sq. in., and as the
ends are fixed, the value of a will be
The steel will be found by the formula :
W x (l + aj[)- (A x c)
112000 x 1 +
14 c
J_
2500
— , therefore
2402\
x-Tp-j- (144x600)
/
A, =
Av -
14 x 6CO
112000 x (1 + -16) - 86400
8400
129920 - 86400
= A, =
43520
8400
8400
Av = 5-18 sq. in.
If four bars are used, then each will
require an area of 1-295 sq. in. A round
bar with a diameter of 1T^- in. has an area
of 1-353 sq. in., and this is the nearest size
obtainable, and the total area of steel
required will therefore be 1-353 x 4 = 5-412
sq. in. As designed, therefore, the column
is 20 ft. high, fixed at the ends, 12 in. square
effective size, and reinforced with four 1T"F
in. diameter bars. This should next be
checked by calculating the safe load it will
carry, and, to be correct, this load must
not be less than 50 tons. Now
A xc+A,, x me
W -
therefore
W =
W -
138-588 x 600 + 5-412 x 15 x 600
1 +
2402
25CO
83152-8 + 48708
W -
131860-8
1-16 1-16
W = 113673 Ib. = 50-7 tons, which is quite
satisfactory, the slight amount over 50 tons
being due to the fact that the actual amount
of steel equals 5-412 sq. in., which is slightly
in excess of the theoretical amount calcu-
lated, namely, 5-18 sq. in.
In order to form a comparison with the
THEORY OF REINFORCED CONCRETE
121
conditions given in the draft regulations of
the London County Council, as previously
stated, it will be interesting to calculate what
load could be carried by this column if the
rules given therein are applied. In the first
instance, it will be necessary to work out the
load that could safely be put on a short
column having the same sectional area and
reinforcement, and then apply the reducing
factor which is dependent on the ratio of
diameter to length, as given in the table
previously.
The safe load for a short column would be
found by the formula :
W = A x c + Av x m c, therefore
W = 138-588 x 600 + 5412 x 15 x 600
W = 83152-8 + 48708 = W = 131860-8 Ib.
This, then, would equal the safe load for a
short column, and the ratio of diameter to
length must next be considered. The length
is given as 20 ft., and the effective diameter
equals 12 in., and the ratio, therefore, equals
20. Upon referring to the table, it will be
seen that where the ratio of diameter and
length does not exceed 21, the safe load can
be taken as -8 of that for a short column.
The safe load for this example will therefore
equal 131860-8 Ib. x -8, which equals
105488-64: Ib., or 47-09 tons. This is slightly
less than the safe load, as calculated by the
formula, which was 50-7 tons and when
working in actual practice, it would be neces-
sary to keep within the limits allowed by the
London County Council rules, if within the
County of London. It must be remem-
bered, also, that the ratio of length to dia-
meter is actually 20, and not 21 as taken,
and this, obviously, will affect the result.
It is very seldom, however, that such a fine
line would be drawn, and the designer in
calculating the loads c nnot work so closely
to the actual loads as to give no margin
whatever. As stated previously, the vari-
ous formulae and methods for calculating
compression members will always be found
to vary considerably, and in this instance
there is only a difference of about 6 per cent.
COLUMNS ECCENTRICALLY LOADED
It is proposed to give a few notes on the
eccentric loading of columns and its effect,
although the value of the eccentricity is
very difficult to obtain, unless such eccen-
tricity is caused by certain definite loads
applied at a certain definite distance from
the centre of the column. Eccentricity in
loading is caused in many ways, a common
case being that where the load from a beam
is transmitted through a bracket projecting
from the column, or by the beam not being
carried right over the column. The eccen-
tricity is sometimes such that it causes one
edge of the column to be in tension, and
sufficient reinforcement must then be pro-
vided to resist same. The fact of the load
not being axial will cause the stress in the
column to vary over the area of the cross
section, and the compression will be at its
maximum at that edge which is the nearest to
the eccentric load, and at a minimum at that
edge which is the farthest away from same.
It is necessary, therefore, to consider the
two extremes, and this can be done by find-
ing the pressure that would occur if the
load were axial, and adding or deducting the
value of the eccentricity. Where the com-
pression is at a maximum, therefore.
Max. c =
W Wy
AE Z
where W = the load, AE = the equivalent
area of the concrete in square inches = the
sectional area of the concrete + 14 times the
sectional area of the steel = A + (w - 1) A,,,
y = the eccentricity of the load in inches,
or, in other words, the distance from its
point of application to the neutral axis of
the section, and Z = the section modulus.
This last expression was explained in the
preceding chapter, and the following values
for reinforced concrete columns are given.
d 2
Eectangular Z =^A.d + ^(m- I) Av -4-,
where A = the total area of the cross section,
d = the outside diameter of the column, and
d± = the distance from centre to centre of
the vertical reinforcement. Circular column
with four bars only
d£
Zj • — a" .A. (t "T ?? \)Tl J. ) -i*-« ^~,
d '
and for circular column with bars arranged
in a circle,
On that edge farthest from the load the mini-
W W?/
mum compression = Mm. c •- — r- =-.
From the above formula may be deduced a
formula for finding W, which is
Max. c x AE x Z
W =
Z + AE x y
It will now be advisable to work out an
example to show the application of these
122
REINFORCED CONCRETE
formulae. Find the safe load that can be
carried by a column 16 in. square reinforced
with eight IJ-in. rods, the load being applied
vertically at a distance of 2 in. from the axis,
and the distance from centre to centre of
Fig. 135. Designing Column Eccentrically
Loaded
rods being 13 in. The maximum stress on
the concrete is not to exceed 500 Ib. per
square inch. The plan of the column, as
given, is shown in Fig. 135, and upon refer-
ence to this the symbols can be followed.
The safe load will be found according to the
formula :
W =
Max. c x A x Z
^ + AE x y
Max. c is given as 500 Ib., and y = 2 in.,
while AE will equal A + (m - 1) Av = 16 x
16 + (15 - 1) 9-8176 = 256 + 137-446 =
AE = 393-445. Av is given by the area of
the rods, the area of one IJ-in. rod = 1-2272
sq. in., and the total area = 1-2272 x 8 =
9-8176 sq. in., as given above. The value
of Z must next be found by the expression
Z = (£ A x d) + % (m - 1) Av x -i
therefore,
Z = (i x 256 x 16) + \ x 14 x 9-8176 x •- --
Z = 682-66 + (68-7232 x 10-56)
Z = 682-66 + 725-717
Z = 1408-377.
Having determined the value of the vari-
ous factors, proceed to find the value of the
weight as follows : —
... 500 x 393-446 x 1408-377
W =
W =
1408-377 + 393-446 x 2
277060148-571
As a check to this, and also as an example
of the use of the formulae for the maximum
and minimum compression, let it be required
to calculate the maximum and minimum
compression on the column above men-
tioned, and carrying the calculated eccentric
load. Then the maximum compression,
which should not exceed 500 Ib., will be
found by the formula :
W W y
Max. c = -j- H — —.
The values of AE and Z have already been
calculated, and therefore,
126207 126207 x 2
~ 393-446 4 1408-377
Max. c = 320 + 179-2 == 499-2 Ib.,
which is within the limit. The minimum
compression will be given by the expres-
sion :
Min. c =
126207 126207 x 2
393446 1408-377
Min. c =320 - 179-2 = 140-8 Ib.
The edge of the column farthest from the
load is therefore not in tension, and the
column is satisfactory. The diagram given
in Fig. 136 shows how the stress varies across
the section, and the length of the vertical
line at any intermediate point represents the
2195-269
W = 126207 Ib., or just over 56 tons.
- 499'2ia»
Fig. 136. — Stress in Column Eccentrically
Loaded
compression stress at that point. The stress
on the steel being fifteen times that on the
concrete the value can be found in any of the
bars by scaling the length of the line coinci-
dent with the axis of the bar, and mul-
tiplying same by 15.
THEORY OF REINFORCED CONCRETE
123
RETAINING WALLS
It is not proposed in these notes to cover
the whole theory and design of retaining
walls, but rather to give some general re-
marks to enable the student to form an idea
of the principles governing the design of such
structures. Walls may be used to retain or
support either earth or water, and the
method employed in reinforced concrete is
very different from that which is customary
with brickwork masonry or plain concrete.
Fig. 137. — Influence of Angle of Repose
and Line of Rupture on Design of
Retaining Walls
In the last-mentioned case the wall is depen-
dent on its thickness and weight, which must
be such that its gravity is not overcome by
the pressure acting on the back of the wall ;
whereas in the case of reinforced concrete
the wall is designed to offer sufficient resist-
ance by its strength. Tension at the back
of a reinforced concrete wall is permissible,
and steel is provided to resist it ; but it is
obvious that a brick wall, for example, would
be capable of resisting very little tensional
stress. A good comparison of the two
methods of design is offered by the front
and back retaining walls constructed at the
Royal Automobile Club (described in a later
chapter), where both plain and reinforced
concrete were used.
In dealing with walls to retain earth, it is
necessary, in the first instance, to investi-
gate the nature and value of the force that
has to be resisted. It will be quite clear
that a mass of earth standing alone without
lateral support will not remain with vertical
faces, but that some of its particles will fall
past one another, since they have little or
no cohesion, and form a mound having slop-
ing surfaces. Sand poured upon a floor forms
a mound having sloping sides, and it will
be seen that there is a certain angle with
the horizontal which the material will main-
tain even without lateral support. The
angle depends on the friction between the
particles, and is called the " angle of repose ";
it varies in different materials, and also in
the same material according to- the state of
consolidation and dryness. The following
table must be taken merely as a guide, and
cannot be considered as fixed by any hard-
and-fast rule : —
Material
Wet clay or vegetable earth
Wet sand
Dry clay, sand, or vegetable earth
Sandy gravel
Clean, firm gravel
Loose shingle
Hard, dry vegetable earth
Clay, well drained . .
16C
20C
30C
26°
45C
40°
45C
45°
This angle can be considered, then, as the
natural angle at which the material will
rest, and this is shown in diagram A (Fig.
137), where the line of rupture is also indi-
cated. It is considered that only the wedge-
shaped portion of earth contained between
the line of rupture and the face of the wall
will require to be supported, as upon the
sudden removal of the wall it is that portion
which would slip, and this has been proved
more or less by actual experience. There will
also be a large amount of friction set up
between the portion that is tending to slide
and the portion that is normally at rest, and
by considering that portion above the line of
rupture only, an allowance is made for
this, and it will be seen in the diagram that
the wedge-shaped portion in tending to slip
downward is really sliding on the inclined
plane represented by the line of rupture.
The line of rupture is found by setting
out the angle of repose and bi-secting the
124
REINFORCED CONCRETE
angle between this and the back of the wall ;
the earth that is contained in this triangle
is calculated and the load worked out
according to the weight per cubic foot of
the particular earth under consideration.
It is usual to take a length of 1 ft. in the
calculations, both in calculating the weight
of the soil to be supported and in the design
of the wall. In the calculations it is neces-
sary to consider both the vertical portion
of the wall and also the base, which requires
to be sufficiently large to distribute the
pressure over such an area of foundation
as to bring the load per square foot within
the safe resistance of the soil. Also the
base must be of sufficient thickness to allow
the projecting portions to act as cantilevers.
The vertical portion will be a cantilever
on the back of the wall will be found by
drawing a line parallel to the line of rupture
from the top of the line representing the
weight until it cuts the horizontal line drawn
through D. This horizontal thrust can then
be multiplied by its distance from the
point of intersection with the base, which
will equal -, and this will give the bending
o
moment to be resisted. The bending moment
will gradually diminish towards the top,
where it will be nil.
With regard to the base, there are three
general types of wall employed, as illus-
trated at A, B, and C (Fig. 138), the selec-
tion of one of which will depend upon cir-
cumstances. The type shown at A, for
example, could not be employed if the
/////'/
V// ///////'
tAEJH
'/ \
'/ N
1 •
\ fCovnjvs.fOK\
1
\
\
\
y
\
'/,
\
'//, A
\
'/ EAKTO
B
Fig. 138.— Three General Types of Retaining Walls.
having a length equal to the height of the
wall, and it is obvious that the greatest
stress will occur at the bottom, and this will
be due to the outward pressure of the earth
which can be ascertained as shown in dia-
gram B (Fig. 137), when the centre of pres-
sure of the triangular prism of earth ABC
is shown at a point equal to --- from the
o
bottom, where h equals the height of the
wall. The value of the pressure will be
found by calculating the weight of the earth
contained in the triangle, and also the centre
of gravity of the triangle as illustrated.
The weight will act downward through this
centre of gravity, and if a line is drawn down-
ward through this point until it cuts the
line of rupture at D as shown, and the
amount of the weight is set up to scale from
this point, the horizontal equivalent thrust
adjacent earth was the property of an
adjoining owner who might raise objection
to the projection under his land. This was
the case at the Koyal Automobile Club, and
it became necessary to design the wall with
the projecting toe extending into the build-
ing as illustrated at B. In the former case
the weight of the earth acting downward
on to the base will tend to prevent the over-
turning of the wall, whereas in B this is
not the case, and sufficient weight must be
provided to prevent the wall from being
thrust over on the extreme outer point of
the toe, which will be the fulcrum of the
lever. The type shown at C is some-
what between the others, having a projec-
tion on both sides. Provision to prevent
sliding on the base is often made by step-
ping the under side in such manner that
vertical surfaces are provided, as shown
THEORY OF REINFORCED CONCRETE
125
by dotted lines in B. The length of the base
for preliminary calculations is often taken
as about one-half of the height, and the
thickness is, of course, calculated from the
bending moment, which it has to resist as a
cantilever.
The type shown at A is often stiffened by
counterforts at intervals, as shown by dotted
lines, these serving as stiffeners, both to
the vertical wall and the base. In London
work it is often necessary to construct
vaults under the pavement, when the outer
Fig. 139. — Retaining Wall with Cantilevers
under Footpath
wall will act as a retaining wall to support
the roadway. A type that is sometimes
employed under such circumstances is illus-
trated in Fig. 139, where the top horizontal
portion acts as a cantilever from the vertical
wall and supports the footpath, while the
base acts as a cantilever from the bottom
of the wall and has to resist the earth pres-
sure. This type of construction was em-
ployed at Messrs. Whiteleys' new building
in Queen's Road, Bayswater, London.
In the case of walls called upon to resist
the pressure of water, matters are somewhat
simplified, as there is no variation due to
weight or angle of repose, and the pressure
will increase according to the depth of the
water and always act at right angles to any
surface. A cubic foot of water weighs 62£ lb.^
and the pressure in pounds per foot against
the wall will therefore equal the depth of the
water in feet multiplied by 62|. It will
be seen, therefore, that the pressure varies
directly as the depth as shown by the shaded
portion in Fig. 140, and the centre of the
whole pressure will be situated at the centre
of gravity of the triangle, which will be situ-
ated at one-third the height from the bottom.
The value of this pressure will equal the
height multiplied by the mean pressure, the
62| x height
latter being = - — - — — . This pressure
jt
multiplied by the height from the intersec-
tion of the wall with the base will give the
Fig. 140.— Retaining Wall to Resist Water
Pressure
maximum bending moment. In the case of
walls where the surface next the water i&
sloping, the total pressure will be greater or
less according to whether the surface is
sloping away from or to the water ; but
as the pressure will always act at right
angles to the surface, the variation in the
angle of the pressure will compensate for
any variation in the amount of the pressure.
The subject of retaining walls is one that
requires very careful consideration when
designing, and the student is advised to
acquire a knowledge of trigonometry before
attempting to become fully conversant with
this branch of reinforced concrete work.
A study of the examples already carried out
in practice will also be helpful.
The Erection of a Reinforced
Concrete Building
IN tliis chapter will be described tlie method
or system of carrying out the construction
of the complete carcass of a reinforced con-
crete building. Attention will be drawn,
not only to the correct way of doing the
work, but also to the faults and pitfalls
that have to be guarded against.
In reinforced concrete work there are four
things that need to be most carefully
observed : (1) strict adherence to the special-
and efficiency of the structure depend, and
if they are not most strictly observed failure
is almost inevitable. Failure, be it always
remembered, not only means great and
unnecessary expense, but involves enormous
risk to life and limb. It need hardly be
said that it behoves all those who undertake
reinforced concrete construction to employ
only reliable supervision and labour.
1st engineer's drawings as to the deposition BUILDING A FACTORY
of the steel reinforcements ; (2) careful and It will be assumed that the building to be
efficient mixing and punning of the concrete erected is of the warehouse or factory type,
being some three or four stories in
1 ^j height (see Fig. 141), and to which
1 !
! . !
will be attached the usual engine and
boiler-house, generally a one-story
structure. It will also be assumed
that the site is a bad one, and the
engineers have found it necessary to
provide a reinforced concrete pile
foundation for the main building, and
a reinforced concrete raft for the en-
gine and boiler-house. To the ware-
house there will be a large basement,
which will involve the erection of a
retaining wall to support the road-
way on the
frontage.
• •
I ' "
1 '
• •
1 " '
' '
-dllllLJUN—, \ Ti, Carting
•
, ! !
1 '- !.
'
a job of this
description,
^ | the first thing
]
,^^-»fl.->-"m«.ilnvii^,""' "4™-
I J
Fig. 141.— Vertical
1 11
I 0
Cross Section
(
m»O^. i'-*1^"^ VWWWjdyj
1 . (
of Typical Fac
Concrete
rsite of all rub-
bish, hack up
all old found
ations, and
» leave the site
tory Building in Reinforced clear for im-
mediate oper-
ations. Next,
so that the steel is entirely surrounded by
this material ; (3) the proper design and
the complete stability of the falsework ;
(4) the proper and careful striking of the
falsework.
On these items the strength, durability,
the general foreman must select suitable
positions for his office and the office of the
clerk of works, and for the building material
stores ; for the last -mentioned it will be
necessary to erect some sheds. The con-
tractor is now ready to receive materials.
126
ERECTION OF A BUILDING
127
Receiving the Bars. — Considering first
the arrival of the steel reinforcement rods,
these should be unloaded carefully so that
the thinner rods are not twisted ; they are
easily put out of shape by rough handling.
All the rods and bars must be sorted and
bundled in their respective lengths and
diameters, a complete list of which must
be kept by the foreman. The sizes should
be indicated by pegs or in some other way,
so that the risk of errors may be lessened ;
the importance of this will be realised when
it is remembered that a unit of one-sixteenth
of an inch is adopted in specifying the bars,
and should bars be in a wrong place or of a
smaller diameter than detailed trouble may
be caused. Bars that arrive twisted and dis-
torted must be put aside to be straightened.
Steel bars are frequently laid down in the
open, but this is not good practice, and
temporary sheds should be erected for their
reception, because exposed steel becomes
badly scaled and pitted with rust, which
ought not on any account to be permitted.
Though it has been recognised that slight
rust is beneficial for the protection of steel
in concrete, deep pitting or rusting means
the reduction of the diameter appreciably.
The amount of rust permissible will occur
while the steel is lying in a temporary
building. Where there is no room for
sheds, the steel should be given a coat of
cement wash and then covered with tar-
paulins. It is a good practice to erect
temporary racks or stands in the sheds for
the reception of the bars, so that they may
be easily lifted down when required ; this
method will prevent excessive distortion in
bars less than f in. in diameter, which get
very badly twisted when laid in rough heaps
of considerable weight ; the wholesale
straightening of bars should be unnecessary.
At the end of each division in the steel rack
a board should be attached with the lengths
and diameters of the rods indicated thereon,
as in the accompanying table, the numbers
in the first column indicating the number of
1 in.
TV in.
l|in.
50 5—0
50 5—6 !
35 6—9
42 9—6
82 11—6
95 20—6
120 10—0
128 12—6
141 26—6
163 27—6
181 30—0
200 31—6
32 14—0
38 16—6
58 17—9
64 18—6
73 19—0
80 20—0
•
bars in stock, and those in the second giving
the lengths in feet and inches.
By this simple means any bar of any par-
ticular length can be obtained at once, which
is impossible when all bars of the same
length are bundled together irrespective of
their thickness.
Aggregate and Sand. — The position
allotted for the deposit of the gravel and
sand should be as near the mixing stage as
possible, so as to save long barrow runs. In
most cases the sand and gravel come to the
job already screened and washed, and in a
large contract this is nearly always advisable,
as otherwise a gang of men would have to
be kept continually washing and screening.
Should the contractor be fortunate enough
to find sand, etc., on the site, the washing,
etc., will be inevitable. The gravel or
aggregate must be deposited in a separate
heap from the sand. Both the clerk of
works and the foreman should see that there*
is no clay or loam mixed with the sand or
gravel. Should broken stone or broken
brick constitute the aggregate, see that it is
free from dust ; in the case of furnace slag
or similar material, no sulphur or other
impurities should be present. In regard to
sand, sharp coarse river sand is preferable,
or good pit sand of various size grains up
to particles that will pass through a J-in.
square mesh, and of which at least 75 per
cent, will pass through a J-in. square mesh.
Sand must be free from all ligneous (woody),
organic or loamy substances.
Cement. — For the cement a specially dry
shed should be erected. The familiar rough
sheds, which are not proof against weather,
should not be permitted. As to the testing
of the cement, that is fully dealt with else-
where in this book ; but it should be said
here that the foreman and the clerk of
works should see that the cement arrives in
sealed bags bearing the maker's name, and
they should from time to time make prac-
tical tests to see that the cement is cool
and in perfect condition. Cement must
always be stored out of contact with wind,
damp air, damp walls, or damp ground, as
otherwise it quickly loses its strength.
Water. — If practicable, water should be
supplied from a main, but if not, it should
come from a good spring or boring. It is
best to have the water laid on by a supply
pipe to the mixing stage, and such a supply
is a necessity when machine mixing —
which is here advocated — is adopted.
123
REINFORCED CONCRETE
Mixing Stage. — The foreman must con-
sider carefully the position of the mixing
stage or the mixing machine, as it is from
this point that the building grows. Prefer-
ably the stage should always be open to
Fig. 142.— Mixing Stage
inspection throughout the entire period of
the job ; certainly it should not be hidden
in an obscure corner, where it is difficult to
watch and check the operations. The ideal
position for a mixing stage is one where it
can remain as long as possible and where
it is convenient for the hoisting of the con-
crete to the different parts of the building.
at each end to form handles (see Fig. 142).
A 2-in. fillet nailed round the board will
prevent the wastage of cement. A level
space should be prepared for the mixing
board.
Hoisting. — Provision should be made for
hoisting the concrete, timber and steel.
On a large job a derrick and Scotchman
should be erected, as well as a concrete
hoist, some efficient types of which have
been introduced.
TOOLS AND APPLIANCES
Generators' tools include many that
are familiar to the plasterer, while for
finishing concrete surfaces he uses hacking
tools (see p. 244), the idea of which has been
borrowed from the mason's kit. The tamp-
ing or punning irons, for ensuring that the
concrete is well tamped around the reinforce-
ment, may be of the shapes shown in Figs.
143 to 146 and 152. Other punning tools,
special spades, etc., are shown by Figs. 147
to 151.
Fi«. 143 Fig. 144 Fig. 145 Fig. 146 Fig. 147.-Perforated
Figs. 143 to 146. — Four Patterns of Iron Tamper Spade
The matter is entirely one for the good
judgment of the foreman. The mixing
board, measuring about 15 ft. by 15 ft.,
should be constructed of good, straight, dry
boards of even thickness, braced together
at each end, and framed up on scantlings of
2 in. by 4 in., the longitudinal ones projecting
For cutting bars, a hack saw — preferably
a machine saw — is useful for large sizes ;
bars can also be severed by nicking and then
breaking. Hand-operated shearing machines
are available for rods of ordinary size.
Small bars used as stirrups can be cut
through with a hammer and chisel, with a
IN PILLARS AND BEAMS
A - the effective area of the pillar.
Ac - area of compressional reinforcement (in
sq. in.).
AE = area equivalent to some given area or area
of an equivalent section or equivalent
area.
As- - cross-sectional area of a vertical or
diagonal shear member, or group of
shear members, in the length p, where
p = pitch of stirrups.
At -- area of tensile reinforcement (in sq. in.).
A7/ - area, of vertical or longitudinal reinforce-
ment in sq. in.
a - arm of the resisting moment or lever arm
(in in.).
a' = arm ratio = a'd . • . a'd = a.
oc = depth or distance of the centre of com-
pression from the compressed edge.
B M = bending moment of the external loads and
reactions (in Ib. in.).
Generally, 6 = breadth.
IN TEE BEAMS
6 — breadth of flange of beam (in in.).
br = breadth of rib of T beam (in in.).
IN BEAMS
c = compressive stress on the compressed edge
of the concrete (in Ib. per sq. in.).
C = total compression on the concrete (in Ib.
per sq. in.).
IN PILLARS
c = working compressive stress on the con-
crete of the hooped core.
cs = compressive stress in the steel (in Ib. per
sq. in.).
IN CIRCULAR SECTIONS
Generally, d = diameter.
IN RECTANGULAR SECTIONS
Generally, d = depth.
IN PILLARS
d = the diameter of the hooped core in in.
IN BEAMS
d = effective depth of the beam (in in.).
dc = distance from compressed edge to centre
of compressional reinforcement.
rf.s -= total depth of the slab (in in.).
IN PILLARS
d1 = distance between the centres of vertical
bars measured perpendicular to the
neutral axis.
E,- - elastic modulus of concrete (in Ib. per sq.
in.).
E, = elastic modulus of steel (in Ib. per sq. in.).
I - inertia moment of a member.
I - length of a pillar or effective length of
span of beam or slab.
Es
TO - modular ratio = vv •
jfic
M S == maximum shear.
n = neutral axis depth — i.e. depth of neutral
axis from the extreme compressed edge
(in in.).
n' = n/d = the neutral axis ratio . • . n'd = n-
O = perimeter or circumference of bars.
P = total safe pressure.
IN PILLARS
p = the pitch of the laterals in in. (i.e. the
axial spacing of the laterals).
IN SHEAR FORMULAE
p = pitch or distance apart (centre to centre)
of the shear members or groups of shear
members (measured horizontally).
IN BEAMS
r = A./bd = ratio of area of tensile reinforce-
ment to the area bd.
R1 = left-hand reaction.
R2 = right-hand reaction.
S = total shear in Ib. at a vertical section.
s = intensity of the shearing stress on con-
crete in Ib. per sq. in.
ss = shearing stress on the steel (in units of
force per unit of area).
T = total tension in the steel (in Ib.).
t = tensile stress on the steel (in Ib. per sq. in. ) .
IN PILLARS
V = volume of hooped core in cub. in.
W = total working load or weight on any
member.
w — weight or load per unit oi length of span.
y = eccentricity of the load measured from the
centre of the pillar (in in.).
Z = section modulus.
In Gordon's formula: — R,;= total safe resistance to compression.
to compression.
total resistance of material
Note. — The above is based on, and modified from, the notation proposed by the Concrete Institute and adopted
by the Royal Institute of British Architects, but certain symbols have been omitted, since they are not used in the
book, and a few others have been added. The notation is built up on the principle of an index, the significant
word in each term having been abbreviated to the initial letter, subscript letters being added in many cases.
Capital letters indicate moments, areas, volumes, total forces, total loads, etc. Small letters indicate intensity
of forces, intensity of loads, intensity of stresses, lineal dimensions (lengths, distances), etc.
ERECTION OF A BUILDING
129
hammer and anvil block, as shown in
Fig. 153, or with cutting pincers or pliers,
several excellent patterns of which are now
available.
bending, especially in the case of bars partly
embedded in concrete. The claw wrench
(Fig. 156) is helpful for bending ends of
stirrups over bars, and is believed to have
Fig. 148. Fig. 149.— Tamper for Producing
-Ross Spade Fine Surface
Fig. 150.— Special
Spade for Facing
Fig. 151.— Andrews
Tamper
been introduced by the Coignet firm. The
wrench shown by Fig. 157 answers a similar
purpose. By means of the key or twister
shown by Fig. 158, the ends of stirrups can
be twisted together. For the wiring together
of the reinforcements where they intersect
one another, practical workers have their
Fig. 152.— Wooden Tamper
Bending appliances include hand tools
and machines. The wrenches shown by
Figs. 154 and 155 are useful for general
9
Fig. 153. — Cutting Anvil and Hammer
own individual methods which they believe
to be best, but there are undoubted advan-
tages in employing a simple tool of the type
introduced in the United States under the
name of the Curry tyer. The Curry tyer
is about 12 in. long, and the method of
using it is shown in Fig 159. The wire ties
used are a few inches long, the actual length
depending upon the thickness of the bars,
130
REINFORCED CONCRETE
Fig. 154
Fig. 155
Figs. 154 and 155.—
Wrenches for Bending
Ends of Bars
Fig. 156
Fig. 157
Figs. 156 and 157.—
Wrenches for Bending
Ends of Stirrups
Fig. 158. — Key or Twister
for Bending Ends of
Stirrups
Fig. 161. — Kennedy Bar Bending Machine No. 1
Fig. 159.— Curry Tyer
Fig. 162. — Kennedy Bar Bending Machine,
Geared Pattern
a
D
6
D
D
-C
It
1
i
A
n~n
--:-"---:::*
_/i\_
B
*
"ttn z '
D
1!
^
!
lf\T
E ''N;\
XC'
-A
/c
.. ^ r
B
Ex F
--F--^"-"-^
f
I !(
1 S \
1"
^
, ;•
1 „ " " m _
XA D
u
U
L~^>
Fig. 160. — Elevation and Plan of Bench Bending Machine
ERECTION OF A BUILDING
and they have a loop at each end. The tool
itself is a twisting appliance resembling in
principle an automatic screwdriver, there
being a sliding handle working up and down
in a spiral groove machined on a revolving
shaft inside the handle. The working end
of the tool has two hooks ; over one of these
is passed one loop of a wire tie, the tie is
then passed around the place of inter-
section, and the other loop is caught over
the remaining hook. The sliding handle is
then drawn back smartly, with the result
-_-
Fig. 163. — Making Bend to Given Inside
Measurement
Fig. 164.— Making Bend to Given Outside
Measurement
Fig. 165.— Making a Double Set
Fig. 166.— Making Sharp Bend in Thin Bar
132
REINFORCED CONCRETE
that the two ends of the .tie are twisted
together tightly. Ties can be made by this
means at least twice as fast as when pliers
and straight wire are used, besides which
the work is more uniform.
In all cases it is advisable to bend bars
cold ; a course that is now easily possible
with bars up to 1| in. diameter. The small
bars (up to 1 in. in diameter) can be bent
on a long, strongly constructed, temporary
bench to which is attached a vice for holding
the bar firmly, the end to be bent lying
between two steel pins fixed in a template
on the bench ; a strong steel lever with a
hole in it to fit over one of the pins must be
used, the bar lying across the lever between
the steel pin on which the lever rotates and
the pin fixed on the lever. By pulling the
lever round, the bar is bent to any angle.
Fig. 160 shows a machine of this type. To
obtain the correct angle, a clear chalk line is
marked on the bench to the angle required,
and the bar is pulled over to lie on it. The
machine above described can be made on
any job at a cost of about 30s., and possibly
less, as the contractor would probably have
sufficient stock material at his workshops to
enable him to fit it up. The bending bench
shown by Fig. 160 and above referred to is
based on one illustrated in the Coignet hand-
book. The rigid bench has a hard-wood top A,
vice B, and bar rest c. The lever D rotates
on a fixed pivot E, and has holes for pins or
rollers F. Such a machine is suitable for
bars up to 1 J in. diameter. A right and left
double bar bender on somewhat similar
lines is shown in the photographic view
(Fig. 167).
Another method of bending bars cold is
by means of a special bending machine, and
Kennedy's may be mentioned as being a
most effective device by means of which cold
bars up to 1£ in. in diameter may be bent.
The Kennedy bar-bending machine is
made in four patterns. No. 1 (see Fig. 161)
has a direct lever, and bends bars up to |-in.
diameter, the smallest radius being \ in. ;
its weight is 25 Ib. As shown, the bar is
held between a stop and a central mandrel
or former, the bending force being applied
by a lever through a pulley in contact with
the bar. No. 2 (see Fig. 162) is geared and
will bend 1-in. bars to a £-in. radius ; its
weight is 124 Ib. No. 3 is much the same as
No. 2, but in addition there is a ratchet
arrangement, while No. 4 is a powerful
worm-geared machine for bending li-in.
bars. To obtain a given measurement inside
to inside of bends A B (Fig. 163), having
made one bend, place the bar as shown, mark
off distance required, mark the line c D
square with A B, and then make the second
bend. When a given measurement E F (Fig.
164) is to be taken over all, make the first
bend, mark off desired measurement G
along the bar, square off from G as before,
taking care to allow for thickness of the bar,
and then make the bend. To make a double
set, having made the first bend, reverse
the bar and place it against the stop, as in
Fig. 165 ; let a straightedge be put against
the mandrel or former, parallel with A, so
as to indicate the extent of the second bend.
For making a sharp bend in thin bars, an
extra former A (Fig. 166), shaped to suit the
section of the material, is inserted, the bar
placed between A and the central former B,
and the lever c, carrying a stop instead of a
pulley, is pushed round.
When large diameter bars have to be bent,
the aid of heat has to be sought, but they
should not be heated above a cherry red —
that is to say, they should be heated only
just sufficient for the purpose. For stirrups
and short lengths of small diameter bars,
such as T3F in. and \ in., a single bend can be
made quickly simply by bending the bar
round a steel pin on a bench by means of a
wrench, as shown in Fig. 168 ; another way
Fig. 168.— Method of Bending Small Bars
is to fix a piece of steel tubing to a vertical
wooden post, nail a stop underneath it, place
the bar in the tube, and pull it over, by
which means stirrups can be made very
quickly.
In almost every case, the bars in a slab or
wall have their ends just bent over for the
purpose of forming a key or grip in the con-
crete ; such bends can be made with a blow
or two from a hammer.
DRIVEN PILES
Now that the contractor has everything on
the site for the erection of his building, the
ERECTION OF A BUILDING
pile making and driving is the first tiling to
which, he must turn his attention.
The Pile Skeleton. — For the steel skele-
ton, the correct bars specified must be
selected and placed on trestles close enough
to prevent sagging ; the bars are then
placed in wood templates, which may
be square or circular according to the
engineer's requirements. These templates
must be removed as soon as sufficient bind-
ing is done to hold the longitudinal bars in
their correct positions. The next process
is to add the binding, which may be done
in several different manners according to
the design of the pile. In the case of a
square pile of the Hennebique type, links
have to be made which wrap round two rods
at a time, as clearly illustrated in Pig. 169.
These links must be exactly spaced at the
given pitch shown in the illustration. For a
circular pile of the Coignet type (shown
by Fig. 170) the links or ties encircle the
bars, and the ends of the links are bent
tightly round a single bar, and at the point
where the links touch the bars they should
be tightly tied with annealed wire. For a
pile of the Considere type, which is octagonal
in shape, the binding is of spiral form, the
spiral being first wound on a drum and then
threaded on to the bars. This type of pile
is shown in Fig. 171. The pitch of the spiral
must be strictly adhered to. Whilst these
skeleton frameworks are being made, the
joiners will be preparing the wooden moulds
for their reception, a detailed description
of which is given in the next chapter. As
the moulds can be used and re-used, it is
at the foreman's discretion how many he
makes, but there should be sufficient to
allow of the work continuing without
hindrance ; the more times one mould can
be employed the greater the economy, and
it will here be useful to state that in ordinary
circumstances pile moulds can be removed
in four or five days after the concreting.
When several moulds are completed, the
steel skeletons, which have been carefully
lifted from the trestles and laid on planks
on level ground, should be taken up and
placed in the moulds, and the cast-iron pile
shoe will then have to be fixed to the bottom
of the steel framework in the mould. Having
made a careful inspection to see that the
steel framework is lying true in the mould,
a gang of concreters with punning rods or
rammers should be set to work ; every bit
of concrete placed in the mould should be
carefully punned so that the steel is com-
pletely covered. Keen supervision should
be exercised, because if by any chance any
cavities form, or there is a large piece of
aggregate loose in a pile, disaster will over-
take that pile under the first blow of the
driving-ram.
In very important work it is better to
cast the piles in a vertical position, this
being considered to give a more uniform
strength over the cross section.
When the concreting is completed, the
uppermost face must be carefully levelled
Fig. 169.—
Hennebique
Square Pile
Fig. 170.—
Coignet
Round Pile
Fig. 171.—
Considere
Octagonal Pile
off, and the pile may be left to dry. In hot
weather the concrete should be watered (by
means of an ordinary watering-pot fitted
with a rose) daily for a week or ten days ;
but this should not be done until the con-
creting has been completed for ten or
twelve hours. Ordinarily, a well-made pile
can be safely driven in six to seven weeks
after making. In cold weather the newly-
made pile must be carefully protected from
frost by covering with sacking, etc. When
the piles have been in the moulds for
four or five days, as already stated, the
moulds may be struck — work that must be
carefully done, so that the arrises are not
chipped off and the steel bared. This
Fig. 172. — Lidgerwood Pile-driving Engine and Reinforced Concrete Pile
134
ERECTION OF A BUILDING
applies to circular as well as square and
other piles, because the " circular " pile has
two flat sides about 4 in. wide for the pur-
pose of facilitating the driving, an arris
being therefore formed at the juncture of
the flat side and the circular side, as shown
in Fig. 170. In cases where a wooden dolly
alone is used without a cast-iron helmet
on the head of a pile during driving, it is
advisable when making the pile to use a
richer mixture for a distance of from 2 ft.
to 2 ft. 6 in. down from the head, the object
being to strengthen the head so that it will
not be seriously damaged under the impact
(a wooden " dolly " actually takes the blow
of the ram or monkey). The proportions
of the concrete for use in piles need to be
determined scientifically.
Driving of Piles. — Eeinforced concrete
piles are driven with a special pile-driving
machine or engine, among the best known
being the Lecour, Sykes, and Lidgerwood.
The last-named is shown in Fig. 172. The
drivers are generally fitted with a two-ton
ram, which is hoisted by means of a steam
winch. The ram can be driven at speeds up
to thirty or forty blows a minute. Care and
judgment must be exercised to drive with
steady, uniform blows with a short drop,
otherwise damage will be done by splintering
and cracking the pile-head, however good
and hard the concrete may be. With a
trustworthy ganger driver who will take
care piles can be driven without a crack.
Owing to the great weight of a long pile,
say one from 30 ft. to 50 ft. in length, special
arrangements should be made for careful
handling, hoisting, and placing in position
for driving. When a pile is hoisted into
its correct position, it must be plumbed up,
bolted on — or rather through — the two
leaders of the piling frame, and then plumbed
up again when lowered before any blows
are given. When a cast-iron helmet is
used, as generally is the case with a Henne-
bique pile, a strong cushion of sawdust in
sackcloth is packed inside the helmet to
relieve the head of the pile from the severe
impact caused by the fall of the ram, thus
saving the head of the pile from shattering.
When a helmet is not used, an elm block or
dolly about 3 ft. long is placed on the head
of the pile for the same purpose as the
helmet. However, Considere piles, by reason
of the special form of their reinforcing, are
generally driven direct, without any helmet
or dolly.
Everything being ready, the pile driver
proceeds with the driving, which is con-
tinued until the specified set or stopping
place is reached ; the set is specified in the
engineer's or architect's specification, and
must always be strictly observed. The test
required for the set of a pile usually is, that
it does not sink more than J in. or J in.
under ten blows of a two-ton ram having
a drop of 3 ft. ; but much depends on the
soil and the nature of the strata through
which the pile has to penetrate.
It is desirable to note a few precautions
that should be taken by the clerk of works
in testing a pile set. The general practice
is first to determine the distance of the drop
of the ram to the head of the pile ; this is
done by clearly marking on the piling
frame by means of a chalk line the height
to which the ram is to be hoisted for each
blow. For marking the pile, a gauge rod
is hung from the leaders of the piling frame
in such a manner that it may swing. A
line is drawn on the pile at the level reached
by the bottom of the gauge. The specified
number of blows is then given, and the pile
is again marked with a pencil at the bottom
of the gauge rod. The space made between
the line drawn on the pile before the delivery
of the blows and the line drawn afterwards
determines the going or sinking of the pile
under the blows.
In witnessing a test, the clerk of works or
inspector should observe the following points
very minutely : — (1) When a helmet is used
for capping a pile, see that the sawdust
cushion packed inside is the same one that
has been used during the driving of the pile,
and that another has not been substituted,
or that a new one has not been packed into
the helmet and the old one placed under it
so that on looking up the inspector sees the
old cushion apparently undisturbed. Should
a new cushion be inserted, the pile does not
get the full benefit of the blow, and conse-
quently the test is not accurate.
(2) See that the ram or monkey is raised
during the test blows to the full height
marked on the piling frame, and see that
the men holding the guy ropes do not regu-
late the fall of the ram so that it loses its
force just before striking the head of the
pile. The precaution should be taken of
checking the length of the gauge rule used
for measuring the fall of the ram with an
ordinary rule. It is as well to measure the
gauge rod every time before it is used.
136
REINFORCED CONCRETE
(3) Watch closely the angle at which the
pencil is held in marking the lines on the
pile, as a somewhat cute device which needs
to be guarded against has come frequently
under our observation. The pencil was held
at an acute angle from the bottom of the
swing gauge, thus marking the pile | in. or
even J in. above what it would be if the
pencil were held at right angles to it ; then,
after driving, the second line was drawn
with the pencil at right angles to the gauge.
Obviously, the space between the two lines
was less than it would be if both of the lines
had been drawn with the pencil at right
angles to the bottom of the gauge. An effec-
tive cure is for the clerk of works to insist
on marking the pile himself.
(4) When pile driving is carried out at
night, see that the pile shoes are not knocked
off before lowering the pile into position for
driving.
Reinforced concrete piles are generally
placed in groups of two, four, or six. It
may happen that in driving the first pile
of the group a set cannot be obtained, and
the pile is driven right into the ground, even
then failing to pass the test, always going,
say, y1^ in. to \ in. more than is permissible.
Where such an event is likely, the driving
of the first pile should be stopped, and the
second one started. The next day, the first
pile should be given about 120 blows with
a 3-ft. drop of the ram before applying the
test, when, in the majority of cases, the pile
will not go more than -^ in. and probably
not as much as that. Should, however, the
pile still continue to go, it must be spliced
or added to in some other way. The splice
must be designed by the engineer ; as a
rule, extra rods are placed at the joint of
the old pile and the addition. The concrete
of the pile must be cut away down for some
distance so that all the reinforcement is
bared. The new steel for the additional
length is properly framed up and joined to
the projecting reinforcement of the pile.
A wooden mould is placed round the steel
framework to the height required for the
additional length, and the whole is now
ready to be rilled with concrete exactly as
for a column (described later). When the
concrete is sufficiently dry, the mould is
taken off, and five or six weeks later the
driving may be re-started.
Cast - in - place Piles. — Concrete piles
cast in place are now frequently used in
foundation work, one of the best known of
the type being the " Simplex " (Fig. 173),
in which system a hollow cylinder is driven
to a bearing, such as ballast, and then
gradually withdrawn, the hole left by the
cylinder being filled in with concrete and
heavily rammed. This cylinder is made of
about ^-in. lap-welded steel about 14 in. to
16 in. in diameter and from 30 ft. to 40 ft.
long, made up in two or three sections cross-
welded. The upper portion
is strengthened by a |-in. riveted
band about 18 in. deep and a
narrower band \ in. thick riveted
to the bottom end just above
the shoe. The pile is provided
with a special alligator jaw or
shoe, which is closed during the
driving to prevent the entrance
of the surrounding materials
into the cylinder or pipe. The
jaw, which is securely attached
to the pipe by cable hinges, and
can be used over and over again,
is composed of two symmetrical
wedge parts, which are kept
closed during the driving by the
earth pressure. In hard soils a
detachable cast-iron point is
often used, it being left in per-
manently. The cylinder or form
is driven by a pile driver, which
is very little different from the
ordinary type, except that it is
fitted with a strong pulling de-
vice attached to the leaders for
the purpose of withdrawing the
form after it has been driven.
When the cylinder has been
driven to refusal, the driving
head is hooked to the hammer
and wire attachment, and both
s
Fig. 173.-
Simplex
Pile
are hauled to the top of the leaders during
the concreting. A batch of concrete of about
5 cub. ft. is dropped into the form, after
which the entire form is raised about 1 ft.
by a pulling device. The concrete is then
rammed by dropping a heavy rammer into
the mass. The impact opens the jaws and
forces the concrete out into the space made
by withdrawing the form. The process is
continued until the whole form can be with-
drawn, and the space it occupied is filled
with concrete, which sets and hardens, and
so becomes a concrete pile. This pile is
sometimes reinforced by bars being inserted
and so arranged as to allow the rammer for
the concrete to be dropped down. By the
ERECTION OF A BUILDING
use of a wet and sloppy concrete the rammer
need not be used, as the concrete will settle
down and surround every part of the rein-
forcement. The rods inserted should pro-
ject through the head of the pile for the
purpose of joining up the beams at their
junction at the pile head.
" COMPRESSOL " CONCRETE PILES
The " Compressol " system is the in-
vention of a French engineer, S. L. Dulac, and
its name indicates the compressing of the
soil. The plant includes three rams which
are of different shapes and sizes, as shown
in Fig. 174. The " borer " is of a long conical
Fig. 174. — "Compressol"
&OE1E.R. Borer, Rammer, and
Tester
shape, with a sharp point, the latter having
a special cavity which brings back a sample
of the soil reached at each blow, so enabling
an opinion to be formed as to the necessity
of carrying the foundation to a greater depth.
The diameter of this ram at the upper part
is 2 ft. 4 in. and the length about 6 ft.,
whilst the weight is 32 cwt. The second
ram is called the " rammer," which has an
ogival shape with a length of 3 ft. and a
diameter of 2 ft. 2 in., the weight being
30 cwt. The third ram is the " tester," and
weighs exactly 1 ton ; unlike the others, it
is made to fall with the large base downwards.
It is shaped somewhat like a frustrum, and
the greater diameter is 2 ft. 8 in. These
rams are worked with a frame, in a similar
manner to that employed for dropping
the monkey in piling work, with a steam
winch and boiler ; the frame is, in addition,
supported on a revolving base plate which,
in turn, rests on a set of wheels in order to
allow the frame to be moved longitudinally,
as well as about its own centre. There are
practically two systems, one of which is
used for deep foundations and the other for
merely a surface compression! of the soil
when the latter is of a poor nature.
In forming the deep foundations the
" borer " is first used, and this is drawn up
the- frame and allowed to drop on to the
soil, the height varying according to the
circumstances, sometimes reaching as much
as 35 ft. Fig. 175 shows the frame and the
" borer " in position during the execution
of the work. An automatic self-acting
grip, supported by a pulley and chain, is
employed, and each ram is provided with a
specially designed rod at the head, as shown
in Fig. 174, which allows the automatic
grip to come into action. As soon as the
ram is dropped, the automatic grip is
lowered to pick up the ram, and upon the
latter being wound up to any desired level
the grip comes into contact with a special
ring, which is fixed to the frame, and this
automatically releases the ram and allows
it to fall ; then the process is repeated.
A sufficient number of drops is given to
make a circular hole in the soil ; and in the
operation all the strata adjoining are com-
pressed to such an extent that the sides of
the hole hold up very well in most cases.
The process of boring and compressing is
continued until the level is reached at
which the foundations are to start, and then
the " borer " is withdrawn and several large
stones are thrown into the circular shaft
and subjected to severe blows with the
" rammer," which forces them outwards
and spreads the bottom of the hole until it
forms a wide base, in some cases with a
diameter of 6 ft. or 7 ft., thus giving a good
footing for the concrete pier, which is to
come above.
When the base is properly prepared the
process of filling in the hole with concrete is
proceeded with, and this is accomplished by
depositing layers about 16 in. to 20 in. thick,
which are heavily rammed with a few blows
of the rammer ; cement concrete is used
where great strength is required. The
effect of the ramming is to consolidate the
concrete and give it increased strength ; at
the same time the surrounding soil is much
compressed by the spreading out of the
concrete, which, when completed, has been
found to measure 6 ft. and more in diameter,
as against the original size of the hole, which
was about 3 ft.
Fig. 176 shows the finished concrete pier,
and indicates clearly that the object of the
ramming is admirably obtained. The photo-
graph shows how the concrete is forced out
to a greater extent when passing through a
138
REINFORCED CONCRETE
soft stratum, and thereby effecting a greater
compression at this point, making a more
uniform resistance at each portion of the
pier and giving projections which materially
assist in resisting any downward pressure.
It is usual, when constructing these piers
in actual building work, to use large stones
at the base of the pier and smaller material
when nearing the top.
when the ram meets the pier ; that is to say,
30 tons for a set of 1 ft. If the set is only
| in. per blow the bearing power will obviously
be 24 times as great, or, in other words,
720 tons. Against this, of course, must be
placed a factor of safety to allow for the
loss of energy due to vibration, the resist-
ance of the air, and other causes, when
testing.
Fig. 175. — " Compressol " Frame and Borer in Ute
Fig. 176. — " Compressol '' Pile
The bearing power of each pier is ascer^
tained by means of the "tester." When
the pier is nearing completion, the rammer
is withdrawn and the tester is substituted,
this being used to give a volley of heavy
blows on the head of the pier, during which
time the set is ascertained. Naturally, the
less the pier is sunk by each blow the greater
the resistance it will offer when carrying a
superincumbent load. The weight of the
ram being exactly 1 ton and the fall being,
say, 30 ft., a force of 30 ton-ft. is obtained
When much water is encountered during
the boring it is necessary to adopt measures
to prevent it from draining into the well,
and this is accomplished in a simple manner.
The hole is rilled with ordinary clay up to
about 8 in. above the water level, and the
boring operation started afresh. The clay
is forced outwards and compressed around
the well, forming a kind of watertight lining,
which is usually sufficient to prevent any
great inflow of water. It may become
necessary in some cases to repeat the opera-
ERECTION OF A BUILDING
139
tion two or three times, but eventually a
perfectly watertight well can be obtained.
In the case of surface compression only
(which is used where the loads are light, but
where the soil is " made " ground, or of a
similar description and needs improvement)
the " borer " is not employed, but trenches
are made about 3 ft. deep and the rammer is
used to make a hole 4 ft. or 5 ft. deep,
which is then filled with dry stones, bricks,
or similar material for about one-third of
the depth, these being subjected to a few
heavy blows with the rammer, which forces
them outwards and downwards all round
into the soil. This process is repeated two
or three times and the tester is then used to
give a few blows, after which a great improve-
ment can be seen in the soil.
This method is employed in different por-
tions of the trenches at such distances apart
as may be necessary under the particular
conditions of the case, which are governed
by the nature of the soil and the loads to be
carried. This is a very cheap and effective
way of improving the foundations for a light
structure.
Pile Gaps. — When all the piles have
been driven and passed, the constructor can
proceed to cap them, and to form the con-
necting beams that will support the bottom
floor. Should the site at the surface be of
fairly solid substance, no bottom planking
will be necessary for the pile caps and beams,
or even for the floor slabs. Fig. 177 is a
plan of a typical pile cap.
For the purpose of constructing a concrete
cap to a pile that has been driven a mould
in the form of a box is made to surround
forcements are placed in the beam moulds,
all the bars properly connected up at their
intersections over the piles (see Fig. 178),
and the concreting begun. While the con-
creting is being done, the spaces between
the beams may be filled in to the level
PILE CAP
2 P/iCS
Fig. 177.— Plan of Typical Pile Caps
and Connecting Beams
of the under-side of the slab, by which
time it is probable that the moulds to the
pile caps and beams may be struck, taken
away, and cleaned for re-use. The filling in
between the beams up to the level of the
bottom of the floor slab should be completed
by thoroughly punning and levelling off
with some good, small, hard material such
as a mixture of cinders and gravel, clinker,
etc., so as to form a good bottom on which
to lay the reinforcement for the floor slab
(see Fig. 178).
THE FOUNDATION SLABS
The placing of the slab reinforcement is
NOTC - PllE OAgJ sm.'t>KO AfJD
TAKiU IHTO CAP
PILES
Fig. 178. — Section showing Pile Caps, Filling, Beam and Slab
each group of piles. The reinforcement has
to be placed in each of the boxes, according
to the engineer's designs. The pile caps are
connected together by beams which are
moulded in wooden forms ; the beam rein-
the next proceeding. The reinforcement
consists of bars laid to form a meshwork, or
of a steel mesh formed by welding or " weav-
ing " fine rods, or by " expanding " steel
plate. Let it be assumed that ordinary
140
REINFORCED CONCRETE
round bars are to be used, spaced at certain
distances, and crossed by distributing bars
wired at the intersections. A practical
method of placing the main bars in their
correct positions is to obtain two straight
pieces of boarding about 4 in. wide, and in
this to cut notches for the bars to agree with
the spacing specified. The boards are
placed, notches upwards, one near each
beam, and the bars laid to fit the notches
on the projecting reinforcement, which con-
sequently is bent anyhow.
Concreting the Slab. — Concreting the
slab reinforcement already laid is the next
process, and arrangements should be made,
if possible, to carry out the concreting
over one complete area at a time, so as to
ensure the work being monolithic. There
are two good practical ways of doing the
work so as to ensure that the reinforce-
Fig. 179. — Foundation Slab Reinforcement held in Notched Templates
(see Fig. 179). By this means the bars will
not get displaced. The notched templates
can be removed shortly after the concreting
has begun, quite a small quantity of concrete
sufficing to hold the rods in place. However,
before any concreting is done, the cross bars
or distributing bars have to be laid and
wired to the longitudinal bars at every (or,
in most cases, every alternate) intersection.
The work is not difficult or complicated, but
it requires great care, and certain precautions
need to be observed. For example, do not
allow men to run indiscriminately over the
steelwork after it has been laid down ;
instead, form runways with planks packed
up from the ground so that they clear both
the meshwork and the beams, it being borne
in mind that, at this period of the work, the
Fig. 180. — Raft Foundation Beam Reinforcement on
Wooden Supports
upper or compression reinforcement and the
stirrups of the beams are projecting above
the level of the concrete in the beams ; fore-
men frequently overlook this, and allow
boards used as runways to rest by their ends
ment does not come out on the surface.
One is to place a round rod, about f in.
in diameter, under the meshwork, and
roll it forward as the concreting pro-
ceeds, this method being best suited for
use on a suspended floor with flat wood
sheeting. The method preferred for a
foundation slab, as in the present case, is
to place a layer of concrete (say about 1 in.
thick) from each barrow load, and to lift
the meshwork through it by means of a
lifting hook (any piece of steel rod hooked
at the end). The hook is caught round one
of the main bars, and lifted about 3 in., the
mesh being given a slight shake and then
lowered gently to rest on the thin concrete
layer. Then the remainder of the concrete
may be added, well tamping all the time,
but not too heavily, or the cement will work
to the top. Punning or tamping needs to
be done lightly but thoroughly. The usual
finish is to level the slab with a straightedge
pressed down on two screeds, and worked
with a short motion.
In concreting a floor, the
concrete should be carried
forward on a straight line
across the whole of the width
of a bay.
In the concreting of the
slab at present being considered, those places
where the columns (over the piles) and the
walls (resting on beams) will come should
not be smoothed over, but left quite rough,
so that good joints can be made later.
-
u
141
142
REINFORCED CONCRETE
The foundation for the main building is
now complete.
The raft foundation slab for the engine
house is constructed in a different manner
from the foundation for the main building.
before doing this a strong grout should be
poured into the mould to strengthen the
key with the slab concrete. To complete the
work, the mould must be filled and the con-
crete carefully punned so that every particle
Fig. 182. — Completed Raft, Kingsway Church, London
The preliminary step is to level the site
either by excavating or by making up with
some good clean material, such as clinker,
gravel, and ashes. The positions of the
beams are next carefully set out, and the
steel skeletons for the beams made in a way
similar to that described on p. 133 for
making a pile skeleton. The skeletons are
placed in their correct positions, and set to
their proper and exact levels on temporary
wooden supports, as shown in Fig. 180. The
slab reinforcement should then be laid in the
same manner as for the floor just previously
described, but in this case the concreting to
the slab must be done
first instead of to the
beams, the concrete
being taken up to the
side of the beam frame-
work, so that when it
set sufficiently to
of the beam reinforcement is properly
covered. The moulds may be stuck four or
five days afterwards.
Figs. 181 and 182 show the raft for Holy
Trinity Church, Kingsway, London, which,
on the completion of the church tower, will
carry a total weight of 2,500 tons, and which
is independent of the foundation of the rest
of the building. A lower slab, 12 in. thick
and stiffened by small cross-beams each
2 ft. wide by 2 ft. 9 in. deep, carries two
main beams each 4 ft. wide by 4 ft. 3 in.
deep, and the reinforcement is by means of
patent indented bars.
s
Fig. 183. — Plan of Retaining Wall with Tapering Counterforts
bear the weight of men
and materials the wooden moulds for
the beams may be erected. As soon as the
concreting is set, the wooden supports to
the beam framework must be withdrawn,
and the framework suspended — or, rather,
steadied — by braces across the top of the
moulds. The moulds having been erected
and fixed, the beams must be concreted, but
THE RETAINING WALL1
In the erection of a retaining wall sup-
porting the roadway, the ground must be
securely shored up and strutted, and pro-
vision must be made, when erecting the
shuttering, to strut the ground up from the
centering, but the shoring must not in the
least bear on the retaining wall. For the
ERECTION OF A BUILDING
purpose of practical illustration, it is assumed
that the vertical bars have already been
placed in and secured to the foundation beam
and slab, which has been specially designed
for this purpose. These bars must be held
upright by means of temporary stays, so
that they do not become twisted.
The wall would probably be about 6 in.
thick and plumb on both sides, with a
horizontal beam half-way up and a capping
beam at the top. There will also be counter-
forts which very much resemble a beam in
a vertical position ; they will probably be
about 2 ft. 6 in. thick at the base, tapering
to the width of the capping beam, which
may be about 18 in. ; moulds will be con-
structed for these similar to those for a
column in a wall. The reinforcements will
be carefully framed up while the concreting
proceeds as the vertical rods of the counter-
forts will have already been fixed, as in the
case of the vertical bars for the wall itself.
The concreting naturally will be brought up
with the wall, so that there will be no joint
between the counterfort and wall. Fig. 183
shows plan of a retaining wall of this descrip-
tion. For a wall of this type, it is advisable
to erect the centering on the external side
to the whole height of the wall, and when
this is done the vertical bars may be fixed
to it at their proper spacing, thus obviating
the use of temporary stays. The horizontal
Fig. 184. — Reinforcement of Retaining Wall, Royal Insurance Building
REINFORCED CONCRETE
r
Fig. 187.— Dia-
gram showing
the above
Retaining Wall
if Built with
Brickwork
Fig. 185 Fig. 186
Figs. 185 and 186. — Reinforced Concrete Retaining Wall for Royal Insurance Building
bars should next be fixed for part of the way
up and securely tied at their intersections
with wire. The wall is now ready for con-
creting, and the shuttering — which should be
about 3 ft. high — should be erected between
the studs, which are generally spaced 5 ft.
or 6 ft. apart. The concrete should be
placed in the wall and carefully punned
with a wall punning iron, doing the whole
length of the wall, if possible, in one day ;
the next day another 3 ft. should be con-
creted, by which time the lower shutters
may be struck, cleaned, and used for the
. third 3 ft., and so on to completion, the
remaining horizontal bars being fixed as
the concreting proceeds. When the posi-
tion for the beam is reached, a two-sided
mould must be securely bracketed out from
the vertical studs supporting the shutters.
The beam reinforcement must be placed in
the mould and connected with the wall
reinforcement, the concreting following.
Scaffolding will have to be erected to com-
plete the wall, and it should be constructed
in such a way that it is independent of the
ERECTION OF A BUILDING
wall except that it may be tied into the
studs supporting the shuttering. Care must
be taken not to shake the wall in erecting
the scaffolding, because the concrete in the
wall is still " green." (Concrete that has
set but is not properly dry is in a condition
known as " green.") The striking of the
outside centering should not be done for
some time, as the wall should be dry right
through before there is any chance what-
ever of any weight coming on it. It is better
to avoid all unnecessary risks. In a few
cases where the wall is erected close against
dug-out ground the centering has to remain
because it cannot be got at, but this is not
often so, there generally being enough room
between the outside of the wall and the
ground for a man to get in.
A retaining wall at the Royal Insurance
Building, Lombard Street, London, is illus-
trated by Figs. 184 to 188, the diagrams
being comparative and showing stability and
pressure on ground of reinforced concrete
and brick walls under identical thrusts.
The space available did not allow of counter-
forts. The wall is 26 ft. 6 in. high from
underside of toe, and is only 21 in. thick
at the bottom, the thickness at the top
being 10 in., there being two offsets. The
wall is vertical at the back, the heel pro-
jecting for 18 in., as shown in Fig. 186. The
reinforcement takes the form of vertical
bars and horizontal stirrups.
COLUMNS
While the retaining wall is being built the
carpenters will be constructing the column
moulds, and the steel workers will be fixing
up the columns. Column reinforcement may
be done in at least two ways : one by erect-
ing the bars in the mould and putting on the
ties or binding as the concreting proceeds ;
the other by framing up the skeleton of the
column on trestles as before described for
Fig. 188. — A Further View of Retaining Wall, Royal Insurance Building, in Course of Erection
10
146
REINFORCED CONCRETE
piles. Probably fewer mistakes are liable to
be made in the second method with regard
to the exact position of the bars and the
spacing of the ties, and also the ties will be
more tightly fixed to the vertical bars. What
is meant is clearly shown in Fig. 189 be-
low. When the skeleton columns are made
they should be carefully stacked in a place
of safety where the ties will not be disturbed ;
this precaution is often neglected, with the
result that the steelwork is trampled on and
displaced as shown in Fig. 189. The skele-
ton columns, having been placed in the
moulds, should be temporarily fixed by
means of wood templates so that they stand
truly plumb on all four sides ; and it should
be seen that the space between the outside
of the vertical bars and the mould is correct.
Before any concreting is permitted, the
CORftKT
Fig. 189. — Cross Sections of Columns showing
Right and Wrong Methods of Placing
Reinforcements
clerk of works should inspect all columns to
see that there is no dirt, sawdust, or other
rubbish lying in the bottom, and that a
good grouting has been poured in to form
the joint between the floor and column or
the joint between the column under the floor
and the column above it, as the case may be.
As a rule the vertical bars of a column pro-
ject through the floor some 6 in. or 8 in., so
that a good connection can be made with
the column above it. The concreting of a
column can be carried out in one of two
ways — either by pouring in the concrete from
the top, or concreting and punning it from
the bottom, one side of the mould being
left open for this purpose in the same
manner as already stated for concreting the
retaining wall. If pouring is the method
employed, no one pouring should fill the
column to a greater height than six times
its diameter, and less is better. In filling
columns it is essential that the concrete is
sufficiently fluid to surround the bars and
ties, and it should be rammed with a long-
handled punning iron. Great difficulty is
caused by the presence of large stones in
the aggregate, and therefore the aggregate
used should pass through a |-in. square mesh.
In the construction of buildings of several
stories, the diameters of the reinforced con-
crete columns invariably decrease as the
building gets higher. The bars of one
column generally project through the floors,
as already stated, and they are frequently
bent in slightly to meet the bars of the
column above. In one system of making
the joint, a sleeve piece is fitted to each bar
of the lower column, and the bottom ends
of the bars of the upper column enter these
sleeves. By another system the bars of the
upper column overlap those of the lower one
for about 12 in., and the two are bound
tightly together with annealed steel wire ;
occasionally the binding is omitted.
WALLS
While the columns are going up, the car-
penters will be erecting the wall shuttering ;
the walls will probably not be more than
6 in. thick. The shuttering, centering, and
concreting for an ordinary straight wall are
worked in practically the same fashion as
that described for the retaining wall, except
that openings will have to be left for win-
dows and doors, and columns (incorporated
in the walls) for supporting the beams and
lintels will be erected practically in the same
way as described for the counterforts to
the retaining wall. Extra reinforcement is
generally placed round the openings ; it may
consist of a bar about f in. to 1 in. in dia-
meter, bent to fit the opening with the ends,
where they meet, overlapping or hooked
through one another ; and both the vertical
and horizontal bars in the wall are bent or
hooked round the bar as shown photo-
graphically in Fig. 527 (p. 324).
In fixing the reinforcing bars in the wall,
close observation should be kept to see that
the spacing or pitch of all bars agrees with
the specification and drawings.
Columns, floors and walls in course of
erection at the Money Order Office, Hol-
loway, London, are illustrated by Figs. 193
and 194 (pp. 148 and 149).
STAIRCASES
During the erection of the walls and
columns provision will have to be made for
the erection of the staircase, which will
either be erected in situ or built up with
separate treads or on reinforced concrete
stringers constructed for their reception.
In the present instance it is proposed to
describe the former system.
ERECTION OF A BUILDING
147
The Factory Act does not allow a straight
flight to contain more than fifteen treads in
a single going ; therefore, in most cases,
when there is a height of from 12 ft. to 14 ft.
between the floors, two flights must be
constructed between two floors, connected
by a half-space landing, with a fireproof
division wall, as shown in Fig. 190. In a
staircase of this type, treads are sup-
ported on a reinforced concrete carriage slab,
on which they are moulded, the half-space
landing being formed in the same way as
CROSS SECTION
PLAN
Figs. 190 and 191. — Horizontal and Vertical
Sections through Typical Factory Staircase
the carriage. The centering for the carriage
slab and landing must be erected, with
moulds for beams supporting the carriage
and landings. The reinforcement is then
laid in position in the same way as described
for a floor, and moulds are then made for
forming the treads and fixed in the right
position, so that the thickness (say, 4 in.)
of the slab carriage can be maintained during
the building of the treads. In some cases
the treads are reinforced with rods laid in
longitudinally and hooked into the walls.
Of course, the walls surrounding the stairs
must be carried up at the same time as the
stairs. The concreting is begun at the
lowest tread and worked upwards, being
carefully punned the whole time. Stairs of
this kind are generally finished in granolithic
about 1 in. thick, a thinner layer invariably
cracking and peeling off. The granolithic
for the riser must necessarily be applied to
the form or mould before the concrete is
deposited ; that for the treads is done
afterwards, but the sooner it is done the
better will be the key between the grano-
lithic and the concrete. Treads are generally
finished off by being grooved with a grooving
roller, so as to prevent their becoming
smooth and slippery. Figs. 190 and 191
show a typical cross section through a
staircase of the type described above.
INTRODUCING THE REINFORCEMENTS
INTO BEAM MOULDS
As already suggested in the case of piles,
the reinforcement for beams, columns, canti-
levers, etc., can be introduced in either of
two ways. One is to build or frame the bars
up in the moulds, and the other is to frame
them up first and then to deposit them
bodily in the moulds. Where members are
loose and not tied to one another, the rein-
forcement must necessarily be built up in the
moulds. Much care and patience must be
exercised in this part of the work. In
building a beam, the bottom bars must be
kept at the proper distance from the bottom
and sides of the mould, and this can be
effected by using reinforced concrete blocks
hollowed out as in Fig. 192 to receive the
STEEL ROD
APPROX
Fig. 192. — Hollowed-out Concrete Block to
Facilitate Spacing of Bars
bars at the correct spacing, two bars for
each beam ; the blocks are laid in the
bottom of the mould, and when the beam
is concreted up the new concrete will adhere
to the other concrete. One firm of con-
tractors always adopts this practice, which
it has found to be effective, although it
might give the impression of introducing a
source of weakness. Another way is to
'49
REINFORCED CONCRETE
place blocks of wood of the right thickness
in the bottom of the mould, and to lay the
bars on them ; these, of course, have to be
removed when there is sufficient concrete
in the mould to hold the bars in the right
position ; as there is, of course, a risk of
the wooden blocks being left in the mould,
the foreman must watch the work very
carefully.
In all cases of building a beam in a mould,
the stirrups have to be placed in first, and
their spacing must be correctly marked on
the centering, each stirrup being adjusted
after a little concrete has been deposited
to hold it in place. The use of loose stirrups
necessitates especial care in this particular,
since it is easy for a careless workman to
knock about the stirrups in all directions
during the process of punning.
VERTICAL 5UPPORT
HORIZONTAL SUPPORT
Fig. 195. — Method of Supporting Reinforcement
in Beam Mould
A method of supporting reinforcements
in beam moulds before concreting is shown
in Fig. 195.
Difficulty is often met with at the inter-
section of beams over a column, there being
trouble in getting the bars into their proper
positions, and particularly in causing them
to maintain their proper distance from the
sides of the moulds. Sometimes the neces-
sary adjustment is made — and made badly
— by the aid of a sledge hammer and a con-
siderable show of temper ! The most effec-
tive method is to bend slightly one or two of
the bars at their ends so enabling them
to overlap properly.
In vertical work, very long bars are some-
times used, and these will always need
to be properly supported temporarily to
prevent their being twisted and entangled.
Long straight bars can safely be bundled
together and fixed to a suitable support,
but when the bars have been bent to a par •
ticular form, they must be supported inde-
pendently.
For the fixing of stirrups and ties of all
kinds when framing up a member in its
mould, their positions must always be
clearly marked in pencil on the mould itself,
so that, as the work is brought up, the ties
can be fixed in their proper places.
FORMING SKELETON REINFORCEMENTS
For framing a member, all the bends must
first be made, the bent bars being then laid
on trestles, framed up in their proper posi-
tions on wooden templates, and then tied
temporarily in their places while the bind-
ings or stirrups, etc., are being fixed to them.
In the case of a column bound spirally, the
bars are placed in wooden templates, in
which is a central hole through which passes
a rod to act as a spindle ; the ends of this
rod are rested on the trestles, so that the
whole framework can be made to revolve.
(This, however, does not apply to spiral bind-
ing as designed for the Considere system,
for which the binding must be carried out
on a drum first and threaded on to the bars
afterwards, as described for the Considere
pile.) The binding is then securely fixed on
a bar of the framework, and so wound round
and round while the framework is made to
revolve. The pitch of the binding is marked
clearly on one of the rods. When a length
of binding is exhausted, the end is turned
tightly round a bar and another length is
begun on the bar before that on which the
first length finishes. The binding is con-
tinued in a like manner till the column is
complete, but it needs to be securely tied
on with annealed wire at every alternate
bar to obviate slipping. When the binding
is in the form of rings the work is done in
the same manner as described for piles (see
p. 133).
For framing up a beam, the stirrups are
cut and bent and then threaded on to the
beam framework, the bars of which are
temporarily placed in their correct positions
on templates, as in the case of a column.
The spacing of the stirrups is marked on
one of the bars running the whole length of
the beam, and the stirrups are tied to the
main bars with annealed wire accordingly.
All independent members can be so framed
up ready for deposition or placing in the
moulds, and if not used at once they should
be carefully stacked away in a place where
they will not be subjected to hard knocks.
ERECTION OF A BUILDING
The bending of small bars round larger
bars is best done with the aid of the wrench
previously described.
Unfortunately it is easy for a man with a
shovel, hammer, or a punning iron to cause
any amount of mischief if allowed to use
these tools carelessly. Extreme care should
be taken in the fixing of reinforcements over
points of support and also in the making of
connections.
MAKING A FLOOR
The very first thing to be considered in
the erection of a floor is the centering or
falsework. Moulds or troughs will be needed
for the reception of the main and secondary
beams, and they will need to be securely
strutted from the floor beneath. They must
be in true alignment and of sufficient strength
to bear the weight of the work that will
come upon them. The wood sheeting is
laid between the beams to support the
round rounds was described in connection
with the foundation slab. For a suspended
or supported floor, the reinforcing will be
carried out in the same way, but, of course,
centering is holding up the floor, whereas a
foundation slab rests on the ground. The
reinforcement may take the form of bars
and rods or of a continuous mesh made with
small diameter rods or from sheet steel. But
before any slab reinforcement is laid, the
beams must be arranged for ; the beam
reinforcements must either be built up in the
moulds or put together as frameworks and
afterwards laid in them. Wooden blocks
should be laid across the mould to support
the top or compression bar of the beam ;
and wooden strips should be inserted on
each side to maintain the correct distance
between the reinforcement and the faces
of the form ; but great care must be taken
that all these blocks and strips are removed
as soon as there is sufficient concrete in the
%3fa?^t'?t:%:rt^
' " " ' ^ • ' ' — "
Fig. 196. — Typical Floor with Continuous Mesh Reinforcement
weight of the floor slab and of the men
working on it. Descriptions of the various
methods of constructing floor centering will
be found in the next chapter.
Before any steel reinforcement is placed
in or on the floor centering, all the moulds
for beams must be thoroughly cleared of all
shavings, sawdust, dirt, or other rubbish
which always accumulates during the erec-
tion of the centering.
For the purpose of preventing concrete
adhering to the centering, it is advisable to
close the pores of the wood by thoroughly
wetting all internal surfaces just previous
to depositing the concrete. When a better
finish is required, the boards may be advan-
tageously limewashed or treated with soft
soap before the steel is placed in position or
the concreting started ; this makes the
centering easier to remove and gives the
work an improved appearance.
The reinforcing of a floor with ordinary
moulds to keep the bars in place. Fig. 196
shows a beam mould with the reinforcement
adjusted by the means just described.
Concreting must not be started until all
the reinforcement is complete and correct.
Obviously, mistakes cannot be rectified once
the concrete has been introduced.
The concreting is next carried out in
exactly the same way as described for the
foundation slab beams, the concrete being
taken up to the level of the top of the moulds
only, and left as rough and as rugged as
possible, so as to make a good key for the
slab concrete, which is added after the slab
reinforcement has been laid.
Continuous wire mesh is generally sup-
plied in large rolls containing from 150 ft.
to 300 ft. run, and varying from 3 ft. to
6 ft. in width. For laying this to form a
floor reinforcement, the rolls are hoisted
on to the centering and placed side by side
at one end. The roll is then undone and
152
REINFORCED CONCRETE
the end taken into the wall (see B, Fig. 196)
and fixed with staples to the centering, the
mesh being next unrolled over the entire
length ; temporary blocks are placed on
the beams (see A), over which it passes to
bring up the mesh to the level required over
the supports, as shown in the engineer's
drawings. The correct length having been
cut off, the free end of the mesh is fixed
into the wall (see B). The other rolls are
laid in the same way, side by side, the space
between them being determined by the pitch
of the main or tension strands of the mesh ;
for example, if the pitch of the mesh is
4 in. the selvedge of one roll is 4 in. from
the selvedge of the other roll. Reinforce-
ment of this type can be quickly laid, since
there is no spacing and tying of rods or bars.
When sheet mesh instead of the wire
mesh is used, first of all it is laid flat on
The concreting of suspended floors is
done in the same manner as described for
the foundation slab in the earlier part of
this chapter ; but it may be well to give in
this place some further particulars of the
process of concreting as applied to floor con-
struction. The concrete should not be so
thin that the water may run, and carry with
it the cement, through any openings and
cracks in the centering, thus causing the
slab when dry to have a honeycomb struc-
ture. The more water used in mixing
concrete, the greater is the shrinkage, but
workmen are more likely to make con-
crete over-wet than over-dry because of the
greater ease in handling and the less punning
that will be required. The concrete should
be sufficiently moist to be worked properly
into the moulds and round the reinforce-
ment.
1-2 °F SPAtS I
_|^g^jfj^^ -
Fig. 197. — Typical Floor with Sheet Mesh Reinforcement
the centering. The sheets may butt over the
supports, for here there is an additional sheet
laid near the top of the slab, these additional
sheets extending over '2 (one-fifth) of the
span on each side of the centre of the sup-
port. Fig. 197 shows a cross section of
this type of floor, and clearly illustrates
what is meant by the above. Other mesh-
reinforced floors are illustrated in a later
chapter devoted to the various commercial
systems.
In laying an expanded metal floor, the
sheets must be so laid on the centering that
the diagonal strands in the sheets all slope
in one direction, otherwise the ends of the
sheets will not properly key into one another
where they overlap. If the sheets bulge
slightly when laid in position, a few nails or
staples should be driven into the centering
to hold the metal down ; when the center-
ing is struck, the staples, etc., will remain in
the concrete, but can easily be clipped off.
As already stated, while the concreting of
this type of floor is proceeding, the mesh
should be lifted and shaken when there is
a layer of concrete f in. or 1 in. thick on
which it can rest.
As far as is practicable, the concreting of
a floor slab over a complete area should pro-
ceed at one time so as to ensure the work
being monolithic ; but when a whole floor
cannot be completed in one day the work
should be divided as follows : — For a main
beam, the concreting must be carried to the
centre of a column. A secondary beam must
be filled up to, and at the same time as, the
main beam to which it is connected. In a
slab, the concrete must be carried to the
centre of a beam. A column must always be
carried to its full height. Walls should be
worked along a complete length, and any
stoppage of work should occur at a horizontal
level.
Concrete spread on a floor should be carried
forward on an even line across the whole
width of a bay.
It will be readily understood that the
remainder of the floors, columns, and walls
in the factory will be carried up to the flat
roof, which is exactly the same as a floor,
except that there may be openings left in
it for lantern lights, that the concrete, in-
stead of being laid level, will be laid to
falls, that gutters will be hollowed out to
ERECTION OF A BUILDING
take the water, and that a parapet will be
erected in continuation of the walls.
THE ERECTION OF CENTERING
From what has already been said, it will
be seen that as the work proceeds all the
centering will have to be struck, cleaned,
and, if possible, re-used — work in which the
foreman will need all his ingenuity to effect
legitimate economy, which is a different
thing from scamping. The foreman must
arrange that the centering is always erected
in advance — well in time for the placing of
the reinforcement. He should strike the
centering in plenty of time for it to be pro-
perly scraped and cleaned before refraining
it, but everything he does must be consistent
with absolute safety. In the case of floors
of large areas, the centering for the height
of and including two floors should be erected
as soon as possible, so that by the time half
of the second floor concreting is done, the
centering from the first floor can be struck
and used for the next ; but he must exer-
cise extreme caution to see that the bottom
planks of the beam forms and their supports
are not removed until it is safe to do so.
All slabs need to be temporarily supported
after the removal of the sheeting. The
sanction of the architect, engineer, or clerk
of works should always be obtained before
any striking is begun.
The ease with which centering is removed
is a good criterion of the quality of its con-
struction ; but, however well it has been put
together, it is necessary to employ a number
of careful men to strike it. At least five
or six should be detailed off for this work,
and it should be definitely explained to
them that the concrete is not yet thoroughly
set, and that they must exercise care to
prevent the jarring or chipping off of arrises,
etc.
Opinions vary as to the period that should
elapse before the false-work is removed ;
but the following is safe practice.
Floor slabs and beams. — The centering may
be loosened in seven or eight days and
struck in ten, except the bottom planks
and supports to beams.
Wall panels. — The strutting can be re-
moved in two or three days in moderate
weather ; the sooner they are struck, the
better chance the wall has of drying out and
hardening.
Columns, etc. — Three to five days under
similar conditions to the above.
It is as well to loosen all centering before
striking, in order to prevent the concrete
from sticking to the boards ; moreover, the
air circulating through will harden the sur-
faces of the concrete, which will not be so
liable to be chipped.
All suspended members must be left
supported by temporary shores placed at
judiciously chosen points, and left for any-
thing from fifteen to thirty days. Where
support is most needed is at the centres of
beams and floor slabs. On no account must
any weight be allowed on any floor or
against any wall for at least three weeks
after the removal of the centering, except
that of the timber for the floor above and
of the men worldng on it.
FINISHING CONCRETE SURFACES
The finishing of concrete is considered in
detail on pp. 241 to 255, but it is desirable
in this chapter to complete the discussion
of the practical work of erecting a reinforced
concrete building by giving a few notes on
the subject. Concrete walls can be cleaned
down, all excrescences removed, and hollows
filled in, and then whitewashed — this, of
course, applying only to buildings of the
warehouse type. They can be decorated
with plaster in the same way as any other
building, but for this purpose the concrete
ought to be hacked or scored to form a
key for the plaster. In this hacking, great
care must be exercised, because a careless
man with a bush hammer or similar imple-
ment can do much damage, although this
may not be visible to the eye. The hacking
must be done while the concrete is green —
that is to say, friable. Take the case of a
reinforced concrete wall 4 in. thick that is
to be plastered inside and finished in stucco
outside, necessitating hacking on two sides.
It is obvious that a man giving the green
concrete wall hard knocks with a hammer
will be causing infinite mischief by dis-
integrating the concrete, and thus destroy-
ing its adhesive strength and monolithic
nature.
THE FIXING OF WINDOW FRAMES,
DOOR FRAMES, ETC.
Provision always has to be made for the
fixing of frames, etc., to the openings left
for them ; an effective method is to insert
screws in their proper positions in the con-
crete, with their worms bound round with
wire, and when the concrete is sufficiently
154
REINFORCED CONCRETE
set, to give each screw a turn ; it will be
found that the wire is a perfect hold for
the screw, and the frames, etc., can be fixed
without difficulty.
The Craig screw-bore (see Fig. 198) is a
great convenience when it is desired to attach
fittings, wood
finishes, etc., to
the concrete.
The method of
using it is first to
insert a master
bolt, as illus-
trated, through
a hole made in
the side of the
form ; the wire
screw - bore is
Fig. 198. — Craig Screw-bore then screwed on
to the bolt, the
reinforcement placed in position, and the
ends of the screw-bore wound around the
reinforcement if this is thought necessary.
Before removing the forms, the bolt is in-
screwed, thus leaving the screw-bore cor-
rectly placed, and, of course, most rigidly
held, and forming a fixed nut into which a
screw threaded the same as the master
bolt is easily inserted.
STEEL CORNER BARS
Steel kerbs or corner bars built into con-
crete steps answer three purposes : as regards
appearance they give a finish to the con-
struction, they provide a durable edge at
the very place where durability is most
needed, and to a great extent they act
as reinforcement. United States firms who
have specialised in these kerbs apparently
agree that the form shown in Fig. 199 is the
best for the purpose, the illustration showing
the Wainwright kerb, which is made of such
a shape as to be well anchored into the con-
crete. The bars are made in various sizes
for various widths of steps. The appearance
of a staircase fitted with such kerbs is shown
in Fig. 200. An interesting application of
the same idea is shown in Fig. 201, which
is a section through a reinforced concrete
column, the four edges of which are built up
with corner bars made of mild steel and
well galvanised. In the illustration, A in-
dicates the corner bars and B spreaders made
of stamped steel 1 in. wide and | in. thick,
which maintain the corner bars in proper
position while the concreting proceeds.
Should additional reinforcement be required,
it may take the form of upright bars, c,
with wire links, D.
Fig. 202.— Ebco
Corner Bar
Fig. 199.— The Wain-
wright Steel Kerb
Fig. 200.— Steps with Steel Kerbs
.
A method that is more generally employed
is to insert breeze bricks at various places
round the opening, so that the frames can
be screwed into them.
Fig. 201. — Section through Column with Steel
Corner Bars
Another type of corner bar — the Ebco —
also provides a rounded corner, but is made
with anchors spaced approximately 17 in.
apart, as shown in Fig. 202.
Forms and Centerings
Design. — Owing to the fact that the con-
struction of the forms, or shuttering and
centering, or horsing accounts for a very
large proportion of the total cost of rein-
forced concrete work, economy in the false-
work is most desirable, and the only proper
way of ensuring this is by the adoption of a
correct design in the first place. This is
recognised in the United States, for the
scheme of the centering is usually decided
in the office before the work is begun. Good
design in centering is evidenced by stability
and rigidity, ease of removal or striking, and
facility of rapid re-erection. In the case of
a building which is regular in form and con-
tains a number of stories which are alike
except as regards dimensions, thickness of
floor and scantlings of beams and columns,
a well-designed centering may be used
many times before the roof is reached. In
the case of members of unusual shape, it is
only by means of proper design that excessive
waste is avoided.
Centering should be designed in such a
way that it may be struck readily without
jarring or hacking the arrises off the con-
crete, and to attain this object as few nails
as possible should be used, thus necessitating
close supervision, for the carpenter never
loses an opportunity of driving a nail. Beam
and column moulds and wall panelling should
be so made that the various members fit
into one another without nailing, the whole
being made secure by bolting, wedging, and
like means.
The contractor is tempted to employ too
light timber, with consequent risk to the
stability, but, on the other hand, too heavy
timber is unnecessarily expensive and
awkward to handle. The timber used should
be strong enough to take, without appreci-
able deflection, any load imposed upon it by
the reinforced concrete, the men working
on it, and the tamping. Where practicable,
the scantlings of the timbers to be used will
be given in this chapter.
It is essential to maintain the perfect
alignment of beams, and to see that all
vertical members are truly plumb, for
reasons that are obvious. Further, all
moulds should be carefully and sufficiently
strutted so as to prevent sagging or bulging
after the concrete has been deposited ; and
all joints should be made tight enough to
prevent the thinner part of the concrete
running through, but at the same time allow-
ances must be made so that when the boards
swell with the moisture they can give but
not bulge.
It is advisable to add angle fillets to beam
and column moulds so as to form a chamfer
on the concrete, (a) thus obviating sharp
arrises which tend to knock off when remov-
ing the centering and so reducing the amount
of patching up to be done afterwards ;
(6) making spalling less easy in case of fire ;
and (c) giving a finished appearance to the
work. It is a good plan to construct all beam
moulds with a camber at the centre of at
least \ in. ; the beam will not come out
straight on the removal of the centering,
but in any case, the camber looks better
than a slight bulge in the middle, and it is
well known that ancient Greek architects
cambered all horizontal lines to make them
look straight to the eye.
Failures may often be quite as much
attributable to weak false-work or to the
removal of the false-work before the con-
crete is sufficiently set, as to such a cause
as bad design. Since any settlement that
may take place when the concrete is newly
made may easily prove disastrous, rigidity
of the forms becomes essential.
The forms and centerings described in this
chapter are taken from actual examples in
practice, this course being considered prefer-
able to the discussion of generalities.
SIZES OF TIMBER
Beams. — Bottom plank, 2 in. to 2f in.
thick. Sides or cheeks, 1J in. for secondary
beams and If in. to 2 in. for main beams.
Columns or 'pillars. — Sides, If in. to 2 in. ;
corner studs, 4 in. by 3 in. and 5 in. by
2 in.
Slab. — 1-in. boarding, maximum spacing
for 1-in. boarding, 2 ft. ; maximum spacing
for If-in. boarding, 4 ft. ; maximum spacing
for 2-in. boarding, 5 ft.
156
REINFORCED CONCRETE
Slab beams. — 6 in. by 3 in., spaced 2 ft. to
2 ft. 6 in. apart, according to nature of slab.
Walls. — Kunners, 9 in. by 3 in.
Mr. Sandford E. Thompson gives the
following safe loads for timber struts in
forms for floor construction : —
Strut
Length
3" x 4"
4" x 4"
6" x 6"
8"x8"
Ib.
Ib.
Ib.
Ib.
14 ft.
500
700
900
1100
12 ft.
600
800
1000
1200
10 ft.
700
900
1100
1200
8 ft.
850
1050
1200
1200
6 ft.
1000
1200
1200
1200
The following tables have been carefully
worked out and will serve as a useful guide :
bridges ; (9) place new posts under girders near
beams with cross heads running along girder
bottoms in same position as original post ;
(10) prop up slab joists temporarily as near
beams as possible; (11) draw nails holding
beam sides to beam bottom ; (12) lower wedges
under posts supporting beams and turn cross
heads lengthwise and wedge up again ;
(13) remove beam sides ; (14) remove girder
sides ; (15) remove slab sheeting and joists.
SETTING OUT
The correct position of the forms can result
only from extremely careful setting out on
a proper system allowing of the main lines
being referred to during the progress of the
work. " Horses " are made with 3-in. by
2-in. stakes driven into the ground, with
floor boards, from 6 in. to 9 in. wide, and any
convenient length up to a few feet, nailed to
TABLE SHOWING DISTANCES BETWEEN SUPPORTS TO BOARDS, &c., FOR FALSE-WORK
Boards,
Planks, &c.
1"
U"
11"
If"
2"
2i"
2J"
3"
Distances
2'0"
2' 6"
3'0"
3f /> n
6
4' 0"
4' 6"
5'0"
5'0"-6'0"
TABLE SHOWING DISTANCES APART FOR VARIOUS SCANTLINGS IN STRUTTING
AND SHORING TO FALSE-WORK
Scantling
(varies with
height)
4" x 2"
4J" x 3"
6" X 2"
7" X 2"
8" x 2J"
9" x 3"
10" X 5"
12" x 6"
Distances
2'0"
3'0"-3'6"
2'6"-3'0"
3'0"-3'6"
4'0"
4'6"-5'0"
5'6"-6'0"
7'0"-8'0"
TABLE SHOWING DISTANCES APART FOR DIE SQUARE TIMBER FOR FALSE-WORK
Scantling
5" x 5"
6" X 6"
7" X 7"
8" X 8"
9" x 9"
Distances
4'0"
4'6"-5'0"
5'6"-6'0"
6'0"-8'0"
8'0"-10'0"
ORDER OF STRIKING THE FORMS
The order of striking forms is thus sum-
marised by Prof. Johnson : (1) Wedges in
column form struck and placed in sacks for
hoisting to next floor ; (2) clearance pieces ;
(3) sides of column form under girder ;
(4) keys at beam sides ; (5) sides of column
form under beam ; (6) post up beam as close
to girder as possible with new post ; (7) re-
move posts under girder bridge ; (8) remove
their tops, as shown in Figs. 203 and 204 ;
and on these horses the significant lines are
accurately marked with a knife. For a
building with square angles (see Fig. 203),
one main line having been determined and
marked on horses, a line is set out at right
angles, by means of a builder's square, on
another row of horses, and the operation is
repeated until the necessary rectangle is
formed. Carefully check the dimensions
FORMS AND CENTERINGS
157
from those figured in the drawings, and do
not trust to scaled dimensions. See that
opposite sides agree in length, and check the
accuracy of the right angles by measuring
Fig. 204.—
Horse used in
Setting Out
lines, the lines of foundations, etc., may now
be marked as in Fig. 203. Acute and obtuse
angles are set out as in Figs. 205 and 206 ;
in such cases the accuracy of the diagonal
dimension is of the utmost importance.
FORMS FOR PILES
For a square pile the mould required
is very simple ; it is constructed with three
sides as in Fig. 207, the uppermost side
being left open for inserting the steel skeleton
and concrete. Angle fillets should be fitted.
The mould must lie perfectly level, and the
skeleton must be securely suspended in the
correct position and be dead level, so that
Fig. 203.— Setting out
Piers of Rectangular
Building
Fig. 206. — Setting
out Obtuse Angle
of Building
30 in. one way and 40 in. the other way, the
diagonal between the two points then meas-
uring 50 in., which equals the square root of
the square of 30 plus that of 40. The outside
lines of the walls and columns, the centre
Fig. 205.— Setting
out Acute Angle
of Building
it may be entirely surrounded by concrete.
The ends of the rods must be properly placed
in the cast-iron shoe at the end of the mould.
An octagonal pile is constructed as
above described, with the exception that
158
REINFORCED CONCRETE
large fillets are fixed to the inside of the
boxes.
For a round pile a mould is constructed
with two circular sides or cheeks, made with
narrow battens after the fashion of a barrel
and bound with iron straps ; the mould must
give the pile two flat sides, so that they can
lie flat up against the guides of the piling
frame. In forming this mould (see Fig. 208)
a straight plank is used, and through it
bolts are passed about 2 ft. apart for the
purpose of holding down the straps that
bind the circular sides. This plank is laid
down dead level on sleepers. The skeleton
frame — the reinforcement — is next laid on
this plank, and then the circular sides or
cheeks are attached and the straps bolted
at the top, as shown. The open space
JfAYS A&OUT 2-4 " PfTCH
•4- .
joists and rebated flooring on which to make
and cure a few piles. Figs. 210 and 211 show
the method. The side shuttering of the
I
Fig. 208.— Bolted Form for Round Pile
piles is formed of 2-in. by 8-in. verticals held
together by 1-in. by 7-in. cleats bid on
rebated flooring. The sides of the shutter-
ing are kept in position by 2-in. by 4-in.
distance pieces, the whole being kept in
position by struts and 1-in. by 7-in. boards
temporarily tacked to the top.
A method of American origin is shown
in Figs. 212 to 214 ; the parts dissociate as
shown, and, for curing, the green piles are
placed on 2-in. by 12-in.
) planks resting on beams
— ' ' — > 3 ft. 6 in. apart.
FOUNDATION FORMS
Fig. 207.— Form for Square Pile
at the top of the mould is left for the con-
creting, and is the same width as the
bottom flat plank.
The same shape can be
made by using wood tem-
plates of the shape shown
in Fig. 209, and fixing the
narrow battens to them.
The two sides are held
together by nailing short
stays on the top side, as
illustrated.
As in the case of a square
pile, the skeleton frame-
work is suspended by hooks
to the top stays to keep it
in correct position, and the
hooks are removed when the concrete is
half-way up the mould, so that there is no
chance of the steel framework being dis-
turbed from its correct position.
Most contractors set up a platform of
In preparing the site
for a foundation slab,
the ground must be brought to a true level
and boarding placed about the footing if the
nature of the soil demands it. If ribbed
foundations are
STAYS A&OVT 24 ' PfTCfr
TEMPLATE
used they must
b e excavated
for and suit-
able boxing
provided. The
concrete is
poured into the
Fig. 209.— Stayed Form for Round Pile
forms in layers and the reinforcements added
at the proper levels. Fig. 215 shows how the
foundation forms are fixed up when the slab
has had sufficient time to set hard enough to
bear the weight of the men and materials.
FORMS AND CENTERINGS
The moulds for the beams are very readily side of the moulds about 2 ft. 6 in. apart,
constructed, it being only necessary to form as shown dotted in the illustration. They
the two sides. When these sides are fitted up can be withdrawn when the sides of the
hE- -
I4*1-
r*:°
-
- 12. CONCRETE.
""
"vVALL
Fig. 210
/DISTANCE PIECE ^
FORM
R3RM
y l/IOIHPH-L rlCl^C • -^
~l"
^ >«
PILE
X xrf
1
M
^ •%;
HLL
1
IE
i
^ ^
PILL
ik A
sV
ij c{
{ 4 if
/ i! tf ij
ZHZ PLANki
WALLPLATE
z
Fig. 211 ( FOLPINC WEPGES
Figs. 210 and 211.— Elevation and Enlarged Cross Section of
Pile-making Platform
• -2*12. t~2*a
Fig. 212.— Plan and Ele-
vation of American
Pile-making Forms
and Platform
Fig. 213.— Enlarged Cross Section through
Form shown in Fig. 212
Fig. 214.— Concrete Piles
and Forms Dissociated
in their proper positions they must be firmly
stayed and strutted to each other, both top
and bottom as shown. By carrying the top
stays over the width of the end beams and
strutting the off-side of these to long pegs
Fig. 215. — Foundation Form Built after Slab is Hard
driven into the ground (if it is firm enough
to hold them), the mould will be secured
without breaking away. If this method of
strutting from pegs is impossible, the moulds
may be held by bolts passing through each
moulds are struck. Fig. 216 is a photo-
graph showing forms for pile caps, etc.
The ordinary box mould for foundations
is simply four sections of the required depth,
two of the sides extending beyond the
others, all bat-
tened and
bolted together
(see Fig. 217).
The 2-in. by
2-in. arris fillet
and the 4-in.
by 2-in. batten
on the ex-
tended side
form a groove
for the bat-
tened section
to slide into. The 4-in. by l|-in. battens
are a 4-in. by 3-in. batten once sawn. The
square-headed f -in. rods have a minus thread,
and 6-in. by 6-in. by J-in. square plates act
as washers. By alternating the fixed square
Fig. 217. — Typical Box Form for Foundation
Fig. 216. — Forms for Pile Caps, Foundations, etc.
1 60
FORMS AND CENTERINGS
161
head of rod and nut the form is made
capable of easy adjustment to bring it
square and can be readily removed and
altered.
In the type of form shown by Fig. 218,
by 4-in. battens to serve as stop pieces.
The four sides having been placed in posi-
tion on the rough base and nailed together
through the stop pieces, 4-in. by 4-in. posts
were skew-nailed on the outside as illus-
trated, and holes bored for the passage of
annealed iron wires, which connected two
facing posts, exactly as shown, and which
were tightened by twisting with a rod. The
external wires were cut when
6>"* fa' &OAKOS the time came to remove
the forms, the projecting
ends being hammered in and
finished ofi with
3*2 &KACL a trowel. It is
advisable to intro-
duce strainers to
take the stress
caused by tighten-
ing up the wires,
and to prevent the
form being pulled
out of shape.
Fig. 218.— Cheaper Type
of Form for Founda-
tions
the 3-in. by 2-in. braces keep the mould
rectangular and also serve as handles. This
is a cheap and effective type of
form which can be recommended for
repetition work.
Form Strengthened by Twisted
Wire. — In the United States, found-
ation and other forms are often held
together by means of twisted annealed
wire, all bolted rods and shoring
being dispensed with. While English
engineers might fear that die-square
results could not be obtained by such
a method, it must be admitted that
it is cheap, simple, adaptable to a
wide range of applications, and de-
serves to be more extensively em-
ployed. Fig. 219 shows such a
form, the detail at the foot of the
figure illustrating the type of found-
ation— square tapering piers arched
together — for which the particular
form shown was required. The
sides consisted of 1-in. boarding
nailed to 2-in. by 4-in. battens.
Two opposite sides were 14 in. wider
than the pier, and to the edges of
their inner sides were spiked 2-in
11
Fig. 219.— Form
Strengthened
witb Wire
Fig. 220. — Typical Column
Form with SHd-in Front
Boards
162
FORMS AND CENTERINGS
163
COLUMN, BEAM, AND SLAB FORMS
The most frequently used forms in rein-
forced concrete are those for columns, beams
and floor slabs, so that any economy effected
whole fomfsides at once with vertical board-
ing held in position by battens, this method
necessitating tha >pouring in of the concrete
from the top, and consequently the use of
Fig. 221. — Typical Column Form with Spiked-on Front Boards
in their design enabling their re-use without
waste is sure to tell favourably in the cost
of the structure.
Three Methods of Constructing
Column Forms. — There are three methods
of setting up column forms.
The first is to construct the forms for the
a wet mixture, because ramming would
probably displace the reinforcements.
The second method is to build up the
whole four sides gradually with horizontal
boards held in position by battens at the
angles, and brought up in short sections as
the work proceeds.
/i
&&OCT
Fig. 222. — Clamped
Form for Short
Columns
164
Fig. 223.— Cheap Type
of Column Form
165
i66
REINFORCED CONCRETE
The third method is the most economical,
and therefore the one most usually adopted.
It consists in erecting only three sides in
position at first, formed of vertical boards ;
the fourth side is gradually brought up with
as the work proceeds, in short lengths placed
between a guide and the angle fillet on each
side. The other portion of the mould is
made with the boards vertically battened
and distance-pieced and bolted into position.
&QAGDS
3*2
Fig. 224. — Column Form with Two Sides held between Fillets and Battens
horizontal boards as the concrete is applied,
thus permitting thorough supervision of the
work as it progresses.
The edges of the columns are usually
chamfered by placing small angular slips
in the corners of the forms.
Fig. 220 shows a form constructed with
three sides only, the fourth being built up
In Fig. 221, the form is built up in board-
ing, 9 in. by'^lj in., all horizontal, strongly
nailed to 4-in. by 2-in. uprights. To make
the front A, the boards are spiked on as the
work proceeds. The back is held together
with 3-in. by 2-in.' battens to allow of
quicker re-use. The angle fillets are lightly
spiked in place.
FORMS AND CENTERINGS
167
In the type shown by Fig. 222, the
boarding is vertical on all four sides and
held together with 4-in. by l|-in. battens
strongly spiked on. In addition to the
ordinary spiking, wooden clamps, out of
3-in. by 2-in. stuff, and bolted together at
the angles, keep the form in shape ; they
rest on the horizontal battens. This method
of construction is employed only for the
shorter height of pillars,|_and has the dis-
arranged at intervals according to the
height of the pillar. The life of this type of
form is not long, because of the spiking on
of the front boards.
In the method shown by Fig. 224, the
9-in. by If-in. boards are all horizontal and
are battened together on sides A, B, and c.
Side D is inserted as the work rises, as short
lengths of If-in. board. In addition to
spikes, the form is bolted through, as shown,
Fig 225. — Column Form with Two Sides held between Fillets and Battens
advantage .of not allowing easy inspection
or close punning, and of causing derange-
ment of the reinforcement in the process of
depositing the concrete in the mould.
Spikes, or nails, clenched at the angles, are
sometimes used instead "of -bolts.
Fig. 223 illustrates a cheap type of form in
which the boards, placed on as the work
proceeds, are simply spiked or strongly
nailed on to the built-up three sides ; 6-in.
by IJ-in. and 8-in. by IJ-in. boards are
held together'with 4-in. by 2-in. battens on
each of the three sides, with £-in. bolt rods
and sometimes a strainer is inserted to take
the pressure of the bolts.
Fig. 225 shows a case in which 6-in. by
2-in. vertical planks are battened together
on three sides of theTJorm and upright
members,r3 in. by 2 in., are spiked on the
sides so as to make with the angle fillets
two sets of grooves to receive the back and
front, the latter being slipped in from the
top as the work proceeds. As illustrated,
the batten on the back is clear of the bolt.
The forms for beams are made in a
number of ways, the best being to have two
i68
REINFORCED CONCRETE
sides and a base easily detachable (see Fig.
226). The 7-in. by 2-in. planks are joined
together by 4-in. by 2-in. battens to form
the base, while 9-in. by IJ-in. planed boards,
with 4-in. by 2-in. battens form the sides.
with 2-in. planks battened together with
4-in. by 2-in. stuff, the battens to the base
being extended and strutted as shown with
3-in. by 2-in. struts. A half-round mould
is inserted in the angles to give a rounded
Temporary ties, say 4£ in. by \\ in., are
nailed across the top before filling in and
punning. The battening is arranged at
3 ft. 6 in. apart. An effective finish to
the'arris of the beam is obtained by insert-
ing"^ hollow mould (see Fig. 228) instead
of the angle fillet.
Fig. 226.— Typical
Beam Form
finish to the beams. This is, how-
ever, a wasteful design, and is not
adjustable to further uses. Fig. 231
shows a form for producing splayed
angles between pillar and beam.
The system of false-work shown by'Fig.
230 illustrates the junction of two beams,
and also the floor centering in -position.
The dead shores are arranged at intervals.
At the edges the IJ-in. boarding is rounded
off as shown to give a good finish to the angles
of floor and beam. The method of striking
»r
/77
Fig. 227. -Boards with Splayed Edges to allow for Expansion
In using dry, sound timber, a provision
to allow for swelling is made by running
the stuff through a mill arranged to splay
one edge, as shown in Fig. 227. It is advis-
able not to run it to a feather edge, but to
allow, say, a TVm- fillet.
Fig. 228 shows a beam form constructed
the floor centering is to withdraw the 4-in.
by 2-in. batten, which is put in in conveni-
ent sections ; drop slightly the 9-in. by
3-in. joists, and then take out the boarding.
Immediately support the floor with head
and cill piece for a further pericd. This
design is open to many objections : splayed
Fig. 228.— Beam Form
Strutted from Ex-
tended Base Battens
J*0££ PtATL
Fig. 229.— Folding Wedges
under Dead Shore
169
iyo
REINFORCED CONCRETE
fillet is better than tlie hollow moulding ;
the side boards had better be horizontal;
and the joists should not rest on the beam
sides but be propped independently, so that
the sides, of the beam can be removed first.
The centering for floor slabs is built up
with a series of deals from beam to beam,
supported by head pieces, dead shores, and
cills on the lower floor at frequent intervals.
wedges. Fig. 229 shows the arrangement at
the base of a 6-in. by 6-in. dead shore, but
it is preferable to have the wedges at the
top of the upright.
Various Beam and Column Forms
Described. — Fig. 232 illustrates the simple
Hennebique type of beam form. A is the
bottom board, 2 in. thick, the exact width
of the beam, and supported by posts F.
Fig. 230.— Form for Two
Intersecting Beams
See that the shoring is taken to a firm
bearing — that is, carried right through all
the floors as in Fig. 246 (p. 176) — not only to
support the weight of the floor being con-
structed, but to ensure that damage is not
done to the floors already in yet still in a
somewhat green condition. The system of
wedging should be arranged in such a way
that it can be eased at given periods, ensur-
ing the beam taking an even setting and
bearing. The cills, and often the heads,
are provided with folding, hard-wood
At the top of the side boards B are nailed
battens or cleats to carry the ends of the
secondary beams D, also supported by
posts F. The upper boards E of the main
beam forms are then placed in position,
together with the side boards H of the
secondary beam form. Finally, the lagging
or floor slab centering is placed on the
top of the side boards of the beams. The
whole is held together with iron clamps, as
shown in Fig. 233. At the bottom of each
strut there should be hard-wood folding
Fig. 232. — Two Cross Sections through Henne-
bique Floor and Plan of one Bay
R
CL/MP
Fig. 233.— Beam Form held by Clamp
BOLT
SCRBff
Fig. 234. — Column Form with
Bolts and Thumb-screws
Fig. 235
7*2
UPRIGHT
WEDGES
Fig. 236
Figs. 235 and 236. — Beam Forms used in Messrs. Sainsbury's Premises, London
172
FORMS AND CENTERINGS
173
wedges, useful in setting and striking the inserting wedges as shown. A section
moulds, and these should be adjusted until through the beam form is shown in Fig. 235.
all is perfectly firm. IE the filling in of the The point to be specially noted is the rebated
Fig. 238.— Plan of Column
Form
SEDUCING
FIUE.T-
Fig. 237 — Beam and Column
Forms used at a Bermondsey
Warehouse
Fig 239.— Plan Showing Posi-
tion of Clamps for Reduced
Column Form
concrete has taken the camber, previously
allowed, out of the beam, the wedges should
at once be driven home, so that the camber
is again obtained.
The beam and column forms adopted at
Messrs. Sainsbury's pre-
mises in Stamford- Street,
London, S.E., are shown
in Pigs. 234 to 236. The
column forms of 1^-in. ver-
tical planed boards were
built up on three sides
complete. A shutter, made
in two heights, of IJ-in.
boards with cross battens
was used for the fourth
side. The whole was
clamped together by 2-in.
by 5-in. clamps 1 ft. 6 in.
apart, adjusted at the corners with bolts
and thumb-screws ; the angles of the
columns were ovolo moulded by the insertion
of corner fillets as in Fig. 234. The columns
can be reduced inside the same clamps by
support to the beam bottom. By means of
fillets of various sizes placed in these rebates,
the beam can be made of less breadth or
depth, and so save cutting up the cheeks of
the beam mould. Fig. 236 shows how the
Fig. 240.— Isometric
Sketch of Forms for
a complete Bay of a
Warehouse Floor
beam form may be reduced in depth and
width without destroying any material.
Figs. 237 and 239 show the beam and
column forms used at Peak, Frean and
Co.'s warehouse in Bermondsey. The beam
174
Fig. 242. — Beam and Column Forms used at H.M.'s Stationery Office
IB.
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Fig. 243 Fig. 245 .
Figs. 243 to 245.— Adjustable Beam Forms designed by H. Kempton Dyson
175
176
FORMS AND CENTERINGS
177
forms are supported by 2-in. by 4-in. uprights
braced by cross battens nailed on. The top
cross brace at the under-side of the beam
is bolted to the uprights, and it supports
the bottom board of the beams. With
reference to the column forms, should it be
desired to reduce the size of the column,
the lower pair of clamps is brought closer
together and the upper pair fitted to
the grooves in the lower pair and wedged
in the new position. The column can be
reduced in the other direction by inserting
Fig. 247.— Form for Twelve-sided Column
3/& DOLTS
FLUTE nOULDS
OF PLASTER OF PAfUS
Fig. 248.— Form for Fluted Column
fillets of the proper size between the clamps
and the boarding as shown. The main beam
forms are supported on 2-in. by 4-in. up-
rights in pairs at every 3 ft. 6 in. of the
length. The top cross pieces immediately
under and supporting the bottom board of
the beam are bolted to the uprights. The
secondary beams rest on fillets nailed to the
sides of the main beams and also on 2-in.
by 4-in. uprights at 3-ft. 6-in. centres. The
lagging of the floor slab is supported on 2-in.
by 4-in. joists 2 ft. apart, the ends of the
joists resting on the secondary beam forms.
12
Fig. 240 gives a good idea of the usual
construction of the centering of floor slab,
girders and secondary beams ; the illus-
tration shows a complete bay of a warehouse
floor in isometric projection.
Beam and Column Forms in H.M.'s
Stationery Office. — Fig. 242 shows the
beam and column forms for the Government
Stationery Office in Stamford Street, S.E.
i
i
. —
TTTTI '
( 1 1 1 1 U
-t
ui'11 ' *
. "
Fig. 249. — Form for Diminished Column
The main beams have a span of 21 ft. 3£ in.,
and the intermediate beams 15 ft 2 in. The
columns are 20 in. by 20 in. There are so
many bays of exactly similar dimensions that
the centering could be used over and over
again. The drawings explain themselves,
but a point to be noted is the method of
supporting the boarding for splaying the
haunches of the beam on 2-in. by 4-in.
fillets nailed to the column. A photograph
showing much of the false- work is reproduced
in Fig. 241 ; the steel gantry, which is a
conspicuous feature in this view^ was
•3
33
c
I
U
178
179
i8o
REINFORCED CONCRETE
employed for raising the building
materials, electric travelling cranes
running upon it.
Beam Forms Designed by H.
Kempton Dyson.— Figs. 243 to 245
show beam forms designed by H.
Kempton Dyson. A point to be
noted is that the beam can be re-
duced in depth by means of a
blocking piece. The cleats on the
uprights can be readjusted to any
height, thus saving the cutting of
the uprights to different lengths
to vary the height. A different
size of base-plate at the bottom of
the uprights may also be employed
Fig. 253. — Form for Column Base at Wesleyan Hall, Westminster
Fig. 254.— Centering for
Floor at Wesleyan
Hall, Westminster
for the same purpose.
The cross bearers under
the beam bottom support
2-in. by 5-in. joists, which
in turn support the 2-in.
by 4-in. joists carrying
the floor slab boards.
These joists as arranged
are not the full length:
between the beams, and
wedges are inserted be-
tween their ends and the
beam boxes to keep the
latter in position. The
boarding carrying the
slab runs parallel to the
beams, and can be used
in fairly long lengths,
thus saving waste in
cutting. The joists being
supported direct from
the uprights, the parts
FORMS AND CENTERINGS
of the beams, floor, and columns can be
easily struck independently of each other.
Various Column Forms. — The form for
of cement. Figs. 250 to 254 are photo-
graphic views showing the false-work to
beams and columns.
Fig. 255. — Centering Resting on Flanges
of Steel Joists
a twelve-sided column, illustrated by Fig. 247>
is made of wooden staves dowelled together
and hooped with adjustable iron straps.
The form for a fluted column, shown in
Fig. 248, consists of narrow laggings dowelled
together and hooped round with adjustable
Fig. 256. — Centering Suspended from
Flanges of Steel Joists
iron bands. The flutes are formed of
plaster-of-paris attached to the inside by
screws which are inserted from
the outside ; in removing them,
the screws are drawn and the
plaster flutes remain as a pro-
tection after striking until the
building is completed.
In the new Wesleyan Hall at
Westminster circular columns
with entasis are used. The
forms (Fig. 249) were made in
sections 4 ft. high, the narrow
vertical laggings being easily bent to the
required curve and secured to horizontal
2 PLANK, 2 8 CENTRES
Fig. 258.— Centering for Arch Ceiling
between Joist Flanges
CENTERINGS FOR CONCRETE FLOORS
CARRIED BY STEEL JOISTS
The simpler varieties of centering for
steel joists will now be briefly dealt with.
Fig. .255 shows a concrete slab lying on
the top flanges of rolled steel joists ;
timber bearers rest on the bottom flanges
and support the lagging. Fig. 256 shows a
similar centering supported on the bottom
flanges by means of , hook hangers or sus-
pended by sb'ngs from the top. It may be
arched as shown by dotted lines. Fig. 257
shows a design by W. F. Kearns for a rein-
forced concrete floor with steel main beams.
The defect of these three floors is that, even
- 2-0 CENTRES
Figs. 259 and 260. — Floor Centering supported by Hangers
2*8 WOOD W4SHERS
ffe BOLTS HAMD NUT
Fig. 257. — Centering for Concrete Floor
having Steel Main Beams
pieces set out with their outer edges
plumb, while their inner edges coincided
with the diameter of the column ; these
horizontal pieces were fixed at intervals of
2 ft. The method gave such good results
that the columns could be finished with in.
although the centering is kept low, the
bottom flanges of the steel joists are not
embedded in the concrete, and therefore are
exposed to the effects of fire ; the defect is
obviated by having a ceiling on iron hangers,
metal lathing being embedded in the plaster.
Fig. 258 shows the centering for an arch
ceiling between the joist flanges, suspended
by f-in. bolts. In the method shown by
Figs. 259 and 260, square-headed hooked
bars are hung on runners which are placed
on the upper flanges of the steel joists,
the bars being placed alternately on each
side of the joist.
WALL AND PARTITION FORMS
The shuttering for walls consists of bat-
tened widths passed down behind quarter-
ing driven into the ground and kept apart
by iron distance pieces at the top, the
i8z
REINFORCED CONCRETE
arrangement for battering the face being on it, to which the uprights, spaced at not
obtained by sloping the uprights to the more than 5-ft. centres, are fixed and braced
required -angle. In the case shown by by runners and struts to keep the horizontal
boards as rigid as possible. The outside
shuttering is carried up to the full height
Fig. 262.— Section of
Wall Form
Fig. 261. — Form for
Wall. Part is Raised
to Second Position
Fig. 261 shutters are made of 7-in. by 2-in.
planks with 4-in. by 2-in. ledgers, and by
employing sleeve pieces over the f-in. rod
bolts fitted with heads, nuts, washers, etc.,
the sides of the form are kept the required
distance apart. In the right-hand part of
the illustration a section of the wall is shown
complete, and the shutter has been raised to
the second position resting on the sleeve
pieces, as further shown in Fig. 262. In the
next illustration (Fig. 263) an alternative,
but inferior, method is shown, the 6-in. by
l£-in. boards having 4-in. by 2-in. continuous
head and cill pieces.
In the type of form illustrated in Figs.
264 and 2(>r> it will be seen that the* floor slab
having first been laid, cill pieces are placed
Fig. 263.— Panel for Wall Form
FORMS AND CENTERINGS
183
the inner shuttering is made in panels 5 ft.
wide and 2 ft. 6 in. high, and built up as the
work proceeds. The size of the uprights is
governed by their distance apart, the height
and thickness of wall, and amount of strut-
ting. This design is not recommended.
For walls from 6 ft. to 14 ft. high, 2-in.
is used the studding can be placed farther
apart, and of course made correspondingly
heavier in scantling.
Fig. 266 shows the false-work for a simple
foundation wall. First, 2-in. by 6-in. hori-
zontal bearers are laid to 2-in. by 4-in. up-
rights, the latter having pointed ends which
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Figs. 264 and
265.— Plan and
Cross Section
of Shored-up
Form for Wall
by 7-in. or 3-in. by 6-in. uprights may be
used if not placed more than 2 ft. 6 in. apart
or 2 in. by 5 in. if spaced not more than 2 ft.
apart. The uprights should be stayed by
one or two rows of horizontal runners
2 in. by 7 in., or flooring boards propped
up by 2-in. by 4-in., 2-in. by 5-in., or 3-in.
by 4-in. struts, whichever scantling is at
hand. It follows that if 2-in. shuttering
are driven into the soil ; the bearers are
kept in position by being secured to stakes
driven into the ground. The uprights are
braced and kept the proper distance apart
by 1-in. by 4-in. battens nailed to their tops,
vertical packing pieces | in. square being
placed between the studs to stay the shut-
tering ; after a section of the concrete has
been finished, the boards are raised to
184
REINFORCED CONCRETE
continue the wall above. The footing is
formed by allowing the first layer of concrete
to flow under the bottom boards.
Ransome's Wall Form. — Fig. 267 shows
Fig. 266. — Form for Simple Foundation Wall
a form much used in America, and patented
by E. L. Ransome in 1885. The vertical
standards are formed of two 1-in. by 6-in.
boards on edge with a slot between, through
which pass the bolts (see Fig. 269). By
undoing the bolt the planks behind the
joints in each row coincide. Long 6-in. by
6-in. timbers are next placed vertically at
the joints, just fitting in the spaces left
between, the short 6-in. by 6-in. ledges,
being 3 in. from the ends of the form. Other
rows are added similarly to suit the height
of the wall. These long 6-in. by 6-in.
verticals are kept in position by 6-in. by
6-in. horizontal wales placed about 2 ft. 6 in.
apart.
This set of operations is carried out on
both sides of the wall ; f-in. diameter rods,
threaded at both ends and fitted with nuts,
are passed through the 6-in. by 6-in. wales,
clear of the 6-in. by 6-in. uprights, through
the 2-in. planking every 4 ft. ; 2-in. by 4-in.
distance pieces, the exact width of the
wall, are placed in the forms quite close
to the bars, and these distance pieces are
knocked out as the concrete reaches them.
After the removal of the forms the pro-
jecting ends of the iron rods are cut off, and
covered in with a little cement put over
the ends. In a particular instance, this
method of holding the forms proved very
successful, not the slightest bulging occurring
although the wall was a thick one.
Fig. 269.— Collar and Set-screw
at X (Fig. 267)
standards can be set free
and the standards raised
as the wall proceeds.
The walls are in 4-ft. sec-
tions. A core box can
be introduced for hollow
walls.
The construction of a
large panel shutter for
walls is shown by Fig. 268.
The panels are made of 2-in. by 8-in. plank
dressed one side. The panels are erected
by placing the bottom row in position and
fixing them by stay lasts. Then another
row is placed on top so that the vertical
:'s Wall Form
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VERTICALS
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Fig. 268.— Panel Shutter
IMITATING MASONRY WALLS
A method adopted by E. L. Ransome is
to fasten wood V strips to the wall forms,
so as to produce an imitation of masonry
(see Fig. 270). The surface may be left
FORMS AND CENTERINGS
185
plain or rusticated by the application of a
pick or chisel.
STAIRCASE FORMS
The forms on which the stairs at Peek,
Frean and Co.'s warehouse at Bermondsey
Fig. 270.— Side of Form for Imitating
Masonry Wall
were made are shown by Figs. 271 to 273.
They are in continuous flights, with half-
space landings. The stringers under the
soffit of the stair are 2 in. by 6 in., and the
carriage 2 in. by 4 in., with cross stemming,
3 in. by 6 in. Horizontal ledgers 2 in. by
5 in. are fixed to stay the uprights and
stringer. The soffit of the steps is formed
by flooring boards being nailed to the
stringer. The form for the steps themselves
is constructed of triangular bracket pieces
nailed to the flooring with a concave moulded
fillet to form the nosing to the riser (see Fig.
273).
In many cases, stairs are made in advance
and erected similarly to stone hanging steps.
In that case the stringer is moulded like an
inclined beam properly reinforced top and
bottom with rods, and rebated on the lower
edge to receive the steps. An examination
of the form for the steps will show at a
glance that the usual back check in hanging
steps is got by fixing in triangular fillets.
METAL FORMS FOR WALLS, ETC.
A recent system of metal forms for walls,
columns, and girders is that of the Blaw
Steel Centering Co., and it is designed to
combine economically the three processes
Fig. 272.— Stringers
and Carriage of Stairs
Fig. 271.— Form and Centering for Staircase
Fig. 273. — Form for
Steps
as shown in the isometric sketch. The
stringers are supported at intervals by _2-in.
by 5-in. uprights ; where the uprights
require to be more widely spaced, they are
of form erection, namely, the assembling of
forms, lining them, and spacing them. The
panels shown in Fig. 274, which make up
the wall forms, are based on a standard
i86
REINFORCED CONCRETE
sized surface of 24 in. square. Forms are Corner panels are supplied with returns 12 in.
furnished in fractional sizes 25 in. by 12 in., on each side and 24= in. high. The panels
UHER3
to IO APART
Fig. 274.— Metal Panel Form for Walls
Fig. 278.— Form for Spandrel
Wall to Bridge
Fig. 275.— Method of
Fastening Panel
Flanges together
Fig. 276.— Detail of
Metal Beam Form
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Fig. 277. — Ransome Form for Cornice
Fig. 279.— Form for Curtain Wall with
Moulded Cornice
24 in. by 6 in., 12 in. square and 6 in. square, have angle flanges on all four sides, these
These sizes enable the contractor to work to being spot welded to sheet steel plates. There
any dimensions which are a multiple of 6 in. are no rivet marks on the face of the forms.
FORMS ' AND CENTERINGS
187
The plates are joined by special fastenings
passing through slots in the flanges. The
flanges are fastened together as shown in
Fig. 275. Adjustable beam and girder
moulds are also made, using wooden planks
' " Rv
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wfl!.
^Bti
Fig. 280.— Two Sections of Form for
Cornice to Hollow Coping
as the bottoms. These pieces are so clamped
together that the sides may be removed and
used in another part of the work, while the
bottom board form remains in place securely
shored until the curing of the concrete is
complete (see Fig. 276).
HINGED
Fig. 281. — Form for Ornamental Parapet
FORMS FOR CORNICES, MOULDINGS,
ETC.
Fig. 277 shows Ransome forms applied
to a cornice. The false-work for a spandrel
wall to a bridge is shown by Fig. 278. The
shuttering is 1| in. thick, kept in position
by 2-in. by 4-in. studs with block pieces.
The mouldings are made by inserting
splayed and ovolo or cavette moulded slips
in the angles of the form. A detail of the
form for a curtain wall with moulded cornice
and frieze is illustrated in Fig. 279, while
Fig. 280 represents forms for the cornice to
a hollow coping.
Cf#r-
Abo
Figs. 282 and 283.— Vertical and Horizontal
Sections of Form for Battered Retaining
Wall
FORMS FOR ORNAMENTAL PARAPET
Fig. 281 shows the form for an ornamental
parapet, 2 ft. high, to a bridge or balus-
i88
REINFORCED CONCRETE
trading. Two boards 7 in. wide,
corresponding to the length of
the panel, one each for the top
and bottom, lap over the side
board, which is 6 in. wide. Two
4-in. boards are used for one
side of the rail, and two 2J-in.
boards to mould the inside edge
of the rail.
For the panel centre two 8-in.
boards joined together are nailed
to the two 4-in. boards. The two
7-in. top and bottom boards are
hinged to this form so as to
fold back to allow the mould to
be easily emptied. The two ends
are of 7-in. timber, hinged to
close in the ends of the forms.
FORMS FOR RETAINING
WALLS
A form for a battered re-
taining wall is shown in vertical
section by Fig. 282, a horizontal
section on line A B being shown
by Fig. 283. In this, 4-in. by
3-in. standards are driven firmly
into the ground and are provided
with a cross-head and 4-in. by
3-in. struts. The 6-in. by^3-in.
planks are battened together and
arranged to come alongside the
4-in. by 3-in. standards. Centre-
bit holes are made through the
batten and standard, and f-in.
bolts, 9 in. long, passed through
and tightened up.
Other forms are illustrated by
three examples, the first of
which is a retaining wall at
Bridlington, carried out by
Ernest Matthews, A.M.I.C.E.,
and clearly illustrated by Figs.
284 and 285. The height of the
wall varies from 10 ft. to 13 ft.
with buttresses at lOJt. centres.
The cutting is excavated, and
IJ-in. by 9-in. rough poling
boards are placed against it with
two rows of wales in the
height. These are shored as
shown with 4-in. by 4-in. struts.
The foundation slab having been
put in, the 2-in. by 5-in. up-
rights, at about 2-ft. centres,
are erected on the foundation
slab and spaced apart sufficiently
V
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BUTTRESS
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9" l!4 POLING BOARDS
Figs. 284 and 285.— Plan and End Elevation of
Form and Centering for Retaining Wall at
Bridlington
FORMS AND CENTERINGS
to allow of the thickness of the wall and
1£ in. of shuttering on each side. The
uprights are held in position by two rows
of 1-in. by 7-in. flooring nailed on and
strutted with 3-in. by 4-in. raking and
horizontal struts to the cutting as shown,
with vertical flooring boards cut to the rake
and nailed to 1^-in. by 5-in. stops, which
are secured to the uprights. The forms for
the buttress should be boarded in on top,
ahead of the concrete laying, with 1-in. by
7-in. flooring on the slope. The forms are
wetted both inside and out before
any concrete is placed in them.
A 2-in. by 7-in. board, acting as a
wind strut, is nailed to the top of
the uprights and the waling at
the cutting.
Retaining Wall at Local
Government Offices. — The forms
for the retaining wall at the
Fig. 287.— Diagram, Plan,
and Details of Centering
for Retaining Wall at
Local Government Offices
Fig. 286. — Part End Elevation and Section of Centering for Retaining
at Local Government Offices
Wall
the shorter ones being 2 in. by 4 in. The
top and bottom portions of the wall should
be strutted independently of each other.
The uprights being securely stayed, the first
two l|-in. by 8-in. boards are placed on the
wall side of the uprights. The back is then
proceeded with similarly.
The two sides of the buttresses are formed
Local Government Offices, carried out
by the Trussed Concrete Steel Co., are
shown by Figs. 286 and 287. The concrete
at the basement level is more than 24 ft.
below the street level, and the excavation
was done in five sections of roughly 4 ft.
each. As each section was excavated, 1-in.
by 7-in. rough vertical sheeting or poling
IQO
REINFORCED CONCRETE
boards were placed against the earth, these
being secured with 4-in. by 9-in. horizontal
wales and 2-in. by 11-in. vertical planks
and raking shores. The shores were
placed 11 ft. or 12 ft. apart. The top
and third shores were formed with
4-in. by 9-in. timbers bolted together,
the intermediate shores being 7-in. by
7-in. balks. As will be seen from the
illustration, the thrust of the earth
against the retaining wall was resisted
by trussed and braced buttressed
division walls with intermediate pilas-
ters.
When the earth was properly held up,
the forms for the wall were begun.
The wall was 5 in. thick with 9-in. by
9-in. and 9-in. by 12-in. pilasters. The
face of the wall is 14 in. from the cutting.
3-in. by 4-in. uprights were fixed up
as shown, the boards being 1| in. thick
and fixed to the uprights a few boards
at a time as the concreting proceeded.
The forms for the pilasters were of
If-in. by 9-in. vertical flooring boards,
the front being added as the concrete
proceeded. The column and strut boxes
were framed on three sides with IJ-in.
by 9-in. flooring boards, the fourth
being added in advance of the con-
crete. The boards were clamped by
1-in. by 6-in. battens nailed on at
2-ft. centres. As the wall proceeded,
the poling boards and wales were taken
out and the back space filled in with
concrete. Only the shores were left in
till the wall was finished, and when they
were taken out the holes left in the
wall were filled up. All the boxing
was of IJ-in. by 9-in. boarding with
1-in. by 6-in. battens ; the arrises were
taken off the columns and beams with
angle fillets.
Another Retaining Wall.— Figs.
288 and 289 illustrate the method
adopted in carrying out the deep
basement in reinforced concrete at the
premises of the British and Foreign
Blind Association, Great Portland
Street, London. The basement has an
area of 35 ft. by 55 ft., and is 31 ft.
out in six sections of 5 ft. deep. Imme-
diately the first section was excavated,
1| in. upright sheeting was laid to the earth,
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C .
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=»
p
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uu ii/. vy uu it., ana is 01 it. p.
below the level of the pavement. The
walls on three sides are reinforced
against a possible 24 ft. head of water.
The fourth side being an existing brick
wall is deep enough to render underpinning
unnecessary. The excavation was carried
. 288 and 289.— Plan and Section of Centering,
etc., for Retaining Wall in Deep Basement
waled in the centre with 10 in. plank, and
strutted right across the basement with
five balks 10£ in. square and 9 ft. apart ;
folding wedges were used to tighten up
FORMS AND CENTERINGS
191
the struts against the brickwork on the
fourth side. Three cross struts, 9 in. by 9 in.,
were used to stay the sheeting on the end
walls, and were continued as stays to the
large lOJ-in. balks, thus making a rigid
] L
Figs. 290 and 291.— Elevation and Plan of Form and
Centering for Silo
rame. The balks and struts had to be
placed so as not to hinder the carrying out
of work on the pillars. The next section
was proceeded with similarly, until the
excavation was carried down to the full depth.
The wall was 14 in. at the bottom, tapering
in. at the top. Half way up the walls,
on the outside, large strap beams were intro-
duced to meet the thrust of the earth and
water, the thrust being further met ,by
introducing horizontal reinforced concrete
strut-beams between the walls and the
internal pillars, which carry
heavy weights from the super-
structure. Horizontally, the
i wall was strengthened by
the addition of pilasters. In
order that the wall should
be monolithic it was carried
up in continuous layers.
The walls and pilasters
were carried up as described
for the preceding example,
the horizontal boarding of
the walls being secured by
upright posts, 2 ft. to 3 ft.
apart, the outer boarding
being added as the work pro-
ceeded. The pilaster sides
were formed by l£-in. ver-
tical flooring, the front of
horizontal boards being added
as the work proceeded. As
previously described, the
poling boards and walls were
taken out at the back as
the work proceeded, and
only the shores were left in
till the completion of the
wall. The boxing of the
strut beams and columns
was carried out with 9-in.
by IJ-in. boarding, and
clamped with 4-in. by 1-in.
battens.
FORMS FOR SILOS
A typical silo erected at the
"Hovis" Mill, Vauxhall, is
shown by Figs. 290 and 291.
It is difficult properly to re-
inforce a rectangular corner,
and for this reason the
columns at the intersections
of the silo walls are on the
angle, as this makes the
strongest job.
The moulds for the walls of each bin were
formed in eight sections, each with planed
1 J-in. vertical boards 3 ft. 6 in. high, secured
to top and bottom horizontal ledgers 2 in.
by 7 in., and a 2-in. by 5-in. middle rail ;
placed against these were planks rebated
to receive the ends of the 4-in. by 7-in.
NOTE FLOORING LAID DIAGONALLY
OVER. RIBS'. OM THI5 FLOOR. WAS
MID THE fOfins FOR- COFFERINQ
FOR/1 roe
Figs. 292 and 293. — Vertical Section and Plan of Centering for Dome
of Wesleyan Hall, Westminster
192
.
FORMS AND CENTERINGS
193
by 9 in. timbers bolted together with
horizontal cills and cross braces, similar to
a gantry for a derrick.
The art of disposing timber to form a
centre capable of supporting the weight of
the material placed on it, and ascertaining
irizontal stays by which the forms were
kept in position, the proper distance apart,
and which in turn were cross-strutted with
4-in. by 6-in. stays. The forms at the angle
column were kept in position with split
2-in. by 7-in. vertical and 2-in. by 7-in.
angle struts placed horizontally, two in the
height of the form. The forms between
two adjoining compartments were kept the
proper distance apart at the head by long
bolts as shown. The concreting was carried
up in heights of 2 ft. at a time.
The forms for the hopper at the bottom
of the silo consisted of IJ-in. dressed boards
on 2-in. by 5-in. raking struts carried down
to the platform and cross braced with
battens. The upper inside boards were
laid as the concreting proceeded.
Forms for circular silos are constructed
as described later for circular tanks.
Ffg. 294. — Centering for Dome at Annapolis
New Academy
the scantling required, demands greater
knowledge than the ordinary builder usually
possesses ; but fortunately, in Great Britain,
at any rate, centres usually err on the side
of safety.
The following examples will give a fair
idea of the nature of dome centering com-
monly met with in practice.
Dome of Westminster Hall. — At the
Wesleyan Hall, West-
minster, is probably the
finest example of a re-
inforced concrete dome
to be found in Great
Britain, its span being
109 ft. over the pen-
dentives. The shell of
the dome is only 4| in.
thick, strengthened by
16 vertical ribs and 7
I\^ ^/ || || jjj horizontal ribs or rings,
Fig 296 Fig. 297 three of which latter
Figs. 295 to 297. -Centering for Octagonal Dome Circular are reinforced to take
in Section the thrust. For this
the bottom main rib is
ORMS FOR DOMES reinforced with an area of 32 in. of steel.
The centering requisite for erecting domes These cross ribs form an ornamental coffer-
is formed of numerous uprights carried up ing which is very effective. The centering
to the springing of the arch from a firm requisite for building this dome was
footing below, and consists either of scaffold carried on a platform which was built
poles spaced at 8 ft. to 10 ft. centres, and up from the concrete floor of the hall,
braced with ledgers and cross braces. In Planks were laid on the floor, and at from
other cases, the uprights are built of 3 in. 8 ft. to 10 ft. centres uprights consisting o
JOU2.
13
194
REINFORCED CONCRETE
three 3-in. by 9-in. timbers bolted together
were erected, together with horizontal
ledgers and cross braces. On the top of
these standards were laid ledgers which
supported the working platform of heavy
planks. Between the central uprights in
the middle of the hall, the lift was erected,
this carrying material right from the bottom
to the top of the building, openings being
left in the floor for this purpose. The plat-
form was at the level of the main cornice at
the springing of the dome. The ribs, which
eight-rib rolled steel joist framework. The
filling in was on the Columbian system of
reinforced concrete by cruciform bars sus-
pended to the ribs by stirrups. An internal
and external shuttering was used. The
inner shuttering was nailed to the horizontal
radiating ribs placed 1 ft. 6 in. to 2 ft. apart,
resting on triangular brackets fixed to
3-in. by 7-in. uprights and raking struts, the
bottom of the latter being secured to planks
laid on the floor. A portion of the external
shuttering sufficient for a part of the dome
Figs. 298 and 299.— Reinforcing the Large and Small Half Cupolas of the Poti Cathedral
consist of two l£-in. by 9-in. pieces, bolted
together, were entirely floored over diagon-
ally, this flooring supporting the forms for
the coffering slabs and beams, treated in
every respect similarly to floors. These ribs
were supported by struts and uprights con-
sisting of two 3-in. by 9-in. timbers bolted
together. Lagging was only erected to
support the underside of the slab. The
upper face of the slab concrete was worked
up from the springing by trowel work. The
centering is clearly shown in Figs. 292 and
293.
Dome of "Morning Post" Building,
London. — This was constructed on an
was made on the ground, it being'formed of
horizontal ribs shaped to the contour of
both plan and section. These ribs were
braced by 3-in. by 7-in. boards. The dome
was concreted in sections between the steel
ribs, the concrete being rammed from the
top, the same portion of shuttering being
used for this purpose. When the lower
portions were concreted the shuttering was
raised by wire guys.
A 20-ft. Saucer Dome. — The forms
for a saucer dome 20 ft. in diameter, sup-
ported on two sides of a barrel vault, which
was carried out at the chapel of St. Charles
College, Netting Hill, London, are very
FORMS AND CENTERINGS
.teresting. A platform was erected at the
level of the springing of the arch. From
this platform, resting on the planks, were
raised :3-in. by 7-in. uprights wedged up to
support at the base of the saucer dome a
circular ring or plate formed of three thick-
nesses of 1^-in. boarding, which in turn
supported the bottom ends of 3-in. by 9-in.
radiating ribs shaped to the contour of the
dome. On these ribs were nailed close-
boarded lagging. The arches to the barrel
,ults were supported on spliced short ribs
enough for it to harden, then raised 2 ft. and
clamped in the new position. The moulds
for the outer form were first put in place, and
the inner forms were hung from them. This
was carried up gradually until the men were
practically working on a floor before the
crown was reached. The base of the dome,
which was 69 ft. in diameter, overhung the
main walls below about 7 ft. all round, the
diameter of the main building being 83 ft.
Thus heavy corbelling was necessitated to
carry the bottom ring of the dome.
Figs. 300 and 301. — Centerings for Belfry and Arches of the Poti Cathedral
measuring 3 in. by 7 in., supported at the
joints by 2-in. by 5-in. struts, the ends of
which rested on a 3-in. by 7-in. tie wedged
up from the platform. The pendentives
were filled in with 3-in. by 9-in. shaped
ribs.
^ Dome of Annapolis New Academy. —
The dome at the new academy at Annapolis,
U.S.A., shown by Fig. 294, is a wonderful
piece of construction. Its peculiarity is
that no permanent centering was used, the
mould and the supporting frame being built
upwards as the work progressed, receiving its
support from the work already done. The
forms were kept on the concrete only long
The cathedral at Poti, Kussia, is described
in a later chapter, but it is convenient to
give here a number of photographs showing
the construction of the dome (see Figs. 298
to 305).
Octagonal Dome. — Figs. 295 to 297
show the centering used for an octagonal
dome, which is circular in section, at a
mortuary chapel in New York. Fig. 296
includes a part-plan of the ribs. The
main section is through the angle ribs, a
section through the common ribs being
shown adjoining. The main dimensions
of the centering are figured in the draw-
ings.
FORMS AND CENTERINGS
197
FORMS FOR TALL CHIMNEYS
The most economical material for chimney
shafts of circular section is reinforced con-
crete, and the simplest forms for shaft con-
struction consist of two sets of inner and
outer moulds, each 3 ft. high (see Fig. 306),
held together by means of latches which
can be readily undone to enable the forms to
be taken away easily. When the concrete is
filled in to the top of the upper ring the
bottom set of moulds is released and placed
Fig. 306. — Centering, etc., for Chimney at
Northfleet
on the top of the upper moulds, left in
position and safely held there by the frictional
resistance of the concrete ; this course is
repeated section by section till the whole is
complete. A circular wooden gauge, made
of two f-in. layers, is placed 6 ft. above
the level of the top form to hold the rods in
true alignment, and it is raised as the work
proceeds.
As the work is carried up from the inside,
only a light scaffold is required, built up
section by section every 4 ft. or 5 ft., the
framing of each section consisting of four
uprights to support a square platform of
stout planks, holed in the centre for hoist-
ing the materials by bucket. Double-ring
chimneys are carried up at the rate of one
form, and single-ring chimneys at the rate
of two forms a day.
The Ransome System.— The moulds
and false-work used by the Eansome Com-
pany in the United States for chimney
shafts are much more complicated. A
square scaffold tower is raised in the centre
of the chimney with four 4-in. by 6-in.
corner posts, and 1-in. by 6-in. cross braces
with 2-in. by 10-in. horizontal braces every
4 ft. or 5 ft., and built up a few feet in
advance of the construction ; the scaffold
tower supports a platform of horizontal
planks. From the tower project beams,
and from these the inner and outer moulds
are suspended by means of four vertical
rods having their upper ends threaded and
engaging in screw wheel bearings so that the
lower moulds can be raised or lowered.
On the top of the form mould on the outside,
light brackets strutted out also support a
working platform. To avoid removing the
cross beams at the head for each set of
moulds, telescoping scaffolding is built in-
side. Inside the chimney a staging is sus-
pended from the tower from which the
workmen place and tamp the concrete.
The shell moulds are 12 ft. high. The six-
sided lower platform is for the workmen
who finish the outer surface of the concrete.
FORMS AND CENTERING FOR ARCHES
AND BRIDGES
In designing and building forms and
centering for arches, care must be taken that
the centering is framed up strongly enough
to take the weight to come upon it ; that,
while the material is used economically, the
work must be capable of being easily taken
down ; that the footing is strong enough
to support the verticals without settlement
when fully weighted ; that the lagging round
the centering is strong enough to take the
weight of the arch ring without deflection ;
that the centering is properly held up in
position by folding wedges, these allowing
the centering to be lowered very gradually
so that the safety of the executed work
can be tested before finally striking.
In general, the forms for arches for rein-
forced concrete are substantially the same
as for masonry and brickwork, and present
no difficulty to anyone acquainted with the
latter. The lagging round the centering
198
REINFORCED CONCRETE
should be dressed to a uniform thickness,
and to a smooth surface.
Centerings for Flat Bridges.— Concrete
bridges may be classified under two heads,
namely, flat bridges and arch bridges. Flat
bridges are either straight flat slabs or com-
2 *4
The false-work is well shown in the photo-
graph (Fig. 310).
Centerings for Arch Ring Bridge.—
The centering for a 40-ft. span arch ring
bridge is shown by Figs. 311 and 312.
8-in. by 8-in. piles are driven in, sawn
ELEVATION.
Fig. 307. — Centering for Flat Bridge
bined slabs and girders, and are adaptable
for spans up to 40 ft. The centering for a
bridge of this latter type is shown by Fig.
307, and, as it bears a strong resemblance
to beam and floor forms already described,
it needs no further explanation.
Centerings for Arched Bridges.—
Arched bridges are constructed with or with-
out ribs. If the arches are reinforced with
steel joists or lattice girders, they are known
as Melan Arches, and the centering is partly
hung from the reinforcement, as shown in
Fig. 308.
Arched Roof Principal at Hammer-
smith Baths. — Fig. 309 shows the forms for
an arched principal in reinforced concrete at
Hammersmith Baths, London, one of the
first of the kind to be carried out in Great
Britain. In accordance with the usual
practice, a working platform was carried
up to the springing. The ribs supporting
the forms were out of 2-in. by 7-in. stuff,
in short lengths supported at the junctions
by raking and vertical struts braced hori-
zontally with 2-in. by 7-in. ledgers. On
the ribs were laid 2-in. by 4-in. lagging, on
which was nailed 1-in. boarding bent to the
contour and forming the bottom. The sides
were formed of IJ-in. boarding clamped
with 1-in. by 3-in. battens, and were sup-
ported on the 2-in. by 4-in. projecting lag-
ging. The sides were battened also on top
to keep the forms the proper distance apart.
off near the top, transverse runners are
bolted to the piles, and large folding wedges
are placed between the tops and the trans-
verse 10-in. by 10-in. beams, which support
the weight of the centering above. The
centering consists of a set of caps or trans-
verse beams resting on the wedges above the
Fig. 308.— Centering for Melan Arched Bridge
pile caps, some shaped bearers notched on
and supported by the upper transonic beams,
and finally of a closely laid lagging resting
on the bearers. The last-named are of
varying size, depending upon the distance
between the verticals and the weight to be
carried. For arches having spans up to
Fig. 309. — Elevation of Centering for Arched Principal, Hammersmith Baths
Fig. 310.— View of Centering for Arched Principal, Hammersmith Baths
199
200
REINFORCED CONCRETE
100 ft. these bearers are from 2 in. to 4 in.
wide and 12 in. to 14 in. deep, and spaced
from 1 ft. 6 in. to 3 ft. centre to centre.
The upper surface of the bearers must be
curved to fit the curvature of the under
the work, when the centering was removed,
was found to be only T3F in.
The main arch, of about 259 ft. span
with a rise of about 87 ft., is designed for
fixed ends without hinges. The roadway is
Figs. 311 and 312.— Centering for Arch Ring Bridge
surface of the arch, and the bearers must be
braced laterally by 1-in. by 6-in. bridging.
The lagging consists of f-in. tongued and
grooved pine or 2-in. spruce with bevelled
edges. When the bearers are far apart, the
lagging may have to be as thick as 4 in.
Teufen Bridge.— The bridge at Teufen,
Switzerland (see the photographs, Figs. 315
22 ft. 7 in. wide, and is more than 216 ft.
above the river. The arch voussoir is
3 ft. 11 in. at the crown and 6 ft. 11 in. at
the abutments. To carry the centering for
this arch, twelve verticals six rows deep were
driven below the river bed and carried up
above the springing as a platform. Each
vertical or standard was formed of two
i :i
1
DETAIL AT B.
Fig. 313. — Part Elevation and Detail of Centering for Bridge at Teufen
and 507) designed by such an undoubted
authority as Prof. Morsch, presents points
of great interest. The centering, Fig. 313,
designed by the professor, is a model to be
safely followed in similar situations, as the
permanent deflection at the completion of
12-in. by 12-in. balks bolted together and
stayed with ledgers and cross braces, both
longitudinally and transversely, at every
29 ft. of height. On top of these standards
were placed 9-in. by 12-in. ties cleated
with angle irons on the under-side to the
FORMS AND CENTERINGS
2OI
verticals ; a 12-in. by 4-in. by 40-in. sole-piece
was bolted to the ties, on which was laid
two sand boxes, except on the end row
adjoining the abutments. A similar sole-
plate rests on the sand boxes, and supports
the tie beam of the arch centering proper.
Pieces of channel iron were employed to
from 4 ft. to 5 ft. wide and the whole width
of the bridge were left unconcreted ; these
Fig. 314.
-Detail of Bridge Centering
(see A, Fig. 313)
distribute the pressure of the vertical and
horizontal timbers on to the sole-plates and
cross beams of the scaffolding direct on to
the verticals, and special steel stirrup ties
held in the feet of the struts, as shown in
!Fig. 314. The main struts were 10-in. by
10-in., and the secondary and small struts sections of the ring were strutted apart by
9 in. by 9 in. and 5 in. by 7 in. respectively, heavy balk timbers, as shown. After the
Fig. 315. — View of Part of False-work for
Bridge at Teufen
VERTICALS
8*8 +
6*4
Figs. 316 and 317. — Centering for Almandares Bridge, Havana
The struts supported the shaped stringers ring had taken its proper bearings, the con-
and laggings, each stringer being composed
of two 13J-in. by 3j-in. planks. The arch
ring was concreted in sections, and portions
creting of the omitted parts of the arch ring
was completed.
Arch bridges formed of a series of arched
Fig. 318.— Centering for Meadow Street Bridge, Pittsburg
STEEL ROLLERS
'•Ll^, ANCHOR/ BOLT3
Figs. 319 and 320.— Centering for Bridge of 233-ft. span
203
u
204
FORMS AND CENTERINGS
205
ribs supporting slabs have the ribs generally
first erected on such centering as has been
described for the Teufen Bridge. For com-
pleting the slabs after the ribs have hardened
BRACES 2*8
Fig. 327.— Centering for Bridge of 80-ft. span
somewhat, false-work similar to that de-
scribed for floors is used.
Seventeen examples of bridge centering
culled from the best Continental and
American practice are shown in outline or
in photographic view by Figs. 316 to 332,
the sizes of the main timbers being given as
far as obtainable.
SUSPENDED CENTERING FOR BRIDGES
In the building of bridges over running
streams great difficulties have been experi-
enced owing to the difficulty of erecting the
centering and owing to the serious settle-
ments of the centering that frequently take
place. American engineers found that the
running water washed round about the base
as at Teufen, and the voussoirs between
these sections not yet concreted in. The
voussoirs were raised to their true position
by screw-jacks, which had to be continually
adjusted to counteract the sinking
still going on. This led to the intro-
duction of flexible suspended center-
ing, and the particular design about
to be described was used in Chick-
ahominy Eiver Bridge, Eichmond,
Va., U.S.A. Fig. 333 shows cables
hung over the piers and anchored at
the abutment of the arches. At-
tached to these cables are the
Fig. 328.— Centering for Bridge of 110-ft.
span
hangers which support the centering shown
in Figs. 334 and 335. In order to pass
)0>JZ
Figs. 329 and 330.— Centering for Bridge at Deer Park, U.S.A.
of the wood piles and destroyed their vertical
friction, in one case causing a settlement of
12 in. Disaster would have occurred had
not the arch been constructed in sections,
the cables through the piers or abut-
ments to anchorage, a small pipe or box
with proper slope is embedded in the abut-
ments when constructing them.
206
REINFORCED CONCRETE
The arch can be raised or lowered to the
proper curvature by lengthening or shorten-
ing the hangers, by means of turnbuckles, or
by differential pulleys for very heavy loads.
Fig. 331.— Centering for Flat Bridge of
42-ft. 7-in. span
It is usual to set up the arch ribs in sections.
The key spaces are then filled with concrete,
embedding the reinforcement and binding
the arch sections together. The arch ribs
being completed, the cables and centering
may be taken down and any superstructure
desired built on the top of the rib.
Figs. 334 and 335 illustrate a panel of
suspended centering which is constructed
of wood firred on top with shaped member.
The furring member can be changed to suit
n arch of different radius. The longitudinal
adopted in Ireland for a foot bridge 172 ft.
long, and 150 ft. above the level of the sea.
The ribs were made in four portions on the
shore, and supported by overhead cables in
position as before described till the key
spaces were filled in. The floor was shuttered
in situ, being supported on the completed
ribs.
Figs. 336 to 338 are photographs which
further illustrate the method of building
a suspended flexible centering. Near the
abutments are frames or towers, from the
top of which hang cables anchored in any
SPAN 88' 6"
Fig. 332.— Centering for Flat Bridge of
88-ft. 6-in. span
suitable way. Wire rope hangers, depending
from the cables, carry cross members of wood,
iron or steel, upon which rest longitudinal
Fig. 334. — Elevation of Panel of Suspended Centering
CABLE
-CABLE
CABLE
i.*"-'*^' •••••' '•j.'./i '•
-^CONCRETE RIB :
v .•^-"s',-' •• \ •. . •.-
J
f
fifASMPe -tF£ET
1
^
I
1
Fig. 333.— One of the Three Spans of the
Chickahominy River Bridge
Fig. 335.— Section of Panel of
Suspended Centering
members are 2-in. by 12-in. planks, over-
lapped at the crossbars and notched to
receive them. The cross bars are 3-in. gas
barrel, 4 ft. long.
A similar style of construction has been
wood or steel shapes so attached that the
whole centering is flexible, and any curve
may be imparted to it merely by varying the
lengths of the hangers ; for the finer adjust-
ments, the hangers have turnbuckles. The
Fig. 336. — Gables supporting Concrete Sections, showing Key Spaces to be filled in
Fig. 337.— Arch Ribs before Striking Suspended Centering
207
208
REINFORCED CONCRETE
end sections preferably rest on the abutments,
the bearings being hinged. The previously
moulded voussoir sections of the concrete
arch ribs, or, instead, the moulds themselves,
smaller ones are constructed by joining short
lengths of concrete pipes together and
sealing the joints, so making one continuous
pipe. It is not good practice to mould pipes
Fig. 338. — Arch Ribs Formed in Suspended Centering
may next be laid in place, being supported
by the side pieces. The approximate lengths
of the hangers is determined in advance, the
final adjustments being made by means of
the turnbuckles or, in the case of very
heavy work, by means of differential pulleys.
Targets hang from the cross bars on supports
of different lengths, and when they all are
at the same level, it is known that the
flexible centering has been brought to its
proper form. An article by Philip Aylett,
in Concrete (Vol. VII., pp. 24-35),
should be consulted for further particulars,
of less than 3 ft. diameter in situ owing to
the difficulty of devising suitable"^ forms.
When more than 3 ft. in diameter, the pipes
can be easily moulded into shapes which
have more stability and efficiency than
have those of circular cross section. By
giving concrete pipes a broad, flat base,
\
RlW
%'BOLTS
Fig. 339.— Centering for 6-ft. Sewer
it s author being the designer of the examples
of flexible centering here illustrated.
FORMS FOR PIPES AND SEWERS
The larger concrete sewers moulded in
place are practically monolithic, while the
SHEET
IRON
Fig. 340.— Centering for 8-ft. 6-in. Sewer
they have a better bearing on the foun-
dation.
Fig. 339 shows the centering for a sewer
6 ft. in diameter, constructed in 8 ft. lengths,
16 ft. being done at one operation. To
facilitate striking the forms they are
FORMS AND CENTERINGS
209
constructed in four portions. The ribs are
2 in. thick, secured to each other by f-in.
bolts at 32-in. centres, and braced by 2-in.
by 6-in. bracing secured to the f-in. bolts.
The top section is secured in position by
shown. The 12-in. by 1^-in. ribs at 18-in.
centres are secured by 2-in. by 8-in. braces.
Fig. 342 shows the form for an arched
culvert 5 ft. wide. After the soil has been
prepared for the concrete, the 4-in. by 4-in.
Fig. 341.— Centering for
Conduit at Jersey City
Fig. 342. — Centering for 5-ft. Arched Culvert
cills should be set and braced with
1-in. by 6-in. braces. The inner
arch form is then wedged up from
the cills. The ribs are shaped out
of 1^-in. stuff and placed 3 ft. apart
centre to centre, this necessitating
thick lagging (2 in. by 3 in.) ; 1-in.
stuff would do if the ribs were only
2 ft. apart. On the outer side, 1-in.
by 4-in. or 1-in. by 6-in. horizontal
rn
Fig. 343.— Centering for
8-ft. Arched Culvert
12-in. by 6-in. wedges.
There is f in. clearance
between the top section
and the side section for
dropping the centre. 2-in.
by 2-in. laggings are fixed
to the ribs • and covered
with No. 27 gauge sheet
iron.
Fig. 340 shows the form for a sewer
8 ft. 6 in. wide. This pipe was constructed
in six sections, and made in 7-ft. lengths.
Fig. 341 shows the centering for the
conduit for water supply for Jersey City,
which is constructed in seven sections as
14
Fig. 344.— Form for
Small Box Culvert
boards are laid to 2-in. by 4-in. braces and
uprights set to the splay of the extrados,
as shown.
Fig. 343 shows the form for an 8-ft. arched
culvert. The 1-in. boards of the wall form
are secured by 2-in. by 4-in. verticals staked
2IO
REINFORCED CONCRETE
into the ground 'and strutted against dump-
ing left in ; a '4-in. by 4-in. cill is fixed,
and on top of this is laid the wedges, at
least 3 in. deep, to facilitate the removal
of the arch forms, which are framed of 1 £-in.
Fig. 345.— Collapsible Steel Centering for Sewer
stuff, 2 ft. 3 in. apart. The side forms can
then be easily removed after the arch forms
are struck.
Fig. 344 shows the form for a small box
culvert 24 in. wide, the top splay giving it
an arch effect cheaply. The outer form of
the wall consists of 2 in. by 10 in. boards
secured to 4 in. by 4 in. posts driven into
the ground and braced by 1 in. by 3 in. cross
pieces. The inner forms are of 1 in. boards
propped up by 2 in. by 4 in. head pieces,
and cills and posts, which are placed without
nailing. The form is struck by pushing the
2 in. by 4 in. head pieces off the side posts.
Collapsible Steel Centering. — A 'great
deal of sewer work is carried out by the
Blaw collapsible steel centering, which con-
sists of flexible steel plates bent cold to the
proper radius and stiffened with channel
ribs (as in Fig. 345). The sections are 5 ft.
long, and held together at the ends by
staples passing through slots on the male
end of each section. The centres are held
in position by two tension rods attached
by a turnbuckle, these being also used for
collapsing the centres. The rivets being
countersunk present a fair face to the
concrete. The following briefly describes
the method of their use. A dish is prepared
in the trench, on which to place the centre.
The concrete is then placed in position and
Fig. 346. — Conduit at Woolwich, showing Collapsible Steel Centering in Use
FORMS AND CENTERINGS 211
jammed ; after it has set, the centres are FORMS FOR TANKS
ollapsed by means of the turnbuckles, which Form for Square Cistern. — Figs. 347
ase them from the concrete without any and 348 show forms for the erection of a
arring or hammering whatever. When
4*2-
p
F — n
\-r-t- .
4*1
\
r^M
) '
9
J [
4*2
Figs. 347 and 348.— Form for Square Tank
be centres are collapsed they are dropped
rollers and pulled along ready for the
ext section. When the centres are very
irge they are mounted on small wheels run
n light rails. The centering is adapted to
small cistern 4 ft. 6 in. by 4 ft. 6 in. by 6 ft.
high. 1-in. dressed boards are cut to the
exact inside dimensions, namely 4 ft. 6 in., on
two sides, and 4 ft. 4 in. on the other two
sides, and they are held in position by
Figs. 349 to 352.— Form for Circular Tank
Afferent shapes for all kinds of conduits
ad for concrete pipe moulds.
Fig. 346 is a reproduction of a photo-
j:aph clearly showing the Blaw co lapsible
i :ntering in use.
,6*6
J ROD
2 *4 LAGGING
Fig. 351
2-in. by 4-in. posts strutted across
with 1-in. horizontal boards at top
and bottom. Eight 2-in. by 4-in.
posts at the corners are set on the
ground, and held in
position by 2-in. by 4-in.
inclined strutting nailed
to planks on the ground
secured by stakes. The
inner and outer forms
are kept the proper dis-
tance apart by 1-in.
battens nailed to both
posts. The posts may
be much higher than the
tank in preference to
cutting the material.
The outside boards can
be allowed to project beyond the corners
and thus save needless cutting.
Form for Circular Tank.— Figs. 349
to 352 show forms for a circular water tank
24 ft. in diameter. Six segments of 60
212
REINFORCED CONCRETE
degrees each form the circumference, and
they are 4 ft. high. There are two bands
of spliced 2-in. by 8-in. planks at top and
bottom shaped to the circle, and nailed to
these at 8-in. centres are 2-in. by 4-in.
laggings to which are secured No. 22 gauge
galvanised iron. The yoke, formed of
6-in. by 6-in. verticals, 2-in. by 6-in. braces
and adjusted by a turnbuckle, is the means
by which the outer and inner forms are
kept the proper distance apart, it also
enabling the forms to be raised as the work
proceeds.
Form for Gasholder Tank.— Fig. 353
represents a section through the walls of
the gasholder tanks at Dubuque, erected for
the Key City Gas Company. The bottom is
about 5 ft. below the outside ground level,
and the walls rise to a height of 21 ft., being
18 in. thick at the base, tapering to 12 in.
at the top. The concrete was first laid over
the entire bottom 16 in. thick, but increased
under the walls to a thickness of 2 ft. 6 in.
round the heads of the posts was also dished
out in order that the posts could be cut
down in the striking of the scaffolding, and
the holes filled up. The whole 264 ft.
circumference of the tank wall was filled
up to the level of the top of the form in one
operation. Forms were used to make a
mortise and tenon joint between one day's
work and the following day's. Between the
outer side of the wall and the earth, up to
the higher ground level, concrete was filled
in, well rammed and tamped, and it was
relied upon as a support to the base of the
wall. Two 2-in. by 12-in. planks nailed on
top of temporary posts were laid radiating
to the tank wall. On top of this two layers
of 1-in. by 6-in., spliced in short lengths and
shaped to the radius of the wall, were fixed,
braced in position with 2-in. by 6-in. up-
rights and 2-in. by 4-in. inclined braces.
The scaffolding supporting the wall forms
on the inside of tank consisted of 4-in. by
6-in. cills laid on the ends of the post, which
projected 8 in. above the floor ; from these
were erected double 2-in. by 4-in. verticals,
with 4-in. by 6-in. ledgers and 2-in. by 6-in.
cross bracing. The outside alternate pilasters
and piers were built up with two sides
complete, and the third side added as the
work proceeded. The shuttering to the
outside wall was similar to that on the
inside and was fastened in position be-
tween and under the edges of the pilaster
forms and lag screwed to the 4-in. posts.
The edges of the inside shutters were bolted
4x4, 6*1 PLANKS
.2*8
~sr
1
\r-
3A HOLE
'/2 POO WITH 3" EYE END
Pit. ^54
2*12
Fi«. 353
Figs. 353 and 354.— Form and Centering for Gasholder Tank
As the foundation was uncertain, piles were
driven 4 ft. 6 in. apart to support the floor.
The last named was roughly dished at the
bottom of the wall in order to give a good
key both to the bottom of the wall and
to support the scaffolding of the wall forms,
which were fixed to temporary posts driven
where shown to carry them. The concrete
to the sides of the inside 2-in. by 4-in.
verticals of the scaffolding, and the little
triangular spaces between the shutters were
filled up with 2-in. vertical strips of steel
on a backing strip of wood 1 in. square.
The forms were raised 3 ft. at one opera-
tion, by blocks attached overhead to a cap
piece, and as the forms were 4 ft. high
FORMS AND CENTERINGS
213
there was an overlap of 1 ft. on the finished
concrete. The forms were cleaned and oiled
after every operation. Fig. 354 shows the
inside shutter for the tank.
Fig. 355. — Form for Gasholder Tank at San Sebastian
Fig. 356. — Gasholder Tank at San Sebastian
A photographic view of
the forms, etc., for a gas-
holder tank at San Sebas-
tian is presented by Fig.
355, the finished tank
being shown by Fig. 356.
Form for Rectangu-
lar Reservoir. — In an
example of a rectangular
reservoir having walls
strengthened by pilasters,
the slab roof was carried
on rib beams. In excav-
ating for the walls, the
footings were made slightly
larger and a 2-in. plank on
edge inserted. After plac-
ing concrete for the foot-
ings, horizontal shutter-
ing of 2-in. board, dressed
inside and propped up by
4-in by 4-in. uprights
placed at 4-ft. centres, was
erected. For the curved
corners thin boards were
bent to the required curve
and secured to the up-
rights, the latter being 2
ft. apart. The posts on
214
REINFORCED CONCRETE
the outside were propped up by raking
struts and wires passing through the wall
to the inside, there being braces connected
to the adjoining columns and walls, and also
struts extending to a proper bearing. As
before, the two sides of the pilasters were
are cheaper, as they require no upkeep.
Figs. 357 and 358 show the form for square
posts. A plank is laid as a base dressed
on the upper side, and on this are set two
tapering pieces, 2 in. by 6 in. at one end
tapering to 2 in. by 4 in. at the other.
5-0
I
jgl
pJA
Fig. 357.— Elevation, Plan, and End View of
Form for Tapered Square Posts
m
Fig. 358. — Cross Section
(enlarged) through Form
for Tapered Square Posts
6-lS1
1
Q
Fig. 359. — Plan and Elevation of Multiple
Form for Tapered Square Posts
completely formed, and the third side added
as the work proceeded. The slab ceiling
and beam forms were carried out as previ-
ously described for floors, with 2-in. boards
dressed on one side, both to the beams
and slab. The beam forms and slab cen-
tering were erected on 4-in. by 4-in. up-
rights resting on the finished floor beneath
and cross braced at intervals.
FORMS FOR FENCE POSTS
Reinforced concrete fence posts have been
used with success, and although their first
cost is more than wood, in the end they
Fig. 360. — Section of Form for
Triangular Posts
These tapering pieces are held in position
without nails by the side fillets which are
nailed to the base board. The tops of the
sides are stayed as shown. Fig. 359 shows
a multiple form for moulding four fence
posts at a time, the posts being tapered
four sides. The divisions separating the
posts are slipped in between cleats at each
end, and held in place by wedges against
blocks nailed to the platform. The illus-
tration is self-explanatory. Fig. 360 shows
a mould for triangular posts. The frame is
hinged so that posts having different angles
can be made in it.
Systems Described
IN this chapter will be found descriptions
of a number of the best-known modern
systems of reinforced concrete, based on
information supplied by various firms in
the industry. It will thus be understood
that where special claims are made for this
or that arrangement, it is simply the in-
ventor's or owner's point of view that is
presented. The explanations have been
kept as concise as possible, since, as most
readers already know, the firms responsible
for the various systems issue lengthy and
well - illustrated descriptions, which may
generally be had for the asking, and it is
therefore a simple matter for the reader to
follow up any special claims or other points
of interest here briefly stated which may
invite his further study.
ARMOURED TUBULAR FLOORING
A- typical floor on this system consists
essentially of three parts (see Figs. 361 and
362), the concrete webs A, coke-breeze tubes
B, and a concrete top layer c. The webs are
reinforced with a ribbed corrugated bar as
shown, and these, together with coke-breeze
tubes, which are 9 in. long, are delivered
complete on the site. First the webs are
placed in the proper position, then the tubes
are put in between, and the top layer of
transverse wires, the two being electrically
welded together at the points of inter-
section. The wire fabric can be supplied as
bright steel or galvanised ; either of the
strands may be of any thickness, from
No. 4 to No. 12 gauge (Imperial Standard
Wire Gauge), but No. 5 is the heaviest that
can be used when both strands are of the
same size. The longitudinal wires are spaced
3 in. apart, and the standard spacing of the
transverse wires is 12 in., 16 in., or 18 in.
The maximum number of longitudinal wires
is 24. The standard maximum width is 72 in.
The greatest width possible is 97 in., and
the wire fabric is supplied in rolls or sheets,
a roll of the heavy gauge containing 150 ft.,
and of the lighter gauges 200 ft.
COIGNET SYSTEM
Beams and Slabs. — The essentials of a
Coignet beam are shown by Fig. 363, in
which A indicates the principal bars subject
to tension, B secondary bars working in com-
pression, C stirrups to resist shear, D principal
bars in floor slab, and E secondary bars in
floor slab. The principle of the beam is the
introduction into the compressed portion of
the concrete of the secondary bars over which
Figs. 361 and 362. — Armoured Tubular Floor
concrete is spread over the whole. The
ceiling side of the floor may be left as laid,
or may be finished by plastering.
CLINTON SYSTEM
Slabs. — This system depends on the use
of an electrically cross-welded steel wire
fabric, consisting of a series of parallel
longitudinal wires spaced at certain dis-
tances apart, and held at intervals by
the stirrups or shear members are hooked.
The stirrups or shear members, which con-
nect together the concrete working in com-
pression and the bars working in extension,
increase the resistance to shearing and also
the compressive resistance of the concrete.
Where the stirrups pass round the bars, they
are fastened by means of annealed wire so
as not to be disturbed when the concrete
is applied. The skeleton reinforcement is
2l6
REINFORCED CONCRETE
prepared in advance, and stacked till ready
for use. As the spacing of the stirrups is
proportional to the shearing stresses, they
are closer together near the supports where
the maximum shearing stresses occur. Much
importance is attached to the fact that both
stirrups and the main bars are of round
section, so that the line of contact between
them is a mere line.
Fig. 363 illustrates an early type of beam,
which has been largely superseded by that
shown by Fig. 364.
In a later type of Coignet beam (Fig. 364)
Fig. 363.— Coignet
Beam and Slab
Fig. 366. — Section
through Coignet
Column
Fig. 367 and 368.—
Base of Coignet
Column
the lower bars shown in the previous figure
are replaced by a group of bars of smaller
diameter, their ends being bent upwards at
an angle of 45 degrees and hooked over the
top bar. The sectional area of the reinforce-
ment is greatest in the middle portion of the
beam where the bending moments are higher,
and the bars are spaced closer together near
the supports as in the previous example.
The bars are bound together with annealed
wire as before. The bars do not lie in con-
tact one with another, there being a space
of at least J in. between them into which
the concrete flows.
The reinforcing of the floor slab has already
been shown in Fig. 363. The principal bars
D are of such diameter and spacing as will
best resist the tension, the secondary bars
E merely distributing the efforts more easily
on the principal ones. At alternate inter-
sections of the bars, they are bound together
with annealed wire. Coignet floor slabs
usually vary in thickness between 3 in. and
6 in. ; but should the thickness exceed this,
a double reinforcement is advised, stirrups
connecting the upper and lower bars at
intervals.
Fig. 365. — Beam Supporting Floor Slab
Centering
Fig. 364. — Coignet Beam Reinforcement
Consisting of Group of Small Bars
Fig. 369 and 370.— Coignet Pipe or
Conduit
Coignet beams may be prepared in advance
and transported and placed in position like
steel girders. They are made in a horizontal
wooden mould, the principal bars being left
protruding for about a foot at each end so
that they may be fixed on to their supports
by means of cement grout. Fig. 365 shows
how the ready-made beams may be used to
support the centering for the floor slabs. The
upper ends of the stirrups project so as to
form a mechanical bond. This method is
seldom used.
Columns. — A simple type is shown by
Fig. 366. In this, the vertical bars are
SYSTEMS DESCRIBED
217
bound by horizontal or spiral hoops or ties
to keep them in their proper position and
also to resist a bursting tendency. The
hoops are fastened to the verticals by means
of annealed wire. The footing of the pillar
where it is necessary to spread the weight
takes the form shown in Figs. 367 and 368.
Walls. — As these are to resist lateral
pressure only, and not intended to support
any vertical load, this being carried by
beams, a meshwork of vertical and horizontal
bars is placed in the centre of the wall to
facilitate concreting, the intersections being
bound together with wire.
Pipes. — The reinforcement takes the
form shown in Figs. 369 and 370, and the
pipe or sewer is constructed in lengths of
about 4 ft. or 5 ft., and connected to the
next section by means of joint and spigot
as shown ; alternatively, the pipes may be
concreted in continuous length.
Piles. — Practically, these are pillars
made in a horizontal mould instead of a
vertical one. Generally, as in Fig. 170
(p. 133), they are circular in section, with
two flat longitudinal surfaces which assist the
proper guiding of the pile during driving.
CONSIDERE SYSTEM
Beams and Slabs. — This system is well-
nigh devoid of fanciful methods of shaping
and arranging the reinforcement, and is
claimed to rest entirely on a scientific basis.
In the case of minor beams, the tensional
reinforcement takes the form of round bars,
some of which are horizontal throughout,
and others are bent up at the ends. Where
a compression member is required, a spiral
supports at each end, there being, in addition,
round bars which cross the other reinforce-
ments at right angles. In large girders
and trusses, as employed in bridge con-
struction, the tension members are rein-
forced in the manner already described,
whilst the compression members are rein-
forced with longitudinal bars and spiral coils.
Columns. — Columns are generally cir-
cular, octagonal, or square in cross section,
and the vertical reinforcements are straight
round bars from 4 in. to 6 in. apart, whilst
the transverse reinforcements consist of
round rods in the form of coils wound
spirally round the vertical reinforcements,
the ends of the coils being bent inwards.
Piles. — Considere has devoted much
attention to the construction of reinforced
concrete piles on scientific principles. His
general form of pile is octagonal, with
longitudinal reinforcing rods, generally num-
bering about eight, inside a continuous spiral
winding of round steel rods, the pitch of the
spiral being about 2 in. at the middle and
diminishing to 1£ in. at both head and foot.
The head is of cylindrical shape and for
about 4 in. is bound with steel coils closely
pitched, this reinforcing the head to such
an extent that it will not fracture under
the driving shock, even though a cap or
dolly is not used. Illustrations showing the
application of the Considere system in prac-
tice will be found on pp. 358 to 360.
CORR BAR
This is an American system of supplying
beam and column reinforcement already
made up and suitable for immediately placing
Fig. 371.— Corr Bar Beam "Unit"
Fig. 372.— Types of
Corr Bars
coil of round steel is inserted near the top
surface. Occasionally beams are addition-
ally reinforced against shear by means of
thin steel rods, which are lapped round the
tension and compression bars. In addition
to the above, Considere sometimes inserts
an extra reinforcement (a spiral coil of
round steel laid nearly horizontal) in that
part of the concrete which is in compression
near the supports of a continuous beam.
The slab reinforcements between the beams
consist of round bars bent up over the
in the forms. Each made-up reinforcement is
called a " unit," and a typical construction
is shown by Fig. 371, this type having been
designed to give, by means of continuous
stirrup, an efficient web reinforcement. The
unit for a round column consists of two or
more verticals with a continuous hooping of
cold-drawn wire. The American company
responsible for this system has introduced
a variety of special reinforcements, including
deformed bars (see Fig. 372) and a kind of
expanded steel known as " Corr Mesh."
218
REINFORCED CONCRETE
DENTILE SYSTEM
Floors. — By means of this system, floors
are constructed having the advantages of
terra-cotta tile floors and those of reinforced
portions fitting together to form a closed
hollow block. The reinforcement can be
placed in both directions. In the case of
Fig. 373.— Dentile Floor with Mitre Tiles F»& 376- — Diamond Mesh Expanded Metal
concrete. Briefly, upon the forms are placed
hollow tile blocks closed on all six sides and
having projecting flanges, which space them
at proper intervals. In the channels so left
the reinforcing bars are placed, the concrete
being then applied and a floor surface being
obtained by spreading a layer of concrete
Fig. 374.— Dentile Floor with Bridge Tiles
right over the tiles. Essentially, then, the
floor consists of hollow tiles with ribs of
reinforced concrete between them united at
the top by a thin slab of plain concrete. On
the ceiling side there is a continuous sur-
face of tile. Clear spans up to 32 ft. have
been built and tested, while for light loads,
Fig. 375.— Dentile Floor with "L" Tiles
and where surrounding conditions are favour-
able, the spans may be as much as 40 ft.
Three shapes of tile are used : mitre tiles
(Fig. 373), consisting of four wedge-shaped
the bridge tile, shown by Fig. 374, the
reinforcement is in one direction only. With
L tiles, (Fig. 375), the reinforcement can go
both ways. Small flat filling pieces are used
to put in the square corners between the
tiles. In all cases the under-sides of the tiles
are roughened so as to take the ceiling
plaster.
EXPANDED METAL SYSTEM
Expanded metal consists of rolled steel
of various thicknesses, cut and expanded
by machinery into meshes of various shapes,
the material being obtainable in a number
of different strengths or weights. Diamond
' mesh is shown by Fig. 376. There are thirty
varieties of this material, the variations being
in the size of mesh, thiclcness of strands, the
Fig. 377.— Rib Mesh Expanded Metal
weight per yard super, etc. The rib mesh
is shown by Fig. 377, this being made in
five varieties ; however, for concrete work
the diamond mesh material is chiefly used.
SYSTEMS DESCRIBED
What is known as the expanded steel bar
is a bar from which a series of meshes has
been expanded (see section, Fig. 378). It
is claimed to be a complete unit, comprising
tension and shear reinforcements, and as its
members are rigidly connected, they cannot
219
e — by a number of stirrups clipped
firmly around the tension bars as shown in
Fig. 380 and bent over at the top so as to be
anchored into the concrete, and they also
insert other bars straight for the greater
part of their length and bent upwards at
Fig. 378. — Section of Expanded
Steel Bar
"*~~ Expanded Metal Lathir.g
esh Expanded Steel"
get out of place during the con-
creting. When expanded metal is
used in connection with these _E
bars, the ends of the sheets may
be wired to or bent over and
lapped on to the web of the bars,
thus securing continuity of rein-
forcement.
Floors. — When these bear on
the walls, it is necessary to Fi& 379.— Four Types of "Expanded Metal" Floors
leave an offsett, corbel, chase,
***» Expanded Metal Lathing
Reinforced Concrete Bel
Expanded Sttel Bars
Ik mesh Expanded Steel
Reinforced Concrete Bea
Reinforcing Rod:
or over-sail. The method of constructing
the floor with expanded metal is described
on page 152, and Fig. 379 presents sec-
tional drawings of a few typical styles of
construction.
Other Applications. — Expanded metal
has been found applicable to a wide range
of work which it is quite unnecessary to
describe in detail.
HENNEBIQUE SYSTEM
Beams. — Scientific designing rather than
the use of steel of special shape or high-
carbon content is the essential of this system.
Thin bars or rods and stirrups of steel are
inserted in just those places where the stress
diagram of the structure shows that the
resistance of the concrete requires to be
supplemented. The experts responsible for
the system believe that if it were practicable
to apply the teachings of theory to their full
extent, the steel reinforcement employed
would be in the form of fine wires receiving
and transmitting stress throughout the con-
crete. They therefore endeavour to recon-
cile theory and practice by using moderately
small bars as the main reinforcement, and
small rods, wire, or steel strips as the con-
necting or auxiliary reinforcement, and all
these are in forms procurable in the open
market. They believe that a beam rein-
forced simply by straight tension bars near
the lower face would not be satisfactory in
practice, and therefore they supplement
those bars — in a beam of the most simple
each end, part of such a bar being shown
by Fig. 381. Diagramatically, a simple
beam of this type is shown in Fig. 382,
which includes a longitudinal section and
three cross sections. The bars near the
lower surface adequately reinforce the
middle portion of the beam, and the inclined
bars and the vertical stirrups supply the
additional reinforcement necessary at the
ends of the beam where tension occurs at
diagonal planes. The company emphasises
their belief that the bars and stirrups rein-
force the concrete against diagonal tension
rather than against vertical shear, and say
that what, in beam tests, is commonly
described as a " shearing " failure, is almost
invariably a diagonal tension failure. The
bent-up ends of the bars lie across the lines
of rupture near the supports of the beam,
and afford in themselves very secure anchor-
age. The spacing of the vertical stirrups is
such as to provide for variations of stress from
point to point. Being vertical, they facili-
tate the operation of ramming the concrete
without risk of displacement, and being
made with a spring clip at the lower end,
they are automatically held in position on
the main bars without either wedges or ties.
The stirrups form an effective web connection
between the tension and compression por-
tions of the beam. The company does not
believe that any stress or vibration developed
under working conditions can possibly over-
come the natural adhesion between the con-
crete and the plain round bars, but the ends
22O
REINFORCED CONCRETE
of the bars are flattened and opened out to
form secure anchorage, and at least half the
bars are bent up towards the supports,
further to ensure perfect security. In beams
that are continuous over intermittent sup-
ports, the ends of the bent-up bars are
carried horizontally across the supports and
terminate near the points of contraflexure
(see Figs. 383 and 334). The inclined bars
dimensions. The double stirrups will be
noticed.
Floors. — Hennebique floors are simple
combinations of beams and slabs. The main
beams receive the load which is trans-
mitted from the secondary beams, the latter
receiving the load transmitted from the
continuous slab connecting all the beams and
receiving the super load placed upon the
Fig. 381.-
Fig. 380. —Henne-
bique Stirrup
round Tension Bar
-Hennebique Tension Bars
and Stirrups
I T
B D
iUr
3*cCion on A-O. Section on CD Section on C r
Fig. 382. — Simple Hennebique Beam
385. — Beam Reinforcement with Com-
pression Bar and Double Stirrups
Fig. 387 and 388.—
Base of Henne-
bique Column
Fig. 389.—
Sheet Piles
z- ai
V
Fig. 390.—
Hollow
Diaphragm
Pile
Figs. 383 and 384. — Hennebique Beams Continuous over
Intermittent Supports
Fig. 386. — Hennebique
Column
in each span, and the horizontal ends of
other inclined bars projecting from the
adjoining spans, then resist the tensile
stress occurring in the upper part of con-
tinuous beams between the supports and
the points of contraflexure.
Beams constructed as above described are
the most economical types, but it is some-
times advantageous to employ compression
bars near the upper surface as shown in
Fig. 385, which type of beam has advant-
ages when a very heavy load has to be
carried by a beam of comparatively small
floor. In a typical instance the main beams
have parallel reinforcing bars, and the
secondary beams have single reinforcement.
In the usual type of Hennebique floor
half the slab panel on each side of each
secondary and main beam constitutes a
compression flange, and, with the beam,
forms a beam of T-section. The longitudinal
bars and stirrups of the main beams pass
into the superimposed slab, and where there
is compression reinforcement, this is entirely
within the slab. The Hennebique hollow-
tube flooring is mentioned on p. 332.
SYSTEMS DESCRIBED
221
Columns. — A typical Hennebique column
is shown by Fig. 386, the vertical reinforce-
ment consisting of plain round bars, and the
auxiliary reinforcement being formed of
links of TVm- steel wire applied in sets of
four, one link passing round the two verticals
on one side. The links brace the longitu-
dinal bars and support the concrete later-
ally. This method was introduced many
years ago, and has given satisfaction in
thousands of cases.
Figs. 387 and 388 show a column base, the
horizontal steel reinforcement distributing
the force transmitted by the vertical bars.
The lower portion of the concrete is rein-
forced by a double system of bars laid at
right angles with one another so as to pro-
vide for the tensile stresses caused by the
bending moment developed by the central
load and the vertical reaction of the ground.
Kesistance to compression is provided
entirely by the upper portion of the con-
crete, and diagonal tension is taken by the
vertical stirrups applied as in beams. A
column base of this description virtually
represents cantilever construction.
Piles. — These can be built of any approved
shape, that shown on p. 133 being typical.
The longitudinal bars are bent inwards at the
toe, but do not bear upon the steel driving
point, this being anchored into the concrete
by four straps bent over at the upper end,
as shown on p. 133. The transverse ties,
resembling those in the column already
described, are spaced more closely near the
top and bottom of the pile than in the
middle.
Sheet piles for retaining walls and coffer-
dams have a head of reduced section (see
Fig. 389), and on two sides of the pile there
is a semicircular groove, the hole formed
by the grooves in contiguous piles being
filled with cement grout to prevent the per-
colation of water. Thus a row of sheet
piles forms a watertight wall. The hollow
diaphragm pile (see Fig. 390) is constructed
on the principle that the resistance to
driving is largely, if not entirely, due to
friction between the external surface of
the pile and the earth, and that a given
volume of reinforced concrete employed in
the form of a hollow cylinder is more effective
than if applied as a solid cylinder. The
reinforcement is practically the same as that
of the solid pile, but, in addition, diaphragms
hold in place a consecutive series of tubes
each about 4 ft. long, the object of the
tubes being to form the hollow core of the
finished pile.
INDENTED BAR SYSTEM
This system depends entirely on the use
of a deformed bar, either square or round
in section (see Figs. 391 and 392). The
square bar varies from J in. to 2 in. square,
and from -24 Ib. to 13-6 Ib. per foot run in
weight, whereas the round bar varies from
fin. to 1| in. in diameter, the weight
per foot run at these sizes being -38 Ib.
and 6'06 Ib. respectively. Strictly speak-
ing, the bar itself is the " system," the
company responsible for it having no
special -method of arranging the rein-
forcement, but preparing their designs
in accordance with approved practice, and
supplying the indented bars to the building
contractor to be used in accordance with the
designs prepared by the Indented Bar Coin-
Fig. 391. — Square Section Indented Bar
Fig. 392. — Round Section Indented Bar
pany or by independent engineers. They
contend that their bar obtains a more
reliable grip on the concrete than does any
smooth bar. The adhesion between smooth
bars and concrete depends, they state, upon
the cement particles entering into the micro-
scopical irregularities in the surface of the
steel. When a steel bar is subject to a
tension of 12,000 Ib., or say 5 tons, per
square inch, it extends by -004 in. per inch of
length, this causing corresponding reduction
in the area of cross section, which, although
extremely small, is sufficient to affect the
adhesion. They instance a number of tests,
the results of which go to show that indented
square bars sustain 1-9 times the stress
required to pull out smooth squares, and
indented round bars 3'7 times the stress
required to pull out smooth round bars.
Further, it is stated that owing to the fact
that at a stress of 3 tons per square inch the
extension in steel is greater than that to
which concrete can be subjected without
cracking, it appears to be undesirable when
smooth bars are used to allow the tension
in the steel to exceed that amount. With
222
REINFORCED CONCRETE
indented bars, however, the continuous and
positive bond provided makes it permissible
for the stress in the steel to be limited only
by the strength of the material used, this
enabling a steel of high tensile strength to
be employed. The draft regulations of the
London County Council propose that there
should be a mechanical bond between the
steel and concrete in cases where steel used
has a greater strength than 72,000 Ib. per
square inch.
When the company's ordinary specifica-
tion is adopted, the bars are of rolled medium
steel with an elastic limit of 50,000 Ib. per
square inch, with an extension of not less than
15 per cent, in a length of 8 in. ; the breaking
strength is 90,000 Ib., roughly 40 tons per
square inch. By the use of such strong
material there is claimed to be a consider-
able saving in the weight of steel required
for a given strength, the factor of safety
can be increased without extra cost, and the
omission of bending, splitting, cranking, and
other devices commonly used for anchoring
smooth bars is a source of economy.
JOHNSON'S LATTICE SYSTEM
This depends on the use of a steel wire
mesh or lattice (see Fig. 393), manufactured
in sheets or panels, and in rolls 6 ft. wide to
about 120 ft., or the rolls can be made longer
than 120 ft. and any width other than 6 ft.,
and the spacing can be varied to suit
individual requirements.^ It is unnecessary
Fig. 393. — Johnson's Steel Wire Lattice
to give full details of the various methods
in which this reinforcement is applied. It
is sufficient to say that it is applicable to
very many purposes, from foundation rafts,
to floor slabs, walls, etc. Figs. 394 to 397
show four different methods of supporting
mesh-reinforced floors.
KAHN SYSTEM
Beams. — The experts responsible for this
system emphasise the fact that a properly
constructed reinforced concrete beam is in
reality a trussed beam, and they state that
stirrups can only transfer stress to the main
tensional member when the two are defi-
nitely and rigidly connected together, the
horizontal reinforcement taking not only the
stress caused from adhesion of the concrete
to it, but also the summation of the hori-
zontal components of the strain in each
of the diagonals. The principles of truss
action occur (or grow) out of this. The
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Figs. 394 to 397. — Four Methods of Supporting
Mesh-reinforced Floors
company does not believe that the concrete
surrounding the bars will prevent the loose
stirrups from slipping, and they therefore
attach the shear members rigidly to the
horizontal reinforcement. In the Kahn
system a bar with two projecting wings or
stirrups is used, the stirrups being bent up
at about 45 degrees, so as to cross the planes
of rupture at nearly right angles (see Fig.
398). At the centre span, where the hori-
zontal tension is greatest, the web is left
intact on the bar, and there serves as an
additional reinforcement (see Fig. 399). The
stirrups are part of the original metal of
the bars, being merely sheared and bent,
whereas in the original Kahn bar, holes were
punched in it and the shear members
fastened in these holes. The Kahn trussed
bar, it will be understood, is sent to the
site complete, ready to be incorporated in
the concrete as a single unit.
The bar undoubtedly possesses great
advantages in lintel construction. The prin-
ciple is stated as follows : A flat arch acts
exactly upon the same principle as a seg-
mental arch, and therefore' -if a section of
the wall above an opening can be converted
into such a flat arch, 'and a tie member suit-
Fig. 398.— View of Kahn Bar
-4»-
Fig. 399. — Section and Elevation of Kahn
Trussed Bar
ably placed so as to receive its thrust, then
that section of the wall has become in reality
an arch. This, in short, is the purpose of
the Kahn lintel, wherein the Kahn trussed
bar is used as a tie member, the diagonals
of the bar taking up the thrusts of the arch.
The lintels can be built for spans as great
as 30 ft. or 40 ft. Tigs. 400 and 401 show
the application of the Kahn bar in lintel
construction.
Floors. — The floor slabs, reinforced with
top and bottom horizontal bars, the lower
Fig. 402. — Keedon Beam Reinforcement
Fig. 403. — Keedon Column Reinforcement
of which is a bar of the type already des-
cribed, form, with a trussed beam, a beam
of T-section.
Columns. — The Kahn bar is also applic-
able to column construction. The prongs
projecting from the bars (the bars are here
223
used as vertical members) reach diagonally
across the column and tie in the main bars
at intervals of 6 in. to 12 in. Some Kahn
pillars have helical hooping.
Figs. 400 and 401.— Lintel Reinforced with
Kahn Bar
"Hy-rib" is a sheet-mesh reinforcement
and lathing introduced by the Kahn
Company.
KEEDON SYSTEM
The peculiarity of this system is the use
of stirrups, hoopings, etc., which are looped
to pass over the main bars, and are rigidly
held to those bars by means of wedges or
keys. Beam reinforcement on this principle
is shown by Fig. 402, and column reinforce-
ment by Fig. 403. The rigid shear members
Fig. 404.
-Column and Beam Reinforcements
Keedon System
or stirrups resist the diagonal tension. Thus,
the stirrups and hoopings form in effect a
series of rigid projections, but the bars, of
course, retain their original cross section,
and are not deformed in any way. All the
stirrups, hoopings, and keys are inter-
224
REINFORCED CONCRETE
changeable, and the work of assembling them
is quickly done. The main reinforcements
are ordinary merchantable steel bars. Fig.
401 shows the Keedon system applied to a
column and beams.
Fig. 405. — Lock -woven Mesh
Fig. 406. — Lock -woven Mesh Floor
LOCK-WOVEN MESH SYSTEM
This depends on the use of a special re-
inforcement, consisting of steel wires woven
together at right angles and secured at the
intersections by means of machine-made
Fig. 408.— Floor Slab Supported by Four
Columns, Mushroom System
knots, which are of three kinds, as shown in
the composite diagram, Fig. 405. The
lengthwise wires, known as carrying wires,
are heavier than the transverse or distri-
buting wires, since they take the tensional
stress, whereas the other wires do little more
than keep the fabric rigid in the course of
laying and distributing any accidental loads
and temperature stresses. The fabric is
sent out in rolls or in the form of sheets
cut to size and shape. The firm responsible
Lock Woven Mesh
Fig. 407. — Fireproof Construction
with Lock-woven Mesh
for the system emphasises the advantage of
using the close-fibre steel, which is drawn
out under considerable pressure, rather than
a material in which the fibres have been
severed by slotting. The method of placing
Fig. 409. — Head of Column, Mushroom
System
in position reinforcements of this type is
explained on p. 152 in the chapter "The
Erection of a Reinforced Concrete Building."
Fig 406 shows a type of floor, with alterna-
tive methods of constructing the beams. The
reinforcement is carried over the steel joists
SYSTEMS DESCRIBED
225
and sags in the centre of the floor, so that
it comes close to the under-surface of the
latter. It is obvious that Lock-woven Mesh
is applicable to a great variety of reinforced
concrete constructions. Fig. 407 shows
methods of fireproofing floor girders and
columns with reinforced concrete casings.
MUSHROOM SYSTEM
This is an American system, in which the
bar reinforcement to the columns are splayed
out radially top and bottom so as to be
thoroughly bonded into the floors. As
shown in Fig. 408, there are four belts of
rods crossing the slab from column to
column, and splayed out over the supple-
the peculiar formation of the rods around
the column head, and is claimed to simplify
centering. It concentrates the maximum
amount of reinforcement around and over
the support where the shear is the greatest,
and eliminates beams and ribs, giving a flat
ceiling which allows of freer illumination
from the windows and more convenient
placing of shafting in the case of factory
equipment. The system has been built
and tested for nearly all spans of from 14 ft.
to 30 ft., and larger spans can readily be
made. Fig. 409 shows the head of a column,
and is an example taken from actual prac-
tice. The columns are spaced 20 ft. 6 in.
centre to centre each way, and the floors
big. 414
Fig. 411.— Paragon
Column, Beam,
and Floor Slab
Fig. 410.— Paragon
Stirrups
mentary cantilever reinforcement at the
top of the column (this description is based
on one by the inventor, C. A. P. Turner, in
his " Concrete Steel Construction," Part I.).
It will be noted from the diagram that in
certain areas the rods are only one layer or
belt in thickness, and a practical test of the
construction up to the yield point of the
steel after the concrete has thoroughly set
shows the development of cracks, due to the
stretch of the steel, approximately along the
dotted lines shown in the diagram, these
lines therefore showing the planes of greatest
weakness, and between them, in the centre
of the slab, there being an approximately cir-
cular flat plate. The inventor demonstrates
that these lines are nearly or approximately
the points of maximum moment in the slab.
The Mushroom system is so called from
15
Fig. 412.— Paragon Figs. 413 and 414.—
Column Hoopings Paragon Helical
Column Wrappings
are designed to carry 150 Ib. per square foot
live loads. Each column is reinforced with
eight 1-in. round steel bars, which project
4 ft. at the top, where, however, they are
bent at right angles over a f-in. by 2-in.
band placed just above the bottom of the
floor slab. The rods flare radially into the
slab from this band, and extend outwards
to a distance of 3 ft. 9 in., while two circles
of steel rods, 8 ft. 4 in. and 4 ft. 6 in. in
diameter respectively, rest on the radial
bars and are wired to them. The reinforcing
bars in the floors extend well over each
column head, and run parallel and diagonal
to the lines of the columns.
PARAGON SYSTEM
Special forms of stirrups, hoopings, and
wrappings are the peculiarities of this
226
REINFORCED CONCRETE
system. Dealing first with the stirrup for
use in beams, heavy floors, and similar
structures subject to flexure or bending
loads, the stirrups are relied upon to resist
the web or shearing stresses ; they lock
tightly to the main bars, and are claimed
to give a perfect mechanical bond dis-
tributing the stresses along the full length
of the beam or slab to ensure that the con-
crete and steel act together in taking the
load. They can be made to lie at any angle
to make the most suitable form of truss and
of any required length. They are so shaped
of hoopings are shown in Fig. 412, and of
these the first and last may be referred to.
The first is for solid work, and has four bars
which pass through the loops shown. The
arms A extend across the area enclosed by
the hoop and diverge, their ends being
turned and bent away from each other. The
last has the arms curved to form a central
ring for use where a central column or pipe
is to be enclosed.
The helical wrappings (Figs. 413 and 414)
are claimed to overcome objections to which
a continuous wrapping is liable — namely,
Fig. 415.— Piketty Beam
Figs. 418 and 419. — Cross Sections of Piketty
Beam with Four Rows of Bars
Figs. 416 and 417. — Piketty Beams with Two and Three
Tension Bars
Figs. 420 and 421. — Square Piketty Column Figs. 422 and 423.— Round Piketty Column
as to give a minimum metal-to-metal con-
tact, and they are available in a variety of
shapes, in which Fig. 410 shows the prin-
cipal five. Fig. 411 is a comprehensive
diagram which includes a beam in which
the stirrups are employed.
Hoopings are used in columns, piles, etc.,
and are claimed to reinforce the core in the
planes between the hoops against the bulging
action, for which purpose they have inturned
arms, or their arms are twisted together to
form spiral binds which lie through the
centre of the core. Each hoop is placed so
that its arms lie in a different direction from
those of the adjoining hoops. Five styles
that the concreting cannot be seen until it
reaches the top of the column form ; that
concrete has to be dropped in from the top,
giving the larger particles a chance of leaving
the finer ones, and so forming a porous
space ; and, thirdly, the wire wrapping may
be injured in the course of tamping, and so
cause the whole length to be materially
weakened. The Paragon helical wrapping
has been sectionised, the ends of the sections
being made to reinforce the core in the form
of bonds similar to the hoopings already
described. A wrapping is threaded by means
of its loop over the end of a bar, fixed in
position, and the concrete placed around it,
SYSTEMS DESCRIBED
227
the tamping being done from the open side
of the form and consequently providing no
excuse for injuring the wrapping. The
figure illustrates the reinforcement for a
circular column, but square columns also
can be made by this method.
PIKETTY SYSTEM
Beams and Slabs. — Paul P. Piketty,
the inventor of this system, considers that
double reinforcement is absolutely indis-
pensable, for reasons which are pursued at
length in the " Handbook " describing the
system, among them being fastening of
stirrups — increase of compressive resistance,
and resistance to secondary tensile stresses
being the result of permanent deformations
in concrete. Fig. 415 shows two views of
the reinforcement of an ordinary beam.
There are two series of bars, namely,
straight bars, and bars bent up at a third
of their length. To resist the shearing
stresses, there are stirrups connecting the
lower with the upper reinforcing bar and
inclined as shown ; their ends are bent over
as shown in the cross section, and all bars are
fish-tailed to resist any tendency to longi-
tudinal sliding. The beam shown by Fig.
416 is on the same lines, but the lower bar
is hooked and fish-tailed. There are three
series of bars in the beam shown by Fig. 417,
whilst Fig. 418 is the cross section of a beam
with four rows of bars. Fig. 419 is the cross
section of a beam with symmetrical reinforce-
ment. The transverse bars are in tension,
whilst the horizontal links hold the four
rows of bars together and increase the com-
pressive resistance of the beam. Slabs on
this system have both lower and upper
reinforcement, the bars of one being generally
at right angles to the bars of the other.
Columns. — Figs. 420 to 423 show pillars
or columns reinforced on the Piketty system.
The square pillar has four vertical reinforce-
ments, and the special shape links will be
noted. The round column has six rein-
forcements, the links being on the same
principle as in the square column.
OTHER SYSTEMS
Bonna Pipes and Conduits. — These are
reinforced with ribbed bars formed into
longitudinals and spiral coils, the two being
notched together and tied at the joints
(see also p. 369).
Chain Concrete Floors. — By means of
special clips, round steel bars laid parallel
in the floor slabs are connected together to
form a continuous sheet.
Columbian Floors. — The reinforcement
comprises ribbed steel bars in conjunction
with rolled steel joists, or heavier section
ribbed steel bars alone.
Dawnay Floors. — The reinforcement
consists of rolled steel joists at 16-in. in-
tervals, or square bars at 12-in. intervals,
laid between other joists.
Ellis Pipes. — Round steel rods reinforce
sections as short as 2 ft. or 3 ft., there being
an ogee joint to each section.
Hodkin-Jones Floors. — These are re-
inforced with corrugated bars placed on
edge and resting on steel girders through
the medium of a bent and slotted plate.
Homan Floors. — These are of many
types. In one, the webs of rolled steel
joists are pierced to allow of the passage of
round bars which project into adjoining
slabs ; and in another, the reinforcement
takes the form of a T-bar with corrugated
web.
Koenen Floors. — Slabs are haunched
at each side near the supporting piers or
joists, the reinforcement of round bars
lying flat between the haunches and being
turned up at the ends so as to be securely
anchored.
Lindsay Floors. — Slabs may be bounded
by girders on two opposite sides, and by
rolled steel joists on the other two. Pairs
of round bars cross, in the vertical plane
in the middle of the slab, one bar passing
over, and the other under, the next
joist.
Potter Floors. — The tensional reinforce-
ment consists of reinforced corrugated rods.
When the span exceeds 12 ft., rolled steel
joists are introduced.
Ridley Cammell Floors, Columns, etc.
—The beam reinforcement is a trough of
dovetail corrugated steel sheeting and angle
bars, the slab reinforcement consisting of
corrugated sheets. Concrete is applied to
both sides of the reinforcement, but center-
ing, etc., is unnecessary, it is claimed.
Column reinforcement is a combination of
the corrugated sheet and bars of various
sections to form a cage. For walls, also,
the sheeting is used, studs, if required, being
provided by bars.
Somerville Floors. — Reinforced flat-
bottomed hollow blocks with curved tops
are supported by the flanges of rolled steel
joists, a top layer of concrete completing
228
REINFORCED CONCRETE
the floor. In another type, the blocks,
much as before, are supported by reinforced
concrete beams, which, when the upper
layer of concrete is in place, become tee-
beams.
Wells Beams, Floors, etc. — The ten-
sional reinforcement of a beam consists of
twin bars connected by a web, the bars being
placed on edge when several are employed.
Some of the bars may be bent up at the
ends, and, in addition, there may be straight
bars to take part of the compression. The
shear reinforcement consists of vertical
stirrups, these being known as " hangers "
when they connect the upper and lower
reinforcements, and as " bonders " when
projecting downwards from the compression
bars to the neutral axis. Slabs connect
main and secondary beams in the usual
way to form floors. Rectangular columns
of moderate size have a round bar near
each angle, the transverse ties being round
links with a hook-and-eye joint. In larger
columns, the links are connected to the
vertical reinforcements.
Wilkinson Floors. — Floor slabs have
round bar reinforcements in both directions,
with their ends bent over for more secure
anchorage.
Williams System. — Small beams are rein-
forced near the lower surface with I-section
bars which take the tension, while the
shear is taken by vertical round bars split
at the ends. Larger beams have I-section
compression bars, and still heavier ones are
reinforced with railway metals, and with
diagonal bars so attached to the reinforce-
ments that the whole is self-supporting in
the mould. The slabs connecting floor
beams contain small rolled joists placed,
parallel and laced together with hoop steel
passing above and below the small joists
alternately. Joists with flat steel bars,
riveted to them are used in piles constructed
on the Williams system, the end of the
joist being shaped to act as a driving point.
The Architectural and Surface Treat-
ment of Reinforced Concrete
ALTHOUGH European and American archi-
tects are now using reinforced concrete
•extensively, it must be admitted that, as far
as buildings of any architectural pretension
are concerned, it is almost entirely used in
hidden situations, such as foundations, floors,
beams, columns, etc., or perhaps as a backing
to the external walling, either brickwork or
masonry, and its use rarely modifies the
external expression of the building, except
in the case perhaps of commercial or indus-
trial architecture. As an example of rein-
forced concrete building clothed by masonry,
attention may be directed to Mr. W. Aubrey
Thomas's Koyal Liver Building, Liverpool
(see pages 329 to 333), in which the external
walls are of granite not more than 14 in.
tlu'ck. In this building the great difficulties,
from an architectural point of view, of deal-
ing with an erection of its height have been
very successfully overcome.
The statement is frequently made that
architects do not like reinforced concrete,
and to a certain extent this is true. Why ?
Probably the reason is to be found in the fact
that concrete has little or no inherent beauty.
Building stones, slates, bricks, and tiles have
a beauty of their own, either of colour or of
texture, or both, irrespective altogether of
the design of which they form a part, and
this beauty is generally increased by the
action of the weather. This is also true of
timber, with its infinite variation of graining
and colour. This being so, architects have
not in the past considered reinforced con-
crete as likely to become a serious competitor
with traditional materials for external walls.
Many who visited the Paris 1900 Exhibi-
tion probably had reinforced concrete
brought to their notice for the first time.
This exhibition had undoubtedly much in-
fluence on the recognition of reinforced con-
crete in England and throughout Europe
generally, and later exhibitions have still
further advertised its claims. The larger
class of exhibition buildings are admirably
adapted to advance the architectural claims
of the "new" material, as in their design a
degree of freedom may be permissible that
would not be allowed in more enduring erec-
tions. This has been, to some extent, taken
advantage of, but not so largely as might
be wished.
In considering the question of reinforced
concrete and the influence it is likely to
have on architectural design in the future, it
must be admitted that a really satisfactory
treatment typical of the material has yet
to be found. This will not be achieved by
any one man, but will only be brought about
gradually, in the course of time, as the
material comes into more general use in
architecture, each architect adding his small
contribution towards the solution of the
problem. In other words, it will be a growth,
just as all past developments have been.
The guiding principle of the civil engineer
is to use as little material as possible, and
this explains why he has so readily adopted
reinforced concrete, for that is precisely the
principle on which it is designed. Whether
it is an altogether good principle from an
architectural point of view is another matter,
since the architect in all monumental work
uses mass as an element of expression, but
it must be remembered that during the
evolution of Gothic architecture the loads
were carried on comparatively small sup-
ports, and these were reduced more and
more as the style progressed, the reason
being most probably purely aesthetic. A
similar tendency to reduce the amount of
material employed to a minimum is seen in
reinforced concrete design ; but in this case
it is for frankly economical reasons. Instead
of continuous walling, the weight of the
building is carried down to the foundations
on piers, external and internal, the floors
resting on internal beams or on wall lintels
spanning from pier to pier, and the exterior
skeleton being united by thin panel walls
which carry only their own weight and are
used merely to keep out the weather. This
thinness of the external walling is one of
the drawbacks of reinforced concrete from
an architectural standpoint, as, owing to the
229
Fig. 424. — The Upper Stories of a Reinforced Concrete Warehouse at Cologne.
Front View
Fig. 425.— End View of Reinforced Concrete Warehouse at Cologne
230
ARCHITECTURAL AND SURFACE TREATMENT
231
difficulty of obtaining effective shadows, a
flat treatment seems almost inevitable, it
being impossible to get any depth to the
window reveals save by subsidiary piers or
other similar expedients. Tor this and
other reasons it does not appear likely that
the traditional materials need fear any rival
for some long time to come in the largest
Fi>. 426
Fig. 427
and most important architectural work. It
seems probable that in work of this class its
greatest opportunities of affecting design will
be in the construction of domes, vaulting,
ceiling lights, roofs, and similar works, for
which the material is so admirably suited
that it is hard to conceive any other method
of building by which such good results could
be so easily attained.
Inasmuch as there is a temptation to
overrate the position of reinforced con-
crete and the influence it is likely to
have over architectural design in the
future, it may be advisable to consider
its possibilities and limitations as a
constructional material. It is frequently
spoken of as though its use enabled
larger spaces to be covered than was
ever before possible ; but this is not the
case, and, although we have now in
engineering constructions some remark-
able examples of the distances that can
be spanned by the scientific use of the
material, the longest span at present is
to be found in the Grafton bridge con-
structed by the city authorities of Auck-
land, New Zealand. The bridge is 960 ft.
long and 40 ft. wide, and the main arch
has a span of 320 ft., although this will
be exceeded by the Albert Bells bridge in
Rome. It is practically certain that it will
always be possible by means of steel to span
larger openings than would be the case were
reinforced concrete employed. But steel,
when used in buildings, must be cased, as,
even if its successful treatment from an
architectural point of view
did not render this necessary,
its lack of fire-resisting quali-
ties would make it impera-
tive. This casing, while pro-
tecting the steel from fire, is
often a source of positive
weakness so far as the action
of the weather is concerned,
as it prevents the periodical
painting of the steel. This
suggests another great advan-
tage possessed by reinforced
concrete, namely, its reduc-
tion of maintenance expenses
to an absolute minimum.
This and the rapidity with
which reinforced concrete con-
structions can be erected are
most important factors when
comparing the cost of erec-
tions by the two methods.
Again, it must be admitted that a rein-
forced concrete beam is a more truthful,
architecturally and scientifically defensible
method of construction than a rolled-steel
joist cased ; and if the former is used in
connection with a continuous floor slab the
beams are all converted into T-beams, which
Fig. 428
Figs. 426 to 428.— Verti-
cal and Horizontal Sec-
tions of Small Hall with
Arched Ribs
Fig. 429. — Interior View of Hall with Arched
Ribs
adds greatly to their strength, the whole
being in monolithic connection with the build-
ing generally. With reinforced concrete, as
with steel, where large spaces have to be
232
REINFORCED CONCRETE
spanned, depth in the beam is indispensable ;
but if the architectural arrangements of the
interior permit, the beams may be most
economically constructed in concrete as
arched beams, so reinforced that no thrust
is transmitted to the supporting piers.
Some good examples of arched beams
carrying very heavy loads are to be found
Fii. 430
'o
Fig. 431
in the Koyal Liver building previously men-
tioned, and in the Wesleyan Central Build-
ings, Westminster (see pp. 337 to 346).
With regard to stanchions and columns,
it will be found that these, generally, when
constructed in reinforced concrete, will work
out to about the same or slightly smaller
sectional area as would have been the case
had a steel stanchion been designed to carry
the same load and then cased with some
fire-resisting material in the usual manner,
so that, as far as proportion is concerned,
the position is practically what it was before.
Yet, here again, the advantages claimed for
reinforced concrete, when used in the con-
struction of beams, could with equal force
be claimed for the material when employed
in supporting them. In connection with
the above remark on column proportions,
the two following examples may prove
interesting. In the first case, the length of
the column was 20 ft. and the load to be
carried 200 tons. By designing in reinforced
concrete the column worked out to 22£ in.
square. By selecting a steel stanchion and
allowing 2 in. all round for fire casing the
result was 18 in. by 17J in. In the second
case, the length of the column was 12 ft.,
and the load to be carried 40 tons ; the
reinforced concrete column worked out to
10 in. square, and the steel stanchion, allow-
ing 2 in. all round as before, to 12 in. by
10 in.
It must not be forgotten that reinforced
concrete is introducing no such revolutionary
or very great change in architectural ex-
pression as that which occurred
when the arch superseded
the lintel. It is interesting
to find Prof. Beresford Pite, in
the course of his Carpenters'
Hall lecture on " What is
Artistic Craftsmanship ? " say-
ing : " If the Greeks had had
the use of steel girders or
ferro-concrete material they
would have been delighted to
find that they could span the
whole of a big front with 'a
beam such as has been em-
ployed in the new Post Office
building in Newgate Street."
It seems practically certain
that the Greeks had a know-
ledge of the arch, and that
they preferred and deliber-
ately selected the lintel for
covering their openings, in spite of the
former rendering larger openings possible
but it does not follow that they would not
have welcomed a stronger beam, although
their desire to span larger openings was not
emphatic enough to force them to adopt the
Fig. 432
Figs. 430 to 432.—
Vertical and Hori-
zontal Sections of
Small Hall with
Barrel Roof
Fig. 433. — Interior View of Hall with Barrel
Roof
arch. Allowing that the physical properties
of marble were the main factors that fixed
the general proportions of Greek architecture,
at any rate as far as the spacing of the
columns was concerned, it must be admitted
ARCHITECTURAL AND SURFACE TREATMENT
233
that no such definite proportions could be
expected by using reinforced concrete. A
marble or masonry beam of a certain sectional
•area is capable of doing a definite amount of
work ; but in the case of reinforced concrete
the strength of the beam depends so largely
on the reinforcement which is hidden, that
•no such definiteness is possible, although
there is a certain proportion between the
•concrete and the steel in a beam which may
be said to be economically ideal.
The following quotation from Garbett's
able book on architectural design is very
suggestive : "A new style requires the
generalised imitation of Nature and of many
Fig. 434
Fig. 435
previous styles ; and a new system requires,
in addition to this (as Prof. Whewell has
remarked) the binding of all together by a
new principle of unity, clearly understood,
agreed upon and kept constantly in view.
Constructive statics afford three such prin-
ciples— the depressile, the compressile, and
the tensile methods — the beam, the arch, the
truss, of which the two former have been
made the bases of past systems ; the third
is ours, to be used in the same manner." In
reinforced concrete, the binding principle of
the truss is introduced into beams by giving
them the rods to take the tensile strain, thus
economising material and rendering larger
spans possible.
It is to some of the larger manufacturing
buildings in which reinforced concrete has
been employed where the mask walls are
uncased, or to engineering works, that we
must turn to find treatments which, although
guided by economical and strictly construc-
tional reasons, may be said to be most
typical of the nature of the material.
Obviously, buildings that are to be hidden
by masonry are more interesting from a con-
structional point of view while in the skeleton
state than after being clothed. The general
resemblance between their proportions and
those of steel framework should be noted,
but we must avoid letting the material in
question have too arbitrary a sway, as we
find that from the earliest times, and during
the most flourishing
periods of architec-
ture, design in one
material has fre-
quently been greatly
influenced by design
in other and some-
times very dissimilar
materials. As an ex-
ample, we have only
to take the Greek
Doric order, with
its triglyphs, guttse,
and mutules, which,
although executed in
marble, are obviously
founded on timber
construction.
Taking, then, an
ordinary reinforced
concrete warehouse,
the piers taking the
weights will probably
be found to show on
both the inside and outside ; the floors, where
they come to the outer wall, will be carried
by wall lintels or beams, which also show ;
the roof will be flat ; the exterior mask walls,
or panelling, possibly about 5 in. or 6 in.
thick, and the window openings fitted with
steel casements. The principal defect (leav-
ing out the question of surface finish) is
generally the lack of proportion ; other
defects are flatness and want of shadows.
The last mentioned might be partly remedied
by keeping the faces of the wall piers, wall
lintels, and curtain walls flush, or almost
flush, on the inside, and by making the piers
of such a shape on plan that they will fit
their position in the general design. This
can frequently be done without in any way
increasing their sectional area. If the main
Fig. 436
Figs. 434 to 436. — Vertical
and Horizontal Sections of
Small Hall with Modified
Barrel Roof
234
REINFORCED CONCRETE
piers are fairly widely spaced, two subsidiary
piers may in some cases be with advantage
introduced between them, and if the inter-
vening windows occupy the whole width
between the subsidiary piers, more satis-
factory reveals can be obtained. The pro-
jection of the cornice could generally be
Fig. 437
rectangular, having small projections at
each end of the main block. The work has
been carried out entirely in reinforced con-
crete subsequently treated with yellow
roughcast, which, being subdued in tone,
gives a really pleasing colour effect. For
the roof rough-dressed local slates of a green
tint have been em-
ployed in an attrac-
tive manner. The
general massing of the
building is good, and
the effect of breaking
up the roof surfaces is
excellent. It is in the
massing of the roof
that the chief charm
of the building lies ;
the bold lines of the
gables and slate-hung
dormers are effective,
and the way in which
the roof surfaces rise
one above the other
gives a charm to the
design. With regard
to the method in
which the slates have
been used there
Fig. 439
Figs. 437 to 439.— Ver-
tical and Horizontal
Sections of Small Hall
with Pierced Arched
Ribs
Fig. 438
greatly increased with advantage to the
building. Another way of obtaining good
horizontal shadows is by means of bal-
conies, where the character of the building
renders them desirable. They can be formed
by continuing the floor slab through the wall,
the reinforcing rods being continued and
turned up in the parapet or balustrade, the
whole being in monolithic connection. Where
subsidiary piers are inadmissible, and very
flat reveals are unavoidable, then probably
casements with broad frames round, in what-
ever material they might be constructed,
would give the best results, just as in a flat
Georgian treatment we have the broad
frames of the sashes with a mould round set
nearly flush with the external face of the
brickwork.
There is an interesting example of rein-
forced concrete construction on the left bank
of the Khine at Cologne ; it is a warehouse,
two views of which are shown by Figs. 424
and 425, p. 220. In plan the building is
workmen. In the
is
much to learn from
the Germans, who are
apparently far more
skilled in the use of
slates than are English
whole roof a visitor
could not discover a single instance of the
Fig. 440. — Interior View of Hall with Pierced
Arched Roof
use of lead. All hips, valleys, gutters, etc.,
are worked with the slates themselves. This
ARCHITECTURAL AND SURFACE TREATMENT
235
masterly use of the slates, together with
the well-designed sprocketing, has the effect
of softening down the roof lines to a remark-
able degree. Taken as a whole, the design
Fig. 441. — Interior View of Hall with Arched Ribs and Vertical
Columns
is most instructive, and helps to show what
vast possibilities there are for fine archi-
tectural treatment of factory buildings,
warehouses, etc., in reinforced concrete.
Turning to engineering works, particu-
larly bridges, some very successful and
architecturally suggestive designs have been
executed in reinforced concrete. In bridges
that are typical of reinforced concrete bridge
design, there are two arched ribs, or, if the
bridge is wide, a series of arched ribs, with
vertical columns resting on them, giving the
effect of open spandrels, and supporting
longitudinal and transverse beams, which in
turn carry the road decking. Some very
light and graceful effects are obtained by
working on these lines, and the treatment
also affords suggestions which should prove
useful in roof designs.
In the designs which will now be referred
to — designs that have been prepared with a
view to their suitability for execution in rein-
forced concrete — the endeavour has been to
show how some of the suggestions previ-
ously made might be allowed to have their
influence ; but, of course, the illustrations
must be taken as merely showing proposed
treatments, and not as working drawings.
Figs. 426 to 429 show a cross section,
plan, the longitudinal section of two bays,
and perspective outline through a building
that would be suitable for a small parish
hall. The springing being so low, the only
form of wooden principal that could have
been used would have been of the hammer-
beam type, and, the
span being 40 ft., this
would have made heavy
buttresses a necessity.
The proposal is to in-
sert reinforced concrete
arched ribs, carried down
right on to the founda-
tions— the pier being a
feature inside as well as
outside — and designed so
as to exert no outward
thrust, connected by con-
tinuous reinforced con-
crete roof and ceiling
slabs, the former of
which come down on to
reinforced concrete beams
between the arched ribs
at eaves level, and all
being in monolithic con-
nection. Below the eaves
the outer walls, which are merely mask walls
and carry no weight, could be constructed
as desired according to the character of the
external design and the funds available.
No reinforced concrete purlins have been
Fig. 442. — Part Longitudinal Section of Hall
with Arched Ribs and Vertical Columns
shown, as the idea was by getting away from
the effect of timber construction to produce
something more typical of the new material ;
but if inserted they would, by making a
236
REINFORCED CONCRETE
thinner roof slab possible, cheapen the cost.
Again, if the arched ribs are looked upon as
only taking the place of timber roof trusses,
the rest of the roof could be finished with
wooden purlins, rafters, etc., in the ordinary
way, and would be still cheaper, though not
nearly so satisfactory.
By means of sections, plan, and outline
perspective, Figs. 420 to 433 show another
small hall, but this time with barrel roof,
the arched ribs showing below same, spring-
ing from piers and being connected at eaves
level by reinforced concrete beam?, and
having continuous reinforced concrete roof
slabbing and barrel vault, the external walls
only keeping out the weather as before.
The plan and sections in Figs. 434 to 436
show a variation of a barrel roof, with flat,
panelled ceilings on each side. In this
•design the whole is intended to be executed
in reinforced concrete, except the mask walls,
which are in brick. The arched rib principle
is abandoned, and the tie-beam adopted, and
although this represents a type well adapted
for construction in reinforced concrete, it
yet does not seem so typical of the material
as the others, but rather more suggestive
of timber.
Figs. 437 to 440, with two sections, plan,
and perspective sketch, show a rather more
elaborate roof, with pierced arched ribs con-
Fig. 443. — Cross Section of Hall with Arched Ribs and
Vertical Columns
nected by reinforced concrete beams at
eaves level, moulded to form interior and
exterior frieze and cornice, with roof and
ceiling slabs, etc., all as before ; but, in
addition, the pierced ribs are connected by
arched purlins, and the piers inside and out
have caps and bases.
A type of roof founded on reinforced
bridge design is shown in perspective, part
longitudinal section, and cross section, by
Figs. 441 to 443. There are arched ribs at
intervals, as in bridges, with vertical columns,
supporting longitudinal and transverse
beams, which in turn support the roof slab.
This corresponds to the bridge decking, save
that it is on the slope, and is treated just
as a reinforced concrete floor would be
treated, with main beams over the arched
ribs, and secondary beams spanning from
main beam to main beam. A continuous
skylight is shown ; the beams supporting it
are connected by a series of arched ribs with
open spandrels springing from main arched
rib to main arched rib. The rest of the
design is sufficiently explained by the draw-
ings. This represents a form of roof design
capable of great development, and perhaps
more typical of reinforced concrete than
any of the others.
Figs. 444 to 448 show a design for an exhi-
bition hall, which is illustrated by means of a
longitudinal section showing two bays out of
three, plan of same, outline perspective, cross
section, and part plan of ceiling. It was sug-
gested by the Renommee Hall, Liege,
designed by M. Paul Jaspar.
As shown, the main hall
is 45 ft. wide, with side
aisles and galleries all round,
16 ft. 6 in. wide. The main
hall is divided into three
bays in the length, each
being 45 ft. square and
having a flat saucer dome,
with ceiling light continuing
the same curve, over. The
saucer domes are of such
a radius that if the curve
is continued the diameter
of the complete semicircle
equals the diagonal of the
bays. The main piers are
connected longitudinally
and transversely under the
saucer domes by semi-
circular arches. The saucer
domes are ribbed and pan-
elled, exhibiting the real construction, and
the ribs, if continued in the same curve, abut
either upon the semicircular arches men-
tioned or upon the main piers, but below the
ARCHITECTURAL AND SURFACE TREATMENT
237
level of the gallery ceiling, which is also the
level of the flat ceiling of the hall. The
panelling between the ribs is omitted ; in
other words, we have what might be called
open ribs we see the flat ceiling of the hall,
which is, of course, of very small area, as
nearly the whole area of the main hall ceiling
is taken up by the domes. This is a treat-
Fig. 4-14. — Part Longitudinal Section of Exhibition Hall with Three Flat Saucer Domes
Fig. 445.— Part Plan of Exhibition Hall
open pendentives, which give a fan-like effect ment which contains many suggestions
to the ribs springing out of the main piers capable of further development. The whole
and continuing those of and supporting the of the work shown is intended to be executed
saucer domes. By looking up between these in reinforced concrete.
238
REINFORCED CONCRETE
A design for a fa§ade suitable for a club or
similar building, to be erected in reinforced
concrete, is shown in Figs. 449 to 452. The
desire has been to produce a design as typical
of the material as possible, and to base the
Fig. 446.— Interior View of Exhibition Hall
the reinforcing rods in the flat roof turned
up into it as shown, the whole being in
monolithic connection. In order to obtain
the effect of a frieze to the building, the
filling-in walls to the top story have been
brought out flush with the
front of the piers, and the
floor slab at third-floor level
being brought beyond the face
of the piers while the wall
lintels are flush, the effect
of a narrow architrave is
obtained. The remarks that
applied to the cornice apply
also to the balcony at first-
floor level. This has been
shown with a panelled front
in the same material, although,
of course, a wrought-iron rail-
ing would be just as suitable
a finish to a reinforced con-
crete balcony as to one in
stone. In order to get some
effective vertical shadows, the
front has been treated with a
feature at each end and a
architectural treatment throughout on the
structural features of the building. The
cornice has been given a good projection, the
blocks under it being merely the floor beams
recessed cen-
tral portion.
The windows
throughout are
Fig. 447. — Cross Section of Exhibition Hall through one
of the Domes
Fig. 448.— Part Plan of Ceiling
of Exhibition Hall
continued through the wall, with brackets
under. The necessity for the blocking being
at the back of the cornice to weigh it down,
as in masonry construction, having dis-
appeared, it has been brought forward, and
steel casements ; those to second floor, hav-
ing no subsidiary piers to give depth to the
reveals, have been given broad frames which
lap over the concrete. The wide ground-
floor windows to the end features are kept
ARCHITECTURAL AND SURFACE TREATMENT
239
nearly flush with the back of the piers, so
that good reveals are obtained. The rest
of the ground-floor windows are also kept
back, but small subsidiary piers are intro-
duced, and the filling in is on the cant. The
first- and second-floor windows to the end
features are treated as flat bays, the filling
in sloping from the small subsidiary piers
to the back of the main piers.
The remaining first-floor windows should
have small subsidiary piers, which are
treated externally as pilasters, with brackets
and hoods ovelr. The hoods are brought
out in a curve from the face of the wall, as
is often done in roughcast, this seeming to
suggest a plastic material.
The filling-in, or mask, walls have been
designed so as to suggest panelling, and the
face of the wall lintels to the second floor
has been kept back from the face of the
piers and flush with the filling, so as to give
more pleasing proportions.
The exterior surface is intended to be
finished without any applied ornament.
The exterior of piers and beams would be
finished with a smooth surface, obtained by
-e
U
Fig. 451 Fig. 452
Figs. 451 and 452. — Detail of Reinforced Concrete Facade in Elevation
and Vertical Section
240
ARCHITECTURAL AND SURFACE TREATMENT
241
ubbing down the surfaces after the removal
if the moulds and floating them with a
hin wash of cement and sand grout. The
ixterior of panels and other sunk sur-
ges would be treated by washing out the
outer film of cement so as to expose the
particles of stone used as matrix and aggre-
;ate.
As forming a contrast to the illustrations
above referred to, attention will now be
directed to Figs. 453 to 457, which, while
scarcely typifying, undoubtedly suggest the
place taken in the past by reinforced con-
crete considered purely architecturally. The
SURFACE TREATMENT
The reason offered by architects why
reinforced concrete is nearly always covered
up is its alleged lack of inherent beauty
as a building material. Fortunately, the
general adoption, some time in the future,
of more artistic methods of finishing will
remove the reproach. Clearly, until rein-
forced concrete is boldly exposed it will
never be able to exert its legitimate influ-
ence over external design, although, even
as now used, it has had a certain influence
on design, particularly in commercial archi-
tecture. Let it be remembered that a build-
Fig. 453. — Small Hall with Mask Walls of Brick and Principals and Roof of Reinforced
Concrete
illustrations show a small public hall or
parish hall apparently built of traditional
materials, the exterior walls being in local
bricks with red facings, and the interior
walls being in Fletton bricks. These walls
are merely mask walls, however, carry-
ing no weight, and they could just as
well, apart from aesthetic reasons, be of
reinforced concrete 6 in. thick, finished, say,
with roughcast. The roof and the princi-
pals are of reinforced concrete, the con-
struction being sufficiently shown by the
cross section, a feature in which is the
arched beam whose virtues have already
been briefly noted.
ing covered with stone will always be
described by the general public as a stone
building, of whatever material the backing
may consist.
The question, therefore, of the surface
treatment of concrete is one of import-
ance to the future of reinforced concrete,
especially in view of the fact that the
repeal of restrictive legislation now per-
mits thin exterior walls (in other words,
mask walls) to be employed. The subject
of surface treatment has already received
great attention, but the matter, is one for
further experiment ; and in the search for
more pleasing finishes — those whose effect
REINFORCED CONCRETE
will be enhanced, not ruined, by the action By " natural finishes " are meant those
of the weather — the knowledge of the obtainable simply by varying the aggregate
chemist, as well as of the practical man, and the nature of the facing in the form, the
should be enlisted. Definite examples of surface left on removing the false -work
the results obtained, as well as of the undergoing either no treatment at all or
Fig. 455
Figs. 454 to 457.—
Longitudinal Sec-
tion, Plan, Front
Elevation and Gross
Section of Hall with
Reinforced Concrete
Principals and Roof
•J*
1
a
Fig. 456
methods ordinarily adopted, should be easily
accessible for inspection by architects gener-
ally.
The methods of finishing concrete sur-
faces will now be considered in detail. They
may be classified under two main headings :
(1) natural finishes, and (2) applied finishes.
Fig. 457
only sufficient to bring into relief the aggre-
gate in the facing or to make the texture
uniform.
" Applied finishes " are those obtained
by the application of plastic materials.
The Untouched Surface. — A common
finish, but one that assthetically fails to.
ARCHITECTURAL AND SURFACE TREATMENT
243
please many people, is to leave the wall just
as it comes from the forms, with all the
board marks showing ; but what it lacks in
beauty it makes up in efficiency. C. K.
Knapp, in a paper read before the National
Association of Cement Users (U.S.A.), men-
tioned an instance in which the original
cement skin had been left undisturbed, and
in which it defied for a number of years the
penetrative power of water which lay in a
pool over a ceiling. He stated that after
protracted storms the surface seems to dry
off instantly, while neighbouring houses of
brick, and even frame construction, retain
evidences of moisture upon the outside long
after the concrete house has assumed its
natural colour. Not only, said Mr. Knapp,
was £40 to £60 saved for tool dressing, but,
in the opinion of the architect, the finish
was more artistic, as well as indicative of
the plastic nature of concrete.
It may be taken as granted, however, in
spite of an occasional opinion to the con-
trary, that the uninteresting nature of the
undisturbed finish, its drabness, and its
entire lack of light and shade effects, are
obstacles to the employment of concrete in
many quarters, and that something different
will nearly always be demanded.
The Brush Finish. — One of the cheapest
and most satisfactory methods of treating
the green concrete is to give it a stiff brush-
ing, taking care to make the effect as uniform
as possible, it being quite easy, either with
an ordinary scrubbing brush or with a wire
brush, to vary the " pattern " as the work
proceeds.
It will be noted that the concrete must
be green ; but it is not possible to give a
definite time from the placing of the concrete
to the commencement of the brushing, as
naturally it will depend upon the propor-
tions of the ingredients, the state of the
weather, etc. Should the brushing be
attempted before the concrete is sufficiently
set, the result will be irregular, as any small
aggregate present may easily be broken
out ; on the other hand, should the brush-
ing be delayed too long, the concrete may
have become too hard to be treated by
this method.
The domestic scrubbing brush answers
when the concrete is very green, and the
wire brush when the concrete is harder.
It is customary to flow water over the
surface of the concrete freely during the
course of the brushing, and this is un-
doubtedly most conveniently done by
means of a hose with a rose head.
It is customary to assist the brushing by
treating the washed surface with dilute
muriatic or acetic acid, which helps to clean
the aggregate and conduces to uniformity
of texture throughout the whole job, which
very possibly has been scrubbed down at
varying intervals of time. One part of acid
diluted with three parts of clean water is
suitable ; but when the concrete has been
made with white cement and white stone
aggregate, it is better to use a 1 : 3 dilute
sulphuric acid.
An objection to the use of acids (muriatic,
acetic, etc.) is the possibility of the forma-
tion of stains and efflorescence, which
certainly are very unsightly. An acid-
treated surface needs to be well washed
with plenty of water.
The Carborundum Finish. — The well-
known carborundum finish gives a light-
coloured surface and, by filling in the pores
with cement, renders the material less
pervious to water. Immediately the forms
are removed, the surface is wetted and then
rubbed with a No. 16 carborundum stone
until a lather forms and extreme roughness
has been removed. The work is washed
down with the help of a brush, and while
wet dusted with a 1 : 2 mixture of cement
and fine sand, which is next rubbed in with
a No. 16 stone, the finish being applied by
rubbing with a No. 30 stone. A committee
appointed by the National Association of
Cement Users (U.S.A.) has recommended
this method, but specified the use in the
early stages of a No. 8 carborundum stone
brick instead of the (finer) No. 16.
The Sand-blast Finish.— Sand-blasting
has proved a quick and convenient method
of finishing, the resultant surface rather
resembling that produced by scrubbing, but
being more regular. The principle is the
abrasion of the surface by particles of sharp
sand carried at a high velocity by a current
of air under pressure. The abrasive removes
the board marks and produces a uniform
matt finish.
Whilst practically any type of sand-blast
apparatus can be adapted to the purpose,
there is much to be said for the Niagara
pattern illustrated by Fig. 458. This is a
simple T-piece weighing about 6 lb., and
it acts on a well-known principle exempli-
fied by the familiar scent-sprayer. The air,
under pressure of from 80 lb. to 90 lb. per
244
REINFORCED CONCRETE
square inch, is supplied through the flexible
l|-in. hose shown, and, in popular language,
produces suction in the IJ-in. vertical pipe,
whose lower end, weighted, is placed in the
Fig. 458. — T -piece for Sand-blast Apparatus
sand. The sand, by the way, can be held
in a vessel or may simply be in a heap.
When the device needs cleaning, the nozzle
is pressed against a hard surface, this
causing the compressed air to find an outlet
through the suction pipe and effectually
clearing this of any obstruction. Obviously,
any similar material to sand can be used with
the device, and it may sometimes be an
advantage to use something with less pro-
nounced cutting properties.
Bush-hammered Finish. — What is
known as "bush hammering " — so-called
from the use of the mason's bush hammer,
one form of which tool is illustrated by
Fig. 459 — destroys the facing film of cement,
and therefore injuriously affects the water-
tightness of the work ; but where this is un-
important, the method can be recommended
as producing an interesting surface, the
cement between the aggregate being chipped
out and the stone aggregate itself being
roughened. The work should be done within
Fig. 459.— A Type of Bush Hammer
about two months of setting, as otherwise
the concrete may be too hard to be affected
by the tool. Stone masons' tools of the axe
and patent-axe type will be familiar to most
readers ; the bush hammer is used in prac-
tically the same way, either by hand or in a
compressed-air machine tool.
Facing Concrete in the Form. — The
full advantage of most of the above methods
of finishing can only be realised when special
precautions have been taken to see that the
work is given a facing — at least 1 in. thick—-
of fine material in the form ; an applied
facing, plastered on the hard concrete, is
not the same thing. The facing is applied
in course of executing the main concreting
by plastering it on the form and then
introducing the body concrete ; or, in-
stead, the body concrete is inserted anc
then pushed back from the form to allow of
the fine stuff being introduced ; still another
method is to use iron plates, as described
later. By "fine stuff" is meant a mortar
or concrete with a small aggregate suitable
to show on the face of the work ; it certainly
does not mean excessively rich stuff, as this
is always liable to go " crazy," that is,
develop hair cracks all over its surface.
1 of cement to from 2 to 3 of fine aggregate
is a suitable proportion. When the facing
is to contain both fine and coarse aggregates,
suitable proportions are 1 : 1J : 3, 1:2:3,
or even 1:2:4, the ingredients being
cement, sand or stone screenings and crushed
stone or screened gravel, and the facing must
be twice as thick as the diameter of the
largest stone in the aggregate.
To facilitate the placing of both the facing
mixture and the backing without mutual
interference, the use of iron plates — " grano-
lithic plates " — is recommended. These maj
be of any suitable depth and length, 12 in.
X 5 ft. or 6 ft. being suitable, furnishec
with a handle near each end and flared oi
at the top, as in Figs. 460 to 462. On the side
opposite to the flare two or three angle-irons
or T-irons are riveted to act as distance
pieces. As many of these plates as requirec
are put into the forms, their ends slightlj
overlapping, with the angles or T's close to
the face of the form, in this way forming
a deep, narrow slot into which the facing
stuff can easily be poured, the flaring of
the plate assisting this. The concrete back-
ing is poured in at the same time, but the
facing mixture is kept at a slightly higher
level to prevent any thin stuff from the
concrete running over the top of the facing
and finding its way to the face of the form.
When the layer is of the desired thickness,
the plates are raised in readiness for the
next layer, and the two materials — the bod)
ARCHITECTURAL AND SURFACE TREATMENT
245
concrete and the facing — brought into
intimate union by ramming.
When the aggregate in the facing material
has been selected especially to give character
to the concrete, care must be taken to ensure
its fineness, and it will be necessary to re-
move the cement skin from the face of the
work. It is usual to use as aggregate stone
or pebbles crushed to pass through, say, a
£-in. screen, but to be retained upon a |-in.
screen. An average diameter of f in. is
about right. The fine stuff is applied to the
face of the form by means of a trowel or
plasterer's float just before introducing the
body concrete ; or the " granolithic plate "
method already described is employed. Care
must be taken to get the two materials
into intimate union. By means of a little
thought it is possible to choose aggregates
a wooden block, or with sandstone and
plenty of water, but this method will not
leave the aggregate in relief. Again, bush
hammering, as already described, may be
adopted. The following suggestions are due
to Henry H. Quimby, M.Am.Soc.C.E. : " If
the height of the wall to be thus treated is
too great to be completed in one day, face
forms must be constructed to facilitate the
removal of the planking without disturbing
the studs or uprights. This is easily accom-
plished by setting the studs 8 in. to 12 in.
away from the face line and supporting
planks with cleats — say 2-in. by 1-in. —
tacked to the studs and the planks. This
permits the lower planks to be removed and
the washing done while the upper planks
are in place and concrete is being deposited.
With the exercise of very watchful care on
fit. 460
Fig. 461
[ C
Fig. 462
Figs. 460 to 462.— Front and End Elevations and Plan of "Granolithic Plate"
of different colours and sizes to produce
excellent effects, and a note may here be
made of the suitability of selected ground
mica for this purpose, this material supply-
ing " life " and sparkle suggesting freshly
wrought granite. The next part of the pro-
cedure will depend largely upon the nature
of the work and the setting power of the
cement used. All that can here be said is
that the surface of the granolithic con-
crete must be exposed while the material
is still friable, so that an immediate wash-
ing with water and a stiff brush will remove
the cement film and expose the aggregate,
which will now appear in decided relief
and of a rough, coarse texture. (The illus-
trations, Figs. 463 to 468, on later pages
are full-size photographic reproductions of
patterns obtained by the above method,
and the inscriptions give all necessary
particulars. They are due originally to
Engineering Neivs") Even when the face has
become too hard for brushing, something of
the effect can be produced by rubbing with
the part of the workmen and unremitting
inspection, two different days' work can be
joined so that, after washing, the joint will
not be unsightly — even scarcely distinguish-
able ; but such work is usually not obtain-
able throughout a structure, and it is found
very easy to obtain thoroughly satisfactory
joints by indenting horizontal grooves at
regular intervals representing courses, and
finishing each day's work at the apex of
a groove. These indentations are made by
means of triangular beads on the face forms.
Usually the bead is the bevelled edge of a
strip set between the face planks and lightly
secured to the planks with partly driven
toe nails, so that, if desired, a plank can be
removed independently of the bead above
it, the bead remaining to set the plank upon
the next course. These grooves in the face
of a wall improve the appearance by re-
lieving the blankness of a large area. It is
found practicable to prosecute the work
with one course of planks where the capacity
of the plant for one day is equal to only one
246
REINFORCED CONCRETE
course of concrete. In this way the same
planks have been used for many different
courses on four or more different structures."
Fig. 463.— Sand Concrete, 1 : 2. Full size
Uniformity of aggregate,
both as regards size and
colour, is of great import-
ance in this method of
finishing. The matter needs
careful attention before
any work is started in
order to minimise the risk
of being obliged to com-
plete the walling with
aggregate of a different
character.
The nature of the tamp-
ing exercises a great influ-
ence on the texture of the
face of the work. For
example, when a large
aggregate is wished to
show in the face of a wall,
the tamping should be done
in the middle, this carry-
ing the stones against the
form. Tamping near the
side helps in getting the
finer stuff towards the face.
In producing horizontal,
flat surface,0,, skilful tamp-
ing will bring the finer stuff to the top,
but a special tamper — the " Andrews," of
American origin, but obtainable in both the
United States and Great
Britain — has been intro-
duced to facilitate this
particular work. As shown
on p. 129, the tamper
has a number of pyramidal
points on its working sur-
face, so arranged that
when a mass of concrete
containing large particles
of stone and gravel is
tamped, these larger par-
ticles are pushed down
further into the mass, and
the thin stuff flows to the
surface. The tool is 8 in.
square, and the points
projecting from the face
are connected at their
bases in such a way as to
prevent particles of stone
from becoming jammed
between them. In use,
the tamper is employed
after the layer of concrete
has been spread and
roughly levelled. After its
Fig. 464. — Crushed Stone Concrete (Cement 1, Yellow Bank
Sand 2, and f in. Screened Stone 3). Full size
ARCHITECTURAL AND SURFACE TREATMENT
247
use, the surface may be smoothed off and
finished like any other finish dressing.
To cause the thin stuff in the wet mixture
Fig. 465. — Pebble Concrete with Scrubbed Surface (Cement 1,
Bar Sand 2, and ^ in. White Pebbles 3). Full size
to flow to the side of the
form, and thus obtain a
fine vertical facing with-
out introducing a second
mixture, the use of a per-
forated spade (see p. 128)
has been suggested ; and
many contractors will find
it convenient to carry out
experiments with the ob-
ject of applying the idea
in other ways. A spade
illustrated on p. 129 is
that known as the Ross.
Its action is to force back
the coarser aggregate and
to allow the finer stuff to
come to the face of the
work. Other tools for a
similar purpose are illus-
trated on pp. 128 and 129.
Pebble Dashing. — To
imitate the effect of stucco
upon which pebbles have
been dashed, the face of
the form is plastered with
well-worked wet clay into which have been
lightly pressed small pebbles, pieces of glass
or glazed tile, marble chips, etc. When,
after the concrete has set,
the form is taken away,
the clay is washed off and
the surface brushed to re-
veal the pebbles, etc., in
the face of the concrete.
Sand Finish. — The me-
thod is on the lines of that
described in the preceding
paragraph. Clay is well
tempered and thoroughly
worked, and then plas-
tered on the inside of the
forms, working it, if de-
sired, into patterns. Sand
is applied evenly to the
wet clay, and then the
concrete is poured in. On
the removal of the forms,
the clay is washed off with
brush and water, the sand
adhering to the concrete.
This method certainly ap-
pears to be unnecessarily
troublesome. Merely to
get a sand finish on plain,
flat walling, it should
suffice to .brush over the
' •*'• KLI ~
mm
f& .'>• '• Y*» 1
A'I'O -r£
Af\*4&;.> &r$«ai
Nt^-C?^,
s»o?
&•-•
•^•••M
Fig. 466. — Granite Grit Concrete (Cement 1, Bar Sand 2, and
i in. Granite Grit 3). Full size
REINFORCED CONCRETE
form with clay water, and apply the sand,
which will readily adhere.
Glazed Finish. — Whilst concrete cannot
be truly polished, it is not difficult in theory
to obtain on it a glazed surface. All that
would be necessary would be to line the
form where required with well-cleaned and
polished glass, or, as an American experi-
menter has proposed, with enamelled iron
plates, of the kind universally employed for
large advertising signs. In practice, the
use of such an untractable and brittle
material as glass would cause never-ending
difficulty, although it might be possible
without much inconvenience to introduce
narrow slips for the purpose of casting
ornamental devices on the work. The
employment of steel forms stove-enamelled
on the working side might provide a solution
to the problem should the production of
glazed surfaces be demanded. A practical
difficulty would be the almost inevitable
formation on the form of air-bells, as these
would, of course, spoil the surface ; but the
trouble might be got over by first lining the
form with fine cement mortar \ in. or less
in thickness, applying it with heavy pres-
sure. It is reasonable to assume that the
polished skin would give the concrete more
than usual resistance to weather and to
mechanical wear.
APPLIED FINISHES
Plastered Surfaces. — When a concrete
surface is finished by coating with plaster
of whatever nature, there is always a risk
of this coming away at a later date should
there be a likelihood of moisture percolating
through the concrete and affecting the back
of the plaster. However, plastering is often
successful, but much depends upon the pre-
paration of the concrete surface and the
kind of plaster used. A good plan is to
wash the concrete with dilute acid, talcing
care to rinse this off afterwards, and then,
while the surface is damp, but not flooded
with water, to apply the plaster. The
moisture in the surface of the concrete
assists the cement mortar to set. The
following method, advanced by an American
contractor, should give good results. The
first coat is a 1 : 3 mixture as dry as it can
be applied, this necessitating considerable
pressure to cause it to adhere well. Let
the coat be thin and scratch it afterwards.
Brush over the first coat with water and
apply the second coat, using as much pres-
sure as possible. If a third coat is neces-
sary, adopt the same precautions.
A concrete surface gives a better key for
plaster after it has been hacked over with
axe or bush hammer, but this necessitates a
thick coat of plaster. If the concrete is soft
enough to be affected by a coarse wire
brush, a rub down with that will answer
instead of hacking, and the plaster coat
can then be thin. The acid treatment and
washing, as described on p. 243, should
precede the plastering.
Tiles, Mosaic, Sgraffito, etc. — In some
situations, tile, or the old Byzantine material,
vitrified mosaic, makes a beautiful and suit-
able finish. In either case, they would have
one constructional advantage, especially in
high buildings, over brick or masonry
casings, in that the weight added by them
to the foundations would be comparatively
trifling. A concrete wall having a true, flat
surface could be prepared for tiling by
rubbing down with a carborundum stone and
then scratching with a brush — bristle or wire,
according to the age of the concrete ; the
wall should be wetted and the tiles bedded
in neat cement, using as small a quantity
as possible to get a good result. Much the
same instruction applies to the mosaic.
It is somewhat surprising that sgraffito is
not more generally adopted as a finish to
concrete work where ornamental effects are
desired ; certainly in some cases it might
be very effectively employed. Sgraffito is
an Italian word meaning "scratched," and
is the name of a very old process consisting
in applying to the surface several coats of
different colours, these coats being brought
to view as required by chipping or scraping.
In the simplest form there are but two
coats — black for the ground, white for the
covering ; and the surface to be treated is
floated to a uniform face, allowing for the
finishing coat, which will be £ in. thick.
The outline of the design is marked on the
face of the floating, as a guide for keying
the J-in. finishing coat, which should be
applied as soon as the colour coat is suffici-
ently hard. Placing the design drawn on
paper in its original position, pounce through
the outline as a guide for cutting through
to the colour coat, using a worn knife to
cut away the superfluous material, which is
removed with a spatula ; the edges of the
work being sloped or inclined according to
the light or shade required. For three-coat
work the colour coat may be left rough,
ARCHITECTURAL AND SURFACE TREATMENT
249
providing it is uniform, a dark colour being
used as a background. This coat is ruled
to within f in. of the finished face, the class
Wjf£* • .
Screened Yellow Pebbles 3). pull
tained by adding a sample of it to a small
quantity of the stuff with which it is to
be used, the material being allowed full
time for setting. For ex-
^•HH^HI ternal work, portland ce-
ment or Aberthaw lime
may be used, in the pro-
portion of 2 parts sand to
1 part cement. Aberthaw
lime may also be used for
internal work, and so also
may Parian cement ; but
for ordinary purposes se-
lenitic may be used with
satisfactory results.
Fig. 469 (p. 251) shows
the excellent effect ob-
tained by the adoption of
inlaid faience tiles and red
bricks, the roof covering
consisting of terra-cotta
tiles.
"Stuc" Work.— This
is a form of interior treat-
ment particularly suitable
for application to concrete
walls and lending itself to
a simple and effective de-
sign. In appearance, it
suggests a good Bath stone.
The material is made with
Sand 2, and
size
of work determining the.
thickness of the various
coats. Apply the succeed-
ing coat, and finish as
soon as convenient, so that
the coats may adhere in
one compact mass. Expe-
dition is specially neces-
sary when treating exterior
work, as water settling on
the incisions of work im-
properly keyed would cause
it to laminate or scale off.
For colouring matter, to
obtain good black use
bone-black — or, for ordin-
ary work, smiths' ashes
as an aggregate ; for red,
Venetian or Indian red ;
for brown, umber ; for
yellow, yellow ochre. When
a neutral tint is required,
a combination of two or
more colours may be em-
ployed. The depth of the
colour should be ascer-
Fig. 468.— Sand Concrete, 1 : 3
250
REINFORCED CONCRETE
a mixture of plaster-of-paris, cement, size and
various colouring matters, the latter vary-
ing slightly according to the desired finished
effect. In the application, a very moist solu-
tion of plaster-of-paris is first stippled on the
walls with a stiff broom to give a key for the
" stuc." The plaster and cement are then
placed in a mixing box about 2 ft. by 1 ft. 5 in.
and about 12 in. deep, and thoroughly
mixed. When this is complete, the size,
water and colours are added, and the whole
thoroughly incorporated, and the mass is
squeezed through the hands until a thick
cream is obtained. The resulting compound
is very quick in setting, and consequently
it requires to be rapidly applied to the sur-
faces, these having been previously screeded.
The hands and a brass trowel are used, the
plaster being literally thrown on. Whilst
moist, it is roughly lined to the screeds, and
the surfaces are planed and scraped to a
fairly even and true face. When set, it is
left for about two months, and during this
period various stains come out to the face
of the " stuc," which assumes a light brown
colour, and becomes very hard. After this
time a scraping plane is employed and the
whole of the outer surface is removed, taking
away all stains and leaving a fine soft stone
effect. During the scraping process, small
pit holes are formed in the surface, which
greatly improves the appearance, and re-
moves the perfectly smooth and artificial
effect otherwise given. The whole surface
is then rubbed down with pumicestone, and
the various joints are set out with chalked
cords in accordance with the design. These
lines are then cut in and filled up with white
plaster, and a final rubbing down with
pumicestone is given. Mouldings and cor-
nices are run in the orthodox manner with
a " horse," several plasterers throwing on
the material at each point, whilst the horse
is worked backwards and forwards.
When using the " stuc " on concrete work
it is advisable to give the walls a thin coat-
ing of Keene's cement to prevent the staining
of the " stuc."
In the execution of the ornament, some
excellent work is seen, as this is not cast,
but modelled in position by the plasterer,
and for this reason every man requires to be
an artist fully in sympathy with his work.
" Stuc " work was extensively adopted in the
decoration of the Koyal Automobile Club,
and it can be said confidently that the work
to the fluted columns round the gallery and
the Eoman-Doric caps, with the enriched
frieze above, are greatly superior to similar
work executed in ordinary fibrous plaster.
The " stuc " work requires neither painting
nor other finish, and retains its fine appear-
ance if merely rubbed down once every three
years.
COLOURING CONCRETE
In response to a demand for a brighter and
more interesting colour than that provided
by natural concrete, there has been a great
deal of experiment to determine the best
methods by which the desired result can be
obtained. Undoubtedly, the most artistic
method, but one not suited to all or even
the majority of situations, is to determine
the colour by the use of a specially selected
aggregate which will give, not only colour,
but texture to the work. Sufficient has been
said on this subject earlier in this chapter.
Another method is to face the walls with
tiles, mosaic, etc., as already described, but
this is expensive, and has the disadvantage
of masking the character of the concrete
surface, and in that particular sense is false
art. Strictly, methods of colouring concrete
include only those by which (a) pigment is
incorporated with the concrete or with the
facing material introduced into the form,
(6) staining or (c) distempering, or (d) oil-
painting the concrete surface.
Body Colours. — But for the fact that the
lime in the concrete has an injurious effect
upon the majority of pigments, there would
be very little to say under this heading.
This injurious action has long been known,
and everyone is familiar with the bleaching
of paint and wallpaper by the lime in newly-
plastered walls. The vegetable and many
of the artificial pigments are the worst
offenders, leaving the selection between the
relatively few mineral pigments. A com-
mittee appointed by the National Association
of Cement Users (U.S.A.) has reported that
the only mineral pigments that should be
used are comprised in the following list :
Lampblack, manganese dioxide, red iron
oxide, English red oxide, brown roasted iron
oxide, brown ochre, yellow ochre, ultra-
marine, chromium oxide, ultramarine green
and violet oxide of iron. This brief list of
pigments does not allow of much latitude.
The committee further reports that the
amount that can be safely used is small
owing to the danger of impairing the strength
of the concrete. Five per cent, by weight
251
252
REINFORCED CONCRETE
to that of the cement should be the limit,
beyond which the impairment of strength
is too great to justify a larger amount,
except in special circumstances, as, for
example, in a thin facing. Even this pro-
portion of those pigments whose colour
differs little from that of the cement does
not produce a marked change. Lampblack,
it is pointed out, is the principal pigment
used for darkening, on account of its strong
contrast and the fineness of its particles.
For lightening the colour somewhat lime is
the best material to use. Colouring matters
can be used in either a dry form or as a
paste. It will generally be found most
convenient for mixing to use the dry form,
thoroughly mixing it with dry mortar before
the addition of any water.
The pigments exercise an influence on
the setting properties. Crimson lake (alum-
ina base) and barium chromate quicken the
cement ; manganese dioxide, red ochre, and
chinese red retard it ; and ferric oxide,
yellow ochre, ultramarine, and chromium
oxide have a slight quickening effect.
In colouring concrete by the admixture of
pigment, the resultant tint must not be
judged by that of the wet mixture, the con-
crete always drying out lighter than that
would suggest. The best course is to mix
together small measured quantities of all
the ingredients dry, add the water, thor-
oughly incorporate, and allow a portion to
set in a mould ; the experiment can be
repeated until the desired result is obtained.
For a white concrete, use 1 of white port-
land cement and 2 of hard marble, screening
to pass through a No. 8 screen and be col-
lected on a No. 40 screen. When the con-
crete is hard, in three or four days, it may
be rubbed with a terrazzo stone to polish
the marble particles.
Before giving a table of concrete colouring
pigments, it is desirable to summarise the
experiments carried out in the United States
by Prof. Charles E. Pellew with the object
of finding the most suitable pigments for
colouring concrete. The question of expense
was of vital importance, and in the matter
of first cost it was evident that surface
colouring would be cheaper. But, unless
there is a thoroughly hard permanent sur-
face for the pigments to adhere to, and
to obviate as far as possible the use of
linseed oil, upon which the lime exerts a
strong action, the body colouring is pro-
bably the most satisfactory. For yellow,
Prof. Pellew thinks that the only available
pigment is yellow ochre, 8 per cent, of a
strong pigment giving a bright tan colour.
This yellow can be used for shading the red,
but is not so effective for this as the man-
ganese brown. Yellow ochre with small
amounts of permanganate brown will give
various shades of yellowish brown or buff
colour. BlacJc is obtained by using carbon
black or lampblack. For a bluish shade a
black iron oxide, imported for the use of
gas works, gave satisfaction. The only red
pigments available are red oxides of iron,
some of them natural, finely ground haema-
tites, and others artificial. They differ
greatly in shade, price, and staining power,
the cheaper pigments being unsatisfactory.
It is best to use a small amount of a strong
though high-priced pigment than larger
quantities of a weak but cheap one. After
experimenting with twenty or more different
colours from various manufacturers, the
best results were obtained from a red colour
at five cents (2|d.) a pound ; from 7J to
10 per cent, (of the weight of cement) was
needed to give a full shade. A slight addi-
tion of permanganate brown gave a red
terra-cotta shade. As regards brown, a
vegetable pigment proved unsatisfactory.
Prof. Pellew tried iron-rust, the action of
which is based on the formation in the
concrete of a reddish-brown deposit of ferric
hydroxide by the action of the lime of the
cement on a soluble salt of iron, like ferric
chloride or ferric sulphate. Unfortunately,
it takes 25 to 30 per cent, of ferric sulphate
(of the weight of concrete) to get at all a
decided colour with this compound, and this
is a serious drawback. He tried manganese
brown, which is based on the formation in
the concrete of brown manganese hydroxide
by the reduction of the salt potassium per-
manganate. The latter possesses a strong
rich purple colour, which in the presence of
oxi disable material, such as organic matter,
turns at once to a full seal brown. In stain-
ing concrete the organic matter must be
supplied in the form of glucose or sugar,
which in quite small quantities will change
the deep purple colour of the permanganate
into a rich seal brown. To get a deeper
colour, Prof. Pellew was obliged to use
24| per cent, (by weight of the cement) of
permanganate and about \ per cent, of
glucose. With regard to a green colour the
high price of chromium oxide, the only green
mineral pigment that will stand the action
ARCHITECTURAL AND SURFACE TREATMENT
253
of lime, prevents its general use. In the
absence of a strong blue that will stand the
action of lime, it is not possible to obtain
a good green by modifying the colour pro-
duced by yellow ochre. Ultramarine blue
is fast to lime, but its staining power is low
when mixed with other pigments, while
Prussian blue is easily attacked by the
cement, and is not, therefore, included in
the table of pigments given on this page.
It seems probable that for greens some
form of surface colouring will have to
answer, but much remains to be done in
the way of experiment.
TABLE OF PIGMENTS
The weights of pigments given in the
following table are those recommended for
adding to each cubic foot, reckoned as 90 lb.,
of cement ; but the table must not be taken
quite literally, as experiments have obtained
widely different results. No greater claim
can be made for the table than that it will
act as a rough guide. It cannot do more,
owing to the great differences frequently
existing between two pigments passing under
the same name. They may be unlike one
another chemically, physically and in their
percentage content of inert cheapener which
so seriously affects the staining power. The
pigments mentioned will certainly give the
tints, but the proportions must be adapted
as required, depending, as above suggested,
on the strength of the pigments them-
selves and also on the proportions of the
concrete.
BLACK . . 10 lb. of manganese di-
oxide or lampblack.
— , BLUISH . 8 lb. or 9 lb. of black iron
oxide.
BLUE . . . 3£ lb. to 4| lb. of ultra-
marine.
, VIOLET . 4J lb. of violet oxide of
iron.
BROWN . . 4 lb. of brown ochre.
, CHOCOLATE 6 lb. of manganese di-
oxide, 4 lb. of red oxide
of iron, and 2 lb. of
black oxide of iron or
copper ; or 3| lb. of
burnt umber.
BUFF . . . 3J lb. of yellow ochre ; or
4J lb. of yellow ochre
and 2£ lb. of perman-
ganate brown.
GREEN . . 10 lb. of chromium oxide
or ultramarine green.
GREY
PINK .
— , DULL
-, BRIGHT
-, DULL
SLATE
-, BLU»E
-, PINKISH
TAN .
TERRA-COTTA
YELLOW
WHITE
1 lb. to 3J lb. of
black or manganese
dioxide.
3 lb. of crimson lake
(alumina base).
2 lb. to 3£ lb. of Venetian
red.
| lb. of burnt umber.
2 lb. to 3J lb. of Chatta-
nooga iron ore or red
iron ore.
5 lb. of Pompeiian or
English red.
5 lb. raw iron dioxide ;
carbon black may be
added.
| lb. to 3£ lb. of lamp-
black or manganese
dioxide.
1 lb. to 31 lb. of ultra-
marine blue.
| lb. to 1 lb. of Venetian
red ; or £ lb. of Chatta-
nooga iron ore or red
iron ore.
7 lb. of yellow ochre.
English red with per-
manganate brown ; or
2 lb. to 31 lb. of Chat-
tanooga iron ore or red
iron ore.
5 lb. to 10 lb. of yellow
ochre.
44 Jib. of barytes (barium
sulphate).
Stains. — Manufacturers have placed upon
the market a number of stains suitable for
use on concrete. A brown stain is easily
made by dissolving iron sulphate (green
copperas) in water, the proportion being
about 2^ lb. per gal. The colour is strength-
ened by repeating the application. The
addition of alum to the above solution gives
a pale yellow ; whilst chrome alum gives a
green. Stains have the advantage over
paints that they do not cover the concrete
with a thick coating, but sink into the pores
and preserve the natural texture, while at
the same time colouring it in rich, deep tones.
They can be applied easily and rapidly, their
covering power is two or three times that
of oil paint, and they cost, bulk for bulk,
much less.
Distempering. — Concrete can be suc-
cessfully distempered with common white-
wash or with cement washes ; as regards
254
REINFORCED CONCRETE
colours, the pigments used must be fast to
lime (see under the heading " Body
Colours"). It will be better to kill the
alkali in the concrete surface with dilute
acid and then well wash with water before
applying the distemper.
' Painting. — By this term, of course, " oil
painting " is understood. Painting is desir-
able only on smooth concrete, the con-
structional nature of which it is not desired
to emphasise. The paint forms a film of
metallic oxides, sulphates, etc., and oxidised
linseed oil on the face of the work, closing
the pores and undoubtedly adding to the
weather-resisting properties. Unfortunately,
the lime exerts a violent chemical action
upon linseed oil, and special preparation of
the concrete surface is absolutely essential.
Usually, this preparation consists in killing
the alkali and then coating with at least
two coats of good white-lead paint before
applying the finishing coat. But a different
style of treatment was suggested some time
ago before the American Society for Testing
Materials by Charles Macnichol, who spoke
from the experience of many years. He
primes the concrete with zinc sulphate
ground up with an equal quantity of water.
Then, after two or three days, the concrete
can be painted on safely. Dr. A. S. Gush-
man suggests the explanation that a chemical
reaction results in the formation of gypsum
(calcium sulphate) and zinc hydroxide
(hydrated oxide of zinc), which substances,
held within the pores of the cement, do not
affect linseed oil injuriously — indeed, they
are common paint pigments. The method
is economical as regards the after coats,
because the suction of the cement has been
already satisfied.
The architect will usually hesitate to
specify any particular formula to be fol-
lowed in painting concrete, and will generally
content himself by specifying one of the
several compositions on the market that are
specially made for the purpose, the con-
stituents of which are, for obvious reasons,
kept secret. Some of them are the result
of very careful trials, and Dr. Macmimilian
Toch, in a lecture delivered some time ago
before the Paint and Varnish Society,
hinted that the best results might be ob-
tained by a proper admixture of Menhaden
fish oil and tung or Chinese wood oil. What-
ever paint or preparation is used, it is
generally recognised that it is useless to
apply it to green or fresh concrete, but an
exposure of at least a year, or even two years,
will render the surface fit for receiving the
paint. In actual practice, however, it is
usually desired to finish the building almost
immediately after it is erected, and in such
a case it becomes necessary to prepare the
surface in order that the paint may per-
manently adhere. Even when the concrete
has been exposed for a year or more the
application of such a preparation may be
made as a matter of precaution to neutralise
any free lime which might remain.
Opinion widely differs as to the com-
position of this preparation. Many practical
men advocate the use of a solution of zinc
sulphate, commonly known as " white
vitriol." This is used in the proportion of
8 Ib. to 1 gallon of water. It is urged against
this method that something more than a
neutralisation of the free lime is required,
and that to obtain satisfactory results a
material should be used which will fill up
the pores of the concrete and prevent
suction. Hence there are advocates of
compositions in which paraffin wax plays
an important part, a method, it may be
remarked in passing, which was successfully
employed a year or so back in connection
with Cleopatra's Needle on the Thames
Embankment.
Under ordinary conditions, however, the
most practical method is to apply a liquid
to neutralise the free lime, and, that object
being thoroughly effected, the paint might
be ordinary white-lead, although much
better results would be obtained by the
use of one of the special " concrete paints 'r
already referred to.
Among the solutions that have been
recommended for use under the coating of
paint are the following : For concrete floors
the use of sodium silicate has been suggested.
Dr. M. Bennett Blackler advocates the use
of copals made up with a small quantity
of oil and diluted with turpentine, and he
expresses his opinion, also, that casein
solutions treated when dry with formaldehyde
would give a coat which is absolutely im-
permeable to water and form a surface
on which paint would stand. In these
opinions Dr. Blackler has been backed up
by many other eminent men ; in fact, it is
not improbable that casein forms the active
base of many of the specialities now on
the market.
Assuming that the surface is in as good1
a condition to receive the paint as can be
ARCHITECTURAL AND SURFACE TREATMENT
255
expected, the next question arises as to
what is the best paint to apply. Many
people believe that when the walls are in
a proper condition the ordinary paint can
be used without difficulty, and the following
formula has been published : — For the
priming coat : 100 Ib. of pure white-lead,
9 gals, of pure boiled linseed oil, and 1 gal.
of turpentine. This may be followed by
using instead of the boiled oil 9 gals, of
raw linseed, and 1J pints of turpentine
drier. Another priming coat recommended
consists of 85 Ib. of pure dry red-lead mixed
with 1 gal. of boiled linseed oil, and \ gal.
of turpentine. The body coat should in
this case be 100 Ib. of pure white-lead,
4 gals, of pure linseed oil (one third boiled
and two-thirds raw), and the finishing coat
100 Ib. pure white-lead, 3| gals, of pure
linseed oil (one-third boiled, and two-thirds
raw), and 1 pint of pure turpentine. The
red -lead in the priming coat above mentioned
would probably be in most cases objection-
able because of its colour.
When the surface of concrete is covered
with cement, the latter may be painted by
giving at first two coats of Alabastine, pro-
ceeding with oil paint in the ordinary way.
W. G. Scott recommends as a paint for
cement the follow ing : 10 Ib. plaster -of -
paris, 2 Ib. portland cement, 8 Ib. whiting.
4 Ib. wheat flour, 6 Ib. zinc oxide, and 2 Ib.
red-lead. This mixture should be ground
in 1 gal. of prepared wood oil, and J gal.
of soya bean oil. The pigments are mixed
separately with the oil, adding them in the
order given, and if the paste is then found
to be too stiff a little more of the bean oil
is added. This paste, Mr. Scott says, grinds
with some difficulty, and it is better to mull
or run through an iron mill first, and then
to grind fine in a stone mill. In use, the
paint is thinned with benzine or turpentine,
or with a mixture of 7 parts of turpentine
and 1 of paraffin oil. The prepared wood
oil referred to may be made by stirring
9 Ib. litharge and 1 Ib. of manganese borate
into 50 gals, of Chinese wood oil, and heating
for five hours at a temperature of 350° F.
(177° C.). After the litharge and manganese
borate have settled out and the oil is clear,
it is ready for use. This paint or cement is
intended for application to porous cement,
and may be used on concrete. The same
author gives the composition of a marine
paint which will set under water, and which
may be quoted here : 10 Ib. portland
cement, 5 Ib. silicate, 3£lb. zinc oxide, 7J Ib.
red-lead, 5 Ib. litharge, and 2 Ib. graphite.
These are mixed with 1 gal. of boiled linseed
oil and ^ gal. of paraffin oil. It sets under
water, and becomes in time as hard as
stone. The same mixture if thickened with
whiting or plaster-of-paris forms a valuable
cement for many purposes.
Some careful experiments conducted by
Mr. Henry Gardner, assistant director of
the Institute of Industrial Research, Wash-
ington, D.C., are reported in his " Paint
Technology and Tests." He carried out
his tests on mortar made of 1 part of port-
land cement, and 3 parts of sharp clean
sand. Omitting the failures, the results
were as follow : — No. 1. Concrete primed
with a 25 per cent, solution of zinc sulphate
crystals dissolved in water. A wide brush
was used for the application, and the spread-
ing rate was approximately 200 square feet
per gallon. Second and third coated on
the second day with the following com-
position : Sublimed white-lead (similar to
" Purex " in Great Britain) 50 per cent.,
zinc oxide 35 per cent., silica and baryte&
12 per cent, and Prussian blue 3 per cent.,
ground in linseed oil, turpentine and drier.
This panel after three years' exposure was
in good condition, although there was a
slight checking. In test No. 2 the concrete
was treated with a 20 per cent, solution of
alum (aluminium sulphate), and a paint of
the same composition as before. The
results were almost identical with No. 1.
In other tests the concrete was primed with
a solution made by dissolving 10 parts of
sodium oxalate in 100 parts of water. The
second and third coats were linseed oil
paints in red, brown, blue and green. The
results are reported as being " very good."
But even better results were obtained when
the second and third coats were composed
of zinc oxide and barytes ground in an oil
(kind not given) having " a low saponifica-
tion value." This paint dried very slowly,
but the results are reported as " excellent,,
extremely glossy waterproof surface pre-
sented."
Durability of Reinforced Concrete
WHEN, recently, a reinforced concrete house,
built fifty-eight years before at St. Denis,
France, by Frangois Coignet, was examined
and the work cut away to disclose the in-
ternal condition, both steel and- concrete
were found to be in an excellent state of
preservation. It is a practical illustration
of this kind, vouched for by a deputation
from the (British) Concrete Institute, that
carries conviction to the minds of those
engineers who are unmoved by mere theories
and by hundreds of laboratory experiments.
That concrete has an effective life of hun-
dreds of years under favourable conditions
is an established fact ; further, those who
have made a special study of reinforced con-
crete know that in ordinary circumstances
the steel will last as long as the concrete in
which it is embedded. The concrete exer-
cises a protective and preservative action
on the steel ; and while he concrete gains
in strength a hundredfold, the steel gains in
durability a thousandfold.
MECHANICAL DESTRUCTIVE
INFLUENCES
Blows and Shocks. — Concrete is not an
ideal material to employ in situations where
heavy direct blows are likely to fall upon it.
Obviously, its brittleness is its greatest
defect, and it therefore follows that while
it makes a durable and wellnigh everlasting
floor for ordinary traffic, it is not suitable
for the floors of workshops in which heavy
iron implements are carelessly dropped
about, or in which heavy trucks with flat-
rim wheels are used, as rough treatment
soon chips the surface. Truck wheels need
to have curved treads or rubber tyres, so that
there are no edges to dig into the concrete
when turning a corner, even when a truck is
tilted considerably. Hence, although con-
crete makes an admirable floor foundation,
it is frequently desirable to have the actual
floor of wood, or to let in iron plates flush
with the surface where strongly abrasive
wear is likely to occur. As a floor material,
too, concrete suffers with all stones, whether
natural or artificial, in lacking in resilience,
the " deadness " of the floor causing the
people who stand or walk on it to become
tired sooner than would be the case on a
wooden floor.
Reinforced concrete is being increasingly
used for dock and harbour works, in which
situations it is commonly subjected to great
shocks. Mr. F. E. Wentworth-Shields
(of the New Dock Works, Southampton)
gives a valuable opinion with regard to its
suitability for such applications. " If rein-
forced concrete is to be used for marine
work," he says, " it must be carefully
fendered at all parts where it is likely to
be struck. In most systems of reinforced
concrete, the steel is placed within an inch
or two of the surface ; and if the structure
is not protected by timber fenders, this
concrete skin is easily knocked away, leaving
the steel naked and liable to rust. At the
same time it is true that steel-concrete does
not spall easily from blows, provided they
do not fall directly on the concrete, and it
will stand a wonderful amount of shocks,
and bending due to shocks, if a wooden
fender is interposed. The coaling-jetty at
Southampton probably suffers as much from
the blows of ships and barges as any steel-
concrete structure yet made. It is pro-
tected by fenders of American elm, and in
spite of blows which have caused the whole
jetty to sway, there is no sign as yet (two
and a half years after completion) of the
concrete spalling off, except at one or two
places where the blow has fallen directly
on the concrete." The same writer gives
an interesting example of the ease with
which reinforced concrete coast construc-
tions can be repaired — the ease with which
repairs can be effected being important
when broadly viewing the subject of dura-
bility. He states that a steamer collided
violently with the quay of this same jetty,
breaking two piles and the beams which they
supported. " On examination by divers it
was found that the piles were broken down
to a level of 12 ft. below low water. The
work of repairing the quay, however, turned
out to be fairly simple ; the broken concrete
256
DURABILITY OF REINFORCED CONCRETE
257
and steel rods were cut away by divers, who
also erected new rods to take their place.
These new rods were joined to the undamaged
part of the old ones by steel tubes, and the
stirrups or binding wire were then placed
round the new rods at correct intervals. A
water-tight casing of timber was then placed
round the pile, which extended from the
undamaged portion to above low water.
From this casing the water was pumped out
and the concrete was filled and rammed into
the box in the ordinary way. Above low
water the piles and beams were remoulded
in just the same way as they had been origin-
ally built."
Earthquakes. — These subject buildings
to every form of stress and shock. Parts of
a building designed to take simple com-
pression are, in a moment, subjected to
enormous bending strains, and floors are
suddenly called upon to act as struts. Whilst
no form of building can long resist severe
earthquake attacks, reinforced concrete has
given as good an account of itself as any
other material, and a better account than
many. There is, unfortunately, a diverg-
ence of opinion on the subject, and, as might
naturally be expected, the brick and terra
cotta interests have made out a strong case
for the superiority of their own materials.
Captain Sewell, who personally studied on the
spot the results of the San Francisco earth-
quake shortly after the catastrophe, stated
in an article in Concrete that at " Palo
Alto, at the Leland Stanford University,
there was a museum building which consisted
of three wings : the central one of reinforced
concrete, and the two side wings of brick-
work, with reinforced concrete floor systems.
This building was not far from the line of
the fault which caused the earthquake, and
it received a severe shaking. Externally,
the reinforced concrete wing appeared to
be absolutely uninjured, except that some
statues were shaken down from the front
parapet wall. The two brick wings were
almost in a state of collapse. An examin-
ation of the interior showed that some
damage had been done in the reinforced
concrete building, in the shape of a few
cracks here and there ; but the sum total
of structural damage was insignificant, and
the writer is inclined to think that one
thousand dollars would cover all the damage
to this wing. The damage to the brick
wings is at least from 50 per cent, to 75 per
cent."
17
In 1908 there was a terrible earthquake
disaster at Messina, in Southern Italy.
Gf. Flament Hennebique, in a paper pre-
sented to the French Society of Civil
Engineers, described how certain Henne-
bique structures at Messina had withstood
the shocks. The more noteworthy instances
are here mentioned. A roadway over the
Portalegni stream remained intact in spite
of the debris which accumulated upon it ;
the floors of the Mandalari Hospital con-
tributed largely to preventing the collapse
of the building ; a 4,000 cub.-metre reservoir
remained perfect ; the walls of the Messina
museum collapsed and carried with them
the floors, which remained unbroken; the
church of the Madeleine collapsed, but the
reinforced floors of the adjoining house
remained intact ; the floor of the railway
electric generating station, situated in the
most severely shaken part of the town,
did not suffer, although the upper parts of
the building collapsed or were badly cracked ;
the hospital attached to the medical school
was destroyed, with the exception of the
reinforced concrete parts, notably the stair-
case, which remained standing in the ruins.
Settlements. — In a milder, slower, but
actually surer way, settlements subject a
building to the same kind of destructive
stresses as those caused by earthquake. An
appreciable settlement will inevitably lead to
cracks, unless it is an even sinking over the
whole area of the site. In a wooden or steel
structure there is a slight accommodation to
the consequences of a local settlement, on
account of the elasticity of the material ;
but there can be none in a monolithic
building. The fact to be borne in mind is
that the foundation should be of such a
nature as to put the risk of settlement out
of all question ; and the use of properly
driven piles or of a reinforced concrete raft
easily ensures that condition of affairs.
Frost. — This is a deteriorating influence
which is at work upon most building
materials. Few substances containing cal-
careous compounds are free from its destruc-
tive effects. Extremely porous substances
are more liable than close-grained ones to
be destroyed by a disintegration of their
particles due to the freezing and consequent
expansion of absorbed water. In the case
of concrete, however, the effect of frost is
only serious under certain limited conditions.
The greatest danger likely to occur from frost
is while the concrete is being gauged up and
258
REINFORCED CONCRETE
before it has set, and, when necessary,
special precautions must be taken at this
stage. The effect of frost after the concrete
has set is of little consequence, as reinforced
concrete of good quality absorbs relatively
little moisture. Where defective cement has
been used, frost might constitute a very real
danger. Valuable information with regard
to the effects of frost upon concrete has been
obtained by submitting briquettes of the
material under varying conditions to different
degrees of cold, and noting the tensile strength
after certain periods have elapsed. Should
a frost occur immediately after mixing, the
strength of the concrete may be permanently
affected ; but should it occur after twenty-
four hours and before the end of the eighth
day, and then be only slight, the setting
will be retarded, but normal strength will be
developed later. After the eighth day, the
frost has no effect, but heavy frost of, say,
15° is likely to injure freshly mixed concrete
permanently. It is wise to refrain from
mixing cement or concrete when the tempera-
ture is below 29° F., which is the freezing
point of the material. The detrimental
effects of frost appear to be due not to
chemical difference or changes, but purely
to mechanical or physical ones, as before the
cement has had time to exert its binding
action upon the aggregate, the frost has
already robbed it of its power to undergo
the chemical changes characteristic of
" setting."
Much of the effect of frost depends on the
amount of water present in the concrete, a
very wet specimen not being nearly so seri-
ously injured as a dry one. No satisfactory
explanation of the action of frost has yet
been made ; it appears to be due to the
great thrust exerted when the water crystal-
lises from the liquid state, but the nature
of this " crystallising force " is, as yet, but
imperfectly known. It appears to be much
more powerful than the increase in volume
which water undergoes on freezing and to
which the disruptive action of frost is com-
monly attributed.
Fire. — The ability to resist the destructive
effects of fire is one of the most important
factors in determining the utility of a build-
ing material. The cement in concrete is
ruined when its water of crystallisation has
been driven off, this occurring at tempera-
tures between 600° and 800° F. (316° and
427° C.) ; but if the dehydrating action
is slow, the material that has undergone
it generally remains and insulates the rest
of the work from the heat. The risk, how-
ever, of extensive damage from this cause is
not great, since serious dehydration requires
an amount of time which would not be likely
to have elapsed before the action of the heat
would be quenched by water applied by
the firemen.
It is impossible to attempt even to out-
line here the numerous experiments con-
ducted under eminent authorities to test
the fire-resisting properties of concrete ;
little more can be done than to say that the
result of the many accurate experiments
has been such as to justify the conclusion
that reinforced concrete is one of the very
best fire-resisting building materials. Tests
conducted by the British Fire Prevention
Committee have shown that reinforced con-
crete suffered no diminution of strength or
decomposition to any appreciable extent
by the heat, although the temperature was
raised to as high a point as 2,000° F. (1,093°
C.). In 1906 Prof. Woolson subjected cubes
of concrete to a temperature of 1,500° F.
(816° C.) for two to five hours, and he found
that, with the exception of specimens in
which quartz gravel had been used in place
of broken stone, the interior of the concrete
did not attain the exterior temperature —
convincing proof of the low thermal con-
ductivity of concrete, and pointing to the
fact that when a concrete wall is heated on
one side no serious rise of temperature is
occasioned on the other side.
Concrete blocks two months old were
placed in boiler flues for twenty-one days
and subjected to temperatures of 1,250° and
250° F. (677° and 121° C.) night and day
alternately ; there was practically no altera-
tion in the condition of the concrete at the
end of the period.
During a fire that occurred recently in a
large warehouse in Dresden, in spite of the
fierceness of the flames, the firemen were able
to keep the fire within the story in which
it had originated, owing to the reinforced
concrete construction. Subsequent examin-
ation showed that the floor was practically
unaffected, although it had been exposed to
a temperature of nearly 2,010° F. (nearly
1,100° C.), and only in one or two places had
small parts of the concrete fallen off, laying
bare the reinforcement, in spite of the fact
that it was acted upon while at these high
temperatures by the firemen's hose.
A few years ago the United States
DURABILITY OF REINFORCED CONCRETE
259
Geological Survey tested thirty panels of
various building materials to determine the
effects of fire and subsequent quenching
with water. In most cases the materials were
subjected to the direct application of gas
furnace heat for two hours and then quenched
with water. Figs. 470 to 475 are photo-
graphs of six of the panels after quenching.
The explanations under the figures should
be read before attempting any comparisons
between them. The report (by Richard L.
Humphrey, an officer of the Survey) con-
tains a number of significant statements, of
•which the more important are here quoted :
" Much of the damage done to the building
materials in this series of tests was occa-
sioned by internal stresses, the gas flame
heating one face of the test pieces much
more rapidly than the other face. All the
materials tested, including hydraulically
pressed brick, cracked . more or less. The
concrete cracked least, but the tests indicate
the necessity for using metal reinforcement
in concrete walls to distribute the effect of
the expansion.
" The average temperature attained by the
faces of the panels ten minutes after the gas
was lighted was about 324° C. (615-2° F.),
and nearly half of the panels had been sub-
jected to freezing weather just prior to the
tests. The average temperature of the face
of one panel of building blocks rose from
<0° to 450° C. (32° to 842° F.) in the first ten
minutes of firing, while that of another panel
of the same material ranged from 22° to
600° C. (71-6° to 1,112° F.) during the same
interval.
" A fact brought out most clearly by these
tests is the low rate of heat-transmission in
portland cement mortars and concretes. This
is one of the desirable qualities in materials
intended for fireproofing purposes. For
marking the cement blocks, linen tags were
fastened by wire nails to the interior walls
at the time of moulding. Most of these tags
remained in place during the test, and when
the walls were dismantled the tags in every
<;ase were found entirely undamaged.
" There was a comparatively slight increase
in the temperatures of the backs of the blocks
during the test. The rise in temperature of
the backs of the building blocks made of
river-sand mortar varied from 25° to 40° C.
(77° to 104° F.), while the rise in the aver-
age temperatures of the faces of the cement
blocks ranged from 650° to 900° C. (1,202°
1,652° F.). The backs of the mortar
to 1,65
blocks made of slag sand showed a rise of
temperature of only 10° C. (50° F.), while
the faces were heated up to 800° C. (1,472°
F.). The low rate of heat conductivity of
the mortar blocks is shown by the slowness
with which the temperature of the backs of
the panels increased in comparison with
that of the faces.
" The damage done to the faces of the
mortar and concrete panels would probably
be caused at a temperature about half that
of the maximum temperatures reached, owing
to the water of crystallisation being driven
from a layer of the mortar.
" The backs of the brick panels also showed
a small increase in temperature, but the
natural building stones and the tiles proved
poorer non-conductors of heat. The temper-
ature of the back of a panel composed of
plastered tiles increased to 128° C. (262-4° F.).
" It was difficult to determine whether the
limestone, granite, gravel, or cinder concrete
sustained the least damage. The faces of all
the panels were more or less pitted by the
fire and washed away by the stream of water.
The test was unfair to the cinder concrete, as
the cinder was very poor, containing a large
percentage of unburned coal ; however, the
sample selected was the best of the six or
eight investigated when purchasing. During
the fire test the coal ignited and left the sur-
face of the concrete very rough and badly
pitted. The limestone aggregate in the face
calcined, and the granite aggregate split and
broke away from the surface mortar. The
granite concrete probably behaved the best.
The damage in no case extended deeply,
probably not more than 1| in. The evidence
shows that even at this depth the tempera-
ture was comparatively low. The high
stresses produced in the panels by the rapid
rise of temperature of the faces while the
backs remained cool caused cracks. On
taking down the panels the blocks of con-
crete were found to be cracked vertically for
some distance back from the face.
" The serious damage to the various
natural building stones precludes any com-
parison among them.
" The concretes tested were pieces of
broken beams that had been tested in the
Government's structural materials testing
laboratories. The pieces were 8 in. by 11 in.
in cross section and of various lengths. They
were fired with the 11-in. face exposed. In
making the concretes, four distinct types of
aggregate had been used, limestone, granite,
Fig. 470. — Single-air-space Mortar Blocks (Sand
and Portland Cement) Laid in Fireclay,
after Firing and Quenching
These 63-day-old blocks were fired for 121 mins. (max.
temp. 700"— 800° F.) and quenched for 5 mins. with
water at 51° F. A strip to the left of the panel was
not quenched. After cooling all the blocks were
found cracked across the web. Surfaces of the blocks
could be broken into small pieces by gentle tapping.
Fig. 471. — Double-air-space Mortar Blocks,
after Firing and Quenching
Fired for 120 mins. (max. temp, about 650" F.), and, after-
lapse of 19 mins., quenched fur o mins. Alter tiring^
vertical cracks showed on each side of the mitre
webs. Richer wet blocks withstood better than the
lean damp blocks. In some cases, end webs split
away from and parallel to faces of blocks, whereas
the middle web remained intact and the face split.
away from it at each side.
Fig. 472. — Terra-cotta Three-air-space Partition
Tiles Laid in Cement Mortar, after Firing
and Quenching
Plastered with ordinary lime plaster } in. thick, and
finished witli a thin skim of plaster-of-paris. Fired
for 121 mins. (max. temp, about 900° F.) and quenched
for 5 mins. with water. Soon after starting firing-,
the plaster came away, and cracks 'developed. In
removing the panel from the furnace, the door stuck
half-way, and the panel was badly shaken in en-
deavouring to prise the door open. Thirty-seven
mins. elapsed before the water was applied, the water
washing away about 65 per cent, of the tiles that
were exposed directly to the stream.
Fig. 473. — Various Building Stones Laid in
Cement Mortar, after Firing and Quenching
" Bedford" indicated in the illustration refers to Bedford
limestone. Fired for 120 mins. (max. temp, about
850° F.)and quenched for 5 mins. with water at 57° F.
During firing, the sandstone spalled ; the limestone
spalled and developed fine irregular cracks ; nearly
all the granite blocks showed small vertical cracks ;
the Bedford limestone showed small vertical cracks
about 8 in. apart ; the marble showed signs of calcin-
ation ; and the wall bulged 1,-^ in., this being reduced-
after quenching to \ in. Alter cooling, the granite,,
limestone and Bedford stone were cracked and had
the faces badly washed off.
260
DURABILITY OF REINFORCED CONCRETE
261
gravel, and cinder. The aggregate was sized
to pass a 1-in. screen and be retained on a
^-in. screen. The concretes had been mixed
in proportions of 1 of cement, 2 of Merrimac
river sand, and 4 of aggregate, and were of
.edium consistency."
F. E. Wentworth-Shields, in the paper
Iready quoted from, says it appears " that
for fire-resisting purposes it is best that the
steel reinforcement should consist of many
bars of small section in preference to few
Tsars of large section, as the first arrange-
ment forms a close network which prevents
instance, at a tobacco warehouse built at
Bristol on the Coignet system, the walls
consist of a framework of steel-concrete
with brick panels. The brickwork can be
arranged, as at Bristol, to conceal the con-
crete entirely ; but the collapse of any part
of the brickwork would not endanger the
floors or the concrete framework."
It will be noted that cinder concrete is
recommended for the outer face — a recom-
mendation which the results of the United
States Geological Survey tests do not seem
to endorse. In the earlier chapter on con-
Fig. 474. — Concrete Blocks Laid in Fireclay,
I after Firing and Quenching
Concrete was of four kinds — namely, limestone, cinder,
granite, and gravel, all being mixed with portland
cement and river sand. Fired for 123 mins. (max.
temp, slightly exceeding 800° F.) and quenched for 5
mins. with water at 52° F. During firing small pieces
fell from the faces, and the limestone, gravel, and
cinder concretes were pitted, the latter two suffering
the most.
.
pieces of concrete from dropping out and
so reducing the strength of the building.
It is also important that the concrete aggre-
gate be crushed small. The ideal composi-
tion for the aggregate seems to be a strong
flint or granite for the hearting, with an
outside sldn of cinder concrete to act as a
good non-conductor. It is essential, too,
that in a building the walls, floors, and
posts should be well tied into one another.
There is a tendency, nowadays, in design-
ing steel-concrete buildings to reject that
material for the walls on the score of ex-
pense, using it only for floors and columns.
Sometimes a compromise is made ; for
Fig. 475. — Granite Blocks Laid in Cement
Mortar, after Firing and Quenching
Firing for 12<H mins. (max. temp. 900° F.) and quenched
for 5 mins. with water at 57° F. During firing, panel
bulged 3 in. and some of the joints and blocks
cracked. Three blocks showed signs of spalling.
During quenching, spalling and cracking became
general. After cooling, the surfaces of the stones
that remained intact could be easily rubbed off with
the fingers : a gentle tapping broke the blocks into
fragments.
crete a note is made of the difference of
opinion regarding this material ; but not
only do opinions differ, even experiments
appear to give contradictory results. For
instance, fire tests of cinder concrete walls
were conducted at Columbia University over
a period of at least two years. The walls of
the buildings used in subjecting floor con-
structions to fire tests were given five fire
tests of four hours each, during which the
temperature was raised to about 1,700° F.
(nearly 930° C.). At the end of each test a
stream of water at 60 Ib. nozzle pressure was
played to and fro over the ceiling for ten
minutes, while the ceiling wall was red hot.
262
REINFORCED CONCRETE
After this severe treatment, the walls, ac-
cording to Prof. Ira H. Woolson, were prac-
tically as good as they were when new,
except that the rear wall was scored by the
water to a depth of from \ in. to 1 in. for
2 ft. or 3 ft. near the top. Prof. Woolson
said he believed the structure to be good,
with occasional plastering, for an indefinite
number of tests. In his opinion, the per-
centage of coal and the amount of fire
material in cinder concrete have, within
reasonable limits, very little if any effect on
its fire-resisting qualities. He believes that
sifting and washing the cinders would be a
useless expense, and injurious to them as
an aggregate for concrete. The pieces of
coal which were close to the surface in the
walls were burned to an ash, which re-
mained in place and acted as a non-conductor
of heat. Plenty of particles of pure coal
could be found in the walls less than 2 in.
beneath the surface.
: It is well known that the water used in
extinguishing a small fire generally does
more damage than the fire itself. Where
there is much wood and plaster construction,
the water swells the wocd and brings down
the suddenly quenched plaster ; it pours
through the ceilings and does infinite harm
in the floors below ; on the subsequent
shrinkage of the wood there is further
trouble, and more plaster is cracked. On
the other hand, the quenching of a localised
fire does not appreciably affect the rein-
forced concrete, and as the floor is water-
tight (practically considered) there is no
harm done to the room below.
CHEMICAL DESTRUCTIVE INFLUENCES
In determining the life of building mate-
rials and their powers to withstand the many
influences which lead to decay, the test of
time is by far the most accurate and satis-
factory ; but the careful study of the
chemical constituents of a building material
may sometimes lead to a means of minimis-
ing the effects of destructive influences. It
will be shown in this section that while
waters or atmospheres impregnated with
chlorides or sulphates and acid-reacting sub-
stances have generally but little effect upon
concrete, there are 'many cases where a
special knowledge of such actions may be
desirable, and, in fact, essential, as in the
construction of chemical factories, public
conveniences, stables, etc., where reacting
gases are likely to permeate the material of
the building. In such circumstances, ample
knowledge of the chemical action of certain
gases and liquids would be useful in arriving
at a means of preventing decay.
Atmosphere. — An average sample of
atmospheric air has the following com-
position per one hundred volumes : Oxygen,
20-61 ; nitrogen, 77-95 ; carbonic acid gas,
0-Oi ; aqueous vapour, 140 ; nitric acid and
ammonia, traces ; carburetted hydrogen and
sulphuretted hydrogen, traces in towns.
The oxygen and nitrogen are without
action upon concrete, but the former, in the
presence of moisture, oxidises iron and steel,
so forming " rust."
Nitrogen is one of the most inert sub-
stances known, and may be entirely disre-
garded so far as concrete is concerned.
Carbonic acid may slightly influence the
condition of concrete. It is produced by
the solution of a gas, carbon dioxide (CO.,),
in water. The cement, which acts as a
matrix in the gauging of the concrete, is
composed chiefly of calcium alumino-silicates.
There is also a certain amount of carbonate
of lime (strictly, calcium carbonate), which
is slightly soluble in water charged with car-
bon dioxide (3 parts by weight in 1,000), the
solution containing the acid calcium carbon-
ate, H2Ca(C03)2. The proportion of car-
bon dioxide in the air varies considerably
with the locality, and in the vicinity of
large factories appreciable quantities of it
are brought down in solution in rain. In
some cases this would be a factor in acceler-
ating the decay of concrete in course of
time ; but, generally speaking, under ordinary
atmospheric conditions it may be entirely
disregarded.
Nitric acid, of which the atmosphere con-
tains traces, is a fuming, powerful liquid
(HN03), which, when brought into contact
with alkaline and basic substances causes
chemical action to take place, resulting in
the decomposition of the substances acted
upon ; but the proportion of this acid pre-
sent in the atmosphere is too small for it to
have any appreciable effect upon concrete.
The other substances mentioned as con-
stituents of the atmosphere are in far too
small proportions to be of consequence in
destroying concrete ; and, speaking gener-
ally, therefore, the atmosphere has no
chemically destructive influence upon con-
crete, a statement substantiated by the
stability of concrete after hundreds of years
of exposure. Of course, where concrete is
DURABILITY OF REINFORCED CONCRETE
263
used in buildings in which it may be exposed
to any gaseous fumes, as in the manufacture
of chemicals, the action of these gases upon
the concrete requires special consideration.
Whether or not the moisture in the air
affects the steel reinforcement will be con-
sidered under the next sub-heading.
Water. — Many authorities have feared
that water must inevitably corrode the rein-
forcement and bring about failure, the danger
being all the more serious because the steel
is completely hidden from view, and the
course of the corrosion cannot be watched.
The fear seems a reasonable one until certain
proved facts relating to the chemistry and
physical conditions of concrete and steel
have been stated. In the first case, a thin
skin or shell of concrete is not waterproof
when new, although it may become so in
the course of a couple of years or so. Natur-
ally, the better the concrete the less will be
its porosity ; but even the best concrete will
absorb a very small amount of water, which
naturally may find its way to the steel.
There are now two facts to remember :
(1) Iron or steel cannot rust ivhen damp
except free oxygen be present. The concrete
used in reinforced work should be of high
quality and therefore of minimum porosity.
Although it may permit the passage of an
extremely slight amount of water, air at
atmospheric pressure could not penetrate it,
and rust cannot therefore form. But per-
haps it will be said that the microscopic
particles of water may carry entrained air
with them. While this is scarcely likely —
probably quite impossible — it could still be
granted, and absolute reliance placed on the
next point.
(2) Steel cannot rust when embedded in a
strongly alkaline substance. In the setting
of portland cement a relatively large
quantity of caustic lime (lime hydrate) is
produced, and the concrete becomes satur-
ated with it. This is strongly alkaline, and
by decreasing the number of hydrogen ions
present makes any corrosion of the steel
impossible.
On the other hand, if the concrete were
acid instead of alkaline, the number of
hydrogen ions would be increased, and,
assuming the presence of oxygen, the steel
would be corroded. Iron rust, it will be
remembered, is not simply an cxide ; it is
a hydrated oxide, one formula being 2FeO;5
+ 3H20 ; this does not express the whole
of the truth, since iron rust contains,
besides the oxide, carbonic acid and am-
monia. It is known that the composition
of rust varies with surrounding conditions.
Of supreme importance is the necessity
of using the best concrete obtainable.
" Rotten," porous concrete, permitting the
free passage of water, would not maintain
the steel bright and free from corrosion,
since the caustic lime is soluble in water,
and in time would all be dissolved away.
Not only does concrete prevent the corro-
sion of the reinforcement, but it removes
any rust that may have been present on
the steel. For this reason no concern is
felt over the use of rusty rods and bars, but
that is a mistake. Thick rust reduces the
sectional area of the steel and lessens the
adhesion of the concrete, although it becomes
completely absorbed.
It is claimed that experiments have
proved that steel reinforcements to concrete
are found to corrode rapidly when an electric
current is passed through them. Such
corrosion can take place only when the
interior of the concrete is damp (that is,
when the concrete is inferior), and need not
be feared when material of the proper
quality is employed. The rods, in the
presence of moisture, would act as poles in
a battery, and hydrogen would congregate
round some and oxygen round others — a
condition conducive to rust.
From certain authoritative experiments,
it appears that the only risk of reinforce-
ments rusting is likely to occur after the
material has been stressed to 35,000 Ib, per
square inch, a condition of affairs which
ordinary designing renders absolutely impos-
sible in normal circumstances, since the
tensile strength of structural steel itself is
regarded as about 60,000 Ib. per square inch ;
the elastic limit is taken at about half that,
and the factor of safety is 4 or 5. It is
obvious that for rusting to be made possible
such a stress must be applied as will give
the steel a permanent set, and, by reduc-
ing its diameter, destroy — or, at any rate,
seriously impair — the cohesion between the
steel and concrete. Such a stress cannot
occur unless the designing be absurdly and
obviously at fault.
It will be of advantage in this place to give
the conclusions arrived at by Ernest R.
Matthews, A.M.Inst.C.E., after a number
of tests, and presented by him in a paper
entitled " The Corrosion of Steel Reinforce-
ment in Concrete," read before the Society
264
REINFORCED CONCRETE
of Engineers. Those conclusions are as
follow : (1) Rusty steel embedded in con-
crete will in a very short time become
bright, regardless of whether the concrete is
in water or air. This point has, in Mr.
Matthews's opinion, been conclusively proved
by his experiments. (2) The application of
cement grout to steel is an effectual safe-
guard against corrosion, but the greatest
care should be taken in the grouting process
to see that every portion of the steel is well
coated ; and before the steel is embedded
in the concrete the cement grout should be
allowed to dry on the steel. (3) If the aggre-
gate used for the concrete is not porous and
the concrete is well mixed, the reinforcement
being well embedded, no cement coating is
needed. This is proved by experiment.
(Seeing that the application of a coat of
cement grout is such an inexpensive proced-
ure, Mr. Matthews makes it a rule in carrying
out work of this kind to have all reinforce-
ments coated in this manner.) (4) No
porous materials, such as coke breeze or slag,
should be used in connection with reinforced
concrete work if such concrete is intended
to be under water or exposed to the air. (5)
Linseed oil or turpentine, or probably any
other coating except cement or lime, applied
to steel before its insertion in concrete,
facilitates rather than prevents the rusting
of the metal. (6) It is of great importance
to ensure that the reinforcing steel is well
embedded in the concrete, so that every
portion is covered with cement. (7) The best
results were obtained with aggregate con-
sisting chiefly of broken stone or brickbats.
Gravel would no doubt answer equally well.
Sea Water. — Whilst pure water is with-
out chemical effect upon concrete, sea water,
acting both chemically and mechanically,
exercises an influence that must not be dis-
regarded. To form some idea as to the
probability of sea water affecting concrete,
it is desirable to know its constituent parts.
An average sample contains per 1,000 parts
the following compounds or salts :
Sodium chloride 27-059
Magnesium chloride . . . . 3-666
Magnesium sulphate . . . . 2-296
Calcium sulphate . . . . . . 1-406
Potassium chloride . . . . . . -766
Calcium carbonate . . . . . . -033
Magnesium bromide . . . . -029
Total salts 35-255
Water . 964-745
In the foregoing list the magnesium and
calcium chlorides and sulphates are the only
compounds likely to have appreciable effect
on the chemical nature of concrete ; such
action as they possess is due to the ions or
acid radicles which they contain, and,
although it is slight, the total quantity
of these salts is so large that the effect
on the concrete is much greater than the
small proportion of them in the sea water
suggests.
As regards the other salts present in sea
water, the chief of which is common salt or
sodium chloride, their action appears to be
of but little consequence. In cases, how-
ever, where sea sand has been used in the
gauging, the concrete takes longer to set,
and it presents a damp surface for some
time, due to the magnesium chloride and
other hygroscopic salts in the sand, derived
from the sea water, absorbing moisture from
the atmosphere and so remaining damp.
A very practical contribution to the dis-
cussion of sea-resisting concrete is a Danish
paper read before the Copenhagen meeting
of the International Committee on Rein-
forced Concrete, by A. Poulsen, who gives
the conclusions drawn from two series of
experiments ; the following is according to
Dr. Desch's summary of the paper : (1)
Good portland cements are very resistant to
the action of sea water. No marked differ-
ence in the behaviour of cements slightly
different in composition has been found,
except that a high proportion of aluminates
tends to cause disintegration. (2) In a
dense mortar, the chemical action is con-
fined to an outer layer of small depth, further
action being checked by the slowness of
diffusion. A porous mortar, by admitting
salt water to the interior, is apt to crack by
expansion owing to chemical change. (3)
The main agency in the destruction of
mortar and concrete in marine embankments,
harbour works, groynes, etc., is not chemical
action, but the alternations of saturation,
drying in the sun, freezing, etc., due to the
alternate exposure and covering by the rise
and fall of the tide. Destruction takes place
sometimes by cracking, sometimes by scal-
ing, the latter effect being produced especi-
ally by frost. (4) The denser the mortar the
better (1 cement to 3 sand is too poor for
marine work). An admixture of fine sand
with the ordinary sand increases the close-
ness of the mixture, but a well-graded ag-
gregate is the most advantageous. (5) The
DURABILITY OF REINFORCED CONCRETE
265
addition of finely-ground silica or trass to
the cement before mixing is often advan-
tageous in the case of the more porous
mortars which are rich in cement. It is
very doubtful whether anything is gained
by adding trass to the denser mortars,
which are rich in cement. (6) Hydraulic
lime mixed with trass, etc., is of Borne
value where a cheap material is required,
in the mild climate and absence of tide of
the Mediterranean, but it is incapable of
withstanding the conditions of coast work
in northern latitudes. (7) The destructive
action of the sea being mainly physical and
mechanical, and not chemical, tests by mere
immersion in still sea water are of very little
value in determining the behaviour of con-
crete in marine engineering works. A mix-
ture which disintegrates under this test is
certainly useless, but a mixture which passes
the test may disintegrate under the more
stringent conditions of practical use. (8) As
long a period as is practical should be allowed
for the hardening of concrete blocks before
placing in the sea. The German custom of
leaving them for one year in moist sand be-
fore setting in place is impracticable in many
places, but should be approached as nearly
as possible. (9) The behaviour of test-
specimens for the first twelve months is very
irregular, and definite conclusions can only
be drawn from the results of long-period
tests.
Acids. — Hydrochloric and sulphuric acids,
while scarcely affecting siliceous materials
such as sand and concrete aggregates,
decompose the cement by dissolving out
the soluble substances such as calcium
carbonate, and they readily attack the
aluminium compounds in the cement. Sul-
phates have a less powerful action than
have chlorides, and they form products
which are largely insoluble, and which,
therefore, serve as protective coatings.
Fortunately, as concrete consists largely of
sand and aggregate, which are scarcely
affected by ordinary acids, it offers a
natural resistance to the common acids
which would be liable to form by the action
of chlorides or sulphates.
The action of hydrofluoric acid is one
worth consideration, as, although a rare acid,
its action is certainly of interest in special
cases. The most characteristic property of
this acid is its readiness to attack silica
either in the combined state as silicates or
in the free state as quartz sand. In fact,
it is the only acid which will dissolve this
substance. The acid may be formed in a
variety of ways, such as by the interaction
of the minerals fluorspar or cryolite and
sulphuric acid, which produces — or at
least releases — the powerful gaseous hydro-
fluoric acid, which is known to attack
almost anything with which it comes into
contact.
As has already been explained, small
quantities of nitric acid are brought down
by the rain, but larger quantities of nitrates
are found in soils, where they are produced
by the decomposition of vegetation and
other nitrogenous matters ; in cases where
the concrete comes into actual contact with
substances of this nature there is no doubt
that in course of time it will be affected by
the chemical action of the nitrates.
Many organic acids are without action
upon concrete, but in the presence of sodium
chloride (common salt) they are capable of
dissolving the cementitious portion of it to
a slight extent. Hence the action of saline
solutions, such as sea water, on the cement
present in concrete, may in some instances
prove serious.
Alkalies. — Such alkalies as strong potas-
sium hydrate (caustic potash) and sodium
carbonate (soda) are capable of dissolving
silica, a substance that is not acted upon
by any acids except hydrofluoric.
Urine. — The action of this substance is of
importance in view of the employment of
concrete for stables, public conveniences,
etc. Possibly, should concrete prove a
durable material for such structures, a suit-
able surface finish will be evolved, so
obviating the employment of the relatively
costly tiling now in common employment.
Urine contains water, urea, uric acid
and certain organic and inorganic salts.
Of these constituents the one most likely
to influence substances of the nature of
concrete is urea. By the action of
moisture, urea (NH2)2 CO, is turned into
ammonium carbonate (NH4)2 C03, which
attacks silicates and substances of a
siliceous nature, and hence tends to
cause the decay of concrete in course of
time. Uric acid is a weak acid, which is
readily changed into urea by exposure to the
atmosphere. The action of urine upon con-
crete would certainly be a slow one, but it
calls for consideration where the durability
of concrete in the construction of stables,
etc., is concerned.
Waterproofing Concrete
THE permeability of concrete has proved a
matter of great concern to many engineers
and users of this material, especially in the
construction of water tanks, sewers, aque-
ducts, gas-holders, etc., and numerous
attempts have been made to render it im-
permeable to water, oil, and other liquids.
The methods used to render concrete
waterproof are of two main classes (a) those
in which a material of an impervious nature,
or one which will combine with an ingredient
in the concrete to form an impervious
material, is applied to the surface of the
finished concrete, and (6) those in which
substances are mixed with the concrete so
that the whole mass becomes impermeable.
The latter method is, in many ways, the
most effective, particularly if the surface of
the concrete is likely to be damaged, as it
gives the whole mass the maximum im-
permeability ; but it is also more costly, and
cannot be applied to the finished work.
SUPERFICIAL WATERPROOFING
Superficial waterproofing may be accom-
plished in a variety of ways, of which the
chief are (a) coating the surface with an
impervious material, such as cement grout-
ing, dense mortar, tar, asphalt, or paraffin
wax dissolved in light petroleum ; or one
which becomes impervious on drying — as
sodium silicate (water glass) and some casein
paints followed by formaldehyde or a solu-
tion of alum followed by a solution of soap,
or soda lye, these alternate coatings being
repeated as often as is considered necessary ;
and (6) applying to the surface a substance
which will enter the pores in the concrete and
will form with the cement an insoluble and
impermeable compound. Sodium oxalate,
which forms the insoluble oxalate of lime in
the pores of the concrete, is typical of this
class of waterproofing material, as is linoleic
acid, which forms an insoluble lime soap ;
but the most largely used substances of this
class are soluble silico fluorides, under the
commercial term of fluates. These fluates
are chiefly composed of aluminium, zinc or
magnesium silico fluorides, and when applied
to concrete they combine with the lime set
free as the cement sets and, by forming double
silico fluorides, they not only render the
material waterproof, but also increase the
hardness and durability of the surface to
which they are applied. M. Merkuloff has
shown that cubes composed of 1 part of
portland cement to 3 parts of sand when
immersed in a dilute fluate solution for
several hours increased 50 per cent, in com-
pressive strength, and that their resistance
to the action of frost was similarly increased.
The surface of the cubes was found to be ten
times as resistant to abrasion as that of the
untreated cubes.
MASS WATERPROOFING
By far the most effective means of render-
ing a mass of concrete waterproof is so to
arrange the aggregate as to obtain a material
practically devoid of pore spaces and with
the closest possible texture. For poor
concretes this may necessitate the addition
of an exceedingly fine, insoluble powder,
such as slaked lime or china clay. A careful
grading of the aggregate and the sand used
will produce a fairly close concrete, and
then a pore filler is quite unnecessary, as has
been proved over and over again. When pore
fillers are employed, they should be added;
to the cement or concrete in a dry state,
and be well mixed before water is added.
Plastic clay is not a suitable pore-filler :
it is too adhesive; but powdered china
clay is suitable. Waxes, greases and oil
are similarly inferior to china clay and to
slaked lime. The only objection to slaked
lime is its tendency to produce a whitish
efflorescence on the surface of the concrete' ;
china clay does not do this, but its slight
plasticity is an almost equal drawback.
If sand or calcined clay should be obtained
in a state of fineness equal to that of china
clay it would be almost perfect as a pore-
filler, but, unfortunately, no grinding plant
will produce such a fine product on a com-
mercial scale at a price at which it can be
used for this purpose.
As showing the differences in" permeability
266
WATERPROOFING CONCRETE
267
due to the use of equal weights of various
substances in the same aggregate, the follow-
ing table (due to Concrete) is interesting.
The figures in the third column represent the
number of cc. of water which penetrated
concrete slabs 10 in. square and 3 in. thick,
subjected to a water pressure on the face of
50 Ib. per square inch during twenty-four
hours. The slabs were taken from the
moulds twenty-four hours after gauging,
and were kept in cold water for twenty days
before use.
Series
3 per cent, waterproofing
material added to cement
A
B
C
D
E
G
H
I
J
K
L
M
Nil
Mixture of slaked lime
and lime soap .
Kesin
China clay
Equal parts resin and
china clay
Paraffin wax .
90 parts slaked lime, 10
parts paraffin wax
Slaked lime
Burned gault clay .
Alumina .
90 parts china clay, 10
parts wax
Equal parts china clay
and alumina
5 per cent, china clay
Permea-
bility
1240
1290
760
865
740
1005
555
450
2370
320
280
372
450
That the quantity of water used in gauging
concrete has a large influence on the im-
permeability of the mass has been well
established, but Cloyd M. Chapman has
shown that the least absorption occurs when
the concrete contains 13 to 15 per cent, of
water, as shown in Fig. 476, which sum-
marises the results of tests on 230 blocks.
The best method of obtaining a water-
proof material thus appears to consist in
using a carefully selected aggregate, together
with the proper proportions of fine material
and cement.
If the proportions of water, cement, sand,
and aggregate are correct, and the aggregate
is properly graded, there should be but little
difficulty in obtaining a mass that is suffi-
ciently waterproof for all practical purposes.
The addition of a pore-filler to such a mass
is unnecessary.
Water-repellent substances are also
used in admixture with concrete, the idea
being that a substance which is not merely
insoluble, but actually repellent to water,
must be more advantageous than such a
substance as clay, silica, or alumina. Most
of these water-repellent materials are of a
fatty or oleaginous nature, oils, soaps, and
the corresponding fatty acids, such as stearic
and linoleic acid, being chiefly used. The use
of blood, milk, and lard is extremely ancient,
being recommended by Vitruvious. At the
present time, mixtures of lime and tallow or
of soap and alum are preferred.
s 10
| 9 "0 II 12 13 14 15 16 17 IB 19 20
fercentagf o] tt/atei uscO in mning Concrete
Fig. 467. — Graph Showing Relative Water
Absorption of Concrete
These materials are usually supplied in the
form of a powder which is mixed with the
cement before gauging, but some workers
prefer to keep solutions of the ingredients
forming the water repellent, and to mix
these solutions separately with the concrete.
Thus, if a solution of alum is first added to
the gauged material and then a solution of
soda, there will be formed in the pores of
the material a precipitate of aluminium
hydroxide. Unfortunately, experiments have
shown that this precipitate is very irregu-
larly distributed, and that it does not
increase the impermeability as much as the
corresponding amount of finely divided
alumina added to the concrete previous to
the latter being mixed.
Emulsions, consisting of oil in the form
of "minute globules suspended in water, are
also used to render concrete impervious to
water. Their use is based on ignorance, as
oils added to the mixing batch are more
convenient, and the emulsion formed during
the mixing process is better distributed
through the concrete than when an emulsion
is added in the first place.
Oils mixed with concrete, in the propor-
tion of 10 to 15 per cent, of the weight of
268
REINFORCED CONCRETE
the cement used, are used to render con-
crete 1 waterproof. The oils used for this
purpose must be of mineral origin, as animal
and vegetable oils are liable to form acids
which disintegrate the concrete. The most
suitable for the purpose are mineral oils
of various kinds, and range from heavy,
black bituminous oils to light, non-volatile
petroleum oils ; white or colourless oils are
used where the colour of the concrete is
important.
It is unnecessary to add oil to good
•concrete ; the addition will render it less
waterproof and will reduce the strength.
A great disadvantage in the use of oil is
the delay which occurs both in the initial
and final setting and hardening and the
loss of strength and toughness which
•ensues.
The chief objection to adding substances
of a water-repellent character to concrete or
to the cement in the form of a dry powder is
that the natural tendency of such materials
is to leave the cement when water is added
and to collect in small masses, whereby a
material of irregular composition is pro-
duced. The same objection holds good in
the case of oleaginous and other substances
which are mixed with water before use ;
the water merely carries the particles hetero-
geneously into the concrete mass, and does
not effect a uniform distribution. Moreover,
the use of materials which possess a strong
water-repellent action undoubtedly tends to
reduce the strength of the concrete with
which they are mixed.
Extensive experiments with a large variety
of substances (each of which was mixed with
the cement in the proportion of 3 of substance
to 100 of cement before use) show that water-
repellent substances, such as oils, greases,
soaps, resins, etc., have no true water-
proofing qualities, and the slight benefit
they confer is more than counterbalanced
by the extent to which they weaken the
•concrete.
MAKING A NON-POROUS CONCRETE
The impermeability of concrete when
obtained is due primarily to the absence of
pores or voids in the material, and this
can only be secured efficiently by a careful
selection of material of suitable grades. For
example, if sufficient cement is used and the
mixing is thoroughly performed, all the pores
into which water can penetrate will be filled.
When, however, insufficient cement is used,
a non-hydraulic material of equal fineness
may be added to fill the pores. The amount
of this material — which, as already indicated,
may be of any convenient composition
providing it does not attack the other
constituents of the concrete — must be ascer-
tained by experiment. The only satisfactory
method consists in making a number of
blocks with aggregates of various grades and
to test the permeability or porosity of the
blocks. The most satisfactory mixture is
then made the basis of a further series of
tests in which different percentages of pore-
fillers are used. In this way, with care, it
is usually possible to produce a perfectly
waterproof concrete with little or no pore-
filler in it.
Concretes that are easily penetrated by
water and other neutral liquids will usually
be found, on examination, to be composed
of badly graded aggregates or to contain a
considerable proportion of very coarse
material. The larger the blocks the larger
may be the size of the coarsest pieces of
aggregate, but blocks of small size should
not contain pieces of material which will
not pass through a J-in. ring. The great
difficulty experienced by some engineers in
obtaining sufficiently waterproof concrete
may usually be overcome by the use of
material which will not pass through a
£-in. or f-in. ring, and by screening out of
the still coarser aggregate (if very large
blocks are used) all matter in it which is less
than £ in. in diameter. The use of too coarse
an aggregate and of an aggregate containing
too large a proportion of fine material are
both equally detrimental to impermeability.
It is important to note that a concrets
should not be judged too hastily as regarde
its waterproof qualities. Until it is set all
cement is porous, and many instances are
known where the " weeping " of water-tanks
ceased after the cement in the concrete
had become properly hardened. Conversely,
concrete which is subjected to the action
of impure water (such as sea water or
spring water highly charged with carbon
dioxide gas) may be rendered permanently
porous, whereas if it had been allowed to
harden properly before being used it would
have proved quite impervious.
It is frequently found that concrete that
is insufficiently waterproof has been care-
lessly made or indifferently mixed. In such
cases any supplementary waterproofing that
may be necessary must be largely of a super-
WATERPROOFING CONCRETE
269
ficial character, and will consist of the
application of fluates, water glass, or some
penetrable paint. The external application
of ordinary paint is useless until the free
lime in the concrete has been neutralised
by one or more preliminary coats of a suit-
able acid. A coating of tar or asphalt is
only available when the pressure of the
water on it is negligible, as in buildings
which are to be protected from rain, or
floors which are liable to splashes. In such
cases the tar or asphalt should be renewed
as frequently as occasion requires, as its
waterproof action is, of course, far from
permanent.
COMMERCIAL WATERPROOFING
SUBSTANCES
It may be helpful to explain briefly how
some of the commercial waterproofing
substances are added to the concrete.
Certsit is a white paste of butter-like con-
sistency which requires to be mixed with
an equal bulk of water and then with from
14 to 19 further parts of water. This is
used instead of ordinary water for mixing
the concrete (for cement mortar, 1 part of
the special material to 10 of water is about
right). The concrete must be made of a
creamy consistence, so that it can be poured
into place. H y dr of uge- Castor is a liquid
bituminous substance which is incorporated
with the mortar or concrete after the wet
mixing has been completed in the ordinary
way. Pudlo is a fine white powder which
requires to be mixed with the cement before
incorporating with any of the other materials.
The water is added to the dry-mixed
materials through a rose, to prevent the
powder being lifted out of the cement.
Specifications, Quantities, Measuring,
Estimating, and Pricing
The Specialist System. — Reinforced con-
crete, when first introduced, was in the hands
of a few specialists, each firm having its own
particular system and type of bar, the latter
being generally patented. These firms
tendered directly for the work, and under-
took to supply the requisite drawings and
•calculations free of charge ; on securing a
contract, they prepared the necessary draw-
ings, calculations, and quantities, which
they submitted for estimates to one or
more firms of builders licensed by them.
This system still obtains, but it has its
objections — (a) The firm responsible for the
design and superintendence of the work
is commercially interested in it. (6) The
system is inconvenient, inasmuch as it
entails the employment of two contractors,
whose work is interdependent, and each of
whom may delay the work of the other.
(c) It is not economical, as it adds an inter-
mediate profit to the cost of the work, for,
in spite of the specialist's claim that he
supplies his design free of charge, it is
obvious that he will only run his business
as long as he makes a profit, and to do this
he must either arrange a percentage to be
included in the tenders of his licensed con-
tractors, or charge sufficient for his steel to
cover profit in addition to the large estab-
lishment charges entailed by the employment
of a technical staff, (d) It necessitates the
selection of a particular system and elim-
inates competition; but this objection has
been partly met by the specialist submitting
tenders from two or more of his licensed
contractors.
As the new construction began to find
favour, and came into more general use,
architects, in their desire to obtain the best
and most economical result, adopted the
system of inviting two or more firms of
specialists to submit schemes and estimates,
and so placed themselves in the position of
having to judge these schemes on two
bases — namely, efficiency and economy — by
striking some sort of a balance, and placing
the schemes in order of merit ; it is open to
doubt whether many of the architects were
in a position to take much practical account
of the first factor, and whether the second
factor was not usually dominant.
A GENERAL SPECIFICATION
It will be obvious that, assuming the
contractors were left with a free hand (as
they undoubtedly were in the earlier
days), the competition was most unfair, as
all of them were not competing on an equal
basis. Therefore, with public departments,
at any rate, it became the practice to issue
with the skeleton drawings indicating the
position and extent of the reinforced concrete
work a specification which gradually became
more and more definite and exact, specifying
no detail which could apply to only one
particular system, but setting forth all
essential conditions and general principles
with which all competitors must comply.
Such a specification should fully describe the
cement, sand, and aggregate ; the composi-
tion of the concrete, and its method of
mixing, whether by hand or machine, and,
if the latter, the type of machine to be used ;
and the description and quality of the steel
and its ultimate tensional strength. The
centering should be specified to be truly laid
with close joints, to be well strutted and
braced, and made perfectly rigid and of
sufficient strength to support the dead
weight of the construction as a liquid,
without deflection. The specification should
mention any portions to be planed and
coated with a wash of lime and clay to
produce a smooth surface in any position
where the concrete is not to be plastered,
that the beam centering is to be designed
to allow of the sides of the forms being taken
down first, the minimum length of time to
be allowed to elapse before the centering
is removed, that it is not to be removed
without the architect's consent, and the
length of time the beams are to be supported
after removing the remainder of the centering.
A full description should be given of the tests
that will be applied for the cement, steel, etc.,
270
SPECIFICATIONS, QUANTITIES, ESTIMATING, ETC. 271
and for the finished construction, stating
the number and dimensions of the test
pieces of steel and the percentage of area
of slabs and beams to be tested, by whom
the cost of the tests is to be borne, and the
procedure in case they prove unsatisfactory.
Every condition which will ensure the
calculations of all competitors being based
as far as possible on the same data should
be specified ; to which end the maximum
safe working stresses to be adopted for com-
pression and shear in concrete, tension in
steel (the tensional strength of the concrete
being neglected), and for adhesion between
concrete and steel should be clearly stated.
The foregoing outline is given, merely to
emphasise the principal points to be attended
to in drafting a specification for the purpose
of obtaining combined schemes and estimates
in competition. It should be written in as
much detail as possible, and embody every
condition capable of general application,
such as accessibility of site, means of getting
in and hoisting materials to the various
levels, provision of sheds for storage, etc.
(unless these are to be provided by the
general contractor, in which case the fact
should be stated), protection of work from
frost, cessation of work during frost, whether
the contractor is to supply water or whether
this will be supplied by the building con-
tractor, whether he is to supply all his own
plant and hoisting tackle, or if and to what
extent he is at liberty to use that of the
building contractor, and what special plant
he must provide, etc., etc. The aim must
be to ensure that all competitors shall
tender on equal terms, so far as this is
possible where each competitor is submitting
a different scheme.
It is also advisable to specify that the
work must be carried on concurrently with
that of other contractors, who are to be
allowed all reasonable facilities for carrying
on their work, and to state the approximate
date at which the various portions of the
building will be ready to receive the rein-
forced concrete, but that the building owner
will not hold himself responsible for any
alteration in those dates due to delay on
the part of the other contractors. A
corresponding clause should be inserted in
the building contractor's contract specifying
these dates, and also indemnifying the
building owner against any claim for delay
due to the reinforced concrete contractors.
A clause should be inserted in each contract
stating that no claim will be entertained for
damage to plant, materials, etc., caused by
other contractors using the site.
In the absence of such precautions,
claims for delay from either contractor will
lie against the building owner, whose sole
remedy is, in turn, to prefer a claim against
the offending contractor, the result of
which is by no means as certain as is the
necessity for meeting the costs of the action.
The contract under the foregoing con-
ditions being for a lump sum, it is very
necessary to attach to the specification a
schedule of prices for measured work to be
adopted in variations, and prices of materials
and labour rates to be adopted in day work ;
these prices (except the labour rates) may
be attached by the surveyor, and thus
become practically a condition of contract,
or they may be left to be priced by the
contractors, in which case they should be
carefully examined before accepting any
tender. The labour rates should always be
fixed by the surveyor, and should be the
net current rates payable in the district.
The percentage that will be added to these
to cover contractor's profit, superintendence
and "sharps" (allowance for sharpening
tools) should be definitely stated.
From skeleton drawings and a specifica-
tion as outlined above, the competing firms
prepare their detailed schemes and estimates.
Public departments and some private archi-
tects require calculations to be submitted
with the estimates, a good practice when the
architect can check them.
QUANTITIES
In order to frame an estmate under such
conditions the reinforced concrete specialists
have to prepare their own quantities, and
this is usually done in the most rough and
ready way. Not only are items of differing
values lumped together, but it is open to
question whether the actual amounts will
bear investigation. All the concrete is
billed per yard cube, even slabs 3-| in. or
4 in. thick ; the centering is lumped together,
little or no differentiation being made be-
tween differing values ; in some cases even
the centering for beams and slabs is lumped
together, no height being stated for the
strutting and bracing, and the steel reinforce-
ment billed together in one item for each
level, or even in one item for all levels.
These quantities are supplied to the licensed
firm or firms on which to estimate for the
272
REINFORCED CONCRETE
actual work, and it will generally be found
that the concrete is priced at one average
price per yard cube, and the centering and
steel each at an average price per yard super
and per ton respectively ; such averages
are rough and not exact. It has been
argued that as reinforced concrete is a
comparatively new material, it is useless
to supply any more detailed quantities, as
the builders have not yet had sufficient
experience to price them accurately. This
contention referred particularly to the
centering, the one item that even at an
early date could have been priced with
reasonable accuracy if properly measured ;
in any case, now that this mode of con-
struction is becoming so general the argu-
ment no longer holds good, and there is no
excuse for continuing these unsatisfactory
methods.
As long as the present system of obtain-
ing schemes and estimates continues, it will
be impossible to supply complete bills of
quantities, because, obviously, a scheme must
be prepared before quantities can be taken
off. Attempts have been made to prepare
bills of quantities for the purpose of obtain-
ing schemes and estimates with a view to
greater accuracy and uniformity of practice
than at present prevails, and, though their
practical utility may be open to question,
instructions will be given in this chapter
for their preparation. The bills would be
supplied to competitors, together with a
set of skeleton drawings and a specification
as previously described, on which to base
the scheme ; or, better still, the specifica-
tion might be incorporated in the preamble
to the bill.
Such a bill would necessarily be in skeleton
form, and should, like the specification, be
in as much detail and as complete as it is
possible to make it in the circumstances.
The preliminary bill should contain the
requisite data for preparing the scheme ;
it should, in fact, be a complete speci-
fication, items which carry a money value
being written in bill form, as for example:
" Provide a close boarded weather-proof
shed, with floor raised above ground, having
a floor area of — ft. super, for the storage of
portland cement, and clear away at com-
pletion " ; where two contractors are em-
ployed, this item would probably be in the
general contractor's contract.
Procedure on the Job. — Before dis-
cussing the measured items of this bill, it
will be advisable to consider the usual mode
of procedure in carrying out a typical
reinforced concrete job, as this will govern
the descriptions, particularly in respect of
the centering. The centering for the lowest
floor is erected first, the reinforcing steel
placed in position, and the concrete de-
posited, tamped in, and levelled. When it
has set sufficiently, sole plates are placed
in position to distribute the weight, and the
centering for the next floor is erected on
them, the steel placed in position, the con-
crete deposited, and the whole process
repeated for the next floor, and so on, care
being taken to place the uprights and struts
for the centering of each floor as nearly as
possible over those on the floor below.
Any deviation from this order, by increasing
the height and scantlings of the posts and
struts, and possibly making the supply of
materials to the lower levels more difficult,
may tend to make the work more costly.
Preparing the Quantities. — The usual
practice of the specialist firms in preparing
their quantities has been to keep the whole
of the work, centering, steel and concrete
separate for each level, this being a con-
venient method, which is correct in principle.
In compiling a bill of this description, it
must be borne in mind that, in addition to
the actual quantity, the description must
impart sufficient information on which to
base the calculations ; for example, to
determine the thickness of a slab and the
amount of reinforcement, it is not sufficient
to give the span only, because slabs " fixed "
at edges, or, what amounts to the same thing,
continuous over supports, are stronger than
those supported only ; and slabs supported
on one edge and fixed at the other hold an
intermediate place between the two kinds.
Top reinforcement must be provided over
the supports in all cases where the slabs are
continuous. Also, rectangular slabs, sup-
ported or built in on four edges, may be
reinforced in both directions; all of which
information must be in the possession of the
designer before he can fill up the blanks left
for him in the bill of quantities. Where
the positions of the beams have been deter-
mined beforehand and indicated on the
skeleton drawings, the dimensions of the
beams and their reinforcement only being
left to the designer, it is possible to embody
the essential conditions in the description ;
for example, in the accompanying sketch
(Fig. 477) of a floor slab over a room or
SPECIFICATIONS, QUANTITIES, ESTIMATING, ETC.
273
apartment 45 ft. long by 20 ft. wide, sup-
ported on two beams across the width of
the room, the centre bay will be continuous
over both supports, the end bays continuous
over one support, and the edges in walls
supported only ; in this case the slab might
be billed in two separate items, each with
its proper description ; but it would convey
a much better idea of the requirements of
the case to the mind of the designer if the
whole slab were combined in one intelligible
item, the description reading thus : "Floor
slab at first floor level over an apartment
45 ft. long by 20 ft. wide, the length divided
into three equal rectangular bays, and the
slab continuous over the two intermediate
supports."
Nothing less than the above description
can, in the absence of illustration, convey
the necessary data.
A much simpler and at the same time
more efficient method of conveying the
conveniently be similar, the various slabs,
walls, columns, beams, etc., being kept
separate under the different reference num-
bers or letters (their sizes being unknown
to the surveyor).
Slabs. — The reinforcement for the slabs
being billed by weight may be in one item at
each level, described as " steel reinforcement
to slabs," and a blank left for the amount.
The centering to the slabs is billed in
squares or yards super, the height of the
strutting and bracing stated and the different
heights kept separate.
Beams. — The concrete in beams is billed
in yards cube, and may, therefore, be in one
item at each level ; the word " cube " should
be inserted in the proper column, the
description written, and a blank left for the
amount.
The steel reinforcement to beams is
described as such and billed by weight, a
blank being left for the amount.
Fig. 477. — Section through Floor Slab supported by Two Beams
information is by lettering or numbering
the various positions on the skeleton plans
and referring to them by corresponding
letters or numbers in the quantities. The
above description would then be curtailed
to " floor slab at ' A ' at first floor level " ;
this method enables the competitor to design
his work from the skeleton drawings, and
then to trace, without a moment's hesitation,
the various items in the quantities and
complete and price them.
GENERAL OUTLINE OF BILL OR
SCHEDULE
In " taking off," the most convenient
order to adopt will be : concrete in slabs,
reinforcement to ditto, centering to ditto,
concrete in beams, reinforcement in ditto,
centering to ditto. One room or apartment
should be completed before starting the
next. Walls and columns should be taken
off in similar order, being separated into
different floor levels, and their heights
stated. External and internal walls should
be kept separate.
The order in the bill at each level may
18
The sizes of the beams being undeter-
mined, the girth of the beam centering is
unknown to the surveyor, and must there-
fore be billed per foot run and described as
" extra materials and labour over slab
centering to casing to beams, — in. on
soffits, and — in. deep," blanks being left
for sizes ; the different heights for strutting
and bracing are kept separate.
Walls. — Concrete in walls should be
billed in yards super up to 12 in. in thickness,
the height of the wall from its floor level
and the height of floor level stated ; blanks
must be left in the description for the thick-
ness. The steel and centering will follow in
the usual order, the description given with
the concrete conveying the necessary in-
formation for computing the height of
hoisting the steel and the amount of strut-
ting to the centering.
Columns. — Concrete in columns is billed
in yards cube, and heights stated as in the
case of walls ; the word " cube " must be
inserted in the proper column, and a blank
left for the amount. The steel and centering
will follow in the usual order.
274
REINFORCED CONCRETE
Where it is decided to strengthen a wall
by means of piers, the concrete in them
should be billed in yards cube, the descrip-
tion stating that it is " in wall piers " ; the
steel and centering will follow as usual. The
size of the piers being undetermined, the
centering will have to be billed in feet run
as in the case of beams, and described as
" extra materials and labour over centering
to flat wall surface for casing to wall piers,
— in. wide on face, and — in. projection,"
blanks being left for the sizes. Where the
arrangement and number of piers have not
been settled it will also be necessary to leave
a blank for the amount.
Centering. — There is some diversity of
opinion as to the amount of detail in which
centering should be measured, some sur-
veyors contending that all extra labours,
such as the intersections of beams, should be
measured ; others measure only the super-
ficial quantity of centering and expect the
price to cover all incidental labours. There
can be no doubt that the more detailed
method is the only one that can be accur-
ately priced, and that the less detail supplied
the more speculative must be the pricing.
It is scarcely possible to measure the
centering for combined schemes and estimates
in as much detail as though the scheme were
already designed ; the most practical method
of measuring for this purpose is to keep the
items of obviously differing values separate,
and to include all labours which cannot be
measured in the descriptions, which should
be as definite as possible. To avoid undue
repetition, the method of measurement
should be clearly stated in the preamble by
clauses such as the following : " All items
of centering are to include the use and waste
of material, all labours in erecting and
removing, and all cutting and fitting at
angles and intersections. The measure-
ments have been taken net as erected."
It would serve no practical purpose to
attempt a list of centerings for various
positions classified according to their value,
but taking the simplest descriptions of two
classes, namely, to plain slabs and to
rectangular beams, it should be a simple
matter to separate and describe others
which may deviate from them in any
particular.
Raking, cutting, and waste should be
measured lineal. Where it is specified that
all internal and external angles are to be
finished with splays 2 in. wide (or as the
case may be), these items should be treated
as extras on the plain centering, and
described as " extra labour forming splayed
internal angle 2 in. wide, between wall and
slab centering," or " between beam and slab
centering," as the case may be, the principal
reason for the distinction being that the
centering at sides of beams is usually
specified to be removable without distu bing
the rest of the centering, in which case the
splay would be removable with the side of
the beam. It must not be forgotten that
in all cases of " extras " the original measure-
ment must include the whole surface ; for
example, in Fig. 478 the beam centering
would be measured the length by the girth
from A to A, and then a lineal dimen-
sion would be taken on both sides of the
beam and described as " extra on centering
to beam for forming_splayed internal angle
Fig. 478. — Gross Section through Beam
Centering
2 in. wide " ; to form the splay on the
external angle an angle fillet would be fixed
in the internal angle of the centering as
illustrated. This should be measured and
billed per foot run, and described as " splayed
angle fillet 2 in. wide on the splay, and
fixing in angles of beam centering to form
splay on the external angle of the concrete."
Other instances where items occur which
might with advantage be measured as
" extras " will be readily distinguished as
they present themselves.
In the case of beams with curved soffits,
the soffits will, of course, have to be measured
separately from the sides, but the two should
be added together and billed in one item,
and described as centering to beams, semi-
circular, segmental, elliptical, or as the case
may be, on soffits.
Not only the centering, but the concrete
and reinforcement should be separated into
classes of differing values, for, in addition to
convenience and the avoidance of repetition
in the descriptions due to the three items
following one another in each class, there is
the fact that the more complex forms will
entail more labour in placing and tumping
SPECIFICATIONS, QUANTITIES, ESTIMATING, ETC.
27:5
the concrete, and more elaborate reinforce-
ment, as well as more expensive centering.
A Detailed Bill Advisable. — Seeing that
labour is the great factor in the cost of
reinforced concrete, and that the amount of
,bour must vary considerably in the different
asses of work, it is scarcely logical to argue
that it is waste of time to differentiate
between them, and that the contractor can
price what is known as a " lumped " bill
with as much accuracy as he can a classified
or more detailed one. Although these
more or less rule-of-thumb methods may
have obtained in the past, there can be little
doubt that, with the more general adoption
of reinforced concrete construction, and the
keener competition as firms of general
building contractors in increasing numbers
become skilled in its use, every opportunity
will be taken of pricing more accurately, and
consequently in more detail ; in short, a
properly prepared bill of quantities for
reinforced work will be priced with as much
care as one for ordinary building work is at
present.
Such, then, are the general outlines of a
special bill — or, as it might be more correctly
styled, schedule — prepared before the re-
inforced concrete construction is actually
designed, for the purpose of obtaining
schemes and estimates for different systems.
Such a bill would undoubtedly ensure the
estimate being made in a certain amount of
detail and the items being more or less
correctly measured. The bill would neces-
sarily contain a considerable amount of repe-
tition— for instance, all the 4-in. slabs could
not find their way into one item ; all the
descriptions would be written by the quantity
surveyor, but of the actual quantities, some
would be supplied by him and some by the
contractor, thus entailing divided responsi-
bility, which would be most undesirable.
Moreover, it must be obvious, from what
has already been said, that in some cases,
more especially those of complete buildings
including walls, so little detail can be actually
known of the completed scheme that it
would be almost a farce to attempt to
measure anything beforehand.
An Alternative to the Bill. — On the
whole, it is open to question whether the
preparation of a bill under these conditions
will pay for the trouble involved, and
whether it is not more reasonable to supply
a specification containing a short schedule
of instructions, naming the items to be
measured and their method of measure-
ment, and attaching the condition that,
before any tender is definitely accepted, the
contractor will be required to submit his
tender on such schedule priced in detail.
To give an example of a schedule here,
or to enter more fully into the details of a
bill, would be premature, as full instructions
for the preparation of a bill from a complete
set of drawings and specification are given
later. From these, in conjunction with the
foregoing instructions, there should be no
difficulty in preparing a bill or schedule for
any given set of conditions.
"One Contractor" Jobs. — It will be
conceded that it is a great advantage to
have the whole of a big job carried out
by one contractor experienced in reinforced
concrete as well as in ordinary construction.
This avoids any division of the responsibility,
and considerable economy both of time and
money results. So many firms now are
prepared to do this that there is no difficulty
in the matter; several have "systems" of
their own, others are licensees of one or other
of the well-known firms of specialists, and
all would doubtless be prepared to carry
out any special system selected or a scheme
specially designed by the architect or his
engineer.
In all cases where the particular system
to be adopted has been decided beforehand
the work can, of course, be properly measured
and incorporated in a bill of quantities, and
if it is decided to invite only builders pre-
pared to do the whole of the work, then one
bill of quantities will suffice ; but the whole of
the reinforced concrete work, including the
steel and centering, should be billed as a
separate trade, and not split up and em-
bodied in the ordinary trades of " concretor,"
" carpenter and joiner," and " smith and
founder."
DESIGNS INDEPENDENT OF THE
SPECIALIST
The ideal method of dealing with re-
inforced concrete work is for the architect
to design his own or employ an independent
engineer to do so, being then free to use any
section of bar considered best for his par-
ticular purpose, or any of the patent bars
(provided the patentees would supply them),
or even to use two or more different bars in
the same building, choosing them for their
fitness for particular positions and uses.
Some, at any rate, of the patentees are
276
REINFORCED CONCRETE
already prepared to supply their steel under
these conditions, and if the system became
in any degree general all would be obliged
to fall into line.
The great obstacle to the adoption of such
a system is the fact that comparatively few
architects can design reinforced concrete
work — nor can they reasonably be expected
to do so, as it is purely engineering work,
foreign to the architect's nature ; why, how-
ever, an architect should not employ an
independent engineer, instead of placing
himself in the hands of commercial firms,
is not quite so easy to see, except for the
difficulty of explaining the situation to the
client, and inducing him to pay the fee, and
the reluctance of the architect to paying it
out of his own pocket. It cannot be too
strongly impressed on clients, however,
that the fee must be paid in any case,
either as such or as extra price of materials.
For example, take a case where reinforced
concrete is used for all floors, beams and
flats in which the total cost of the reinforced
work, exclusive of all ordinary builders'
work, is £1,000 ; assume that 450 cwt. of
steel (a reasonable average case) will be
employed. This steel would be supplied to
the builder at, say, £14 per ton (this is one
of the actual rates), to which he would have
to add the cost of hoisting and placing in
position and profit, an average of 25s. to
30s. per ton. Now, £8 per ton would be a
very good price for ordinary steel rods, or,
allowing for extra labour in bending and fixing
stirrups, say £9. The addition of £1 10s.
to this ought to be sufficient to cover the
extra value of any patent bar. Assuming,
then, for the sake of argument, £10 10s. as
a proper market price for the patent bar,
instead of £14, a surplus of £3 10s. per ton,
or a total of £78 15s., equivalent to 7| per
cent, on the total cost of the reinforced
concrete work including the centering, will
remain to pay for drawings, superintendence,
etc., by an independent engineer.
Reinforced concrete being as much a
mode of building construction as is a system
of brick walls with wood or steel joists, it
is difficult to believe that architects will
continue to ignore its study ; in any case,
there can be no reason why public depart-
ments should not see that a section of their
large technical staffs is skilled in this branch
of work, and so make themselves independent
of commercial firms as far as design and
calculations are concerned.
MEASURING
For the purpose of measuring and estimat-
ing, at any rate, it will be necessary to assume
a settled and complete design before the
system of measurement can be explained.
There are many instances, particularly,
perhaps, in England, where reinforced con-
crete construction is favoured for floors,
flats, roofs, partitions, columns, and beams,
where its use for external walls would not be
considered. In such cases there will be
several items of builders' work in connection
with the reinforced concrete; where there
are two contractors these items will be
executed by the building contractor, where
only one contractor is employed these items
must still be measured, but should be billed
in their proper place in the ordinary trades,
and not under the heading of " reinforced
concrete " ; these items include such as
"cutting or leaving chase -- in. deep for
edges of floor slabs and making good,"
" cutting or leaving recesses for ends of
reinforced concrete beams and padstones
(here state dimensions), and building in
in cement," etc., etc. ; similar items occur in
the case of ordinary concrete and steel joist
construction, and therefore they will not be
further referred to here, it being assumed
that the reader is already acquainted with
the modes of measurement of ordinary
builder's work.
As already stated, it is the practice of
specialists, when taking their own quantities,
to measure all concrete cube ; while this
practice is here deprecated, no undue or
unnecessary elaboration will be advocated,
but the builder should be given the oppor-
tunity of judging what class of work he is
estimating for. The fact of a contractor
pricing two or more separate items at the
same rate is not necessarily a criticism on
the work of the surveyor ; the contractor
should be given the opportunity of judging
their relative values for himself, and it is
not a good practice for the surveyor to do
it for him. Again, the contractor may not
have considered these items of equal value,
but may have priced them at an average
value after noting the ratio between the
quantities of the different descriptions, in
which case, assuming a variation on the
contract entailing an addition to the more
expensive and an omission on the cheaper
description, the contractor would, under the
usual terms of a building agreement, stand
to lose, as all variations would be valued at
SPECIFICATIONS, QUANTITIES, ESTIMATING, ETC. 277
the rate stated in the bills ; he has, however,
been provided with the requisite data to
enable him either to price separately, or to
form a true average, whereas, when items
of differing values are combined under one
description, the contractor has no option
but to affix an average rate, and a specula-
tive one at that.
There is every reason to suppose that
reasonably detailed quantities and increasing
competition will gradually lead to more
careful and accurate pricing.
Measuring Concrete. — The concrete in
walls and partitions up to 12 in. thick
should be measured super, also rafts up to
12 in. in thickness. Over this thickness,
these items should be measured cube ; con-
crete in foundations, columns, piers and
beams should also be measured cube. The
work should be kept separate at the different
levels ; floor levels will form convenient
divisions, but their height above ground
should be stated. The different classes of
work should be kept separate ; for example,
slabs on the slope (as to pitch roofs, etc.)
should be kept separate from horizontal
slabs. Note that slabs laid to a pitch of 45°
or less may be laid on single centering, but
that to steeper pitches the concrete will have
to be packed between centering; the two
classes of work must be kept separate as the
labour is different. The descriptions of the
two classes respectively would be somewhat
as follows : " 5 in. concrete slab laid sloping,"
and " 5 in. concrete slab sloping and packed
between centering." Arched beams should
be kept separate from straight ditto ;
circular walls and partitions separate from
straight ditto, and the radius stated.
Vaults should be measured super, stating
whether segmental, semicircular, etc. ; and
thickness given. Domes should be measured
super, stating the thickness, unless this
varies, in which case it will be necessary to
measure them cube, stating that it is in
domes, the thickness varying from — to —
(the superficial area of a dome is the same
as that of the circular portion of a cylinder
exactly enclosing it). Concrete in curved
ribs should be measured cube, and described
as in semicircular or segmental ribs, or as
the case may be, and the radius stated. The
extra concrete forming internal splays to
angles may be measured either per foot run
and sizes given, or per foot cube and de-
scribed ; the latter method is preferable, as
the concrete is only worth the same rate as
that in the wall or beam to which it is
attached, the extra labour being on the
centering.
In measuring concrete, no deduction is
made for the space occupied by the re-
inforcement.
Small Holes. — Small holes required to
be left in the concrete for the passage of
pipes, etc., should be numbered, and their
size given, the wood plugs or drums fixed
on the centering being included in the
description of the holes.
Every precaution should be taken against
having to cut into the concrete after it is
laid. It is better and cheaper to bed bolts
with back plates in the concrete, and leave
them projecting the requisite distance for
fixing. These bolts have to be fixed in
position in the centering. The bolts them-
selves are usually provided under the
building contract, but they must be numbered
for the labour of "bedding in." To cut a
hole for a bolt or any other purpose in
reinforced concrete is an expensive item
of labour, the material being so very hard.
Measuring Steel Reinforcement. — The
steel reinforcement is measured per foot
run and billed in cwt., qr. and lb., billing to
the nearest 1 lb. ; for example, 54 lb. would
be billed as 2 qr. ; 60 lb. as 2 qr. 1 lb. ; 59 lb.
as 2 qr. ; 73 lb. as 2 qr. 14 lb., and so on.
Care must be taken in measuring the steel
to add for all laps and, unless these are
clearly shown on the details, the necessary
information must be obtained from the
designer. The weights of the bars per foot
run may, in the case of standard bars with
rigidly attached web members, be obtained
from the trade catalogues ; in the case of
loose stirrups, these will have to be separ-
ately measured and weighted out ; they
should be measured from an actual sample
bar, in order to obtain the correct length
around the bends or twists ; they may then
be weighted out from tables of weights or
some sample 12 in. pieces actually weighted ;
in the case of standard stirrups of any
particular system the weights may either
be obtained from the trade catalogue or by
actual weighing ; stirrups may either be
measured per foot run or numbered, which-
ever lends itself the more readily to the
subsequent weighting out ; in the case of
bars and rods it is only necessary to remember
the weight per foot super of 1 in. plate, and
the weight per foot run of 1 in. diameter
rods to obtain the required weight of any
278
REINFORCED CONCRETE
other sizes ; for example, the weight of
1 in. by 1 in. bar per foot run will be r\th
that of 1 in. plate per foot super ; that of
1 in. by 2 in. £th ; that of 1 in. by ^ in.
Jjth and so on ; in the case of rods, seeing
that the areas of circles vary as the squares
of their diameters, it will only be necessary
to multiply the weight of 1 in. rod by the
square of the diameter of any other size to
find its weight ; thus the weight of a £ in.
diameter rod will be £th that of 1 in. ;
that of a 2 in. rod four times that of 1 in. ;
and so on.
The steel reinforcement should be kept
separate for the different positions and
classes of work. In a job where bars with
fixed web members, such as the Kahn, are
used and it is necessary in certain portions of
the work to use loose stirrups, this work
should be billed separately and properly
described, there being considerably more
labour in placing steel bars and loose stirrups
than in placing a bar having fixed stirrups.
Measuring Centering. — Centering is not
only a very important item, but also a
very costly one. Moreover, the value of the
different classes of work varies so consider-
ably and extra labour forms such an appre-
ciable item that it should be measured as
carefully and in as much detail as the cir-
cumstances of the case permit. There have
probably been more surprises at the end of
a job due to the speculative nature of the
items of centering than to any other cause.
" Use and Waste." — The concrete and
the steel can be priced with reasonable
accuracy, even when they are not taken in
as much detail as they should be, but not
so the centering. That must be, unless
carefully measured in detail, a very specu-
lative item. The materials are charged as
use and waste only, but the centering has
to be very carefully erected, and labour
enters largely into the cost. No definite
proportion can be fixed for " use and
waste," as this differs so widely in different
classes of work ; in the case of centering to
a plain floor slab, there will be a certain
amount of waste in cutting to the required
lengths, but nearly the whole can be re-used
by further cutting on another job, or possibly
in another position on the same job. The
larger the areas of the slabs the less waste
will there be. The labour, also, in erecting
slab centering is comparatively simple.
But in beam centering there will be much
more cutting and \vaste and more labour.
As the dimensions and span of beams
decrease the waste and labour increase ;
in elaborate works, much of the centering
will scarcely be worth the cost of carting
from the site, and though still technically
described as " use and waste," its full price
ought to be included in the estimate.
First-class Centering. — Headers will
.know that a perfectly true and finished
surface on walling and mouldings, and even
on carving (so called), provided there is no
undercutting, can be left direct from the
centering or moulds without any subsequent
rendering ; but for this purpose the false-
work must be wrought on one side and
jointed up as a first-class piece of joinery,
the carved work being cut in reverse in
wood blocks, which are inserted in their
proper position in the wall centering. Such
centering will, of course, be far more ex-
pensive than the ordinary style, and more
labour and care will be entailed in packing
the concrete.
Centering required to be planed and
covered with a wash of lime and clay to
leave a smooth surface on the concrete may
be kept separate and described, but it is
preferable to measure the planing and coat-
ing separately.
In order to form a key for plastered ceil-
ings, it is sometimes specified that a layer
of fine breeze is to be spread on the slab
centering before filling in the concrete, this
should be measured separately and billed
in yards super.
Centering for a scheme already designed
can and therefore should be measured in
more detail than when measuring for com-
bined schemes and estimates.
" Centering " Clauses in Preamble. — In
the case of ordinary building works, it will
often be impossible to separately measure
the bearers, struts, and braces to the center-
ing, as it is unusual to supply drawings for
this, but any clauses in the specification
governing the design and erection of the
centering should find their way into the
preamble. In the absence of any more
definite specification, some such clause as
the following should be inserted in the
preamble : " The centering to be well
strutted and braced and made perfectly
rigid and strong enough to sustain the weight
of the slabs, etc., as a liquid mass, without
deflection." This clause, used alone, throws
the responsibility for the efficiency of the
centering on the contractor. To avoid
SPECIFICATIONS, QUANTITIES, ESTIMATING, ETC.
279
repetition in the descriptions, the following
clause may be inserted in the preamble :
" The centering to include all bearers, struts,
braces, etc., wedging, and all ironwork
required unless otherwise stated " ; it will
then only remain to state the purpose of
the centering and the height of the strutting
in the description.
Specially Designed Centering. — Where,
as in the case of heavy engineering works, or
from the special nature of the work, the
engineer or architect elects to design his
own centering it will be advisable to mea-
sure it in detail as designed, otherwise it
would be difficult to enforce the work being
carried out to drawings without incurring
an extra. When measuring strutting and
bracing in detail, it is advisable to measure
the timbers lineal and to state their scant-
lings, rather than to measure them cube
as in the case of ordinary carpentry. The
architect's permission should be obtained
to insert a clause in the preamble to the
effect that the scantlings given for the
timbers to the centering are minimum
ecantlings, but that the contractor is at
liberty to use other scantlings of equal or
greater strength with the architect's ap-
proval, provided no extra cost is incurred.
The object of this clause is to enable the con-
tractor to use such scantlings as he may have
in stock or as are most easily procurable, so
as not to increase unnecessarily the cost of
the temporary work.
Centering should be measured super
(except as hereafter stated) and described,
and the height of the strutting given ; that
at different levels and to different descrip-
tions of work should be kept separate.
Openings. — All openings should be de-'
ducted where it is practicable and convenient
to omit the centering, but in the case of
small openings, particularly in horizontal
slabs, it will often be cheaper to run the
centering through than to provide extra
supports or trim the bearers round the
opening, in which case it should not be
deducted, but the vertical form to edges
should be erected on it. It will be necessary
to keep some openings clear for access and
working purposes ; staircase and lift open-
ings are usually the most convenient for this
purpose in the horizontal slabs, and door
openings in the walls. In all cases where
the centering is deducted, an item should
be taken of "cutting and waste around
openings " ; with regard to doors and windows
these may be numbered and the average
size of openings given, stating whether one
or both sides are measured, or the dimension
may be taken run, and a similar dimension
taken on slabs around openings for stairs,
skylights, etc., etc. Keep the centering to
edges of openings in narrow widths separate.
The vertical centering around openings
in slabs should be described as such, and
measured super ; where 9 in. or less in
height, it may be measured run and the
height given.
Centering to Walls. — Vertical centering
to walls and partitions should be measured
to both sides ; it is not a good practice to
measure one side only, stating in the descrip-
tion that " one side is measured for two,"
because the amount often varies considerably
on the two sides of a wall, especially if it be
an external wall with cross walls abutting
against one side only.
Centering to external walls should be
kept separate. There are not quite the
same facilities for getting at both sides of
the centering and the upper stories can only
be strutted from one side.
Moulds for Architectural Features. —
Where centering has to be formed into
moulds for architectural features it should
be kept separate, as more labour and waste
is involved. It may either be separately
measured and the vertical wall centering
behind it deducted or measured as extra
on the vertical centering ; in most instances,
the latter method is to be preferred. A
typical case is that of a shaped core for a
moulded cornice to be finished in cement,
the simplest and probably the best method
of measuring which is per foot run as extra
material and labour over vertical wall
centering, stating in the description both
the height and the girth of the centering,
so as to enable the estimator to ascertain
exactly the extra net amount of material
involved and to judge the extra labour and
waste. By making it a lineal dimension,
there is also the added advantage that the
mitres, etc., may be "written short" in
the bill. It should be particularly noted
that this item is described as extra materials
and labour; this makes it perfectly clear
that no deduction has been made from the
vertical centering for that portion imme-
diately behind the cornice ; nor, on the
other hand, has any material been previously
measured to cover the projection.
Centering to Inclined Slabs.— Centering
280
REINFORCED CONCRETE
to inclined slabs (as for pitched roofs, etc.)
should be kept separate from horizontal
slabs. Centering to slabs inclined at a
steep pitch (more than 45°) requiring
centering both sides should be kept separate ;
in this case the description must be carefully
worded to avoid any ambiguity, thus :
" Centering to both sides of inclined slabs
(both sides measured)."
Centering to Beams. — Centering to
beams should be measured super and the
different descriptions of beams kept separate ;
as previously explained, the super is obtained
by taking the length by the girth of the
centering on the outside of the sheeting.
Internal and external chamfers on beams,
etc., should be measured as has been pre-
viously explained on p. 274. Intersections
of beam centering with beam and of beam
centering with wall or padstone centering
should be measured where the end of one
beam abuts on another beam, padstone or
concrete wall. A piece has to be accurately
cut out of the centering of one of the beams
or the wall, and the end of the abutting cen-
tering accurately and closely fitted thereto.
The most comprehensive way of measuring
these items is to number and describe them,
stating the girth of the centering of the
abutting beam ; see example_ below : —
cleanly from the concrete should be either
included in the description or separately
measured.
Moulds for Cornices, etc. — The moulds
for cornices, string courses, etc., should be
measured lineal as extra on the vertical
centering, internal and external mitres, and
mitred and returned ends numbered and kept
separate. Any ends fitted against plain
centering should also be numbered, such as
an internal angle where the cornice is not
returned, in which case the vertical centering
would have to be cut and scribed to the
outside of the cornice moulds ; these
numbered items should be " written short "
in the bill. Blocks for carving should be
numbered and described, and the size stated.
The actual carving of the blocks must be
treated as a labour item, and is best provided
for by a provisional sum, unless it is of a
geometrical or other simple nature such as
could be dealt with by a description or
sketch. The typical example on the next
page will serve to illustrate the descriptions
and method of billing. The reference letter
" A," in addition to the purpose it serves
in this section of the bill, will be found even
more useful in saving repetition in the
descriptions of similar centering at other
levels.
No.
Intersections of beam centering 27 in. \
girth J
-Ditto ditto with wall centering \
30 in. girth J
Ditto ditto with padstone cen- \
tering 24 in. girth /
8.
Centering to Padstones. — This should
be measured super and kept separate.
Centering to Columns and Piers. —
This should be measured super, each descrip-
tion, such as " to columns," " to piers," and
" to wall piers," etc., being kept separate and
properly detailed. They should be billed
in their proper order of floor levels, and their
heights stated. All junctions of columns or
piers with slab centering should be numbered
and their girth stated.
Centering to Produce Fine Finish. —
Centering, such as that described on p. 278
to produce a finished face on the concrete
direct from the moulds, should of course be
kept separate and carefully described ; any
coating of the centering or other device
to allow of the centering coming away
ABSTRACTING
In many cases an abstract will scarcely
be necessary, but in others it may be found
more convenient to have one. It is assumed
that the reader is acquainted with the
methods of measuring, abstracting and
billing ordinary builder's work, and it will
therefore be unnecessary to do more here
than to suggest a convenient form for
abstracting reinforced concrete work. A
perfect abstract should present the items
in the correct order for billing ; it has been
stated before that it is the custom of special-
ists to group the three main items of concrete,
steel, and centering in sections according to
levels. Floor levels are adopted as far as
possible, as being convenient divisions.
This method, although preventing all the
SPECIFICATIONS, QUANTITIES, ESTIMATING, ETC. 281
Yds.
200
Ft.
100
In.
Sup.
Run
Centering to outer face of external 9-in."
walls from first to second floor levels, the
boards tongued and grooved and carefully
jointed and clamped, the inner face to be
planed to a true and even surface, so as to
leave no marks of this jointing in the surface
of the concrete
Extra materials and labour over vertical''
centering as " A " for centering to moulded
cornice 24 in. girth 12 in. high as sketch,
carefully jointed to the vertical centering ;
all joints to be close and to be grooved
and tongued, and the internal surface of
the mould clean and true, so as to leave
no markings on the concrete
No. 5 Internal mitre
No. 10 External mitre
No. 2 Mitred and returned ends
No. 1 End fitted to vertical centering andl
the vertical centering cut and scribed /
No. 3 3 in. blocks, size 2 ft. 0 in. by 3 ft. 6 in.,
carefully jointed up and prepared for
carver, and cutting out for and inserting
and carefully jointing same to vertical
centering as " A." (Vertical centering not
deducted.)
s. d
items of either material being billed con-
secutively, places each in its proper rela-
tion to the others in a very comprehensive
manner, and is to be commended for the
main items. It is advisable for the sake of
clearness to apply the system to the main
items only, and to bill all incidental items
and labours under a further sub-heading of
" Sundries," which will comprise more
particularly such items as are not affected
by the level at which they are executed.
The setting out of a portion of an imag-
inary abstract on pp. 282 and 283 will illus-
trate what is meant.
BILLING
When one contractor is to carry out the
whole of the work, including the reinforced
concrete, all preliminary works, whether in
connection with the general or the reinforced
work, would find their way into the one
preliminary bill. Many of the items, such,
for example, as " water," would be covered
by the usual clause, and would obviously
apply to the whole of the work.
When the reinforced concrete work is to
be made a separate contract, it must have
its own separate bill of preliminary works,
and it is often necessary to refer from one
contract to the other in order to make it
clear to each contractor exactly how much
is expected of him and how much the other
will have to do, as, for instance, in the
provision of water, scaffolding, hoisting
tackle, etc., previously referred to (see
p. 271).
The Preamble. — Having written the
" Preliminary Bill," the " Bill for Reinforced
Concrete Construction " must in either case
start with a " Preamble," most of which
will be taken direct from the " specification."
Judiciously worded clauses in the preamble
will often save repetition of descriptions in
the bill. The system of reinforcement to
be adopted should be stated in the preamble,
as the amount of labour in placing the steel
varies with the different systems. Where
it is found necessary in a few positions to
adopt a special system of reinforcement
the words " except where otherwise stated "
should be added after the description of
the system, and the special description of
the exceptions given opposite the items. The
nature of the clauses forming the preamble
is indicated when treating of specifications,
but clauses which relate solely to design
will, of course, be quite superfluous. The
following example will serve as a guide, but
282
REINFORCED CONCRETE
«*/- q. £ &<*£
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it must be added to, and altered as
necessary to meet the special re-
quirements of each case : —
No. 2 — REINFORCED CONCRETE
The reinforced concrete construction
will be on the — system (except where
otherwise stated) and the whole of the
steel is to be obtained from
The materials including the cement are
to be tested as hereafter specified and
the cost of all such tests whether ac-
cepted or not are to be borne by the
Builder.
The cement to be portland from an
approved manufacturer to be slow setting
in accordance with the British Standard
specification and capable of meeting the
tests therein specified.
The sand to be river or pit sand
coarse and gritty with grains of various
sizes and perfectly clean.
The aggregate to be composed of clean
gravel carefully selected and screened to
pass a J-in. mesh.
The concrete to be 'composed as fol-
lows : —
Gravel 27 cub. ft.
Sand 13J cub. ft.
Cement 5i cwt.
to be thoroughly mixed in a mechanical mixer
approved by the Architect.
Steel must have an ultimate tensile strength
of not less than 28 tons or more than 32 tons
per square inch with a contraction of area at
fracture of not less than 40 per cent, the fracture
not to show more than 10 per cent, of granular.
Samples of the steel bars must be able to stand
being bent cold until the ends close over a bar
of one and a half times the diameter of that of
the bar to be tested.
The Architect or his representative will select
and stamp such samples as he may consider
desirable at any time either at the steel works
or on the site and the Builder will be required
to send such samples carriage paid to Messrs.
or other approved testing and
experimenting works and the reports obtained
shall be accepted by the Builder.
No welding of the rods or bars will be
permitted. They are to be bent
cold.
The steel is to be brought upon
£57"^ the site as required and no steel is
to be fixed in a dirty condition.
The steel reinforcement to slabs
is to be protected by a minimum thickness of
J in. and that to beams by a minimum thick-
of U in. c
c.
. .
.
/o- a '
SPECIFICATIONS, QUANTITIES, ESTIMATING, ETC. 283
total area) as the Architect
shall direct in the following
manner : A dead load equal
to the calculated super load
plus 50 per cent, shall be
evenly distributed over such
parts of the floor etc. as are
to be tested and the deflec-
tion shall in no case exceed
sinjth part of the span of the
slab or beam under test. The
extra load of 50 per cent,
shall be removed immediately
Q after the reading and the cal-
Vy*^/^. r~-*_ cuiate(i \oa^ snan be left for
at least 12 hours in position
The workmanship throughout is to be of the when it shall be removed ; fresh readings taken
very best description and to the entire satisfac- 24 hours later shall record no appreciable set.
tion of the Architect who shall have full power No cracking or scaling of the concrete shall be
to have any parts of the work which are in his developed under any test. ft
opinion unsatisfactory re-executed to his satis- In the event of the aforesaid deflection being
faction and the Builder is to bear the expense. exceeded or an appreciable permanent set being
The centering to be true and rigid and of found to remain or of the results of the test of
sufficient strength easily to carry the dead weight any portion being otherwise unsatisfactory to the
of the construction as a liquid without deflection ; Architect that portion at the expiry ofjsix
the posts or struts to be
properly braced in both
directions. All joints to
be tight so as to prevent
leakage. The centering
to be so designed that
the sides of the beams
can be taken down first
then the slab centering
complete.
No centering is to be
removed in less than four
weeks' tune after con-
creting or until the con-
crete has thoroughly set
and has aged to give it
sufficient strength to
carry its own weight and
in addition whatever live
load is liable to come on
the work during the
course of construction.
Beams are to remain
supported at least two
weeks after all other false-
work has been removed.
Columns shall not be
given their full loading in
less than five weeks
after concreting. In no
case shall any centering
be removed without the
consent of the Architect.
The Builder shall at
his own expense test such
parts of the work (not .
being more in the aggre- *~ ' a*y£* J
gate than 5 per cent, of the
284
REINFORCED CONCRETE
weeks from the date of the first test shall be
again tested in a similar manner. Should
further deflection take place during the second
test or further permanent set be found to exist
or should the work in the opinion of the Architect
still prove unsatisfactory then and in such case
the Architect shall have full power to require
the removal of the unsatisfactory portion and
its reconstruction in a satisfactory manner at
the expense of the Builder.
The test load shall not be applied to any part
of the works until a period of two calendar months
shall have elapsed since the deposit of the
concrete in situ.
No cranes or other machinery shall be placed
upon the reinforced concrete work and the
extent to which the beams floors etc. may be
loaded with materials during the progress of the
work shall be subject to the approval of the
Architect.
The price of the steel is to include for all
labour in bending and fixing.
Other Clauses in the Preamble.—
The preamble to a bill should include all
clauses from the specification which may in
any degree affect the prices of the measured
items in the bill, as well as any general
descriptions of material or labour which may
save repetition in the descriptions of the
measured items. It is not usual to include
in the preamble any items requiring a price
to be attached and for this reason in a bill
of quantities for ordinary builder's work
it has been the custom to place any clauses
for testing work in situ (as, for example, in
the case of the plumber's bill) at the end of
the trade and to start the item with the
words " Allow for testing, etc., etc."
The Testing Clauses.— The testing
clauses in a reinforced concrete bill are not
at present usually priced, but in a large job
they must carry an appreciable money
value which cannot reasonably be spread
over the various items in the bill ; the
proper method of dealing with this item
would be to put the clauses describing the
method of testing in the preamble as in the
foregoing example, and to insert as the last
measured item in the bill the following :
" Allow for testing the reinforced concrete
construction as described." The contractor
will then be at liberty to price this item or
not, as he thinks fit. As competition gets
keener, and with more detailed bills to help
him in pricing each item as closely as possible,
it may well be that he will embrace the
opportunity of attaching a separate price
to the testing.
Examples of Billing Measured Items.
— It is always advisable in billing to adopt
some definite order, but it is by no means
essential to adopt that suggested in the
section on " Abstracting." The descrip-
tions, however, must be absolutely clear
and definite so as to leave no doubt in the
mind of the estimator as to what is meant,
and they should also be as concise as is
consistent with this rule. The example
below illustrates the suggested system of
billing the measured items following imme-
diately after the preamble.
Then proceed to bill the next floor in a
similar manner, and so on, floor after floor,
finishing with the roof or flat. Having
billed the whole of the principal items in
this manner, proceed to bill the incidental
and labour items under separate sub-head-
ings of " Sundries in Reinforced Concrete,"
and " Sundries in Centering" (see p. 215).
These two examples illustrate the prin-
ciples to be followed in billing, and from
/to
^ cjL^u^-^f *° *» <^*6^v 7
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S*—8-e
SPECIFICATIONS, QUANTITIES, ESTIMATING, ETC.
285
far
*..
be assessed after considerable experience
and close observation ; and there are no
reliable data at present available for general
use.
In the case of a standard bar with fixed
shear members, the relative value of fixing
in different positions will vary comparatively
little, but the value of hoisting will, of course,
vary with the height.
The centering, if properly measured, can
be priced with reasonable accuracy, and a
little experience should enable the estimator
to judge the relative proportion to be adopted
for use and waste in different positions.
Prices for Concrete. — Concrete com-
posed of 5£ cwt. of cement, 13| cub. ft. of
sand, and 27 cub. ft. of gravel, is equivalent
to a proportion of 1 : 2 and 4 by measure ;
a cubic yard of this finished concrete requires
nearly 1£ cubic yards of dry materials,
say 40 cub. ft., which, divided up into the
given proportions (1 : 2 and 4), would give
— cub. ft. of cement, — = —
7 7
sand and
them the student should be able to bill and
describe any other items, always bearing in
mind that the object of a bill of quantities
is to convey to the mind of the estimator in
the most concise manner possible exactly
what the builder has to do and how much
of it.
A schedule of rates to be paid in day-
work similar to that described on p. 271
should be given at the end of the bill.
PRICING
With regard to pricing the estimate, it is
only necessary to examine a dozen tenders
for the same work to see that pricing is not
an exact science, and therefore any prices
which may be given can only be approximate
or average prices. The method of arriving
at the total value of each item is by analysis,
making due allowance for any special
circumstances peculiar to each job. The
total quantity of reinforced concrete will
determine whether it will be more economical
to mix the concrete by hand or to employ a
mechanical mixer (if this has not been
already settled by the architect).
The value of a cubic yard of concrete of
any given composition will remain more or
less constant, and may be readily ascertained,
but the relative value of hoisting, depositing
and tamping in various positions can only
cub. ft. of
cub. ft. of gravel, = 5f
cub. ft. of cement at 90 Ib. per cub. ft.
weighs 514 Ib. ; these data, together with
the prime cost of the materials, including
cartage to the site and the cost of mixing,
will give the prime cost value of a cubic yard
of concrete. The cost of water will be
covered by the item for water in the pre-
liminary bill if this item has been priced ;
if, however, the contractor has only attached
a price to this item to cover fees and the
cost of the temporary plumbing, the concrete
286
REINFORCED CONCRETE
will have to bear its proportion of the water
charges. In cases where the water is paid
for by meter, about 30 gallons should be
allowed per cubic yard of concrete. The
cost of mixing will depend on whether it is
mixed by hand or by machine. The cost of
labour in hoisting, placing, and tamping
will depend upon the height of the hoisting,
and whether the work is in walls, slabs, slop-
ing roofs, beams, etc., etc. To the cost of
the concrete in situ so obtained must be
added a percentage to cover builder's profit
and establishment charges. The following
example, to which average London prices
have been attached, will explain the principle
of analysis of a cubic yard of concrete : —
s. d.
Portland cement, 514 Ib. at 30s. per
cement ton delivered . 7 0
Sand, 11 f cub. ft. at 6s. 6d. per
cub. yd. ... 29
Gravel, 22 £ cub. ft. at 7s. per cub. yd. 5 11
Labourer measuring and mixing,
3 hrs. at 7d. . . . J. 9
Prime cost of a cub. yd. of concrete
exclusive of hoisting and placing 17 5
To this price must be added the value of
hoisting, placing, and tamping in various
positions, and a percentage to cover profit
and establishment charges. For example,
a yard super of floor slab 6 in. thick will take
•Jth of a cubic yard of concrete, and the
analysis will be as follows : —
s. d.
•Jth of a cubic yard of concrete as
above 2 11
Concretor placing and tamping, and
Labourer hoisting not exceeding
20 ft., and attending upon con-
creter 4
~3 3
Profit and establishment charges
15 per cent. 6
In a similar manner may be found the
price of a floor slab of any other thickness
or at any height, or the price of any item of
concrete, whether billed in yards super or
yards cube ; the prime cost of a yard cube,
as given above, is the basis in every case.
The above analysis is for hand-mixed
concrete. If a machine mixer is used, the
cost of the motive power per hour, and the
wages of the attendants must be ascertained,
to which must be added a proportion of the
cost of installing the plant, and a percentage
on the capital cost of same to cover wear and
tear ; from the cost of running the mixer
thus found, and the amount of concrete
which it is capable of turning out per hour,
the cost of mixing per yard cube is readily
ascertained.
Prices for Steel Reinforcement.—
The cost of steel for the reinforcement,
delivered on the site, will depend upon the
bar used, and the distance for carriage and
cartage ; to the cost of the steel as obtained
from the makers, and carriage, must be
added the cost of labour in hoisting and
placing according to height and position ;
the amount of such labour will vary with
the system adopted, from the minimum
amount in the case of bars with fixed shear
members to the maximum in the case of
ordinary rods where the shear members have
to be bent to pattern and fixed in position.
The Kahn trussed bar is an example of a
bar with shear members in one piece with
the bar.
The bar is rolled with side flanges which
are machine cut ready to be turned up at
an angle of 45° to form the shear members ;
the price of the patent bar is about £14
per ton, or 14s. per cwt., at London station,
to which must be added the cost of cartage
to site, hoisting, fixing in position, and profit
and establishment charges. The total cost
of the bar, in London, fixed in position,
averages for floor slabs and rectangular
beams supporting same 15s. 3d. per cwt. ;
for padstones at ends of beams, 15s. 6d. ;
for ground floor walls 15s.
Prices for Centering. — The cost of
materials for centering remaining constant,
the value of centering in any given position
will be governed by the proportion to be
allowed for use and waste, the amount of
labour involved in that particular position,
and the quantity of bearers, struts, and
braces per unit of area. Centering to floor
slabs 12 ft. high will require, on an average,
about 18 cub. ft. of timber for bearers,
posts and braces, and the cost of materials
for a square of centering will be as follows :
s. d.
1 square of 1 in. boarding at 13s. per
square 13 0
18 cub. ft. of timber in bearers, posts,
etc., at Is. 7d. per cub. ft. 28 6
Cost per square 41 6
which is equivalent to 3s. 9d. per yard super.
SPECIFICATIONS, QUANTITIES, ESTIMATING, ETC.
287
It is usual to charge one-third the cost of
materials for centering of this description
for use and waste, on the assumption that
they may be used three times before being
discarded, each re-use entailing, of course,
further cutting and waste to suit the several
positions ; on this assumption the cost of a
yard super of centering to floor slabs 12 ft.
high would be made up as follows
d.
^rd of 3s. 9d. for use and waste of
materials
Cost of labour and nails erecting and
removing at 7s. 6d. per square
1 11
Add 15 per cent, for profit and
establishment charges 3^
Cost of centering and strutting per
yard super 2 2£
Where heights vary the timbering will not
vary in the same ratio, for the reason that,
while the joists and horizontal timbers'
remain the same, the posts and braces will
vary both in length and scantling. The
amount of timber making up the 18 cub. ft.
per square of centering in the foregoing
example may be split up nearly into 7 cub. ft.
in horizontal members and short battens,
and 11 cub. ft. in posts and braces, costing
respectively lls. Id. and 17s. 5d. per square,
which is equivalent to Is. and Is. 6d. per
yard super, ^rd of which for use and waste
amounts to 4d. and 6d. respectively. The
latter prices may, within limits, be taken
for all practical purposes to vary directly
as the height, and so charged as £d. per foot of
height ; any considerable increase in height
will, however, necessitate an increase in
scantlings, and will, therefore, upset this
ratio. Beam centering or centering for any
other purpose may be worked out in a
similar manner ; the relative amount of
labour will vary in each case, so will the
proportion to be adopted for use and waste,
as previously explained, and the value of these
these two factors can only be ascertained
by experience. They can never be stated
with mathematical accuracy, because they
will be found to vary more or less on every
job, and the amount of labour particularly
will vary under different foremen. It is a
good practice to take every opportunity of
noting on each job for future reference the
relative amount of waste and cost of labour
in different positions.
It is a good exercise for the student to
make drawings, however rough, of centering
and strutting for different positions and
heights, and to price them in detail in
accordance with the foregoing principles.
Approximate or Average Prices. —
As before stated, the prices given must be
taken as approximate or average only.
Prices of material and labour are continually
changing and competitive prices vary still
more, according to the state of the building
trade. Builders, when trade is slack, are
willing to cut their profit to almost vanishing
point in order to keep their works going.
From two recently priced bills, it is found
that in one concrete varies from 21s. 9d. to
23s. 6d. per yard cube, and centering to
floor slabs is priced at a uniform rate
throughout of 2s. per yard, which is about
22s. 2|d. per square ; while in the other
concrete is priced at a uniform rate of
28s. 6d. per yard cube, and centering to floor
slabs at 17s. per square for a height of
12 ft., and 17s. 6d. for a height of 14 ft. It
will be obvious from this one instance that
any list of prices can only be taken as the
merest guide. Moreover, the object here
has been not to compile a price book, but
to state the methods of measuring reinforced
concrete work and to enunciate the principles
on which the various prices are built up.
In all new methods of construction it is
essential that the quantity surveyor should
keep himself acquainted with the broad
principles governing their design and execu-
tion, in order that he may measure them
intelligently and properly separate the
different values. The estimator must keep
himself posted in the current prices of
materials, and by keen observation gradually
build up data of the amount of labour
entailed in the different items. He must
also make himself acquainted with the cost
of installation and removal of plant, per-
centage for wear and tear, etc., either for
insertion in the preliminary bill or appor-
tionment to the various items affected, as the
case may be.
Arches and Bridges
DESIGN OF ARCHES AND BRIDGES
THERE is probably no branch of reinforced
concrete work that shows the advantage
of this material more fully than that of
arches and bridges, as it lends itself readily
to scientific design and architectural treat-
ment, as will be seen upon referring to the
many examples illustrated in this chapter.
Some of the bridges that have been built,
especially those of the open spandrel type,
are really excellent in design and worthy
of close study, being graceful and light,
while at the same time satisfying the eye
that they are sufficiently strong. Apart from
being far more beautiful than steel bridges,
the cost of maintenance with a concrete
bridge is practically nil, and this is an
Fig. 479.— Horizontal Thrust at Grown of
Arch
the abutments on either side ; this is due
to one half of the arch bearing upon the
other half, as shown in Fig. 479, while the
three forces acting on one half of the arch
are shown in Fig. 480. The weight, how-
ever, will not be concentrated at one point
in this manner, but will be distributed over
the extrados of the arch, except in the case
of live loads, which will be concentrated at
every point in turn according to the progress
of the load. It is usual when this is the
case to calculate the arch to resist the load
when it is at the most damaging point, and
this will usually occur at one-fourth of the
important consideration when comparing
the two materials.
In order that the main principles which
govern the design of arches may be under-
stood, it will be advisable to give .a few notes
on the theory of arches in general, as the
same principles will apply, regardless of
the material employed. It is obvious that
the arch is kept in equilibrium by the action
of three forces, namely (1) the load or
weight, which is equal to the weight of the
arch itself and the superimposed load ; (2)
the reaction at the support or abutment ; and
(3) the thrust at the crown. In a sym-
metrical arch which is symmetrically loaded
it is clear that the same line of thrust will
occur on each side of the centre, and the
thrust at the crown must therefore be hori-
zontal and acting from the centre toward
288
Fig. 480. — Three Forces acting on Half of
Arch
span. In the case of distributed loads,
however, the horizontal thrust at the crown
and the weight acting vertically at the
various points will cause an oblique line of
thrust according to their respective amounts.
This line of thrust in arches built up of
separate voussoirs should always pass
through the middle third of the arch ring,
but in the case of reinforced concrete this
is not strictly necessary, as steel can be
introduced to take tensional stress if the
same should be found to be present upon
calculation. The line of thrust should not,
however, fall outside the arch ring.
This line of thrust is found graphically,
as illustrated in Fig. 481. Here one half
of the arch is shown, and it will be seen
that the arch has been divided into equal
portions and the forces acting downward
ARCHES AND BRIDGES
289
represent the load on each portion. The
thrust at the crown is produced to meet
w1, which represents the weight on the first
portion next the crown, and the two are
combined to give the resultant Rr This
resultant is then again produced to meet
the line of w2, and these are combined to
give R2 and so on until the abutment is met,
thus giving a line of thrust or resistance
throughout the whole arch. If the arch
were divided up into an infinite number of
layers, then the line of thrust would become
a curve, and in this particular instance a
parabolic curve could be drawn without
any other working, and this would be cor-
rect. There is one point, however, to be
borne in mind, and that is the fact that
there is no definite proof as to the point
through which the horizontal thrust at the
crown will pass, and the same remark
applies to the thrust at the skewbacks, and
although it is taken somewhere within the
middle third of the ring, there is no law
whereby the points can be determined with
accuracy, and the line of thrust will be
dependent on the points selected. The usual
method in practice is to take the horizontal
thrust at the crown as acting through the
upper edge of the middle third of the ring,
and at the skewback to take the lower edge
of the middle third. There being three
forces acting on the arch to produce equili-
brium, it will be clear that the value of two
if a live load exists this should be considered,
and, if necessary, substituted by equivalent
dead load, in order that the centre of gravity
of the whole mass may be found. When the
centre of gravity has been ascertained and
Fig. 482. — Forces Acting on Arch to Produce
Equilibrium
put on the diagram as shown at c G, a ver-
tical line is drawn through this point, indi-
cating the downward tendency of the
weight. The line of horizontal thrust is then
drawn through the selected point within the
middle third at the crown, and produced
until it cuts the vertical line drawn through
Fig. 481. — Finding Line of Thrust in Arch
of these will be dependent upon the other the centre of gravity as shown at A. The
one, which will be the weight. The value of selected point within^ the middle third at
the maximum thrust will require to be the skewback is now joined to the point A,
found, and this can be done as illustrated and the directions of the three forces is
in Fig. 482, where the arch is drawn to scale shown. The amount of the weight will be
and the weight to be carried is also shown ; known, and consequently, if this is set
19
2QO
REINFORCED CONCRETE
downward to scale from the point A, giving
the point B, and lines are drawn from the
latter point parallel to the lines repre-
senting the direction of the other forces, as
shown by the dotted lines, then the amount
Fig. 483.— Three-hinged Arch
of the horizontal thrust at the crown will be
represented by the distance A c to the same
scale as that employed in setting down the
weight, and the thrust at the skewbacks will
be represented by the distance A D. If these
three forces are set out separately they will
give a closed triangle, which shows that
equilibrium is produced, and the student
willj'at once recognise an example of the
When the thrust has been determined the
compression per square inch on the concrete
can be found by taking the total sectional
area of a width of the arch equal to 12 in.
plus 14 times the area of the steel and divid-
Fig. 484. — Two-hinged Arch
ing the same into the thrust as given for
compression members. When the r.rch is
unequally loaded in such manner that the
line of thrust passes outside the middle
third of the ring and produces tension, the
moments must be calculated and the arch
designed according to the formulae given
in an earlier chapter for double reinforced
beams.
In addition to the type of arch described
Fig. 485. — Foot-bridges at Railway Terminals, Vera Cruz
triangle of forces. The thrust at the skew-
back will always be in excess of the thrust at
the crown, and for this reason the area of
the concrete is often increased by making
the arch of greater thickness at this point,
as illustrated.
above, arches with hinges are sometimes
employed, the introduction of hinges allow-
ing the line of resistance to be absolutely
determined, as it must pass through the
definite points given by the hinges. Fig.
483 shows a three-hinged arch, and Fig. 484
ARCHES AND BRIDGES
291
a two-hinged arch. It will be
clear that there can be no
bending moment at these
hinged joints, whatever the
condition of loading may be.
It is absolutely essential that
the abutments are unyielding,
and these must be properly
designed to be safe against
crushing or sliding. Provision
to resist shearing and tem-
perature stresses must also be
made. Great care and expe-
rience are necessary in the de-
signing of^arches and bridges to
produce satisfactory work, and
the student is recommended
to take a few simple cases,
without introducing complica-
tions, and work these out and
gradually lead up to the more
advanced work, at the same
time studying some of the
many examples which have
been carried out in practice.
EXAMPLES OF ARCHES
AND BRIDGES
Foot-bridges at the railway
terminals Vera Cruz, Mexico
(see Fig. 486), are interesting
constructions, one of them
being no less than 993 ft. long,
there being sufficient clearance
for four tracks. Two ex-
pansion joints provide for
variations in length. In con-
structing the railing, the top
and bottom rails, with the
uprights for one panel, were
moulded on the ground and
afterwards set in the forms
for the railing posts, which
were poured in place. The
panels are 6 ft. 6 in. long, and
can be lifted jy by three men.
Much shorter bridges at the
railway terminals are those
between the Customs ware-
houses (see Fig. 485). These
were designed for a loading of
200 Ib. per square foot, and
also for a track loading of
push cars carrying 4-ton loads.
A foot-bridge over the Weis-
seritz, at Cotta-Dresden, is
shown by Fig. 487. It is of
292
ARCHES AND BRIDGES
293
85-ft. span, and was erected on the Moller deflection amounting to only a little more
system of horizontal girder bridges, by than T\- in.
Messrs. Rud-Wolle, of Leipzig. A bridge in Cowley County, Kansas, con-
Fig. 488. — Moortown Bridge, Wimborne, Dorset
The bridge at Wimborne, Dorset, shown
by Fig. 488, is one of two bridges in reinforced
concrete erected at Wimborne, one on the
Moortown Road and the other on the Ring-
sisting of a small steel span with 28 ft. of
wood trestle approach, was converted into
a reinforced concrete structure (see Figs.
489 to 491). The existing structure was
Fig. 489. — Decking and Parapets of Bridge in Cowley County, Kansas
wood Road. The bridges were designed and
executed by the Cubitt Concrete Construc-
tion Co., and the test load was carried out
six months after completion, the recorded
dismantled, the abutments and piers re-
paired, the steel work cleaned and replaced,
the additional reinforcements put in posi-
tion and wired to keep their place. Timber
Fig. 490. — Underneath View of Main Floor Span of Bridge in Cowley County, Kansas
'- eif » iPL:«io"«'V
1 Ik
SLEVATIOM — — MAJLF SECTION
Fig. 491. — Elevation and Details of Bridge in Cowley County, Kansas
294
ARCHES AND BRIDGES
295
for the forms was 1-in. dressed boards for
the side pieces, the braces to prevent bulg-
ing being 2 in. by 4 in. and 2 in. by 6 in.
The span is 50 ft., the approach 28 ft. 9 in.,
and the road bed is 14 ft. wide, and it was
estimated that the cost of the work was
two-thirds that for a new concrete structure.
A bridge over the Merrimac River, New
Hampshire, U.S.A. (Fig. 492), is an example
of -a bridge built under extremely difficult
conditions, inasmuch as it replaced an old
iron bridge and was built underneath it, so
middle span was raised 7 ft. above its original
grade, and the shore ends were put up 5 ft.
The struts, however, began to buckle, and
the bearings of the end-posts displayed a
tendency to work out of line before the
spans were half-way up. The light struts
were stiffened by embedding them in con-
crete. Forms were built about them and
concrete poured, while the traffic passed
continuously over the structure, causing
heavy vibration of all members. Neverthe-
less, the concrete set perfectly. One abut-
Fig. 492. — Bridge over Merrimac River, New Hampshire
as not to stop the traffic. It was designed
by W. M. Denman, of Springfield, Mass.
The grade of the roadway of the new bridge
was required to be 4 ft. higher than the
road level of the old iron bridge. Accord-
ing to Engineering News, the old iron
spans were 150 ft., 168 ft., and 170 ft. long,
with an 18-ft. roadway and one footpath.
The portal struts were very light, being
only 18 in. deep, made of the lightest angles
and lacing bars. In order to build the new
bridge under it, it was necessary to raise
the old spans, and the light portal struts
made this a very difficult undertaking. The
ment for the new bridge rested on rock just
within the old abutment, which is used as
backing to resist the thrust. It was found
necessary to underpin this abutment in sec-
tions with concrete. Two of the piers also
rest on rock at 4 ft. to 6 ft. below low water.
The two other piers are on piles in 10 ft. to
15 ft. of water. The piers of the new
bridge were so spaced as to permit of their
construction without interfering with the
old piers. The arches of the new spans
necessarily passed through the spaces be-
tween the tops of the old piers and the old
truss bridge. This was done by boxing in
Fig. 493. — Foot-bridge at Mizen Head
Fig. 494.— Erecting the Ribs for Foot-bridge at Mizen Head
ARCHES AND BRIDGES
297
the 4 ft. by 4 ft. columns of blocking under
each end-post and concreting. the arch ring
around the box, diverting
the steel reinforcement at
that point. After the arches
had hardened, the fill was
made and the old bridge
let down on the new, so
that it could be dismantled
and removed. The columns
of blocking were taken out
and the holes plugged with
concrete. The new bridge
has spans of 88 ft., 94 ft.,
and 100 ft. The roadway
is 30 ft. wide in the clear,
and the footpath is 6 ft.
wide.
A one-span foot-bridge at
Mizen Head, Ireland, giving
access to Cloghan Island,
is shown by Fig. 493 ; it was designed by
M. Noel Ridley, the contractors being Alfred
Fig. 495. — Suspended Centering for Bridge
Fig. 496.— Bridge Grossing the Bremba, Italy
Thorne and Sons.
The span is 172 ft.,
and the height above
the sea 150 ft. The
bridge is of the
through-arch type,
the ribs being of rec-
tangular section, 3 ft.
by 1 ft. at the spring-
ing and 1 ft. 6 in.
by 1 ft. at the crown.
The following are
the chief dimen-
sions: Bibs, 5 ft.
6 in. apart, centre to
centre, where par-
allel in the centre
portion, and 15 ft.
apart at the abut-
ments. Footway,
4 ft. 6 in. wide be-
tween horizontal
beams (15 in. by
9 in.), and 5 in.
thick at centre.
Trestles and wind
bracing, 9 in. by 9
in. Hangers, 9 in.
by 6 in. Handrails,
4 in. octagonal, with
posts 9 in. by 6 in.
and 6 in. by 6 in.
The work wai dene
without centering,
the ribs being made
on shore and built
298
ARCHES AND BRIDGES
299
out from the abutments. The various parts
were placed in position by means of a wire
ropeway (see Fig. 494.) Only the floor and
hangers were made in situ. Under a distri-
buted test load of 150 Ib. per square foot,
the maximum deflection at the centre was
about ~ in.
Reinforced concrete is a popular material
for bridge construction in Italy. Occasion-
ally the designs are striking and even dar-
ing, an instance being the bridge that crosses
the Bremba at the Gorge of Sidina (see Figs.
495 and 496). The first photograph gives
some idea of the exceptional difficulties of
the construction and shows the suspension
of the centering from the old brick bridge.
The arch, which has a span of 37 '5 metres, is
shown complete in the second view. The
bridge was constructed by F. Leonardi, of
Milan.
A triple-hinged bridge, giving distinct
architectural effect, and built by Weirich
and Eeinken, of Kiel, is shown by Fig. 497.
Its span is 27 metres, the rise being 5
metres. The parapet and roof are of
wood, and the steps are of plain concrete
formed direct into the arch.
A bridge over the San Luis Rey River at
Oceanside, California, is shown by Fig. 498.
It is a 700-ft. bridge of three arches, and
its members were built on the ground, left
there to harden, and then hoisted into posi-
tion, the object being to obviate the exces-
sive cost of false-work. According to Con-
crete, the arch ribs were poured in two
sections, and the spandrel posts cast sepa-
rately. After these were put in place on
piers and abutments the forms were bolted
in place for the deck of the bridge. The
bridge was designed on the W. M. Thomas
system (patented), the expected load being
250 Ib. per square foot. It consists of six
spans, four of which are 103 ft. long, the
two end spans being 107 ft. The bridge is
19-4 ft. in width over all, with 18-ft. road-
way.
The foundation for the piers and abut-
ments consists of thirty piles to each pier
and abutment, the piles being cut off 2 ft.
below mean low tide, the foundations thus
extending down about 3^ ft. below the river
bed. Each pile was calculated to sustain a
17-ton load. The foundations of piers are
12 ft. wide and 26 ft. long, and there is no
reinforcement in the piers. The arch ribs or
beams were made on the ground under the
respective positions which they were later
to occupy in the completed structure ; they
are 11 ft. on centres, and are reinforced
with a steel framework consisting of twelve
f-in. round rods and £-in. by IJ-in. flats,
the reinforcements being connected at the
crown end of the beam to cylindrical plates
of steel having a ball joint mated into a cup
in the opposite beam. The semi-spherical
protuberances at the reverse end of the
beam are £-in. semi-circular plates, form-
ing portions of the hinges at skew back on
the piers. The hinge shoe on piers consists
of 1-in. bolts, the reds extending deep into
the concrete. The spandrel posts, also
made separately on the ground, were poured
in 12-in. by 16-in. sections. The longer
posts are reinforced with four f-in. round
rods, and the shorter posts have four |-in.
rods. These rods were left protruding from
the sides, top, and bottom of the posts. The
rods extending from the top form a tie to
connect the spandrel arches with the span-
drel posts ; the rods protruding from the
bottom have a thread, and they extend
through the rack beam and bolt the spandrel
post to the beam ; the rods extending
through the sides of the beam are embedded
in diaphragm cross- braces, and hold the
posts to a rigid position. Holes were left in
the spandrel posts and the wooden center-
ing for spandrel arches to provide means
for firmly bolting the floor supports to
the spandrel posts ; the monolithic concrete
deck was then poured in the usual manner.
The floor was reinforced with f-in. round
rods running transversely across the bridge,
and the outside cantilever portion of the
floor has rods running longitudinally 6 in.
on centres.
The Scenery Hill Viaduct, Philadelphia
(Fig. 499), includes an arch having a span of
33 metres over a railway line, and carries a
roadway which is 32 metres above the rails.
The construction is on the Hennebique
system, with plain bars, to which are
hooked stirrups acting as a secondary
reinforcement.
The Meadow Street bridge, Pittsburg
(Figs. EOO and 501), is 454 ft, long, having a
30-ft. roadway and two 10-ft. footpaths,
with a maximum clear height at the centre
of the arch of 78 ft. The main arch span
is 209 ft. long, with a rise of 4614 ft., and
consists of three arch ribs, the two outside
ribs varying from 5 ft. deep at the crown to
6 ft. 2 in. at the springing line. The main
arch ribs are connected at the panel points
ARCHES AND BRIDGES
by twenty-six horizontal struts, each 12 in.
wide, which provide additional lateral
strength. The main arch ribs spring from
two main piers of cellular construction
37 ft. 9 in. long and 14 ft. in width. Between
the main piers and the abutments are three
viaduct approach arches, each having a
span of 21 ft. The front and side walls of
the east abutment are 13 in. thick, and are
designed for vertical pressure and also to
301
of the east abutment which has been
already mentioned.
The roadway of the bridge, which is of
reinforced concrete slab and floor-beam
construction, is supported on spandrel girders
supported by columns 24 in. by 24 in.,
spaced 15 ft. centre to centre, resting upon
the main arch ribs. The roadway on the
approach spans is supported in a similar
manner, except the span is 21 ft., and the
Fig. 499.— Scenery Hill Viaduct, Philadelphia
act as a beam if necessary, the tops of the
walls being covered and tied together with
a reinforced concrete slab, varying in thick-
ness from 8| in. at the centre line of bridge
to 12J in. at the curb lines, similar to the
type used on the main spans, and extending
28 ft. back from the face of the abutment.
Beyond this point the side walls are designed
as ordinary retaining walls with spread foot-
ings and to act as cantilevers. The side
walls on the west abutment are 37 ft. long
and designed as ordinary retaining walls,
the same as the portion of the side walls
columns, which are 24 in. by 36 in., extend
to the rock below, the greatest length of
the column of the approach spans being
39 ft., and on the main span 29 ft. in
length.
The footpaths, which are reinforced, have
a uniform thickness of 6 in., with a slope
of J in. to the foot to provide drainage, are
10 ft. in width, and are supported on beams
acting as cantilevers spaced 7 ft. 6 in. on
the main span and 7 ft. on the approach
spans.
The reinforcement for the main arch
302
Q
•o
en
-
3°3
304
REINFORCED CONCRETE
ribs consist of eight l|-in. grooved bars,
arranged in four pairs, each pair connected
with £-in. lacing bars, clamped into the
grooves on the 1^-in. bars, and spaced about
1 ft. apart, making a latticed girder effect,
placed vertically and curved to fit the arch
rib. The strut reinforcement, which was
made in two pieces of latticed girder frames,
similar to that of the ribs, was stapled to the
longitudinal bars in the main ribs, the struts
being placed at panel points on the bridge,
the main bars being 1| in. The column
reinforcement is IJ-in. vertical bars, stapled
at 9-in. intervals, and clamped around the
hooped bands, which were IJ-in. rounds.
ness of 8J in. The reinforcement for the
footpath slab is f-in. grooved bars, placed
6 in., centre to centre, in a 6-in. slab of
7 ft. 6 in. span, between footpath brackets.
The reinforcement for the floor system was
set in place and wired to the forms before
concreting. Round bars were used in the
piers at the end of the main span near the
surface of the concrete, to prevent cracking
of the thin walls. The reinforcement was
f-in. round rods, spaced 24 in. apart and
wired in place. Further particulars of this
higlily interesting construction will be found
in an article (from which the above details
have been abstracted) by N. S. Sprague in
Fig. 502.— Grafton Bridge, Auckland
The spandrel girders, which were built above
each arch rib, spanning from column to
column on 15-ft. centres, were reinforced
with five IJ-in. bars laid horizontally, and
two l|-in. bars curved to fit the soffit of
the arch. Top reinforcement was placed
over the columns, although the spandrel
girders were designed as simply supported.
The floor beams were supported by the
spandrel girders already referred to, and all
of them were reinforced with four 1^-in.
bars and one 1-in. bar, grooved as before,
negative reinforcement being placed over
the centre column.
The reinforcement in the roadway floor
slabs consisted of f-in. grooved steel bars,
spaced 6 in. apart, centre to centre, on a
span of 7 ft. 6 in. centre to centre, between
the floor beams in a slab of a minimum thick-
Engineering News and also in Concrete (Vol.
VI., pp. 203-209).
Grafton bridge, Auckland, is shown by
Fig. 502. Its total length is 960 ft., a short
span having been added at the Grafton
Road end, and there is one span of 35 ft. ;
one span of 37 '5 ft. ; four spans of 75 -5 ft. ;
one main arch span of 320 ft., flanked on each
side by two main piers occupying at top
18 ft. each ; two spans of 78'5 ft. and 80'5 ft.
respectively ; and one span of 42 ft. All
approach spans consist of straight girders,
the three shorter ones being of solid design
and the others of open panel. There are
three girders to each span. The following
particulars are due to Concrete :
The main feature of the bridge is, of
course, the 320-ft. 3-hinge arch (with a rise
of 84 ft. from centre of abutment hinges
ARCHES AND BRIDGES
305
to centre of middle hinge), which consists
of two parallel ribs ranging in size from
6 ft. by 4 ft. 2 in. at the abutments to
9 ft. 9 in. by 4 ft. 1 in. (in the girder) to 5 ft.
by 4 ft. at the centre hinge, connected by
wide ties of reinforced concrete 8 ft. 4 in.
by 20 in., haunched 18 in. each way into the
rib and spaced at 21-ft. horizontal centres.
The end hinges rest on solid abutments or
skewbacks of concrete, with a small amount
of reinforcement, there being 720 cubic yards
of concrete in the main abutments. These
abutments rest upon a foundation consisting
of a sort of sandstone, known locally as
" papa " rock, and are quite independent
of the large piers previously referred to, so
that the movement in one would not neces-
sarily affect the other. The hinges them-
selves, which consist of cast steel, rest on
grills made up of steel H -girders and plates,
the centre hinge being treated in like
manner, the grills being built into, and sur-
rounded by, concrete. The superstructure
consists of seven piers of T-section, the
T being 3 ft. by 6 in., and the remainder
2 ft. 6 in. by 10 in. spaced 18 ft. apart in the
clear. At the abutment the decking is
carried by the cantilever, which projects 7 ft.
from the end pier, a similar cantilever being
constructed from the main pier over the
abutment to meet the one referred to.
The three highest piers on each half are
braced by horizontal transverse beams of
T section, spaced at intervals of approxi-
mately 30 ft. On each pier rest transverse
girders, 3 ft. 7 in. by 10 in., with haunches
at each bearing ; the actual deck system on
the main span consists of five secondary
beams, each 1 ft. 8 in. by 10 in., running
parallel with the direction of the bridge,
the deck itself being of reinforced concrete
fully 6 in. thick. The kerbing is situated
over the outer beam in each case, the foot-
path being carried by a cantilever consist-
ing of a deep haunch occurring at each
transverse girder and by a decking 5^ in.
thick in between. A further secondary beam,
12 in. deep and 6 in. wide, connects the
haunch beams under the parapet.
The open panel girders are 9 ft. deep by
15 in. wide, there being three girders in the
width of the bridge. These girders are sup-
ported by piers. The piers are formed by
two solid piers, 5 ft. by 4 ft., slightly rein-
forced with steel and connected at top by a
deep girder or curved beam to carry the
middle girder. The short spans, as before
20
described, are ordinary solid girders, the two
at the Symonds Street end being 3 ft. 9 in.
by 15 in., and that at the Graf ton Road end
being 3 ft. 9 in. by 15 in.
The deck system in the case of the straight
girder spans is the same in all instances,
and consists of transverse secondary beams
2 ft. 6 in. deep at the middle girder, diminish-
ing to 1 ft. 6 in. depth at the outer girder
and of a uniform width of 6 in., and con-
tinued beyond the outer girder in the form
of a cantilever to support the footpath and
parapet. The decking on the carriage way is
6 in. thick, that of the footpaths being 5|in.,
including a IJ-in. layer of fine concrete.
The parapets are 4 ft. 9 in. in height, and
constructed throughout of reinforced con-
crete. In the top member of the parapet is
fixed black iron tubing to receive the cables
for the electric lighting of the bridge. The
construction of the bridge occupied two
and a half years, and was carried out by
the Ferro-Concrete Company, whose Chief
Engineer, K. F. Moore, was responsible for
the design, most of the calculations in con-
nection with the bridge having been made
by C. Rosegger. Under a test load, equal
to 113£ Ib. per square foot, the total amount
of deflection registered | in., but from vari-
ous causes too much notice must not be
taken of this figure. In a later test, con-
sisting of passing over the bridge simultane-
ously two steam rollers, weighing together
32 tons, the maximum deflection registered
on the main span was -^ in., and on the
girder span TV in.
A bridge over the River Werra, in Thu-
ringia, Germany (Figs. 503 to 505), was built
by Christiani and Nielson. In design it con-
sists of a rigid arch, the two-arched beams
being united by transverse struts, and
carrying the reinforced concrete roadway
by means of steel ties. To prevent injury
from variations of temperature, two end
sections are rigidly connected with the
arched beams, while the middle section
hangs freely, engaging with the end ones by-
means of indentations. The clear span is
53 m. The decking is 12 cm. to 14 cm. (5| in.
thick), and is carried by four longitudinal
beams, which in their turn rest on trans-
verse supports at intervals of 4-15 m. (13 ft.
7 in.). These supports are 80 cm. (2 ft. 7 in),
wide in the middle, tapering to 50 cm. (1 ft.
7| in.) at the ends. The rails are continu-
ous, and are carried on sleepers, which are
readily removed sideways if required. The
3o6
REINFORCED CONCRETE
f, ,'^n
gravel ballast is 30 cm. to 40 cm. (12 in. to
16 in.) deep. A photographic view on p. 203
shows the very interesting type of centering
employed, this being of parabolic form, with
a maximum excess of height in the centre
of 50 mm., diminishing towards the end.
The supports rested on wedges, of which the
central ones were the first to be driven out.
During the concreting the centering settled
35 mm. at the crown of the arch, and when
the centering was removed after the con-
crete had hardened for five weeks there was
a further settlement of 11 mm. The load-
ing tests, which were applied on January
20, 1911, the work having been begun
on April 15, 1910, were conducted with
three loaded railway trucks, each weighing
about 48 tons, and a locomotive weighing
34 tons, and the results indicated a modulus
of elasticity of 300,000 kg. per square centi-
metre (roughly, 1,904 tons per square inch).
Walnut Lane Bridge, Philadelphia (see
Fig. 506), was designed by George S.
Webster, M.Am.Soc.C.E., in conjunction
with Henry H. Quimby, M.Am.Soc.C.E., res-
pectively chief engineer and assistant en-
gineer to the Department of Public Works,
Philadelphia. It crosses the Wissahickon
Creek and ravine, and connects Germantown
and Koxborough, the steepest grade of
approach to the bridge being 4| ft. in 100
descending from Germantown. The road-
way is 40 ft. wide between the kerbs, the
side walks being each 8 ft. wide. The main
arch is 233 ft. clear span between the piers,
the deck of the bridge being 147 ft. above
the creek and the underside of the arch
133 ft. above the creek. The springing point
of the arch at the face of the piers is 34 ft.
above the drive. Economy and lightness
are secured by designing both the main arch
and the bridge arches in the form of twin
supporting arch ribs floored across the top.
Over each main arch rib the floor is carried
on eight piers of spandrel or relieving arches,
which support the four longitudinal walls
upon which the floor will rest. The whole
is hollow underneath clear up to the floor,
and as no earth filling is required, there is
a great saving in weight. Each of the five
bridge spans is 53 ft. clear between the piers,
and those next to the main span are about
70 ft. clear above the ground.
A bridge near Teufen, Switzerland, is
illustrated by Fig. 507. The following
particulars of it are reproduced from an
article in Concrete by Willy Obrist,
ARCHES AND BRIDGES
307
who was in part indebted to an article
the Schweiz. Bauzeitung, by Prof.
in
Morsch, the designer of the bridge. The
contractors were Frote Westermann and
Co., Zurich. The bridge, which replaces an
iron bridge erected in 1856, crosses the
Sittertobel in one large arch, 79 m. span
steam roller of 20 tons weight and a crowd
amounting to 450 kg. per square metre (92
Ib. per square foot) were considered, provi-
sion also being made for a variation of tem-
perature of 20° C. each way. The form of
the arch is that of the curve of equilibrium
for the dead load. The thickness at the
Fig. 504. — Elevation and Plan of Bridge over River Werra
r^
i
R
m
505.— Details of Beams for Bridge over River Werra
and 26-5 m. rise, which is succeeded on the
left bank by four arches 10-25 m. in span
and on the right bank by two such arches
(Fig. 507). The roadway is about 66 m.
above the level of the river, and has a width
of 6-9 m. between the parapets, including
two footpaths, each 60 cm. wide. The
main span is treated as an arch with fixed
ends and without hinges. As live load, a
crown being 1'20 m. and at the abutments
2-13 m., the maximum and minimum
stresses were calculated to be 31 -3 and 04
kg. per square centimetre (445 and 5*7 Ib. per
square inch). Although no tensile stresses
should theoretically occur, yet in order to
secure a proper distribution of the loads
due to the spandrel columns, the arch was
reinforced with rods of 28 mm. diameter,
308
REINFORCED CONCRETE
there being at the crown ten such rods above
and eighteen below, in the quadrants fourteen
above . and below, and at the abutments
eighteen above and ten below. The breadth
of the arch is 6 -5 m. at the crown, and this
gradually increases, in order to secure lateral
stability, to 7-50 m. at the abutments.
The foundations are in interstratified marl
and sandstone, and sustain a maximum load
of 6'5 kg. per square centimetre (92 lb. per
square inch). The roadway consists of a
reinforced plate 20 cm. thick and four longi-
tudinal beams, and is supported from the
arch by columns of 50 cm. by 50 cm. section.
The beams are treated as continuous, and
are respectively 149 m. and 1'37 in. apart,
4'5 m. span, 65 cm. high, and 25 cm. broad.
The connection with the main columns is
made by means of a so-called " pendulum
wall," 25 cm. thick, the elasticity of which
allows it to follow all the deformations of
the roadway due to changes of temperature.
The smaller arches are also so formed that
only compressive stresses occur ; all are
reinforced with five rods of 14 mm. diameter,
the thickness at the crown in each case
being 35 cm.
The centering used was a model of its kind
(see p. 200). The timbers were so arranged
as to be subject only to compression, all
loads being directly borne. Channel irons
were placed between the vertical and hori-
zontal timbers to distribute the loads. The
six ties, 1-4 m. (4 ft. 7 in.) apart, were bedded
on sand boxes at a height somewhat above
the springing of the arch. As the timber-
ing stood without load from December, 1907,
to the following March, it was necessary to
provide for proper resistance to wind pres-
sure. Special struts were therefore placed
perpendicularly to the ties, supported below
at 10 m. to the right and left of the axis of
the bridge. The good construction of the
centering is clear on consideration of the
deflections observed. During the concret-
ing a depression of 32 mm. (1J in.) occurred,
and on removal of the centering a permanent
deflection of 5 mm. (T37 in.) was found.
The construction of the work was begun
in March, 1907, and the bridge was taken
over on November 7 of the next year.
The concrete for the reinforced portions
was composed of a mixture of 1 yd. gravel,
£ yd. sand, and 600 lb. portland cement.
THE ROYAL AUTOMOBILE CLUB'S
BUILDING
Excavations and Retaining Walls.—
The Club House of the Eoyal Automobile
Club, erected in Pall Mall from the designs
of Messrs. Mewes and Davis and E. Keynes
Purchase (the joint architects), in 1909-11,
undoubtedly ranks as one of the finest build-
ings in London, and the many and varied
examples of reinforced concrete work com-
prised by it give it an interest, from our
point of view, surpassed by no other building
in London. The site, 228 ft. by 140 ft., was
Fig. 508.— Retaining Wall with Asphalt
Damp-proof Course
formerly occupied by the old War Office
buildings, adjoining the Carlton Club.
Excavations, etc. — The soil throughout
is a good London ballast, which, although
virgin soil, required careful strutting at all
parts in order to prevent it running down ;
and one feature of the work was the very
heavy timbering, some idea of which can be
gathered from the photographic views.
Water was reached at a level of about 30 ft.
below the pavement line, and as some of the
foundations had to be carried to a depth of
more than 47 ft., two pumps were constantly
at work in order to keep the water below the
level at which the operations of excavating
and concreting were being executed. This
was done by sinking a pit at the lowest level
to which the water drained, and then con-
tinuously pumping to prevent the water
rising above a certain height. The second
pump kept the trenches dry and prevented
any portion becoming flooded.
In a few cases the foundations are carried
right through the overlying ballast down
to the London clay, this great depth being
necessary as there are two floors below the
pavement level ; and owing to the high
level of the sewer in Pall Mall all the low-
level drains are carried to two pneumatic
ejectors which raise the sewage to the re-
quired height. These ejectors are placed in
a specially constructed chamber at a depth of
20 ft. below the basement floor, and in order
to prevent any risk of sliding, the grillage
foundations under any adjoining stanchions
are carried down to the same depth as the
ejector chamber. Owing to the presence of
water, and the basement floor being below
the level of the sewer, the whole building
is erected in what constitutes a huge under-
ground tank, an asphalt damp-proof course
(see Fig. 508) being continuous and carried
under and around all grillages, walls,
chambers, and subways of every description,
so as to prevent water from any quarter
penetrating the building after completion.
The horizontal asphalt under floors was
£ in. thick, and under foundations or where
vertical f in. thick ; the horizontal asphalt
under all floors is covered with 12 in. of
cement concrete in order to prevent it being
raised by the water pressure ; the head
of water was not great enough to justify the
reinforcing of this concrete.
Retaining Walls, etc.— An interesting
feature of the work is the construction of
the retaining walls, which are excellent
examples of plain concrete and reinforced
concrete work. The manholes in some cases
are so deep that the walls had to be calculated
as retaining walls ; in several instances the
Fig. 509. — Timbering to Support Earth, before Building Retaining Wall (R. A. G.'s Building)
Fig. 510. — Shuttering and Reinforcements for Retaining Wall (R. A. C.'s Building)
312
VARIED APPLICATIONS AND EXAMPLES
walls are 3 ft. 6 in. thick at the base.
The retaining wall at the rear of the site is
of reinforced concrete, 200 ft. long and nearly
40 ft. high, and supports Carlton Gardens,
which adjoin the site on the south. Fig. 509
shows the rear of the site as well as the
timbering erected to. support the earth before
the wall was put in. The other trenches
shown are for the grillage foundations of the
•stanchions carrying the main back walls
of the building. The great width of the
trenches can be seen and also the timbering
313
curves into the heel of the wall, which
projects under Carlton Gardens, and into
the toe on the opposite side of the wall ;
and are tied together with horizontal bars
throughout. Some of these bars are of
such a length and shape that it was found
impossible to cart them to the site in the
form required, and therefore they were bent
on ^ the works by a hydraulic press, three
bars being shaped at a time.
Fig. 510 shows the shuttering for the wall
and some of the vertical bars in position.
Fig. 511.— Retaining Wall Complete (R. A. C.'s Building)
that was required, this consisting of poling
boards, waling pieces, and struts where
above water level ; whilst below that level,
runners were driven down as the work
proceeded in order to keep the earth back.
The reinforced wall, although of such
great length and height, is only 3 ft. to 3 ft.
6 in. thick at the base, with a vertical face
-against the gardens and a battering face
next the new building. The whole is con-
structed of portland cement concrete,
machine-mixed, in the proportion of 1, 1§,
•3£, reinforced with indented steel bars
varying from J in. to £ in. in thickness.
The vertical bars are carried with various
Near the top of this view will be noted some
horizontal pieces of wood, which are notched
in order to receive the various bars and keep
them in position, and also to maintain
sufficient space between the bars and the
shuttering so as to ensure the former being
covered with concrete in the finished wall.
Small pieces of brick are placed under the
bottom of the bars with the same object.
Although not necessary for purposes of
strength, the horizontal and vertical bars
are wired together where crossing each
other, the object being to ensure that the
reinforcement shall not be moved out of
position during the filling in of the concrete.
314
REINFORCED CONCRETE
This is certainly the best method of carrying
out the work, although in this instance it
meant some 15,000 wirings. These walls
and foundations were executed by Messrs.
Trollope, Colls and Sons, Limited, of Gros-
venor Koad, Pimlico.
A view of the completed reinforced retain-
ing wall is shown by Fig. 511 ; the strut-
ting was removed in sections, the bottom
row being taken away first, and then the
second row, and so on until the whole had
been dispensed with. The first tier was
removed six weeks after the concrete had
found to work out at 6 ft., and as the space
in the front vaults was not quite so im-
portant as that in the rear of the main
building it was decided to use plain concrete,
Had reinforced concrete been employed, a
much thinner wall could have been con-
structed, as the back wall is 40 ft. high and
only 3 ft. 6 in. thick at the base, whilst the
front wall is only 25 ft. high and yet needs
a thickness of 6 ft. at the base. At the
same time, of course, it must be borne in
mind that the front wall has much heavier
work to do owing to the roadway which it,
Fig. 512. — Inside Face of Retaining Wall, and Methods of Supporting Mains (R. A. C.'s
Building)
been rammed in position, and each hole
was carefully filled up with concrete im-
mediately the strut had been taken away.
An interval of seven days was allowed to
pass between the removal of each row in
order that the weight coming upon the wall
might be gradually applied.
A good comparison may be made between
the back and front retaining walls. The
front wall forms the outer wall of the vaults
under the pavement and supports the road-
way in Pall Mall, and is executed in plain
cement concrete (1, 2, 4). The total length
is 228 ft., and the height 25 ft. Upon
calculation, the thickness of the base was
supports. The wall has a vertical face on
the inside, and on the outside, next the
roadway, has two offsets, the first of which
reduces the thickness to 4 ft. 6 in., and the
second to 2 ft. 6 in. There is no heel to
the wall, but a toe is formed at the base
which projects under the vaults to the extent
of 2 ft. 6 in. More than 30,550 cubic feet
of concrete were required in the construction
of this wall, but the work was speedily
carried out, owing to its simple character,
although great care had to be exercised in
providing temporary supports for the road-
way and the numerous gas, water, telephone,
and electric light mains met with.
VARIED APPLICATIONS AND EXAMPLES
3i5
Fig. 512 shows the inside face of the front
retaining wall, after the shuttering had been
removed, and the various methods that were
employed for holding up the mains that come
over the vaults. It also shows the wall of
the vaults that faces the club house (there
being an area along the front of the building
and is provided with openings for coal
shoots, etc.
Foundations. — Before describing the
foundations and other work below the base-
Fig. 513. — Section showing Front Vaults
(R. A. C.'s Building)
3 ft. 6 in. wide), which is built in white
glazed bricks for the sake of cleanliness
and light. The asphalt damp-proof course,
previously mentioned, is carried over the
walls and roofs of the vaults, passing under
the retaining wall, and up on the outside of
il'~4'lx
Fig. 514.— Plan of Grillage Foundation
(R. A. C.'s Building)
ment floor level, it will be interesting to give
some idea of the accommodation provided
on this floor.
The central space is occupied by an
elliptical vestibule, 48 ft. by 32 ft., from
which the various corridors lead to the other
^6"<OW<:eJK]-6-
Fig. 515.— Section through Top Layer Beams
of Grillage
it, being finished under the pavement,
against the stone balustrade, which forms
a finish to the vault wall, as shown in
Fig. 513. The sloping roof over the vault is
constructed of rolled steel joists and concrete
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Fig. 516.
Section through Bottom Layer
Beams of Grillage
portions of this floor, and the main staircase
leads to the ground floor. On the east side is
a Turkish bath, and on the west are three
racquets courts and a gymnasium, the latter
being connected with a swimming bath,
3i6
REINFORCED CONCRETE
wliicli occupies the rear of the building.
The front of the building on this floor pro-
vides private dressing-rooms for the members
and a large dining-hall for the servants,
while the boiler-house for heating purposes
is also situated in this part of the building.
The space in the, front vaults is used for
coals, stores, large workshop, and lavatories.
In addition to this accommodation there are
several lavatories, dressing boxes, locker
rooms, attendants' rooms, and a large
servery.
The grillage foundations (Figs. 514 to
The loads on the stanchions, of course, vary
considerably, but in some cases over 500
tons has to be carried, and consequently it
will be seen that an ordinary plain concrete
foundation would have been so large and
the thickness so great as to be almost im-
practicable. It was decided to use grillage
foundations, and, although this is quite a
common form of construction in large build-
ings, a detailed explanation is given here
because they are a typical example.
The grillages are so large that, although
the stanchions are spaced at fair distances
Fig. 517.— Trench for Grillages under Stanchions (R. A. C .'s Building)
516) are carried to a great depth in order to
obtain a good bottom below any adjoining
drains, underground flues, and pits of
various kinds. The building is constructed
on the steel frame principle, where all the
walls, partitions, and floors (with the excep-
tion of the party walls) are carried on steel
girders at each floor level, the total weights
being brought down by the stanchions alone,
which, consequently, carry very heavy
loads.
There are as many as 112 stanchions
which actually run down to the soil, whilst
there are a few more which are carried by
large plate girders higher up the building.
apart, one continuous trench from 12 ft.
to 15 ft. wide is necessary to execute the
work, as can be seen in Fig. 517, which is a
view showing the trench for the grillages
under the stanchions which carry the main
back wall of the upper part. It also gives
a view of the site looking towards the north-
east corner, and in the distance can be
seen a full-size model of one bay of the pro-
posed new front elevation, which was erected
in wood, in order that a true conception of
the finished building might be obtained with
regard to detail and proportion.
The plan of the grillage foundation shown
in Fig. 514, on the preceding page, is repro-
VARIED APPLICATIONS AND EXAMPLES
duced from an actual drawing of one
foundation, and may be regarded as a
typical case. The safe load put upon the
soil can be taken approximately at about
3 tons per square foot, and, consequently,
as the foundation proper measures 10 ft.
6 in. by 10 ft. 6 in., it may be assumed that
the load carried by this particular stanchion
is 110 sq. ft. multiplied by 3 tons=330 tons.
This weight has to be brought down and
spread over the required area of soil by a
stiff foundation. Figs. 515 and 516 show
how this is accomplished by placing the base
of the stanchion directly on to three 24-in.
by 7^-in. rolled steel joists, which are tied
together and the webs stiffened by three
pairs of f-in. cast-iron separators and f-in.
bolts. The webs are further stiffened against
any buckling by the application of angle
stiff en ers (not illustrated) 3| in. by 3| in.
by | in. under the base of the stanchion,
317
three rows of f-in. tie rods, with pieces of
gas barrel between the webs to keep the
joists rigid ; these latter act as small canti-
levers on each side of the top layer and
distribute the load over the concrete and
soil beneath. Under the joists is placed
WIDJH OF PILE.
ctif> 70 KKH J«wf
Fig. 519,— Section of Steel Pile
cement concrete 18 in. thick, and the whole
of the joists and stanchion base are filled
in with and surrounded by concrete.
Before placing the grillage in position,
a pit was formed with a concrete bottom 6 in.
thick and brick sides 4£ in. thick, lined with
an asphalt damp-proof course as illustrated
Fig. 518.— Asphalt Damp-proof Course under Base of Back Retaining Wall (R. A. C.'s
Building)
twelve of these being used in each case.
The steel joists forming the top layer of the
grillage act as cantilevers on two sides of the
stanchion base and distribute the load to the
second layer, which in this particular instance
is composed of ten 12-in. by 6-in. rolled steel
joists spaced slightly over 12 in. centre to
centre. These joists are tied together with
in the drawings. Below the water level, the
bottom and sides of this pit are increased to
12 in. and 9 in. respectively. The view given
in Fig. 518 shows the asphalt damp-course as
it was applied under the base of the back
retaining wall. The excavations between
the grillages, where these are close together,
are filled in with gravel well rammed (not
REINFORCED CONCRETE
with concrete) in order to keep each grillage
foundation separate and prevent any unequal
loading or settlement.
Steel Piling for Foundations. —
Although various forms of steel piling have
been used extensively in the construction of
•sea walls, docks, collieries, and such work,
where great strength and permanence are
required, steel piling for foundations is quite
a modern feature in London building work.
The form of pile used in this particular
instance is indicated in Fig. 519, which is a
horizontal section showing the piling and
the method of forming the angles. The
piles are of corrugated steel J in. thick, but
the lap given to each pile is so great that
in the finished work the thickness of the
piling in every case is equal to J in. Thus,
if it were necessary to obtain finished steel
piling, 1 in. thick, |-in. piles only would be
Fig. 520.— Top of Finished Steel Piling
used, and the lap employed as shown. The
piles are 3 ft. 6 in. wide, with corrugations
about 4J in. deep and 1 ft. 4 in. centres, the
object of the corrugations being, of course,
to increase the lateral strength of the piling.
The length of the piles employed is 15 ft.,
and Fig. 520 is a view taken about 15 ft.
below the basement floor level, showing the
top of the finished piling along one side of
the chamber.
The driving of the steel piles is effected
;•
Fig. 521. — Cast-iron Gap to Head of Steel
Pile
in a similar manner to that employed for
ordinary timber piling, but a very heavy
monkey is used (as the piles are not pointed
in any way), weighing 1| tons, and this is
given a drop which varies from 1 ft. to 4 ft.
The head of the pile is protected by a specially
made cast-iron cap (see Fig. 521), weighing
about 3 cwt. ; the under side of the cap is
shown in this view, and its application to
the pile is obvious. The greatest distance
that a pile is driven by one blow is J in.,
and great care is exercised during the driving
to prevent the pile being turned out of its
course by any obstruction it may meet.
The vibration caused by the monkey is
considerable, owing to its great weight.
When the four sides of the pilings are com-
plete, the whole of the excavation can be
carried out within the piling, and the new
work executed without any further timbering
or supports.
The makers of this form of piling have
now brought out one of a different section,
considered to be superior to the earlier one.
Floors. — The floors and roofs through-
out the Royal Automobile Club's building
are of reinforced concrete, with the exception
of the. main girders, which are of steel alone.
The principle is that of a steel framed
building, with the floor panels filled in with
reinforced concrete, the latter being a separate
construction, which receives its support from
the main steel girders ; but it cannot be said
that the whole floor forms one homogeneous
structure, as occurs when the main girders
and panels are constructed of reinforced
VARIED APPLICATIONS AND EXAMPLES
319
concrete at the same time. Generally
speaking, the steel girders are fixed at a
uniform level of 4| in. below the floor line
to the top flange, while the top of the con-
crete is finished 3| in. below the floor line ;
thus a covering of 1 in. of concrete is
obtained over the main girders ; and this
allows some of the reinforcing bars to be
carried over these, as can be seen in Fig. 522,
which shows the reinforcement and centering
occurs on one side of the girder only, the
reinforcement is cranked well over the top
flange and thus hooked to the girder. Fig.
522 shows that a certain amount of reinforce-
ment is provided at right angles to the main
reinforcing bars. These are bars £ in.
square spaced at 2-ft. centres, to form a tie
and distribute temperature stresses which
might cause cracks.
The type of reinforcement used was the
Fig. 522.— Reinforcement and Centering for Floor (R. A. C.'s Building)
in position previous to the filling in of the
concrete.
The small wire clips on the main steel
girder support the concrete casing, which
protects the lower part, and are quite en-
veloped in the concrete filling when the latter
is complete. It will be noticed that every
third bar is carried over the steel girder and
allowed to run for some distance into the
adjoining panel, thus to a certain extent
making the floor continuous and preventing
cracks from occurring over the girder, as
would otherwise be the case, and at the same
time giving some provision against shear
near the girder. Where the concrete floor
patent indented steel bars of |-in. and £-in.
section, and the tests imposed were sufficiently
severe to ensure the steel being free from any
brittle tendency. The size and spacing of
the bars, of course, varies according to the
span, which, generally speaking, ranges
from 8 ft. to 14 ft., but as a general rule the
distance from centre to centre is 6 in., while
the thickness of the concrete varies from
4| in. to 7 in. The ground and lower ground
floors were calculated to carry a load of
220 Ib. per sq. ft., and the upper floors and
flat roofs a load of 160 Ib. per sq. ft. The
concrete was composed of 1 part portland
cement, 2 parts sand, 2 parts clinker, and
320
REINFORCED CONCRETE
2 parts ballast or broken brick, and the
whole was mixed by machinery.
The cement was required to comply with
the British standard specification for medium-
setting cement, with the exception that the
tensile strength was to be at least 600 Ib.
per sq. in. after twenty-eight days' setting,
Fig. 523. — Elliptical Reinforced Concrete Lintels
instead of 500 Ib., as called for in the British
standard. The sand was screened to pass
a yVm- mesh, and contained a mixture of
fine and coarse sand ; this was mixed
with the cement in the dry state before the
addition of the coarse aggregate. The
gravel was screened and crushed to pass a
|-in. diameter ring, and the clinker was
required to be broken to the same state and
be free from all coke breeze, ashes, or coal.
The whole of the operations of crushing,
screening, gauging, and mixing were accom-
plished in a specially erected machine at one
point, immediately under efficient super-
vision, and from this point the concrete was-
rapidly transported to the different part s of the
works as required ; and in no case was more
than half an hour
allowed to elapse be-
tween the time of mix-
ing and the time of
placing in position.
Where new work was
placed next to the sur-
face of existing concrete
the latter was cleaned
of all loose particles
and dust, and given a
thick coat of neat ce-
ment grout before the
new concrete was
rammed in position.
Care was taken to
strengthen all portions
of the floors, where
small holes were re-
quired for pipes and
similar purposes ; and
where the holes were
large proper steel
framing was provided.
Lintels, Ceilings,
etc. — Fig. 523 is a
view showing the rein-
forced concrete lintels
which occur over the
window openings in
the three large ellipti-
cal bays to the swim-
ming bath in the base-
ment floor. The steel
framing to the terrace
above at the ground-
floor level was erected
as shown and sus-
pended in position until
the walls were built
up to receive same. In addition to the
floors and roofs, which are all in concrete,
special suspended concrete ceilings are
constructed in the majority of the rooms on
all floors. These are required owing to the
varying size of the main girders, which
project below the concrete floor and the
numerous ventilation ducts, pipes, and cables
carried about in all directions, thus rendering
it quite impossible to form a satisfactory
VARIED APPLICATIONS AND EXAMPLES
ceiling line without special construction.
These ceilings are formed of pumice concrete
slabs, 2i in. thick, and they are suspended
from the concrete floor and girders above,
with steel hangers at intervals which carry
inverted steel tees at their lower ends, on
which the slabs rest, wire lathing being
provided to give a key for the plaster on
the under-side of these steel tees.
Staircases. — The staircases provide some
very interesting forms of construction, both
321
the ground to the fifth floors, and this is
situated close to, and leading from, the
large elliptical vestibule, 48 ft. by 32 ft.,
which occupies the central portion of the
ground-floor plan. The staircase well on
the upper floors is 27 ft. 6 in. long and
16 ft. 6 in. wide between the walls, while on
the ground floor it is 43 ft. 6 in. long and
terminated by a semicircular end. The
flights themselves are 5 ft. 6 in. wide in the
clear, and the treads are 12£ in. wide with a
Fig. 524.— Shuttering to Curb of Skylight over Elliptical Vestibule (R. A. C.'s Building)
in the nature of their planning and the
materials employed in their execution,
which vary greatly with the importance of
the staircase. In a building of this size
many staircases are necessary, and there are
no less than fourteen in all, exclusive of the
small flights which lead to the terraces, and
others similar. The form of construction
employed is, generally speaking, that of
concrete steps, strengthened by the addition
of steel joists, channels and tees, but rein-
forced concrete, wood, and plain concrete
are also used in different cases.
The main staircase is that leading from
21
rise of 6 in., while the lowest steps, which land
on the ground floor, are gradually increased
in width until the bottom tread is 18 in. wide,
and these lower steps are also shaped in
curtail fashion to discharge everyone toward
the centre of the staircase hall and away
from the wall. The stairs are formed in
concrete with rolled steel joists at each
half landing and floor level, to support the
10-in. by 3^-in. raking steel channels, the
latter carrying the concrete stringer and
the outer ends of the steps ; the inner ends
being supported by 14-in. brick walls sur-
rounding the staircase itself. These raking
322
REINFORCED CONCRETE
channels are cleated and riveted^ to the
steel joists with all necessary ramps, and the
steps themselves are further strengthened by
the addition of 2-in. by £-in. steel tees, which
form a core, and f-in. steel tie rods which
serve to tie the channels to the walls. No
projections such as beams are allowed to
show below the soffit, as all the steelwork is
entirely enveloped in concrete sufficiently
thick to give one continuous flowing line,
which follows the pitch of the stairs, while
the same curves. The landings, treads, and
risers are finished with Eschaillon stone,
2 in. and 1£ in. thick, this being a very
hard cream-coloured French marble which
is used unpolished, while the walls and the
stringers are finished in stucco, closely
resembling stonework.
Of the other staircases, that running
from the basement to the fifth floors will
not be used for much continual work, service
lifts being provided for the upper floors, and
Fig. 525. — Shuttering and Reinforcements for Sloping Roof (R. A. C.'s Building)
the concrete stringer, which is about 2 ft.
2 in. deep and projecting 2 in. above the steps
and the same distance below the soffit, is also
continuous and carried round all landings and
floors with a graceful curve. This necessi-
tates the use of more concrete than would
be necessary if the beams were allowed to
project below the soffit, but the effect
obtained is a very pleasing one, and the idea
of graceful and easy ascent is conveyed to
everyone looking up the staircase. The
handrail is carried by an iron balustrade
fixed to the concrete stringer and following
consequently it is constructed of concrete,
on the cantilever principle, each step obtain-
ing its support from one wall only, with a
certain amount of bearing on the step below,
an open well between the up and down
flights being maintained. The concrete
used is composed of granite chippings and
portland cement in the proportion of 4 to 1 ,
and the steps, which are 2 ft. 9 in. wide, are
strengthened by the addition of steel tee
cores. They are finished with Mason's
patent treads and 1J in. of granolithic.
Reinforced concrete is employed in two
VARIED APPLICATIONS AND EXAMPLES
323
instances on account of its adaptability
under peculiar conditions. One of these
cases is that of a staircase which leads from
a lobby adjoining the lounge on the ground
floor to a restaurant placed on the first
floor, the shape being very winding and
curious, and as such great difficulty would
have occurred in
shaping ordinary
steel channels and
joists.
The other in-
stance is that of a
small staircase 2 ft.
6 in. wide which
has a support on
one side only, and
in addition has to
pass over a large
ventilating duct
where every inch
of space was of
great value. A
portion of the steps
required to be de-
signed as canti-
levers, and these
are reinforced with
two ^-in. indented
steel bars in their
upper surfaces,
while a portion of
the flight itself was
required to act as
a beam where pass-
ing over the duct
previously men-
tioned. This ne-
cessitated some of
the treads forming
an abutment for
the beam portion,
and these were re-
inforced with three
£-in. bars instead
of two, as the
longitudinal rein-
forcement was so designed to distribute the
pressure over three treads in each case. The
bottom flight, which is about 12 ft. long, was
designed to act as a beam without any appre-
ciable support at the sides, £-in. bars spaced
at 3-in. pitch being provided longitudinally
for the full length of the flight with trans-
verse bars at 12-in. centres, while the mini-
mum thickness of the concrete was 5 in. The
remaining staircases are constructed of
steel framing enveloped with concrete, and
no detailed description is necessary.
Central Elliptical Vestibule. — Fig. 524
illustrates the work over the large central
elliptical vestibule, 48 ft. by 32 ft., which
occurs on the ground floor. This is carried
right up through the first floor, with a gallery
Fig. 526. — Shuttering and Reinforcements for Sloping Roof (R. A. C.'s
Building)
around at this level, and finished midway
between the second and third floors with a
large skylight and a ceiling light under.
Owing to a large cove which was required
on the inside, all the steel framing had to
be bent to the required curve, and this
rendered the work very complicated. The
photograph is taken above the second floor,
and shows the shuttering for the concrete
curb to the large skylight.
324
REINFORCED CONCRETE
Sloping Roofs. — Sloping roofs occur on
both of the elevations. Fig. 525 shows the
shuttering and reinforcement in position,
and a portion of the concrete during the
execution of the roof facing Pall Mall.
The fourth floor only comes within the roof
at this point, and the height is 10 ft. 6 in.
from the floor level to the asphalt flat above.
The pitch of the roof is practically 60 degrees,
and it is carried through in one straight line
for the entire length of 228 ft. without any
the dormer windows, these being formed in
such a manner that the windows are set back
within the roof and not allowed to project
in the usual manner. This necessitated
forming the cheeks and the small flat at the
bottom of each window in concrete also, and
the same thickness is employed as in the
main slope. The cross bars are all turned
round into the cheeks and, in addition,
vertical bars are provided. The small flat
at the bottom of each opening is reinforced
Fig. 527. — Reinforcement around Elliptical Openings in Sloping Roof (R. A. C.'s Building)
break as shown. The thickness of concrete
employed is 4£ in., and the reinforcement
consists of J-in. indented steel bars placed
vertically at 10-in. centres, these being
turned over the rolled steel joists, which
act as trimmers at the intersection of the
flat and sloping portions of the roof. They
are also secured in a similar manner to the
steel joists placed at the foot of the slope.
In addition to the vertical reinforcement,
J-in. bars are placed horizontally at 2-ft.
centres to distribute temperature stresses.
Numerous openings are provided at
regular intervals throughout the length for
with £-in. bars at 10-in. centres, and similar
bars, two in number, are placed at the head
and bottom of the opening in the sloping
concrete to act as a tie, and round these are
bent the vertical bars which are cut off on
account of the opening. An excellent idea
of the shuttering adopted can be obtained
from Fig. 525.
The work to the back portion is of a
slightly more complicated nature, owing
to the fact that two floors occur in the roof,
thus the length of the slope is considerably
more. The two floors are the fourth and
the fifth, and the total height is 20 ft. 6 in.
Fig. 528. — Shuttering for Swimming Bath
Fig. 529.— Reinforcements for Swimming Bath (R. A. C.'s Buildinj
325
326
REINFORCED CONCRETE
from the fourth floor level to the finished
roof level. It will be seen, however, on re-
ferring to Fig. 526, that steel framing
is introduced at the fifth-floor level, and
thus the long slope is cut up into two
portions, although every alternate bar is
made continuous. This roof, unlike that
facing Pall Mall, is not continuous, but is
divided up by one central and two end
pavilions, which rise above the general roof
level about 4 ft. 6 in. The central pavilion,
the opening. The various' bars are secured
at the crossings to prevent displacement
when concreting.
The concrete was composed of 1 part port-
land cement, 2 parts sand, 2 parts clinker
and 2 parts ballast, mixed by machinery, and
the whole operation of crushing, screening,
gauging and mixing was accomplished at one
point in a specially erected machine.
Swimming Bath. — The construction of
the swimming bath affords an excellent
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Fig. 530. — Reinforcements for Swimming Bath (R. A. C.'s Building)
which is curved in section, and one end
pavilion are noticeable in Fig. 526. The
concrete to the central pavilion is built on
the curve, and a portion of the shuttering
for this can be seen. The general part of
the roof between the pavilions is built to a
pitch of 65 degrees and the end pavilions
to a pitch of 75 degrees. Another feature
which entailed careful execution is the
introduction of elliptical and circular open-
ings for the windows, and a detail view of the
work around two of these openings is shown
in Fig. 527. The reinforcement is hooked
over a curved bar which follows the line of
example of the adaptability of reinforced
concrete to work of all kinds. The bath is
situated in the central portion of the south
side of the basement floor, and its size is
86 ft. 9 in. by 30 ft. 8 in., these measure-
ments being to the face of the concrete work
only. The depth varies from 4 ft. 4 in.
at the shallow end to 8 ft. 10 in. at the
deepest point, the latter being at a distance
of 18 ft. from the diving end, which is 7 ft.
4 in. deep, thus giving a maximum depth of
water at the point required by the bather
when diving from the highest stage. These
measurements are from the floor level to the
VARIED APPLICATIONS AND EXAMPLES
bottom of the bath, the water level being
10 in. below the floor line.
Two flights of steps are provided on each
side near the ends, and these do not project
into the bath in the ordinary way, but are
recessed in the sides in order to keep the
entire surface of the bath free from obstruc-
tion. The reinforced wall is carried round
on the outside of these steps, and the latter
are carried on plain concrete stepped as
required.
One of the principal points considered in
327
30 ft. 8 in., and thus thick walls could not
have been adopted such as would have been
necessary with brick or plain concrete.
Another point in favour of reinforced
concrete was the fact that a subway was
required around all four sides of the bath
on the outside, and this was limited in
width by the many grillages and other
underground work in the vicinity making it
necessary to retain every inch of space that
was possible, and this subway prevented
the wall of the bath from receiving any
Fig. 531. — Reinforcing Sides of Bath and Concreting First Layer of Bars in the Bottom
the constructional work to the bath was that
of keeping the side and end walls and the
bottom entirely free from the adjoining
stanchions and grillages, in order to avoid
the possibility of any settlement of the
latter from affecting the bath, especially as
this would be liable to crack the marble
lining and cause serious defects. There are
twelve stanchions which run down at different
points round the bath, and these are situated
close to the edge, as will be seen from the
fact that the distance from centre to centre
across the bath is only 33 ft. 9| in., while
the width of the bath itself in the rough is
assistance on the outside, and, in addition,
necessitated carrying the reinforced floor
over the subway.
The whole of the space to be occupied by
the bath and subway was first prepared, and
the continuous asphalt damp-course, which
is carried under the entire building, was
formed and laid to the required levels to
suit the work of the bath proper.
Pig. 528 shows a portion of the shuttering
for the concrete work being erected, and a
portion of the subway is just noticeable.
The last stage of the pumping is also depicted,
with the asphalt damp-course completed with
328
REINFORCED CONCRETE
the exception of the small piece surrounding
the well, from which the water is being
drawn. This well was formed to allow the
pump to work and keep the surface water
from flooding the work, as the bottom of
the bath is below the water level, and the
pumping was continued until the last possible
moment. When the suction pipe was
removed, the pipe leading to the well was
sealed with a special cap, the concrete filled
in, and the asphalt damp-course made good,
thus completing the huge underground tank
in which the entire building stands.
After the asphalt was complete, the con-
crete work was begun, and carried out as
rapidly as possible. The thickness of the
side and end walls is 12 in., while 18 in. of
concrete is employed for the bottom. The
concrete is splayed at the junction between
the vertical and horizontal work, and it is
also splayed back at the top of the walls.
To prevent the necessity of forming a hole
in the reinforced work and in the asphalt
damp-course below for the outlet to the bath,
which is situated at the deepest point, a dip
or trench is formed in the two former to allow
the water pipe to be carried through to the
valve chamber above them, and the work is
brought to the level as required with plain
concrete filling. The reinforcement to the
bottom consists of f -in. square indented steel
bars spaced at 6-in. centres across the width
of the bath, and similar bars longitudinally at
12-in. centres. These are placed in the
under surface of the concrete, and a similar
amount of reinforcement is employed in the
upper surface.
The reinforcement of the walls consists of
vertical bars £ in. square, spaced at 6-in.
centres, with the same size bars placed
horizontally at 9-in. centres. The splayed
portions are also reinforced, and the manner
in which this is done is clearly illustrated in
Fig. 529, which is a view showing the first
layer of reinforcement in position at the
shallow end of the bath. An excellent idea
of the work in progress can be gathered from
this photograph, indicating, as it does, the
shuttering, the reinforcement in position,
the recess for the steps, and the small blocks
used for levelling purposes. The hole
which is shown through the shuttering at
the end of the bath is one of the four openings
required for the inlet pipes. The bars were
securely wired at every crossing, this entailing
about 20,000 wirings in all, the object being
to prevent any possible displacement of the
reinforcement during the filling in of the
concrete. The concrete was composed of
1 part portland cement, 2 parts sand, and
4 parts of ballast ; and the lower layer of
bars was first concreted in the bottom, and
then 1 ft. of the sides, returning again to the
bottom when the second layer was encased.
After this, the sides were completed, and
during the whole time occupied in the execu-
tion, the whole of the concrete in position
was watered night and day in order to form
one solid mass when complete, and avoid
the possibility of the work being in separate
layers without proper cohesion.
Fig. 530 gives a good idea of the reinforce-
ment to the whole bath, looking from the
shallow end, and it shows the bars as placed
approximately in position previous to the
wiring together at every crossing. Fig. 531
illustrates the reinforcement to the sides
and shows the concreting of the first layer
of bars in the bottom, and the shuttering
for the lower part of the sides.
The total weight of the reinforcement
used in the bath is about 40 tons. After
the completion of the concrete work, a layer
of asphalt was applied in the inner surface,
to form a water-tight lining and prevent any
possibility of the water in the bath from
percolating through the surrounding work.
The finished bath is lined throughout with
white Sicilian marble, 1J in. thick, laid in
slabs, 3 ft. wide and the full depth of the
bath, without horizontal joints. The scum
trough and spittoons are in similar marble,
as are also the floor paving around ; outlets
from the first-named are taken through into
the subway previously mentioned, and carried
into a drain running round the bath. The
steps leading to the bath have marble treads
and risers.
Procedure in Executing the Work.—
The following is a brief description of the pro-
cedure in executing the reinforced concrete
at the Koyal Automobile Club. The cement
upon arrival at the site was tested. Careful
attention was paid to the selection of store,
and a clean, dry vault was used, with facili-
ties provided for covering up in frosty and
mild weather. The bags were only released
in the quantity required from time to time,
and not left lying about the site. The aggre-
gate for the reinforced floors and roof was
as follows : 1 cement, 2 sand, 2 broken
brick, 2 clinker.
The broken brick was of approved quality,
not soft, all traces of lime and dirt removed
VARIED APPLICATIONS AND EXAMPLES
329
and screened to pass a f-in. mesh, the dust
resulting from the crushing being removed.
The clinker was carefully inspected for
impurities and live matter (such as sulphur
and coal), and if not well burnt was con-
demned. No coke breeze or ashes were
allowed on the job. The sand was clean
and sharp, and passed a TVm- rnesh. For
the small amount of concrete which was
mixed by hand, gauge-boxes were measured
and cubed, and, if found accurate, were
marked, and only those so marked were
screen, falling thence into a hopper below,
the outlet of which was directly over the
gauge-box, the last-named being worked on
a centre pivot. Upon lifting two iron
shutters fitted at the base of these outlets,
the ballast and clinker fell into the gauge-
boxes, the pin was withdrawn, and the
material precipitated into the mixer below.
The sand and cement were gauged on the
staging and shovelled into the mixer. The
mixed concrete was shot into a skip on the
lift hoist, and raised to the required floor,
allowed to be used. The clinker, broken
brick and sand having been gauged, the
cement was thrown over the mass, the whole
was turned twice dry, water added by means
of a rose watering-can and then turned
twice wet, the water being taken from the
company's main. Most of the concrete used
on the job was mixed by machine.
Fig. 532 is a diagram showing the concrete-
handling and conveying plant in use on the
Royal Automobile Club building. The
ballast and bricks were shot from the skips
of the steam crane to the staging of the
crusher. The aggregate from the crusher
was shovelled into the revolving 2-in. mesh
Fig. 532.— System of
Handling Concrete
at Royal Automobile
Club's Building
by which method the whole of the
building was concreted from the one
spot.
THE ROYAL LIVER BUILDING
This well-known building in Liver-
pool is constructed entirely of reinforced con-
crete, and may be regarded as the first
" skyscraper" erected in England. The site,
which was formerly occupied by St. George's
Dock, is about 301 ft. long and 175 ft. 6 in.
wide. The architect was Mr. W. Aubrey
Thomas, of Liverpool, and the Hennebique
system of construction was employed. The
height from the street level to the main roof
is about 167 ft. ; to the centre of the clock,
226 ft. ; and to the top of the Liver bird is
310 ft. There is a basement floor 12 ft.
below the street level, while the depth of
the foundations varies from 18 ft. to 40 ft.
below the floor level, giving a total average
330
REINFORCED CONCRETE
distance of 351 ft. from the foundations to
the top of the tower. There are eleven
stories in the main building, and seventeen
stories in each main tower. There has been
much discussion at various times as to the
relative advantages of reinforced concrete
and structural steel when rapid construction
is an important consideration, and as an
example of the possibilities of the former, it
surrounded by retaining walls, which in
reality act as deep beams spanning the
distance between the foundation piers, the
latter having an average distance of 30 ft.
apart. The subsoil water level actually rises.
to a height of 10 ft. above the basement
floor line, and the walls were formed with
5-in. panels only, stiffened by vertical
counterforts at short intervals apart. A
Fig. 533. — Columns and Arched Ribs in the Royal Liver Building
is interesting to note that in this building,
which covers an area of about 45,000 sq. ft.,
the first seven floors were constructed in an
average time of nineteen working days per
floor. Very large spans were adopted in
order to give the maximum amount of clear
floor "space, and this necessitated the carry-
ing of very heavy loads by the columns. The
foundations were formed by large concrete
piers, which were sunk to the level of the
old red sandstone, and connected at the top
by reinforced concrete slabs. The site is
continuous coping, 18 in. wide and 8 in.
deep, was constructed at the top of the wall,
and the base consisted of a wide toe and a
slightly projecting heel. This portion of the
work was very difficult on account of the
water. In some cases, the foundation piers
were not available as supports for the
retaining wall, and cantilevers were then
formed which projected 11 ft. from the
main columns and carried the wall, together
with the basement flooring.
The columns provide some interesting
Fig. 534. — Arched Beam (before Concreting) in Royal Liver Building
Fig. 535.— " Skeleton" of Royal Liver Building
333
REINFORCED CONCRETE
construction on account of the exception-
ally heavy loads to be carried ; the interior
ones beneath the main towers sustain a
total load of 1,200 tons each, while the
corresponding columns against the external
walls support a total load of 1,500 tons
each. These columns carry a load of
900 tons at the ninth floor level, and they
are 3 feet square at this point, while in the
basement they have an extreme size of 8 ft.
by 3 ft., with reinforcement consisting of
twenty-five bars If in. in diameter, and
sixteen bars If in. in diameter, tied in every
direction by J-in. diameter steel links spaced
at 3-in. centres. The remaining columns in
the exterior walls, which carry the granite
cases, and in addition support is given to
two large arched beams which each carry
1,480 tons. These arches have a clear span
of 60 ft. and a rise of 32 ft., thus extending
from the ninth floor level to 2 ft. 9 in. above
the eleventh floor level. The thickness of
the concrete is only 1 ft. 9 in., and they
form an interesting example of scientific
construction which would be impossible in
any other material. Other examples of
arched members are provided in the glass
roof over the ground floor. The clear span
is 58 ft., and the total depth required was
only 23 in., and the width 7 in., this giving
the maximum amount of glass area, and
allowing tlie construction of members which
Fig. 536. — Cathedral at Manila, Philippine Islands
curtain walls and the floor loads, support a
total load of about 750 tons each at the
lowest level, where they are 8 ft. by 1 ft.
9 in., while the average size at the top is
14 in. by 12 in.
The floor beams, generally speaking, have
a span of about 40 ft. with a width of 9 in.
and a depth of 34 in., including the thickness
of the floor slab. The reinforcement con-
sists of four bars in the tensional area and
two bars in the compressional area, pro-
vision against shear and diagonal tension
being made by turning up two of the tension
bars at the ends of the span and by the
introduction of vertical stirrups. Some
large beams were required at the ninth and
tenth floor levels in the west main tower,
these having spans over 40 ft. long in some
are slender and in keeping with their function.
The floors were constructed throughout of
the Hennebique hollow tubes, which are
admirably suited for a building of this
description, where efficiency, combined with
lightness, are essential. These tubes are
formed of concrete or fireclay, and they are
made about 20 in. wide by 24 in. long
by 7 in. high, and are placed in parallel
longitudinal rows between the secondary
beams, the latter being moulded in position
between the tubes, which are temporarily
supported on planks carried at each end by
the main beams. The tubes are formed
with sides which are inclined slightly out-
wards towards the top, thus preventing any
tendency to drop out on the under-side of
the floor, and the top of the tubes are
VARIED APPLICATIONS AND EXAMPLES
slightly arched to increase their strength.
A minimum covering of 1£ in. of concrete
was employed above the tubes, thus form -
ing a floor construction 8| in. thick. This
system has the advantage that no elaborate
centering is required, as the sides of the
tubes form a permanent shuttering for the
secondary beams, while the tubes can, if
333
The towers are finished with domes con-
structed in reinforced concrete and covered
with sheet copper, while large circular
openings were required for the clock dials
which are slightly larger than those in the
Clock Tower of the Houses of Parliament.
Many interesting staircases are formed in
the building, and all of these are in rein-
D'O
SCCTIOH B~8.
HALF SECTION THROUGH fiAvc LOOKING TOWARD CHAHCCL
Fig. 537. — Various Sections through Nave of Manila Cathedral
necessary, be made on the site, thus avoiding
any possibility of delay.
The external walls throughout are con-
structed with a granite facing which, gener-
ally speaking, has a thickness of 14 in.,
and this is carried at each floor level
by the reinforced concrete construction.
This method enabled several floors to be
built simultaneously, thus saving a great
deal of time in the finishing of the structure.
forced concrete, those in the upper part of
the towers being of the spiral form. Figs.
533 to 535 are photographic views which
give some idea of the magnitude of the
building.
CATHEDRALS AT MANILA AND POTI
Figs. 536 to 545 show the Episcopa
Cathedral of St. Mary and St. John erected
at Manila, in the Philippine Islands, the
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VARIED APPLICATIONS AND EXAMPLES
337
architects being Messrs. Sturgis and Barton,
of Boston, and the contractors the Atlantic
Gulf and Pacific Company. The illustra-
tions give a good idea of the construction.
The cathedral, which seats 1,000 persons,
is built entirely of reinforced concrete, the
vertical walls being hollow. The rein-
forcement used was mild steel in the form
of round rods.
Figs. 546 to 549 (originally due to Le
Beton Arme) show a cathedral at Poti,
Kussia, erected in reinforced concrete. It
was designed by Prof. Marfield, and the
construction is in accordance with the
concrete engineers, The Trussed Concrete
Steel Co., Ltd. ; constructional steelwork,
Dawnay and Sons, Ltd.) is shown in plan and
elevation by Figs. 550 and 551, details being
illustrated by Figs. 552 to 565. It stands on a
solid concrete platform, varying in thickness
from 15 in. to 7 ft. 6 in., the under-side of the
platform being at one level throughout, and
the additional thickness being applied to the
upper side. Half-in., f-in., and 1-in. trussed
bars reinforce the concrete foundation,
where extra loading or reduction in thick-
ness necessitates. A feature of the building
is the large dome which is carried on steel
Fig. 546. — Longitu-
dinal Section through
Poti Cathedral
Hennebique system. The walls, though
shown as solid, are built hollow, there being
an outer wall 10 cm. thick, an air space of
18 cm., and an inner wall 8 cm. thick ; the
pillars also are hollow, the spaces being
utilised in the ventilating and heating
systems. The principal arches have a
span of 12.30 metres, and the vertical
distance from the springing to the topmost
point of the grand cupola is 29.10 metres.
Figs. 298 to 305 (pp. 194 to 196) show the
erection of the dome.
WESLEYAN HALL, WESTMINSTER
This building (architects, Messrs. Lan-
chester and Eickards, F.K.I. B.A. ; general
contractors, Messrs. Dove Bros. ; reinforced
22
lattice stanchions, which are borne by grillage
foundations, each pair of stanchions resting
on eight 10-in. by 5-in. rolled steel joists,
which are at right angles to seven 14-in. by
6-in. joists about 16 ft. long underneath,
the whole of the steelwork in the foundations
being enveloped with concrete.
In the basement of the building there are
two large tea-rooms, each about 65 ft. long
by 41 ft. wide, with ceilings in the form of
reinforced concrete domes, carried in each
case by six piers connected by curved rein-
forced concrete ribs. The elliptical dome in
the centre of the ceiling springs from a
rectangular plan, and the rise is only about
6 ft. ; the concrete is 9 in. thick, reinforced
with |-in. trussed bars at 12-in. centres.
Figr. 547 and 548. — Elevation and Horizontal Section of Dome of Poti Cathedral
Fig. 549.— Ground
Plan of Poti
Cathedral
338
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Figs. 550 and 551.— Cross Section and Plan of Wesleyan Hall
339
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REINFORCED CONCRETE
Above the dome, as shown in the illustrations,
comes the ground floor, consisting of a 5-in.
thickness of concrete reinforced with J-in.
bars at 12-in. centres, there being an open
spandrel between this floor and the dome
below. The piers which carry the weight of
dome and floor are 4 ft. square and are rein-
forced by eight 1-in. trussed bars and 1-in.
and IJ-in. spiral hooping (pitch, 3 in.). On
the next floor (the first floor) is a large
conference hall, the floor of which is carried
by reinforced concrete columns which have
been illustrated in an earlier chapter (see
pp. 177 to 180). Their height is about 28 ft.,
and their diameter 2 ft., the reinforcement
girders being also used in places. The front
of the gallery is carried by a reinforced
cantilever, 14 ft. 6 in. long, the sloping
portion of this making an angle of about
52 degrees with the horizontal and carrying
the tiers for the seating accommodation.
There are domes to the foyers in the front
of the building, and there is a dome — a very
conspicuous feature of the building — in the
centre over the main hall. The latter is of
double construction, that seen from the
inside being of reinforced concrete, that seen
from the outside consisting of a light steel
framework. The concrete dome, of 114 ft.
diameter, is carried on four concrete girders
Fig. 566. — Free Church, Hampstead Garden Suburb, in Course of Construction
consisting of eight 1-in. ribbed bars, hooped
spirally with Ty-in. steel at a 6-in. pitch, the
bars being 1J in. from the face of the work.
The entasis of each column was formed in
the mould itself, and was not built up by
the plasterer. The forms are described at
the reference above given. Throughout the
building the floors are of reinforced concrete,
lightened in a large number of cases by the
introduction of 8-in. or 12-in. hollow tiles
on edge in the lower surface of the floor,
the 6-in. space between them being occupied
by the concrete and trussed bars.
The conference hall, with domed roof and
galleries, affords a number of interesting
examples of reinforced concrete work. The
galleries are of reinforced concrete, steel-plate
of 68 ft. span, and 18 ft. deep at the centre.
They are supported on eight steel lattice
stanchions, on each pair of which there is
an estimated weight of 1,100 tons.
FREE CHURCH, HAMPSTEAD
Fig. 567 presents the chief elevations of
this church, while Fig. 566 is a photograph
of the centering for the inner dome. The
architect was Mr. E. L. Lutyens, F.K.I.B.A.
The church has a seating capacity of 1,100
in addition to the choir gallery. The
columns, beams, and domes are constructed
in reinforced concrete. The eight columns
in the nave are about 28 ft. high, and, except
in the case of those under the dome, they
are 2 ft. 9 in. square for a distance of 9 ft.
VARIED APPLICATIONS AND EXAMPLES
347
from the floor, and circular above that height.
They are reinforced by four |-in. square bars
in the corners, and by eight similar bars
uniformly spaced on the circumference of an
internal circular cove. The eight bars rein-
force the circular part of the column, the
square bars bending into the circular portion.
The vertical reinforcements are hooped with
columns carry reinforced concrete arches, of
which the rise is 13 ft., the depth at crown
4 ft. 6 in., and the thickness 2 ft. The arches
themselves support the inner dome, having
a radius of about 13 ft. and a rise of about
12 ft. Above the pendentives, which are
4 in. thick and reinforced with expanded
metal, the angles are cut off by beams 4 ft.
Fig. 567. — South, West and East
Elevations of Free Church,
Hampstead Garden Suburb
links of ^-in. steel, spaced 12 in. apart.
Each column rests on a reinforced concrete
base measuring 8 ft. by 5 ft. The columns
carry 18-in. square beams of 22-ft. span.
The columns that carry the dome are also
about 28 ft. high, the"jnrst 9 ft. being 2 ft.
9 in. square, the upper portion being 2 ft.
4J in. square. » The reinforcement is very
similar to that of the round columns. These
6 in. deep and 2 ft. thick, which convert the
plan to an octagon. There is a 6-ft. opening
in the dome, the ring being reinforced with
two l|-in. rods and two 1-in. rods. The
main portion of the dome is reinforced with
T5F-in. rods, circumferentially and radially.
The outer dome has a timber framing which
is supported by the frustum of a cone in
reinforced concrete 30 ft. 6 in. high.
349
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350
VARIED APPLICATIONS AND EXAMPLES
ST. BARNABAS' CHURCH, JAMAICA
Figs. 568 to 575 show a small church (St.
Barnabas', George's Plain, Jamaica) erected
to the designs of Mr. Hastwell Grayson, M.A.,
351
but later it was found desirable to design the
principals in the same material to provide
greater stability. The spars and joinery are
in hard wood, as soft wood is quickly
Fig. 575. — Sections through Nave and Chancel, St. Barnabas' Church, Jamaica
F.R.I.B.A., the reinforced work having been
designed by Mr. Parr Emett. The church
destroyed by white ants. The donor was
anxious that through currents of air should
was planned for a coloured congregation of be encouraged, and that no inaccessible
230, including choir. George's Plain is close spaces should be left where scorpions might
to, but not actually in, the districts liable to breed. The whole of the reinforcement
Fig. 576.— Balcony and Cantilevers in Majestic Theatre, Los Angeles
hurricanes and earthquakes. In the first
instance, concrete was adopted for the walls
only on account of the heavy charge for
freight and cartage on other walling materials,
consists of ordinary commercial bars^ in
rather long lengths, bound together with
J-in. wire. The bars in the arches and across
the floor for tying in the feet of the arches
352
REINFORCED CONCRETE
are 1 in. by 1 in. square, those in the founda- 1-in. square twisted bars, the two sets being
tions, 1 in. by i in., and those in the walls connected by stirrups. To span the audi-
1 in. by J in. torium there are three trussed girders (see
Fig. 578.— Trussed Girder at
Majestic Theatre, Los
Angeles
Fig. 577. — Cantilevers at
Majestic Theatre, Los
Angeles
T
MAJESTIC THEATRE,
LOS ANGELES
Whilst this theatre
(see Figs. 576 to 580)
is built of reinforced
concrete entirely, only
one or two of the most
prominent features need
be alluded to here.
Fig. 576 is a view of
the balcony and of the
cantilevers which carry
the gallery. The bal-
cony, which has a free
overhang of 30 ft., is
carried by six canti-
levers, the two side ones
being brackets 18 ft.
long and projecting
from the truss columns
on each side of the
auditorium, whilst the
four central ones (Fig.
577), having a clear
projection of 29 ft.
and 30 ft., rest on in-
dependent columns, ex-
tend back over the foyer, and are anchored
in the wall at the rear of the auditorium.
The cantilevers are 78 in. deep at their
maximum section, the tensional reinforce-
ment is seven If-in. square twisted bars,
and the compression reinforcement three
Figs. 578 to 580) having horizontal chords,
and carrying a three-story building. Their
length is 71 ft. ; depth, 10 ft. ; and widths
at top and bottom 24 in. and 18 in. These
trusses are reinforced at the top with twelve
l|-in. square twisted bars, and at the
VARIED APPLICATIONS AND EXAMPLES
353
bottom with fifteen If -in. bars, and the bars
are bent as shown in the diagram. It was
thought desirable to splice the steel, the lap
joints being 5 ft. to 6 ft. long, with three
clamps each. The centering having^ been
removed six weeks after pouring, each of
the trusses carried a load of 500,000 Ib.
without measurable deflection, an additional
250,000 Ib. on one of them causing a deflec-
tion of less than ^T in. The
diagram shows that the trusses
are carried by columns 36 in.
by 60 in. in section, the chief
reinforcements of which are
carried down into the footings.
As the columns are more than
80 ft. long, the steel bars had
to be jointed, laps of about
4 ft. being allowed, with three
clamps to each. Only one bar
in four has a joint at any one
level. The columns, with the
section slightly reduced, are
continued up to the next floor
level, where there is produced
a thrust which is assumed to
equal that caused by the maxi-
London, an extension that includes rein-
forced concrete columns, floors and roof
constructed on the Kahn system. The
floors and columns do not require description,
but the roof and dome deserve attention.
The roof principals, of the arched type, act
as girders, tie-bars being thus unnecessary,
as the beam takes up the whole of the stress
due to the thrust of the arch. (The con-
Figs. 579 and 580.-
Fig. 580
-Trussed Girder in Majestic Theatre,
Los Angeles
mum moment the elongation of the column
is capable of carrying. The floor beams are
built as horizontal columns to take the
thrust ; otherwise the large negative
moment in the trusses at the support
would have necessitated the employment
of columns of immense sectional area.
NATIONAL GALLERY EXTENSION
The illustrations (Figs. 581 to 590) show
the extension to the National Gallery,
23
Fig. 579
struction is on the same lines
as that of the arched principal
at the Hammersmith Baths, de-
scribed on p. 198.) The dome
has a span of 32 ft., and is
34 ft. 6 in. above the floor.
The eight ribs are supported
by four beams. At the top,
the thrust of the ribs is taken
by a reinforced ring round
the turret ; at the bottom,
rods, which form a ring round
the dome, take the thrust.
This, with other details, is
made clear in the illustrations.
A slight detail, but one of
practical importance, is the
leaving of bolts in the concrete
for the more convenient fixing of grounds
for fibrous plaster, blind fittings, etc.
ALUMINIUM WORKS AT KINLOCH-
LEVEN
The works of the British Aluminium Com-
pany, at Kinlochleven, are not of reinforced
concrete throughout, but a great many of the
details — columns, bunkers, lintels, galleries,
floors, etc. — are of that material, having
been constructed on the Coignet system.
— ELEVATION AT P. F. —
354
SECTION OF RIB
Fig. 582. -Details
of Intermediate
and End Ribs of
Dome Roof to
National Gal-
lery Extension
Figs. 583 and 584. — Dome Roof to National Gallery Extension
355
35$
VARIED APPLICATIONS AND EXAMPLES 357
Probably owing to the unlimited supply reinforced concrete was only -55 that for
of local rock which was crushed to form the the same work in steel. The bunkers are
aggregate, the estimate for bins, etc., in of large capacity, one of them having a
Fig. 592. — Section through Bunker at
Kinlochleven Aluminium Works
Fig. 593.— Detail of Beam of Bunker (A,
Fig. 592)
cubic content of 80,000 cub. ft. ; Figs. 591
to 593 show the construction of one of them,
the beam at A (Fig. 592) being shown in
detail by Fig. 593. The concrete was in the
Fig. 591. — Bunker at Kinlochleven Aluminium Works
358
REINFORCED CONCRETE
proportions of 3 : 5 : 10, and the aggregate
was most carefully graded as follows :
Passed f-in. mesh and retained on f-in.
mesh, 80 parts ; ditto, f-in. and £-in.,
90 parts ; ditto, £-in. and f-in., 570 parts ;
and ditto, f-in. and J-in., 260 parts ; total,
1,000 parts. Similarly, the sand was graded :
250 parts, between J-in. and J-in. ; 300
parts, between £-in. and TVm« ; 200 parts,
between TVm- and ^Vm- ; and 250 parts,
between 2Vm> and gVm- 5 total, 1,000 parts.
ROYAL INSURANCEIBUILDING, LONDON
The Koyal Insurance Building in Lombard
Street^ (architects, Messrs. Gordon and
Gunter ; reinforced concrete engineers, The
Considere Construction Co., Ltd. ; contrac-
tors, Trollope and Colls) is constructed in
steel as regards the general framing, much
of the remainder of the work being executed
in reinforced concrete. There being two
floors below the pavement level, the lower
one being 21 ft. down, it was necessary to
build a very strong retaining wall, the
drawings and photographs of which will be
found on pp. 143 to 145, where it is shown
that reinforced concrete possessed undoubted
advantages over brickwork. Part of the
horizontal portion of the retaining wall, as
illustrated at the above reference, is con-
Figs. 594 and 595. — Plan and Section of Foundation Slab, Royal Insurance Building
VARIED APPLICATIONS AND EXAMPLES
structed as a beam with the object of dis-
tributing the pressure from the stanchions
which carry the main walls of the super-
structure. The retaining wall is undoubtedly
an extremely interesting piece of work from
the point of view of design. Figs. 594 to 598
illustrate some of the reinforced concrete
work in the building. The drawings of the
foundation slab illustrate the large number
of stirrups used, this being a strong point
of Considere designs. Reinforced concrete
foundations were adopted for the steel stan-
chions throughout. The largest foundation
is nearly 10 ft. square at the bottom, the off-
sets reducing this to 4 ft. square at the top.
This base is reinforced with forty-four bars
near the lower surface, and with forty-eight
bars elsewhere, as illustrated in Figs. 594
and 595. Illustrations below show the strong
room situated in the sub-basement floor, and
measuring 8 ft. 10 in. high in the interior,
divided into two compartments by a 10-in.
wall, in which there is a small doorway. The
outer walls and roof of the strong room are
359
10 in. thick and reinforced with both
horizontal and vertical bars, which are
connected by stirrups 4 in. apart. Rein-
forced floors, lightened by means of tiles, are
used, the thickness of 7 in. being made of
2| in. of concrete, and 4| in. of hollow tile
(4| in. by 9 in.), but on both sides of the
steel joists that carry the sides of the bay
the concrete was kept solid for 18 in., and
into this the lower bars were anchored. Bars
also pass over the tops of the steel joists.
Over the Lombard Street entrance is a rein-
forced concrete dome of 12 ft. diameter, the
concrete being 4 in. thick, and reinforced
circumferentially by TVin. bars at 12-in.
centres, whilst the radial reinforcement is
T5g- in. in diameter and spaced at 6-in. centres.
Arches 5 in. thick are formed on four sides,
so as to adjust the dome to the square plan
(see Figs. 597 and 598).
WALL WITH POST BUTTRESSES
Figs. 599 and 600 show walls that are not
of the continuous plain wall type, but are
Fig. 596.— Details of Strong Room in Sub-basement, Royal Insurance Building
;i
PL/IN XT CBOWW FLO0J5
Figs. 597 and 598. — Vertical and Horizontal Sections of Dome over Main
Entrance, Royal Insurance Building
360
Figs. 599 and 600.— Walls with Post Buttresses
361
- 4 — *•»" J. J--J
Fig. 603
1
Fig. 602
Figs. — 601 to 604. — Moulding Box for Fence Posts
i Fig. 604
Fig. 60S.— Fence Posts
362
VARIED APPLICATIONS AND EXAMPLES
363
divided into bays by posts. These posts
were manufactured at a depot in moulding
boxes (see Figs. 601 to 604), and conveyed
to the site. The design of the posts, as will
be seen from Fig. 605, includes a groove on
two sides ; these are the sides into which
the wall keys. Another and more particular
object of the groove is that the contraction
of the wall, if any, that may take place will
be here, where it will not be seen, instead of
cracking in any part of the wall, as might be
the case in continuous concrete walls. A
piece of wall of this description, after having
been up for twelve months, showed no
cracks in any part, but there was a slight
contraction noticeable at some of the posts.
The posts have several holes through them ;
these are for the horizontal reinforcing wires.
The posts themselves have the usual rein-
forcement (see Fig. 606). The posts are sent
to the site and erected just in the same way
as ordinary fence posts.
The horizontal reinforcing wires, eight in
number (providing for a wall 6 ft. high
above ground line), are run through each
post, and are strained tight with a special
iron straining post, illustrated by Fig. 607,
erected temporarily for that purpose. When
this is done the result is a highly efficient
strained-wire fence supported by concrete
posts. Vertical rods are next interlaced
with the horizontal wires ; these
have the effect of stiffening the wiring
considerably. The shuttering may now
be placed in position for moulding
the wall. Ten bays of shuttering
are necessary to keep a gang of five
men going. Details of the shuttering are
shown by Figs. 608 to 610, designed to
fit up to the posts and leaving the desired
thickness of the wall between them ; the
concrete is then put in in small quantities,
and this, when well rammed with special
/Bmkr Nztz gauge wirt
as the concrete may be rammed with the
special spades to such an extent as to
bend or break almost anything. The wood
becomes wet through, and in that condition
it bends very easily ; plenty of stiffening
uprights are needed, as shown, with bolt-
through, to keep the shuttering from spreads
ing. In making the shuttering the boards,
if dry, should not be cramped up tight, but
should be just butted up to each other and
the edges bevel faced, as otherwise, when
they become wet and swell very much, they
get badly out of shape ; the bevel edge
Fig. 607.— Iron Straining Post
allows the boards to swell without buckling,
and should they run over each other slightly
they can be planed down. The boards for
the shuttering should be selected from the
best mernel or pitch pine, free from shakes,
knots, and other imperfections, as the
Fig. 606. — Reinforcement for Fence Posts
spades, gives a face to the wall as smooth
and hard as polished steel, no touching up
being necessary. The richer the concrete
the better the face ; 4 to 1 gives a splendid
result, in proportions of 5 of stone all passed
through 1-in. sieve, 3 of sand, 2 of cement.
The shuttering must be made very strong
lightest imperfection on the surface of the
boards spoils the face of the wall ; bad
defects in the face of the boards will bring
the face of the concrete away with them.
The holes for the bolts should all be burnt
out so that the bolts may work freely in the
holes when required to be removed. The
— ^
r
rr
71
4-<-
Fig. 608
ii
i—
r
Fig. 610
hil— .
"1
Fig. 609
I AnCHOf B/9/rs
Figs. 608 to 610. — Reinforcement and Shuttering to Wall Fig. 612. — Reinforcement
of Heel of Retaining Wall
Fig. 611. — Retaining Wall, Dilworth Street, Pittsburg
364
VARIED APPLICATIONS AND EXAMPLES
365
bolts should have a square, quick-acting
thread to enable the wing nut to be mani-
pulated expeditiously. The washers for the
bolts should be screwed on to the wooden
uprights ; they will then always be in the
place where they are required.
The whole height of the shuttering has
to be erected straight away on account of
the bolts and upright timbers, as when these
are once put in and screwed up no other
boards can be put in. Therefore, with the
full height of the shuttering erected, it is
necessary to have long spades to ram the
concrete at the bottom, shorter ones being
used as the top is reached. With such a
thin wall the space between the shuttering
Fi
Figs. 613 to 616. -Photographs, Elevation, Plan and Sections of Spiral Staircase
366
REINFORCED CONCRETE
and the wire reinforcement is less than
1£ in. ; therefore a simple plain spade of
hard-wood or iron, free from projections,
must be used.
RETAINING WALL
Fig. 611 shows part of a retaining wall
erected in connection with the construction
of Dilworth Street, Pittsburg, in 1909. The
wall is built on the site of a steep hill, upon
which had been deposited clay and rubbish
to a depth of from 8 ft. to 10 ft., and at a
depth of 9J ft. below this shale of good
quality was found. The reinforced wall was
designed for three depths — 35 ft., 30 ft., and
21*5 ft. respectively from top of wall to
bottom of floor, and it rests upon piers ex-
tending along the front of the wall and
underneath each rib. The required width of
base, as computed, was 17 '2 ft., and the
width taken for the design was 17 '5 ft.,
including a 6-in. toe projection. The front
wall transmits the lateral thrust to the ribs,
and is designed as a slab fixed at the ribs.
At the ribs the reinforcing rods are placed
through holes punched in angles made of
structural steel, the object being to assure
the alignment of the rods and to tie the
front wall to the ribs. A double line of
reinforcement is used in the front wall. The
ribs were computed as cantilevers fixed at
the base, where they are tied into the floor,
which is held by the superimposed load of
earth. They were spaced 9 ft. apart. The
rib rods are run down the back of the wall,
some of them extending into the projection
at the heel, and some of them being placed
under transverse 1-in. rods, which in turn
engage the reinforcement in the floor.
Vertical ties were added. There is a
double reinforcement in the floor, so as
to provide for negative moments at
the base of the ribs. The design of
the heel projection is shown in Fig. 612.
The concrete in the reinforced parts of
the structure was made in the propor-
tions of 1 : 2| : 5 ; elsewhere it was
1:3:7.
STAIRWAYS
The Franco-British Exhibition at Shep-
herd's Bush, London, W., in 1908, contained
a few examples of reinforced concrete, one
of the most interesting being a spiral stair-
way (see Figs. 613 and 614) of fifty-four
steps, designed and erected on the Henne-
bique system. The line drawings (Figs. 615
and 616) show how graceful and striking the
design was ; and it will be understood, of
course, that in practice such a stairway
would be in monolithic connection with the
floor beams, both at the top and the bot-
tom. The stairway was exhibited as here
illustrated, it standing on a relatively small
area and supporting the spiral coils and
the projecting platform solely by the elastic
strength of the reinforced concrete con-
struction. The height from ground to plat-
form level was 27 ft., the over-all diameter
of the spiral 16 ft. 4 in. The two curved
stringers were 8 in. wide, by 2 ft. 4 in. deep.
The distance between them was 4 ft., the
risers were 6 in. deep, and the treads varied
from 7J in. wide at the inner curve to 15f in.
at the outer one. The cantilever platform
is bounded by a kerb 8 in. wide by 1 ft. 6 in.
deep, and finished by a 4-in. slab. The foun-
dations were only 3 ft. deep below ground
level, and consisted of a rectangular caisson,
16 ft. by 10 ft. in plan, formed of longitudinal
and transverse beams connected at the
top by a continuous slab. The calculations
showed that the centre of gravity when the
stairway was not loaded or carried a uni-
form load, passed through a point the posi-
tion of which guaranteed stability. The
result of calculations, assuming the stairway
to be heavily loaded at one side of the
centre line, showed that the centre of gravity
would pass through a point which was
only a few inches away from that corre-
sponding with the centre of gravity for uni-
form loading.
The stairway shown in Fig. 617 is in the
Mclntyre building at Salt Lake City, U.S.A.
The photograph shows the work in an un-
finished state. As finished, the treads and
risers are of marble.
Staircases at the Eitz-Carlton Hotel, New
York, have some features of interest (see
Fig. 618). They are straight until the first
floor level is reached, but from there to the
top they are of the spiral shape. Unlike
the staircase at the Franco-British Exhibi-
tion previously referred to, the reinforced
concrete construction is not relied upon to
take the whole of the stresses due to load-
ing, inasmuch as between each two floors
the staircase is supported at three points
by iron hangers cast into the concrete and
suspended from the floor above, as illus-
trated. By this method there is obtained
a beam construction with two end supports.
The following particulars are due to
VARIED APPLICATIONS AND EXAMPLES
367
Engineering Record : The linings are on the
cantilever principle. The forms for these
staircases were quite a little problem in
themselves. The soffit was moulded on a
f-in. layer of 1 : 3 mortar, applied to J-in.
wire mesh, which was attached to transverse
pieces. This mortar was allowed to take
a hard set, after which the longitudinal and
transverse reinforcement was wired in, and
the final cast of a complete staircase made
at one operation. The soffit was plastered
in the upper stories and basement, while
in the cellar it was simply rubbed to give a
finished appearance. No form was used
for the treads, and the surface for the entire
width of 3| ft. was hard trowelled. All
risers and exposed strings were rubbed until
smooth : the stairs were primed and en-
amelled. The lower and upper ends of each
staircase are supported directly on the floor
and beam construction of the building. All
reinforcement consisted of square, cold-
twisted steel bars, on which a unit tensile
stress of 16,000 Ib. per square inch was
allowed. The longitudinal reinforcement
consisted of -^-m. square bars, spaced 2f in.
Fig. 617.— Stairway in Mclntyre Building, Salt Lake City
368
REINFORCED CONCRETE
on centres on the two side sections of each
staircase, and TV~m- ro^s ^ m- on centres
for the middle portion.
CHIMNEY-SHAFTS
A chimney at the Northfleet works of the
Associated Portland Cement Manufacturers is
shown photographically by Fig. 619. It is 247
ft. high above the bottom of the foundations,
and its internal diameter is 8 ft. 6 in. The
foundations, 18 ft. by 18 ft., are 2 ft. thick,
tapering to 4 ft. thick at the bottom, which
is 35 ft. below ground level ; they are rein-
forced by IJ-in. by IJ-in. by TVin- T's. Up
to a height of 62 ft., the outer shell is 12 in.
thick ; there is a cavity of 4 in. and an inner
shell, 4 in. thick, and above that height the
chimney is a single shell, 9 in. thick. The
vertical work is reinforced by steel rings
f in. diameter, spaced 18 in. apart, and by
vertical T-bars of the same dimensions as
those in the foundation. Fig. 306 (p. 197)
Fig. 618. — Staircases in Ritz-Carlton Hotel, New York
VARIED APPLICATIONS AND EXAMPLES
369
shows the template employed to keep the
vertical bars in position.
In a 144-ft. 9-in. stack, reinforced with in-
dented bars and built on a site near Drury
Lane, London, by Trollope and Colls (engineer,
Mr. Oscar Faber,B.Sc.), the inside diameter
is only 2 ft. 6 in., and the lower portion, up to
the flue entrance, is 1 ft. 3 in. thick, and the
diameter at the base is 5 ft. ; with brick-
work, according to Mr. Faber's calculations,
the least permissible diameter at the base
would have been 12 ft. Above the flue
entrance, the thickness of the outer shell is
only 6 in. next coming an air-space 4 in.
thick, and then a firebrick lining, 5 in.
thick, consisting of special firebricks with a
raised rib, 2 in. by f in., fitting into a recess
in the next course. By means of vent-holes
left near the flue entrance, an air current
is induced between the lining and the shell.
Timber rings are embedded at intervals on
the inside face of the concrete shell for the
purpose of reducing to zero the stresses due
to expansion, this system being in accord-
ance with a method patented by the designer.
In the firebrick lining there are special bricks
at 10-ft. intervals, with side projections,
between which projections and the outer shell
there is a clearance of f in., to allow for
expansion. The foundation was taken 15 ft.
below basement level, a total of 29 ft. 9 in.
below ground level, and a solid block of con-
crete, 16 ft. 6 in. by 14 ft., was made, this
serving as a foundation block and also as
underpinning to existing walls. A staircase
formed out of the block gives access to an
engine room below the yard. Fig. 620 pre-
sents a number of sections through the
shaft.
PIPES, SEWERS, AND CONDUITS
The construction of heavy pipes, sewers,
etc., on the Bonna system was the subject of
a report presented by Mr. Arthur E. Collins,
M.Inst.C.E., Engineer to the City of Nor-
wich, to the Sewerage Committee of that Cor-
poration, following on a special visit paid
by him to France. His report is summarised
as follows : "I visited Maisons-Alfort in
the Department of Seine, where a water
main was being laid. The pipes were being
constructed in a yard adjoining the road
where they were to be laid. They had an
internal diameter of 2 ft. 7 in., with thick-
ness of walls of 2f in. and length of 10 ft.
The length of main was five miles. The
internal pressure was 120 ft. head. At
24
about mid-thickness of the pipe walls are
complete cylinders of sheet steel ; these
cylinders are made at the spot, and are
jointed by means of a special fusing ap-
paratus, which fuses the two surfaces of
steel into one at the junction. The apparatus
in question consists of a mechanically-
Fig. 619.— 247-ft. Chim-
ney at Northfleet
Fig. 620.— 144-ft. 9-in.
Chimney near Drury
Lane, London
Fig. 619
operated blow-pipe, using a mixture of
acetylene and oxygen. Before being fused
together the edges of the sheets are turned
up about | in., so as to give a convenient
surface upon which the fusing apparatus
may operate. This arrangement also gives
a flexible or expansive joint, the value of
which is apparent when the next process
of manufacture is seen. This consists of
Fig. 621. — Sewer Pipe Reinforcements, Paris
Fig. 622. — Sewer Construction at St. Louis, U.S.A.
370 '
VARIED APPLICATIONS AND EXAMPLES 371
placing within the steel tube a cage com- pitch of spiral depend on the diameter of
posed of steel of + section wound into a the tube and the intended working pres-
spiral with longitudinal rods at about 9-in. SHT-P T^ ct00i f,,^« ~t ^^
spiral with longitudinal rods at about 9-in.
intervals ; the longitudinal rods and the
sure. The steel tube is of the same length
as the pipe, and is slightly flanged at each
. , . , ----- , Ulic pipe, auu IB isiignny nangea at eacn
spiral are wired together at a sufficient end ; the internal cage is of almost the same
Figs. 623 and 624. — Sewer Construction at Acton, London
number of intersections. The springiness
of the steel tube enables it to be drawn
over the internal cage, whilst at the same
time it is in close adherence thereto. A
similar cage, but of heavier section of steel,
is drawn over the outside of the steel tube.
The weight of the steel + section and the
length as the pipe ; the external cage is
slightly shorter than the pipe. When the
tube, with its internal and external cages,
has been assembled in the manner stated, the
assemblage is placed upright on a platen or
table ; a collapsible core of very ingenious
construction is placed in position inside the
Fig. 625. — Cross Section of Culvert at Kilton
3 •>'• 23- o IN BACK i £•!'• 20°-O IN FRONT.
Fig. 626.— Part End Elevation and Part Cross Section of Culvert at Kilton
372
VARIED APPLICATIONS AND EXAMPLES
assemblage, leaving sufficient space all round
to be occupied by concrete ; a cope in three
parts is placed around the outside of the
outer cage, leaving sufficient space for con-
crete there ; concrete is then mixed by hand
Fig. 627. — Gross Section of Conduit at
Woolwich
on a concrete mixing trough on an elevated
platform running on rails, and provided
with cranes and other conveniences. The
trough is mounted to tip to either end, and
is 12 ft. by 4 ft. by 1 ft. 8 in. deep. A
special mixing paddle is used somewhat
similar to a small spade, but with slots in it.
Four men mix the concrete dry, and six
men mix it again wet. When thoroughly
mixed so as to be in a practically liquid con-
dition, by tilting the trough and opening
a hatch in the lower end the concrete flows
between the core and the interior of the
assemblage, and between the cope
and the exterior of the assemblage.
During the pouring the cope is
struck a series of blows with wooden
mallets to assist in consolidation
and in disentanglement of air."
Fig. 621 shows the reinforcements
for a concrete sewer which comprises
the main outfall in Paris (engineers,
Bechmann and Lanney ; contractors,
Messrs. Chassin and Sons). The
skeleton reinforcements are clearly
illustrated. On this system a pipe
line, 2 kilometres (roughly 1 J miles) long was
constructed, the diameter being 2 m. (nearly
79 in.).
Fig. 622 shows a sewer at St. Louis, U.S.A.,
in course of construction. The immensity
of the work can be gauged from the heavy
centering employed. The reinforcement
was by means of indented bars.
373
Figs. 623 and 624 show a sewer con-
structed for the Urban District Council of
Acton by the Columbian Fireproofing Com-
pany. It is a pipe 4 ft. 6 in. in diameter,
constructed on the " Bonna " system. The
pipe has a semi-circular crown with a seg-
mental invert, this shape having been
adopted because the sewer lies near the sur-
face of the road, and it was desired to give
the pipe as much strength as possible with-
out diminishing the bore. The thickness of
the concrete at the crown is 6 in., the
reinforcement being 2-in. cruciform bars.
Fig. 346 (p. 210) is a photographic view of
a conduit at Woolwich in course of erection,
the illustration showing the Blaw collapsible
steel centering being pulled forward for the
next section. Fig. 267 shows a cross section
of the pipe. For concreting the invert, the
half-round centres, 2 ft. 6 in. in diameter,
were set up on a dish of concrete, and the
work was then concreted up to the spring-
ing line. Then for concreting the crown,
as soon as the work would allow, the centres
were reversed and set up on timbers, about
20 ft. of pipe being done at a time, the centres
being then collapsed and pulled forward on
ponding angle iron is placed on the centre,
angle irons as illustrated on p. 210. A corres-
and the rolling forward is made possible by
means of balls 1J in. in diameter.
Drawings of a reinforced concrete culvert
at Kilton are presented by Figs. 625 and
626 ; this culvert was built under the North
Eastern railway line between Skinningrove
and Kilton, to provide a waterway at the
Fig. 628.— Wharf at Dennemont
bottom of a valley formerly spanned by a
viaduct. The work was designed by the
Trussed Concrete Steel Company, Ltd.,
under the supervision of Mr. W. J. Cudworth,
late chief engineer of the North Eastern
Kailway. The reinforced concrete at the
crown of the culvert is 18 in. thick, and the
total length of the culvert is 450 ft.
374
REINFORCED CONCRETE
WHARF AT DENNEMONT
A wharf at Dennemont, near Mantes,
built on the banks of the Seine, at the works
of a cement manufacturer, is shown in Figs.
628 and 629, the construction being accord-
ing to the Coign et system, and the designer
being E. Candlot. The wharf is 8-75 m. long
and 9 '5 m. wide from front to back, the
decking being 3-7 m. above normal water
level. It is carried on twelve reinforced
concrete piles, each 30 cm. in diameter,
placed in three rows, twelve columns being
built up from the piles. At a uniform level
at the tops of the piles there are both longi-
tudinal and transverse members, and there
are smaller members at the tops of the
VARIED APPLICATIONS AND EXAMPLES
375
columns, the latter members supporting
the decking. The horizontal members be-
tween the tops of the piles are 16 cm. wide
and 40 cm. deep, and are reinforced by four
bars of 14 mm. diameter, two near the top
and two near the bottom surface of the
concrete. These members are not designed
to act as beams, but as struts, and it will be
noted that there are vertical reinforcements
along these struts consisting of steel wire
6 mm. in diameter and spaced 10 cm. apart.
The construction of the piles, of the columns
over the piles, of the lower horizontal mem-
bers, and of the top beams and decking is
well shown in Fig. 629. The columns, 23
cm. square, contain four vertical reinforce-
ments, each 16 mm. in diameter, these being
wound spirally with 6-mm. wire, the coils
being 10 cm. apart centre to centre, as
indicated in the sectional views. The deck
beams have a lower reinforcement of a
32-mm. diameter bar and an upper reinforce-
ment of a 16-mm. bar, the two being con-
nected by vertical ties of 6-mm. wire. The
decking panels are reinforced by sixteen
10-mm. bars, half of them at right angles to
the others, curved upwards to pass over the
top reinforcement of the main beams.
FRONT CLEVATION
LONGITUDINAL SECTION XV-B
'.' J '.'
CROSS srr.Tiorn c-n
E
_JL
3
5
1
V J
-
J
CROSS SCCTiors
Fig. 630. — Details of Jetty Head at
Thames Haven
DCTAIL5 or PiLCS
Figs. 631 and 632. — Jetty Head at Thames Haven, in Course of Construction
376
VARIED APPLICATIONS AND EXAMPLES 377
JETTY HEAD AT THAMES HAVEN plete structure in itself and independent of
Detail illustrations of a jetty head at the approach jetty, which could not be
Thames Haven are given by Figs. 630 to 634. reckoned on for support. It being impos-
This jetty replaces an old T-head composed sible to fix concrete bracing below low-water
of cast-iron piles with wrought-iron bracings, level, it was decided to make the main sup-
a great part of which was carried away as port of the structure composed of two piles
the result of a collision with a large vessel, which were surrounded by a reinforced con-
what was left of the structure,
with the addition of temporary
wooden piles, serving as a stage
irom which to construct a new
jetty head in reinforced con-
crete. The work in course of
progress is shown by Fig. 631,
in which illustration some of
the centering still in position
will be noticed. More of the
centering round the piles and
bracing is illustrated in Fig.
632, whilst piles and bracing
are shown by Fig. 633, the
skeleton reinforcement for this
work being shown by Fig. 634.
The following description of
the work is quoted from
Concrete : "The jetty head is
crete cylinder 5 ft. in diameter,
and then filled in solid, so as
to give great lateral stiffness to
each support. A general plan
and elevation is attached here-
with. There are three main
columns in the width of the
structure, the centre column
being midway between the
outer columns, with horizontal
and diagonal bracing in all
directions. It will be seen
that this method of construc-
tion and bracing has the effect
of distributing all shocks over
a large number of the columns,
and thereby giving great rigid-
ity to the structure.
" The columns are 5 ft. in
diameter up to low- water line,
after which they are reduced
to an oval section of about 5 ft. by 2 ft. 6 in.,
and continued in this section up to the
under-side of the deck. All the horizontals
and diagonals are also spirally and longi-
tudinally reinforced, so as to take stresses
of either compression or tension. Timber
fenders are bolted to the columns to act as
rubbers against the jars of ships.
" The method of procedure in carrying out
the work was as follows : Piles 45 ft. long
Figs. 633 and 634. — Piles and Bracing of Thames Haven
Jetty Head
136 ft, long by 32 ft, wide. The deck level
is 5 ft, 6 in. above high-water spring tides,
and about 45 ft. to 48 ft. from the bed of
the river ; the spring tide range is about
18 ft. Vessels up to 7,000 tons or 8,000
tons burden are accommodated, so that the
structure was needed to be specially strong
in cross section, in order to take the shock
resulting from such large vessels. The jetty
head was required to be designed as a com-
378
REINFORCED CONCRETE
were driven 18 ft. to 20 ft. into the bed of the
river in groups of two, as shown on the
plan. The temporary steel casing was
then dropped over the piles and bedded
about 2 ft. or 3 ft. into the bed of the river.
Inside this steel casing and over the piles an
enveloping reinforcing, composed of longi-
tudinal rods and hoops, was dropped, and
the casing was filled up with concrete 3 to 1,
deposited through the water by means of
shoots, to 3 ft. above low-water level. The
columns, braces, and deck were then built up
in situ with the usual timber shuttering.
" The temporary steel casing was made in
the pile. The piles withstood this driving
very well indeed, and, where the dolly was
used, the head did not suffer in the least,
although, in some cases, the piles were
driven to a set of 100 blows for the last inch
before they were regarded as being driven
to refusal. In the cases of direct driving,
without any dolly at all, the piles were
simply broomed up for about 12 in. of the
head in a similar manner to timber piles."
LIFEBOAT SLIPWAY
The photographs (Figs. 635 to 639) show
a reinforced concrete slipway that has been
Fig. 635. — Lifeboat Slipway, Ackergill
two halves, with vertical joints bolted to-
gether, so that it could afterwards be taken
away, when the concrete was set, by means
of a diver unbolting the vertical joints. The
piles were 15-in. diameter octagonal section,
made on the Considere principle ; the re-
inforcing was composed of f-in. diameter
steel, spirally wound with 2-in. pitch, and
eight longitudinal rods of 1-in. diameter.
The ground into which the piles were driven
is composed of Thames ballast with an over-
lay of about 4 ft. to 6 ft. of soft, sandy mud.
The piles were driven with a 30-cwt. monkey,
falling 6 ft. to 7 ft., mostly with a timber
dolly intervening between the head of the
pile and the monkey, but in some cases with
the monkey falling direct on to the head of
built at the Royal National Lifeboat In-
stitution's station at Ackergill, near Wick,
North Scotland. The illustrations show the
various stages in the construction, and give
much information as to the manner in which
the concrete is reinforced ; in particular, the
spiral reinforcement of the columns and
bracings will be noted. The total length
of the slipway is 194 ft., and, excepting for
the mass concrete toe in the sea, the whole
of the construction is of reinforced concrete.
The octagonal columns, 20 ft. apart, are
14 in. in diameter, and have bases 2 ft. 6 in.
square, which go down 2 ft. into the solid
rock. The extra precaution was taken of
boring holes in the rock, so that the rein-
forcing rods go down for a further 18 in. and
REINFORCED CONCRETE
are cement grouted into place. The platform,
which is well shown in one of the photo-
graphs on p. 370. accommodates the lifeboat
and the hauling winch ; it is carried 16 ft.
above the shore on three rows of columns,
and is 58 ft. long, 16 ft. wide, and the slabs
that form the decking are 5 in. thick. The
. sloping part of the slipway has a gradient
of 1 in 7. To facilitate the moving of the
lifeboat, there are longitudinal keelway
beams in pairs, these being 12 in. wide and
20 in. deep, between which are a number of
rollers to support the keel of the boat.
ROOFED RESERVOIR
Figs. 640 and 641 show part of a service
reservoir at Kloof Nek, Cape Town, 900 ft.
above sea level. The work was carried out
by the engineering staff of the Cape Town
City Council, Messrs. Murray and Stewart
being the contractors and the reinforce-
ment being expanded metal. The ninety-
six columns are of rectangular cross section
varying from 18 in. by 18 in. to 30 in. by
18 in. ; some of them are shown in the photo-
columns and the arched roofing is presented
by Fig. 641. The columns are reinforced
by f-in. diameter rods at each angle, spread
outwards into the concrete base, the rods
being wired together with J-in. diameter
wire, the pitch of the spiral winding being
9 in. In plan the columns form five con-
centric circles, 15 ft. apart, and they serve
as supports for the semi-circular arched
Fi(J. 641
i- m ^
l
Fig. 640 and 641.— Columns and Arched Roofing of Kloof Nek Reservoir
graphic view (Fig. 640)," from which it will
be understood that the reservoir is circular
in plan. The inside diameter is 162 ft., the
height to top- water level is 24 ft., and the
total capacity is 3,000,000 gallons. An
outline diagram, showing the rectangular
walls, which are level at the top, to carry
the roof, which is constituted of segmental
arches springing radially from a eentral dome
to the concentric walls in series, the outer
arch, as shown in the diagram, merging into
the main outer wall. The roof is reinforced
VARIED APPLICATIONS AND EXAMPLES
38i
with expanded metal of 3-in. mesh laid with
a minimum lap of 6 in. The reinforcement
is laid as near the intrudes of the arches as
possible, and there is a further layer of
expanded metal near the extrados of the
haunches of the segmental arches, and
turned in to meet the reinforcement at the
intrados. The rise of the arches is 2 ft., the
minimum thickness 6 in., while the thick-
ness at the springing is 9 in. The concrete
used throughout consisted of 1 of cement,
2 of sand, and 4 of broken granite, the
latter not exceeding 1 in. for the roof arches
ami 1J in. for the rest of the work. Stone
obtained at a quarry one-third of a mile dis-
illustrated by the part perspective and part
sectional view shown by Fig. 642 is actually
a bridge of peculiarly distorted shape. It
forms %part of the Brooklands motor track
at Wey bridge, at the place where the track
crosses the river Wey. It was necessary to
make provision for a considerable volume
of overflow water which comes down occa-
sionally. Viewed in plan, the track exhibits
two curves, one with a radius of 1,000 ft.,
and the other with a radius of 2,000 ft.,
and at one end of the bridge the outer edge
of the track is banked up to the height of
40 ft. above the datum line, which is from
5 ft. to 8 ft. below normal ground level,
Fig. 642.— Bridge forming part of Brooklands Motor Track
tant from the reservoir was used. At the
vertical joints the usual V-shaped bond was
strengthened by inserting two 1-in. steel
rods, 4 ft. long, for a distance of 2 ft. on each
side of the joint. In view of the known
porosity »f concrete, the method of finish-
ing is of interest. The whole of the inside
of the reservoir, to a height above water
level, as well as the extrados of the roof
arches, was hacked, thoroughly cleaned
down, and then rendered in cement mortar
of 2 to 1 with a thin skimming of neat
cement. The roof of the reservoir is covered
with earth to a minimum thickness of 15 in.
SPECTATORS' STANDS, ETC.
While coming under the heading of
stands, etc., the particular construction
whilst the inner, edge is only 12 ft. above
the datum line ; at the other end of the
bridge the corresponding heights above the
datum line are 32 ft. and 10 ft. 6 in. The
bridge has a length of nearly 200 ft., and
provides a track which is uniformly 100 ft.
wide. The bridge was designed to carry
a super load of 112 Ib. per square foot, and
a rolling load of 2 tons per square foot. The
reinforced concrete construction was on
the Hennebique system, and the founda-
tion includes forty-two piles, mainly 14 in.
square, their heads being in monolithic
connection with the beams and columns of
the superstructure. The tops of the columns
are joined by longitudinal and cross-beams,
whose upper surfaces were curved so as to
be adapted to the concave road decking, this
382
REINFORCED CONCRETE
consisting of slabs from 4| in. to 6£ in.
thick, sub-divided by secondary beams into
panels roughly 17 ft. by 5 ft. To hold up
the earthen embankment at one end of the
structure, there are panels of reinforced
concrete, and the end retaining wall is
extended by means of wing walls in order
to prevent the material flowing down to-
wards the channel of the river. The con-
struction at the lower end of the bridge
is much the same, but the section
illustrated is taken on a line which
precludes the panelling, etc., being
shown.
Fig. 643 shows the Stadium at
Shepherd's Bush, London, in almost
a finished state, the roofing being in
course of construction. It is inter-
esting to note in connection with
this structure that at an early stage
pass a £-in. ring, with the addition of one-
third sand. The timber seats are bolted to
studs embedded 1 in. in the concrete, an
arrangement that has given complete satis-
faction.
A view of a stand erected for the Bradford
City Football Club, Ltd., at Midland Koad,
Bradford, is shown by Fig. 645, the photo-
graph having been taken in the course of
construction, and Figs. 646 to 650 are some
STUDS FOR FIXING SEATS.
'INDENTED BAR.'
Fig. 644.— Platform Con-
struction, Stadium
9»3CHAMNE.I_.
ZO-O'SPAM.
Fig. 643. — Stadium at Shepherd's Bush, London
it was intended to build it entirely of
reinforced concrete, but that ultimately
the stand was built in steel, and only
the platforms were of reinforced concrete.
For the purpose of placing this in position
there was an inclined tramway running up
the Stadium. The reinforcement consisted of
indented bars, £ in. square, the bars being
placed 12 in. apart, and every fourth one
being carried through the channels and
turned over as shown in Fig. 644 in accord-
ance with a request of the London County
Council. The slabs are 2 in. thick, and the
concrete for this consisted of 1 part of
cement to 4 parts of an aggregate consisting
of Thames ballast crushed and graded to
of the working drawings. The stand is a
striking example of open framework con-
struction, the pillars being sufficiently
strong to withstand lateral stresses, and thus
rendering unnecessary the employment of
diagonal struts ; the horizontal struts for
bracing the pillars together are " sym-
metric beams." The architect was Mr.
Archibald Leitch, M.I.Mech.E., the work-
ing drawings of the reinforced concrete were
got out by Messrs. F. A. Macdonald and
Partners, Kahn steel reinforcement was
used, and the responsible contractors were
Messrs. John Ellis and Sons, Ltd., of
Leicester. With the exception of the roof
the whole of the construction is of reinforced
Fig. 645. —Bradford Football Stand, in Course of Construction
E.L.EVA.T1OH AND
ENLARGED SE.CT1OM
- OF *•
• 8 EA.M
p;x 646 —Floors, Columns, and Beams, Bradford
Football Stand
383
f.«--J
HrHf
Fig. 647. — Details of Foundations and Columns, Bradford Football Stand
Fig. 648.— Plans of Bradford Football Stand
Fig. 649.— Details of Beams and Brackets, Bradford Football Stand
Fig. 650. — Various Sections through Bradford Football Stand
25
385
386
REINFORCED CONCRETE
concrete, the length of the stand being
33(Ht., and the breadth 27 ft. 4 in., diminish-
ing "uniformly to 13 ft. 4 in. The decking
slab has an effective depth of 4 in., and is
reinforced by a network of bars over its
whole area ; the secondary beams are in a
transverse direction, and are therefore
inclined to the horizontal. The adaptability
of reinforced concrete to awkward forms of
construction is demonstrated by the fact
that as the platform is partly inclined and
by means of Kahn bars. The stand has a
length of 800 ft., a width of 240 ft., and a
height of 105 ft., its sixty tiers of seats and
two rows of boxes having a seating capacity
of about 30,000. The soil on the site con-
sisted of dry, hard sand, and in this were
dug a number of pits about 8 ft. deep, in
which were built reinforced concrete piers
— some of them 10 ft. wide — to act as foot-
ings for the columns, which numbered 156.
Speed being of prime importance, work was
Fig. 651. — Grand Stand, St. Paul, in Course of Construction
partly horizontal, the secondary beams were
formed in such a way that their depth varies
in the length, as illustrated. The main
beams and lintels run longitudinally, and
in section they are 12 in. broad by 21 in.
deep, measured on the vertical axis of
the beam. The clear span of the beams is
26 ft. 3 in., and the columns are at 27 ft. 6 in.
centres.
Views in course of construction and three
part elevations of the grand stand at Minne-
sota State Fair race track, St. Paul, are pre-
sented by Figs. 651 to 654, the architects
for this structure being Messrs. Reed and
Stem, of St. Paul, the reinforcement being
carried forward night and day, the whole of
the false- work being left in position until con-
creting was finished, so as to obviate delay
caused by shifting and repairing the forms.
The columns have both spiral and vertical
reinforcements, and they are cylindrical
except the walls. They were moulded in
knock-down steel forms made in telescopic
sections about 5 ft. long, overlapping about
4 in. on horizontal seams and 3 in. on ver-
tical ones, the forms being clamped into
position by hinged collars of 4 in. by ^--in.
steel with a hasp-like connection, locked by
a tapered key. The joints were stiffened by
wooden fillers 3 in. thick, inserted between
387
388
REINFORCED CONCRETE
the collars and the steel sheets. The knee-
brace sections between the columns and
beams were made by extending the beam
moulds. The projecting reinforcements from
the foundation piers were connected to the
vertical reinforcements of the columns by
means of sleeves 2 ft. long, the same method
of joining being employed at the various
floor levels between the columns of one
story and those of the next. The method
take the tread and riser and supported at
frequent intervals on horizontal cross pieces
forming parts of triangular offsets nailed to
the upper sides of false-work beams inclined
parallel to the slope of the platform. At first
the weight of the wet concrete produced a
heavy downward lateral thrust, which de-
flected the riser forms ; but this was cor-
rected by the use of a tier of very heavy
upper inclined beams, to which the riser
Fig. 653.— East and West Elevations of Grand Stand, St. Paul
Fig. 654. —Half-elevation (to street) of Grand Stand, St. Paul
of carrying on the work was in every sense
expeditious and up-to-date, and the extent
to which the concrete was placed in posi-
tion without hand labour was remarkable.
Concrete was hoisted in buckets, automatic-
ally dumped into cars running on a 24-in.
gauge Koppel service track, and conveyed
to the work and shot into the forms through
tapered shoots, the only hand work being
spreading and light tamping. Terrace steps
in the upper platform were cast on corru-
gated iron sheets bent at right angles to
forms were secured in the same way as
those of the supporting forms below. The
lower ends of the upper inclined beams en-
gaged a heavy horizontal girder, tied back
at frequent intervals by horizontal tension
rods, which transferred the thrust across the
building to heavy columns or other suitable
members.
The method of finishing some of the sur-
faces is interesting. All the floor slabs were
cast and finished at one operation, the con-
crete being made especially rich for the
VARIED APPLICATIONS AND EXAMPLES
389
purpose. The extra amount of floating had
the effect of bringing more moisture than
usual to the surface, and this sometimes
caused shallow pitting, which, however, was
not regarded as serious. Other exposed
concrete surfaces were stippled with cement
grout by dipping a bunch of stiff twigs in
the grout and striking it sharply with a rod,
causing the cement to spatter over the
surface. In all cases where the surface had
Fig. 655. — Railway Sleepers
been thoroughly wetted, the cement adhered
perfectly.
RAILWAY SLEEPERS
The drawings (Fig. 656) and the photo-
graph (Fig. 655) show reinforced concrete
sleepers designed on the Italian model,
and adopted in an experimental track laid
between Dresden and Kotzschenbroda,
Saxony. The reinforcement is clearly shown
in the drawings, in which the U-shaped
stirrup placed where the maximum shearing
stress occurs will be noted. To receive the
screw bolts there are wooden blocks of
dovetail shape about 4 in. deep wholly
embedded in the concrete except for the
top surface. The sleepers are roughly
6 ft. long and weigh 209 lb., of which the
reinforcement accounts for 10| lb. The
drawings show that the rails do not rest
on the concrete ; instead, the pressure is
spread by means of an oak block about
3 in. high and 10 in. long. As illustrated,
the rails are held by 7 in. screw bolts, but
in a later system clamps are employed.
At the end of a year, parts of the track
were broken up for the purpose of closely
examining the work, and the sleepers were
found to be in perfect condition. The
screws were tight, and the rails were not
perceptibly corroded.
GRAIN SILOS AT SILVERTOWN
Reinforced concrete has proved itself
to be an admirable material for the con-
struction of silos, an excellent example of
which is a granary erected at Silvertown
on the Hennebique system, and consisting
of eight cylindrical silos, each having a
height of 80 ft. and a diameter of 20 ft.,
and having a storage capacity of 505 tons,
the total capacity of the eight silos being
about 18,000 quarters of grain.
The silo walls are only 6 in. thick.
The architect was Mr. F. E. L. Harris,
A.R.I. B. A., and the contractors were
Messrs. A. Jackaman and Son. The
construction is clearly shown in the
photograph and drawings (Figs. 657
to 664). Reinforced concrete piles
to the number of 124, each 14 in.
square in cross section, support a
general foundation slab, immediately
underneath which are tunnels or
subways for the transport of the
grain. Care was taken to see that
the [projecting reinforcements from
ELEVATION s, LONGITUDINAL SECTION
i.,44 •(«-• 42? ----
zoo — -H K- -oo ~
SECTION A B SECTION C D
Fig. 656.— Details of Sleepers
the heads of the piles were well bonded
into the columns, slabs, etc., so as to
make the whole structure monolithic.
With the same object in view the walls
of the silos are connected one with the
other, as clearly shown in the horizontal
section, in which also the vertical ribs or
counterforts to the outer walls of the silos
will be noted.
Figs. 657 and 658.— Two Sections through Silos
c.
X
4 : ; '•
; i •;.. ...
1
i
e 1 l -1
j. i j ;.,»...
1 • ;
i i ',""
! i r
V
1 J
Fig. 661. — Horizontal Section through
Silo Roof
Fig. 659.— Section through Silo Walls
O
IH
Fig. 662. — Horizontal Section through
Silos (looking upwards)
Fig. 660.^Horizontal Section through
Silos
Fig. 663. — Horizontal Section through
Silos, showing Tunnel Arrangement
390
VARIED APPLICATIONS AND EXAMPLES
WATER TOWER AT SINGEN
Figs. 664 and 665 are photographs, and
Figs. 666 to 668 working drawings of a
water tower erected in 1909, at a factory
at Singen, Germany, the architect being
H. Steub, and the designers and contractors
391
the base, tapering to 2 ft. by 1 ft. 6 in. at
the top. The ring-shaped foundation has
an outer diameter of 41 ft., and an inner
diameter of 16 ft. 6 in. As shown in the
vertical cross section, the floor of the tank
is built in panel form, the panels being 9i in.
Figs. 664 and 665.— Water Tower at Singen
being the firm of Josef Krapp, of Karlsruhe,
whose engineer (F. Guske) has contributed
to Beton und Eisen an article which has
served as the basis for the following descrip-
tion. The total height is 141 ft., the base
of the tank or reservoir being 98 ft. 5 in.
above the ground. There are six columns,
6 ft. 6 in. by 1 ft. 8 in. in cross section at
thick, and the ribs 2 ft. 3 in. thick. There
are three floors below the tank level, the
means of communication between them
consisting "of a spiral staircase carried on
concealed corbels. Surrounding the tank
is a gallery, also carried on corbels, which,
in this case, are a feature of the eleva-
tion. The tank, 23 ft. 9 in. in diameter, and
392
REINFORCED CONCRETE
21 ft. 8 in. high, is built of steel plate with
a casing of movable slabs of pumice-stone
SECTION J K
Fig. 666. — Vertical Section of
Water Tower
concrete inserted between very thin vertical
concrete supports, the space between the
steel and the slabs being filled with dry peat.
The roof is of wood, covered partly with slates
and partly with copper. In designing the
tower, a wind pressure of 30 Ib. per square
foot was allowed for ; the load on the gravel
soil is 23 Ib. per square inch, and the greatest
compressive strength in the structure is
467 Ib. per square inch. On the completion
of the work, water was run into the tank
SECTION G H
SECTION E F
Figs. 667 and 668. — Three Horizontal
Half-sections of Water Tower
until a height of 20 ft. 4 in. was reached, the
centre of the ribbed floor then showing a
depression of 0'124 in. The dimensions
indicated in all the drawings shown on
this page are in metres and centimetres.
Index
ABSTRACTING for quantities, 280,
281
Acid steel, 60
Acids, effect of, on concrete, 265
— — used in surfacing, 243
Adhesion and shearing stress, 2, 112
: working data, 84
jEsso-concrete, 2
Aggregate, artificial, 14, 15
basalts, 14
blast-furnace slag, 15
broken bricks, 14
concrete, 15
burnt clay, 14
chalk, 13
cinders, 15
clinker, 15
coke breeze, 15
definition of the word, 12
fire-resisting, 261
flint, 13
gauges, 27, 28
granite, 13, 14
gravel, 12
lavas, 14
limestone, 13
natural, 12-14
position on site of, 127
quartzite, 13
residues, 14
sand as, 15
sandstone, 12, 13
shingle, 12
sizes of, 15
sulphur in, 14
terra-cotta, 14
voids in, 25-27
volcanic rocks, 14
, washing, 15, 16
Albert Bells Bridge, Rome, 231
Alkalies, effect of, on concrete, 265
Allen's floor, 7
Almandares Bridge, Havana, 201
Aluminium works, Kinlochleven,
353, 357
American Association of Portland
Cement Manufacturers, 29
American inventions, 10
pile forms, 159
Andrews tamper, 129, 246
Angle of repose, 123
Annapolis Academy, 195
Arched beams, measuring, 277
Arches and bridges, examples of,
291-310
— , forms for, 197-
208
, design of, 288-291
, Melan, 198
Architectural treatment, 229-241
" Armoured Concrete," 2
Armoured tubular flooring, 215
Aspdin, Joseph, 5
Aspdin, William, 6
Aspdin's cement, 17
Atmospheric effect on concrete,
262
Aylett's flexible centering, 208
BALL mill for cement, 20, 21
Barker and Hunter mixer, 31, 32
Barrows, concrete, 46
Bars, bending, 132
, cutting, 128, 129
— , receiving, on site, 127
Basalts as aggregate, 14
Basic steel, 60
Batch concrete mixers, 31-42
Beam, bending moment at various
points in, 65-68, 69-71
— centering, measuring, 274,
280
with double reinforcements,
calculating, 99-104, 111
forms, 167-177, 179, 180
, introducing reinforce-
ment into, 147, 150
, timber for, 155
, framing up, 150
, moment of inertia in, 72-4
, resistance of, 77
reactions, 64
, safe load on, 77, 78
, section modulus of, 76
sections and their moments
of inertia, 74
, shearing stress in, 79-82,
112-117
with single reinforcement,
calculating, 85-95
, tee, calculating, 104-112
Bechmann and Lanney, 373
Bending appliances, 129
bars, 132 •
bench, 132
moment, 65-72
• at centre of slab, 95
: working data, 85
tests for steel, 54
Bessemer steel, 59
" Beton Coignet," 6
Bill, preamble to, 281-284
of quantities, general outline
of, 273, 274
, testing clauses in, 284
Billing, 281
measured items, 284
Binding for skeleton reinforce-
ments, 150
, spiral, for column, 150
Bladder, Dr. M. B., 254
Blast-furnace slag as aggregate, 15
Blaw Steel centering, 185, 210,
373
Blister steel, 59
Boat, Lambot's, 6
393
Bolte mixer, 43
Bonna system for pipes, 227, 369,
373
Bradford Football Club stand,
382-386
Brannon's Pile, 7
Bremba (Italy), bridge over, 297,
299
Brick aggregate, 14
Bridges, architectural treatment
of, 235
, design of, 288-291
— , examples of, 291-310
, suspended centering for,
205-208
Bridlington, retaining wall at, 181
British Fire Prevention Com-
mittee, 258
and Foreign Blind Associa-
tion's premises, 190
standard specification for
cement, 21, 22
Bronze-concrete, 2
Brooklands motor track, Wey-
bridge, 381, 382
Brush finish, 243
Bunker at aluminium works, Kin-
lochleven, 353, 357
Burke, E., 265
Burnell, G. R., 5
, H. H., 5
Bush-hammer, 244
Bush-hammered finish, 244, 245
CALCULATIONS, data for, 83-85
Californian State Normal School,
San Jose, 249, 251
Cantilever, bending moment in,
68-69, 71, 72
— reactions, 63
, shearing stress in, 80, 81
in theatre construction (Los
Angeles), 352, 353
Caps for reinforced piles, con-
structing, 139
Carborundum finish, 243
Carey-Latham mixer, 44
Cast crucible steel, 59
Cathedral at Manila, 333, 337
Poti, 195, 196, 337
Ceiling (see also Floor)
slab, Leconte's, 5
Ceilings at Royal Automobile
Club's building, 320
Cement, analytical test for, 22, 23
, British standard specifica-
tion for, 21
clinker, producing, 19, 20
— - — , composition of, 17, 18
, fineness test for, 22
gauges, 28
, iron portland, 24
, Le Chatelier's test for, 22, 23
394
INDEX
Cement manufacture, ball mill
for, 20, 21
chalk in, 18, 19
dry process, 19
Griffin mill for, 20
grinding clinker in, 20
Hoffmann kiln for, 20
pendulum mill for, 20
principles of, 17, 18
rotary kilns for, 20
Slurry, 19
washmill for, 18
wet process, 18, 19
" natural," 23, 24
puzzolanic, 24
setting of, 21
" setting time " test for, 23
on site, 127
slag, 24
soundness tests for, 23
specific gravity test for, 22
specifying proportions of, 27
tensile strength test for,
23
testing, 21-23
weight of, 22, 26
Cementation steel, 59
Centering, 155-214 (see also
Forms)
beam, measuring, 280
, sizes of, 155
billing, 274
Blaw steel, 185, 210, 273
column, measuring, 280
, sizes of, 155
economy in, 155
erection of, 153
failures in, 155
first-class, 278
for bridges, 197-211
culvert, 209, 210
domes, 192-196
engineering works, mea-
suring, 279
— floors, 151, 153, 181,
319
— good design in, 155, 278
measuring, 274, 278-281
— Melan arch, 198
Morsch's, 200
— pricing, 286
— removing, 153
setting out, 156, 157
sizes of timber for, 155
slab, sizes of, 155
specification for, 270
— suspended flexible, 206
timber sizes for, 155, 156
wall, measuring, 279
Chain concrete system, 227
Chains embedded in concrete, 5
Chalk as aggregate, 13
in cement manufacture, 18,
19
Chapman, Cloyd M., 267
Chateau d'Eau, 10
Chemical destructive influences,
262-265
Chicago cube mixer, 32
Chickahominy Bridge, Richmond,
Va., 205, 206
Chimney at Drury Lane, 369
forms, 197
at Northfleet, 368, 369
Church at St. Barnabas, Jamaica,
347-351
Cinder concrete, 15, 261
Cistern, square, form for, 211
Claw wrench, 129
Clay, burnt, as aggregate, 14
Clinker as aggregate, 15
, cement, 19, 20
Clinton system, 215
Co-efficients of expansion, 2, 56
Coignet, Edmond, 9
, Frangois, 6
system, 6, 133, 215-217
Coignet's first British patent, 10
Coke breeze as aggregate, 15
Collapsible steel centering for
sewers, 210
Collins, A. E., 369
Colouring concrete, 250-253
Coltrin mixer, 43, 44
Columbian system, 194, 227
Column, billing, 273, 274
, bursting tendency in, 117
, calculating, 117-122
, diminished, form for, 181
, eccentrically loaded, 121,
122
, fluted, form for, 181
forms, 162-167, 173, 180-181
, introducing reinforce-
ments into, 147, 150
'—, measuring, 280
-, removing, 153
limited loading of, 119
proportions, 232
reinforcing, 117-118, 145,
146
safe loads on, 119-121
spiral reinforcement for, 150
stress in, 78, 79
twelve-sided, form for, 181
Compression bars (struts), 78, 79
Compressol piles, 137-139
Concrete, 12-49
adhering to centering, pre-
venting, 154
aggregates, 12-15
, bonding, 49
, co-efficient of expansion, 2
, colouring, 250-253
compared with steel, 2
, compressive strength of, 12
, conveying, 46-48
, definition of, 12
, distempering, 253, 254
, elasticity of, 84
, finishing surface of (see
Finish)
, fire effects on, 259
, lime, 24
mixers, 30-45
, painting, 254
, placing, by gravity system,
48
— , pricing, 271, 285-286
, properties of, 4
, proportioning, 25, 26
, quality of, 12
Concrete, strength of : working
data, 84
, strong, 12
, watering, 133
Concrete Institute, 256
Concreters' tools, 128-132
Concreting floor, 151, 152
slab reinforcements, 140
— suspended floors, 152
Conduit at Woolwich, 210, 373
Considere octagonal pile, 133
system, 217
Considered tests, 9
Constants or factors, 61
Continuous mixers, 42-46
Conveying concrete, 46-48
Copenhagen International Com-
mittee, 264
Corner bars, steel, 154
Cornice forms, 187
, measuring, 280
, moulded, measuring, 279
with hollow coping, form
for, 187
Corr bar system, 217
Corrosion of steel reinforcement,
264
Craig screw bore, 154
Cudworth, W. J., 373
Culvert, centering for, 209, 210
at Kilton, 372, 373
Curry Tyer, 129, 130
Cushman, Dr. A. S., 254
Cutting of bars, 128, 129
DAMP-COURSE, Asphalt, 317
Dawnay system, 227
Day's reinforced slabs, 9
Deer Park (U.S.A.) Bridge, 205
Denman, W. M., 295
Denmark, reinforced concrete in,
10
Dennett's system, 6, 7
Dentile system, 217, 218
Desch, Dr., 264
Diagonal tension, 112
Distempering concrete, 253, 254
Dock and harbour works, 256
Domes, examples of, 193-196, 333,
337, 338, 346, 347, 355, 359,
360
, forms for, 192-196
— , measuring concrete of, 277
Door frames, fixing, 153
Drum mixers, 31, 32
"Dry" mixing, 29, 30
Dubuque gasholder tank, 212 ""
Dulac's compressol piles, 137-139
Durability of reinforced concrete,
256-265
Dyson, H. Kempton, forms de
signed by, 180
EARTH, angle of repose of, 123
Earthquakes and reinforced con-
crete buildings, 257
Ebco corner bar, 154
Eclipse mixer, 32
Edwards, F. G., 9
Elasticity of concrete, 84
. steel, 84
Elliptical vestibule at'RoyarAuto-
mobile Club's building", 322,
323
Ellis system, 227
Emett, Parr, 347
Empirical formulae, 61
Emulsions for waterproofing con-
crete, 268
Engine-house, raft foundation slab
for, 142
Erection, practical, 126-154
, procedure in, 272
Exhibition hall, 236, 237
Expanded metal system, 152, 2, 18,
219
Expansion, co-efficient of, 2, 56
Express mixer, 40
FABER, OSCAR, 369
Fagade for club, 238-240
Facing concrete in the form, 244,
245
Factors or Constants, 61
Factory buildings, erecting, 126
, example of, 234, 235
False- work (see Centering, Forms,
etc.)
Fawcett mixer, 40
Fence posts, forms for, 214, 362,
363
Fenders, timber, 256
" Fine Stuff," 244
Finish, applied, 248-250
brush, 243
bush-hammered, 244, 245
carborundum, 243
glazed, 248
interior, " stuc " work for,
249, 250
mosaic, 248
pebble-dashed, 247
plastered, 248
sand, 247
sand-blast, 243
sgraffito, 248, 249
, tile, 248
Finishing concrete, 153, 242
. , acids used in, 243
. : tamping, 246
: untouched surface, 242
Fire resistance, 258-262
Fire-resisting aggregate, 261
building, 261
reinforcement, 261
Flint as aggregate, 13
Floor, Allen's, 7
, centering for, 151, 153, 181,
319
erection of, 151, 152
Kearns', 181
Lythgoe and Thornton's, 7
Meyenberg's, 9
Monier, 7
reinforcement, 151
, concreting, 151
, continuous wire mesh
for, 151
: expanded metal, 152
, sheet mesh for, 152
, suspended, 151, 152
, concreting, 152
INDEX
Floor, at Royal Automobile Club's
building, 318
slabs, removing centering
from, 153
— , quantities for, 272, 273
tubes, Hennebique, 332
, Wayss and Koenen's, 9
Fluted column, foim for, 181
Foot-bridges (see Bridges)
Forms, 155-214 (see also Center-
ing)
for arches and bridges, 197-
208
, beam, 167-177
for circular tank, 211, 212
, column, 162-167
for cornices, 187
, correct position of, 156
for curtain wall with moulded
cornice, 187
diminished column, 181
domes, 192-196
, facing concrete in, 244-246
— for fence posts, 214, 362, 363
• fluted column, 181
foundations, 158-161
gasholder tank, 212,
213
• girders and beams; 177
, metal, for walls, 185, 186
for mouldings, 187
ornamental parapet,
187, 188
, panel, for walls, 185, 186
for pile, 157, 158 »
caps, 160
• pipes and sewers,
208-211
rectangular reservoir,
213
retaining walls, 188-
191
for silos, 191, 193
spandrel wall to bridge,
187
square cistern, 211
— staircases, 185
, striking, 156
for tall chimneys, 197
twelve-sided column,
181
walls and partitions,
181-184
Formulae, Empirical, 61
, rational, 61
Foundation, box mould for, 159
forms, 158-161
, grillage, 315-317
, ribbed, 158
slab reinforcements, 139, 140
1 steel piling for, 317
Franco-British Exhibition, 366
Frost, effect of, on concrete, 257,
258
Fuller and Thompson's experi-
ments, 15, 16
GARBETT on architectural design,
233
Gasholder tank, form for, 212, 213
Gaspary mixer, 41, 42, 44
395
Gauge boxes, 27
Gauges, aggregate, 27, 28
, cement, 28
, water, 28
Gauhe mixer, 32, 33
Girder (see Beam)
Glazed finish on concrete, 248
Golding's metallic lathing, 8
Gordon and Gunter, 358
Gordon's formula, 79, 119
Grafton Bridge, Auckland, New
Zealand, 231, 304
Grain silos at Silvertown, 389-390
Grand stand at Minnesota, St.
Paul, 386
Granite as aggregate, 13, 14
Granolithic plates, 244, 245
Gravel as aggregate, 12
Gravity mixers, 42, 43
system of placing concrete, 48
Grayson, Hastwell, 347
Grillage foundations at Royal
Automobile Club's building,
315
H.M. STATIONERY Office, London,
177
Halls, architectural treatment of,
235, 236, 241, 242
Hammersmith Baths, 198, 199
Hampstead Garden City Free
Church, 346, 347
Handling concrete, plant for, 329
Harris, F. E. L., 389
Heat, effect of, on concrete, 258
Hennebique beam form, 170, 172
first patent, 9, 12
floor tubes, 332
, Frangois, 9
, G. Flament, 257
square pile, 133
system, 219-221, 301
History of reinforced concrete,
5-11
Hodkin- Jones system, 227
Hoffmann kiln, 20
Hoisting appliances, 48
Holland, reinforced concrete in, 10
Homan system, 227
" Horses " used in setting out, 156
Hovis Mill, Vauxhall, 191
Humphrey, Richard L., 259
Hungary, reinforced concrete in, 8
Hyatt, Thaddeus, 7-9
Hydrofluoric acid, effect of, on
concrete, 265
"Hy-rib," 223
IMPERMEABILITY of concrete, 267,
268
Indented bar system, 221, 222
Inertia, moment of, 72-74
Iron portland cement, 24
Iron, rusting of, in concrete, 263
JASPAR, PAUL, 236
Jersey City, conduit at, 209
Jetty Head at Thames Haven,
376, 377
Johnson, Prof., 156
Johnson's Lattice system, 222
396
INDEX
KAHN system, 11, 222, 223
Kansas, Cowley County, bridge
at, 293, 294
Kearns' floor, 181
Keedon system, 223
Kennedy's bar-bending machine,
132
Kent mixer, 44
Kerbs, Wainwright, 154
Kiel dockworks bridge, 298, 299
Kiln for cement manufacture, 20
Kingsway Church, London, 141, 142
Kinlochleven Aluminium Works,
353-357
Knapp, C. R., 243
Koehring mixer, 33
Koenen, M., 8, 9
Koenen system, 227
LAMBOT'S concrete boat, 6
Lanchester and Rickards,
FF.R.I.B.A., 337
Lanzoni's system, 8
Lascelles, W. H., 8
Lathing, Golding's, 8
, Scarles', 8
Lavas as aggregates, 14
Laying concrete, 48, 49
Le Chatelier's tests for soundness
in cement, 22, 23
Leconte's ceiling slab, 5
Lecour's pile-driving machine, 135
Lee, T. A., 10
Leitch, Archibald, 382
Levers, orders of, 62, 63
Lidgerwood pile-driving machine,
134, 135
Lifeboat slipway, Ackergill, 378-
380
Ligno-concrete, 2
Lime, blue lias, 24, 25
— concrete, 24, 25
Limestone as aggregate, 13
Lindsay's system, 8, 227
Lintels at Royal Automobile
Club's building, 320
Load (see Safe Load)
to be carried, ascertaining,
83, 84
Local Government Offices, 189
Lock-woven mesh system, 223
Loudon, J. C., 5
Lutyens, E. L., 346
Lythgoe and Thornton's floor, 7
MACNICHOL, C., 254
Majestic Theatre, Los Angeles,
352, 353
Manila Cathedral, 333-337
Marfield, Prof., 337
Marine work, reinforced concrete
>-« for, 256
Marsh-Capron mixer, 33, 34
Mason mixer, 42
Masonry walls, imitating, 184
" Matrix " defined, 12
Matthews, E. R., 181, 264
Matthews' system, 7
Mayoh's floor, 9
Mclntyre building, New York,
stairway at, 366 367
McKelvey mixer, 34
Meadow Street Bridge, Pittsburg,
202
Measuring centering, 278-281
to beams, 280
, clauses in, 278
to columns and piers,
280
to inclined slabs, 279
to walls, 279
concrete, 377
cornice-mould centering, 280
and estimating, 276, 277
moulded cornice, 279
moulds for architectural fea-
tures, 279
steel reinforcement, 277
Melan, T., 9, 10
arches, 198
floors, 10
Merkuloff, M., 266
Merrimac River Bridge, New
Hampshire, 295
Mesh (see Wire Mesh)
Messent mixer, 34
Mewes and Davis, 311
Meyenberg's floor, 9
Mill, ball, 20, 21
, pendulum, 20
Mixers, concrete, 30-45
, , choosing, 30
Mixing concrete by hand, 28, 29
by machine, 30
, "dry," 29, 30
efficiency, testing, 46
stage, 128
, "wet," 25, 26, 29
Mizen Head foot-bridge^ 296, 299
Modulus of elasticity of steel, 53
— , section, 75-77
Moller system, 293
Moment of inertia, 72-74
of resistance, 75, 77
Moments, principle of, 61
Money Order Office, Holloway, 146,
148, 149
" Monier concrete," 2
floor, 7
system, 7-9
Moore, R. F., 306
Morning Post building, London,
194
Morsch, Prof., 307
Morsch's centering, 200
Mosaic finish, 248
Mouchel, L. G., 10
Moulded cornice, measuring, 279
Mouldings, forms for, 187
Moulds (see Forms)
Mushroom system, 224, 225
NATIONAL Gallery extension, 353-
356
" Natural " cements, 23, 24
Neumann, Paul, 9
Neutral axis, finding position of,
90-92
Niagara sand-blast, 243
Nims mixer, 45
Notation, key to (see Inset after
OCEANSIDE, California, bridge over
San Luis Rey River, 299
Oehler mixer, 41
Oil for waterproofing concrete,
268
Openings in centering, deducting
for, 279
, reinforcement at, 146
Owens, Dr. J. S., 30, 46
mixer, 43
PADSTONES, measuring centering
to, 280
Painting concrete, 254
Panel shutter for wall, 184
Pansy mixer, 41
Paragon system, 225, 226
Parapet, form for, 187, 188
Paris Exhibition of 1900, 229
Parker's experiments, 17
Partitions, forms for, 181-184
Pebble dashing as finish, 247
Peek, Frean & Co.'s building,
Bermondsey, 173, 177, 185
Pellew, Charles E., 252
Pendulum mill, 20
Perfect mixer, 44, 45
Permeability of concrete, 267
Philadelphia : viaduct at Scenery
Hill, 301
: Walnut Lane bridge, 307,
308
Piers for rectangular building, set-
ting out, 156, 157
Pigments for colouring concrete,
252, 253
Piketty system, 227
Pile, Brannon's, 7
caps, 139
, forms for, 160
cast in place, 136
Coignet, 133
Compressol, 137-139
Considere, 133
constructionplatformfor,158
driven, 132-139
driving, 133, 136
machine, Lecour's, 135
-, Lidgerwood, 134,
135
-, Sykes', 135
, Dulac's Compressol, 137-139
forms, 157, 158
, American, 158
, Hennebique, 133
, octagonal, mould for, 157,
158
, round, mould for, 158
set, testing, 135, 136
, simplex, 136
, skeleton framework for, 133
, square, mould for, 157
Piling, steel, 317, 318
Pioneer mixer, 35
Pipe (see Sewer)
forms, 208-211
Pipes, Bonna system for, 227, 369,
371 373
Pite, Prof. Beresford, 232
Pittsburg : bridge at Meadow
Street, 301-303
Plaster for concrete surface, 248
Plates, granolithic, 244, 245
Portland cement (see Cement)
Posts, fence, forms for, 214
Poti Cathedral, Russia, 195, 196,
337
Potter system, 227
, T.. on use of coke-breeze
as aggregate, 15
Poulsen, A., 264
Pricing centering, 286
concrete, 271, 285, 286
estimate, 285-287
steel reinforcement, 286
Principle of moments, 61
Procedure in erecting building, 272
Proportioning concrete, 25
Punning tools, 128
Purchase, Keynes, 311
Puzzolanic cements, 24
QUANTITIES, 271-275
Quartzite as aggregate, 13
Quimby, H. H., 245, 307
RAFT foundation slab for engine
house, 142
reinforcement, 141, 142
Railway sleepers, 389
Ransome, E. L., 8
, Frederick, 7
cart, 48
false-work for chimneys, 197
forms for cornices, 187
mixers, 35-37
tower, 47
twisted square bar, 8
wall forms, 184
Rational formulae, 61
Reactions, 63-65
Rectangular building, setting out
piers of, 156, 157
Reinforced concrete defined, 1, 2
, properties of, 4
Reinforcement, calculating amount
of (see Beams, Slabs, Columns,
etc.)
Renommee Hall, Liege, 236
Reservoir at Kloof Nek, Cape
Town, 380, 381
, rectangular, form for, 213
Residues, furnace, as aggregates,
14
Resistance, moment of, 75, 77
Retaining wall at Bridlington,
181
, designing, 123-125
— , forms for, 188-191
at Pittsburg, 364, 366
at Royal Automobile
Club's building, 311-
315
at Royal Insurance
building, 145, 358,
359
reinforcement for,
142-145
— , types of, 124, 125
Ridley Cammell system, 227
, M. Noel, 299
Roll mixer, 37
INDEX
Roofs, architectural treatment of,
236
— , examples of, 323, 324, 353-
356, 380, 382
, sloping, at Royal Auto-
mobile Club's building, 323,
324
Ross spade, 129, 247
Rotary cement kiln, 20
" Rotten " porous concrete, 263
Royal Automobile Club's building
described, 250, 311-328
Royal Insurance building, 145,
358-360, 363
Royal Liver building, 229, 329-
333
Rusting of reinforcement, 263
SAFE load on beam, 77, 78
Sainsbury's building, London,
172, 173
St. Charles' College, Netting Hill,
194, 195
San Sebastian, gasholder tank at,
213
Sand, 15
finish, 247
substitutes, 16
, voids in, 25, 26
, washing, 16, 17
Sand-blast finish, 243
, Niagara, 243
Sandstone as aggregate, 12, 13
Scarles' wire-lathing, 8
Scenery Hill Viaduct, Philadelphia,
301
Schule, Prof. F., 10
Schuster, Rudolph, 8
Scott's system, 7
Screw bore, Craig, 154
Sea-resisting concrete, 264
Sea-water, effect of, upon con-
crete, 264
Section modulus, 75-77
Setting out building, 156
" Setting time " tests for cement,
23
Settlements, 257
Sewell, Captain, 257
Sewer at Acton, 371, 373
, Bonna system for, 227, 369,
371, 373
, forms for, 208-211
in Paris, 370, 373
pipes, early, 8
at St. Louis, 370, 373
Sgraffito finish for concrete, 248,
249
Shaler, I. A., 10
Shear, horizontal, 79
, vertical, 79
Shearing stress, 79-82, 112-117
Sheet mesh for floors, 152
Shingle as aggregate, 12
Siegwart beam, 10
Silos, 389-390
, forms for, 191, 193
at Hovis Mill, 191
Simmons, William, 8
Simplex piles, 136
Singen, water tower at, 391-392
397
Skeleton reinforcements, forming,
150, 151
- for piles, 133
Slab, bending moment at centre
of, 95
, billing, 273
— , calculating, 95-99, 102-104
— , foundation, 139, 140
— , measuring, 277
— moulds, timber for, 155, 156
reinforcement, concreting,
140
reinforcements for founda-
tions, 139, 140
, raft foundation for,
140, 142
Slag as aggregate, 15
cement, 24
Sleepers, railway, 389
Slurry, 19
Smeaton's experiments, 17
Smith mixers, 37, 38
Somerville system, 227
Soundness tests for cement, 23
Spade, perforated, 128
, Ross, 129, 247
Span, effective, 65
Spans : working data, 85
Specialist system, 270
, designs independent
of, 275, 276
Specific gravity test for cement, 22
Specification for cement, British
standards, 21, 22
• centering, 270
steel, 52
, general, 270, 271
Sprague, N. S., 304
Stadium at Shepherd's Bush,
London, 382
Stage, mixing, 128
Stains for concrete, 253
Staircases, 146, 147, 365-368
, forms for, 185
at Royal Automobile Club's
building, 320-322
Stanchion proportions, 232
Stand at Bradford, 382-386
Stands, grand, 382-389
Steel, 50-60
, acid, 60
•, adhesion of : working data
84
, alloy, 56-59
, annealed, 51
, basic, 60
• , open hearth, 60
, bending tests for, 54
, Bessemer, 60
, blister, 59
, carbon in, 57-59
: carbon content, 50, 51
, cast crucible, 59
, cementation, 59
: co-efficient of expansion, 2,
56
, composition of, 50, 56-59
, corner bars, 154
, corrosion of, 26
, ductility of, 51
, elasticity of, 52
INDEX
Steel, fatigue >-•.
- , hanlnc — oi. .'>!
-- , high-speed. .">•. ,">;'
- : how heat atVects physical
proper! lea,
- : limit of ehstieity. ."'_
- , malleability of. .">0
- , manganese in. od
- : modulus of elasticity
- , open hearth. dO
- , phosphorus in. ,">7
- , piling lor foundations at
Automobile Club's
building. 151 7
- , properties of. 4
- , quenched. .">!
- , reinforcement. measurins:.
177
-- , pricing. I
-- , speeiticat ion for. .">_
- ; rc>istanee to repeated
stresses, 55
- rusting of. L'd:1.
- shearim; strength of. ,V>
- silicon in, 56
- strength of : working data,
sulphur in. ,">7
- tenacity of, 60
- testing, .">'.'
- toughness of, 53
Steps, steel eorner bar> for. KM
Stirrups, measuring. L'77
Stress, 01-82
- , working. s!
Striking forms, l.'.i-
Struts (see Columns \
- . stiv» iii. 7S. 70
" Stuc " work for interior finish,
•-MO. 250
Sulphur in aggregate. 14
Surfaee treatment. "J I 1 LV'-O
Suspended eentering for bridges.
•Jo. -Jos
- floor reinforeement, 151
Sykos' pile dri\ ing maehine, 135
SyMcms deserihed. lM."> .'.S
Swimming bath at Royal Auto-
mobile Club's building. S'J I
Switzerland, reinfoived e one rote in,
10
. (Mi ' quantities. -J7I?
Tampers, UN. I-M. -jj,;
Tamping eon. :
Tank, circular. form for. ill. -2\'2
- gasholder, form for. iM'J. '2\3
Taylor mixer*. 36
Tedesco, N. de. ;>
Tee beams, ealeulating. 101 1 TJ
Tensile strength test for eement.
Terminologx . -
Terra-eotta as aggregate. H
Test in>: eement. -\. --
steel. :,-2
"Test ins: Clauses" in Keinfoieed
Conerete Hill. "JM
Teufen IM- rland. L\>0.
;uui. :?o;i
Thaeker. !•].. 10
Theatre. 1 .os An-eles. :!,M\ ;?;,;;
Thomas. \V. Aubre>. L'L'S». :Ji'.»
\V. M.. s\>tem. LW
Thompson's. Sandfoul F... safe
loads for timber struts. I. "id
Thornton and l.ythgoes' lloor. 7
Thuasne system. .">
Thuringia. bridge over l\iver
Wen a. SO."'. SOii
Tile finish for eonerete, 'Jl^
Timber fenders.
struts, safe loads for. lot)
Toeh. Pr. M.. L'.M
Tools, eoneretors'. 1'JS l.'5'J
To\\er. Kansome. 17
Trump mixer, l.">
Trussed Conerete Steel Co.. 11. ISO.
S7S r"' "/"•<> KahiO
Tunnalt. K.. Tappen. -'»>,".
Turner's mushi\>om system.
Twelve-sided eolunin. form for. IS I
Tyer, Curry. I •_".'. ISO
I'uixr. elTeet of. on eonerete. ^|i,">
U.S. Geological Survey. L\".'.». 'JtU
VATTLTINOS, early, 8
Vaults, measuring, L'77
N'era Cm/,. M<-\ieo. foot bridges at,
291
Viaduet at Seenery Hill, Phila-
delphia. :?!>;>. :icl
Vietoria mixer. :!S. :;-,i
Visintini system. 10
Voids in aggregate. •_',">. L'C
: detention of term. 2,">
in sand, -ti
Volcanic rocks as aggregates, 14
\\ u\\\ ui.:iir kerbs. 154
\\alls. billing. -2i:\
, cinder eonerete. 2l>l
forms. IS I 1S1
, imitating masonry. IS I
— , metal forms for.
Walls with moulded cornice, form
for. 1S7
Walls, panel shutter for. 1S1 ISd
panels, removing centering
from. l."'l>
, with post buttresses, 359,
:uii- >:;
, ivinforcement, lid
, retaining ^ .• K e t a i n i n
Wain
Walnut l,ane Hrid^e, Philadelphia
•JO I
Warehouse, architectural defects
in. -':?:?
at Cologne. i:U>. i:? »
Washing aggregates, !."> Id
sand.' Id '
\\ashmill. eement. IS
Water absorption of eonerete. id7
, effect of, on concrete. I'd;!.
MM
gauges. iS
for mixing eonerete.
on site. Ii7, IiS
Waterproofing concrete. idd ii>0
, emulsions for. idS
, mass, idd
materials, id7, idS
. oils for. idS
— , superticial, -dti
Water repellent substances, 'Jd7
Water tower at Singen. SiM .'?;':.'
\\.i\-- aiul Koenen's tloor, 0
s system. 7. 0
Webster. 'C,. S.. :U>7
Weisserit.'. fi'ot -bridge over, '2\>'2.
Wells 'system.
Wentworth-Sheilds. !• K.. ±">d. id I
Werra. Thuiin^ia, bruise oxer.
na
Wesleyan Hall. Westminster. 17S-
isi. ioi. los. :i:i7. :;;;;> ;;id
Whalley mixer. 41. 4i
Wharf at IVnnemont. ;>7 t, :'7.">
Whew ell. Professor, il!;;
\\'hit;> concrete. if>i
Wilkinson's system, d. iiS
Williams' system. iiS
Wimborne. bridge at. i;>:>
Window frames, fixing. 1 ."<:?. l.">4
Wiiv mesh, continuous, for lloors.
Wiiv-stivncthened form. 101
Woixl. \\. K . S
Woolson. Professor Ira H.. ioS.
Woolwich, conduit at. ilO. S7:>
Wnvn. Sir Christopher, o
\\'ivnehes for beiulnm bars. li!>,
iao
Wiinseh's system. S. i>
n nr CASS: v rt HILL, LONDON
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