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'Qnivereitf of TlSltsconsln

STRUCTURAL DESIGN

VOLUME I

ELEMENTS OF STRUCTURAL DESIGN

HORACE R. THAYER

Assistant Profbssor of Structural Design

Carnbgib Technical Schools

Pittsburg, Pa.

LONDON

CONSTABLE & COMPANY LTD.

1912

D. VAN NOSTRAND COMPANY

THK ftCIKNTIFIC PRKSS

ROBKRT ORUMMONO AND COMPANY

BROOKLYN, N. Y.

(^re-^^.;^"?^

173122

APR 11 1913

PREFACE

It might be well to say here what is repeated in the text:
that this work presupposes a knowledge of mechanics, stresses,
and the mathematics on which they depend.

The experience of the author in teaching and in practice
has led him to believe that no available book presents all
structural subjects so concisely that, they can be covered in
the time usually allotted by technical schools. Moreover,
the fundamental principles of shop practice and erection, which
govern the designer at every step, are not clearly developed.
These faults the author has attempted to correct, using only
orthodox methods of presentation.

This volume considers wooden structures and the fundamental
principles of design in steel. Should this work receive recognition,
it is intended to follow it by a second volume on the " Design
of Simple Structures," plate girders, viaducts, truss bridges,
mill buildings, high office buildings, and standpipes, and a
third on the " Design of Advanced Structures," cantilever,
movable, and suspension bridges and arches. It has been
thought best for the present to omit the data usually found in
handbooks.

As a considerable portion of this treatise is original data,
corrections and suggestions will be especially welcome. How-
ever, a great deal of pains has been taken to eliminate errors.

The writer wishes at this time to acknowledge his references.
These include almost every American authority on the subject.
Many of them have been referred to at the proper place in the
book. Particular mention should be made of the Engineering
News J Engineering Record ^ and Transactions of the American
Society of Civil Engineers. The last two kindly allowed the

■ • ■
Ul

IV PREFACE

reproduction of their illustrations. The aid rendered by various
manufacturers is acknowledged in the text.

The author desires to thank the following gentlemen for
assistance in revising manuscript: Registrar Tarbell and Profs.
McCullough, Jones, Lose, and H. S. Domberger of the Carnegie
Technical Schools and Messrs. Gordon, Allen, and RI S. Dom-
berger. • H. R. T.

Ca&negie Technical Scbools,
Pittsburgh, Pa., March 15, 191 2.

CONTENTS

CHAPTER I
Materials

ART. PACK

1. Introduction i

2. Growth and Characteristics of Timber 2

3. Faults of Timber 5

4. Preservative Processes for Timber 8

5. Varieties of Timber 10

6. Strength of Timber 11

7. Uses of Timber 13

8. Cast Iron 13

9. Wrought Iron 15

10. Bessemer Steel 16

ir. Open Hearth Steel 17

12. Cast Steel and Alloys of Steel 19

13. Paints 20

CHAPTER II

Commercial Shapes

14. Handbooks Units and Dimensions 22

15. Commercial Shapes of Wood 24

16. Commercial Shapes for Cast Iron and Steel Castings 25

17. Rolling 27

18. Circular Shapes > 29

19. Rectangular Shapes 3^

20. Angles 31

21. I-Beams and Channels 33

22. Occasional Shapes 34

23. Rare Shapes 3^

vi CONTENTS

CHAPTER in
Wooden Structukes

ART. PAGB

24. Principles of Design 37

25. Accessories of Other Material 40

26. Joints 42

27. Designs of Timber Structures 51

28. General Description of Roof Trusses 54

29. Computations for a Roof Truss 57

30. Trussed Beams 62

31. Description of Bridges 67

32. Computations for a Bridge 69

33. Trestle Bents 75

CHAPTER IV

Fabkication o? Structukal Steel

34. Organization of Administration 79

35. Plant in General 80

36. Stock Yard 83

37. Main Shop 85

38. Machine Shop 95

39. Forge Shop 98

40. Templets 100

41. Methods of Cutting Material 102

42. Methods of Bending 104

43. Process for Upsetting 106

44. Methods for Making Holes 108

45. Layout and Assembly 109

46. Fastenings for Steel Work no

47. Methods for Riveting 113

48. Inspection Painting and Shipment 117

49. Erection 1 19

CHAPTER V
The Engineering Department

50. Specifications 131

51. Problem of Design 140

52. Economical Relations 143

53. Estimating 146

CONTENTS vu

JiRT. PAGB

54. Design of Beams «... 148

55. Design of Tension Members , 157

56. Design of Compression Members i6z

57. Strain Sheet 172

58. Detailing 172

59. Design of Splices and Beam Connections Z79

60. Design of Riveted and Pin Joints in Trusses J83

61. Shoes 186

62. Structural Drawings X93

63. Auxiliaries — ^Bills of Materials 198

64. Bilk of Eyebars, Pins and Accessories 201

65. Other Bills 203

66. Checking 204

67. Other Steps 207

68. Examination of Structures in Use 208

69. Failures 209

ELEMENTS OF STRUCTURAL DESIGN

CHAPTER I

MATERIALS

Art. I. Introduction

This work is intended for students and draftsmen who
understand analytical mechanics, mechanics of materials, and
methods of determining stresses either by graphics or by com-
putation. On this account, only a synopsis of results will be
given in sample designs. However, the reader should check
each operation to be sure that he thoroughly imderstands it.

The writer believes that the difficulty usually found in teach-
ing structural work arises from the fact that fundamental
principles are not clearly developed and illustrated by simple
examples at the start. It is axiomatic among practical men
that a thorough knowledge of details should precede any attempt
at design. Yet this well established principle is often ignored
in technical training.

We shall consider first the materials and their commercial
shapes, which, often altered in manufacture, are used in struc-
tural work; next, the organization of companies to handle
them; afterwards, their machines, their capacities, and the way
in which they operate to transform the original shapes; then the
means by which a member carrying a given stress can be n:ost
economically fabricated to carry its load; next the methods
of fastening together these different members; and finally,
the design of the finished structures.

In order to serve as a reference book for the student and
draftsman, much data will be given which will be supplanted
in practice by the slightly different standards of the locality or

2 ELEMENTS OF STRUCTURAL DESIGN

of the company concerned. For example, the common sizes
of timber as given in Art. 15, vary somewhat with the locality
and date. They are inserted, however, to give an idea about how
they run, to serve as a standard for student designs, and to be
used where other information is not available. Data of this
sort should not be memorized.

Art 2. Growth and Characteristics of Timber *

The advantages of timber are that it is light, cheap, abimdant,
and easily worked or altered. On the other hand, it is accessible
to vermin and insects, is combustible, quite perishable, and
weak in shear and bearing perpendicular to the grain. These
weaknesses are more fully discussed in Chapter III. The
approaching exhaustion of^ our reserve supplies has partially
neutralized its advantages of abimdance and cheapness.

Timber is cut from the trees in a manner familiar to all.
Those woods which are used in construction are almost entirely
exogenous. These grow by formation of new wood each year
on the outer surface. On a transverse section of the tree, they
appear as rings whose number equals its age in years. Each
ring is composed of two parts: an inner portion called spring
wood which is soft and larking in strength; and an outer part
called summer wood which is hard and strong. Hence the
more of the latter, the stronger the timber is. In the conifers
(pine, spruce, and hemlock), the summer wood is dark and the
spring wood light; in the broad leaved trees (oak, maple, birch),
the reverse is true. When these annual rings are narrow, wood
is said to be fine grained; if wide, coarse grained. When fibers
are not parallel to axis, as in hemlock, it is called cross-grained;
when wavy as in maple, wavy grained or curly. The figuring
of bird's-eye maple is due to the concentric circles appearing in
a tangential section of unevenly growing wood.

A zone of the wood next to the bark and i to 3 inches wide is
lighter than the remainder and is termed sapwood because it is

• References for Timber: Bulletin No. 10, U. S. Forestry Div., Agricultural
Department, 1895; Johnson's "Materials of Construction"; Kidder's "Building
Construction and Superintendence," Part II. Snow's "Principal Species of
Wood."

MATERIALS 3

active in carrying the sap. The interior of the tree called
heartwood is inert and fulfills only the mechanical function of
helping to sustain the tree. Sapwood is weak and subject to
rapid decay owing to the great amount of fermentable matter
contained therein.

Outside of the tree is the bark, a rough scaly covering whose
appearance is often of assistance in determining species. In
preparing the tree for use as lumber, the bark should be sawn or
stripped off, as it interferes with seasoning.

Inside of the bark, sap and heartwood, spring and summer
wood alike, is made up of " wood fibers " and " medullary "
or " pith " rays. The former are small hollow cells about
O.I inch long and .001 inch in diameter. Their greatest length
is ordinarily parallel to the axis of the tree. Pith rays are
somewhat similar but smaller cells, extending radially. Much
the greater part of the timber is composed of wood fibers and this
gives rise to some of its structural peculiarities.

In the first place, as we should expect, its principal strength
lies in a direction parallel to the axis of the tree. Other ways
it is weak, as in shearing along the grain or in bearing per-
pendicular to the grain.

The shrinkage of timber is also affected by this peculiar
composition. Roughly, green timber is one-half water; 2 per
cent is a constituent part of the -cells of the sapwood; 18 per cent
saturates the walls df all cells; the remaining 30 per cent fills
the cavities. All but a small part of this moisture may be
expelled by dr)dng. However, on exposure to the air, dried
lumber takes up moisture until a 10 to 15 per cent content is
reached. This is the standard for seasoned Imnber.

There are three reasons for seasoning. First, to increase
its strength (Art. 6). Next, the large amount of water and sap
in green stuff affords a favorable condition for the growth of
those germs which cause decay (Art. 3). Finally, the removal
of moisture as it dries out, alters the dimensions of the timber.
Let us study this change with care.

As one might expect, the relative change in the direction of
the main axis of the cells is small; perpendicular to said axis, it is
large. Radially, Fig. 2a, it is held in a measure by the pith
rays. Tangentially there is no restriant and more of the shrink-

4 ELEMENTS OF STRUCTURAL DESIGN

age takes place in this direction. For this reason, green lumber
of the shapes seen in Fig. 2by takes, after seasoning, forms
exaggerated in Fig. 2C. The longitudinal shrinkage of timber
will average about one one-thousandth of the length. Approx-

f((ro^)i)

Fig. 26,— Original Shapes — Green.

vmco)»i"

Fig. 2a. — ^Tangential and Radial. Fig. 2c. — ^Resulting Forma — Seasoned.

imate values of radial and tangential shrinkage, deduced from
U. S. Timber Tests, follow:

Variety.

Soft pine, cedar, cypress, spruce

Hard pine

Ash, elm, poplar, walnut, maple

Chestnut

Oak

Radial.

TangentiaL

.020

.04

.025

OSS

.030

.070

.040

.080

.060

.120

There are two methods of seasoning: (a) Air, (b) kiln
drying. In the former, the lumber is stacked in the open and,
as far as possible, arranged to shed water yet give the air free
access. This usually takes from two to three years and the
lumber is never fit for some purposes. A kiln is a tight chamber
through which a current of air of about 150 to 180° F. is passed.
Hard woods should first be air-dried. Steaming is often employed

to prevent checking or case hardening (see next
article). Four to six days are required for i inch
material and longer for larger stuff.

At the junction of the limb and stem, fibers
on the lower side run into trunk as seen in Fig. 2d.
On upper side fibers are not continuous. For
this reason a split made above does not run into
the knot, while one made below does. When limbs die, they

Fig. 2d. — Grain
at a Knot.

MATERIALS 6

break oflf, and are finally covered by the growth of the trunk,
and give rise to the annoying dead or loose knots. Their
weakening effect is fully explained by the structure of the timber
at this point.

The color of the timber serves as a characteristic mark
for many species, is a help in detecting decay, and adds to the
appearance of certain finishing woods. Odor is of importance
for the first two reasons. Cjrpress and hard pine, much alike
in appearance, may be distinguished by the resinous odor of the
latter. Resonance is that property which enables a substance
to transmit sound. In timber, knots and othei; irregularities
lessen this transmission. Decaying wood gives a dull heavy
sound when struck with a hammer.

Values for the weight of various kinds of wood in pounds per
cubic foot are as follows :

Variety.

Hickory, oak

Ash, elm, cherry, maple, walnut

Hard pine, Oregon pine

Norway pine, cypress, hemlock .

Cedar, spruce

White pine, pedlar

Weight, green.

Weight, seasoned.

S6 to 64

42 to 48

48 to 56

36 to 42

40 to 48

30 to 36

32 to 40

24 to 30

24 to 40

18 to 30

24 to 32

18 to 24

Art 3. Faults of Timber*

These may be divided into four groups:
(i) Defects of growth.
. (2) Faults due to improper handling or seasoning.

(3) Bacterial decay.

(4) Injury caused by worms and insects.

Prominent among the first named are sapwood, shakes, and
knots.

As already noted, sapwood is the weaker part of the tree.
However, the rejection of pieces solely on this account is justified
only in timber used for special purposes. ^

Shakes are of two kinds, the heart, Fig. 3a, and the cup,

* Year Book, 191 1, Am. Soc. Test. Mat., pp. 166-172.

6 ELEMENTS OF STRUCTURAL DESIGN

Fig. 36. Only the larger trees are subject to them. Small
shakes do no appreciable harm; those extendmg through the
piece are serious and justify its rejection.

Knots are a grave source of weakness in timber. Whether
in tension, flexure, or compression, the safe strength varies
considerably with number and size of knots. A rough measure
of their effect in weakening a piece is to consider them as open
holes of equal size. In many cases they are objectionable for
appearance^s sake. It is generally impracticable to exclude
knots altogether; however, they should be limited in amount.

(2) Faults due to improper seasoning and handling are
wane, checking, and case hardening.

Where a portion of the exterior surface of the log appears
on the sawn piece, as in Fig. 3c, it is called a wane. It signifies

Fig. s<^.
Heart Shake.

Fig. 36.
Cup Shake.

Fig. 3c.
Wane.

Fig. ^d.
Checking.

the presence of sapwood, lessens the amoimt of timber, and may
interfere with some uses.

Fig. 3d shows checking which is a separation of the wood
fibers on the end of a stick, extending back into the piece but a
short distance. It is caused by kiln drying and may be pre-
vented by first seasoning in air for three to six months.

Case hardening i^ seasoning of the outer layers before the
interior. The latter, as the operation goes on, shrinks and
checks badly. It may be prevented by steaming, or, better
still, by a preliminary air drying.

(3) There are three ways in which bacteria cause the decay
of timber:

(a) Fungus growth.

(b) Dry rot.

Fungus growth may attach itself to the living tree, forming
a knob on the exterior, while filaments from the same, in appear-
ance much like roots, eat into the tree* and devour sap and fiber.

MATERIALS 7

Another form attacks the sawn timber in much the same manner
as the mould on a piece of bread. Moderately warm localities
with moisture, but not immersion, and untreated lumber are
fields suitable to its growth.

Dry rot is caused by bacteria which exist where the gases
of decomposition cannot escape. Affected wood looks fairly
firm, but readily crumples to powder when squeezed between the
fingers. It spreads rapidly, contact not being necessary for
infection. Moderate warmth, lack of ventilation, and moisture
but not immersion, favor its growth.

A different kind of bacteria causes wet rot in timber. This
variety thrives only where exposed to circulating air. The
infected portion is generally wet, dark or dirty looking, and
falls to pieces in the hand. It spreads by contact only. Moder-
ate warmth and moisture without continuous immersion are
favorable conditions, Wood is particularly susceptible to
this form of decay when exposed to alternations of wet and dry,

To prevent decay due to these causes just outlined:

(a') Season, thus in a measure depriving bacteria of their
food.

(6') Use preservatives. These are invariably disinfectants,
that is, poison for the bacteria. See Art. 4.

{c') Keep under fresh water. In tidal seas, there is danger
from marine worms. (See below.)

(d') Avoid conditions favorable to their growth as given
above.

Of these, {c') is the only method that is always dependable.
Removal of wet rot already existing and observance of any
one of above rules should prevent its spreading farther. For
dry rot, in addition, all pieces in vicinity should be scraped
and washed with acid.

(4) We shall limit ourselves to the consideration of marine
worms.* Although there are insects which cause damage,
they are largely limited to tropical climates.

The principal of these pests are:

Teredo or ship worm, about six inches long, and one-eighth
inch in diameter.

* Colson's "Notes on Dock and Dock Construction." Eng. News, Vol. XL^
P-34.

8 ELEMENTS OF STRUCTURAL DESIGN

Liznnoria Terebrans, a small insect about one>sixth inch
long resembling a wood louse.

Chelura Terebrans, in appearance like a sand shrimp. It is
about one-quarter inch long and very destructive.

They inhabit salt water, attacking the timber between low tide
and the bottom of the sea. Here they bore in the wood, in some
cases leaving but a small percentage of the whole. For protection:

(a) Leave the bark on (partiaJ).

(ft) Use only certain kinds of woods, for example, palmetto,
cypress, pine. (Of doubtful value.)

{c) Drive flat-headed nails of iron, copper, or zinc close
together. They should extend from a foot below the ground
to high water.

(d) Cover timber with tarred paper and zinc or copper
sheathing from a foot below the ground to high water.

(e) Creosote the timber. This is the best remedy and will
be taken up in the next article.

Art 4* Preservative Processes for Timber

The lack of durability in timber is caused by (i) rot, either
wet or dry, (2) marine worms, and (3) fire. Means for preventing
destruction by these agencies will be discussed in this article.

The life of timber if fully exposed to marine worms, is
very short, say a year or so. For an untreated railroad tie
it is six to twelve years, according to species of timber and
location. The use of tie plates is expected to increase this.
The life of untreated timber if exposed to the weather is ten
to twenty years; if housed, forty years or more. Preservatives
will considerably prolong durations given above.

If timber be kept under water it will last indefinitely. Well
preserved pieces of wood have been foimd in the wet strata
of bygone geologic ages.*

Timber to be buried in the ground should be charred or
dipped in coal tar. In the latter case, seasoning is necessary;
in the former it is advisable.

Creosoting, if well done, is the best of all preservative
processes. The material, creosote, is obtained from the dead

* Eng. News, Vol. LIV, p. 555.

MATERIALS 9

oil of coal tar by distillation. The idea of the treatment is
to introduce it into the pores of the wood. As long as it remains
there, the pests will not attack it. The creosote does not
penetrate deeply, hence care must be taken in making fresh
cuts in timber, as they may afford entrance to its enemies. The
proper way is to cut and remove bark before creosoting. The
creosote should not have more than 2| per cent of water and
a specific gravity of not less than 1.04 at 100° F.

In the process, timber is first air dried for several months,
then placed in large cylindrical vessels, subjected to steam at
a pressure of 15 to 40 lbs. per square inch; next to a negative
pressure of 12 lbs. per square inch; afterwards to creosote oil
at a temperature of 120° F. and a pressure of 150 to 200 pounds
per square inch. Five to twenty-five pounds per cubic foot^
of timber should be absorbed, several hours being required
for the saturation. The larger amounts are for protection
against marine worms. Creosote seems to make timber brittle
and ill adapted to resist abrasive forces. The cost of th«
treatment is i.o to 1.5 cents per pound of creosote.

Various modifications of this process are in use but above
is typical.

Bumettizing: In the same general manner, other chemicals
may be introduced into the wood. In bumettizing, about
J pound chloride of zinc per cubic foot is injected at a cost of
2§ to 6 cents per cubic foot. It bleaches out very rapidly
with moisture or water, hence its field is very limited. Is likely
to render timber brittle.

Kyanizing: Here the chemical is bichloride of mercury,
but like the chloride of zinc, it rapidly dissolves out under
the action of water.

Zinc-Creosote Process: For the sake of economy, the creosote
and emulsion of chloride of zinc are simultaneously injected.
Results, however, are not so favorable as for creosote alone.

Zinc Tannin, or Wellhouse Process: In this case, chloride
of zinc is injected, followed by glue and tannin, these two
latter substances forming an artificial leather which plugs up
the pores in the outside so as to keep in the zinc chloride.

Other Processes: Many other process have been used
among which we will mention:

10 ELEMENTS OF STRUCTURAL DESIGN

(a) Creo-resinate — creosote, resin, formaldehyde.

Q)) Water creosote — emulsion of creosote and water.

" Fire Proof " Wood.* Wood impregnated on pressure
with such salts as those of alum, ammonia, and the phosphates
bums with much more difficulty than before treatment. It
acts in two ways: first, a deposit is formed in the cells which
retards the flame; second, a gas is given off that hinders com-
bustion. This gas is sometimes quite offensive. There is no
such thing as fireproof wood. It is simply fire retarding.

Art 5. Varieties of Timber

White pine is an evergreen tree with a needle-like leaf.
Timber is a light whitish color, does not warp or check, and is
hence a good finishing lumber. Is easy to work but lacks
strength. First-class white pine is scarce and expensive.

Hard, or long leaved Southern, or yellow pine timber is
heavy, free from knots, has a reddish-brown tint and a resinous
odor. Trees from which it is cut grow in the Southern States.
It is very durable, very strong, very stiff, and stands well,
but is hard to work. It is our best structural timber.

Norway pine is common along the Canadian border. The
timber is a white wood with a reddish tint, and is soft and
durable. Its strength and other characteristics are intermediate
between those of white and yellow pine.

Oregon pine or Douglas fir is a western timber. It is much
like yellow pine, except that wood is coarser grained. It is
stiff, strong, and durable, and except for difficulty in working,
an ideal construction timber.

There are three varieties of spruce; white, black, and red,
all much alike. Timber is a light whitish or reddish color, soft,
easy to work, of medium strength, warps and twists a good deal.

Hemlock grows in the northern United States and in Canada.
Wood is light, of reddish gray color, lacking in strength, moder-
ately durable, cross-grained, rough, and splintery. It shrinks
and warps considerably in seasoning.

There are several varieties of cedar, all of which are light,

♦ Eng. News, Vol. LIV, p. 353.

MATERIALS 11

soft, grayish brown or red woods. Timber is durable, seasons
rapidly, shrinks and checks but little.

Several different species of cjrpress are found in the swamps
of our Southern States. Wood is light, soft, easily worked,
straight grained and free from knots. It warps and shrinks
little and is used in finishing.

There are six varieties of ash of which the principal are
white and black. Timber is heavy, tough, strong, and hard.

Three kinds of oak occur, white, red, and live. The latter
may be distinguished by its very crooked limbs; the others,
by the color of the timber. Oak is hard, tough, and strong.
It is especially prominent among timbers by reason of its high
shearing strength. It is difficult to work, shrinks and cracks
badly in seasoning, but once seasoned, it stands well. Live
oak is now very expensive and is used only for special purposes
such as in wooden boats. Both white and red oak are extensively
employed, but the former is more desirable in every way.

Beech is a white to light brownish timber, coarse textured^
heavy, hard, and strong.

Chestnut is a light, soft, coarse-textured wood. Possesses
only medium strength but is very durable.

Poplar or whitewood is a white or pale yellowish timber,
very free from knots. Timber is light, soft, and weak, shrinks
badly and warps considerably.

Maple makes a hard, tough, strong timber, white in color.
It seasons and stands well.

Art 6. Strength of Timber

Allowable values are in pounds per square inch, for good
merchantable timber as received from the lumber yard. Formula
for flat-ended columns:

d
Where,

Sc-^a-h

5<j = allowable compressive unit stress;
a, 6 « constants given below ;

/ . t r i- . unsupported length in inches

—= greatest value of fraction — . , — -^^ — ; — ; s

a least breadth in inches

ELEMENTS OF STRUCTURAL DESIGN

BUILDINGS

Variety.

Ten-
sion.

Flex-
ure.

Compression.

. Bearing
... to grain.

Shear-
ing.

Modulus
of

Per.

Par.

Elasticity.

Chestnut

White oak

Red oak

Hemlock

Spruce

600
1000
900
400
600
600
800

IIOO

1200

750

1000

900

600

7SO

750
900

IIOO

1500

600
1000
900
400
600
600
800
IIOO

1200

6
12
II

4
6

300
SOO
400
200
300
200
300
300
600

1000
1500
1200

750
1000

750

900

1400

1500

ISO

140

70
100
100

1,200,000
1,600,000
1,500,000
900,000
1,200,000

White pine

Norway pine

Oregon pine

Yellow pine

1,100,000
1,300,000
1,500,000
1,600,000

TRESTLES AND BRIDGES

Variety.

Chestnut

White oak...
Red oak . . . .
Hemlock. . . .

Spruce

White pine. .
Norway pine.
Oregon pine .
Yellow pine. .

Ten-
sion.

400

750
600
250
400
400
SOO
700
800

Flex-
ure."

Compression.

SOO

400

750

600

400

250

soo

400

soo

400

600

soo

900

700

1000

■

800

4
8

7
3

5
6

9
10

Bearing
. . to grain.

Shear-
ing.

Per.

Par.

200
400
350
ISO
200

150
200
200
400

7SO
1000
800
SOO
7SO
soo
600
900
1000

40
100
90
40
SO
40
SO
70
70

Modulus

of
Elasticity.

1,200,000
1,600,000
1,500,000
900,000
1,200,000
1,100,000
1,300,000
1,500,000
1,600,000

Above values may vary considerably. Notice the marked
weakness of timber in shear and bearing perpendicular to the
grain.

The lower part of the tree is stronger than the upper. In
a transverse section, the heart is stronger except in an old tree
where it has begun to lose its vitality.

Tests have shown that boxing a tree for turpentine does
not affect its strength. Time of felling makes no difference
except as it may influence seasoning. Calling the strength of
timber with 10 per cent moisture 100, we may say very roughly:

MATERIALS 13

with 50 per cent moisture, its strength is 50; 40 per cent moisture,
555 30 P^r cent, 65; 20 per cent, 80. Whether water is original
or absorbed seems to make no difference. Resisting power
does not seem to vary with the size except as the latter affects
seasoning. Let us call the ultimate load for the usual accelerated
test for a wooden piece 100: if the test lasts one day, the load
becomes 75; one week, 65; one year, 60.

Art. 7* Uses of Timber

In all locations, durability and economy must be considered.
In various situations, we have the following special requirements:

For posts, girders, joists, trusses, and roof timbers:

If stresses are light, ease of framing is the principal requisite;
hence use spruce and hemlock.

For heavier stresses, strength is desirable and yellow pine,
oak, or perhaps spruce or Norway pine may be employed.

For large timbers, either Oregon pine or yellow pine can be
obtained in lengths up to sixty feet.

For imder flooring, a cheap timber such as spruce or hemlock
will do. A wood that will stand and wear well is demanded
for the upper floors and thresholds; such are quarter sawn
white oak, maple, and yellow pine.

For shingles use cedar, cypress, or white pine; for siding
and clapboards, the last two may be employed; for doors,
sash, blinds, inside and outside finish, use white pine or cypress.

Piles and cribbage may be designed of oak, elm, hard pine,
cjrpress, spruce, and hemlock. For bridge ties, use hard pine
and white oak.

Art 8. Cast Iron'''

Iron ores are dug from the earth, and placed in a blast
furnace with coke or some other fuel and a flux like limestone
to carry away the impurities. The resulting product is pig
iron. This is heated in a cupola which is something like a small
blast furnace. The melting is also similar except that charges

•Reference for irons and steels: Johnson's "Materials of Construction";
Campbell's "Manufacture and Properties of Iron and Steel"; Stoughton's
"Metallurgy of Iron and Steel."

14 ELEMENTS OF STRUCTURAL DESIGN

of coke and limestone are much smaller. The slag and the
iron are tapped off, the latter into ladles which are poured as
explained in Art. i6. After cooling, the box is taken apart,
the projections on the casting cut off, and it is then placed in a
tumbler to remove the sand. This is commercial cast iron.
Let us examine it with especial reference to those imperfections
which so limit it in structural work.

Cast iron consists of about 93 per cent of iron together with
at least 1.5 per cent of carbon. The remaining portion is largely
silicon, phosphorus, and manganese.

Carbon may occur in two forms, the combined and the
graphitic. The former is the important element in cast iron,
the effect of the other elements being, in general, to increase
or decrease it and in that way influence the properties of the
metal. A small amount makes a gray soft iron, easily worked
and comparatively strong in tension. On the other hand, a
large amoimt makes a hard brittle iron.

The effects of silicon and aluminum are similar, each tend-
ing to eliminate blowholes. A small amoimt of silicon diminishes
combined carbon and hence softens the cast iron. A larger
dose seems however to make it brittle.

Sulphur also makes iron hard and brittle and should not be
allowed above .10 per cent. Phosphorus up to .70 per cent
does not injure the metal, but helps it to fill the moidd.

Manganese when alone seems to harden cast iron, but with
much silicon present may soften it. Tends to coimteract sulphur
and silicon.

The strength of cast iron in tension varies from 10,000 to
40,000, with an average of 20,000 lbs. per square inch. In
compression, tests on small, short pieces show an ultimate
strength of 50,000 to 200,000. On full-sized columns, how-
ever, this falls to 20,000 to 40,000 lbs. per square inch. The
reasons for this extraordinary drop will be discussed presently.
The flexural strength will vary from 10,000 to 60,000 lbs. per
square inch. An average value of the modulus of elasticity is
15,000,000 lbs. per square inch.

It is hard and resists fire and corrosion better than either
wrought iron or steel. It cannot be hammered, bent, rolled,
or forged. It is very likely to be brittle and it possesses little

MATERIALS 15

. elasticity or resistance to shock. It is liable to blowholes, to
segregation,* and to stresses due to the shrinkage of the interior
after the exterior has cooled. The last named are often termed
" shrinkage " or " initial " stresses. Displacement of core in
castings may occur and leads to the extremely objectionable
eccentric sections.

As might be expected, such a material has proven imsatis-
factory for engineering purposes. It is employed largely in
locations where the stresses are small, compressive, and quiescent
or nearly so; for example, in bearing blocks and washers.
Cast steel is now displacing cast iron in many places.

Castings should be of tough gray iron with not over o.io
j>eT cent of sulphur. They must not contain any blowholes
or other flaws. Test pieces i in. square must show a modulus
of rupture not less than 40,000 lbs. per square inch.

Art 9* Wrought Iron

Wrought iron may be defined as iron almost chemically

f>ure, intermixed with more or less slag. A tjrpical wrought
ron will contain about 0.06 per cent carbon, 0.09 per cent
silicon, 0.15 per cent manganese, 0.009 P^^ cent sulphur, and
0.12 per cent phosphorus.

Pig iron from the blast furnace and iron ore are heated
together in a puddling furnace. The resulting metal is squeezed
and rolled out, giving it that fibrous quality which is char-
acteristic of wrought iron.

The influence of carbon and silicon is to make the iron
hard and brittle. There should not be over 0.25 per cent
phosphorus, as it causes " cold shortness," that is, brittleness
while cold. Sulphur should be limited to 0.05 per cent, as it
is likely to cause " red shortness," or brittleness when hot.

The strength of wrought iron in tension along the grain
varies from 45,000 to 55,000 lbs. per square inch, elastic limit
from 23,000 to 40,000. Tensile strength crosswise of the grain
will average 80 per cent of the above. Percentage of elongation,
S to 30; reduction of area, 10 to 40 per cent. Shearing strength
is, in either direction, 80 per cent of tensile.

* The concentration of certain elements in a part of the casting.

16 ELEMENTS OF STRUCTURAL DESIGN

Due to the ductile nature of the material, short spedmens
do not fail in compression but grow stronger with increasing
loads. Elastic limit is about the same in tension and compres-
sion. Longer specimens fail by buckling. Tetmajer gives:

Sc— 43,000—1845, 5 = 10 to 112,

= 282,000,000/^2 >II2,

where 5 is the slendemess ratio.*

Certain shapes, like rounds or squares, will not break in
flexure owing to the ductility of the specimen. For some rolled
sections or built beams, there is a chance for failure. If properly
designed, they will show a modulus of rupture of 40,000 to
50,000 lbs. per square inch and a coefficient of elasticity of
25,000,000.

Wrought iron is a ductile metal; it can be welded, rolled,
or forged; it will stand a great deal of abuse without injury.
It probably resists corrosion better than steel. From above
properties, it may be seen that it is an excellent structural
material; however, the greater strength of steel has given it a
preference over wrought iron. Its use since 1900 has been con-
fined largely to blacksmith's work.

The following specifications for merchant iron. Grade "A,**
were proposed by Association for Testing Materials.

Tensile strength, 50,000 lbs. per square inch or more.
Yield point, 25,000 lbs. per square inch or more.
Elongation in 8 ins., 25 per cent or more.

Must show a long clean silky fiber when nicked and broken.
A piece shall bend cold 180 degrees flat on itself without fracture.
Must be straight, smooth, free from cinder spots or flaws,
buckles, blisters, or cracks.

Art. 10. Bessemer Steel

Melted pig iron is introduced into the converter and a
blast of air is passed through it. After this has oxidized out
the impurities, spiegeleisen, an iron rich in carbon and manganese,

unsupported length
* Maximiun value of fraction

least radius of gyration

■

MATERIALS 17

is added. The latter element unites with the oxygen, while
the former gives to the steel the proper carbon content.

There are two methods of manufacture: the acid and the
basic; in the latter, calcined lime is added to the molten steel
to eliminate the phosphorus. While there is a slight preference
for the basic on account of the lessened danger of an excess of
phosphorus, engineers usually fail to specify either kind, but
state permissible limit of this objectionable element.

The product may be divided into soft steel, containing
0.15 per cent carbon; medium, 0.30 per cent; hard, 0.50 per cent.
The lower carbon content give us a ductile metal, possessing
almost imlimited capacity for abuse. It is, however, weak
compared with the hard steels which, on the other hand, are
quite brittle. For structural work, we use either soft or medium,
preferably the former if there is much forging. Where high
stresses are to be resisted, medium steel is better. This paragraph
applies also to open hearth steel as considered in next article.

Silicon increases strength and hardness and decreases ductility.
Manganese in small quantities makes metal hard and malleable.
Sulphur and phosphorus are both objectionable elements,
making metal hot and cold short respectively. Both should
be limited to very small amounts.

For reasons which will be given in the next article, the
Bessemer process is used for structural steel only in inferior
work. A great deal of the rail tonnage is Bessemer steel, but
even here, it is being displaced by the open hearth process.

Strength and tests will be discussed under the head of
open hearth steel, which it closely resembles. For rails, it
is usual to specify the drop test and a chemical composition
about as follows: carbon, 0.45 per cent; manganese, 0.90 per
cent; silicon, not to exceed 0.20 per cent; phosphorus, not to
exceed o.io per cent.

Art II. Open Hearth Steel

In this process, pig iron, scrap, and iron ore are subjected
to an oxidi^g flame in an open hearth furnace. When carbon
has been lowered to the proper amount, spiegeleisen or ferro-
manganese is added which combines with the oxide of iron and

ELEMENTS OF STRUCTURAL DESIGN

prevents further loss of carbon. As in Bessemer steel, we have
the acid and the open hearth processes, also soft, medium, and
hard steels. The effect of the elements is substantially the
same in both cases.

Bessemer is the cheaper method but gives poorer material.
Open hearth is more uniform and more reUable. The process is
such that it can be better regulated to produce required com-
position of metal. Always specify open hearth for important
structural work. We may use medium, soft, or rivet steel, the
latter being very soft and ductile.

American Association for Testing Materials recommend
following specifications:

No grade should have more than 0.08 per cent phosphorus,
not more than 0.06 per cent sulphur. The finished material
should be free from injurious seams, flaws, or cracks. In
addition the following requirements should be fulfilled:

Tensile strength, lbs. per sq.in.
Yield point, lbs. per sq.in. (not

less than)
Elongation in eight inches (not

less than)
Cold

Bend'

Rivet Steel.

50,000 to 60,000

one-

180**
flat on itself.

Soft Steel.

52,000 to 62,000
half tensile stre

180**
flat on itself.

Medium Steel.

60,000 to 70,000

ngth

22%

180**

around its

own thickness.

The strength of steel will vary according to its impurities.
Carbon is the most important element in its influence on strength.
This in tension for a small specimen may be gauged by the phys-
ical requirements just given. Shear will average 80 per cent
of the tensile stresses. Compression ' for small specimens has
about the s^me elastic limit as in tension. For full-size pieces,
we find a large reduction in breaking loads, particidarly in com-
pression. While data for definite conclusions are lacking, it
looks as though the ultimate stress in a full-size column might,
even with what are now considered sound details, fall as much
as 50 per cent below that for a small-sized specimen. (Art. 69.)

MATERIALS 19

The coefficient of elasticity is in the vicinity of 30,000,000 lbs.
per square inch.

Many shop processes such as punching, shearing, and
bending the metal, either hot or cold, cause stresses while still
without load. These are called *' initial stresses." Another
cause is the rapid cooling of metal after heating for forging. To
remove this undesirable condition, metal is heated to about
1200° F., and allowed to cool very slowly and uniformly. This
process is called annealing.

Cast iron costs about 2.5 cents per pound when patterns
are furnished; cast steel under the same circumstances, 4 cents.
Steel or wrought iron will cost 1.2 cents as rolled or about 2.5
cents as fabricated, f.o.b. cars at shop.

Art 12. Cast Steel and Alloys of Steel

The process of casting as outlined briefly in Art. 16, gives
notable economy in the fabrication of shapes possessing an
intricate form. But, as already pointed out, cast iron has
faults which limit its application in structural work. Of late,
the practice of making the castings of open hearth steel has grown
steadily in favor. As in structural shapes, the material may be
soft, medirnn, pr hard steel. The latter, like rolled stuff, is
unsuitable for structural purposes. The properties of the result-
ing castings are much the same as those of the metal from
which it is poured. Thus, if soft steel be used, it will be ductile,
show a large resistance to impact tests, have a high elastic limit
and a definite yield point. Blowholes, cracks, and segregation
are the faults to be guarded against. Complicated castings
should be annealed.

We shall speak of but two alloys of steel, vanadium and
nickel.* Both seem to increase strength to a remarkable degree
without interfering with its toughness. In fact it is claimed
that vanadium increases it.

Nickel steel has actually been used in bridge work, and it
is doubtless the material of the future for long-span structures.
The usual percentage is three to three and one-half and the
steel to which it is added commonly open hearth. More car-

• See WaddelPs paper, "Nickel Steel for Bridges," Trans. A.S.C.E., Vol. LXIII.

20 ELEMENTS OF STRUCTURAL DESIGN

bon may be used than would be allowable without the nickel.
Such an alloy will have an elastic limit about equal to the tensile
strength of the steel and an ultimate strength 80 per cent
greater. In compression, the excess will vary from 50 to 75
per cent, the latter for short struts. The coefficient of elasticity
is unchanged. Nickel steel does not stand shop abuse as well
as carbon steel, but it is nevertheless satisfactory. Shopwork
such as punching, drilling, and chipping, will be more expensive.
Nickel steel seems in general to resist- corrosion better.

Art 13. Paints *

Very little is known in regard to the theory of the preserva-
tion of wood by the use of paint. For steel, however, con-
siderable has been done in this respect. It is now considered
that rust, the principal enemy of steel, is due to the electrolytic
action between the hydrogen of the water and the iron. Oxygen
must be present. Also, some acids, for instance the carbonic
acid always in the air, accelerate the rusting. To prevent this
action, we have " inhibitors " or rust preventers. Alkalies
act as such, also chromic acid and its salts. The former
cannot be used with ordinary paints made of linseed oil, because
they imite with the latter to form soap. This objection does
not hold for the chromates, and they make excellent inhibitive
paints.

Carbonic acid, oxygen, and moisture are always present in
the air, and hence unprotected steel will rust. One excellent
preventive method, encasing in concrete, will be taken up in
Vol. II. We will now consider the other method, protection
by paint.

This usually consists of an aggregate of pigment with a
cementing material of linseed oil. Pure linseed oil is made by
crushing flaxseed, and allowing it to stand and settle and thus
purify. In this form, it is known as raw linseed oil, which dries
or oxidizes very slowly. This process may be hastened by
adulterating with japan drier or by boiling. Former makes
oil poorer as a paint, while latter process is expensive.

* Cushman and Gardner's " Corrosion and Preservation of Iron and Steel.''
Ketchum's "Steel MUl Buildings," Chap. XXVII.

MATERIALS 21

Pigment shoiild be finely ground, preferably in oil. For
it, the following substances may be used:

White lead (hydrated carbonate of lead), is employed for
wood and for finished surfaces in steel. Disintegrates when
attacked by corrosive gases and does not make a good bottom
coat.

Red lead (lead tetroxide), is very stable, either on exposure
to light or the weather. Is probably the best paint for metal.
It is mildly inhibitive, but is improved by the addition of 3 per
cent of zinc chromate.

Zinc oxide has a tendency to peel but when mixed with
red lead makes a good paint for metal surfaces.

Iron oxide is sometimes used. It should be free from the
hydrated oxide.

Carbon, when mixed with linseed oil, has a large covering
capacity with a correspondingly reduced protection.

Structural work will average 150 to 250 sq.ft. per ton of
metal. Conmion practice is to estimate i gallon per ton per
coat.

CHAPTER II

COMMERCIAL SHAPES

Art 14. HandbookSi Units, and Dimensions

The leading manufacturers publish books which give the
details and properties of the different shapes rolled by them,
and a great deal of other data which are very useful to the drafts-
man and designer. Prominent among these are the handbooks
prepared by the Cambria Steel Co., the Bethlehem Steel Co.,
and the Carnegie Steel Co. This additional information usually
consists of safe loads for different shapes, either as a beam
or as a column; radii of gyration and capacity of common
'types of built-up columns; values for rivets and pins; details
of bolts, rivets, nuts, upset ends, eyebars, tumbuckles, sleeve-
nuts, clevis nuts, pins, loop rods, and nails; weights and areas
of plates and round or square bars.

A problem of frequent occurrence is to find the hypothenuse
of a right-angled triangle when both legs are given in feet,
inches, and fractions of an inch. This is a very cumbersome
operation without the aid of tables of squares. Hall's Tables
($2.00) may be recommended, while Smoley's ($3.00) are still
better. The latter also contains logarithms of numbers and
sines, tangents, and secants, which may be used to advantage
in figuring triangles. For large distances and for bridges built
on a curve, a seven-place logarithmic table of numbers and
trigonometrical functions is advisable.

A number of very good books have been written with the
idea of still further assisting the draftsman. Such are Godfrey's
"Tables," Osborn's " Moments of Inertia," and Sample's
" Properties of Steel Sections."

In continental Europe, the metric system is employed in

structural work. It would be convenient here, but the expense

attendant upon such a change of units has hitherto prevented

its adoption.

COMMERCIAL SHAPES 23

Dimensions of wires and thin plates are commonly given
in gage nimibers. For sheet steel, the United States Standard
Gage is used and a sheet may be specified thus — i PL, i6"X
No. 20 U. S. Standard gageX3'-4". Unfortunately, there
are several different " standards," no two of which are alike.
This makes it necessary to name the one employed, as above,
imless it is definitely understood by all concerned. A new
gage, the ** standard " decimal gage, has been recently adopted
by the Association of American Steel Manufacturers, in which
the gage is expressed in even decimals of inches. Its imiversal
adoption would obviate the confusion now arising from the
multiplicity of systems. Tables giving the equivalents of
the different gages may be found in structural handbooks.

Except as above noted, the units of measurements are the
foot, inch, and the thirty-second of an inch. Save the dimensions
of pin-holes which will be taken up later, the following rules
must be observed:

(i) All distances are to be computed to the nearest thirty-
second of an inch. ,

(2) All distances except dimensions of castings, shapes, and
plates, when 12" or over must be expressed in feet and
inches, — thus, i'-6V, not 18^'.

(3) The fractions of an inch must be reduced to its lowest
terms, thus, r'-6i", not i'-6/y".

(4) Machinists and pattern makers prefer dimensions up
to two feet in inches. In structural drawings

involving these classes of work, this rule may or
may not be followed.

(5) The method of giving the dimensions of
plates and shapes stated in the following articles p^^ ^ —Method

must be used. of Specifying

The only angle which the workman is sup- Angles.
posed to understand is 90 degrees; all others
must be given in bevels, that is, by drawing a right triangle
with one side of the angle as the hypothenuse and the two
legs drawn parallel and perpendicular to the other side. For
example, an angle of 56 degrees and 30 minutes is expressed
as shown in Fig. 14, the longer leg always being 12".

ELEMENTS OF STRUCTURAL DESIGN

Art 15. Commercial Shapes of Wood

Fig. isa shows the ordinary way of cutting up logs;
the timber is then known as "bastard sawed." Fig. 156
shows the method taken to produce " quarter " or " rift "
sawed lumber. The middle boards in Fig. 15a are sometimes
sold for quarter sawed stuflf. Bastard sawed is cheaper but
does not stand well, on account of the tangential shrinkage.

With few exceptions, timber is sawn into rectangular shapes.
Rough boards are commonly \'\ i}", i\'\ 2", or 2\" thick.
In the following table, a * indicates that the given size may
usually be obtained, although the list will vary somewhat with
the locality. Larger sizes may be had, but they are to be used
with caution since they are more expensive per foot, B.M.,
more likely to season improperly, to contain sap-wood, decayed

Fig. 15a. — ^Bastard Sawed. Fig. 15 J. — Quarter Sawed.

heartwood, or other faults. Sizes in the lower left-hand comer
are not common, since their use is inadvisable. Besides the
reasons just given, a beam whose thickness is less than one-
seventh the depth has a tendency to buckle. While good size
timbers may be obtained up to a length of 60 feet, cost per
foot for any given size increases rapidly above 20 feet. Stock
lengths are usually 10, 12, 14, and 16 feet.

THICKNESS

Depth.

10"

I a"

14"

16"

10"

12"

14"

16"

■ * ' * *

COMMERCIAL SHAPES 26

When it is desired to give the wood a smooth finish, it is
planed, A" to i" (varying with size) being taken off for each
planing. Hence in specifying planed stuff, we should make
'^ it such dimensions that it could be readily cut from stock
material; thus we specif y flooring as |", ij", if", if", and so
forth, to be made from \*\ \\!\ ij", and 2" plank.

Figs. 15c, d, and e give forms for tongued and grooved
timber. TTie purpose of the groove in the bottom of \<^d is
to lessen the effect of warping. •

Shingles are wedged-shaped pieces of wood, A" to J" thick

\ \ ^ ^ ^ f

Fig. ISC. Fig. i%d. Fig. 15^. Fig. 15/. Fig. 15^.

Tongued and Grooved Timber. Clapboard. Mouldings.

at the butt, 14 to 16" long, and 3 to 14" in width. A clapboard
may be defined as a shingle 6" long and 4' wide.

A piece of wood, small in section and used for trimming
is called a moulding, Fig. 15^. There is dmost infinite variety
to their shape, and they may often be obtained ready made
from the lumber dealer, or ordered from the planing mill.

Art i6. Commercial Shapes for Cast Iron and Steel Castings

Molten iron or steel is poured in a space formed by burying
in sand a piece of wood called the pattern and then withdrawing
it. This pattern is a dupUcate of the desired rough piece
except that it is a trifle larger to allow for the shrinkage of the
hot metal. In order to make this space, the box which contains
the sand should be cut by one or, for intricate castings, two
or more " parting lines " or lines at which the box separates
to remove the pattern. Holes are usually formed by " cores "
which are prisms of a section same as the desired shape of the
hole. These extend into recesses left by the pattern in the
sand.

The principles which are of importance follow:
(i) The parting line must be so chosen that the pattern
may be withdrawn. This is important since economy demands
as few parting lines as possible.

26 ELEMENTS GF 8TBUCTUEAL DESIGN

(z) Surfaces which are shown on the drawing as parallel
to the line of withdrawal of the pattern are tapered by the
pattern maker about A" per foot to prevent disturbance of
the sand when the pattern is taken out

(3) : The • thickness of the metal of the casting should be
between |" and ij", preferably between
i and i". If smaller than J", it should
not be used in important positions; if
larger than i|", make holes enough in it
to cut down metal, taking care to con-
serve the necessary strength. Thus if it
were required to make a casting 2'-o"X
Fig. 16.— Typical Casting, i'"*" and 4" high, it should not be made
solid but somewhat as shown in Fig. 16,
the number of ribs being dependent on the strength required.
This is done to avoid the initial stresses caused by the unequal
cooling of different thicknesses. These alone are sometimes
sufficient to break a casting. As noted in Art. 14, 1-4" is often
expressed as 16" for convenience of pattern maker.

(4) Surfaces of revolution and plane surfaces are easiest
to make and to handle, hence they should be used where
possible.

(5) Two plane surfaces are not allowed to come to an
intersection, but are joined by a curved surface of perhaps
i" radius, as the casting tends to crack at a sharp angle. The
draftsman gives dimensions to the meeting point, but should
show the rounding of the edges, without giving the radius,
as the pattern maker takes care of this.

(6) Surfaces which must be exact and for which small
variations of A to \" would not be permissible, should
be marked " finish " or some abbreviation therefor. Designer
should give dimensions of finished piece and the pattern maker
will add the necessary amount. He also takes care of the
shrinkage by using a shrink rule which is Just enough longer
than the ordinary rule to allow for the shrinkage of the metal
when coohng, usually about i" in a foot.

(7) Holes for bolts, pins, and so forth, may be marked, —
" Core for ... .dia. bolts," in case a rough fit is desired; or
" Drill for .... dia. bolts," in case more exact work is wanted.

COMMERCIAL SHAPES 27

Wtcjri designed for important dutiies, a bolt may be turned
down and the hole made .002 to 003" larger.

(8) Castings may be riveted but bolting is much more
common.

Art 17. Rolling

•

An ingot weighing twenty to thirty times as much per foot as
the product and of'sufficient length to furnish the desired amount
of iJie finished shape, is passed between the rolls. These are
so shaped that the piece is reduced to its proper dimensions
by gradual steps. The hot metal tends to squeeze in between
the rolls leaving projections called " fins/' Fig.
176, which tendency may be reduced by rolling
with the joint at a different place.

Auxiliary rolls, at right angles to the main
ones, are often used to advantage. An example
of this is the " universal mill,'* which rolls edges Fio^i^ya.— Amnge-
of plates as well as its flat sides. Fig. 17a. ment of Rolls in

Rolling iron or steel is a big trade in itself Universal Mill,
which we cannot enter into here. We will only
attempt a few of the general principles as a means of under-
standing the common shapes and as a guide in case new
sections are desired. The latter should be avoided except
where the tonnage will be sufficient to justify it.

(i) Metal must not be too far from the axis of rolling;*

I c I — I

Fig. 176. Fig. 17c. Fig. 17^. Fig. ije. Fig. 17/. Fig. 17^. Fig. 17A.

-r for the consequent variation in the lineal speed of the rolls
injures the metal.

(2) Projections at right angles to this axis must have a
bevel. Thus a tee must be rolled as shown in Fig. 17c, and
not as given in Fig. 1 7^.

(3) Reentrant angles can be made only by the use of auxiliary
rolls after the final or " finishing " pass. Thus the shape shown

* The c^ter of gravity line of the rolled shape.

28 ELEMENTS OF STRUCTURAL DESIGN

in Fig. lye must be rolled as seen in Fig. 17/ and bent by
the auxiliary rolls.

(4) Sections should be so designed as to cool evenly and
thus avoid curling after rolling and the initial stresses due to
one part cooling after another. A square rod must be rolled
as shown in Fig. ijg, for, if first made square, it will shrink
to the form seen in Fig. 1 7 A.

(5) Plates ordered to a certain thickness may overnm
their theoretic weight by from 3 to 10 per cent or even
more. Actual amount for different sized plates may be taken
from the handbooks.

(6) Unless a special price is paid, material is likely to vary
from the specified length. The amount changes with the size,
shape, and length, see Art. 58.

The limitations above given, especially (i), interfere with
the development of an I-beam which possesses maximum
economy as a beam. The Grey process of rolling, compara-
tively new, obviates this.

" The method of rolling comprises essentially a set of rolls
with axes placed parallel to the web and working the inner
profile of the beam, and a second set of rolls with axes ncrmal
to the web which works the outer faces of the flanges. The
speeds of the two sets of rolls and their diameters are so related
as to produce homogeneity of structure. A feature of commercial
importance is the adjustable support of the rolls, j)ermitting,
for instance, a variation in weight by increase of flanges alone,
or by increase of flanges and web m any specified proportion
and that without changing roUs." *

Claims are made by those who own and control the patents
that this process produces a superior metal, free from internal
stresses. This is disputed and the counterclaim advanced that
the metal is not as strong inch for inch as the old method of
rolling. Tests which will settle the question of superiority
should be awaited.

Rounds are often cold rolled. In this process, the hot
roUed product is pickled in add to remove the scale and rolled
cold between chilled cast-iron rolls. Shafts made in this way
may be obtained up to five inches in diameter. Rounds less

♦ Eng. News, Vol. XLVI, p. 387.

COMMERCIAL SHAPES

than one-quarter inch in diameter are wire drawn, that is,
drawn cold through a groove smaller than the original diam-
eter.

Steel which has a varying section or that which is too large
to be rolled, must be cast or forged. The preceding article
explains former process; in the latter the hot ingot is hammered
into the required shape.

In the manner just indicated, many different shapes are
rolled. For the present we shall limit ourselves to those used
in structural work ITiey may be classified as common, occa-
sional, and rare, in accordance with the frequency of their
occurrence in this treatise.

Common.

Occasional.

Rare.

Circular

T-beams

Deck beams

Rectangular

Z-bars

Bulb angles

Angles

Rails

Oblique angles

I-beams

Trough sections

Splice angles

Channels

Column sections
Pipe

The actual sections in which these shapes are commonly
rolled may be obtained from the handbooks. The maximum
lengths are also sometimes given there, or they may be secured
by consulting the mills.

We shall now take up some of the principal facts in relation
to each shape.

Art. i8. Circular Shapes

These, as their name implies, are true cylinders. They
may be drawn, hot rolled, cold rolled, or forged. Circular
shapes are termed wires if less than one-quarter inch in diameter;
if more, rounds or rods. We seldom use less than one-half inch
in structural work.

By " one inch rod," we mean that its diameter is one inch.
In ordering or in specifying on plans, always describe thus:

so ELEMENTS OF STRUCTURAL DESIGN

S'Rounds, 7r dia. Xi'-6|";
or,

8 0s 7rdia. Xi'-6|",

alwajrs placing the length last.

Ciroilar shapes may be obtained in all sizes from a fine
wire up to a shaft two feet in diameter. Above 7", rounds are
forged; from J to 2", they vary by sixteenths; from 2" to
7", by eighths; practice being slightly diflferent for each com-
pany. There is no method of increasing area with the same
grooves as in angles, Art. 20.

Wire is found in the cables of suspension bridges, while
rounds are used in shafts, in rods for carrying tension, and for
making bolts, rivets, pins, and rollers.

Art 19. Rectangular Shapes

As their name indicates, these have a rectangular cross-
section. They may be made by grooved rolls, by flat rolls, or
by the imiversal mill as already explained. In the last two
cases the increase in thickness is made by simply spreading the
rolls.

A rectangular shape which has its width and thickness
the same is called a "square"; if the larger dimension is
greater than eight inches, it is termed a " plate" ; if it is less,
it is designated a "bar" or "flat," the latter term being usually
applied to a thin section less than three-sixteenths of an inch
in thickness.

Material is specified thus, 2 Pis., 36''Xi"X8-4i", the first

dimension being always in inches and parallel to

^"''^^ the rolls. In this case, the 36" dimension would

\^,^ have rolled edges,* the others being sheared. If

Fig. 19. above mentioned plate is irregular, as shown in

Irregular Plate. Fig. 19, all edges are sheared edges. This is

important since sheared edges do not make a nice
fit or a neat appearance. Moreover, it is expensive to correct

* That iS| it would if rolled in a universal mill. In "sheared plate" it is rolled
somewhat larger and sheared to size.

COMMERCIAL SHAPEQ 31

by milling, Art. 37. As already noted m Art. 14, the thickness
of thin plates or flats is often g^ven ,in gages.

Common sizes are about as follows. Squares: from A"-
to A" by 32ds; from A" to 2" by i6ths; from 2" to 3" by
eighths; from 3" to 5" by quarters. Plates, bars, and flats
usually vary by U. S. Standard gages where leSs than i'' thick;
above that to 2^" in thickness by sixteenths. Above 15'' in
width, there is seldom a call for a plate more than one inch thick.
Any width may be rolled in a xmiversal mill, or may be sheared
out. Common widths nm from 2" tp 5'' by half inches; 5 to 12''
by inches; 12 to 36" by even inches; 3 to .10 feet, by half feet.

These shapes in their different forms occur frequently in
structural work: squares are used for ties; diagonals and chords
eanying tension only are often made of bars; plates are employed
in built-up girders, compression members, connectioii plates,
and other places too nimierous to mention.

Art. 20. Angles ,

The §hape of the minimiun thickness of an angle of a givers
length of legs is essentially that of two rectangles, joined together
at right angles, Fig. 20a. Th6 comers on the inside of the

.\<s^^^^^css^^v\^^^

Fig. 20a. — ^Typical Fig. 20b, — ^Method of Fig. 20c. — ^Method of

Angle. Rolling. Increasing Section.

angle are eased by curves of a small radius, the values of which
may be found in the handbooks. These curves must not be
forgotten in detailing.

Fig. 206 shows the finishing rolls in position for the minimum
thickness. Let the unshaded part of Fig. 20c represent a
4"X3''XA"- Then aA=V', he=z'\ and the thickness of
either leg is A''- Suppose it be required to roll a 4"X3"Xf".
We then raise the rolls an amount ai=ef=s€c 45° *XA" = |'^

* Not always 45°, but near enough for illustrative purposes.

32 ELEMENTS OF STRUCTURAL DESIGN

The distance ig = ah and gf—he, the interior lengths and radii
remaining undianged. It will be noted, however, that the
extreme length of each leg has been increased by ft" making
the real size 4ft''X3A"Xi", although it is spoken of as a

This theoretic shape is modified by the inaccuracies of
workmanship, by the flowing out of the metal, or by a " finishing
pass." The above reasons make the length of the leg of the
angle somewhat uncertain; hence, in design and detail, we
allow for a possible overrun. For the Stime reason, dimensions
perpendicular to the edge called gages are always given from
the sharp comer.

An angle is designated by the length of each leg and the
common thickness. If one leg is longer than the other, it
always comes first; if each is the same, both must be given,
thus: •

2Ls,6"X3i"xrX24'-^i".
iL,4''X4" xrXi'-o^

Angles are sometimes specified by weight, but both weight and

thickness should never be given.

Thus,

iL,4"X4"Xi2.8#Xi'-o".
Never,

iL, 4''X4"xr Xi2.8#Xi'-o".

In the handbooks, angles are divided into equal and unequal
legged, although the same general principles apply to each,
and they are equally important. They may be obtained,
increasing by small amounts, from a f'xf'Xi" to an 8"X8"
Xii". Regular sizes vary only by sixteenths of an inch in
thickness.

Angles are very common: alone or with 2 or 4 riveted together,
they may be used for small tension or compression. They are
employed for the flanges of girders and stringers. In built up
columns and tension members, they fasten the plates together.
These are but a few of their many applications.

f *•

COMMERCIAL SHAPES 3*

Art 21. I-Beams and Channels

If we neglect the curves which are used instead of sharp
angles at the inside comers, an I-beam is made up of one large
rectangle (the web), two smaller rectangles and four triangles
(the flanges). The bevel of the sloping parts is 2" in 12'' for
standard sections. See Fig. 21a.

I-beams are rolled horizontally as shown in Fig. 216. In
order to increase the weight per foot, the rolls are simply spread
farther apart, thus changing only the flange width and web

Fig. 2ia. Fig. 216. Fig. 21c,

Typical I-beam. Method of Rolling I-beazn. Typical Channel.

thickness. The amount, w', of increase in pounds per lineal
foot, divided by the height in inches times 3.4, equals the increased
thickness in inches, either of the web or flange. That is.

They are specified by their depth and weight per foot, thus,.

i/, iS"X42#Xii'-4";

the dimensions of the American standards, adopted January,
1896, are thereby understood. The sizes vary from a 3"Xs.s#
to a 24"Xioo#. They are used for beams, footings, and singly
or latticed together as columns.

The Grey process, already explained in Art. 17, has made
possible larger Is and those which contain more material in the
flanges than the American standard. The former are theoret-
ically far more economical either for a column or for a beam than
the latter. If the ney method will roll as good a quality of,
steel as the old, it will extend the new shapes into fields hitherto-
occupied by built-up sections.

?4 ELEMEI^TS OF STRUCTURAL DESIGN

Cut an I-beam in two along the web and we have a channel.
Fig. 2IC. Methods of rolling and increasing the section are
similar to those for I-beams.

Channels are specified by their depth and weight per ioot,
thus:

4 [s, i2"X2o.5# X3o'-o".

Sizes vary from a 3"X4.o# to 15" Xsstf. Shapes some-
what resembling channels are employed for small work as in
expanded metal partitions. Where not otherwise stated, the
dimensions of the American standard are understood. Channels
are used for columns and for beams in places where an I-beam
is not so convenient, for example, against a wall.

Art 22. Occasional Shapes

T beams are composed of a flange, abCy decreasing in thick-
ness from the center, and a stem, bd, increasing towards the top,

Fig. 22a. Fig. 22b. Fig. 22c. Fig. 22^.

Typical T beam. Typical Zee bar. Typical Rail. Typical Trough Section.

Fig. 22a. There is no method of enlarging by increasing the
distance between rolls.

In case ac equals bd, the tee is called equal legged; if dif-
ferent, unequal legged. They are best designated thus:

2Ts,3"X4"X9.3#Xi2'-6",

the length of flange always being given before the depth of
leg. Sizes vary from a i"Xi"Xi.o# to 4i"X3J"Xi5.9#.
They are used prindpally in the roofs of buildings.

Zee bars, Fig. 22ft, are composed of three rectangles of
the same thickness with some of the comers eased as shown.
Methods of rolling and increasing size are similar to those for
angles. They are specified thus,

4 Zs, 3ft"XsA"X3ft"xr X24'-2".

COMMEfiCIAL SHAPES 135

First the flange, ab; then the web, bd; next the flange, de; and
- last, the common thickness. It wiU be noted that the increase
in the length of the legs is shown, differing in this regard from
angles. Since the two flanges are usually the same, one of
them is sometimes omitted, thus,

4Zs,sfir"X3A"xrX24'-2".

Sizes vary from a 3"X2tt"x} to aei^Xafxr. Their
principal use is in columns.

Rails, Fig. 22c, of many different kinds are rolled but the
standards of the American Society of Civil Engineers are usually
specified, as they may be more readily obtained. Sizes vary
from an ij" rail at 8# per yard to a 6" at 150^. For the
Society standards, the inclination of the top of the base and
the bottom of the head is 13 degrees with the horizontal, It
is not customary to spread the rolls to increase the weight.

%©/'""'"\^'^ ^^t»i«i^

Fig. 22e.

Fig. 22/.

Fig. 22^.

Fig. 2 2 a.

Typical Trough

Portion of

Special

• Typical

Section.

Phoenix Column.

Cohimn.

H section.

Rails are specified by their depth, weight per yard, and name
of standard, thus,

10 Rails, s" X8o# per yard, A.S.C.E., X3o'-o".

The depth is sometimes omitted. Rails are used for railroad
tracks, crane tracks, and in bearings and spread footings.

For the purpose of making a solid steel floor with suflScient
strength to carry a load for a span of several feet, special shapes
are rolled, so designed that they may be readily fastened together.
See Figs. 2 2d and e. Angles and plates riveted to form trough
sections, are now the standard construction for this purpose,
see Vol. II.

Special column sections are rolled, the idea being to obtain
a strong post with a mim'miun of riveting. The old Phoenix

36 ELEMENTS OF STRUCTURA.L DESIGN

column consisted of four or eight shapes fastened together as
shown in Fig. 22 A It is now obsolete on account of the dif-
ficulty of inspecting, repainting, and making connections thereto.
Jones and Laughlin roll a patented shape, a bent I-beam, which,
when riveted to a similar shape, give us a post as shown in
Fig. 22g. The recently introduced H-shape, Fig. 2 2 A, has
become very popular as a colunm. (Art. 56.)

Pipes are sometimes used in structural work, but the method
of manufactiuing them does not belong here. Their actual
diameters differ somewhat from the nominal, and tables in
handbooks should be consulted in case their exact dimensions
are required.

Art 23. Rare Shapes

Among such may be mentioned the deck or bulb beam,
Fig. 23a, which is simply an I-beam roimded off on the bottom.
Manner of rolling, of increasing weight, and of specifying, are

T 1

Fig. 23a. Fig. 236. Fig. 23c.

Typical Bulb Beam. Typical Bulb Angle. Typical Splice Angle.

same as for I-beams. They are employed in place of the latter
on ships.

In this place bulb angles are also used. These are angles
with a swelled and rounded end, as shown in Fig. 236. Manner
of rolling is same as for angles.

Angles with the legs at more or less than ninety degrees may
be had, but it is generally cheaper to bend an angle or plate
to the required shape.

Splice angles. Fig. 23c, which are angles with the outside
comer rounded so as to fit the inside of an angle, may be
obtained; but it is preferable to plane off the comer and shear
the edges of an ordinary angle.

CHAPTER III

WOODEN STRUCTURES •

Art. 24. Principles of De^ga

(i) Timber as ordinarily received is likely to be a little
less than nominal size.

(2) The dimensions of timber perpendicular to the grain
will vary with the amount of moisture it contains. If green
timber be placed in a warm dry

building, it will shrink; seasoned *"''

stuff out-of-doors will absorb moist-
ure and swell. In a direction par-
allel to the grain, there is little
alteration. (Art. 2.) Hence, where
settlement is to be avoided, put
grain of timber in line with the
loads. If some pieces must be
placed otherwise, make them as thin
as possible. For example, the cracks
in the plastering of dwelling houses
are largely due to unequal settle-
ment. This is caused by resting

the studs on top of the joists, Fig. ^J-^ a^h'^^

24(1, mstead of passing between
them as in Fig. 24*. "*""* rf SuppobIdj Fh», h

(3) When practicable, timber

should not be placed where it will be subjected to alternations
of wet and dry. This is in order to prevent wet rot, Art. 3.
The expense of roofing wooden bridges has been considered
advisable in many instances in order to prolong the life of the
structure.

* "Structural Details," by H. S. Jacoby; " Building Construction and Super-
intendence," Part II, by F. E. Kidder; "Architects' and Builders' Ptteket BooL,"
by F, E. Kidder; "Roof Trusses in Wood and Steel," by M. A. Howe.

38 ELEMENTS OF STRUCTURAL DESIGN

(4) Keep timber well ventilated to prevent dry rot. (Art.
3.) If it be necessary to build a member of two or more pieces
side by side, they should be held an inch or so apart by blocks
or washers. A hole is sometimes bored lengthwise through
posts with crossholes top and bottom, partly for the same reason.
The bricking up of the ends of joists or trusses, while common,
is open to this objection.

(5) Where it is desirable to- keep out the rain, arrange joints
so that water would be compelled ta run up hill befbreente^ring
the structure. Thus, In the cornice shown in Fig. 24c, instead
of details in 24^, we may employ- arrangement of 24^. How-
ever, the rain which drove against the upright board would still
tend to follow along the bottom of the horizontal board and into

Fig. 24c.
Cornice.

FlG. 24J. Fig. 24^.
Incorrect. Incorrect.

Biuim

Fig. 24/. Fig. 24^.
Good. Good.

Details of Cornice.

the wall. To prevent this, the cornice is designed as shown in
Fig. 24f. Another method is to cut a small groove on the imder
side near the outer comer as in Fig. 24 g.

(6) The exclusion of birds and vermin must be borne in
mind especially in residences and office buildings. Rat-proof-
ing in San Francisco is now compulsory. Sparrows' nests in
covered Howe trusses are a frequent cause of fire.

(7) A structure properly designed at all points for the
usual loads is reasonably secure against hurricanes or earth-
quakes. In localities where these are especially prevalent,
higher values should be assumed for the wind load and extra
pains should be taken to fasten together the different parts.

(8) To protect a wooden structure from fire:

(a) Avoid small sections. A io"Xio" wooden post* will

WOODEN STRUCTURES

stand a fire longer than an unprotected steel beam of equal
capacity.

' (b) Kfeep surfaces as unbroken as posable. A Toof inade
x)f 3'' plank on purlins is a much better risk than one of i'^
boards on 2"X8" rafters.

((i) These enclosed spaces, if extending from floor to floors
are even more objectionable as they help the spread of flames.

(9) The strength of the wood in bearing perpendicular to
the grain is much smaller than its compressive strength. A
column is frequently designed as shown in Fig. 24^* However^

Fig. 24^. FiG. 24*. ' Fig. 24/.

Post with Bolster. Cast-iron Base. Splice at Base.

except for very long colimms, the bolster is much the weaker
part. This may be remedied by making the latter of strong
material, white oak for example. Also a casting as shown in
Fig. 24/ may be used or bolster and column spliced as in Fig. 24^1
(10) Lack of strength in shearing along the grain is perhaps
its most important characteristic, weakening it much as a
structural material. In splicing or joining a tension member,
this fault makes it possible to secure but a fraction of the orig-
inal strength. For the same reason notching a beam either
top or bottom, produces, especially when near the ends, a large
loss in capacity.

40 ELEMENTS OF STRUCTURAL DESIGN

Art 25. Accessories of Other Material

Steel, wrought iron, and cast iron are valuable aids in the
design of wooden structures. In this article, when not other-
wise mentioned, either of the first two may be used.

(i) Tension Members. For the reason given in (10) of
preceding article and also that in Art. 27, iron finds frequent
application in the transmission of tension. Fig. 25a shows
how a rod may be used for this purpose. The bearing area at
the notch should have a strength equal to that of the rod. If
in addition, the latter be upset, Art. 43, its full strength will
be conserved. In upsetting, the blacksmith enlarges each end
of the rod until the excess is suflScient to provide for the cutting
of the thread and the weakening due to forging. Dimensions

Fig. 25a.— Use of Rod Fig. 256. Fig. 25c, Fig. 25^. — ^Hangers,
as Tension Member. Upset End.

of upsets can be obtained from handbooks. It does not pay
if the pieces be short, small, or few in number. Fig. 25 A shows
an upset with a thread turned thereon. Eyebars, Art. 43,
or loop rods. Art. 42, may also serve as tension members. In
either case, pins, large special bolts, Art. 46, are used at the
joints.

(2) Connection Plates. These are usually plates or bars,
either bent or straight. Some wiU be taken up in later articles
in this chapter. Of the many useful applications, we will call
attention to the hanger, which may be found where one beam
frames into another at right angles. Fig. 25c shows the single
type; 25^, the double. The twist should be so made that the
upper parts of the hangers lie flat against connecting beam.
The size of the bar varies from 2"Xi" to 4"X|". The strap
bolt, Fig. 26uy is another excellent form.

(3) Reinforcement for Beams. For example, Fig. 25^
shows the manner in which the steel is placed in a " flitched "

WOODEN STRUCTURES 41

beam. Plate should be f to |" thick; the total width of the
timber, about twelve times that of the plate; the plate, i"
less depth than the wood on account of overrun of steel and under-
run and shrinkage of timber. An alternate spacing of i8"
suffices for the f " bolts.

Fig. 2se, — Flitched Beam.

(4) Bearing Blocks. These are usually of cast iron. Their
function is to distribute the pressure over a larger area of masonry
or timber. The latter case has already been discussed in (9)
of the preceding article. Fig. 241 in that paragraph represents
one type of a bearing block. A style known as the angle block
is shown in Fig. 261;. Still another angle block is seen in Fig. 3 id.

Fig. 25/. Fig. asg. Fio. 25*. Fig. 25*. Fig. a$j.

Washers.

(5) Washers. The ratio of the bearing strength of timber
perpendicular to grain to that of iron is very roughly one-twenty-
fifth. If we allow for the screw thread, we find that about
fifteen times gross area of rod is required in bearing. As this
is an extreme case, standards are often a little less. For bolts
where they are not in tension, full development is not necessary.
Here the plate washer, Fig. 25/, or square washer, Fig. 25^,
suffices. Either may be cast with a cored hole or made of rolled
stuff with a punched hole. At any rate it should be J'' greater
than threaded end. The diameter of the washer is two or
three times that of the bolt. The thickness must not be less
than J". The remaining washers are cast. Fig. 25^" shows
an ogee washer, a very common form. Its thickness equals
diameter of bolt equals one-half diameter at top equals i diameter
at bottom. Fig. 25* represents a lighter form, while Fig. 25/
gives a washer where the rod enters at an angle.

(6) Keys. These are simple prisms of a uniform rectangular

42 ELEMENTS OF STRUCTURAL DESIGN

section, usually of cast iron. However, they may be made of
some wood which, like oak, possesses a large resistance to shear.
Fig. 26ife exemplifies their use.

(7) Fastenings.

{a) Nails, either cut or wire, are familiar to all. They
are specified as twopenny (2d); threepenny (3d); etc., sizes
varying from 2d to 6od. Dimensions are given in handbooks.

(6) Bolts. These are rounds with one end headed up and
a thread turned on the other. ITie head is a prism either
square as in Fig. 25^, or hexagonal. On the threaded end is
placed a nut which also may be either square or hexagonal.
Bolts are specified by diameter of cylindrical part and length
/, Fig. 2sk, thus, 240 bolts, i"x6" u.h., (under head) with
square heads and nuts. Usual sizes of bolts are, f, |, i, i}, ij"^
diameters. Up to 24" in length, they are carried in stock.
Holes in wood are usually made the same size as the bolt.

a feS B a Qp«* (Dc=w— Ox

Fig. 25^. Fig. 25/. Fig. asm. Fig. 2S«.

Bolt. Lag Screw. Wood Screw. Drift Bolt.

{c) Lag screws are bolts with the screw end pointed, but
without nuts (Fig. 25/). They vary in size from JXii" to
iXi2"u.h.

(d) Wood screws. Fig. 25m, are somewhat similar except
that the head is differently diaped and has a slot for driving.
They are made in all sizes up to 6" in length.

(e) Drift bolts. Fig. 2sw, look very much like large spikes
and are driven in a similar way into a bored hole. The diam-
eter of the round hole should be 30 per cent less for the round
bolt and 15 per cent less for the square. Ragging, that is,
roughening the bolt, lessens its holding power.

(/) Dowels or dowel pins are double-ended drift bolts.

Art 26. Joints

While almost any joint may be made entirely of timber,
it will be generally possible to conserve a large percentage of
the original strength only by the use of steel.

WOODEN STRUCTURES

Where the members are fastened together by overlapping
and bolting as in Fig. 260, it is called a scarf joint.

A fish joint is one where the members abut and are fastened
together at the side by timbers or plates called fish plates.

I>t us take now the very simple fish joint shown in Fig. 266.

Fig. j6o.^
Scarf Joint,

F10.16B.
Fish Joint.

S„ 5/ -shearing miit stresses of timber and metal respectively.
Si, S,' -tensile unit stresses of timber and metal respectively.
5/, 5/flexural imit stresses of timber and metal respectively.
Si,, 5h'— bearing unit stresses of metal against timber and metal
respectively.

44 ELEMENTS OF STRUCTURAL DESIGN

The true distribution of loads on the bolt with the diagram
of bending moments is given in Fig. 26c. Fig. 26d represents
our asstunption; Fig. 26c, that usually employed. The latter
arrangement gives much higher bending stresses than either of
the other two. To neutralize this, it is common to employ high
flexural stresses in connection with such assumption. Let
us now investigate the safe capacity of such a bolt, using Fig.
26d, Here the bolt of diameter d will be loaded with the safe
bearing pressure for such a distance / that the safe shearing or
flexural strength is equaled. This distance I must not exceed t
in Fig. 26b. By equating shear and moment to the resistance
of the section of the bolt we obtain :
For shear

For moment

/=d(x5//325,,)».

Fig. 26/ shows the forces where the steel fish plates are
used. Herfe above equations become,

By these equations the safe lateral resistance in pounds
may be computed for a one-inch round bolt, drift bolt, spike,
nail, wood screw, or lag screw. Other sizes will carry a load
in proportion to the diameter squared. For double shear,
double values. We have also added safe unit resistance to
withdrawal. For screws this is based upon the area of the
circiunscribing cylinder. In nails and screws, area of the point
is not considered. The quantities given are for buildings, for
bridges use two-thirds of same. Amoimts below are based
upon proper design of e, />, and tj Fig. 266.

This distance e should be such that the safe shearing stress
is not exceeded. Assiune this to be carried in the same depth
as the bearing,

S^=SM2d
or

e^dSj2S,,

a mean value of which is 6d for presstire parallel to grain.

WOODEN 8TBUCTURE8

46 ELEMENTS OF 8TEUCTURAL DESIGN

On account of washer and nuts, p cannot be much less than
^d. Art. 25 (5). As in riveted joints, the idea is to make strength
of bolts equal to that of the net section. Economy of material
for a tension joint may be promoted by arranging it as shown
in Fig. 26g.

Above analysis gives imiform weight of bolts, no matter
what diameter may be taken. Hence use as large a dze as is

Fig. i6g.— Efficient Jotat. Fig. 36A.— Fish. Fic, 361.— Scarf,
Joints for ConipKssion Memben.

convenient and will afford a satisfactory distribution of the
stresses.

We will now consider the following cases:

(1) Splicing a compression member.

(2) Splicihg a tension member.

(3) Splicing a beam.

(4) A tension member entering a piece.

(5) A compression member entering a piece.

(6) A beam framing into another beam.

(i) Compression members may be spliced by:

(a) Fish plates on all four sides, Fig. 26/t. Each plate
should have not less than two rows of not less than two bolts
each on each side of jomt.

(6) Scarf joint which should be parallel and perpendicular
to the compression as shown in Fig. 261.

Abutting surfaces are supposed to carry the load and the
bolts are put in to render the member continuous under com-

WOODEN STRUCTURES

pression. Properly built joints have an efficiency dose to lOo
■per cent.

(2) Tension members are spliced by:

(a) Plain joint with 2 or 4 fish plates, either of timber or
steel. Fig. 266 would do for small tensile stresses.

(6) Scarf joint, Fig. 267.

Fig. 26;. — ^Scarf Joint for Tension.

{(£) Fish plates if of steel may be bent and, if of timber, m^y
be notched into the tie. Fig. 26/ shows the latter case.

The author prefers (a). It should be remembered that the
notching and fitting of complicated joints are expensive* Some

Fig. 26A. — Combination Joint for Tension.

Fig. 26/. — ^Notched Fish Plate Joint for Tension.

of these may show higher efficiency in an analysis based on
all elements working in unison. This, however, is an ideal
which is not reached in practice. For example, in Fig. 26k,
it is quite difficult to notch out so exactly that the load will
be divided among the keys and bolts in proportion to their
strength.

To illustrate the computation of a joint in tension, let us

48 ELEMENTS OF STRUCTURAL DESIGN

determine the capacity of splice shown in Fig. 26jt. Timber,
white oak; cast iron keys; and i" steel bolts. Piece is 8"x8",
hence will have two rows of bolts. Using stresses for buildings,
these two rows of five bolts each will carry,

10X2X1500=30000 lbs.

The maximum stress on the net section of the 2} X8"-fish
plate occurs at a, and is,

15,000/(2} X6) - 1000 lbs. per sq.in. O.K.

Taking the Eeys as i" in width, the capacity of each is,

8X7X150=8400 lbs.

Half depth of key must be such that allowable bearing is
not exceeded.

8400/8X1500=0.70 in.

We will make them ij" deep. The total capacity of the
joint is then,

30000+16800 = 46800 lbs.

To obtain this capacity, fish plates must be extended to
include four more rows of bolts to
the right of the upper a and to
the left of the lower one.

(3) Splicing for bending is to

be avoided where possible. If

necessary, use methods given

in (s). As before, (a) is preferable.

The plates may be planes parallel

Fig. i6«.-jomt in Beam with Steel *« ^^ moment or In planes per-

Fish Plates. pendiculaT thereto. In the former

case, they must be treated as

shown in Art. 59 provision being made for the fact that

the resultant pressure for some of the bolts is not parallel

to the grain. To illustrate the latter case, let it be required to

WOODEN STRUCTURES

splice a loXio" spruce for a building to conserve the strength
of the net section, taken as 8Xio'', Fig. 26m. Allowable
bending moment is,

Af =56A2/6 = 75oX8XioXio/6«iooooo in.-lbs.

Let us make plates and bolts of steel. For the former we will
require,

100000/10X16000=0.62 sq.in.

We will use 10X3/8" plates, furnishing 2.91 sq.in. net area.

Fig. 26n. — ^Mortise Fig. 260, — ^Fish
and Tenon Joint. Joint.

Fig. 26^. — ^Joint with
Bent Connection Plate.

The stress in the plate equals 100,000/10 equals 10,000 lbs.
Number of i" bolts required is,

10000/1900 = 6,

arranged as shown in Fig. 26m. The bolts at d should be inserted
to prevent the buckling of the plate in compression.

(4) A joint where a tension member enters a piece may be
handled by :

(a) Mortise and tenon joint. Fig. 26n. This is expensive
and lacks strength as an analysis will show.

(6) Plain fish joint. Fig. 260. This joint is objectionable
on account of the weakness of the bolts which are in bearing
almost perpendicular to the grain and the tendency to split
the horizontal piece.

(c) A rod as given by Fig. 2$a. This is a good detaiL
Where bearing is inclined, allowable values may be interpolated
according to the inclination in degrees as shown in the example
below.

ELEMENTS OP STRUCTUEAL DESIGN

((/) Bent strap of iron or steel as represented in Fig. 26p.
This is also an excellent detail.

Let us exemplify computations by designing rod in Fig.
250 to carry 10,000 lbs. when used for a roof truss of wrought
iron and yellow pine. The size of the rod is

ic(P/4 = ioooo/i2ocx>=o.83 sq.in. Use lA" dia. round rod.

Taking inclination of rod with horizontal as 60 degrees.
Allowable bearing pressure =600+ (900 X 30)790 = 900 lbs.
per sq.in. ^

Area required = ioooo/9oo=ii.i sq.in.
An ogee washer 4" in diameter might be used.
(5) Where a strut enters a piece, the compression may be
taken by:

(a) Notching, Fig. 26q and r. vThe former is the proper
design for small stresses, acting at an
inclination. It should be nailed in. Fig.
26r has the weakness spoken of in Art.
24 (9), and may be strengthened as there
Fig. 36j. Fio. i6f. noted.
Notched Joints. (b) Where there is little or no hori-

zontal component, the notching may be
omitted. The strut is then held in place by toenailing or drift
bolts.

Fig. 36s. Fig. 16I. Fig. 26U. Fig. rf».

Methods Eoi Carrying Compression at an Angle.

(d) Bolting inclined piece as represented in Fig. 26/. A
somewhat similar scheme, employing the strap bolt, is given
in Fig. 26M.

(«) TTie use of cast or wrought-iron shoes. See Fig. 26r
and ^id. This is probably the best method for large stresses.

WOODEN STRUCTURES 51

Combinations of several types are frequent. However, as
already pointed out, there is difficulty in making different
parts work together.

As an example of tlie computations for these joints, let us
consider Fig. 26q when built in red oak for a railroad bridge.
Let the inclined member be a 6"X6" at an angle of 30° with
the horizontal, and let its stress be 12,000 lbs. Its horizontal
component is then 10,500 and vertical 6000. At 650 and 350
lbs. per square inch respectively, there are required 16. i
and 17. 1 sq.in. The depth of the notch must then be 2f",
while tlie length of inclined bearing should be not less than 3".
A more exact method is indicated in Fig. 29^.

(6) A beam franung into another beam, usualty at right
angles. There are three conrnnon methods:

(a) Mortise and tenon,^ similar to Fig. 26».

(6) Toenailing (nailing on side).

(a) is expensive and weak, (6) is cheap and weak, while
(c) is expensive but strong, conserving, when properly designed,
the full strength of both beams.

One of the best methods is to use a built-up section. For
example, an 8"X8" timber may be made from planks, each
2X8"Xi6'-o" securely bolted together with one joint every
4 feet. With good abutting joints this should be as strong as
an 8"X8" in compression. In tension it should have a net area
of about 6"X6". Also joints (4) and (5) may be designed by
making one member double and passing the other through it.

Art 27. Design of Timber Structures

The objects of design are: \

To provide a structure safe under any probable circumstances.
To do this as economically as possible,
llie latter is fulfilled by making the sum of the following
annual charges a minimum :

(a) Interest on first cost.

(b) Maintenance.

ELEMENTS OF STRUCTURAL DESIGN

{d) Operation.

Suppose two alternative schemes proposed for a drawbridge
to be as follows: No. i will cost $200,000; repairs, painting, and
so forth, $400 per year; bridge is estimated to last 30 years; it
will be operated by 9 men at a total cost of $20 per day. For
Scheme No. 2, the corresponding quantities are: cost, $150,000;
repairs, $300; duration, 25 years; operation, 12 men, $27.
Taking masonry as alike in both cases and interest at 4%, we
may compare as follows:

Scheme.

a.
Interest.

b.
Maintenance.

Sinking Fund.

d.
Operation.

Total Annual
Charge.

$8000
6000

$400
300

$3560

3600
See Trau twine,
p. 46.

$7300
9850

$19,260
19JSO

Our analysis shows a slight preference for No. i.

It is necessary for a student to obtain an elementary knowl-
edge of many subjects and time can seldom be afforded for
alternative designs. However, in practice, several should be
drawn up on detail paper and thoroughly examined to eliminate
all waste and weaknesses. Then, after comparison of costs as
above outlined, the best is selected for the finished drawing of
the proposed structure.

To compare wood and iron, let us take their cost when
fabricated at $50 per M and 3 cents per pound.

1 sq.in. of iron will carry 12,000 lbs. tension and wUl cost
10 cents per foot.

27 sq.in. of wood will carry 12,000 lbs. tension and will
cost II cents per foot.

2 sq.in. of iron will carry 12,000 lbs. compression and will
cost 20 cents per foot.

15 sq.in. of timber will carry 12,000 lbs. compression and
will cost 6 cents per foot.

The above is quite rough since allowance has to be madef
for excess of gross over net area, the reduction of allowable
stress in compression, and so forth. It indicates clearly why

WOODEN STRUCTURES 53

wood is cheaper than iron in first cost and also why the latter
is often employed in tension members.

In the preceding articles, iron has been freely used in framing
joints. Its shearing and bearing strength, large as compared
with wood,- make it particularly valuable. The objections
are:

(i) It is largely blacksmith's work and therefore expensive.
(Art. 42.)

(2) Small pieces are likely to be lost in shipping. (Art. 48.)

(3) In the case of error or change in design, iron is not
as easy to alter as timber.

(4) Another material adds to the difficulty of handling
the job. However, its advantages are such that it is used
considerably.

If much framing is required and the timbers are small,
spruce, white pine, Norway pine, or hemlock may be employed.
All frame easily but the latter is often weak and treacherous.
For heavy pieces and small amounts of notching and cutting,
use yellow pine and white oak. These are strong woods but
they frame with some difficulty. For long pieces, take yellow
or (Oregon pine. Sleepers and posts are made of cedar, chestnut,
and cypress. Keys and fish plates, if of wood, are commonly
of white oak.

On account of the rule that safety of the structure must
not depend upon friction, bolts and screws make a much better
design than spikes or nails. One of the former must be used
when in direct tension and they are preferable in shear. Drift
bolts, dowel pins, and nails may be employed where the stress
is carried mainly in bearing. A cheap and rapid method for
temporary construction is the fastening together by spikes.
A disadvantage, particularly with wire spikes, is the lessened
salvage value. Lag screws are difficult to drive in hard woods
like oak.

In Europe, labor is cheap and lumber dear, hence elaborate
joints are often made to save material. Here the reverse is
true, therefore plain joints with ample connecting plates are
economical. / .

Ties exposed to the wind and compression members must
be thoroughly braced. It is sometimes necessary to use knee

54 ELEMENTS OF STRUCTURAL DESIGN

bracing. Fig. 27a, but it is not as strong as the X bracing in
Fig. 27A, and it introduces large bending

n|\/| stresses. Weil-nailed boarding is con-
l/\J sidered equivalent to an X bracing in its
Fig na Fic nb P'*"*' ^^ '^ much more efficient if laid

Knee Bracing. X Bracing, diagonally.

It is customary to make no provision
for the alteration of length of timber with change of tempera-
ture and to neglect consideration of the stresses caused thereby.

Art 28. General Description of Jtoat Trusses

Underneath the slate, shingles, or other covering, lies the
boarding, running parallel to the peak. This boarding, or
sheathing, as it is sometimes called, is planed on one side and
nailed to the raf-
ters which sup-
port it, Fig. 286.
The latter are
" sized," that is,
are notched a
small amount, to
bring their tops
to a uniform level
over the purlins.
These are beams
running parallel j^^ ^g^

to the peak m Typical Arrangement of Root Tmsa.

turn resting on

the trusses. They are preferably placed vertically or perpen-
dicularly above the joint in the truss on which they rest.
If otherwise located, top chord must be computed to cany the
combined compression and bending.

Another method is to run the boarding perpendicular to
the peak and to rest it directly ufKjn the purBns, omitting the
rafters. Although more material will be used this way, it
requires less work and is a better fire risk. Art. 24, (8), (a) and
(b). Or both purlins and rafters may be omitted and sheath-
ing run directly from truss to truss, resting on the top chord.

WOODEN STRUCTURES 65

The thickness of the boarding varies from |" to 3"; it is
often assumed and allowable span computed. This should be
such that deflection is not more than 0.2 per cent of the same.
This condition will be fulfilled by making ratio of span to thick-
ness not more than 30. The rafters are usually 2X4", 2X6/'
or 2X8/' with larger dimension vertical. 8X8", 8X10",
and 10X10" are common for the purlins, the latter in each case
being perpendicular to the roof. Two purlins are used at the
peak, one on each side, Fig. 28a. They have the same depth as
the others but only i to J the width on account of the lessened
load. At this place, it is customary to insert between the rafters
the ridgepole. It is made a little deeper to ensure full bearing
and 2" thick. At the eaves where the truss meets the wall,
the rafters are notched over a piece called the plate which lies
horizontally. They are then extended to support the gutter

^^i^iw

Fig. 2&/. Fig. iSe. Fig. 28/. Fig. 28^^.

Forms of Roof Trusses.

and the finish around it, the three together constituting the
" cornice,'' Fig. 28c. If this plate rests upon the brickwork,
it is made of sufficient size to distribute the load over the
masonry. A 4X8" laid flat would do. In the design of the
cornice, care should be taken to provide enough waterway and
to exclude weather and rain from the building.

The purlins usually extend over a single " bay," as the
distance center to center of trusses is called. This should be
chosen to utilize some stock length of timber, such as 11 '-6"
to use 12' stuff, 13'-^", 15-6", etc. It may be determined
by conditions within the building, such as. location of windows
in supporting wall. When there is no such limitation, several
different lengths may be tried, the cost of roof trusses esti-
mated and the most economical chosen. This is usually about
15 feet decreasing somewhat for small spans. In a like way the
most efficient arrangement for rafters, purlins, and trusses may
be investigated.

Figs. 2&d, e, /, and g show types of wooden roof trusses;

56 ELEMENTS OF STRUCTURAL DESIGN

/ representing what is perhaps the most common one. Light
lines indicate those members which are usually made of iron
in combination trusses, that is, in trusses made of iron and tim-
ber. However, all tension members are sometimes made of
steel or iron. Like the roof systems, various trusses may be
tried to secure the most favorable design. Most economical
inclination is one-third pitch, that is, making the ratio of height
at center to span one-third.

Bracing is usually omitted where the truss rests on hiick
walls. Li this case the sheathing is supposed to give sufficient
stiffness. Where trusses rest on isolated columns, the latter
must be braced both ways. X bracing is best- but knee bracing
is often used on account of the clearance required.

The dead weight per inclined square foot for various kinds
of roofing exclusive of sheathing is about as follows: Shingles,
2 to 3 lbs.; slate, S to 8; tiles, lo to 40; tin, i to 2; corrugated
iron, I to 3; gravel, 6 to 8. The weight of timber per foot
B.M. (board measure) may be estimated as, oak, 4.5. lbs.;
hard pine, 4; cedar, cypress, hemlock, spruce, and chestnut,
3; white pine and poplar, 2.5. The weight in pounds, Wy
of the truss alone may be obtained from Jacoby's formula:

1^=0.505(1+0.155).

Here a is the length of bay while 5 is the span, both in feet.
The weight of snow in pounds per horizontal square foot, w,
may be taken from the formula:

w=(/— 25) cos f,

where / is the latitude and i is the angle of inclination of the
roof with horizontal, both in degrees. This allowance will
vary somewhat with the climate. The pressure of the wind,
taken as normal to the surface, may be obtained from Duchemin's
formula:

^=C2 sin 6/i-f sin^.

/>= pressure in pounds per square foot, 6= angle of inclination
^th horizontal, and C equals a constant, a mean value for which

WOODEN STRUCTURES 67

is 40 lbs. It will be noted that p equals C for a vertical
surface.

In computations many engineers consider the resultant of
vertical and perpendicular loads. This we do not favor, as the
usual arrangement of the rafters takes care of component parallel
to roof. A very simple method is to add directly the entire
weight of roof, snow, and wind, and consider them as acting
perpendicularly on every part of the roof surface. To be sure
this is higher than the actual load on boarding and rafters, but
it serves as an impact allowance and also to take care in a way
of a concentrated load. If without knee braces, the same method
may be pursued for ordinary trusses. The writer, however,
considers the latter poor engineering. For knee-braced trusses,
it is never allowable. The proper way then is for all framed
structures to obtain maximum stress of each kind after a full
consideration of all possible loadings.

Art 29. Computations for a Roof Truss

Let us suppose trusses of the type shown in Fig. 28/ to be
of 40' span, i3'-6" c. to c, and angle of rafters with horizontal,
30 degrees. Roofing, slate weighing 8 lbs. per square foot.
Material, spruce with wrought iron rods upset at ends for ten-

Fig. 29a. — ^Boarding. Fig. iqb. — Rafter. Fig. 29c. — Purlin.

Load Diagrams.

sion members. For the computation of sheathing, rafters, and
purlins, we will allow 32 lbs. for wind, 15 lbs. for snow, and 13
lbs. for dead load, making 60 lbs. per square foot, perpendicular
to the roof. Note that the actual computation below shows
the dead load to be 15.5 lbs. per square foot. See Fig. 29/" for
finished design.

(i) Allowable distance center to center of rafters, Fig. 29a.

5r=6Jlf/6A2=3tt;P/4j^ or / = 2//(65//3ze;)*.

ELEMENTS OF STRUCTURAL DESIGN

Let us try J'' boards. Then, taking a strip 12" wide, A = i",
6 = 12", tt^ = 5 lbs. per linear inch, 5/= 750 lbs. per square inch,
hence /=42.9". To prevent excessive deflection, this distance
must be reduced. J"X3o = 26.2, we will make it 6 rafters to a
bay equals 27" spacing.

(2) Size of rafter (Fig. 296). Make 2" wide.

Sf^iwP/^hK^ or h-^o.sKiw/hSf)^.
Span =/ = 240 sec. 30 deg./3=92.4".

2«;=6oX27/(i2Xi2) = 11.25 lbs. per linear inch.

5/= 750 lbs. per square inch.

Substituting, 4 = 6.93", ^^d we will use 2"X8", the next
size above.

(3) Size of purlin (Fig. 29c). Carpenter usually arranges

rafters to suit himself. Most unfavorable case is shown in the
figure. Its maximum e£Fect is the same as that of a imiformly
distributed load of equal amount.

5/=3wP/4JA2 or bh? = iwP/4Sf.
/= 162", 5/= 750 lbs. per square inch.

Hence,

Tt;=92.4X6o/(i2Xi2)=38.5 lbs. per linear inch.
6A2 = loio. Use 10" X 10", JA^ = 1000.

(4) Size of members. Estimated weight of roofing is:
slate 8 lbs., sheathing 2.5 lbs., rafters 2 lbs., purlins 3 lbs.; total

WOODEN 8TBUCTUKE8 59

15.5 lbs. per inclined square foot. Estimated dead weight of
truss is j 13.5X40(1+0.15X40)= i890#. Dead panel load

= -^+i5.5X 13.5 X 7.7= 1920S. Panel load for snow = 15

Xi3.5X4o/6 = i35oS. Wind per inclined square foot is 4oXi
sin 30 deg./i+sin^ 30 deg.= 32#. Wind apex load= 32 x 7.7
Xi3.5™3340#. Consider truss as fixed at both ends.

(5) Joint ADM. Make like Fig. 261;. Bearing area required
ior the stress in DM is 25.05 cos 3o°/iooo = 2i.7 sq.in. Use
two notches, each 8" wide by i§" deep, affording 22.0 sq.in.
bearing area. Shearing length reqidred is, 25.05 cos 30°/8X8o
=34.0". Make two of i'-6" as shown in Fig, 29/. For stress
in AD, 25.05 cos 3oV770 = 28.2 sq.in. Use 3J''X8" bearing.
Allowing 3000 lbs. per sq.in. shear, necessary thickness of
casting is, 25.05 cos 30°/30ooXS=o.9i". Using an allowable
ffexural stress of 5000 lbs. per sq.in.,

i-(3W;/65/)'.

Here, W = 2s.o5 cos 3o° = 2i,7oo lbs., l^-^-s", 6 = 8", 5/=5ooo
lbs. per sq.in. Substituting, ( = 2.39". In Fig. agf, casting is

ELEMENTS OF STRUCTURAL DESIGN

made ij" thick. Since required thickness varies as distance
from the top, the lower half would be deficient in strength.
The filleting of the comers of the casting, Art. i6, would help
some by increasing strength of iron and lowering point of
application of resultant pressure. Also stiflFening ribs might be
employed. In any event, supplement design as shown with a

Fig. 29^.

special detail of the casting, giving necessary dimensions at
important points.
(6) Area washers.

For DE and FG, 3 5 50/300
For HH\ 10370/300

1 1.8 sq.in., use 4" O. G. Washer.
34.6 .'* '' 4.S"X8" Washer,

special.

(7) Notches for EF and GH. (Fig. 29^.)

Total
Pressure
in Kips.

Area
Sq. in.

Pressute

in Lbs. per

Sq. In.

Inclin.

with Grain.

Degrees.

Allowable

Pressure. Lbs.

per Sq. In.

7.40

49.8

148

•

430

8.20

13s

610

620

p«

9.80

384

^SS

440

4.80

12.0

400

390

360

48.7

390

6.90

9.0

770

6.80

30.0

230

490

6.20

12.0

510

530

WOODEN 8THUCTUEE8

62 ELEMENTS OF STRUCTURAL DESIGN

(8) Splice at center. Use i" steel bolts and two fish plates,
each 3"Xio". Value of bolts, Art. 26, is 2000 lbs. each.
Number required is 17,970/2000 = 9.

Art 30. Trussed Beams *

Figs. 30a and c show the styles known as the king and
queen post truss respectively. They are trussed beams only
when horizontal chord is continuous. Computations often

assiune them to be jointed structures. Where
the angle of inclination, 0, is 20° or more, with
horizontal, the error will not be a serious one.
Fig. 30a.— King We sometimes find them inverted, but the depth
Post Truss. is then made such that they need not be con-
sidered as trussed beams. Taking the usual
case of constant sections and uniform load, the correct method
for either of the above trusses is as follows:

(i) Assume a jointed structure and determine sections.
This assumption is for the purpose of obtaining sizes and is
carried no farther.

(2) Compute the deflection at panel points caused by any
uniform load, PTi, acting on entire length of top chord, con-
sidered as a simple truss. Reactions must be obtained by
treating beam as continuous. ,

(3) Compute the uniform load, W2, which will cause equal
deflections at panel points when the top chord is considered
a simple beam for its entire span.

(4) Then, for any imiform load, W2/(Wi+W2) of that
load acts as if on a simple beam of length equal to the span.
Wi/{W\+W2) of that load acts as in a continuous beam.f
From the reactions for the same, stresses for the truss may be
determined.

(5) Those in the beam are then the result of compression
due to truss action, plus bending due to W2XLo?A/{Wi+W2)

*See "Modem Framed Structures," by Johnson, Bryan, and Tumeaure,
Part II, p. 408. While theory given here* bears a strong resemblance to that
in above reference, it was consulted only after manuscript was finished.

t This follows from the fact that deflections due to truss action and that due
o beam action must be the same at panel points.

WOODEN STRUCTURES

on simple beam of span length, plus bending due to Wi xLoad/
{W1+W2) on a continuous beam with supports at each panel
point of the truss.

(6) If stresses so found are close to allowable values, details
may be determined for the final design; otherwise, revise and
recompute.

As an example, let us take a beam whose span is 20 feet
with a depth at center of 2 feet as shown in outline in Fig.
30a and as detailed in Fig. 306. Let the load be 2000 lbs. per
lineal foot.

(i) Assuming discontinuity in top chord, stresses are;
bCy 20,000 #C; ab, 50,000 *C; ac, 51,000 ^T; bending in ab,
20,000X120/8=300,000 in.lbs. Use yellow pine. Allowable
compression, 1200— 15//^; flexural stress, 1500; bearing 600
and 1500; £, 1,600,000. For iron, 10,000 in tension and
£ = 25,000,000, all in pounds and inches.

For be we will try a 4"Xi2'', larger than necessary but
affording a good design. If we take the top chord as made
of* two timbers, each 8" wide by 10" deep, its stresses will be

50,000/160+6X300,000/16X10X10 = 1437 lbs. per sq.in.
Required area of rod = 51,000/10,000 = 5.1 sq.in.
Use one round, 2 9/16" dia., area = 5.16 sq.in.

(2) Deflection due to a uniform load of 1000 lbs. Load
on truss, 625 lbs. Everything in pounds and inches.

Member.

Stress.

I
length.

Area.

E
Mod. Elas.

sn/AE

ab
be
ac

1562
62s

1593

240

245

160

48
5.16

1,600,000

25,000,000

Total

2.29
0.12

4.83

7.24

Deflection equals 7.24/625=0.0116",

64 ELEMENTS OF STRUCTURAL DESIGN

(3) Deflection=o.oii6=— T ,, ^,^ — ;-.

^'^'^ 384 X 1,600,000 X 16 Xio»

Hence W2= 137 pounds.

(4) Of this uniform load of 2000 pounds per lin. ft., 137/1137
Equals 240 lbs., acts as on a simple beam, while 1760 lbs.
affects truss. Stresses in the latter are:

aJ = 5S,ooo#C, &c = 22,000 #C, ac = 56,100*7.
Unit stress in he equals 22,000/48 = 460 lbs. per sq.in. C.
" '' ac '' 56,100/5.16 = 10,900 '' '' '' T.

(5) Max. moment in ab occiu^ where the shear equals zero.
This happens at center and also at i2/w;= 9000/2000 =4. 5 feet
from either support.

For the former, M = 9000 X 10 — 20,000 X 5 = — 10,000 ft. lbs.
For the latter, 3f =9000X4.5— 9,000X2.25 = 20,250 ft.lbs.
Stress in a&= —55,000/160^6X20,250X12/(16X10X10)

— 344=fc9ii = 1255C or 5677 in lbs. per sq.in.

(6) The stress in oc is greater than that allowable. As we
make this member larger, still more of the load will come on
the truss. About a 2 11/16'' rod is required, we will make it
2f' ' diameter and recompute. For the deflection in (2), we get
.0106". The uniform loads on beam and truss are 220 and
1 780 lbs. respectively, and total stresses are, —

^* = SSJSO C, be = 22,300 C, ac = 56,900 T.

The imit stresses are, in lbs. per sq.in., —

aJ= 1236 C, bc= 465 C, ac= g$8oT.

A 2 11/ 1 6'' rod might have done but ab and be have reserve

WOODEN STRUCTURES 66

Strength and it is best to have a Uttle in ac. It would not be
advisable tostrssoft the full allowable Sexural value of 1500 lbs., ,
as it is in part a compression member. Use casting at c designed
for a load of 22,300 lbs , concentrated below, and uniformly
distributed over 12" above. Bearing values must be tested
at shoulders on be and for rods at a.

Problems of this kind may often be advantageously solved
by the aid of the method of least work, This principle, with
which the student should already be familiar, states in brief
that the load is so divided among statically indeterminate

oi King Post Truss Shown in Fig. ioa.

systems as to make the sum total of the work a minimum.
Iwet us then apply it to this case.

Let £1, Ea, Ea, be moduli of elasticity for ah, be, and ac resp.
Ai, A2, Az, be areas for ab, be, and ac resp.
Li, L2, La, represent spans ab, be, and ac resp.
Ji be the moment of inertia of ab about an

horizontal axis through the center of

gravity.
P be the load on the truss.

If =-3wLi- total load.
9 = tan~^6c/(iJ.

Further, let M" equal the bending moment in beam at a
point distant x from either reaction, and let K represent the
work done. Then

£ = a C'M^ix/i2EI)+h^S'L/AE,

66 ELEMENTS OF STRUCTURAL DESIGN

where S designates the total stress in bars. Now substituting
for 5, the stresses in the different bars, and for M, x — Jzew^,

1 J W^L^ PWLi^ P^Li^ WwLi^ wPLi^ vr^Li^\
a/A 12 6 12 8 "*■ 8 "^ 20 /

i_ P^Li tan e . P^Li (esc? 6 sec 6 . cot^

2 A2E2

P^Li / csc^ e sec e cot^ e \
4 \ AsE^ AiEi/'

Following the rules of calculus, we differentiate this expression
with regard to P and place it equal to zero to get the minimum
value of K, and obtain

/ Li^ tan e csc^ 6 sec 6 cot^ 6 V
^ ' ^\6EiIiA2E2 2AzEz "^ 2A1E1)

Substituting values given in problem above, we compute P
to be 22,200 lbs., in substantial agreement with the method
of deflections.

By putting an initial camber, that is, an upward curvature '
in the stringer, its stresses may be changed a considerable
amount. The deflection in the problem just considered is
0.0106X40,000/1 1 25* = .38'', about f . If now the nuts zX
the end be screwed up before the load is put on so that it has
a camber of 0.38X2000/1780=7/16'', the load acting as a
simple beam will be zero.

Any load for the king post truss or a symmetrical load for
the queen post truss may be similarly treated. For the latter
under unsymmetrical loads, above analysis will not hold, since
horizontal component of stress in bottom chords must be con-
stant. We proceed as follows:

(i) Assiune jointed structure with a diagonal, ce^ compute
stresses and determine sections making

ab = bc=cd, ae = ef=fd, and be=cf,

* Recomputed value for 113 7 in (4).

WOODEN STRUCTURES

This assumption is for the purpose of obtaining sizes and is
carried no farther.

(2) Taking (uf as a simple beam, compute deflections at
b and c caused by given imsymmetrical loads, and call the
mean of these, Z>i.

(3) Still considering ad as a simple beam, compute upward
deflection, ^2, at either b or c due to

two equal loads, P, at b and c.

(4) Taking Fig. 30^ as a truss,
find downward deflection, ds, due to Fiq. 30c.— Queen Post Truss,
the loads P at 6 and c.

(s) Note that the truss offers no resistance to an upward
movement at b, accompanied by an equal downward movement
at c. Also that the average deflection at panel points, Z>i,
due to whole load acting on beam of entire span, minus upward
deflection, Z>2, of beam due to reactions, T, carried by truss,
equals downward deflection, D3, at same points due to the truss
action. That is, Z>i— Z>2=^3. But, because of the propor-
tionality of deflections, D2 = Td2/P and Dz = Tdz/P. Sub-
stituting, T = PDi/{d2+dz).

(6) The stresses in the truss may then be determined. In
the top chord, besides its direct compression, it will be a simple
beam of length ad subject to the given loads plus the two upward
ones, r, at b and c just obtained.

(7) If stresses are satisfactory, problem is completed; if
not, revise and recompute.

Art 31. Description of Bridges

/m^ mmmm wbMAM

Fig. 31a.
Queen Post Truss.

Fig. 316.
Lattice Truss.

Fig. 31C.
Howe Truss.

The above represents the leading t)rpes of bridge trusses:
a, the queen post truss mentioned in the preceding article;
J, the lattice; and c, the Howe: the latter is the best and most
common form.

68 ELEMENTS OF STRUCTURAL DESIGN

There are usually two similar trusses. Between them are
the* floorbeams, generally having the larger dimension for their
depth. On top of these floorbeams and parallel with the truss,
are stringers with a depth two to six times their width. On
these are laid the ties in a railroad bridge or the plank in a high-
way bridge.

Greater economy may be obtained by the use of deep
stringers but i6" is about the greatest depth that can be easily
obtained and its width must not be less than one-sixth the
depth on account of the tendency of narrow beams to buckle.

In a highway bridge, the dead floor load should be computed
as it varies a great deal with the width. For railroad single
track bridges, 400 lbs. per lin.ft. may be used for the weight
of one track, and one-fifth the live load for the weight of floor.
The weight of the trusses for any wooden bridge may be taken
as their live load burden times one three-hundredth of the span
in feet. Here the weight is for the same number of trusses
and for the same length as the load. If we take live weight
in poimds on each lineal foot of truss, we obtain weight of truss
in pounds per lineal foot.

In a railroad bridge, ties are usually spaced 12'' on centers,
are 9 to 12 feet long, and 6X8'' in section with the latter
vertical. However, they should be tested for their loads.
About nine inches inside of each rail are placed the guard rails,
both these and the main raUs being spiked to each tie. Approx-
imately 20'' center to center outside of main rails are placed
the guard timbers, about 6" vertical by 8'', which are bolted
to every third tie.

Six stringers commonly support the latter. Of these, two
are placed under each rail and are computed to carry the loads
therefrom. The other two are put at either end of the tie.
The stringers rest on the floorbeam, these in turn rest on' the
bottom chord or are hung from the verticals. When the floor-
beams are not placed at a joint, the chord must be designed
to carry the bending moment as well as the direct stress.

In a Howe truss, verticals are ordinarily of iron or steel
and are upset at their ends. Special washers of wood or iron
are often necessary to take the stress into the chords, these
washers being figured to carry their loads when uniformly dis-

WOODEN STRUCTURES 69

tributed. The verticals usually pass through the block of oak
or cast iron on which the diagonals rest, and also through
the bottom chord.

If cast iron be used for the angle block, it need not be
finished. The lugs at the top, Fig. ^id, should have a hole
for a bolt to fasten in the diagonals. Those
at the bottom should be figured to furnish
sufficient bearing area on timber. The thick-
ness of the cast iron at this point should

suffice for the shear and moment. Some- Picairf. ^Angle Block

what similar castings are placed at the Howe Truss,
bottom of the top chord and they may
also be used as bearing blocks for the laterals.

These are best kept in the plane of the chords. X bracing
should be employed at the panel points of deck bridges, with
knee bracing at the entrance to through bridges.

Chords are often made of three or more pieces. They are
spliced as near a joint as possible. If compound, pieces should
be kept at least two inches apart and occasional ^vooden fish
plates inserted between them. Splices should be arranged to
stagger, that is, they should be so arranged that no two will
occur at same or nearby points. Diagonals are usually of
two timbers, while between them run the counters of one
piece each, the two sets being tightly bolted together at their
intersection.

No provision is made for the expansion due to the change
of temperature.

Art 32. Computations for a Bridge

(See Figs, sid and e)

Let it be required to design a through Howe truss for a
single track railroad. It is divided into eight panels of i2'-o"
each making a total of 96'-o". We will make distance center
to center of trusses i6'-o", thus providing the necessary ij^-o"
in the clear. Headroom above base of rail should be
2o'-o'', hence center to center of chords will be made 24'-o".
The live loading is 4000 lbs. per lin.ft. plus a concentration

ELEMENTS OF STRUCTURAL DESIGN

of 8000 lbs. for floor system only, both per track. For wind
load, use 100 and 300 lbs. per lin.ft. on top and bottom chords
respectively, the latter to be treated as live. Material, hard
pme with wrought iron tension members.

(i) Size of stringers. Using four floorbeams to a panel
length, the span of the stringer becomes three feet. Allowing
200 and 100 lbs. per lin.ft. per rail for track and stringer
respectively, the latter is a beam subjected to a i&iform load
of 6900 lbs. and a concentrated load of 4000 lbs. The max-

FlG. 32a.

iTniim shear is then 7450 lbs. and maximum moment 67,000
in.lbs.

Least allowable value for shear, bh = 1.5 X 7450/70 = 160
* * ' * moment, JA^ = 6 X 67 ,000/ 1000 = 402

These requirements will be satisfied by using under each rail
two 8'' X 10" with the latter vertical.

(2) Size of floorbeam, span 16', Fig. 32a.

P = 2300 X 3 +4000 = 10,900 lbs.

Uniform load, estimated, 100X16 = 1600 lbs.
Maximum shear, 11,700 lbs. Maximxun moment, 758,000 in.lbs.

Required in shear, bh =1.5X1 1,700/70 = 250.
' ' ' * moment, bf^ = 6X 758,000/1000 =4548.

Use two 9"Xi6", former horizontal.

WOODEN STRUCTURES

(3) Loads.

Dead load per foot per truss for track

floor
truss

=: 200 lbs. on bottom chord.

=400

= 2000X96/300.

= 640 lbs., J top, J bottom.

Dead upper panel load is 0.32 X 12

'' lower '* '* 0.92X12

Live lower *' *' 2.00X12

Wind upper *' '' 0.10X12

lower '* ** 0.30X12

3-8

kips.

II.O

24.0

1.2

3-6

(4) Stresses. Values are in kips and kip in.
(See figure below.)

Upper chord. . .

Member.

Lower chord. .

Diagonals.

Counters.

Verticals.

UxUt
UtUt
UzUi

LqLi
LiLt

UiLo
UtLx

UiU
UOa

UiLi
UtU

Dead.

25.9 C
44. 4C

sssc

25. 9T
44. 4T
SSST
59. 2T

57. 9 C

41. 4C

24.8 C

8.3 C

41.4 T

24. 8T

8.3T

48.0 T
33. 2T
18. 4T
II. oT

Live.

42.0 C

72. oC
90. oC

42. oT
72. oT
90.0T
96. oT

94.2 C

70.5 c
50.3 c

33.6 C

3.4C
lo.iC
20.2 C

84. oT
63. oT
45.0 T
30.0 T

Wind.

Maxxmum.

2.2C
3.SC

12. 6T*
19.3 T*
23.3 T*
24.6 T*

9.1C

67.9 C
118. 6 C
149.0 C

80.5 T
135. 7T
168.8 T
179. 8T

161. 2 C
III. 9 C

75.1 c

41. 9C

II. 9 c

132. oT
96. 2 T
63 -4 T
41. oT

* Three kips is due to the effect of the wind on the upper chord.

ELEMENTS OF STRUCTURAL DESIGN

Stresses in portal. See Fig. 32c.

In a, 2.1X13.4X1.414/5=8.0 T or C.
Max. moment in UiLo is 3.5X5.0X12 = 210.

'* '' i/it/'i or LoLo is 2.1X60 = 126.

** ** bottom chord. See Fig. 326.*

18.5X54-8.0X36 = 711.

ao XLO ao 6.0

^ Jg.Q' ]

Fig. 326. — Bottom Chord.

Fig. 32c— Portal.

Fig. lid. — ^Diagram Howe Truss.

*Two kips has been added here to cover weight of bottom chord and
extras and to allow of possible mi^lacement of floorbeams from assumed
position.

WOODEN STRUCTURES

(s) Table of unit stresses.

Member.

Max.

Kipe.

Hor.Vert.

Unit Stresses
in Lbs. per Sq. in.

Unsup-
ported
Length.
Inches.

Allow-

Direct.

Second-
ary.

Total.

able.

UiUt

67. 9C

i2"Xio"

566

• • •

566

144

656

UtUt

118. 6C

2o"Xio"

593

• • •

593

144

656

UtUi

149.0 c

24"Xio"

621

• • •

621

144

656

LoLi

80.5T

3o"Xi6"

168

556

724

• • •

800

LiU

135. 7T

34"Xi6"

250

490

740

• ■ •

800

168. 8T

38"Xi6"

277

440

717

• • •

800

179. 8T

38"Xi6"

297

440

737

• • •

800

VxU

161. 2 C

26"Xl2"

516

15s

671

161

666

UtLi

111.9C Twoio"Xio"

560

• • •

560

161

639

u^u

75. iC

" 8"X 8"

586

■ • •

586

161

600

u,u

41.9 c

" 6"X 8"

436

• • •

436

161

530

UxU

• • • •

One 6"X 8"

• a •

* ■ •

• • ■

161

530

UtU

■ • ■ •

'* 6"X 8"

• • •

• • ■

• • •

161

530

ViU

II. 9C

'* 6"X 8"

248

• • •

248

161

530

VxU

132. oT

Two 3 J" rounds

6860

• ■ •

6860

■ • •

7000

UtU

96. 2 T

*' 3" '*

6800

• • •

6800

■ « ■

7000

UzU

63. 4T

It 2j// <i

6450

• • •

6450

« • •

7000

u,u

41. oT

<< 2" *'

6530

• « •

6530

• « •

7000

Top diags. .

3.8C

One 6"X 6"

• • ■

120

600

Top ties

1.8T

** r round

4100

• • •

4100

• « •

7000

UiUi

6.7T

One 8"Xi2"

660

730

• • •

800

«, Fig. 32c 1

8.oT^
8.oC/

'' 4"X 6"

333
333

• • •

• ■ •

333
333

• • «

800
590

Bolt, diags.

15. 8C

" 6"X 6"

440

■ • •

440

120

600

LiU

9.4T

One i|" round

6350

« • •

6350

7000

6.7T

*' I J" "

6770

• • •

6770

7000

4.ST

it -N <<

5720

• • •

5720

7000

. Li/:*

2.7T

li a// 11

6140

■ • •

6140

7000

LoU

II. oT

a 8"Xl2"

660

770

800

In the design, Fig. 32^, the bridge must be built to conform
to the above results. The stress in wooden tension members
is kept low to allow for the joints. In computing U\Lq for

ELEMENTS OF STRUCTURAL DESIGN

if! >il

WOODEN STRUCTURES

moment, it is considered as a solid beam, a little different from
its actual construction but probably agreeing closely with its
strength. It is better to nail diagonal batten plates of say one
inch stuff along top and bottom, leaving small spaces between
for ventilation. For LqZo the floorbeams are used as they
will be amply strong. Bolts and other details are not shown
in the drawing. At all joints, packing blocks should be used
and pieces thoroughly bolted together. In built up compres-
sion members, care should be used that the slendemess ratio
for individual members does not exceed that for the column
as a whole.

Art 33. Trestle Bents.* (Fig. 32f)

While these may be computed and designed like any other
structure, they are usually built from standard plans evolved
from experience. Standardization is rendered necessary by
the need for keeping in stock the timber for renewal.

The ties, rails, and guard timbers should be the same as for
a Howe truss. Stringers are usually built of three timbers,

Fig. 33a.

Fig. 336.

Details at Cap.

each 6 to 8" wide and 14 to 18" deep. They are well bolted
together and packed by fish plates at caps and spools (cast-
iron washers), elsewhere to keep them 2" apart. The span
varies from 12 to 16 feet. Separate timbers may extend over
one or two spans, in the latter case breaking joints. They may
rest directly on top of the cap of the bents as shown in Fig.
33a, in which case they are drift bolted to it. Or " corbels "
may be placed underneath as seen in Fig. 336, when it is

*See "Theory and Practice of Modem Framed Structures," by Johnson,
Bryan, and Tumeaure, 1904 ed., Chap. XXV.

76 ELEMENTS OF BTRUCTURAL DESIGN

bolted to Stringers and drift bolted to the cap. Corbels add
to the cost, contribute to shrinkage and decay, and should be
avoided.

The structure which supports the stringers is called a " bent."
These are usually built about as represented in Fig. 33c. The
posts are under or nearly under the rail, while the battered
posts are placed just outside at the top. The inclination of
the latter is 2 U) 3" per foot. Sometimes a square bent is
used, but it is suitable only for tangents and small heights,
Fig- 33<i-

The cap may be made whole as shown in Fig. 33a and b.
In this case, it is usually about i2"Xi2"Xio' to 14'. Or
the posts can be mortised into the cap as seen in Fig. 33$. In

, Fic. isc. Fio. iid. Fig jje

Typical Bent. Square Bent, Post Mortised

into Cap.

the latter event, two 6"Xi2" are firmly bolted to the posts,
which are notched to receive them. These posts are usually
i2"Xi2" mortised or doweled into the sill, also a i2"Xi2"
extending 2 or 3 feet outside of the joint. The sills rest on pile
or masonry foundations, preferably being firmly bolted thereto.
The bracing in the plane of the bent is called " sway bracing "
and may be omitted if batter posts are used and if the height
of the bent does nor exceed 20 feet. Each diagonal should be
2 or 3"Xi2", firmly bolted at each end, and spiked at each of
the other posts. Cross girts, horizontal transverse members,
may be in pairs bolted to the posts; or single members, framed
in between and toenailed to them. Sometimes the bents are
made in stories, the cross girt forming the sill fo'r one story
and a cap for the one below it.

WOODEN STRUCTURES

78 ELEMENTS OF STRUCTURAL DESIGN

The bracing in a plane parallel to the track is called the
longitudinal bracing. Diagonals are about 6''X8'' and girts
about 6"Xi2". It is placed in the plane of the center of track.
Plank bracing in plane of vertical or battered posts is frequently
used and should be just as good if an efficient connection
between cap and post is obtained.

Usual size of drift or other bolts is f '' diameter.

Except when using standard plans, the safety of which in
similar locations has been proven in practice, loads should be
estimated and every piece and joint carefully proportioned
not to exceed allowable limits. In designing the longitudinal
bracing, stresses due to the braking of the train must not be
forgotten. This may be estimated at one-fifth the total live
weight. The sway bracing has to carry the wind load, a high
value of which should be chosen to provide strength to resist
the buckling of the posts.

CHAPTER IV

FABRICATION OF STRUCTURAL STEEL*

Art. 34. Organization of Administration

We pass now to the design of structures largely of steel.
Let us first take up the company, its men, machines, and
methods by which the rolled shapes are fabricated into bridges
and buildings.

The company may be divided into the following departments:

Executive, Ordering, Shipping,

Sales, Operating, Erecting,

Engineering, Inspecting, Financial.

In the executive department are the president, vice-president,
general manager, purchaser, and treasurer. They decide upon
the policy of the company, appoint minor officers, and exercise
a general supervision of all employees. Those who occupy
these positions should have capacity for handling men and good
business judgment in addition to a technical education and a
thorough knowledge of the work. However, they are not
directly productive and we shall not consider them further.

The sales department endeavors to secure business for the
company, that is, to obtain contracts for making bridges and
buildings. In competition they submit figures based on esti-
mated weights and drawings obtained in the engineering depart-
ment. See Arts. 51, 53, and 57. If the salesmen " land " the
job, the engineering department also makes the preliminary
order, Art. 63, the detail drawings, Arts. 58 and 62, and the
final lists of materials, Arts. 63, 64, and 65.

The order department relists material and procures it from
the mill. Operating department (the shop), cuts, punches,

*See "Roofs and Bridges," by Merriman and Jacoby, Part III, Chap. IV.
See Eng. Record, Vol. XLVTII, pp. 360 et seq., pp. 620 et seq.

80 ELEMENTS OF STRUCTURAL DESIGN

rivets, and paints, thus transforming this material into beams,
ties, and struts. Inspection department examines them to be
sure of their accord with drawings and specifications. When
passed, shipping department weighs them, then loads on cars
which take pieces to their destination. Here erection depart-
ment put them into final position. Financial department col-
lects the contract price, pays the men, and subdivides cost.

Art 35. Plant in General

The site of a plant should be approximately level and must
have ample shipping facilities. Freight charges on raw material
from mills to plant plus that on finished material from plant to
site is to be kept as low as possible. Abundance and cheap-
ness of labor are important.

The framework of the building is usually steel. Any standard
roofing or siding may be employed. For the latter, brick or
concrete is preferred, but they are expensive and hard to alter.

The grouping of the buildings should be such as to reduce
cost of handling to a minimum. It is usual to so arrange
the yard that raw material begins at one end and gradually
works through to the other, coming out as the finished product.
This keeps length of haul down to a minimum. To economize
on cost, systems of narrow gage yard tracks should be installed,
see Fig. 35. In connection with these, there are numerous
bridge and jib cranes, Fig. 36. Together the track system
and the cranes do all of the heavy moving and a great deal
of the lighter.

Raw material is unloaded by cranes from cars on the siding
to stock yard. Art. 36. When shop is ready for fabrication,
the cranes load it on the push cars whence it is taken through
the shop. At each place where work is to be done, it is lifted
off cars and afterwards replaced. Another crane at the other
end of the works, loads on cars for shipment.

The principal buildings are:

Offices, Main Shop, Forge Shop,

Power Plant, Machine Shop, Foundry.

Templet and Pattern Shop,

FABRICATION OF STRUCTURAL STEEL

82 ELEMENTS OF STRUCTURAL DESIGN

The offices are preferably located at entrance, near enough
to the works to be convenient, yet far enough away to escape
the smoke and noise. A large part of this building, usually
the upper floors, will be devoted to drawing rooms. These
should be light and well ventilated. Space must be provided,
not only for the regular estimating and detailing forces, but
also for the erecting and operating departments. The former
designs tools and appliances for erection work, Art. 49. The
latter performs a similar office for the equipment of the
shop.

In the power house are located the necessary boilers and
engines. The steam which the former produces may be used:

(a) Directly.

(i) To warm buildings. (2) Power. (3) Water supply.

(b) Or converted into:

(i) Electricity for light. (3) Pneumatic pressure.

(2) Electricity for power. (4) Hydraulic pressure.

(ai) and (^3) are outside the scope of this book. (02) is
not common. Electric lighting consists of arc lamps for general
lighting and incandescent for the individual tools. Electricity
for power may be furnished by motor to machines through
direct connection or line shaft and belts. Generally speaking,
compressed air is used for portable drills, reamers, riveters,
chippers, and so forth, also for furnishing draft, for sand blast,
and for painting. Hydraulic pressure is employed in forging,
upsetting, and riveting — ^80 lbs. per sq.in. for pneumatic and
800 for hydraulic represent average practice. Occasionally
two systems of different pressures, either in air or water may
be used.

We shall not consider the foundry here as its principles
have been taken up in Art. 16.

FABRICATION OF STRUCTURAL STEEL S3

Art. 36. Stock Yard. (Fig. 36.)

Material is unloaded by cranes from railroad cars and
deposited in the stock yards. Size, length, and contract number
are marked thereon and it is placed where it can be found when
needed. The yard may or may not be under cover. Some

Fig. 36. — Pennsylvania Steel Co,

shelter must, however, be provided for the machines. These
are principally straightening rolls. There are two kinds, one
for plates and another for angles. Either consists of a number
of adjustable rolls between which steel is passed. Other shapes
may be straightened by hammering. Shears and cold saws
sometimes occur but they are more appropriately described
under the head of " Main Shop,"

ELEMENTS OF STRUCTURAL DESIGN

III "

FABRICATION OP STEUCTURAL STEEL

Art 37. Main Shop. (Fig. 37a and b.)

Here the main operations of fabrication are carried on.

At the end nearest the stock yard, a space is reserved for
laying out material, either on the steel itself, or by templet,
Art. 40.

Next come the shears. These may be

(1) Beam, (3) Plate, (5) Cross.

(2) Angle, (4) SpUt,

FiC. ijb. — Main Shop, Pennsylvania Steel Co., Harrisburg, Pa.

In shearing, two strong plates are so adjusted. Fig, 37c,
that they just slip by one another. When cut is made, it must
begin at edge, and successively shear off the material. The
supports and means for handling different shapes vary.

In the beam shear, a number of different blades are necessary

86 ELEMENTS OF STRUCTURAL DESIGN

in order to fit various sizes of I beams and channels. It is
not a common tool.

An angle shear is shown in Fig. 37^. May be single or double

as shown. In the better class of machines,

^_. — J I they are made to revolve so that a skew cut

~| [I may be made without moving piece They

should be powerful enough tocut an8"x8"Xi"

ShearBl^s. ^ngle.

Fig- 37* represents a plate shear. This too
may be fixed or revolving. A fully equipped shop will have
one machine capable of cutting i2o"Xi" plate.

Fig. 37<(.— Double Angle Shears, Long & Alstatter, Hamilton. Ohio.

There are also small shears as seen in Fig. 37/. The blades
may be parallel to the axis of the machine (cross shears), or
perpendicular thereto (split shears).

Among the accessories, we may name the circular cold
saw and a machine for planing the rough edges of sheared
plate. Also the coping machine, Fig. 3jg, which shears off the
flanges of I beams and channels; after this has been done,

FABRICATION OF STRUCTURAL STEEL 87

remainder is a plate and may be easily taken off by an ordinary
shear which is a part of the machine. See Fig, ng'.

Next come the punches, which may be classified as fol-
lows:

88 ELEMENTS OF STRUCTURAL DESIGN

Fig. 37/.— She&r, Bdrd Machlnci)' Co., Pittsburg, Pa.

Fic. 37g,-K;oping Madiine, Long & Alstatter Co., H&milton, Ohio.

FABRICATION OF STRUCTURAL STEEL 89

(i) Single. {3) Rack or spacing (4) Multiple.

(2) Gang. tables. {5) Special devices.

Essential idea of a punch is a rod of steel passing through

Fig. 371. — Single Punch, Long & Alstatter Co., Hamilton, Ohio.

a hole just a trifle larger. If now a plate be put between the
two, the former forces out an irregularly shaped piece of metal
as shown in Fig. 37A.

90 EUIMENTS OF STRUCTURAL DESIGN

(i) is shown in Fig. 37*. Twice the distance between tool

and back of throat is the width of the largest plate it can punch.

(2) is similar to (i) except that there are several punches

instead of one. It may be used for any standard grouping
of holes.

In (3) there are a number of punches in a line, usually

four, with a device that enables any combination to be used

FABRICATION OF STRUCTURAL STEEL 9!

at one stroke. Tlie shape which is being punched is fed forward
by a mechanically operated table. One type has, at the side
of track for table, numerous pieces of steel. A lever in the
side of table engages one of these pieces. When released by
attendant, it catches on next piece. Of course, these must
be carefully set.

(4), Fig. T,TJ, is like (3) except that it is larger and has more

Fig. 37*- Fig. 37/.

Punching Washer Fillers. Punch for Lattice Bars.

punches. They sometimes have sufficient width for 120"
plate.

Among the special devices, we may mention a machine
with two dies of different diameters. This punches out material
as shown in Fig. 37ft. The numerals sipiify the number of

Fic. 37ni.— Pneumatic Drill, Chicago Pneumatic Tool Co., Cbic^o, El.

the Stroke. Another machine has a punch as shown in Fig.
37^. It is used to cut out lattice bars as shown in Fig. 56X.

Drills are used for boring holes in metal. Reamers are
fluted tools employed to enlarge a hole already formed. Both
may be used in the same machine. There are four common
types;

(i) Pneumatic drill. (3) Radial drill.

(2) Ordinary drill press. (4) Boring mill.

92 ELEMENTS OF STRUCTURAL DESIGN

(i), Fig. 37m, b a small compressed air engine or turbine,
turning a shaft to which is attached the tool. It obtams its
air from the supply lines by means of armored hose.

(2) as shown in Fig. 37», is the usual drill press.

Fig. 37«.— Ordinary Drill, Baird FiG. ,170.— Radial Drill.

Machinery Co., Pittsburgh, Pa.

(3), Fig. 370, is hardly less familiar. It consists of small
drills on swinging arms so arranged that they can move back
and forth. They are mounted in gangs and their support may
be movable.

(4) There are two kinds of boring mills: the horizontal and

Fig. S7p. Fig. 37?.

Toob foe Boring Mill.

the vertical. They are much like drills and can be employed
as such. In general, they are used for large holes. The tool
for boring eyebars is shown in Fig. 37^. One like Fig. 37?
will enlarge a punched hole.

FABRICATION OF STRUCTURAL STEEL 93

Methods of driviog rivets will be taken up later. There
are four types of machines;

(,i) Percussion. (3) Pneumatic hydraulic.

(2) Toggle joint. (4) Hydraulic.

A percussion riveter, Fig, ^■jr, is a piece of steel forced back

Flfi. 3Tr. — PneumSilJc Riveter (without tool), Chicago Pneumatic Tool Co., '
Chicago, m.

and forth like a steam pile driver or a rock drill. This impinges
on a stationary die which forms the head of the rivet. Motive
power is compressed air from the company mains.

In (2), Fig. 37s, an air piston works on a toggle joint, thus

Fio. 371. — Toggle Joint Riveter, Chic^o Pneumatic Tool Co.,

Chicago, III.

producing required pressure. Satisfactory adjustment is quite
difficult

(3). Here the air piston acts on a plunger which in tium
compresses oil. This drives the piston carrying the riveting die.

94 ELEMENTS OF STRUCTURAL DESIGN

(4), Fig. 37^. This consists of two jaws, on each end of which
is a die. One of these is attached to a powerful hydraulic piston.
Air may also be used here.

(i) is portable. (4) is
usually fixed, while (2) and {3)
may be either. Whether it is
best to make riveter portable
or not depends upon size of
piece handled.

Among other machines, we
may mention the end milling
machine, or rotary planer.
Fig. 37W, in which a rotating
wheel carries in its circum-
ference teeth which remove

„ , ,. „. n",etal to form a smooth sur-

FiG. 37i. — Hydrauiic Riveter. , ,- ■ .

face, bometmies there are

two planers so made that they can be put a fixed distance

£^>art. Again they are mounted to rotate in order to plane

to a bevel.

I Fig. 37i(.— Rotary Planer, NOes-Bement-Pcmd Co., NewYork.

m ^

Yia. iyv Fic. 37ai. Fic. 37J.

Stiffeoers. Tool for Fitting. Tool tor Fitting

Chipping work is done by a pneumatic tool similar to
riveter except that a chisel is substituted for the die.

FABRICATION OF STRUCTURAL STEEL 95

' Another machine which may be found in main shop is one
for fitting the stiffeners of a plate girder into the flange angles,
Fig. 3711. The superfluous metal may be removed by a special
milling machine with a cutter like Fig, 37«i, or a shaper with
tool like Fig. 377, See next article for a deflnition of these
machines.

Art 38. Machine Sbt^

The functions of a machine shop are threefold:
(1) To do mechanical work on structural parts. Such are
drilling, boring, planing, turning, and' milling. It will be
noticed thai; many of these operations occur in other shops.
This is because it saves time and labor when handling heavy
pieces to keep tools near by. We find then that the machine

Fig. jSa. — Lathe, Baird Machinery Co.

shop takes up this work only when members are light or there
is a great deal to be done on them. Among such, we may
mention the turning of pins and rollers, the threading of pins,
pin-nuts and pilot nuts, the planing of base plates and cast-
ings, and so forth.

(j) To do machine work where a part of a structural job.
Such, for example, is the machinery in turntables and movable
bridges. Another example is the making and repair of erection
tools.

(3) To manufacture and maintain such machines and tools
in the plant as are not purchased outside.

The idea of machine work is either to cut holes or recesses
in metal, for example, a slotted hole, Fig^ 61b, or a key seat; or

96 ELEMENTS OF STRUCTURAL DESIGN

to make a surface exact. Thus a roller must be truly cylin-
drical, and surfaces which slide must be exact planes.
The principal machines found here are:

(i) Drill presses. (3) Lathes. (5) Planers.

(2) Boring machines. (4) Shapers. (6) Milling machines.

(1) and (j) have already been explained in preceding article.

Lathes, Fig. 38a, have a revolving support and a tool-holder

which travels parallel to piece but is made adjustable to bring

Fig. jSi.— 34" Crank Shaper. Queen City Machine Tool Co.

tool nearer to or iarther from work. As is obvious, they finish
a surface, generated by the revolution of a straight, broken,
or curved line about the axis of rotation. They may also be
employed to make screw threads.

In shapers, Fig. 386, piece has slow lateral motion, while
tool in an adjustable support moves across the work. This
vnll finish plane and cylindrical surfaces generated by a straight,
broken, or curved line; its principal use is for plane surfaces.

Planers, Fig. 38c, form the same kind of surfaces as shapers.
However, the piece moves instead of the tool which has a slow

FABRICATION OF STRUCTURAL STEEL

Fig. 38£.--Cindnnati Planer Co.

ELEMENTS OF STRUCTURAL DESIGN

lateral motion and is adjustable. In general, planer is employed
on large work.

Milling machine, Fig. 38^, is one for forming exact surfaces
by means of revolving tools or cutters.

An average allowance for plates is 1/16" for each planing.

Fig. sSe,

Fig. 38/.

Plain Rollers.

In ordering hot rolled rounds, add f" to turned diameter; for
forged, add i'\

As an example of machine work, let us take rollers as used
for large expansion bearings. All exterior surfaces except

Fig. 38g.

Segmental Rollers.

Fig. 38A.

ends must be finished. For Figs. 38^ and /, rollers are sawn
from roimds, and then turned in a lathe. Figs. 38^ and h may
be planed and turned from a round, from a rectangular bar,
or from a forging of about the same shape.

Art 39. Forge Shop. (Fig. 39.)

Here we make rivets and bolts, upset eyebars, manufacture
miscellaneous forgings, and do general blacksmith work. We
find in this shop, rivet- and bolt-making machines into which
the heated rod is thrust and from which it emerges in small
pieces, cut to proper length, and with one end upset to correct
shape. Here are located machines for making nuts and putting
thread on bolts.

Where the eyebars are made, we find first the furnaces

FABRICATION OF STRUCTURAL STEEL 99

in which the raw material is heated, next the upsetting machine
which enlarges head to the final shape. Then we have a punch
which, while the head is still hot, trims off the irregular pro-
jections and punches a hole one inch smaller than finished
diameter. There are also straightening machines as already de-
scribed and annealing furnaces to take out any internal stresses

Fig. 39. — PartiaJ View of Forge Shop, American Bridge Co., Ambridge, Pa.

caused by preceding treatment. Near them is located a boring
mill, composed of two drills, the distance between which is
adjustable.

For the production of miscellaneous forgings and general
blacksmith's work, we find the usual equipment. Special
mention might be made of the steam hammer and machines for
bending steel to a form, " bulldozers."

100 ELEMENTS OF STRUCTURAL DESIGN

Art 40. Templets *

Steel work consists in the main of the rolled shapes, which
we have considered in Chap. II, cut to exact dimensions. For
the purpose of fastening these different shapes together, holes
are placed at frequent intervals. These cuts and holes, made
as shown on drawings furnished by the engineering department,
are located in four ways:

(i) By laying out on the steel directly.

(2) By templets.

(3) By rack or multiple punch. (See Arts. 37 and 44.)

(4) Special processes.

(i) is the best method where there are only a few holes
in a long piece, or where there are only one or two pieces. The
mechanic works directly on the steel putting a punch mark
wherever a hole is to go while the cuts are indicated by a series
of such punches; (2) Templets (" template " is also correct)
are pieces of wood designed to be clamped on the steel while
the necessary dimensions are transferred from it. Holes about
i'' diameter are bored in the wood opposite those which are
to be punched in the steel. Pasteboard or cast iron may also
be used as a material.

As an example of (4), we may mention a pimch with a
special table, carrying a plate to be punched and a pattern
plate. When a die attached to machine is placed in any hole
in pattern, the punch is directly over same spot in other plate,
and accordingly clutch may be thrown in and hole made as

usual. Thus this machine saves
preparation of templet, and the
work of laying out.

A very common term in tech-
nical work is right and left.
Fig. 40a. ^^ about any plane as an axis.

Left. Right. we construct a solid symmetrical

with a given solid, the original
is called " right " while the other is " left," Fig 40a.

Axiom I. The position of the axis is immaterial, as the
resulting left will be the same for all positions.

* Eng. News, Vol. LV, p. 326.

FABRICATION OF STRUCTURAL STEEL

101

Axiom 2. The left of a body containing a plane of symmetry
is the same as its right.

Axiom 3. The left of a composite body is composed of
parts each of which is a left of the corresponding parts in the
original.

The templet for a small plate is a piece of wood of exactly
the same area but not necessarily the same thickness. That
for a large plate is usually framed together like a truss with the
pieces so placed as to contain the holes.

The templet for an angle is made as shown in Fig. 406.
If the angle be right and left, it is constructed like Fig. 40c;
in case there are holes in one leg only, as in Fig. 40^; if there
are no holes in a part of one flange, that portion of the templet
may be omitted altogether; or the one piece may contain
layout for both legs.

KS3BS

UUUll

I..

Fig. 406. Fig. 40c.

Fig. 4od. Fig. 4oe. Fig. 40/.
Templets.

Fig, 40^.

The templet for the top of a channel or I beam is a plain
flat board. For the web of the latter it is a board to which
are nailed cross pieces containing the holes and just fitting
the fillets, Fig. 40^. Templets for channel are seen in Fig. 40/.

On the templet is marked the size of section, length, diameter
of holes, identification mark, and nmnber required.

Templet making is expensive and therefore should be econ-
omized as far as possible.

(i) Design as many pieces as possible so that they may go
through the rack punch.

(2) If there are two members of the same length but slightly
different loads, it is customary to make them alike but strong
enough for either. Often a great deal may be saved by bearing
this point in mind during the design.

(3) If pieces are not alike, they may be made enough so
that a single templet can be used for both. For example

102 ELEMENTS OF STRUCTURAL DESIGN

the stresses in the laterals of a bridge decrease toward the
center. One templet will do for all panels, however, by using
the same sized legs throughout and varying their thickness
and by omitting certain rivets; the templet should be marked
to show variations. Cuts occurring in one member but not
in another may be indicated as in Fig. 40^.

The engineering department gives measurements in either of
two ways:

(i) Give dimensions sufficient to completely determine
everything.

(2) Give general center to center distances, size of all
material, number of rivets required, together with maximum
and minimum edge distances.

(i) is generally the best method since in case repair or
additions are decided on, (2) gives no definite knowledge of
the location of the rivets.

A templet shop is usually a well-lighted room with plenty
of space for full size layouts. It should contain a full assort-
ment of carpenter^s tools, numerous benches, a boring machine,
and a knife for cutting off material.

Art. 41. Methods of Cutting Material *

(See Art. 37 for description of machines)

Shapes other than plates, angles, I beams, and channels
are usually sawn. However, squares and rounds may be
sheared by a special blade. One important point comes in
here. It is practically impossible to shear off a distance less
than half the thickness of tlie metal.

Plates are cut either by punching out and chipping, or by
one of the shears. The latter should always be used where
possible, as the former is very expensive. To see if a given
plate can be cut by the shop equipment, draftsmen ought to
have a scale drawing, such as is shown in Fig. 41a, of the various
shears. Note that piece must be reversed to finish shearing.
A trial with the plate drawn to the same scale will soon show
if it may be cut off in the machine. If impossible, it must be

* Reference for Arts. 41-47, Eng. News, Vol. LV, p. 356.

FABRICATION OF STRUCTURAL STEEL

103

punched out as shown in Fig. 416, and chipped ojBF, which is
very ejqpensive.

Avoid re-entrant cuts, abc, Fig. 41c. Sinee the shear cracks
material beyond where it has cut, abc must either be punched
out and chipped or else a hole made at b and then ab and be
can be sheared. This is expensive on account of extra process

WisS^''

Fig. 41a.

Diagram of Gate
Shear.

Fig. 416.

Fig. 4i<;.

Punching Out Re-entrant
Plate. Cut.

Fig. 4i(f.

Diagram of Angle
Shear.

involved. Do not make any angle much less than 90°, as such
a comer tends to curl up when shearing.

Angles are usually cut by the angle shears. It saves a
great deal of time and often obviates shearing entirely if they
have square ends. As will be seen by a study of Fig. 4 id,
there is a limit to the skew at which an angle may enter the

Fig. 4ie.

Fig. 41^.

Fig. 4 1 a.

Cuts for Various Shapes.

tool. If beyond the limit, angle should be cut off square,
and one leg trimmed in plate shear as shown. Fig. 4ie, two
processes instead of one, and that much more expensive.

Fig. 41/ shows three kinds of skew cuts. The " front cut "
is the usual one and angle shears are designed for it. . The " back

104 ELEMENTS OF 8TRUCTUBAL DESIGN

cut " is more costly but can be done. The re-entrant cut is
entirely impracticable.

An I beam or phannel may be cut in four ways:

(i) Beam Shears, (2) Coping Machine. (3) Cold Saw.
(4) Punching.

(4) is ejqpensive and is done only where the other equipment
is lacking. (3) does good work but takes time, (i) and (2)
are cheap and good enough. If the beam is to be cut by a
plane perpendicular to the web but beveled to the plane of
the flange, it may be cold sawn or coped out approximately
to requ red lines as shown in Fig. 41 g. If cut by a plane per-
pendicular to flange but beveled to web, it may be cut by cold
saw, if latter is large enough or coped out to lines shown in
Fig. 41A.

Important principles developed here are:

(i) Avoid small cuts.

(2) Do not use re-entrant angles.

(3) Keep cuts to a minimmn.

(4) Use square ends imless important reasons determine
otherwise.

Art. 42. Methods of Bending

Small rods and thin plates are bent by drawing into position
in the assembled structure. In other cases the part must be
heated. If much bending of a certain radius is required, two
dies of that radius are prepared, one of which is fastened to a
plunger. The heated section is placed between them and the
stroke of the plunger completes the job. Or the hot shape
is hammered or forced by levers into the desired form, the
templet being used as a pattern.

A shape may be bent in a plane containing its axis or so
as to change the section. As an example of the latter class
let us take the connection of two I beams framing together
at an oblique angle, Fig. 42a. If angle of intersection lies
between 85 and 95 degrees, it is customary to use a bent angle.
Otherwise a plate is bent the required amoimt. It is very
difficult to make a sharp corner of the latter construction.
Generally it will be rounded to a radius of a few inches.

FABRICATION OF STRUCTURAL STEEL

105

Crimping, Fig. 426, is the offsetting of an angle and is best
done by a machine.

Loop rods may be either circular or square in section. The
latter gives better contact and is preferred for that reason.
They are used only for the purpose of carrying tension between
two pins and are commonly made adjustable as explained in
Art. 43. To fasten them, the rod is bent back on itself and firmly
welded, the distance between pin and weld being equal to

Fig. 42a. Fig. 426. Fig. 42c.

Bent Connections. Crimped Angle. Single Loop Rod.

Fig. 42</.
Double Loop Rod.

two or three diameters of the former. This fork is sometimes
made double as shown in Fig. ^2d,

Hangers are short members carrying a direct load. A
common construction is to make them like a short non-adjustable
rod. The breaking of a hanger supporting a floor beam in a
deck railroad bridge near Forest HiUsi Mass., on the Boston
and Providence R. R. caused a bad wreck, Art. 69, and hangers
have not been used in first-class construction since. Indeed,
in a well- designed structure,
little reliance is placed on a
weld joint, its most important
allowable duty being for wind
stresses.

Buckle plates are plates stiff-
ened by pressing between large
dies into form shown in Fig. ^2e.

When bent cold within the elastic limit, the length of the
neutral axis remains imchanged; above it, the tension side
stretches while the compression side does not alter very much.
When bent hot, much depends on the manipulation of the
metal by the blacksmith.

Forge work injures steel considerably, although subsequent
annealing may remove some of this.

A shape can be bent to almost any radius, but the sharper

Fig. 42e. — Buckle Plates.

106 ELEMENTS OP STRUCTURAL DESIGN

this becomes, the greater the cost and the loss of strength
and the less desirable is the appearance.

We find then that it is. difficult to do good work in bending;
that it is wasteful and weakening. This conclusion will
apply to all hand forgings, hence hand work in the forge shop
is to be made a minimum.

Art 43* Process for Upsetting

This has already been 4!^fined in Art. 25. We there spoke
of one of the two cases, the other being that of eyebars. The
upset for the screw end, whether the shape be round, square,
or flat, is made roimd and of sufficient size after thread is cut
to give at least 20% excess of area, this being to allow for loss
of strength due to forging, as explained in preceding article.
The length of the rod necessary to form this upset may be
readily computed if we allow 10% for losses due to heating
and trimming.

Adjustable members are sometimes made by cutting in two
an ordinary member, upsetting and threading each end so cut^
and inserting a tumbuckle, Fig. 43a, or sleeve-nut, Fig. 436^

^31

Fig. 43a. — ^Tumbuckle. Fig. 436. — Sleeve Nut.

In each, the; thread must be right hand in one half and left.
in the other. The former has the advantage that the position
of the ends may be determined by inspection. Eyebars are
made by placing a bar in a furnace, heating the end for several
feet, and upsetting. Bar is next annealed, that is, heated to a
dull cherry red and then allowed to cool very slowly after
which they are taken to the straightening rolls. Sometimes
a hole somewhat smaller than final is punched while metal
is hot and then it is drilled to exact size, or sometimes entire
hole is drilled' at one operation.

The bar now presents the appearance shown in Fig. 43c.
The diameter of the pinhole is commonly made 1/50 to 1/32

FABRICATION OF STRUCTURAL STEEL 107

of an fnch larger than size of pin. The thickness of the bar is
usually the same throughout, but the head is sometimes
thicker than the body of the bar. The net area of the hole
should be at least 30% in excess of the area of the bar itself.
The head is usually made of arcs of circles, as shown.

><^t

Fig. 43c. — Eyebar.

Length of upset; diameter of thread; dimensions of turn-
buckles, sleeve-nut, and eyebar heads may differ a little, each
company having its standards.

An adjustable member may also be made by upsetting and
threading each end and attaching thereto a clevis nut which
connects with a pin which in its turn passes through the con-
nection plate. Method of manufacture is as follows: The
iron or steel is first piled up as shown in Fig. 4^6. In the die
imder the steam hammer, it takes, after being heated, the
shape seen in Fig. 43^. Reheated and hammered in another
die it takes the form of Fig. 43/. The smith then bends the

(inr rn

Fig. 43^ Fig. 43^. Fig. 43/. Fig. 43^.

Successive Stages in Manufacture of Clevis Nuts.

ends close to the center imtil they become parallel. The
holes for the pin are next drilled and the end is threaded, and
the nut appears in its finished form as shown in Fig. 43^. Each
company has its own standards, the same clevis doing for several
sizes of threads or pins, the details being such as to make the
nut stronger than the bar.

108 ELEMENTS OF STRUCTURAL DESIGN

Art 44. Methods for Making Holes

(See Art. 37 for machines)

Holes may be either punched or drilled.

A punch is likely to break if metal is thicker than diameter
of hole. Do not try partial holes, Fig. 44a; it is difficult to
seciu-e good work and the punch will not long endure this
treatment.

In detailing members where rack or multiple punch will
be used, care must be taken to keep rivets in horizontal and
vertical lines. There is a minimum allowable distance :n each
direction, due to the construction of the machine. If spacing
is less than that, templets must be used, adding quite a bit

Fig. 44a. — Partial Hole. Fig. 44b. — ^Typical Matching of Holes.

to the expense. Rack or multiple punching does not pay
where there are but few alike, for small pieces; or for skew
work. Other cases should be so arranged that shop can use
it if they so elect.

Punching is the cheaper process, but it distorts and injures
metal, and holes are likely to match poorly. Further, when
several pieces are joined together, the resulting hole, Fig. 44J,
is irregular and rivet does not thoroughly fill it. To overcome
these objections, we may:

(i) Drill from solid, or

(2) Enlarge by reaming.

For the former, there are four methods:

(la) Pneumatic drill.

(16) Drill press.

(ic) Radial drills.

(id) Boring machines.

(16) and (id) are much alike, the latter being used for large
holes, as already noted, (la) is portable but not as effective

FABRICATION OF STRUCTURAL STEEL 109

as (lb), (ic), however, combines the good points of both.
Pieces may be laid down underneath the radial drills, while
the latter, attached to a small gantry crane, are moved along
it, drilling holes as they go.

Now drilling from solid is quite expensive and is seldom
done except for cast iron or where metal is almost as thick as
diameter of the rivet. The advantages of drilling without its
disadvantages may be obtained by sub-punching and reaming,
that is, by punching a hole about | inch smaller than original
diameter of rivet, and then enlarging it to 1/16 inch more.
This enlargement may be effected by using either of first three
machines mentioned in drilling, with the same or different
tools. For reaming also the radial machines are best.

Reaming is done after assembly because it insures a good
rivet and takes care of several holes at once. It is often required
for field rivets. There are two methods of doing this: by
reaming all parts which connect, to a common iron templet;
also to put them together at shop, ream, and then matchmark,
so the same connection will be made in the field. The latter,
while expensive, is obviously preferable and is now quite common.

Pinholes in eyebars or built-up members are punched out
and then enlarged in a boring machine. A slotted hole, Fig.
616, may be cut out by a special punch, or two holes may be
made at each end, and remainder cut out by shaper.

Important principles are:

(i) Avoid partial holes.

(2) Holes having a diameter greater than thickness of
material must be drilled to avoid breaking punch.

(3) Different sized holes are a fruitful source of expense
and annoyance. See also Art. 47.

Art. 45. Layout and Assembly

Material which requires bending is sent to blacksmith's shop
and thence to the room of the layer-out. Straight stuff passes
directly thereto. Some pieces will be taken to spacing tables,
others punch-marked by measurements on steel itself and
perhaps the larger part will be laid off by templet. Art. 40.

Layer-out is supposed to be as economical as possible of mate-

ilO ELEMENTS OF STRUCTURAL DESIGN

rial. In a large job, involving a great deal of, let us say, 1 2" X i'^
plates, it will be ordered in about 30 foot lengths. It may be
that there are many different lengths and hundreds of pieces
to be cut from this. Perhaps there is only one possible way
of doing this. The bills of material, Art. 63, give what he is
to cut with as little waste as possible. He must also make
necessary allowances for fitting and milling. Irregular plates
may often be sheared to save a great deal of material. Thus
a plate like Fig. 45a should be cut out as seen in Fig. 4S&,

/\\

Fig. 4sa. Fig. 456.

Method of Shearing Skew Plates.

The material is next sheared, Art. 41, then punched or
drilled, Art. 44, and passed on to assembly. Surfaces which
will afterwards be in coixtact now receive a coat of paint. The
different parts are next " assembled," that is, fitted together
and fastened with a sufficient nmnber of bolts to hold firmly
while rivets are being driven. Though forbidden by most
specifications, holes which do not fit well are persuaded by
tiie use of " drift pins " (pointed pieces of steel), backed up
by sledge hammers. A better way and one which is now used
more and more, is to sub-punch and ream out after assembly
(Art. 44).

It is particularly desirable for this class of work that drawings
should be clear, views properly shown, and notes explicit and
easily imderstood.

Art. 46. Fastenings for Steel Work

While there are rivets having different styles of heads as
shown in Fig. 46(1, they are not common in structurai work,
the button head being the universal type. This is slightly
less than a hemisphere. It may be modified as indicatal in
Art. 47.
- Bolts, Art. 25, 7&, are also used in steel work. The hexagonal

fAPRICATION OF STRUCTURAL STEEL

111

head is lighter and is preferable on account of its better appear-
ance and lesser clearance required for tightening. Unlike the
wood the hole in the steel must be made some, larger, and since
the bolt is not heated for driving, it does not fill the irregulari-
ties as does the rivet. This objection may be overcome by drilling
the hole and turning the bolt a few thou-
sandths smaller. It is then called a '^ turned
bolt.'*

A tap bolt in steel corresponds to a lag
screw in wood. It may have a square or
hexagonal head but no nut, and is used to
fasten one object to another where it is Unusual Rivet Heads,
impracticable to get at the nut end. The
piece that carries the screw end is said to be tapped, that is,
it has a hole bored in it a little more than the length of the
bolt and a female thread is then turned thereon.

A stud bolt is a tap bolt with a thread and nut instead of
a head on the outer end. A hook bolt. Fig. 4.6b, has instead of
its head a form bent as shown. It is often used in fastening
ties to the beams on which they rest. A U bolt is made as
shown in Fig. 46c.

Fig. 46a.

Fig. 46^.
Hook Bolt.

* «

Fig. 46c.
UBolt.

Fig. 46J. Fig. 46^.

Ragged Bolt. Swedged Bolt

Fig. 46/.
Expansion Bolt*

Foundation bolts are made in various styles, although the
plain bolt is about as good as any of them and much more
economical; however, it is sometimes threaded;, sometimes
ragged, Fig. 46^, swedged. Fig. 46^, or is made as shown in
Fig. 46/, expansion bolt.

Pins are large specially designed bolts with both ends threaded
and a nut placed on each. The usual t)TDe of a pin is shown
in Fig. 46^. Each company has its standards, but the
distance fe is usually equal to the metal which it grips, while
h and h are each made equal to thickness of the nut plxis a

112

. ELEMENTS OF STRUCTURAL DESIGN

quarter of an inch or a little more. The nut is always hexagonal^
and has a long diameter of about 4/3 that of the pin and
thickness of ij". 62 is about 3/4 of di.

Pins are turned and threaded from rounds of medium steel
1/16 to 3/16" larger, this amount varying with shop practice
and with size of pin.

Sometimes a cast-iron washer about i" in thickness and
slightly larger than the large diameter of the pin is placed
at one end. In that case I2 and either h or h are each made

F16. 46g. — ^Typical Pin. Fig. 46/1. — Lomas Nut. Fig. 46». — Special Pin.

y longer. The idea of this is to provide for variations in grip
from its computed length. A still better method of accom-
plishing the same purpose is to use the Lomas nut, Fig. 46A, which
is about the same size as the usual pin nut except it has a
recess of Y' to J" as shown.

Occasionally, for the purpose of providing a smaller hole
in a plate, the pin is turned to a lesser diameter usually to that
of the thread. This may be done on either or both ends, the
latter case being shown on Fig. 461. Or, at the support, projec-
tions beyond the thread may be made in order that jacks be
used to lift the bridge.

Suppose in Fig. 46; that it is necessary to keep the outer
eyebars 2" outside of the inner, as shown. If pin and nut as

given above be used, the eyebars
might move around or rattle.
It then becomes necessary to
use washer fillers, Fig. 46)^.

The p ate is usually J" thick,

and the inside diameter is

about 3/16" more than that of

the pin. In this case we would

order, "2 Pis. is''XJ"Xif '," the 15" being given as the

plate width since the same size pins are often used throughout

a job, hence, all plate for this purpose will have the same width.

Fig. 46;.
Pin Joint.

Fig. 46k. — Washer
Filler for Joint.

FABRICATION OP STRUCTURAL STEEL

113

In order to facilitate erection and protect the threads of
the pin, a point and a cap must be provided. The point is
called a pilot nut and may be either short or long as shown in
Figs. 46/ and m. It is cast of iron or steel and has its outside

•O J 1

Fig. 46/. Fig. 46111.
Short. Pilot Nuts. Long.

Fig. 46n.
Driving Nut.

Fig. 460.
Pin Ready for Driving.

diameter equal to that of the pin, while a female thread to
fit that on the end of the pin is turned inside. The cap is called
a driving nut and is shown in Fig. 46W. As in the pilot nut,
the driver fits both at the thread and on the outside. The
pilot and driving nuts now give the pin the appearance shown
in Fig. 460, and render erection quite easy.

Cotter pins are commonly used with clevis nuts. They
derive their name from the cotter, a small piece of bent wire
as shown in Fig. 46/^, which is inserted at
one end to prevent the pin from coming
out. The other end is about J" larger
in diameter than the body, the whole
pin appearing as shown in Fig. 469. Iz is
usually made J", I2 a trifle more than the
thickness of metal to be gripped, while h
is about i". The material is ordered exact to length and of a
size of round equaling diameter of head. It is then turned
down and a 7/16" hole for a f " cotter pin placed as shown.
Fig. 46q.

Fig. 46^.
Cotter.

Fig. 4W.
Cbtter Pin.

Art 47. Methods for Riveting

(For machines, see Art. 37)

The rivets described in preceding article may be driven by :

(i) Machine riveter. (2) Pneumatic hand riveter.

(3) Hand tools.

The hot rivet is " entered " into hole from one side and
firmly held there until tool on other side has upset the head.

114 ELEMENTS OF STRUCTURAL DESIGN

At each end is a bar with a nearly henuspherical recess as seen
in Fig. 47a. Two sides may be cut away or an octagonal bar
used. The head mashes up so that f " should be employed
instead of i" in figuring clearances. , . .

In (i), both ends are held by machine, large power is exerted,
and it makes an excellent rivet very economically. Hence
always design so a machine riveter can ht employed as much
as possible.

In (2), cylindrical end is upset by the riveter. The other
end may be held by a, short heavy bar about in line with rivet,
by a longer piece called a dolly bar, Fig. 476, used as a lever,
or by a pneumatic " holder-on." The latter is a bar with a
die in the end which is forced into position against the rivet

Fig. 47a. — ^Rivet Die. Fig. 47b, — Dolly Bar.

and held there by pneumatic pressure. There are several
forms of- holding bars in order to provide for driving in difficult
locations; they may be bent or inclined at an angle to the
rivet.

Thisr process makes good rivets but they are not as strong
as those driven by machine if proper care is used in adjustment
of the latter. The advantages of this process are the portability
of the riveter and its adaptability to almost any condition.

In (3), same tools are employed on holding end. The
driving is accomplished by a die set in a handle. This die
is much like those we have already studied except that it is
quite short and head is shaped to receive the blows of the sledges.
Resulting rivets are weaker and more expensive than those
driven by air and this method is now seldom used; only in small
erection work and in locations not accessible to the pneumatic
riveter.

In driving the. rivet, it is pushed in on the holding side
and upset on the driving side. It is best when it can be entered
and driven either way. In machine driving, some feet are
required in both directions for clearance in line with rivet axis,
rather more, being necessary on driving side. Measurements

FABRICATION OF STRUCTURAL STEEL

115

will vary for different machines, and actual details of riveter
should be consulted in doubtful cases. For pnemnatic riveting,
24" is desirable on driving side and 12" on holding, but both
can be lessened by special tools, a riveter of this type bringing
former down to 12". This applies to cases where rivet is
backed by a pneumatic " holder-on." This is not always
necessary, but should be employed if pneiunatic process is used
in driving long rivets. On the holding side distance can usually
be made as small as entering of rivet will permit. The latter
applies to hand riveting also, but on the driving side, room
must be provided to swing sledges.

Fig. 47c. Fio. 47^. Fig. 47*.

Transversely to the axis of the rivet, clearance must be
provided for die as already described. In certain locations,
parts of machine are to be cleared. Consider, as an example,
the vertical rivets in the flanges of plate girders, as seen in Fig.
/^^c. Here the distance d for machine driv'ng should be about
2" and e 2J". In determining clearances, consider only rivet
heads that are close by, say within 2 or 3 inches. Those 8 or
10" away may be ignored entirely. Thus in Fig. 47^, to drive
a after other rivets, consider head h but not d, and probably
not c. This interference might be prevented by staggering
as in Fig. 47^. Following table gives approximate value for
clearances:

Hand Driving.

Machine Driving.

Diam. Rivet.

Minimum.

Desirable.

Minimum.

Desirable.

i"
1"

li"

li"
ij"

li"
li"
i|"

li"

Clearance required for driving rivets, for erection, or perhap>s
for other purposes, may require a head somewhat less in height

116 ELEAIENT8 OF STRUCTURAL DESIGN

than full head described in preceding article. It may be^
Fig- 47/:

Tzr MI \J V7

Fig. 47/. — Special Heads.

1234
Flatten Flatten Flatten Countersunk

tof". toi". toi". and chipped.

(i) Head flattened to f ''.

(2) Head flattened to i", and hole countersunk to obtain
the necessary strength.

(3) Head flattened to J". Here rivet is pounded down
about flat but is not chipped off, care being taken that the head
does not exceed J".

(4) Head countersimk and chipped flat.

Note well — (i) involves hammering down rivet either by
hand or a flat die. (2) and (3), hammering down and boring
out of hole (countersinking). (4) means hammering with
special die, countersinking, and chipping if special head is on
the driving side or a countersunk head rivet and countersinking
if on entering side. From the above, it will be seen that coimter-
sinking rivets adds largely to the cost.
Points to be kept in mind are:
. (i) Be sure to provide sufficient room in all directions
aroimd riveter. Be a little liberal in cases where there is close
work on more than one side. Provide for entering and holding
one way and driving on the other. Proceed similarly for bolts
except to substitute clearance for turning nut in place of driving.

(2) If thickness of material gripped by rivet (grip), be
more than four diameters, there is likelihood of unsound work.
Avoid therefore long rivets.

(3) Countersunk rivets shoxild be reduced to a minimiun
and if possible eliminated altogether. The additional operations
are but a small part of the expense; it is the changing of dies
and the care necessary to look after these special rivets.

(4) Different sized rivets in the same piece mean different
dies for both pimching and riveting. This is costly and a source
of much bother. To avoid proceed as follows: For each job,

FABRICATION OF STRUCTURAL STEEL 117

pick out a certain sized rivet, usually the largest which may
be driven. This is about J" for small work, J" for mediimi,
and i" for heavy construction. Design sections so that one of
these may be used throughout. Sometimes it will be cheaper
to increase a section and use more metal than to have varying
sizes of rivets. When the connections between parts of a structure
are few, different sizes may then be used in those parts. In
this case, the line of demarcation should be so chosen that the
fewest pieces in number and the smallest in
size will have to be pimched and riveted twice. ^m/^

(5) If rivet spacing exceeds 6" or 16 times yig. ah-
the thickness of Uie thinnest outside plate, there Buckling of Plate,
is danger of buckling as shown in Fig. 47^.

(6) Use only as many rivets as are necessary for strength
and stiflPness. Shop driving alone costs 2ff apiece, and total
expense is not far from 5ff.

Art. 48. Inspection, Painting, and Shipment

Inspection is of two kinds; that controlled by the structural
concern and that in the interest of the purchaser. We shall
consider inspection of material as outside of this work.

The inspector for the structural company examines each
piece and ascertains if outside dimensions and open holes agree
with drawings. His principal objects are:

(i) To ensure acceptance of the piece by purchaser.

(2) To correct deviation from plans which would delay
completion in case his concern erects bridge.

The work of the purchaser's inspector is much more difficult.
Moreover, he is judge between structural company and pur-
chaser as to inspection and fulfilhnent of contract as shown
in the plans and specifications. His field is:

(i) To allow only approved material to be used. Ordinarily
this too has been inspected.

(2) To ensure agreement of material with that called for
by strain sheet.

(3) To go over joints and ascertain if pieces will fit together

* Reference for Shipping and Erection, Eng. News, Vol. LV, p. 381.'

118 ELEMENTS OF STRUCTURAL DESIGN

in. the field. Not only should measurements be compared with
those on the drawings, but the latter should themselves be
checked with one another.

(4) To enforce specifications in regard to workmanship.
Material must be straight, holes properly punched, and surfaces
inaccessible after assembly painted. Inspector should watch
milling, boring, planing, and riveting, testing the latter by
hammer to be sure that they are tight.

(s) Just before painting, it his duty to make a detailed
comparison of each piece with plan,* and have any errors cor-
rected.

(6) To inspect painting, weighing, and shipment.

On important jobs there may be an inspector of erection.
His duties will be to see that steel work has not been injured
in transit, that pieces are put in proper position, and all field
rivets are well driven.

Material should be cleaned of rust before painting. On
important work, it may be removed by pickling or sand blast;
it is sometimes taken off by a coat of gasoline before the paint.

Paint is usually applied by hand with a brush. It may
be done by compressed air, which sprays paint on the steely
(also on anything else in the vicinity). It is wasteful, unhealthy^
and does not do the work as well but is economical of labor.
Pins, pin holes, screw threads, and rollers should be coated
with white lead and tallow.

Steel is next weighed and this compared with computed
weight, a variation of 25% being allowed. It is then loaded
on cars and shipped to its destination.

We shall take up details of preparing pieces for shipment
only so far as it concerns designer and draftsman. Pieces
less than 40 feet long, 8 feet wide, and 9 feet high may be
shipped almost anywhere. If the width or height of a piec^
exceeds these limits, examine clearance diagram, Art. 50, sa^
for roads over which the job must travel. Longer parts njay
be supported on two cars. Pieces as long as 130 feet may be
carried by inserting idler or spacing cars. For such lengths,
conditions on curves must be investigated. Plate girders are
placed upright on blocks about 6" high to which they are
firmly braced. ' .

FABRICATION OF STRUCTURAL STEEL 119

The following points should be borne in mind when shipping:
(i) Pieces which, like long plate girders, are difficult to

handle, should be shipped so that turning end for end at site

will be avoided.
. (2) Small parts are likely to be lost in shipment; they

may be boxed up, bolted together, or fastened onto a piece

With which they connect.

(3) Ship projecting plates or shapes only when necessary
to avoid expensive field riveting; otherwise they may suffer
injury or interfere with economical shipment.

(4) Freight rates run about as follows: The minimum rate
per pound can be obtained when loaded with not less than 30,000
for one car or 45,000 for two. If cars are all occupied, not
less than these amounts must be paid for. If only a portion
of one car be taken, shipment is made at " less car load " rates
which are higher per pound. It is hence sometimes better,
in spite of the objectionable field riveting, to ship a small job
" knocked down " (in small pieces) and pay pound prices for
it rather than full car rates. Remember, however, that the
latter is prompter and less likely to result in the loss of a
piece, while the former is much easier to team and to handle.

(5) Sometimes purchaser pays freight. Do not forget to
guard the interests of your client as far as it is honorable to do so.

Shipping bills are described in the next chapter, Art. 65.
They are used as a guide by inspector, shipper^ and ..erector. .
When everything on this bill has .left the shop, its part of the
contract is completed. In the iiext article, we shall take up

the work at the final site.

"■*•'•■■ * ' ' "

Art. 49. Erection *

Assembling the structure on its site and fastening together
is styled the erection. In this article, we shall describe methods
for plate girders, viaducts, and simple bridge trusses. Processes
peculiar to other types will be considered in chapter dealing
with same.

The heavy weights may be handled by block and tackle^
jacks, gin-poles, derricks, gallows frames, and travelers.

♦ See Dubois* "Mechanics of Engineering," Vol. II, Chap XIII.

120 ELEMENTS OF STRUCTURAL DESIGN

HydrauHc jacks have a short stroke and are not thoroughly
reliable. On account of the latter, blocking must always be
used to take up the lift. They may be had with a capacity of
800,000 poimds.

A gin-pole, Fig. 49a, is a sunple strut of timber or steel,
guyed by two or more ropes at the top and supported at the
bottom. The hoisting rope runs from a crab near the foot
over a sheave at the top and down to the load. It is employed
to raise or lower a weight, and is limited as to height of lift by

Fio, 4911,

upper sheave. Horizontal motion is difficult but can be obtained
by manipulating guys with block and tackle. May be moved
from place to place by lifting bodily with a derrick, by sliding
foot and paying out or pulling in guy lines, or by taking down
and setting it up again.

An A frame is much like a gin-pole except that mast is made
of two inclined struts braced together and that less guys are
needed, one being sufficient. Methods of lifting and moving
are also similar.

Steel and wooden derricks, Fig. 496, guyed either by wire
ropes or stiff legs are familiar to all. They are moved like gin-

FABRICATION OF STRUCTURAL STEEL 121

Fig. 49&— Derrick Erecting 85-foot Plate Girder on Western Maryland R,R,

Fig. 4<)«. — Derrict Car, American Bridge Co., Ambridge, Pa,

122 ELEMENTS OF STRUCTURAL DESIGN

poles. These not only raise or lower the load, but they can place
it almost anywhere within a hemisphere whose center is the
foot of the mast (the upright piece) and whose radius is the
boom (the movable piece). This is likely to be somewhat
limited by (a) Lack of strength in derrick for certain positions
of the load, (b) Construction. For example, guys might inter-
fere, (c) Objects on the ground.

Derricks mounted on a car, that is, derrick cars. Fig. 49c,
are very efficient machines. They are commonly provided with
a hoisting engine, air compressors, and full sets of tools. The
mast is designed to collapse and the boom if a long one is made
telescopic; hence they may be shipped from shop to site like an
ordinary freight car. It can move slowly under its own power

even with load. Its field of action
is a semi-cylinder whose axis is a
line through the foot of the mast and
parallel to the track, and whose radius
is the boom. When raising loads at
one side, the other may be prevented
Fig. 49J.-Use of Auxiliary ^^om upUft by loading an auxiUary
Boom. boom. Fig. 49^, by fastening one

wheel to track, or by " outriggers.'*
The latter are beams temporarily bolted transversely to the
car with ends supported, loaded, or fastened, to counterbalance
the eccentric load.

A gallows frame is shown in Fig. 49^. It is guyed at the top
and supported at the bottom. A limited amount of motion
may be secured by manipulating guys or by varying pulls on
the two ropes which support the load. In the main, however,
its office is to raise or lower an object.

Travelers are very common in bridge work. There are three
types: the ordinary traveler, the cantilever traveler, and the
creeper.

The former. Fig. 49/, consists of several gallows frames
braced together so as to be independent of guying. Sheaves
at the top provide necessary number of hoists, while wheels
on the bottom allow of longitudinal motion. It can carry a load
between any two points within its limits. The inside lines must
clear the truss which it is proposed to erect.

FABRICATION OP STRUCTURAL STEEL

Fic. 49e.— Unloading Plate Girders by Gallows Frame.

Fig. 4f/.— Ordinary Traveler, American Bridge Co., Ambridgc, Pa.

124 ELEMENTS OP STRUCTURAL DESIGN

A cantilever traveler, Fig. 49g, is one which overhangs. Its
essential elements are two projecting trusses well braced together
and mounted on wheels, with sheaves at one end and engine
and counterbalancing weight at the other.

A creeper is shown in Fig. 49A. It is a small derrick, mounted
on wheels which run on the top chord of the truss.

There is a great deal of variety in the design of these erec-
tion tools, and capacity and measurements differ widely. Space

Fig. 4gj, — Garttilever Traveler. American Bridge Co,, Ambridge, Pa.

does not admit our taking up the subject of design of even the
framed structures. In general, these may be made of either
wood or steel. Follow conventional methods and use customary
allowable values for highway bridges. (Vol. II,) Special
care must be taken to get maximum stresses imder the varied
conditions met in service.

There is even more variety in the situation of the site and
in the ingenuity displayed in overcoming obstacles of various
sorts. We shall attempt, however, to give only common repre-
sentative methods.

FABRICATION OF STRUCTURAL STEEL 125

Three factors are of very great importance:

(i) If the site can be reached by rail or navigable waters,
it may be termed accessible. Otherwise, it must be shipped in
small sections.

(2) If the traflSc must be maintained except for some inter-
ruptions of a few hours each, it might be designated continuous
traffic; such are the difficulties attending construction of this
sort Uiat it frequently pays, particularly with highway bridges.

Fig. 49A. — Creeper Traveler Erectfag Cantilever Bridge at Beaver, Pa.
Taken from A. R. Raymer's paper on the "Pittsburgh and Lake Erie
Railroad Cantilever Bridge over the Ohio River at Beaver, Pa.,"
in Vol. LXXIII. Trans. A.S.C.E., opp. p. 156.

to build a temporary structure near by for use during erec-
tion of bridge. It may then be treated as a case of interrupted
traffic.

(j) Delivery of material. It may be brought onto tracks
running over structure which it will replace, or alongsidej under-
neath, or at one end of final location.

126 ELEMENTS OF STRUCTURAL DESIGN

ERECTION OF PLATE GIRDERS *

There are five methods, the first three for new structures
or interrupted traffic, the fourth and fifth for continuous traffic.

(j) Launching, Fig. 49*, for inaccessible positions where
derricks of sufficient capacity are not available and where
material is delivered at one end.

(2) Girders may be lifted directly into position (Fig. 49a).

(j) Tracks, supported by a temporary wooden structure
called falsework are built across proposed span. Girders are
then suspended, one on either side of flat cars, brought directly
over their final position, and carefully lowered.

{4) Girders may be brought to site on flat cars and unloaded
at or near their final position by overhead hoists supported by
falsework.

Ill v^

Fig. 49J. — Erecting a Plate Girder by Launching.

(5) Girders may be unloaded at one side and shoved or
lifted into place as old bridge is removed, travel being temporarily
suspended.

Up to the capacity of shipping facilities and erection tools,
entire structure is preferably riveted up complete. The girders
themselves can be shipped in parts, but this is very rare.

The girders may be placed a little more than their proper
distance apart, the intermediate pieces put in position, and then
the former moved up to fit. A much better way is to make a
design in which all intermediate pieces may be " swung in "
with girders in final position.

ERECTION OF VIADUCTS f

These have seldom been built to replace old structures.
Material is commonly imloaded at one end and there reloaded

* Engineering Record, Vol. LIX, p. 494 et seq.
t Ibid, Vol. LXI, p. 429.

FABRICATION OF .STRUCTURAL STEEL 127

onto contractor's cars, which deliver it to erector's gang. The
latter generally employ a derrick car or a cantilever traveler.
Beginning at one abutment, they erect in order: the first bent,
the first span, second bent, longitudinal bracing, second span,
third bent and so on. Or the first tower, the first span, the
second span, second tower, and so forth.

ERECTION OF TRUSS BRIDGES

Small trusses may be shipped complete or riveted up at the
site and handled like plate girders.

There are four methods of erecting larger bridges: *

(i) Falsework, Fig. 49;. This is the usual way. The
falsework is composed of wooden trestle bents. Posts and caps
are about i2"Xi2" with 3"Xio" bracing, resting upon piles,
or, in favorable ground, mud sills. On top of these bents are
supported the traveler and the blocking on which the bridge is
placed. One proceeding is to begin at or near center, erect
panels there on both sides, connecting up with lateral bracing.
From there trusses are erected towards fixed end. Traveler is
next brought back to center and erection carried on towards
free end. Another method is to erect floor system, fasten trusses
to floorbeams, in order previously given. After the latter are
complete, blocking is removed and bridge settles onto its shoes.

Other three methods are employed where falsework would
be impracticable on accoimt of depth or rapid movement of
stream.

(2) Cantilever. This case is shown in Fig. ^gk, where the
two shore spans would be erected by falsework. The center
span would be built as seen in the picture. Toggle joints are
provided at a and wedges at ft. Trusses are built a Uttle high
and are dropped into position to connect by means of toggles
and wedges. One disadvantage of this method is the extra
material sometimes needed to carry the erection stresses.

(j) In end laimching, the bridge is erected on shore. When
finished, it is pushed forward on rollers, the projecting end
being sustained by a float.

{4) In floating, the bridge is built on falsework carried by
scows sunk in the water. Latter is pumped out and structure

ELEMENTS OF STRUCTURAL DESIGN

Fig. 43r. — Falsework Supporting Bridge during Erection, American Bridge
Co., Ambridge, Pa.

FABRICATION OF STEUCTURAL STEEL

12&

is towed to a position just over the final one, and valves in the
bottom of scows are opened.

Pins are fitted with pilot and driving nuts (Art. 46) and driven
by means of a wooden maul or a heavy suspended timber.

When assembled, bolts are placed in the rivet holes. Gen-
erally but a fraction of the open holes are so filled. Enough,
however, must be inserted to carry the maximum stresses which
will occur before riveting.

Rivets may be driven by hand or a pneumatic hammer
(Art. 37). The former is now confined to very small jobs or
places not accessible to the latter. Field riveting is much more
expensive than shop, costing 5 to 15 cents apiece, or-even more.
Jt varies a great deal with circumstances.

Fig. 49ife. — Erecting a Truss by Cantilever Method.

The designer should always consider erection and have at
least one simple and safe method in mind. Take the plans of
the apparatus which will be used and see how every piece will
be placed in position. Bear in mind the following principles:

(a) Make field riveting a minimmn. It is much more expen-
sive than shop yet not as strong.

(b) However, this should not make pieces too large or too
heavy to be readily handled or shipped.

(c) Avoid, if possible, groups of a few rivets in inaccessible
locations. It may often cost more to build the platform on
which the workmen stand than it does to drive the rivets.

(d) Consider how each bolt, pin, and field rivet will be
entered and fastened. For an example of difficult driving, see
inside field rivets in Fig. 49/.

(e) Avoid entering joints. Fig. ^gm. This might be obviated
by attaching one or both angles to projecting webs.

(J) Allow ample clearances where it will do no harm. Do

130

ELEMENTS OF STRUCTURAL DESIGN

not attempt to get closer to an interference than one-half Inch
unless it will look bad or weaken the structure.

(g) Where a horizontal member frames into a vertical
surface, for example, connection of stringer and floorbeam, an
angle on which to rest the former is a great help.

Fig. 49/.
Interior Field Rivets are Hard to Drive.

Fig. 49m
Entering Joint.

(A) Consider how anchor bolts will be placed. The best
method is to set them in holes drilled after erection. See that
enough room is allowed for drill and hammering same.

(i) Avoid members very much alike but not exactly so; a
nusplacement may be very expensive.

CHAPTER V

THE ENGINEERING DEPARTMENT

Art so. SpedJBcations *

The clauses which define a contract are called the Speci-
fications. In its completest sense, the word covers the neces-
sary legal forms, statements of amoimts and limits of work,
permissible materials, and rules of procedure. It is mainly
in the latter that we are interested. These are used not only
as a part of the agreement, but as a guide to contractor's engi-
neers. They may specify simply the finished istructure, leaving
designer and builder free to exercise their judgment; or they
may cover the minutest details and processes. An intermediate
course is better. Specifications should be complete, concise,
and clear. Useless directions add to the cost, while meager
allow inferior work.

As an example, we will give a set of specifications for raibroad
bridges. This will follow somewhat closely present (191 2)
average practice. Notes of occasional differences and other
comments will be enclosed in brackets.

Sample Specifications for Railroad Bridges

(a) Description

(7) Up to 30 feet. Rolled I beams.

Preferred 30 to 100 feet. Plate girders.
Types 100 to 175 feet. Riveted Trusses.

Above 17s feet. Pin-connected trusses.
Deck bridges shall be used where conditions
permit.

* See Cooper's 1906 Specifications for Railway Bridges; Ostrup's Standard
Specifications.

131

132

ELEMENTS OF STRUCTURAL DESIGN

(2)

Preferred
Depth

(?)

Stringer
Spacing

(4)
Spacing of

Girders and

Trusses

Depth shall not be less than following

amounts:
Plate girders, one-tenth span.
Trusses, one-sixth span.

Stringer spacing shall be 6 feet 6 inches
center to center, except for curves or
some other special reason.

Center to center spacing shall be as follows:
Deck girders, one-twelfth span but not less

than 6 feet 6 inches.
Through girders to suit clearances.
Trusses, not less than one-twelfth span.

Fig. 50a. Fig. 506.

Typical Clearance Diagrams
Single Track. Double Track.

(5)

Clearance
Diagrams

Ties and
Guard Timbers

Bottom line in Figs. 50a and b represents
base of rail. Widths to be increased to
give the necessary clearance on curves.

Ties shall be of white oak or yellow pine
eight inches wide and spaced as near six
inches apart in the clear as is practical.
Make depth one-tenth the span but never
less than eight inches. Minimum notch
over supports, one-half inch. Fasten every
third tie to stringer or girder by |" hook
bolts. Guard timbers to be eight inches
wide and six deep notched to 4" over ties
and fastened by J" bolts to every third
tie.

THE ENGINEERING DEPARTMENT

133

(I)
Dead

Loads

(2)

Own
Weight

Live
Load

(J) Loads

Dead loads shall be computed in all cases.
Allow 125 lbs. per ft. per track for rails.
Timber to be considered as green and
estimated as given in Art. 2. Assume
ballast to weigh 1 10 lbs. per cu.f t.

Stresses due to own weight must be allowed
in the design.

[Lnpact, which is often included in specifi-
cations, is covered by formula for imit
stresses.]

The live load per track shall be taken as
Cooper's E 50, as given in Fig. 50^. Weights
are in thousands of pounds.

^§^§

H 13 ^ i ^ § ^ e ^ § § Hi

jfi 9099 00094 9999 99 99 1

(4)

Centrifugal
Force

.(5)

TRACTIVfe

Force

Fig. soc. — Loading Cooper's E 50.

Centrifugal force, F, shall be assumed to
act at a point 6 feet above base of rail.
Let Z?= degree of curvature, IT = live load
on bridge, ?> = velocity in feet per second.
(Assume a speed in miles per hour of 60— 3Z)).
F = Wv^/gr. Substituting :
z> = (60—51)) 5 280/3600 feet per sec.
g = 32.2 ft. per sec. per sec.
r = (s76o/Z)) in feet.
We may derive the following formula :
F^.042DW{i-o.iD+o.oo2sD^).

A force equal to one-fifth the live load on
the bridge shall be considered to act
horizontally along base of rail in either
direction.

The wind shall be taken as 30 lbs. per sq.

134

ELEMENTS OF STRUCTURAL DESIGN

(6)
Wind

ft. acting on structure and train or 50 lbs.
per sq.ft. on former alone. Consider all
trusses but only one plate girder to receive
this pressure.

Let us take C to represent the safe unit
stress for quiescent tension equals 9000
for soft steel, 10,000 for medium steel, and
15,000 for 3 per cent nickel steel. Let

(I)
Allowable

Unit

Stresses

Combinations

(5)

Other Allow-
able Values

3f = i+4

minimum stress

^maximum stress'

i?=maximiun slendemess ratio

_ unsupported length
least radius of gyration'

Tension on net section, CM

Compression on gross section, CAf(i — .oo62?)
Shear on shop rivets, pins, and

gross section of webs, 2CM/^,

Shear on field rivets and bolts, CM/2.
Bearing on shop rivets and pins, 4CM/3.
Bearing on field rivets and bolts, CM.
Bending on pins, 1.5 CM

Other flexural stresses, CM.

The fraction minimum/maximum shall be
considered as negative if there is a reversal
of stress. Members are to be designed
for either (a) 80% of maximum stresses
due to dead, live, centrifugal, tractive, and
wind, or (b) the first three. The larger of
these two values must be employed.

For wood, see Art. 6. Allowable stress per
lineal inch on medimn steel rollers is 300
times diameter in inches. For bearing
on masonry, 300 pounds per sq.in.

THE ENGINEERING DEPARTMENT

135

Accessibility
AND Drainage

(2)

Minimum
Thickness

(3)
Grip

(4)
Section

Design

(d) Design

Sections and details shall be accessible for
cleaning, painting, and inspection, and shall
not retain water.

Except for fillers and lacing bars, minimum
allowable thickness of metal is | inch.

The grip of the rivets shall not exceed 5
times its diameter (Art. 47).

Avoid large sectional areas which receive
their stresses indirectly [e.g., cover plate
on top chord, Fig. sod, should be kept as
thin as conditions permit. This is because
compression distributes itself unequally,
giving the heaviest unit stresses in the webs,
W, and the least in plate, P.]

Fig. sod* — Distribution of Stress in a Built-up Section.

(5)

Angle
Connections

Net Area
AT Pins

(7)

Net
Section

(8)
Counters

If angles be connected by one leg only,
its area will be considered as that leg.
This clause shall not operate, however, to
reduce the radius of gyration.

Riveted tension members shall have a net
area at pin equal to five-fourths that in
body of piece.

In deducting rivet holes, diameter is to be
taken as one-eighth inch greater than
nominal diameter of rivet. In staggered
spacing, use actual area along the zigzag
line if it will be less than that in one plane.

Counters shall be designed for 25% addi-
tional live load and a 25% increase in allow-
able stresses.

136

ELEMENTS OF STRUCTURAL DESIGN

(si)

Maxdium

Length

Compression

Members

(lo)

Transverse

Loads

Top Flanges

Plate

Girders

(12)

Top Flange

Rivets

(t3)
Computation

OF Plate

Girders

Maxiinuin slendemess ratio for members
carrying wind or tractive forces only, 125;
for other pieces, 100. K slendemess ratio
for any part of a compound column exceeds
that of column as a whole, the ratio for that
part shall be used. The two compression
chords of truss or plate girder bridges shall
be similarly treated as a large column with
wind bracing for lacing. [Clause 4a is
intended to eliminate the possibility of
lower stresses by this method.]

Bending stresses due to own weight in com-
pression members may be neutralized by
eccentric arrangement of joints (Art. 56).
Other transverse loads are preferably
avoided but must be allowed for if they
occur.

Top flanges of beams and plate girder must
have their allowable stress reduced by the
compression formula if unsupported for a
distance greater than 15 times width of
cover plate.

Rivets in the top flanges of deck plate girders
and stringers shall be computed for result-
ant shear and vertical load. The heaviest
wheel of the loading is considered as dis-
tributed over three feet.

In computing plate girders, web shall be
designed to carry its share of the bending
moment. Compression and tension flanges
shall be made alike, and not less than one-
third of flange area shall be in angles and
side plates.

Stiffeners shall be in pairs. At bearings
and points of concentrated loading, they
shall have sufficient capacity to carry the
load as a colvmm. If thickness of web is

THE ENGINEERING DEPARTMENT

137

(14)

Stiffeners

(15)

Bbaong

Rivet
Spacing

(2)

Field Rivets

Latticed

Compression

Members

more than one-fiftieth of the depth, other
stiffeners may be omitted. If not, they
shall be spaced one-half to one-third depth
apart at ends and depth apart at the middle.
^At intermediate points, use a proportionate
spacing.

Complete upper, lower, and sway lateral
systems shall be provided where possible;
under other circumstances as in through
structures, bracing will be designed to be of
the necessary strength and as eflGicient as
practicable. Solid floor shall be considered
as a lateral system in its own plane. Struts
located at or near shoes shall be capable
of resisting temperature stresses,

(e) Details

Rivets may be |, J or i inch diameter, d.
Let / equal thickness of thinnest outside
plate, then:

Max. edge distance = 8/,
' ^ spacing = 16/, but not more

than 6 inches.
Min. edge distance = 1.5^,
' ' spacing = 3d.

When staggered, spacing is the shortest
distance center to center of rivets. Above
rules for maximum do not apply to two
angles riveted together.

Nimiber of field rivets shall be kept as low
as possible. (Art. 49.)

A latticed compression member shall be
figured for a uniform transverse load equal
to 1/30 strength of member as a short
strut. (Art. 56.) It shall have as near
ends as practical, batten plates not shorter
than greatest width of member, and not

138

ELEMENTS OF STRUCTURAL DESIGN

(4)

Riveted

cobcpression

Members

(5)
Spuces

Rigid
Meicbers

(7)
Camber

(8)
Shoes

thinner than 1/50 of transverse distance
between rivets. Longitudinal sparing on
batten plates shall not exceed 4J.

The transverse distance between rows of
rivets in plate shall not exceed 40/; longi-
tudinally, the space will not be more than
4d for a distance from ends equal to twice
the greatest width of the member.

All joints must be fully spliced except the
milled ends of short well-braced columns.
For the latter use two rows of rivets on
each side of joint on all four faces.

Verticals which carry tension shall be designed
as stiff members. In riveted trusses,
tension members must be battened or
latticed. Connection of floorbeam and
trusses must be rigid.

To provide a camber for trusses, make top
chord longer than bottom by J inch in
every 10 feet.

Provision for an expansion of J inch in 10
feet shall be made. Spans over 75 feet
must have hinged shoes, fixed at one end
and rollers at the other. The diameter
of these rollers in inches shall exceed by
3 the span ia feet divided by 100. Rollers
and pins shall be made of medium steel.

(2)

Punching AND
Reaming

(/) Workmanship

Workmanship shall be first-dass in every
respect.

Where material is thicker than diameter of
rivet, hole must be drilled from the solid.
All other holes for shop and field rivets
shall be punched J inch smaller than nomi-

THE ENGINEEBINa DEPARTMENT

139

Rivets

(4)
Turned Bolts

(5)
Sheaked Edges

(6)
Stiffeners

(7)
Upset

Ends

(8)

Holes

Adjustable
Members

nal diameter and reamed to i/i6 inch
larger after assembly.

All rivets must be tight, completely fill the
hole, and have full round concentric heads.

When replacing rivets, bolts must be turned
to fit.

Sheared edges of medium steel plate over f
inch thick shall be planed.

Stiffeners and their fillers must be fitted to
flange angles.

Welds are forbidden. Eyebars and upset
ends shall be annealed. Strength of either
of the last two must exceed that of body
of bar.

Holes shall be placed in center of member
unless otherwise shown; clearance of pin
in hole shall be 1/50 inch if diameter be
less than 5 inches; 1/32 inch, if more.
Distance between holes shall not vary
more than one-twenty-thousandth from
true length.

Adjustable members shall be avoided where
possible. When necessary, use turn-
buckles.

Shop
Paint

(g) Painting and Erection

All steel before leaving shop shall be cleaned
of rust and given one coat of paint. Sur-
faces which will be in contact afterwards,
must be painted before assembly. Pins,
pin holes, screw threads and rollers shall
be coated with white lead and tallow before
shipping.

ELEMENTS OF 8TEUCTURAL DESIGN

Parts not accessible after erection shall

(a) receive one coat at shop and one at site

Field before erection. Other parts shall receive

Paint two coats in their final position. Painting

shall not be done in wet or freezing weather.

(j) Pilot and driving nuts must be used in

Pins driving pins.

[For the saie of conciseness, we have omitted
paints and other materials. In specifying
them, use, if possible, some standard
specifications.]

Art 51. Piobiem of Design

The site of the proposed structure should first be surveyed
and a map prepared. The object of this is:

(/) To locate the structure in the most economical position.
{2) To enable its details to be specified in advance.
(j) To determine quantities and estimate cost.
Property lines, buildings, contours, soundings, and borings
should be taken. The latter ought to extend to rock, and if
there is doubt as to its integ-
rity, should be examined by
the diamond drill, which with-
draws a core for examination.
Consider every possible loca-
tion, a few rough measurements
often showing the impractica-
bility of some designs. It
may be necessary to place the
structure at a particular spot,
but it is even then better to
make the survey for reasons
Fio. SI.— Alternative Routes for (^) ^"^ 0)-

Railroad Location. To indicate the necessity

for thorough information, let

us take the case shown in Fig. 51. The problem is to locate a

railroad between the two towns and design the necessary struc-

THE ENGINEERING DEPARTMENT 141

tures. The cost of operating would be least if alinement and
grade were straight from one town to the other. But first cost
would probably be lessened by:

(ai) Going farther up the valley and using a shorter bridge
but a longer Une.

(^2) Cutting deeper into the hill at the towns, and so lessen-
ing depth in valley.

(as) Lowering grade in valley.

Various modifications and combinations of these should be
worked out.

(61) Consider now that alinement and grade for railroad are
chosen — shall we use a viaduct or a bridge with abutments or
fill all the way across, leaving only a culvert? In the first two
cases, where will it be economical to stop the fill and begin the
viaduct or bridge?

(62) Suppose location and ends of fill to be settled, shall we
use timber, steel, stone, concrete, reinforced concrete, or com-
binations of these?

(63) After material is selected, what shall be the spans?
Shall they be imiform, making superstructures aUke, or shall
they increase as valley becomes deeper? We might use one very
long span or many very short ones, but obviously the former
would result in an exceedingly expensive bridge, while the latter
would give many abutments and pierSj thus increasing cost.

(64) Closely allied to this is the type of bridge. Considering
now only a steel bridge, shall it be arch, suspension, cantilever,

or simple truss? Shall we use the through or the deck structure?

(65) Type having been determined upon, what should be the
depth and what the panel length?

(Je) Coming down now to the design of the members of the
truss and taking the individual pieces, what combinations of
structural shapes will carry the stresses economically and make
easy and efficient joints?

(67) Even in the details of the latter, there are various types
each having its own advantages.

In this example, we have indicated only a few of the questions
which are interwoven with the problem of design. Some, it
will be noticed, are a little outside the province of the structural
engineer. We will answer them all in the same way:

142 ELEMENTS OF STRUCTURAL DESIGN

Choose the most economical design which gives the

KEQXJIRED DEGREE OF SAFETY.

For elements of cost, see Art. 27. (ai), (a2), and (az) belong
to Railroad Engineering. In (61), speaking roughly, we would
stop fill at a depth where cost of fill per lineal foot became
greater than cost of bridge. But it must be remembered that
maintenance charges are much lower for a properly built fill
and its sinking fund would be .zero. These operate therefore to
increase depth where fill stops.

(62) Timber is cheaper in its first cost but maintenance and
renewal charges are higher. Properly built masonry bridges
have neither maintenance nor sinking fund charges, but their
first cost is high, they cannot be used for long spans, and they
are ill suited to foundations on compressible soils such as are
quite common. We shall confine ourselves to steel structures
in the remainder of this treatise.

The rest of our work will be taken up in discussing the
remaining points for different structures. Special reference
may be made to Art. 52 for (fta) and (65); Arts. 54, 55, and 56,
(fte); Arts. 58, 59, 60, (67). ^ •

Very often considerations other than economical ones will
govern. Such, for instance, are the architectural appearance
or legal difficulties. All possible alternatives, however, must
be very carefully investigated. Do not make arbitrary decisions
but prepare estimates of cost \mtil certain that the most desir-
able scheme has been found.

Plans are better if completed before construction is begun,
although it is true that many changes will have to be made
as the work progresses. Estimates of cost can then be finished
with much more ease and economy. These do not have that
uncertainty which is likely to mean high bids from responsible
contractors.

To estimate the cost, quantities are computed from the
preliminary plans. Since these will probably be changed more
or less in actual construction, rules which are only approximately
correct are often employed. Each quantity multiplied by its
estimated price gives the total amount to which something like
ten per cent should be added for contractor's profit.

Often the designing and estimating for the steel work is

THE ENGINEERING DEPARTMENT 143

done by the fabricating company. However, if there is no
competition, the purchaser will pay dearly for advice obtained
in that way. If competitive designs are requested, the buyer
really pays for them all. Structiural steel is let either by the
pound or the lump sum, the latter signifying a fixed amount
for the whole job. In the former case, the contractor may try
to use as heavy material as he can; in the latter, as light as
possible, that is, he tries to " skin " the bridge. To prevent
this, plans and specifications should be thorough and explicit,
simply allowing the necessary latitude for varying shop practice.
Best method is then as follows: The general design and
sizes of all material are determined by purchaser's engineer.
Plans and specifications are next prepared and sent to'prospect-
ive bidders. The latter take off the quantities in the estimating
room and make their own estimate of cost, usually on the basis
of structure ready for traflSc. This with an allowance for profit
is submitted as a bid. The lowest responsible bidder is then
given the job.

Art. 52. Economical Relations

Given a structure of a certain type, there are relations between
measurements which produce the most economical design.

(j) As an example, let us take the case of a plate girder of
a given span and loading. The weight of the web and its
fittings is about constant per square foot. Letting h represent
depth, we may then say:

Total weight of web=CiA.

Using approximate method of computation, Art. 54, the area
of the flange and hence its weight will vary inversely as the
depth, or

Total weight of both flanges = C2/A,

Total weight of girder =Ci A +C2/A.=W

Placing first derivative equal to zero to obtain value of h
which renders weight a minimum:

8TF/8A=Ci-C2/A2=o, .

or Cih=C2/h.

144 ELEMENTS OF STRUCTURAL DESIGN

Hence, make depth of girder such that weight of both flanges
equals that of the web and its fittings.

The above proof assumes, as often happens, that minimum
thickness of web suffices. Where it does not, W=Ci+C2/A.
Hence, for the latter case, increase depth as much as practical
or until minimiun thickness of web is reached.

(2) Let a river crossing have x spans of length L/x and
suppose the foundations to be the same throughout its length.
The cost of the steelwork for the floor will be constant and we
will call it F. That of trusses will vary as square of span and
we will represent it by CxL^/oc^=Cl?/x. The cost of each
pier will be about the same whatever the span and we will call
each P and total amount Px,

The entire structure then has a cost:

£=F+CLVx+Pjc,
8£/8a:=-CLV^+^=o or Px--CD/x.

Hence cost of piers equals that for trusses in an economical
structure.

(j) For the comparison of trusses, we may find the smn of
the products of maximiun stress in each member by its length.
That truss for which this sum is a minimimi is the most econom-
ical. , However, this method does not take into accoimt one very
important fact: that for the same stress, steel compression
members are a great deal more expensive than tension members.
The following example will explain not only how this may be
allowed for but also show method of applying calculus.

Everything in kips and inches.

Allowable imit stresses:

Tension =8.0,

Compression =8.0(1 — ) ,

\ 160 0/

if p = — , then
4

Compression = 8.o( i

40 -4/

THE ENGINEERING DEPARTMENT

145

or
or

Let 5= allowable unit stress in tension or pure compression;

T = total stress ; length of member = L.
Then volmne in tension = TL/S,

Then voliune in compression =AL=TL/Sii jl

SAL^TL+sb/40

A^T/S+L/40,
V^AL^TL/S+Ly40.

Fig. 52. — Economical Height, Pratt Truss.

Mem-
ber.

Total Stress,
T

-6W

+3^^

(i>'+A*)i

-W

+2PF

(^'+A«)*

Length.

(^*+A»)*

(^•+/k*)*

Volume of x,
V

Num'
ber.

6W p^-^-hy p*-\'h*
S A "^ 40

Sh ^ ^40

6Wp*
Sh

$Wk

Wh 1^
S ■*'4o

2W
Sh

(^»+A«)

Total

Total Volume.

S ' h "^ 20
Sh ^ ^20

Wp'

Sh
6Wh

S 40
Sh ^^ S ^10^40

Ih Sh^ S 20 '

146 ELEMENTS OF STRUCTURAL DESIGN

or iSfi^+46oWfr^ = ii2oWp^. '

Here TF = lo dead. S = 8.0. p = 240.

Substituting, A = 247 = 20' — 7".

For doubtful or irregular cases where above methods cannot
be used, make several designs and estimate quantities and
costs. The greater the magnitude of the work, the more need
for time. Do not spend a large amount of money upon a small
or typical structure but follow current practice. A saving of
$50 in material at a cost of $100 in drawing room is poor engi-
neering.

There are very many practical points interwoven with this
question. For example, the most economical length of an I
beam span will be one where a standard depth of the minimum
weight is just loaded to its full capacity. Again in a viaduct of
varying depth, span should change to correspond as indicated
in. (2) of this article. However, spans thus determined might
be too long to be easily erected, and a great deal of time might
be saved in designing, detailing, and fabricating, if they were
made alike or perhaps in two or three different lengths.

■

. Art. 53. Estimating

While, strictly speaking, this includes only the approximate
determination of weights and cost, the estimating room of a
structural company has following functions to perform :

(i) To determine what kind of a structure shall be used
in a given locatioii and what its general dimensions shall be.
(Arts, 51 and 52).

(2) To specify loads and unit stresses. (Art. 50).

(3) To compute total stresses in various parts of the struc-
ture.

{4) To design these members. (Arts. 54, 55, 56).

(5) To outline in a rough way the details.

(6) To estimate weight and cost of the different items.

(j), (2), and (j) ought to be done by purchaser's engineer,
and {4) and (5) are often so handled.

An estimator must hence be thoroughly posted on both

THE ENGINEERING DEPARTMENT 147

theoretical and practical work. His is the highest position in the
engineering department which does not require the handling
of men.

{6) is best done by writing a rough bill of material and thus
getting weight and cost. This, carefully made, is accurate and
detailer can keep his work close to the estimated figures. How-
ever, it is usually done in a hurry, and there is seldom time for
anything but the roughest sort of drawings. Hence the esti-
mator tends to forget certain parts and therefore underestimates.

Time is often too short even for the above. Then the
section may be considered as nmning from joint to joint and
either a fixed length, a fixed amount, or a percentage then
added. This varies a great deal with so many conditions that
we cannot attempt to give values here. It should be figured
out for typical members or abstracted from similar cases. Weight
of rivet heads are often added as a percentage and this too is
quite changeable.

An even more approximate method is to use formulae express-
ing some relation between weight and the dimensions of the
structure. Such is:

1^=0.505(1 -[-0.155.,

as already given for wooden trusses in Art. 28. Or we may
estimate from diagrams plotted from actual weights of com-
pleted structures.

Allowance should be made for waste and for possible varia-
tion of weight in rolling. Where work is to be let by the pound
price, only relative amounts are necessary and accuracy is not
so essential. The cost on each different class of raw material
is determined by consulting price card of steel company. For
other items, a number of very important points arise :

(a) Are members unlike one another or may they be made
the same? The latter lowers cost everywhere and especially
in pattern, templet, and drawing room.

(6) Is the structure skew or square? The former involves
a great deal of expensive blacksmithing, and notably increases
labor in detailing and fabricating.

148 ELEMENTS OP STRUCTURAL DESIGN

(d) Are erection conditions favorable? Is the site handy
to the raihroad? Will unusual equipment be required? Is
there danger of the structure being carried away by floods or
ice?

(e) Is enough time allowed for economical work?

Very rough average cost for erected steel is about as follows —
1912.

Cents per lb.

Material 1.20

Engineering 0.15

Templet 0.08

Shop 0.80

Erection 0.60

General expenses 0.17

Transportation Variable

Or 3.0 cents per pound plus transportation.

The pieces of which a structure may be composed are beams,
tension members, and columns.

Art 54* Design of Beams

Beams may be made of angles, I beams, channels, T beams
zee bars, rails, trough sections, or built-up members.

Plates are sometimes used for flooring. As an example of
their use, let it be required to find allowable span of | inch
medium steel plate when subjected to a load of 500 lbs. per sq.
ft., using specifications of Art. 50. This is a imiformly loaded
beam. Considering a strip one foot wide: Total load = (500+
i5)/i2=43 lbs. per lin.in.

M=Sbh^/6, 5 = 10,000. 6 = 12, A=|.
= io,ooo- 12 •9/(64' 6) = 2810 Ib.-in.
/ = {8M/w)^ = (8 • 2810/43)* = 23 inches allowable span.

Although probably continuous, American practice is to
regard it as a simple span. The minimiun is so small that it
is considered as zero in formula for allowable stress.

Angles, zee bars, and channels are not symmetrical about
center line. There is danger that the load 'may be applied

THE ENGINEERING DEPARTMENT

149

eccentrically and thus cause an injurious twisting, Fig. 54a.
Nevertheless, they are used quite a bit in situations where
their form renders them more convenient. As an example, let
it be required to design a soft steel angle to carry a load of 300
lbs. per lin.ft. for a span of 8 feet. Allowable stress in flexure,
16,000; shear, 12,000, both in pounds per sq.in.

Maximum moment = Wl/S = 2400 • 96/8 = 28,800 in.-lbs.

//c=Af/5= 28,800/16,000= 1.8.

Use s"X3"X5/i6" weighing 8.2 lbs. per lin.ft. with long leg
parallel to the load. Testing for shear:

5= FZaz/6/* = i2oo- 1.66- 3.32 •0.31/(0.31 -6.26) = 1050 lbs.

per sq.in., max. shearing stress. O. K.

t
^

Fig. 54a. — ^Application of Load to a
Channel Used as a Beam.

Fig. 54^.
Separator.

A symmetrical section could be made out of this by fastening
together two angles whose joint capacity would be 600 lbs. per
lin.ft. However, an I-beam would be more economical as the
angles would have to be riveted together and the former would
weigh less.

Thus to design an I-beam equivalent to the two angles
whose combined weight is 16.4 lbs. per lin.ft., we look for one
with a section modulus equal to or greater than 3.6. We might
use a 4" at 10.5 lbs. or a 5" at 9.75 lbs. Assuming as usual
that shear is uniformly distributed over the web, the shearing
xinit stresses on gross area are 2400/(4-0.41) or 2400/(5-0.21) =
1460 or 2280 lbs. per sq.in. They are both O.K. even allowing
for possible rivet holes. It will be noted that while the 5" beam
has less weight it is not quite J" thick. It is likely, therefore,

* Merriman's " Mechanics of Materials/' Art. 108.

150

ELEMENTS OF STRUCTURAL DESIGN

to lack stiffness and has less resistance to corrosion. Either
would effect a marked saving in weight.

Two I-beams may be used as a single beam. They should be
united by bolts which pass through separators, Fig. 546. The
idea is to stiffen the top flanges and make the two act together.
Two I-beams are not as economical as one, but they are sometimes
used where head room is limited or where one is not sufficient.
It should be remembered that the Bethlehem Steel Co. is now
rolling some sections deeper than 24 inches and also special
sections of less depth which have larger resisting moments than
the standard. (Art. 21.) As a measure of this increased
capacity, the following table shows safe loads for spans of twenty
feet; allowable fiber stress, 16,000 lbs. per sq.in. :

Standard Sections.

Bbthlbhbm Sections.

No.

Depth.

Weight

per Foot.

Iba.

Total

Weiffht

per root.

lbs.

Caj^dty.

No.

Depth.

Weight

per Foot.

lbs.

Caj^ty.

24"
24"
18"

80
80

240
160
1 10

278,400

185,600

94,000

I
I
I

30"

24"
18"

200

140

325,000

187,000

95i000

The common type of a built-up beam is shown in Fig. 54c.
The web is usually kept away from the back of angles |'' to J".
Sometimes it is made flush and chipped or milled off. The idea
is to prevent pockets for the accumulation of dirt and moisture.
This makes a good but expensive job. Occasionally the web is
allowed to project. Such may be the case in the supporting
beams (stringers) for a railroad bridge where it makes the neces-
sary notching of the ties (called dapping) easy. This can be
done only where there is no cover plate. The rectangular
plates shown dotted may be added to increase its strength.
Ordinarily, they do not extend the entire length of the girder,
but are cut off to correspond with variations of the bending
moment.

The angles may have equal or unequal legs; in the latter
case, the longe/leg is placed horizontally since it is more effective
in this way.

THE ENGINEERING DEPARTMENT 151

For heavier beams, there are a number of arrangements,
Figs. 54d and e being representative types. Where a heavy load
is to be borne and the depth is limited, a box. girder, Fig. 54/,
may be employed. Sometimes more than two webs are used.

Considering now built-up sections, there are two methods of
computation, the exact and the approximate. The former
assimies that the stress varies as the distance from the neutral
axis, the web as well as the flange bearing its share of the moment.
The latter supposes entire moment to be carried by the flange.

^(Mfvm^ ^flKtan^ ^lyrtamw

rWBD

vweb

Fig. 54^. Fio. 54^. Fig. 54^. Fig. 54/.

Typical Beam Sections.

(a) Design of Web

In either event, shear is considered to be imif ormly distributed
over the web. This is approximately true. We have the formula
for shearing imit stress in lbs. per sq.in.

S^VXaz/bl.

where V = total shear on vertical section in lbs. ;

Zaz = statical moment about center of gravity of part of
section above point where shearing imit stress is
desired — computed in inches;
/= moment of inertia. about center of gravity of entire

section in inches;
6= breadth at same point in inches.

Now as we pass up from center of a beam shaped like a
plate girder, there is no change in V, /, or 6, and there is little
change in 'Eaz imtil flange is reached when stress drops off very

152 ELEMENTS OF STRUCTURAL DESIGN

rapidly as shown in Fig. 54;. That is, as the shear over the
web is uniform and as there is little on flange, we assume entire
amount to be uniformly distributed over web.

(6) Exact Method of Finding Flange Area

Taking up now the exact method, we assume a composition
of the flange which we estimate to be sufficient. We then com-
pute / and c as though it were solid except for rivet holes, and
determine unit stress in lbs. per sq.in. at outside fiber of beam
from the formula:

S'-Mc/I,

where if = bending moment at section in in. lbs.;

c=max. distance in inches from center of gravity to
outside fiber;

7=moment of inertia about center of gravity in inches.
If 5 comes too high or too low, we revise and recompute.

S-tA— Part Fio. 541.— Part of a BijUt

In the formula, S=V"Laz/bI, if we omit b, it represents
the total shear per lin.in. That is, if we should cut an I beam
as shown in Fig. 54A and then load it, we would find longitudinal
motion along plane of cut. To prevent this we would need a
force for every lineal inch, equal to VZaz/I. This force, it
will be noted by referring to the proof, is equal to the difference
per lin.in. of total stresses in cut off part. Similarly, in a built-
up beam, the tendency of some of the shapes to shear off is given
by the same formula. Thus, in Fig. 54*, enough rivets must
be passed through the angles to safely carry this force.

THE ENGINEERING DEPARTMENT 153

If, as often happens, there is a vertical load on top or bottom

of the girder, the vertical stress per lineal inch must be combined

with the horizontal to obtain resultant. It is usual for ease

. of fabrication to make both flanges and their rivet spacing alike,

hence only spacing on loaded chord need be computed.

(d) Approximate Method of Finding Flange Area

In the approximate method, the moment of inertia is twice
the net area of the flange, a, times the square of half the dis-
tance, A, between centers of gravity of flanges. This neglects
moment of inertia of web and flanges about their own centers
of gravity as already explained. Moreover, it assimies that
distance to most strained fiber is equal to \h. Substituting in
flexure formula already giveh :

^ Mc Mih ,-., , ,^,«,

S-=—=—~^^M/ah or a=M/Sh.

(e) ApproxinuUe Method of Determining Rivet Spacing

To investigate the rivet spacing by this method, let a built-
up beam. Fig. 54^, be subjected to the bending moments Af 1
and M2 at distances I apart. Let V be the average shear for
that interval. As already shown the flange rivets have to carry
the change in stress between the two sections. Now the stress
at left section is Mi/h\ at right, Afa/A; their difference is:

(M2-Mi)/A.

But M2=Mi+F/,

Mi + Vl-Mi VI

hence, 5=

A A*

Shear per lin.in.=5=5// = F/A. This must be combined with
vertical shear as before.

(/) Intermediate Method of Finding Flange Area

An intermediate method of finding flange area is to consider
that the web carries its shar^ of the moment and that the center

154

ELEMENTS OF STRUCTURAL DESIGN

to center of gravity of flanges, the depth of web, and twice the
distance from neutral axis to most strained fiber, are all alike.
Then,

M th

)■

where / is the thickness of the web.

To allow for holes in the web, \ is made J. That is, in this
method, we use approximate formula except that we deduct \
gross area of web from flange area required.

(g) Example Showing Approximate Method of Computation

Let us now design in mediiun steel a beam of 30 feet span
to carry a dead load of 700 lbs. and a live of 5000 both per lineal
foot. Use approximate method and stresses as given in Art. 50.
Rivets, J inch dia. Estimating dead load of beam at 300 lbs.
per lin.ft.:

. Min. 1000

iH :rz — = lH : = l.o8.

2 Max.

2-6ooo

To obtain economical depth, we will assume a |" web and
multiply its area by 1.65 to allow for stiff eners and fillers. For
flanges, we will use for comparison net area required.

Af= 6000 .30 -30 -12/8 = 8,100,000 in. lbs.

M/Sh

Depth,
Web.

Area Web.
Sq. In.

ArcftXi.6s.
Sq. In.

Effective
Depth.

Flange

Area

Required.

Sq. In.

Area X 3.
Sq. In.

Total Area.
Sq. In.

30"

11.25

18.6

28"

26.8

53.6

72.2

13 SO

22.3

22.0

44.0

66.3

15. 75

26.0

18.7

37.4

63.4

18.00

29.7

16.3

32.6

62.3

20:25

33-4

14.4

28.8

62.2

22.50

37.1

12.9

25.8

62.9

THE ENGINEERING DEPARTMENT . 155

The theoretic minimum occurs between 48'' and 54'' and
verifies proof in Art. 52. We will take the former. Its weight
per foot adding 20% for rivet heads and bracing, will be :

62.3 •1.20-3.4 = 254 Ibs-

Proceeding with design :

Gross area required in shear = 6000 -30/2 -7 200 = 12.5 sq.in.
Necessary thickness of web = 12.5/48 =0.26". Use f, the
minimimi allowable. Net area required in each flange is 16.3
sq.in. Using a section like Fig. 54^;, and deducting one i"
dia. rivet hole in each angle and two in each plate,

2 Ls, 6''x6"Xi". Gross area = 11.50. Net = 10.50 sq.in.
I PI. 14 Xi''. 7.00 6.00

Total, 18.50 16.50

Back to back of flange angles will be made 48.5". Distance
between centers of gravity is then:

48.5 — 2(10.50-1.68 — 6.00- 0.25)716.5 =46.5".

End. 6' out. I a' out.

Vertical force on rivets per lin. in 500 500 500

Maximum shear in pounds 90,000 57}000 30,000

Horizontal shear in pounds per lin.in. » Vjln . . . i ,960 i ,240 650

Resultant shear per lin.in 2,020 1,340 820

Spacing, rivet value 4710 lbs 2.34" 3.50" S.7S"

Values of rivet, i*f '14,400 » 4710 lbs. in bearing
2x(7/i6)*' 7,200=8640 lbs. shear

Computation for vertical rivets is similar. Considering
increments of stress as proportional to areas, which is approx-
mately true, shear per lin.in. between plate and angle equals :

area of cover plates V
area entire flange K

The vertical force is, of course, zero.

Distance from end o 6 12 feet

Vlh i960 1240 650 lbs. per lin.in.

, . ^ 6.00

Area cover pls./area flge. = -- — — .364

16.50

.3647- 710 450 230 lbs. per lin.in.

Value of two rivets in angle shear 8640 lbs.

8640/shear per lin.in.= spacing^ 12.2 19.2 37.5 ins.

156 ELEMENTS OF SfRUCTURAL DESIGN

(A) Same Problem, — Exaci Method

Now let us try the same problem by the exact method,
assmning equal depth. Computation for the web will remain
xmchanged. We next test the flanges, assimdng 7/16'' metal
therein :

I for web =|*'i/i2-f'48*48-48 = 259o

1 for 4 flge.Ls=4-4.62(24.25 — 1.66)2 = 9430

2 cov. pis. =12 '1(24.25+0.22)2 =6310

Total in inches, 18,330

S = Mc/I=8,ioo,ooo« 24.69/18,330 = 10,900 lbs. per sq.in.

Spacing of horizontal rivets in flanges:
Statical moment: For 2 Ls 2-4.62(24.25 — 1.66) =209

I PI. A* 12(24.25+0.22) = 128
Total, Xaz =337

Distance from end, feet o 6 12

Vertical shear per ]in.in $(» 500 500

Shear '..... 90,000 S7,ooo 30,000

Xaz/ 1 = .01 84. Hor. shear per iin.in. = .0184F = i ,660 i ,050 550

Resultant per Iin.in IJ30 1,160 740

Spacing » 47 lo/resultant 2.72 4.05 6u^o

Spacing of vertical rivets:

Here there will be no vertical force.

Distance from end, feet o 6 12

Shear as before 90,000 57)Ooo 30,000

/ 128

Xaz/ 1 — =.00698

18330

Hor. shear per Iin.in 630 400 210

Value of two rivets in single shear 8640

8640/shear per lin.in.» spacing 13.7 21.6 41.0

, However, spacing must not exceed 6" or 4^" if staggered
(Art. 50 ei).

In above problems it has been assumed in all cases that
top flange is properly supported. (Art. 50 du,)

Tables in handbooks are often a great help in the computa-

* Deductg Jin for holes.

THE ENGINEERING DEPARTMENT 157

tion of beams. Capacity under uniform loading for all the shapes
and some girders may be found in Cambria or Carnegie.

Many other examples will be found in Vol. II. Especially
important are those given in the Chapters on Plate Girders and
Office Buildings.

Art 55* Design of Tension Members

These may be of round rods, square rods, rectangular bars,
angles, and built-up shapes. There are three ways in which the
stresses due to its own weight may be combined with tension.

(i) Add maximum flexural stress to the tensile unit stress.

(2) Take account of the fact that the weight and pull cause
a deflection at the center, at which point the moments are of
opposite kind. (Fig. 55a.)

Fig. ssa. — ^Tie Acted upon by Fig. 556. — Weight in an

its Own Weight. Indined Member.

(j) Consider the varying eccentricity of the pull.

Many engineers ignore these stresses largely or wholly,
(i) is good enough for comparatively short and deep sections.
(j) is too^complicated to be used in practice and is a needless
refinement.

Taking up the second method, let w be the weight of the
bar per lineal inch, / its moment of inertia in inches, E its modu-
lus of elasticity in pounds per square inch, a its area in square
inches, and / its length in inches. The maximum moment
occurs at center of span and is:

M^iwP/8)-Pd.

Deflection due to a bending moment .

{m=wP/&)

is 5W/V384E/ = (m)(5/2/48£7).

158 ELEMENTS OF STRUCTURAL DESIGN

Assuming now that the deflection, d, bears a similar ratio to
its moment, M:

M = (wP/S)-sPl^M/4&EI or M--wP/S{i+5PP/48EI)

S = Mc/I = wPc/&{I+sPP/4»E) .

For a rectangle this becomes,

S^SwPh/Ub/fi+SPP/E).

where J = breadth and A = depth, both in inches.
To obtain maximum stress, this must be added to direct stress.
In the case of an inclined member, Fig. 556, weighing w per
lineal imit, let any elementary weight, m, be resolved into com-
ponents m sin 6 causing direct stresses which are small and may
be ignored, and m cos 6 causing bending stresses. The maximum
bending moment will occur at the middle and be equal to

wP cos 6

where / is inclined length of member.

Square or round rods are easily fabricated and erected.
They are usually adjustable and hence may work loose. Also
their lack of stiffness makes them likely to rattle. Square rods
can be turned with an ordinary wrench whereas a pipe wrench
is required for a round. However, the latter fact may be an
advantage if they are within ready reach.

Let it be required to design a steel rod for a tension of 46,000
lbs. Allowable stress, 12,000 lbs. per sq.in. £ = 30,000,000
lbs. per sq.in.

Area required=46,ooo/i2,ooo = 3.83 sq.in. Use 2" square
or 2J" round. Supposing now the square rod to be 20 feet
long, let us investigate stress due to own weight which is 1.13
lbs. per Un.in.

^wPh _ 3 -1.13 -240- 240- 2^

4bti? + sPP/E 4 • 2 • 8 + (5 • 46,000 • 240 • 240) 730,000,060'

= 774 lbs. per sq.in. and the rod should be made 2 A"
square instead.

THE ENGINEERING DEPARTMENT 159

For locations wHere members will always be in tension,
rectangular bars are now the accepted design. Where idle or
likely to take some compression, the best practice favors using
compression shapes. Sometimes bars themselves are " counter-
braced " that is, fastened together to resist some compression.
These bars have their ends enlarged to provide connection for
pins and are then known as eyebars, Art. 43. Thickness should
not be more than i its depth in order to give a well packed joint;
it should not be under 1/7 to prevent weakness on compression
side as a beam under its own weight.

Let us design an eyebar for a pull of 88,000 lbs. and an allow-
able stress of 15,000 lbs. per sq.in. Bar is 30 feet long, and
inclined at an angle of 30 deg. with horizontal. Use first method.

88,000/15,000 = 5.87 sq.in. area required.
Try a 6"XiA'', acrea = 6.37 sq.in. Wt. per lin.in. = i.8i lbs.
Direct stress = 88,000/6.37 = 13,800 lbs. per sq.in.
Bending moment = wP cos 6/8 = i .81 • 360 • 360 • .866/8.

= 25,400 in. lbs.
S = 6M/bcP = 6 • 2 5, 400/ J .06 • 6 • 6 = 4000 lbs. per sq.in.

As this stress will remain constant for tne same depth, we
can increase width and allow 11,000 lbs. per square inch for
direct stress.

88,000/11,000 = 8.00 sq.in., use 6"Xif".

Probably a 7" bar would be more economical.

Angles, singly or in pairs, are used a great deal, either for
wind bracing or small trusses. They do not bend or rattle
like rods and they will stand some compression. Let us design
a pair of angles for the same data as the square rod.

46,000/12,000=3.83 sq.in. Use 2 Ls 5"X3''xA".

Gross area, 4.80 sq.in.

Deducting 2 holes, each J''xA" for J" rivets:

Net area =4.26 sq. in.

Direct stress =46,000/4.26 = 10,800 lbs. per sq.in.

160 ELEMENTS OF STRUCTURAL DESIGN

Stress due to its own weight, assuming longer legs vertical;

(5 • 46,000 • 240 • 24o\
12.52+=^—- l = 76olbs.per
S4.m. 48-30,000,000 /

Total stress is 10,800+760 = 11,560 lbs. per sq.in.

There are two common types of the built-up tension member^
the I, Fig. 56/ or g, and two channels, either rolled. Fig. 56; or
k, or built up as in Fig. 56/ or w. The former consists of four
angles united either by a single continuous plate, by batten
plates, or by lacing. Cover plates may be added top and bottom.
The I is a favorite section for bracing capable of canying com-
pression, and for the web tension members of riveted trusses.
The solid I should not be used where its web will be horizontal
because so placed it retains water.

For heavier stresses, use one of the two channel sections.
These are united top and bottom by lacing or batten plates.
Flanges may be turned either way as determined by conditions
at joints or clearance for riveting. Plates shown dotted may be
added to increase strength.

As an example, let it be required to design in soft steel
according to specifications. Art. 50, a built I section for a max-
imum tension of 142,000 lbs. and a minimum of 20,000. Member
will be 25 feet long and vertical.

Allowable stre^ is 9000(1 +20,000/2 -142,000) =9650 lbs.
per sq.in.

Net area required = 142,000/9650 = 14.7 sq.in.

Use I PL 14" X A". Gross area 6.12 deduct for 2 i" rivets.

4Ls5''X3''xr. 11.44 '' 4 ''

Net area, 15.1Q O.K.
Where a riveted section is horizontal or inclined, stress
due to own weight may be very nicely taken care of by giving
connection such an eccentricity that its moment balances that
of the weight.

THE ENGINEERING DEPARTMENT 161

Art. 56. Design of Compression Members

The desirable features in a column are:

(i) Favorable disposition of the metal, that is, that dis-
position, which, for a given area, makes radius of gyration a
maximum.

(2) Economy of shopwork. Coliunns are usually expensive
to fabricate.

(j) Easy end and intermediate connections.

(4) Connections which give centrally applied loads. Even
if eccentricities balance, live load on one side only may change
this.

(5) In many places, it is desirable that the column should
be as compact as possible.

{6) A section enclosed on all sides is objectionable, since
it is inaccessible either for inspection or repair.

(7) Out of doors, all steel work should be so designed that
it will not retain water.

Let us now consider the different types of coliunns in the
light of the above. Taking up first, columns of a single shape.

(a) Angle. (6) Zee bar.

These have small radii of gjrration and are limited to short
columns and low stresses. Except for (i), they are desirable
in every way.

This has a small radius of gyration one way, but a large
moment of inertia the other. It is hence suited for small
compressive stresses when combined with bending. Employed
for struts at eaves of buildings and ends of bridges. Desirable
in every way except (i).

(d) I-beam.

This is much like the channel but it has a larger radius of
gyration and better withstands bending. We find it used for
larger compressions and bending moments as in a column
supporting the roof of a mill building. It is not as convenient
for shopwork and connections on two sides are quite eccentric.

{e) H section.

The rapidity with which this column has sprung into favor

162 ELEMENTS OF STRUCTURAL DESIGN

for moderate loads is explained by its advantages. The metal
is well disposed, there is no shopwork except at connections
and these are quite easy and fairly central, they are compact,
are not enclosed and will not hold water unless placed horizon-
tally. We know of but one objection to them, they cost about
$0.20 per hundred weight more for the raw material.

Taking up now the built-up sections,

(/*) Two angles riveted, Fig. 50a.

The piece between the angles shows a washer filler, Fig.
37Jfe, which may be inserted at intervals or omitted altogether.
This section is more economical if placed with the short legs
outstanding. Its characteristics are much like those of a single
angle column but the radius of gyration is a little larger and
shop work is more expensive. Suitable only for short members
and light loads. Employed for wind bracing and roof trusses.

(g) Two angles and a plate. Fig. 566.

-L • JL ^

is^

Fig. s6a Fig. s6b. Fig. s6c.

Two Angles. Two Angles and a Plate. Four Angles Riveted.

Column Sections.

Much like (/) except that it is des'gned to carry bending
moment in addition to compression. Used for the top chord
of roof and riveted trusses. This section might also be employed
for tensile stresses.

(A) Four angles, riveted, Fig. 56^.

These are fastened together as shown with batten plates
at intervals. It is economical of space, but metal is very
poorly placed, shopwork is fairly high, connections are diflScult
and eccentric, and column is full of pockets. An imdesirable
section.

(i) Four angles latticed or battened. Figs. 56^ and e.

The material of the section is well placed, but the proportion
of details which do not carry weight is very high, shopwork
is expensive, connections are difficult and costly. Used
principally in long unbraced columns where it is important
to keep weight low as in derricks.

THE ENGINEERING DEPARTMENT 163

(j) Built I-beam, latticed, Fig. 56/.

The shorter leg of the angle should be parallel to lacing
for economy. Material is not well disposed and cost of details
is high. It is used in locations where the stress is small and
depth is demanded for sake of connections ot to resist stresses
due to own weight as in the bracing for a bridge.

(k) Built I-beam, solid. Fig. 56^.

Here there is less waste material than in (j). Plates shown
dotted increase allowable unit stress as well as area. The
radius of gyration, however, still continues small, and it is not a
very economical section. Shopwork is moderate for either.
Connections are easy but eccentric on two sides. Used as col-
umns in buildings and in web members of riveted truss bridges.

Fig. 56J. Fig. s6«. Fig. 56/. Fig. s6g. Fig. 56k. Fig. 56*.

Four Angles Latticed or Built Fs Three I Two Channels

Battened. Latticed Solid. Beams, and One I.

Column Sections.

(/) Three I-beams, Fig. 56A.

This section seems quite desirable but, on account of the
slightly greater advantages of (m), is not common.

(m) Two channels and one I-beam, Fig. s&i.

As in (0, section is very well placed, it is economical of
shopwork, and easy to connect with. While either is likely
to have eccentric connections on two sides, they are well able
to bear this. It is not a closed section, and it does not hold
water if placed vertically. It does, however, occupy con-
siderable room, (/) slightly more than (m), and former is also
a little harder to handle. They are ideal where a moment in
both directions is to be carried, as in columns for cranes and
elevated raibroads. Obviously, either (/) or (m) may be built up.

(w) Two channels, latticed. Figs. 56; and k.

Metal is well placed but shopwork and weight of details
are moderately high. Connections are fairly easy and this
type is frequently used as, for example, in parts of bridges

164

ELEMENTS OF STRUCTURAL DESIGN

and columns for buildings. Extra plates may be added either

outside or inside as shown by dotted lines.

(o) Two built channels, latticed, Figs. 56/ and m ,

Very much like (n) except that shopwork is higher and that

it is used where the rolled shapes will not provide the necessary

area.

nnnn

Fig. 56;. Fig. 56*

Two Channels,
Latticed.

Fig. 56/. Fig. 56m.

Two Built Channels,
Latticed.

Column Sections.

Fig. s6». Fig. 560.

Two Channels and
Cover Plate.

(p) Two channels and a cover plate, Fig. s6n.

Channels are turned as shown and may be rolled or built-
up. Plates may be added as in (o) or, if nothing interferes,
angles may be added on the inside, Fig. 560. Material is well
placed, shopwork rhoderate, open for inspection, and especially
suited for easy and efficient connections in the top chords of
bridges where it is the accepted design.

(q) Built I and two built channels, Fig. s6p.

Fig. 56^

Built I and Two Built Channels.

Fig. 56^.
"Box."

Fig. 56f .
Zee Bar.

Column Sections.

To still further increase capacity, one or more built I's
may be inserted between channels in (0), and in a similar way
extra plates could be added. As in (p), one might rivet on a
cover plate in place of lacing.

(r) Box column, two channels and two cover plates. Fig.

569. Channels are either rolled or built up. Material is very

well placed, shopwork moderate, connections easy, but they are

.quite eccentric, and it is a closed colimm. Used in building

THE ENGINEERING DEPAETMENT 165

work, where it should always be incased in fireproofing to
ensure against rust.

(s) Zee-bar column, Fig. gfir.

Here, the theoretic disposition of the metal is very good,
but under test the outside comers fail first because they are
insufficiently stayed. Connections are central and easily
arranged. Is accessible for inspection except when plates
shown dotted are added to secure increased strength.

Among the remaining types of columns, we may mention
the Phoenix, now obsolete, and the Larimer,
Fig. 22g, both of which are explained in '

Art. 22.

As an example of how large columns are
built up, we give Fig. 561.* This shows a
section of the bottom chord of the Quebec „ <«— Serf
cantilever bridge, the failure of which caused the Bottom Chord
the wreck of the structure. (Art. 69.) The of the QuebecCan-
outer ribs were made of 2 L's, 8"x6"xi8", tilever Bridge.
1 PI. S4"xr. 2 Pis. 54"XW', and i
PI. 37j"XiB". The inner ribs were 2 L's, 8"X3i"xH", 2
Pis. 54,"xW, 2 Pis. 46"xW- Lacing was double, 45 deg.,
of I L 4"X3"X|" and cross struts, i L 3§''X3"x|".

In design, the author prefers the straight hne formula,

P=CiAii-C2l/p).

where P is total load and A gross area; l/p, the greatest
slendemess ratio, that is, the greatest value of the fraction,
unsupported length divided by corresponding radius of gyration,
Ci is the unit strength for a short strut and Ca is a constant
for a given material which will bring the straight line tangent
to Euler's curve for long columns. We have made C2 a httle
large, about .006, in order to discourage the use of long slender
columns.

The stress due to own weight may be taken care of as in
tension members. However, the moment due to the load P
(see preceding article), is plus instead of minus and hence the
formula becomes :

S=wPc/&iI-sPP/4&E).
• Engineering News, Vol. LVIII, p. 320.

166 ELEMENTS OF STRUCTURAL DESIGN

It is customary in main members to give the connection just .
enough eccentricity to balance the stress due to own weight as
aheady mentioned for ties.

Let us take up now the fastening together of the parts of a
built-up column. In the first place, the distance between the
points at which the parts are riveted should be such that the
slendemess ratio for none of those parts exceeds that for the
column as a whole.

To obtain strength required for this fastening whether of
lacing or rivets, let us consider the above formula,

P = Ci^(l-C2//p).

The reason why long columns fail at lower imit stresses
than short ones of the same cross-section is that irregularities
of manufacture cause an eccentricity, the effect of which is
similar to a imiform load appUed at right angles to the colunm.
The transverse load causes a bending with compression on one
side and tension on the other, and post fails when siun of com-
pressions due to flexure, 5i, and axial load, 52, reaches Ci.*

Ci=5i+52=5i+Ci(i-C2//p)
or

S\ =CiC2//p.
But

assuming that load which causes bending moment is uniform.
Where Pr = uniform load appUed transversely, and

c = distance from neutral axis to most strained fiber*
Equating these two values of 5i, and substituting 4p/3, a
rough value, for c,

W = 6ACiC2.

Taking for C2, the value .006,

W = .oz6ACiy

or about one-thirtieth load on a short strut of same section.
Some of the assumptions made in the proof might be questioned
on theoretic grounds but the fact remains that the rule here
deduced agrees very well with present practice.

THE ENGINEERING DEPARTMENT

167

In addition to specifications in Art. 50, €3, and C4, the
follow.'ng should be noted. Lacing may be single, inclined at
an angle of about 60° with axis, and, if there are two sets,
staggered as shown in Fig. 56^; or double, at an angle of 45°,
Fig. 56U. In either case, it should be figured as a truss. How-
ever, as it is more economical to make all pieces atike, only

Fig. $6t. Fig. 5611. Fig. sOe. Fig. s6w.

AirangBmcDt of Lattice Ban. Showing Use of Batten Plates.

maximiun stress in end member need be computed. lattice
bars are often placed as in Fig. 568, but it does not seem like an
effective arrangement; we must admit that it is employed in
good work. As already mentioned, occasional batten plates
are sometimes inserted in place of lacing, Fig. $6w; nevertheless,
as may be inferred from above analysis, it is Inefficient and
imsuitable for important locations.

The lattice bars vary in size from ij"xj" to aJ"X|"
and even larger for built-up columns. Angles may be used to

Fig. s6x.
Fonns of Lattice Bars.

Fio. s6y. Fic. 562.

Airangements of Batten Plates and Lattice Bars.

advantage in very large compression members. Fig. 56a:
shows different ways of making the bars. The center of the
curve is seldom at the rivet but nearer the outer end.

The latticing should start either from the batten plates,
Fig. 56y, or from a rivet as close to it as clearance will allow,
Fig. 562. Lattice bars may be used where the surfaces with
which they connect are not in exactly the same plane. For
example, the lattice bar in Fig. sfi^i might run from on top
of the batten plate to top of channel.

168

ELEMENTS OF STRUCTURAL DESIGN

Let it be required to design a medium steel single shape
column 8 ft. long according to specifications in Art. 50, Load
is 50,000 lbs. dead.

Allowable stress, 15,000 (i — .006 //p).

Maximiun //p = ioo, minimum p=o.96".
All zee bars and channels have lower radii of gyration, but we
can use any equal legged angle above a 4^X4'' and any I beam
above 10" in depth.

Section.

♦iL SXSX 7/8
*iLsXsXis/i6
1L6X6X11/16
1L6X6X s/8
1L6X6X 9/16
iL 8X8X 1/2

il 10

il 12

Weight
per Foot.

Allow.

Ar^A.

Total

Stress.

27.2

0.96

6000

7 99

48,000 #

28.9

0.96

6000

8.50

51,000 #

26. s

I. 17

7620

7.78

59,300 %

24.2

I. 18

7680

7. II

54,600 %

21.9

I. 18

7680

6.43

49.300 #

26.4

1.58

9S70

7.75

74,000 #

25.0

web too t

hin

♦

35.0

0.99

6270

10.29

64,700*

Remarks.

Too low
O.K.
Too high
O.K.
Too low

Everything in pounds and inches.

Number 4 should be used. We need not have tried numbers
6 and 8, after we had found their weight unless a surplus of
strength were desirable. In this case we should use 6,, as it
has a larger capacity and less material than 8.

Next we will compute the size required for a medimn steel
column of two channels latticed with flanges turned out. Speci-
fications as in Art. 50. Post is vertical and 25 feet long .Loads
range between 112,000 compression and 25,000 tension. Estimat-
ing allowable stress at 6000, about 19 square inches will be
needed. Let us try 2 channels i5"at 33 lbs.

Allowable stress

= 10,000(1 — 25,000/2 • 1 1 2,000) (i — .006 • 300/5.62)

= 6040 lbs. per sq.in.

Area required = 1 1 2,000/6040 = 18.5 sq.in.

* Special sections and undesirable on that account. Also so thick it would
have to be drilled (Art. 44).

THE ENGINEERING DEPARTMENT

169

19.8 are furnished, so this is O.K. There is plenty of metal for
tension.

To design the latticing, let us take it as single and inclined
at 60° with the axis. Gage to gage will be about 13.25" and
the unsupported length of the bars will be 13.25 sec 30° = 15.3".
Assuming f" thickness, the allowable stress is,

10,000(1 — 25,000/2 • 1 12,000) (i — .cx>6 • 1 5.3/. 107) = 1260 lbs. per sq.in.
Transverse load is

(10,000/30) (i — 25,000/2' 112,000) (19.8) = 5850 lbs.
Stress at either end is, 5850 sec. 30^/4 = 1690 lbs.

1690/1260 = 1.34. Use 3XA", area 1.31 sq.in. Amply safe
with depth larger than assmned.

Now let us determine sizes for a two-channel and a plate
section of medium steel, Fig. 56n. Length, 20 feet; maximum
load 280,000 lbs. C; minimum load, 80,000 lbs. C.
Specifications as in Art. 50.

Assume allowable stress to be 9000 lbs. per
sq.in.

Then area required will be 280,000/9000 equals
31 sq.in. Let us try section shown in Fig. 56aa, horizontal
distance in clear between plates, 10".

Sis6.

Area.

Distance to
Center of
Gravity.

Static

Moment.

/
Moment.
YY

a. I PL 18XI

6-75

8-31

+56. 1

466

182

b. 2PIS. 16XI

12.00

256

323

c. 2Ls. aJXaJxf

4.96

7. II

+35. 3

257

212

d. 2Ls. aJXsJxf

7.96

7.02

—56.0

402

335

For entire section

31-67

1. 12

+35. 4

1381

1052

Everything in inches.

Ixx about c.g. = i38i— 31.67' 1. 12 •1.12 = 1341,
p = (//i4)* = (ioS2/3i.67)*=s.75 in.

170 ELEMENTS OF STRUCTURAL DESIGN

Allowable unit stresses

= io,ooo(i +80,000/2 ■28o,cxx))(i — . 006 ■240/5.75).

=8570 lbs. pier sq-in.
Total allowable stress

=31.67-8570 = 272,000 lbs.

This is too low. We will use bottom angles H" thick, making
total quantities:

Area, 32.39; distance to center of gravity, 0.96"; statical
moment about XX, 30.9; / about center of gravity, horizontal

Fig. s^aft- — Typical Column. Upper Chord of Pin-conDected Truss Bridge,
American Bridge Co., Ambiidge, Fa.

axis, 1383; lyy, 1096; p, 5.80"; allowable unit stress 8590;
total allowable stress, 278,000 lbs. ; will do.

Allowing 40% for extras, weight per foot will be 154 lbs.
Deflection = $WP/3&4^I

= 5 ■ 3080- 246- 240- 240/384 30,000,000 - 1383 = .013"
M due to own weight = 3o8o'24o/8=9240o in. lbs.

THE ENGINEEEINQ DEPAETMENT 171

Amount which the center of gravity of the column should
be placed above the intersection
point is,

.013+92,400/280,000 =

.342 or about j".

It will be noticed that any
additional plates that may be
necessary will be located on the
side at a distance from the center
about equal to the radius of gyra-
tion. Therefore allowable unit
stress wUl be changed little when
reinforcement is tacked on. In
practice, for a case Hke this, allow-
able stress need not be recom-
puted.

Latticing must be figured for a
transverse load of,

10,000(1 +80,000/2 ■ 280,000)

32.39/2-30 = 6200 lbs.,

the other half being carried by the
top plate. Maximiun shear is 3100
lbs. at each end. Latticing is
usually made double, 45°, and
riveted at the center. Stress in
end bar is then 3100 sec. 4sV2 =
2190 lbs. Unsupported length is
about 7^ sec. 45°= 11".

Let us try a 3 XA". p=o.o9".
Allowable total stress equals 2860
lbs. O.K. A 2iXi" would be as
cheap and much more efficient.

While lacing bars are usually
weakest in compression, rivets and tensile stresses should also
be watched. 1

Fig. s6ac.— Typical Coliunn. Ver-
tical Post in Drawbridge, Aracri-
caa Bridge Co., Ambridge, Pa.

172 ELEMENTS OF STRUCTURAL DESIGN

Art. 57. Strain Sheet

" Stress Sheet " would be more appropriate but we follow
custom.

The strain sheet gives a line drawing of proposed structure
with its principal dimensions. There should also be a state-
ment as to loads assimied, total stresses resulting therefrom,
unit stresses allowed, and sections designed for the different
pieces. Second and last are usually placed directly on the
members to which they belong. Title should give name and
location of designer and purchaser. Date, scale, and name of
maker ought also to appear on the drawing. For lettering,
see Art. 62, also sample strain sheet, Fig. 57.

Sometimes important or peculiar connections are drawn out.
Often all details are worked up, and leading dimensions and
material given. It partakes then of the nature of a detailed
drawing and is called a " general plan."

Art 58. Detailing

Detailing may be divided into two parts :

(a) The design of the small parts of the structure — rivet
spacing, connections, shoes, and so on, which will be taken up
in Arts. 59, 60, and 61, and also in other volumes.

(b) Structural steel consists of rolled shapes, cut, forged
or bent, punched or drilled, and machined. Specifying and
locating these form the second part. See also Art. 40.

There are two general methods of accomplishing (fc). Take,
for example, a roof truss. In the first method, we show by
sketch number of rivets and method of arranging connections,
give center to center distances, state material required, also
maximum and minimum spacing and edge distances and leave
templet shop to arrange details on full size layout on shop floor.
This method is cheap, especially in the drawing room, but
it renders control by the engineer more difficult. Also in case
of repairs or alterations, expensive measurements on site may
be necessary.

THE ENGINEERING DEPARTMENT

■n

•IS5

ill

■?

i V5
I f

174

ELEMENTS OF STRUCTURAL DESIGN

The second method gives all dimensions required to work
out each piece separately. Fig. 580 shows three different ways
in which this may be done for skew measurements, (r) is
more expensive and liable to error and is used very seldom and
only to prepare for rack punch. (2) and (j) are both good;

(i) (2) (3)

I" rivets H". open hol^.

Fig. 5S11. — Methods o( Detailiiig Cocuectioii Plates.

latter is a little easier in drawing room and a little more difficult
in shop.

There is still another subdivision. Structures may be detailed
" in place," that is, with the pieces in the relative position
which they will occupy in the field. This is easier to under-
stand, but the manual drawing is more diffi-
cult and it takes additional space. Or
" knocked down " with pieces taken apart
and given in their most convenient position.
Fig. 586 shows difference. It is not common
to take apart a shipping piece.

Consider now the simple cases given
above. Fig. 58a shows details of a plate;
c, an angle; d, an I-beam; and e, a plate
girder. To enable shops to fabricate these
or other structures;
(i) Bill material. Standard fittings need only be named.
For example, rivets are specified by diameter, given in note
as shown. Other pieces of metal must be billed as mentioned
in Chapter II. This material is lettered on the drawing in
such a place that there will be no doubt as to which piece is

THE ENGINEERING DEPAETMENT

intended. Often a table is made at the side giving the informa-
tion, but unless an assembly or identifying mark is used, or

^ iu,P,--1^-.,ii m

lLa-.3t\|-i.i

•4 LATERALS L6(|2

Fig. s8c.— Detail of an Angle.— Open Holes, H".

except for very simple structures, the writer does not approve.
It is best to place notation as near piece as is feasible, with
arrows if necessary.

opm now* V + SCAM5 B 12

Fig. sSif.— Detail oi an I-Beam.

(z) In these shapes, holes will be drilled or punched.
Longitudinal spacing must be given in all cases. If more than

Fig. s8<.— Detail of a Plate Girder.

three are alike, they should not be repeated, 3", 3", 3," 3", 3",
but marked 5 at 3" = i'-3". Rivets alternately spaced on two

176 ELEMENTS OF STRUCTURAL DESIGN

lines as seen in Fig. 58^ and in top view of Fig. 58J are located
so many alternate spaces. Note that distances given are not
the center to center measurements but are taken along
dimension line from one rivet to a point opposite the next.

Suppose now we have an angle 20 feet long with 40 rivets,
6'' apart. Assume first that the angle goes in a space just
2o'-oi" long. Over all distance or two edge distances must
then be marked " not more." Second, suppose piece is to be
machined to be 20 feet long when finished. Both ends are then
marked " mill," both edge distances given, and shop takes care
of it if enough material is ordered. Third, let variation either
way of a quarter inch be unimportant. Practice varies here,
two, one, or none of the edge distances being given, as it is left
largely to the discretion of the shop whether a piece should be
cut to exact length or not.

This brings us to the subject of mill variation, see Art. 17.
Shapes from the mill do not come exact length unless a pro-
hibitory price is paid. The schedule of variation is quite
complicated. We will state but one very important item, —
I-beams and channels may come f " long or short. Hence we
detail them so this uncertainty will do no harm and mark the
allowable variations, usually A", on each end. Fig. 5&/. In
case it is required to mill the end of an I-beam or channel,
it must be ordered long enough so there will be sufficient metal
even if it comes f " short.

(j) Next transverse spacing must be given. For each size
I-beam, angle, and channel, there are certain standard spacings
(see hand-book). These are determined by edge distance and
clearance for driving. In case there is more room than needed,
variation may be made from the standard. However, this
should be done only by an experienced draftsman.

Fig. 58/ shows how dimensions are given for shapes other
than rectangles. In I-and T-beams, spacing in flanges is sym-
metrical. If dimensions marked x are given, it means shape is
to be milled or cut to this dimension. Avoid, if possible, as
it is very expensive.

(4) Cuts may be located by three methods as illustrated in

Fig. 58?.

(5) Bends are specified by bevels, by radius of curvature

THE ENGINEERING DEPARTMENT

177

and bevels, or by the dimensions of the piece on which
it goes.

(d) Overall and center to center dimensions of finished
piece must be given. Distances between groups of field rivets
are also important. Besides their convenience in the drawing
room, they are an aid to the inspector. Always give hack to back
of angles. In case of long members 'with complicated spacing,
locate intermediate points by a separate line of dimensions.

fe^

Fig. 58/. — Transverse Spacing of Shapes.

•Let us take up now the question of assembly marks. Sup-
pose a job involves 1000 tons and 60 sheets of drawings. Cer-
tain details exactly alike and forming part of pieces to be shipped
occur on different plans, often several times on the same sheet.
These may be handled in the ordinary way or method of assembly
marks may be used. The latter system is about as follows:
Give all except main members a mark. Those on first
sheet will be ai, bi, . . ., aai, abi, . . ., bai, 66/, and so on. If

L2lul

Fig. s8f . — ^Methods of Detailing Cut.

now on sheet two, a part occurs which is the same as on one,
it is given its old mark, ami for instance. Details not like any-
thing previous will be given new ones, a2, b2, . . ., aa2y etc.
Parts that are alike have the same mark and conversely. Those
which are right and left must be so designated. Details are
given where piece first occurs; elsewhere only enough is furnished
for other connecting parts; except that material is billed once,
on each sheet which contains it, thus, iL, 4''X3"Xf Xi'— o"
bmj. Elsewhere on sheet, it is simply bmj. In bills of material,
bmj is listed in every place where it occurs. At the first of these

178 ELEMENTS OF STRUCTURAL DESIGN

places, total number of bmj is given. In the templet room, the
workman who is assigned to sheet 3 makes templet; if it occurs
on other sheets, the templet maker knows from its number that
it has already been taken care of. It causes some additional
work in the drawing room but is economical and efficient in the
shop.

Every piece which is shipped must have its mark for identifica-
tion, during erection. This should be suggestive, G for girder,
5, stringer, and so forth. In trusses, joints are sometimes
lettered Uo, Ui, etc., above; Zo, Li, etc., below. Ui Lo is then
the endpost. These marks should be given directly under
member and in letters somewhat larger tiian rest of drawing.
Writer prefers the form,

S Girders G14 | ^f.

Among the many other ways in which it may be written, we
mention,

^. J • J ( 3 as shown mark G14R

5 Girders reqmred •{ .1 1 1 1 TT r
'^ ^ ( 2 other hand mark G14L.

But every experienced man knows what the first inscription
means and no additional information is given to pay for extra
space and time consumed by the second.

To distinguish between assembly and shipping marks,
observe that several of the former are riveted together to make
one shipping mark, and a number of latter when fastened together
form finished structure.

Important points to be borne in mind in detailing are:

(i) Dimensions must be accurate to A".

(2) All necessary measurements must be given, but,

(5) Avoid needless repetition.

{4) Everything must be clear and condse.

(5) Show connecting work detailed on other sheets in red.
And in the design of details,

(tf) Use as few shapes as possible in addition to those
employed for sections.

THE ENGINEERING DEPARTMENT 179

Art. 59. Design of Splices and Beam Connections

The following principles are important for all joints, whether
splice, riveted connection, or pin joint:

(i) The joint should be economical of material and shop-
work and erection, the latter two factors being the more im-
portant.

(2) Be careful to make details of sufficient strength. Two
methods of computation are employed :

(a) Make details as strong or stronger than the main
members.

(b) Make details as strong as stresses. Sometimes for
small stresses, specifications for minimimi sized material make
much larger members obligatory. The difference between
(a) and (6) is then considerable. The author prefers to use
(a) when the structure thus strengthened is worth enough more
to pay for the additional outlay.

(5) Consider erection very carefully.

{4) Make joints as rigid as possible.

(5) Compactness adds to rigidity, economy, and strength.

{6) Important members should meet at a point except as
necessary to balance moment due to own weight. (See
Arts. 55 and 56.)

In addition for riveted joints.

(7) Keep field riveting to a minimimi as already noted.
(Art. 49).

{8) One rivet is not enough and two are too few for important
work.

Practice has established a number of conventional rules
for the computation of rivets which do not accord with actual
conditions. Nevertheless, customary methods seem safe since
constants are derived from tests making same assumptions.
These are:

(a) That rivets completely fill the holes; as they are driven
hot and afterwards shrink, this cannot be true.

(b) While considered to fill the holes, their capacity is com-
puted from the original diameters.

180

ELEMENTS OF STRUCTURAL DESIGN

and bearing, they actually hold by the friction of the cooling
rivet.

(d) While we neglect bending, there must be considerable
in the body of the rivets.

(e) In tension, stress is considered as uniformly distributed
over net area with holes i" greater than nominal diameter
of rivets.

(/) Compression is considered as distributed imiformly
over gross area if rivets are driven; otherwise over net area.

leoobT

°°9-'"»°^^

(^yw>* 'A/vio Ct9non L|(

Fig. 59a. — Assumed Distribution of Stress.

(g) Shear is sometimes taken as distributed over net area
and sometimes as over gross; the former seems more logical.
(A) That the stress in a group of rivets is um'formly dis-
tributed is far from the truth. First, the end rivets of a straight
axial connection must carry more than middle. If stresses

in rivets were equal, we would have
impossible conditions, shown in Fig. 59a.
Secondly, loads are often applied eccen-
trically. Let us suppose a load of 24,000
lbs. to be applied to the group of rivets
shown in Fig. 596. The stress assuming
equal distribution is now 4000 lbs. in
each rivet. Suppose, however, that load
is appUed lA" to right of right hand row
of rivets. The eccentric application of the
load now causes a moment of 24,000* 2A =
62,000 in.-lbs. Let i? be the stress on the
4 outer rivets; then that on the other two
will be i.i2i?/3.2, assuming stress to vary
as distance from center of gravity of
group. The moment of 4R is 4-i?-3.2 = i2.8 R. That of the
other two is 2'i.i2-i.i2-jR/3.2=o.78i?. Total = 13.62? = 62,000.

i?=455o lbs. stress in outer rivets.

1.12R/1.2 = 1600 lbs. stress in other rivets.

Maximum will be the resultant of 4000 and 4550 = 7050 lbs..

Fig. 59^. — Two Stresses
in a Connection Angle.

THE ENGINEERING DEPARTMENT 181

Joints may be divided as follows:

(i) Tension.

(a) Splices (2) Compression.

is) Bending.

(b) Beams connectii^ to other pieces. (i) Resting upon it.

Subdivided actording to position (2) Framing into it.
with regard to other piece. (3) Suspended from it.

(2) Pin.
(c) Will be the subject of the next article.

In (or) and (02), it is customary to splice by providing plates
on all sides, each with not less than two rows of rivets. For
the former, Fig. 59c, number of rivets must be figured. In
the latter, Fig. 59^, we may (j) shear ends and compute the

i-l U'^i zi IS-

Fig. 59c. — Splicing a Tension Member. Fig, jgi. — Splidng a Column.

necessary rivets, (2) mill ends and put in about two or three
rows of rivets each side of the joint as shown in the figure,
or (j) design rivets to be as strong as member in bending.
Common practice is to use (2) and make sphce as near a sup-
port as connections will permit.

(aj) Splicing of beam occurs seldom except for plate girders,
under which head it will be taken up in detail. To illustrate
method, let it be required to sphce a 24" I at 80 lbs. to conserve
its full net strength. See Fig. 59*. On each side, we will
use a 2o"x|" plate: I, gross = 25o; /, net, estimated = i7s
in.* for each. I/c for a 24" I at 80 lbs. = //c for splice— 174.0.
Estimating c at 13.0", necessary 7 = 2262, or 1762 must be
furnished by cover plates. Assuming their center of gravity
to be at 12.5", A for one flange is 1762/2* 12.5- 12.5 = 5,65 sqia.

182

ELEMENTS OF STRUCTURAL DESIGN

Use two plates each 8"xi", gross area 6.00, net, 4.50 sq.m.,
deducting 2 i" boles. This computation equates the resisting
moment of gross areas which is only approximately correct
The rivets in this fiange will be in shear. To develop 4.5 sq.in.
there will be necessary 4.50-1.5, or 6.75 sq-in. shearing area'

Fig. sge. — Splidng a Beam.

This will mean 12 J" shop rivets on each side of the joint as
shown.

Theory for rivet splice at sides is similar to that for riveted
connections just considered. Here the value R will be that
for bearing and is equivalent to JXjXi. 5 = 0.66 sq.in. area for
flexural stresses. 1.5 represents ratio of allowable bearing to
bending if we assume shop rivets. In other words, one rivet
is just as strong as 0.66 sq.in. at same point. o.66(S.5''+5.s^
+2.5^') = 72 = / forhalfrowof rivets.
There will be required 350/144 = 3
rows of rivets. The correct de-
duction for holes in side plates
is 2(i)-i-(8.5^+s.5= + 2.52)=8i,
leaving net / as i6q, substantially
as assumed.

Where a beam rests on top of
a piece, (bi), it is preferably fast-
ened thereto by holes through its
base. The objections to this meth-
od are, — danger of buckling of
webs which should be figured for
compression and, if insufficient,
Fig. 59f.— Suspended Floor Beam, stayed by stiffeners; lack of rigidity
in beams which can be prevented
by X bracing at each end; and want of support for piece to
which it connects.

THE ENGINEERING DEPARTMENT 183

(bj) lacks rigidity and is now considered poor design. It
is sometimes seen in the connection between the floor beams
and trusses of old highway bridges, Fig. $gf.

The standard connection, (62) is good. Fig. 59^ shows
common method. Sometimes a " shelf angle" is placed just
below the beam and this, in con-
junction with angles as in figure or
an angle riveted onto top, forms an
acceptable type. In latter case, upper

angle must not be figured to carry !

any part of the load. Shelf angle i

alone makes an inferior connection

for reasons given for (61). When Fic. 59;.— Beam Connection.
beam coimects to a truss joint, it

may fasten to one of the members, or to their pro-ongations, or
be riveted direct to the plate. (62) is the common style.

Art 60. DesigD of Riveted and Pin Joints in Trusses

The members of a truss which meet at a riveted joint are
fastened together by means of one or two plates. The former
is used for members composed of one or two angles. The latter
when connecting to I beam and two chaimel sections. For
this case, distance between plates at different joints should, if
possible, be made the same and all truss members arranged to
go outside or inside. For clearance, allow A" or J" if an
entering joint, Fig. ^gm, is necessary. Fastening plates to
one of the pieces in the shop increases cost of shipping but
lessens field rivets.

The thickness of plate unless determined by connections is
preferably made such that resistance of rivets in shearing and
bearing are about the same; however, it is seldom advisable
to make it thick enough so material cannot be punched. Art.
44. Its length and breadth are determined by the necessary
niunber of rivets. Plates are often irregular; if material cut
from a rectangular plate will pay for extra labor (scrap is now,
1912, worth i cent per pound), it is generally better to shear
it off. (Art. 63.)

Let us next design joint shown in Fig. 60a which represents

184

ELEMENTS OF STRUCTURAL DESIGN

a portion of a strain sheet. To get proper thickness of plates,
/, we will equate bearing and shearing values of rivet, assuming
unit stress for former to be twice the latter.

2 X Rivet area in bearing = Area in 2 shears, or,

2/d = 2x(P/4 or <=o.79J.

d=o.7S, hence t=o,$g'\ use A" plate.

Assmning allowable shearing and bearing values of 10,000
and 20,000 lbs. per sq.in. respectively, and for field rivets f of

ll'rivetd

Mcxx. Dif rercncc cjoooT

Fig. 60a. — Portion of Strain Sheet. Fig. 606. — Corresponding Detail.

these amounts, we may prepare the following table and then
design our joint as shown in Fig. 66b.

Member.

Stress.

Rivet.

Value.

No. Rivets
Required.

48,ooor

Field

6330

8,iooC

3310

34,30oC

6330

AD-DO

67,ooor

Shop

8440

Everything in pounds. The last stress is really the maximum difference of
stresses.

Pin Joints

In the other t)^, the pin passes through the members
directly. Compression pieces should be designed to facilitate
this. Tension members are eyebars, and as these may be out
of parallel by one-eighth of an inch in a foot, less difficulty is
experienced in locating them. To explain how " packing plan "
(a drawing showing arrangement of members aroimd joints)
influences design, let us examine typical joints of an ordinary
Pratt truss. " Figs. 60c, d, and e give packing plans for hip,
shoe, and a panel point in the lower chord respectively.

THE ENGINEERING DEPARTMENT 185

TTie upper chords are made ot such width that there is
room for the diagonals outside the posts. The same position

Fig. 6oc.— At Hip.

Fre. 6ad.—At Shoe.

Fic. 6oyii. Fio, 6oi.

Typical Riveted Joint. Typical Pio Joint.

American Bridge Co., Arobridge, Pa.

is maintained at bottom. The eyebars constituting bottom
chord are passed outside the diagonals and inside end post.
Webs of latter should be opposite those of shoe.

186 ELEMENTS OF STRUCTURAL DESIGN

A pin is treated as a round beam acted upon by forces at
various angles. We determine, often after several trials, which
loading or loadings stress the pin most; then resolve all
forces into horizontal and vertical components; find horizontal,
vertical, and resultant shear, and horizontal, vertical, and
resultant moment at salient points. From maximum resultant
shears and moments, proper size of the pin may be determined
by the usual rules of Applied Mechanics. The second volume
of this work will give sample computations.

Pin connected structures are easier to erect. Also, their
joints are more like the hinges assumed in computation, hence
cause smaller bending stresses due to deflection. Riveted
trusses deflect less and are stiffer.

Art 6i. Shoes*

Masonry is comparatively weak and is capable of bearing
in compression but a very small part of load carried by an
equal amoxmt of steel. It is therefore necessary to build out
at supports, and the structure for this purpose is called a shoe.
It should be capable of transmitting the following forces:

(/) The vertical reaction from the girder to the masonry.

(2) A sidewise pressure caused by tJie wind or centrifugal
force from the bracing to the anchor bolts.

(5) An uplift caused by wind from girder or truss to nuts
of anchor bolts.

Short spans are sometimes fixed at both ends. The usual
case, however, is that one end is fixed and the other is free to
move. For the former it must carry:

{4) The longitudinal force caused by traction or applica-
tion of the brakes.

We will next give desirable qualities for ideal shoes. First,
for the free end:

(a) It should move with very little friction.

(b) It should be accessible for cleaning, oiling, or repair.
Next, for both ends:

♦ See Part HI, "Details of Bridge Construction— Plate Girders," by Skinner.

THE ENGINEERING DEPARTMENT 187

detached from its foundation. As dnders, dirt, and moisture
are likely to gather around the shoe, (6) and {c) need careful
attention.

{d) Details should distribute pressure imiformly either before
or after deflection.

The requirement {S) is usually ignored on short spans
although the concentration of pressure caused thereby on the
outside edge of the masonry must be considerable. For longer
spans, we use the pin joint.

We shall classify as follows:

Type (i) Fixed without pin.

(2) Free without pin, sliding joint.
(5) Free without pin, rolling joint.
{4) Fixed with pin.
(5) Free with pin, rolling joint.

Type(i) Fixed without pin. (Fig. 6ia.)

This is used for spans of less than 75 feet. Here a sole
plate, J to i" thick, is countersunk riveted to the bottom flange
and placed upon a bearing plate of same thickness and dimen-
sions. Planing is not necessary. Bolt holes about ij"
diameter for 2. anchor bolts ii"Xi2" or thereabouts are pro-
vided for fastening to the girder. In bridges built on a grade
one of these plates is planed to allow therefor.

In a bridge where stringers are used, at each abutment, a
shoe must be provided for every stringer or else a floorbeam
be used at the end. The latter method is probably the better.
It makes all the stringers alike and it cheapens the masonry,
but it uses more steel, and the connections of the end floorbeam
are often troublesome.

The bearing plate may be steel or cast iron. Where a
high shoe is required, it may be made either of several plates
riveted together or of cast iron, the latter detail being more
frequent. In case the height is much in excess of 2 ins., it is
usually made hollow. Underneath the sole plate is sometimes
placed a sheet of lead and occasionally it is grouted up.

Type (2). Free without pin, sliding joint.

One method is to make as shown in Fig. 6ia, the only dif-
ference being that both plates must be planed on their surface

188

ELEMENTS OF STRUCTURAL DESIGN

of contact and the holes in the sole plate instead of being cir-
cular as in the fixed bearing are slotted as shown in Fig. 6ib.
The distance e is made equal to the expansion of span for max-
imum range of temperature. In this locality, (Pittsburgh)
— 20 to 1 20° F. are about the extremes. This gives us a varia-
tion of (140 X. 0000065 = . 00091) Xlength. From this is derived
an approximate rule; J in. for every 10 feet. This allowance is
also supposed to cover inaccuracies of fabrication and change
in lengtii due to deflection.

The distance d is made i" larger than bolt for li" and
smaller; i" being added for ij" or larger. Holes where d = ij
and ^ = 1" would be noted as,— "Slotted holes ii"X2i''."

wmfJiMMmwrntmu'
Section AA

Fig. 61a. — Simple
Fixed Bearing.

]

Section B9

Fig. 616. Fig. 6ic. — Notched Ex-
Slotted Hole. pansion Bearing.

Fig. 61J.
Cast Base.

Another method is to notch a plate into the flange or sole
plate of the girder as-shown in Fig. 61c, The notch in the flange
should exceed in length parallel to bridge projection in the
bearing plate by. an amoxmt equal to e, Fig. 616. The plate
shown dotted is advisable to provide against uplift. However,
it is often omitted.

Either type (i) or (2) may be used in a simple form for the
shoes of stringers, the slotted hole being the most common detail.

An example of a high cast-iron base is shown in Fig. 6 id.
This, it will be observed might serve equally well for a fixed
end, the only difference being the holes in the sole plate which
are slotted for the free end and roxmd at fixed. Principles
stated in Art. 16 must be carefully followed.

Bearing surfaces for cast iron shoidd always be planed.

THE ENGINEERING DEPARTMENT 189

Sliding or rolling surfaces must be planed either for steel or
cast iron. Sole plates are often omitted for short spans and
cheap work.

Type (j). Free without pin, rolling joint. For spans over
say 75 feet, friction caused by a sliding joint is too great and
rollers (Art. 38) are substituted. For plate girders and small
trusses these are usually 3 to 5" diameter and droUar. A
large proportion of this cylinder does no useful work and, if
cut away as shown in Fig. 6ie, it continues to act as before,
but occupies much less room to transmit a given pressure.
At extremes of heat or cold, the rollers are inclined as shown by
Fig. 6ie; by cutting out as seen in Fig. 6I^ we have another
type which admits of larger expansion for a given distance
center to center, or of closer spacing for a given expansion.

I o s a ^ o a o f

Fig. 6ie. Fig. 61/.

Segmental Rollers. •

There are two ways of providing for requirement (2) , which
calls for the transmission of forces acting horizontally and at
right angles to the bridge. The first method is to fasten some
shape, usually an angle, on the base plate so as to bear against
the sole plate above as shown in Fig. 61/. If the fastening be
tap bolts, the shapes may be readily removed so that the rollers
can be cleaned, repaired, or oiled. It also helps to keep out
cinders, dirt, and so forth.

In the second method, a strip about 2i"Xi" is either riveted
on or planed out from the bottom of the sole plate and the
top of the bearing plate. A corresponding recess about A"
wider is turned in the rollers, Fig. 38^. It may be made exactly
the same depth and then count as a part of the rollers in the
computation for the length required; or, what is more common,
a clearance of say A" vertically may be allowed, but the
width of the slot is then considered to carry no load.

Some means must be provided to keep the rollers the proper
distance apart and parallel. For this purpose, circular rollers

190

ELEMENTS OF STRUCTURAL DESIGN

need but one guide bar on each end. It may be either fastened
to the rollers by tap bolts which are loose in the guide bars and
tight in the female thread of the roller, Fig. 6igy or the roller
may be turned down to a shoulder and enough allowed to pro-
ject beyond the guide bar to allow a cotter pin to be inserted
through a hole drilled for the purpose, Fig. 6iA. Sometimes
these shoulders are cut off flush with the edge of the guide bars

Icr

Tap Bolt
OuideBor

Rolkr
Gukk ear

'Cotter
Fig. 6ig. Fig. 6iA.

Details of Rollers.

and the latter are fastened together by two rods passing through
pipes whose inside diameters are shghtly larger than those of
the rods between the bars. Fig. 6ii.

In the case of segmental rollers, it becomes necessary not
only to keep their axes, but also their planed faces parallel.
The well-nigh universal way of doing this is shown in Fig. 6ie.
Clearance between upper and lower bars must be such as to
allow the proper expansion. Rollers are tap bolted to each of
the guide bars. The former are usually not less than 6" high
in order to allow room for two bars.

^Fv y y y y ^gJL

Fig. 6y.

Fig. 6iife.

Roller Bearings.

Provision against uplift is frequently omitted altogether.
It is best made, however, by prolonging the anchor bolts and
passing them through longitudinally slotted holes in the sole
plate, or by using Z bars. Fig. 6i*, which while allowing the
necessary longitudinal play hold it securely against any upward
movement.

Type {4). Fixed with pin.

In the bearings thus far considered, the deflection of the
girder, if originally true, concentrates a large part of the pressure

THE ENGINEERING DEPARTMENT

191

on the bridge end of the abutment. This may be prevented
by giving the girder a camber, that is, an upward curve sufficient
to make girder true when fully loaded. But the pressure is
uniformly distributed then only for one loading.

A much better method is to use a pin bearing. The shoe is
made in three parts. The upper part is firmly fastened to the
girder and has one or more ribs arranged symmetrically about
the center line of the girder, a diagrammatic view being given
in Fig. 6i/, which also shows the pin perpendicxilar to the plane
of the girder carrying the stress which it receives to the lower
part. This is generally much like the upper portion and rests
directly on the imder surface which as well as the top surface
of the shoe should be planed. Transverse " diaframs,*' explained
below, would be necessary for shoe seen in Fig. 6i/. These

Fig. 6i/. — ^Pin Bearing.

Fig. 6im. — ^Diafram. Fig. 6i». — Gusset.

are not shown in drawing. They must be computed to carry
entire load as shoe would otherwise have little capacity. In
pedestal, they are needed to stiffen plate.

The breadth of the base shoxild be about twice the distance
from the pin to masonry; if made much larger, too much pres-
sure will be concentrated on the area directly xmder the pin;
if made much smaller, there is danger of overturning, although,
of course, a great deal depends on the structural arrangement
for carrying these stresses. A base plate more than i8" in
largest dimensions shoxild be not less than f thick and no
plate should project more than 4" from a support and these
supports should be not more than 10" apart. When the ribs
are not sufficient for this purpose, gussets. Fig. 6i«, may be
built out from them or partitions commonly called " diaframs "
or " diaphragms," Fig. 6iw, placed between them. Both

192

ELEMENTS OF STRUCTURAL DESIGN

serve the important purpose of stiflfening the ribs. When unsup-
ported, the thickness should be at least one-twelfth the length.

The pin is of medium steel and should fasten both shoe and
pedestal together to prevent a possible uplift, although the pin
is often set in two half holes.

The shoe and pedestal may be cast or riveted. In the
former case it may be cast iron or of cast steel, the latter being
better but more expensive. The usual thickness of metal
is i} or li". Four bolts rj" diameter are common for anchor
bolts. *

Riveted shoes can be made lighter than cast shoes and
are less likely to injury from impact. Both shoe and pedestal

Fig. 610.— Cast-Shoe of Type (4). Fig. 61^.— Riveted Pedestal o£ Type (4).

are composed of horizontal plates to which ribs parallel to plane
of girder or truss are attached by means of angles. These
ribs are made of one heavy plate or 2 or 3 lighter ones riveted
together and are planed where they bear.

Diaframs are made of 4 angles and a plate, Fig. 6im,while
gussets are composed of 2 pairs of angles and a triangular plate,
Fig. 6i«.

Rivets in the base plate of the pedestal should be counter-
sunk. Hence, as they have scarcely any stress, use maximum
allowable spacing. The locking together is accomplished and at
the same time a minimum of stress put in the pin by passing it
through the outside plates of the pedestal and the inside plates

THE ENGINEERING DEPARTMENT 193

ot the shoe as shown in Fig. 6il. A clearance of about i"
should be provided between the plates.

Type (5) Free with pin, rolling joint.

This type is made like type {4) except the roller nests and
plates on which it bears, which are Hke type (j).

—Typical Shoe, Type (4). ShowiDg Two Shoes Connected by an End
Strut, American Bridge Co., Ambridge, Pa.

Art. 63. Structural Drawings

The organization of a structural drawing room is about as
follows:

(i) Chief Engineer, in general charge of all engineering
work, and particularly interested in questions of design and
securing new contracts.

(2) Head Draftsman looks out for the minor questions of
design, but his duties are largely executive, allotting work,
handling correspondence, hiring and discharging men. He
should be capable of encouraging a spirit of loyalty in every
subordinate.

(J) Squad Bosses are placed in charge of a group of four to
twelve men, and report to the head draftsman. They are
expected to do actual work besides superintending that of the
other men.

194

ELEMENTS OF STRUCTURAL DESIGN

(4) Estimators, see Art. 53.

(5) Checkers, see Art. 66.

(6) Detailers make the shop drawings from strain sheets
and write biUs.

(7) Tracers copy drawings already worked out in pencil or
write simple bills. They gradually take up detailing.

{8) Stenographers, blueprint boys, errand boys, and so on.

The drawing is preferably made on dull side of tracing
cloth in order to show pencil marks clearly. An experienced
detailer will make a drawing directly on it. Parts of simple
structures may be inked in direct if no changes are likely. The
common and about the best size is 24^X36" outside with one-
half inch border all the way around. Sheets about io''X

Rivets #'
open hol«» )t*

461 ic

42 DeAM5 4DZ ^
4B5

Fig. 62a. — Detail of Several Kinds of a Beam on One Sketch.

12" may be used for detailing I beams, channels, and H sections.
Often beam .sketches with dimension hnes are printed thereon.
Clearness and neatness are essential qualities of a structural
drawing. Use J" = i' for large or very simple work, and i" for
other ordinary cases. Special larger scales are employed only
for complicated details or machining. Where work is of a
uniform nature, a break may be inserted and only a portion of
the length shown. Sometimes center lines of truss are made
to one scale while details around each joint are much larger.
Ignore scale where a clearer drawing may be secured. In
changing dimensions, we seldom alter sketch. Do not attempt
to crowd the drawing. Of course, every additional tracing
means much more blueprinting. However, we believe that no
money is gained by putting objects so close together that
there is not the proper space between views and the different
pieces. Always allow more room than you think, you need,

THE ENGINEERING DEPARTMENT 195

because changes, errors, and lack of foresight tend to increase
amount to be shown on a drawing.

In case several pieces are somewhat alike but not exactly
so, they are detailed by one sketch, notes and separate sets of
dimensions showing difference. Fig. 62a. Notice carefully and
consult many other drawings because no set of rules, nothing
but observation and practice, can make a draftsman.

Always follow a definite order. Fig. 626 shows method for
various views. Omit those that are not necessary. If both ends
are alike, omit one end view, if nearly so, show difference by
notes. It is better if possible to place vertical members with their
tops at the top of the sheet, and
to draw horizontal members hori- f Top view |

zontally. It is wise in many cases

to note "West" or *'Mark this Eievowon

end ^ Top ' " for convenience in

End End

erection. In case structure is view |9echtono« PoHomvk^ vj««v
symmetrical about center line or fig. 626.— Arrangement of Views,
nearly so, note it thus and detail

left half. In constructing views, parts which would obscure
or unnecessary things which would take too much time are
often omitted. Do not shade except for curved surfaces, and
in general avoid artistic or decorative work except perhaps in
dealing with non-technical men in securing contracts.

Conventional signs are very little used except for rivets.
Open holes, the rivets for which are driven in the field, should
be drawn to scale and blackened. A shop rivet is drawn to
scale of head and left open. Flattening the head is shown by
small lines at an angle of 45°: inside the rivet if on inside or far
side; outside if on outside or near side. The number of lines
show the number of eighths of inches in height of rivet head;
if countersunk and chipped an x is used thus:

Field rivet, full head •

Shop rivet, full head O

Field rivet, countersunk and chipped far side, (§)

Shop rivet, flatten to J" near side Q

Field rivet, flatten to f " both sides ^

Fig. 62c,

196 ELEMENTS OF STRUCTURAL DESIGN

A " marking diagram," a small line drawing of complete
structure, is advisable in many instances. Members detailed
oil drawing are shown heavy therein. Small pieces which are
bolted to larger ones for shipment must be so marked, Art. 48, (2).

The lines showing the object whether broken or full should
be of medium weight. If a full even Ine is not secured, the
ruling pen may be dull or the tracing may need a more thorough
rubbing with pounce powder. On the other hand, dimension
lines, center lines, and so forth, should be made as light as pos-
sible to ink readily.

Visible line, 1/80'' thick i'Vi'i'k'i'etc

Invisible line, 1/80" thick "" "~ ""
Dimension line, very thin p

Center line, very thin 1 — ^^;ftl!til_k; — e^.

Section line, very thin k' k' i' k' k' V etc

Fig. 6 2d.

Draw these out to indicated measurements on a piece of paper
and then keep as near as possible thereto by eye. Uniformity
and correctness of proportion are essential even in dotted
lines.

The salient points of lettering are execution, form, spacing,
and general arrangement. The one idea is uniformity. Better
a lettering which is imiformly poor than one which is partly
good and partly bad.

(a) The work done by a man in tracing an ideal bit of
lettering may be termed his execution. Lines must not be
blotted, blurred, or even ragged. For design work, they are
made of mediimi weight. Here it is of utmost importance that
width be uniform. Use a mediima pointed pen which makes
proper weight without pressure. Ruling pen must be employed
altogether or not at all. DiflSiculties encountered by a novice
are a lack of steadiness which practice will generally correct,
and a blurring due to an unclean pen or a partially dried-
up ink.

(6) Form. In every system of lettering, there are certain
proportions which give best results. Beginners must master
and use them. Furthermore, we must have uniformity in :

THE ENGINEERING DEPARTMENT 197

(i) Breadth of letter. Do not mix broad and narrow
letters or figures. Except where crowded, make relative dimen-,
sions the same throughout the drawing and contract.

(2) Height. Make same class of letters same height for
same job. For the bulk of the work, heights should be: for
letters A and f"; whole numbers, A"; fractions^ A". Some
draftsmen do very well without guide lines, but most men need
them. At any rate adjoining letters must have equal heights.

(j) Vertical or slanting letters may be used but inclination
must be kept constant for same class of lettering and particularly
in the same note or line of dimensions.

We give below, first correct lettering and then in turn errors
(&), (i), (2), and 6).

Rivet Rivet Rivet RiVet /Pivet

Fig. 62c.

(c) Spacing between letters of the same word and between
different words should appear equal. This does not mean that
they will be equal.

Rivet not Rivet

Fig. 62/.

(d) General arrangement should harmonize with the drawing.
Place lettering near object to which it refers but avoid crowding
as much as possible. For a long object use a long title and
vice versa.

Notes should state material, size of rivets, and open holes,
paint, and any imusual or important point in the specifications.
They ought always to be placed at the same part of the drawing.

Title should give name of purchaser, location, span, and
kind of structure, name of fabricator, scale, date, when made
and by whom, when checked and by whom, and name of squad
boss in charge.

Besides the drawings of structural steel, the detailer prepares
those for the machinery, bills of various sorts, and the erection

198 ELEMENTS OF STRUCTURAL DESIGN

diagram. The latter is a large line drawing for the use of the
erector and checker. It gives principal dimensions of the
structure, shows each part in its finished position, and states
any directions which may be made necessary by any peculiarities
of design. It should also have a list of drawings with number
and contents.

Art. 63. Auxiliaries— Bills of Material

Auxiliary to the drawing are the following bills:

Material Clevis Nuts

Eyebars, Plain Bent List

Eyebars, Adjustable Castings

Pins and Accessories Field Rivets and Bolts

Pilot and Driving Nuts Shipping Bill

These are written or lettered in ink on transparent paper so
that they may be printed. At the top is the name of the com-
pany, its location, and so on, also a blank for name of pur-
chaser, his location, date, number of sheet, name of draftsman,
checker, and so forth.

The bill of material lists the steel required and combines
it to make the mill order. If a logical procedxire were followed,
this woxild be written after details were made, checked, and
accepted. Instead the order is commonly written as soon after
contract is signed as a draftsman can be secured. When the
drawings are accepted, enough material may be on hand to start
job. At any rate, valuable time has been saved, but at an
increased cost in drawing room. Rough layouts are made to
determine length of sections and details at critical points. Due
to the hurry and the rough nature of the work, errors are quite
probable. Also purchaser's engineer may require changes that
afifect order. If material has not been shipped from mill, it
may be changed. When done this way two bills are written;
the first a rough one in pencil, the second a fbsiished one in ink.
The two mill orders must correspond except for changes.

The finished bill is as follows omitting heading.

THE ENGINEERING DEPARTMENT

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200 ELEMENTS OF STRUCTURiiL DESIGN

At head of mill order, write kind of material. Next the
pieces and mark as shown. Then list main members and after-
wards details in some definite order.

If a shape be changed, for example, a 6''X6" angle sheared
down to 6"Xsi'', note should be made under "Remarks,"
"Cut to 6"Xs|"," while order column reads 6"X6". If
piece be bent, it is mentioned imder .the head of " Remarks."
Such material should be ordered separately from the straight
stuff because former alone is sent to the blacksmith shop.
Also material bolted for shipment must be noted.

Sketch plates are those of irregular plan which are ordered exact
size from the mill. When cost of rectangular plate is greater than
cost of sketch plate at lo cents per hundred poimds additional price
plus value of scrap cut from rectangular plate, about ^ cent per
poimd, sketch plates should be ordered. Also where plates are
too large for shop shears. Its plan must then be placed under
" Description," and " Mill Order " marked " Sketch Plate."

Pieces planed on ends should be ordered about J" long for
each mill; if on flat sides, add ^ to J" for each. For plain
stiff eners, increase length by y"; if they contain crimps, add
i'' for each plus its depth.

The length of bent angles for ordering should be computed
on center of gravity line. Several inches are added to allow of
manipulation in the forge shop.

I beams and channels are purchased in lengths called for
on the drawings, allowance being made for a variation of f "
either way as already explained. Art. 58. Bars, plates, and
angles, less than 15 feet long, are ordered in multiple, that is,
to be cut from longer pieces. The most convenient lengths are
about but not much over 30 feet. Keep number of items
down by combining mill order for each sized section as much as
possible. Do not order lengths exceeding limits published
in hand-books without consulting mills. Try to keep mill order
in even figures, never using fractions less than quarter inches.
If necessary to have two rolled edges, plates must be marked,
" U.M.," (universal mill, Art. 19); otherwise either U.M. or
sheared plate may be furnished.

Latticing should be ordered as so many lineal feet.* Allow
for xmused ends and waste in cutting.

THE ENGINEERING DEPARTMENT 201

Structural companies carry steel in the more common
sizes and some accumulates from errors and alterations in plans.
Unless specifications for material prevent, use all of this which
will fit, and write " stock " xmder " Order No."

Art 64. Bills of Eyebarsi Pins, and Accessories

In the bill of eyebars just below the heading is placed a
sketch of a short eyebar, Fig. 64.

In the atove dimensions, W and T are given by the strain
sheet, Pi and P2 are made A" or 5V'' larger than pin which
is computed as given in Art. 60. Ti and T2 are usually made the
same as T, the exception being where the pin does not furnish
sufficient bearing. This is usually safeguarded by the rule that
it shall be at least iW. Di and D2 are determined from the
standard size of dies, tables of which are furnished the drafts-
man. These are so made that there is at least 30% excess
metal in a section through the pin. See Art. 43.

The length Li is the center to center of joints. Then

r i^l+il T ATS ^1+^2 . r
Li H =1,2 and Li + gives L3.

2 2

To obtain Ai and A 2, compute gross area of eyebar outside
of the width W running from center to center pin holes. Add
10% to allow for burning and waste and divide by W.

The bill for adjustable eyebars is similar to that of the
usual fixed type except that at one end is an upset with thread
for which length, diameter, and mmaber of threads per inch
must be given. One of the threads must be left handed, while
the other is right. The tumbuckles or sleevenuts are specified
by calling for certain standard sizes, for which adjustable ends
must be fitted. The area left at the root of the thread must be
at least 30% in excess of that in the body of the bar. The
length must be such as to give the tumbuckle plenty of play.
About 3'' should be left between the ends of the eyebars. Li, L2,
Zs^and A , are taken from upset end of bar.

Similarly in the bill of pins a sketch of pins and pin nuts
and the necessary dimensions determined as stated in Art. 46

202

ELEMENTS OF STRUCTURAL DESIGN

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THE ENGINEERING DEPARTMENT 203

are given. As in the bill for eyebars, material is ordered on same
sheet.

For pilot and driving nuts, it is simply necessary to call for
the kind and give size of pin.

Art 65. Other Bills

While a clevis nut may be billed in a manner similar to
ej^ebars, it seems an unnecessary amoxmt of trouble since they
are usually one of several standard sizes, the only variation
being in the size of thread, and opening of jaw. As in tiun-
buckles and sleevenuts, they must be threaded right and left
on the same member. The bent list is a rough description of
the work to be done by the blacksmith shop.

Castings may be placed on sheets for purposes of filing, or
they may be detailed there without placing upon drawings at all.

The shop takes care of shop rivets and bolts. Those which
are driven in the field must be listed, however. This is done
on a bill which has columns for number, location, diameter,
grip, and length imder head. The latter exceeds the grip
(thickness of metal grasped by rivet) by an amoimt suflScient
to fill the hole plus enough to make the head.

At the end all rivets of the same length u.h. (under head)
are summed up, and 10% added, to allow for errors and lost or
condemned rivets. If i{i a place where extra rivets can be
readily obtained, this bill need not be checked; if otherwise, a
larger percentage may be added and the list carefully reviewed
For erection in inaccessible locations, add liberally to longer
lengths of rivets; in an emergency, a few might be cut down
with cold chisel.

The shipping bill contains the marks of all parts and mate-
rial to be shipped. The bulk of this is known simply by its
marks. Often a rough description of each piece is added together
with a statement of any pieces which may be bolted for shipment.

204 ELEMENTS OF STRUCTURAL DESIGN

Art 66. Checking

After the drawings and their accompanying bills have been
completed, the checker examines them thoroughly and has the
draftsman correct the errors. He places a small red check-
mark or dot just over each correct word or dimension. Any
which are wrong, he encircles in ordinary or blue pencil, writing
correct amoimt. The author prefers to cross mark the dimension,
perhaps explaining the reason to the draftsman, retaining
the correct figure in his own note-book. This ensures proper
re-examination by the detailer, which is often slighted. Further-
more an ignorant man is likely to erase or even change figures
in correcting the drawing. Some companies keep " field
checkers," who examine field holes and field clearances. We
believe that it is imnecessary if precautions stated in this article
are followed.

Some men check very roughly, running over the dimensions
and examining a few points here and there. Others give atten-
tion largely to small matters which would not amoimt to much
anyway. They are very fond of changing dimensions by a
thirty-second of an inch; as a matter of fact, in their zeal for
extreme accuracy, much more important matters may escape
them. There is a great deal of friction between detailer and
checker. Actual errors must, of course, be corrected, but the
trouble arises over the design of the details. To avoid this:

(/) Let both detailer and checker consider the best good
of their employer.

(2) If checker cannot convince a reasonable detailer of the
wisdom of a change, it is better to let it go as it is. If the
latter feels that his work will not be materially altered, he will
take much more interest.

Next let us consider the good checker. He seldom finds it
necessary to change dimensions by a thirty-second of an inch.
In conference with the draftsman, he discusses minor errors
of design and suggests means for avoiding them in the future.
They consider together the more serious mistakes and the best
method of correcting them. When cost of making change in
drawing room equals or exceeds amount to be saved, he leaves

THE ENGINEERING DEPARTMENT 205

it alone unless deficient in strength. He is quite insistent on
vital matters. No weak points, no infringement of specifica-
tions, no impracticable shop or field work escapes him. Espe-
cial care is taken with open holes and field clearances.

The following system may be used to great advantage. It
is better to go over the entire drawing or set of drawings
considering only one point at a time. Before finally signing
the drawing, be sure that every word and dimension is check
marked.

Schedule for Checking

(i) Check Principal Dimensions. Read correspondence to
be sure no changes have been made by letter. Then check
principal dimensions to agree with purchaser's drawing and also
any interdependent sheets of details already checked. Above
all, compare carefully with masonry plan. Want of agreement
with this is certain to cause trouble. Be sure to examine arrow-
heads at the same time. The latter applies to all dimensions.

(2) Check Agreement with Specifications. If sizes were
given in stridn sheets or general plans forming a part of con-
tract, they should now be compared. Otherwise, they must
be designed from stresses or loads. In the same way, each joint,
splice, connection, and rivet spacing in built-up girders, must
be compared or thoroughly tested. Read specifications and see
that every clause governing the work is satisfied. Particularly:

(a) Are ends of columns milled?

{b) Does thickness of material lie between minimxmi allowable
and maximum practical? (Arts. 50 d2, 44.)

\ {d) Edge distances should lie within permissible values.
(Art. 506/.)

(3) Views and Notes.

(a) Are views properly shown?

(b) Are rights and lefts given where necessary and are
they correct where stated? (Art. 40.)

306 ELEMENTS OF STRUCTURAL DESIGN

(d) Is each necessary mark given, no two being alike?

(e) Are pieces to be bolted for shipment so designated?
(Art. 48.)

(/) Do the remaining notes convey the correct information
concisely?

(g) Are any other notes necessary?
(A) Check title.

(4) Shop rivets and bolts.

(a) Is templet work economized? (Art. 40.)
(6) Is spacing suitable for rack work? ( Art. 44.)
(c) Can rivets and bolts be easily driven? Are rivets
staggered where advisable? (Art. 47.)

(5) Miscellaneous.

(a) Have you good shop clearances?

(b) Is all material listed?

(e) Can lengths as detailed be secured from mill?

{6) Erection.

(a) Are pieces such as to admit of economical shipment?
(Art. 48.)

(b) Consider some easy method by which structure as
detailed could be put together. (Art. 49.)

(d) Test clearances during and after erection. Do this
on each connecting piece, thus reviewing it at least twice.

{e) Can it be painted after erection?

(7) General.
We may now proceed to check lines of dimensions and see
that they add up. Field holes should receive special attention
and, like field clearances, be verified every place where they
occur. For each group check,

(a) Vertical position and spacing,
(6) Horizontal position and spacing.
' (c) Arrangement, how staggered,
(d) Diameter of holes.

THE ENGINEERING DEPARTMENT 207

(8) BiUs.

Bill of rivets need not be checked when located where same
may be readily purchased. Omit also the reviewing of shipping
bills for jobs in the vicinity. Other bills must be checked.
Use above outline where it will apply but in general these bills
are simply listing. Ensure that everything is taken off by
following your own procedure.

Above method is rather slow but in competent hands will
obviate field checking and lessen bill for field extras. The
latter is a statement of extra cost during erection caused by
mistakes in drawing room and shop.

Art. 67. Other Steps

After the drawing is signed by the checker, the squad boss
takes it to the head draftsman or chief engineer of the structural
company, who examines the drawing, usually confining his
criticism to matters of strength and shopwork. After his
approval, the drawings are sent, usually all or a large part of
a contract at a time, to the engineer of the purchaser. His
examination is particularly for strength. If his concern does
the erecting, he should investigate field connections and clear-
ances; for, although the specifications frequently contain
clauses making the seller responsible for any errors, it is better
for both parties that they should be discovered in time. If the job
is for a lump sum, he is careful that details are not skimped;
if for a pound price, he guards against an excess of material.
Changes ordered by either engineer are promptly made. Extreme
care must be taken to see that everything affected is correctly
altered.

As soon as both signatures are attached, blueprints are
taken of all the drawings and bills. As each part of the works
will need a print, from 10 to 20 copies of each must be taken.
Any alteration, whether caused by error or by a change in design,
must be corrected on the tracing and date noted in the title.
Either all the prints which have been sent out must be called
in and destroyed and a new set made from the revised tracing,
or else " change slips " must be sent out to be pinned on the
drawing. These give the alteration to be made and date and

208 ELEMENTS OF STRUCTURAL DESIGN

name of checker. They are regarded in much the same light
as notices from the registrar's office at a imiversity.

Art 68. Examination of Structures in Use

When called upon to examine a structure, the engineer
should obtain the following data :

(/) General dimensions.

(2) Cross sections of members.

(j) Details of these members.

{4) Connections at joints.

(5) Accessories, such as flooring, ballast, etc.

Preceding information may sometimes be obtained from
drawings. Very great care must be taken not to use proposed
plans and even working details are likely to have been modified
during or after construction. Often no trustworthy data can
be found and complete measurements must be had if possible.

{6) Character of material.

(7) Deterioration through rot, rust, or other causes.

(8) Wear caused by traffic,
(p) Workmanship.

Number (6) may be found on plans, or material taken from a
part of the bridge may be placed in a testing machine. The
latter is the approved method of determining injury caused by
fire or an accident of some kind. (7) is obtained for steel by
scratching off rust and comparing former and present thickness.
The condition of timber may be well tested by boring a hole in
a position where it will least weaken the piece. (8) may be
very easy to see as in the planking of a highway bridge or very
difficult as in the shank of rivets. In general the wear of struc-
tural work or its overload is hard to detect by visual examina-
tion. Loosened rivets and excessive deflection are common
signs. Stresses near or beyond the elastic limit result in those
manifestations familiar to all — necking down in tension and
bending in compression. And finally:

(lo) Loads to which the structure will be subjected.

Taking now a standard set of specifications, compute
permissible stress in weakest part of each member. Allowance
must be made for deterioration, wear, or poor workmanship^

THE ENGINEERING DEPARTMENT 209

if such be present. Now compare this with actual total stress.
If excess of latter over former be less than io%, it is all right;
lo to 20%, it needs close supervision; 20 to 40%, is dangerous;
above 40%, must be replaced at once. Of course, these are
general rules to be used with judgment. Carefully consider
such points as efficiency of supervision, loss of life, limb or
property, in case of accident, and probability of occurrence
of maximum load.

Art. 69. Failures

Space will not permit us to discuss all prominent structural
failures. We shall take up only a few which will be instructive.
One purpose is to give the young designer a proper apprecia-
tion of the responsibility which he bears.

We think the record is not a bad one when considered as a
proportion. Furthermore, the continual changes in the art of
bridge building have deprived us of the light of experience.
Pressure for extreme economy in engineering, material, and
construction have made very narrow margin for the designer.
What wonder if the bounds are overstepped at times? Such was
the cause of the disaster to the Quebec bridge, the most important
of recent times.*

This structure, Fig. 69a, was a cantilever span across the St.
Lawrence River, with two anchor arms of 500 feet, two cantilever
arms of 562.5 feet, and one suspended span of 675 feet. The
latter two made distance between piers 1800 feet, 90 feet longer
than the Forth Bridge, the longest existing span. The Quebec
Bridge was 67 feet center to center of trusses and had a max-
imxmi depth of 315 feet. It was intended for two railway
tracks, two electric car tracks, two roadways, and two foot-
ways. While building out as a cantilever beam, Aug. 29, 1907,
it fell into the river, 150 feet below. Fig. 696. Seventy-four
lives and two million dollars were lost.

The causes were: Assumed dead load was only about 75%
of the actual, too high stresses were allowed, and, by far the
most important, insufficient latticing was provided for com-
pression members. The. lattice bars of a bottom chord member,

* See Engineering News, September 5, 1907, et seq.

Fig. 69a. — Quebec Cantilever Bridge before its Fall,

Fig. 696.— Wreck o( the Quebec Bridge.

THE ENGINEERING DEPARTMENT 211

Fig. 565, broke when its stress was 14,000,000 lbs., or 17,900
lbs. per sq.in. This and other columns in the vicinity suffering
about the same imit stress had been showing signs of overload.
Tests on a model post, one-third size, showed an ultimate com-
pressive strength of 22,000 lbs. per sq.in. A better latticed but
somewhat similar column bent in body at 30,000 lbs. per sq.in.
In other words, the compression strength of large pieces of good
design is not far from 25^00 pounds per square inch.

We may divide failures into two classes, those which result
from:

(7) Some fault of design or maintenance, and

(2) A very unusual occurrence. Such are earthquakes,
extreme storms, or floods, derailment or collision on a bridge.
Provision can be made for these accidents and sometimes this
is done. However, generally speaking, one is not justified in
spending money for remote contingencies.

We may further subdivide (/) :

(la) Ignorance. Such was the case in the disaster which
occurred on the Boston and Albany Railroad at Chester, Mass.,
in 1893,* Here were located two through skew spans, each of
riveted quadruple latticed trusses. Workmen
were strengthening the bridge by placing addi- m«bb
tional cover plates on top chord. Fig. 69^. To IT

do this, old rivets were driven out, replaced by | |

bolts, the plates added, and rivets substituted ^^ 6o<:— Section
for bolts, a few at a time. The foreman does of Top Chord of
not seem to have fully appreciated the function of Chester Bridge!
the rivets and allowed too many holes to remain
empty. A train of an engine and eight passenger cars was
passing over the thus weakened structure when it broke, killing
17 and injuring 32.

{lb) Economy in two forms:

(767) Saving in first cost so extreme that failure results as
in the Quebec bridge.

(762) Keeping a structure in service under loads much
heavier than those for which it was designed. The writer once
visited a wreck caused by a mistake of this sort. As near as he
can recollect, it was a railway deck plate girder of 65 feet span.

••Engineering News, September 7, 1893, et seq.

212 ELEMENTS OF STRUCTURAL DESIGN

Web was 5 feet deep, made up of 36"xA"Xs'--o'' plates,
spliced by two bars and a single row of rivets on each side. ^
Flanges were each of 2 Ls, 6''X6"XA" and 2 cover plates
14'' X A"- Stiff eners were 6 to 9 feet apart.

(ic) Lapses.

We shall so term cases where an engineer, otherwise skillful,
has shown incompetence or forgetfulness in one particular
respect. As such we shall class the Tay Bridge disaster.*
This occurred at the Firth of Tay in Scotland, Dec. 29, 1879.
The structure was a viaduct about two miles long and contained
85 spans varying in length from 27 to 245 feet. The 13 that
fell were through riveted lattice trusses of 245 feet span. They
were supported on hexagonal towers whose legs were of cast
iron. On the night in question, a storm was raging and the
wind was blowing against the truss with a speed estimated at
72 miles per hour. When the engine with seven coaches had
almost reached the middle of the thirteen spans in question,
the whole blew over. Not one of the 75 persons on board sur-
vived. The structure, otherwise well designed, lacked strength
to resist wind stresses. Failure was probably due to weakness
of laterals and their connections by lugs to the<cast-iron columns.

(id) Unforeseen conditions.

We refer here, not to the extremely improbable conditions
which we have already discussed but rather to those which are
probable and should be guarded against.

Such a case occurred in the Home building fire in Pittsburgh
in May, i897.t A large part of the damage was caused by the
fall of a water tank on the roof. This was carried by naked
beams protected only by a suspended ceiling below. When
fire occurred, it wrecked this ceiling, and the heated beams
allowed tank to fall. Beams themselves should have been
fireproofed.

We will give in addition two more historic failures.

The Ashtabula accident happened on the night of Dec. 29,
1876. t The bridge was a deck, double track, Howe truss built
of iron. The trusses had 14 panels at 11 feet, were i9'-9"

* Engineering News, January 3, 1880.

t Engineering Record, Vol. XXXV, p. 537.

t Engineering News, January 6, 1877 et seq.

THE ENGINEERING DEPARTMENT 213

high, located i7'-2" c. to c. Compression members were each
jnade of several small I beams with practically nothing to cor-
respond to modem latticing, and details were very poor. As a
heavy train with two locomotives and eleven cars was passing
slowly over the bridge, it collapsed just as first engine had
nearly reached the other abutment. The train fell 75 feet and
was consumed by fire. Of 209 persons on board, 92 were
killed and 64 injured. It is supposed that failure occurred
in the top chord under the first locomotive.

The Bussey Bridge near Boston, Mass., was a skew bridge
of 104. feet span.* One truss was a deck Whipple of 16 panels
at 6.5 feet and depth 12.5 feet. Other was 4 panels at 26 feet
and depth 16 feet. Pin through floorbeam was attached to
pin at hip joint by an eccentric loop welded hanger made of two
bars about if'XiJ". This was improperly designed and
broke, allowing latter part of a long train to fall through the
bridge; 32 were killed and 70 hurt.

We shall not attempt to enumerate the troubles of stand-
pipes and highway bridges. Failures of the former are frequent
and not always easy to explain. The weakness of .latter is
caused by:

(a) The election of poUticians rather than business men or
engineers to committees having the matter in charge.

(b) The tendency of the layman to rely rather upon the
advice of salesmen than upon that of reputable designers.

{c) Attempts to economize. Except when aided by experts,
this is very poor policy. Only a trained man can tell whether
a low price represents inferior work or not. Usually it does.

(d) The economy which precludes expert advice, operates
in inspection of details and field erection.

By far the greater portion of accidents that have come
to the author^s attention have been due to slowly applied or
quiescent forces. Some of the structures mentioned above
had at times carried rapidly moving loads. However, the
failure did not take place then but later with less impact.

* Engineering News, March 19, 1887 ct seq.

INDEX

A frame, 120

Add process, 17

Administration of plant, 79

Allowable unit stresses, 134

Allowable unit stresses in timber, 11

Alloys of steel, 19

Angle block, 69

Angle shear, 86

Angles, 31

Angles for beams, 148

Angles for columns, 161

Angles for tension, 159

Annealing, 19

Ash, II

Ashtabula bridge disaster, 212

Assembly, no

Assembly marks, 177

Assumptions for rivets, 179

Auxiliaries, 198

Auxiliary boom, 122

B
Bars, 30

Basic process, 1 7
Bastard sawed timber, 24
Batter posts, 76
Beam connections, 179
Beam shears, 85, 104
Beam splices, 181
Beams, design, 148
Bearing blocks, 41
Beech, 11
Bending, 104
Bent list, 203

Bents, trestle, 76

Bessemer steel, 16

Bethlehem sections, 28, 33, 150

Bevels, 23

Bill of castings, 203

Bill of clevis nuts, 203

Bill of eyebars, 201

Bill of material, 198

Bill of rivets, 203

Bill of pins, 201

Bill, shipping, 203

Billing material, 174

Boarding, 54

Boarding, design of, 57

Bolster, 39

Bolts, 42, no

Bolts, strength of, 44

Boring machines, 92, 96, 108

Boring mill, 92, 96, 108

Box colimm, 164

Box girder, 151

Boxing timber, 12

Bridges, 67

Buckle plates, 105

Built I-beams for columns, 163

Bulb angles, 36

Bulb beams, 36

Bumettizing timber, 9

Bussey bridge failure, 213

Camber, 138
Cantilever traveler, 124
Caps for trestle, 76
Case hardening, 6

215

216

INDEX

Cast iron, 13, 19

Castings, 25

Castings, steel, 19, 35

Cedar, 10

Centrifugal force, 133

Change slips, 207

Channels, 33, 34

Channels for beams, 148

Channels for columns, 161

Channels for tension members, 160

Checking, 204

Checking in timber, 6

Chester bridge failure, 211

Chestnut, 11, 12

Chief Engineer, 193

Chipping, 94

Circular shapes, 29

Clapboards, 25

Clearance for riveting, 114

Clearance diagram, 132

Clevis nuts, 107, 203

Cold rolling, 28

Cold saw, 86, 104

Cold shortness, 15

Color of timber, 5

Column sections, 35, 161

Column splices, 181

Columns, design of, 161

Columns, details, 137, 138

Combination trusses, 56

Commercial shapes of timber, 24

Compressed air, 82

Compression members, design of, 161

Compression members, details, 137,

138, 167
Computations for roof truss, 57
Computations for bridge, 69
Connection plates, 40, 46-49
Conventional signs, 195
Coping machine, 86, 104
Corbels, 75
Cores, 25
Carnice, 38, 54
Cost of erected steel, 148
Cotter, 113
Cotter pins, 113
Countersinking, 116

Creeper traveler, 124
Creo-resinate process, 10
Creosoting timber, 8
Crimping, 105
Cross girts, 76
Cross shear, 86
Cypress, 11

Dapping ties, 150

Deck beams, 36

Derrick, 120

Derrick cars, 122

Design in general, 51, 140

Design of beam connections, 182

Design of beams, 148

Design of columns, 161

Design of sections, 135

Design of splices, 181

Design of tension members, 157

Detailing, 172

Details of columns, 137, 138, 167

Diaframs, 191

Dimensions, 23

Douglas fir, 10, 12

Dowel pins, 42

Dowels, 42

Drawings, 194

Drift bolts, 42

Drift pins, no

Drills, 91- 96, 108

Driving nuts, 113, 140

Dry rot, 7, 38

Durability of timber, 8

Economical relations, 143

Electric lighting, 82

End milling machine, 94

Engineering department, 79

Entering joints, 129

Erection, 80, 119, 139

Estimating, 146

Examination of structures in use, 208

Executive- department, 79

Expansion, 138, 188

INDEX

217

Expansion bolt, iii
Eyebars, 98, 106
Eyebars, bill of, 201

Failures, 209

Falsework, 127

Field extras, 207

Financial department^ 80

Finishing pass, 27

Fins, 27

Fireproof timber, 10

Fish joint, 43

Flats, 30

Flitch plate girders, 40

Floating method of erection, 1^7

Floorbeams, 67

Floorbeams, computations for, 70

Forge shop, 98

Foundation bolts, iii

Four angles as a column, 162

Framing timber, 53

Freight rates, 1 19

Fungus growth, 6

Gages, 23

Gallows frame, 122

Gang punch, 90

Gate shears, 87

General plan, 172

Gin-pole, 120

Girder, box, 151

Grey process of rolling, 28, 33, 150

Grip of rivet, 1x6, 135

Guard timber, 132

Gussets, 191

H-section for columns, 161
H-shape, 36
Handbooks, 22
Hangers, 40» 5i» loS
Hard pine, 10, 12
Hard steel, 17
Haseiman process, 10
Head draftsman, 193

Heartwood, 3, 12

Hemlock, 10, 12

Holder on, J15

Hook bolts, III

Home building fire, 212

Howe truss, 67

Howe truss, computations for, 69

Hydraulic jacks, 120

Hydraulic pressure, 82

Hydraulic riveter, 94

I-beams, 33

I-beams for beams, 149

I-bcams for columns, 161

I-beams for tension members, x6o

Inhibitors, 20

Initial stresses, 19, 26

Inspection, 117, 20S

Inspection department, 80

Iron, cast, 13

Iron, wrought, 15, ig

Jacks, hydraulic 120
Joints, 42, 184

K
Keys, 41
Kiln drying, 4
King post truss, 62, 66
Knee bracing, 54
Knocked down, 1 74
Knots, 4, 6
Kyanizing timber, 9

Lag screws, 42

Lathes, 96

Lattice truss, 67

Latticing, design of, 166

Launching method of erection, 127

Layout, 84, 100, \og

Least work, 65

Lettering, 196

Lines, 196

Linseed oil, 20

218

INDEX

Loads for railroad bridges, 133

Lomas nut, 112

Long leaved Southern pine, 10, la

Loop rods, 105

Lumber, see Timber.

Lump sum, 143

Machine shc^, 95

Main shc^, 84

Maple, II

Marine worms, 7, 8

Marking diagram, 196

Medium steel, 17, 18

Medullary rays, 3

Methods of bending material, 104

Methods of cutting material, 102

Methods of erection, 119

Methods of making holes, 108

Methods of riveting, 113

Methods of upsetting, 106

Metric system, 22

Mill variation, 176

Milling machine, 94, 98

Mortise and tenon, 49

Mouldings, 25

Multiple punch, 91, 108

N
Nails, 42
Nickel steel, 19
Norway pine, 10, 12

O
Oak, II, 12
Occasional shapes, 34
Odor of timber, 5
Offices, 82
Ogee washer, 4!
Operating department, 79
Order department, 79
Oregon pine, 10, 12
Organization of administration, 79
Outriggers, 122
Overrun, 28

Painting, 118, 139

Paints, 20

Parting lines, 25

Pattern, 25

Percussion riveter, 93

Phoenix columns, 35

Pigment, 20

Pilot nuts, 113, 140

Pui joints, 184

Pine, 10, 12

Pins, III

Pins, bill of, 201

Pins, computations, 186

Pipes, 36

Pith rays, 3

Planers, 96

Planing, 25

Plant, 80

Plate, 54

Plate girders, economical depth, 143

Plate girders, erection, 126

Plate shears, 86

Plates, 30

Pneumatic drill, 92

Pneumatic hydraulic riveter, 93

Pneumatic pressure, 82

Pneumatic riveter, 93

Poplar, II

Portal, design of, 72

Posts for trestles, 76

Power plant, 82

Preliminary plans, 140

Presefvation of timber, 8

Principles for cutting material, 104

Principles for castings, 25

Principles of design in wood, 37

Principles for detailing, 178

Principles for dimensioning, 23, 102

Principles for erection, 129

Principles for joints, 179

Principles for making holes, 109

Principles for riveting, 116

Principles for shipping, 119

Principles for templet making, loi

Punches, 87

Punching, 102, 108, 138

INDEX

219

Purlins, 54

Purlins, design of, 58

Quarter sawed timber, 24
Quebec bridge, 165, 209
Quec(n post truss, 62, 67

Rack punch, 90

Radial drill, 92, 108

Rafters, 54

Rafters, computation, 58

Ragged bolt, iii

Rails, 35

Rare shapes, 36

Rat-proofing, 38

Reaming, 108, 139

Rectangular shapes, 30

Red shortness, 15

Re-entrant cut, 103

Resonance of timber, 5

Ridge pole, 54

Rift sawed timber, 24

Right and left, 100

Rivet steel, 18

Riveted joints, 183

Riveters, 93

Riveting, 113

Rivets, no, 136-139

Rods, 29

Rods for tension members, 158

Rollers, 98, 189

Rolling, 27

Roof truss, computations for, 57

Roof truss, description, 54

Roofing, weight, 56

Rot, wet and dry, 7, 37

Rotary planer, 94

Roimds, 29

Sales department, 79
Sap wood, 2, 5
Saw, 86
Scarf joint, 43

Seasoning timber, 3, 12
Segmental roUers, 98, 189
Segregation, 15
Separators, 149
Shakes in timber, 5
Shapers, 96
Sheared plate, 30
Shears, 85, 102
Sheathing, 54
Sheathing, design of, 57
Shingles, 13, 25
Shipment, 118
Shipping bill, 203
Shipping department, 80
Shipping marks, 178
Shoes, 138, 185, 186
Shop, 8s

Shrinkage of timber, 3, 37
Sills for trestle, 76
Sizing, 54
Sketch plates, 200
Skew cuts, 103
Sleeve nuts, 106
Slotted holes, 109, 188
Snow, weight, 56
Soft steel, 17, 18
Spacing tables, 90
Specifications, 131
Spiegeleisen, 16
Splice angles, 36
Splices, 181
Split shears, 86
Spools, 75
Spruce, 10, 12
Squad bosses, 193
Squares, 30
Steel alloys, 19
Steel, Bessemer, 16
Steel castings, 19, 25
Steel, hard, 17
Steel, medium, 17, 18
Steel, nickel, 19
Steel, open hearth, 17
Steel, rivet, 18
Steel, soft, 17, 18
Steel, vanadium, 19
Stiff eners, 136, 139

220

INDEX

Stock yard, 83
Straightening rofts, 83
Strain sheet, 172
Strength of timber, 11
Stringers, 67
Stringers, design of, 70
Stringers, for trestles, 75
Structural drawings, 193
Stud bolt. III
Sub-punching, io(>
Swedged bolt, iii

T-beams, 34

Table of squares, 22

Tap bolts. III

Tay bridge disaster, 212

Templets, 100

Tension members, design, 157

Three I-beams for columns, 163

Ties, 13, 68, 132

Timber, allowable stresses, it

Timber, color, 5

Timber, commercial shapes of, 24

Timber, durability, 8

Timber, faults, 5

Timber, fireproofing, 10

Timber, grain, 2

Timber, growth, 2

Timber, knots, 4, 6

Timber, odor, 5

Timber, preservation, 8

Timber, principles of design, 37^

Timber, protection from fire, ^S

Timber, resonance, $

Timber, sapwood, 2, 5

Timber, seasoning, 3

Timber, shrinkage, 3

Timber, strength, 11

Timber, uses of, 13

Timber, varieties, lO-

Timber, weight, 5

Toggle joint riveter, 93

Tongued and grooved timber, is

Tracers, 194

Tractive force, 133

Traveler, 122

Traveler, cantilever, 124

Traveler, creeper, 124

Trestle bents, 75

Trough sections, 35

Truss bridges, erection, 127

Truss, weight, 56

Trussed beams, 62

Trusses, economical relations, 144

Tumbuckles, 106

Turned bolts, iii, 139

Two angles and plate for columns, 162

Two angles for columns, 162

Two channels and I-beam for columns,

163
Two channels and plate for columns,

164
Two channels for columns, 163

U-bolt, III
Universal mill, 27
Upset ends, 40, 106, 139
Upsetting, 106
Uses of timber, 13

Vanadium steel, 19
Variation, mill, 176
Varieties of timber, 10
Viaducts, erection, 126

Wane in timber, 6
Washer fillers, 112
Washers, 41, 112
Water creosote process, 10
Weights of roofing, 56
Weight of snow, 56
Weight of timber, 5
Weight of trusses, 56
Wellhouse process, 9
Wet rot, timber, 7, 37
White pine, 10, 12
Whitewood, 11
Wind pressure, 56, 133
Wire drawing, 29
Wires, 29

INDEX

221

Wood, see Timber.

Wood fibres, 3

Wood screws, 42

VV^ooden bridges, 67

Workmanship in ^>edfication8, 138

Wrought iron, 15, 19

X-bracing, 54

Yellow pine, 10, 12

Zee bars, 34
Zee bars for beams, 148
Zee bars for colmnns, 161
Zinc creosote process, 9
Zinc tannin process, 9

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