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REPORT 



ON THE 



■lACKWELL'S ISLAND BRIDGE 

(QUEENSBORO BRIDGE) 




C. KUNZ 

Chief Engineer 



THE PENNSYLVANIA STEEL CO, 



fernia 
laJ 



And a Commission consisting of 
CHARLES MACDONALD, Consulting Engineer 

Past-President Am. Soc. C. E. 

C. C. SCHNEIDER, Consulting Engineer 

Past- President Am. Soc. C. E. 

H. R. LEONARD, Consulting Engineer 
J. E, GREINER, Consulting Engineer 



Steelton, Penna.y March 24, 1909 



REPORT 



ON THE 



BLACKWELL'S ISLAND BRIDGE 

(QUEENSBORO BRIDGE) 



By F. C. KUNZ 

Chief Engineer 

THE PENNSYLVANIA STEEL CO. 



And a Commission consisting of 
CHARLES MACDONALD, Consulting Engineer 

Past-President Am. Soc. C. E. 

C. C. SCHNEIDER, Consulting Engineer 

Past-President Am. Soc. C. E. 

H. R. LEONARD, Consulting Engineer 
J. E. GREINER, Consulting Engineer 



Steelton, Penna., March 24, 1909 



UlSount Ipleasiant f^ttii 

J. Horace McFarland Company 
Harrisburg, Pennsylvania 



5RLF OtI SS&^^^'^O 



URL 



INDEX 



PAGE 



General Elevation and Progress Photograph of Bridge .. Frontispiece / 
Queen's Approach, View Looking through Bridge Showing'Main Road- 

wa}^ Frontispiece II 

Letter Transmitting Chief Engineer's Report to Commission 5 

Report of Commission 7-13 

Chief Engineer's Report 15-46 

Supplement to Chief Engineer's Report 47 

Appendix A: Extracts from Reports of Experts 49-51 

Appendix B: Extracts of Specifications for the Steel Superstructure 52-57 
Cross Section of Bridge with Two Rapid Transit Railroad Tracks... 58 
Cross Section of Bridge with Four Rapid Transit Railroad Tracks. . . 59 

Photograph of Crowd ^in Coney Island 61 

Photograph of Crowd on Ferryboat 63 

Photograph of Gathering of Teams 65 

Table 1: Diagram of "Continuous" Live Load for Chords. 
Table 2: Diagram of "Continuous" Live Load for Web Members. 
Table 3: Diagram of "Discontinuous" Live Load for Chords. 
Table 4: Diagram of "Discontinuous" Live Load for Web Members. 
Table 5: Unit Stresses for Various Conditions of Loading with Origi- 
nal Paving. 
Table 6: Unit Stresses for Various Conditions of Loading with Final 

Paving. 
Table 7: Diagram Showing Congested Loading. 



BlackweU's Island (Queensboro) Bridge 



THE PENNSYLVANIA STEEL COMPANY 

OFFICE OF THE VICE-PRESIDENT 



J. V. 



W. Retnders 

Vice-President 



Steelton, Pa., December 22d, 1908. 
Mr. Charles MacDonald, 
Mr. C. C. Schneider, 
Mr. H. R. Leonard, 
Mr. J. E. Greiner. 

Dear Sirs: The Pennsylvania Steel Company, on November 20th, 1903, 
contracted with the City of New York to furnish the steel superstructure 
of the Blackwell's Island Bridge, in accordance with plans and specifications 
prepared by the Department of Bridges, under the commissionership of 
Mr. Gustav Lindenthal. On December 15th, 1904, the city entered into 
a supplementary contract with the Steel Company, providing for certain 
work not originally contemplated, including the addition of two elevated 
railroad tracks. The work was completed June 15th, 1908, and a certificate 
of acceptance issued by the Department of Bridges. 

In the spring of 1908 articles appeared in one of the New York daily 
papers criticizing the design of the bridge and drawing analogies between 
it and the Quebec Bridge, which collapsed some months previous. Sub- 
sequent investigations of the structure, conducted at the instance of the 
Department of Finance and the Department of Bridges, of the City of New 
York, by Prof. William H. Burr and Messrs. Boiler & Hodge, have led to many 
serious misconceptions in the public mind. Proper appreciation of the find- 
ings of these engineers presupposes a clear understanding of the original 
assumptions upon which the specifications and general plans were drawn, 
and the significance of which must be read into the computations which 
formed the basis of the reports. 

As far as that part of the work is concerned for which the steel contractor 
was responsible, the reports are uniformly favorable, the following con- 
clusions of Messrs. Boiler & Hodge being characteristic of both reports, viz.: 



"(Second) That the steel manufactured for this structure is first- Conclusions i 

class bridge material and in accordance with the specifications. Boiler & ' ' 

"(Third) That the workmanship of this structure is first-class Hodge ; 

and in accordance with the requirements of the specifications. i 

"(Fourth) That the erection and field riveting of the structure \ 

appears to have been done in a first-class manner. j 

"(Fifth) That the actual sections of the various members agree | 



6 LETTER TRANSMITTING CHIEF ENGINEER'S REPORT 

with the sections ordered on the working drawings and shown on 
our sheets Nos. 8 and 9, and that the shipping weights are correct." 

(See Appendix A, page 49, for other extracts.) 

While, therefore, from the point of view of a contractor we are not in- 
volved in any of the issues that have been raised, it is proper that our knowl- 
edge of the situation should be made available both for the information of 
the engineering profession as well as the general public, whose sense of security 
in respect to this structure has been unduly disturbed. 

From this point of view, we have asked our Chief Engineer, Mr. F. C. 
Kunz, to prepare a report setting forth all the salient points that have been 
raised from time to time, with such information as we are able to supply 
in regard to the same, and we now ask that you, as a Commission, carefully 
examine this report, advising us whether you agree or disagree with the 
conclusions as set forth therein, and stating briefly the grounds upon which 
your opmion is based. 

Very truly yours, 

J. V. W. REYNDERS, 

Vice-President. 



Report of Commission 



The Pennsylvania Steel'^^Company, 

J. V. W. Reynders, Vice-President, Steelton, Pa. 

Dear Sir: Since the failure^of the Quebec Bridge, public confidence has 
been somewhat disturbed as regards the safety of bridges of unusual mag- 
nitude. This feeling of distrust has been aggravated by the opinion expressed 
in the report of the Royal Commission, appointed to inquire into and report 
on the cause of the failure of the Quebec Bridge; this report was published 
and has been extensively quoted by the technical journals in this country 
as well as abroad. The unwarranted remark contained in this report, that 
"under extreme conditions, the Quebec Bridge stresses are in general har- 
mony with those permitted in the Black well's Island Bridge," produced 
the impression in the minds of the New York public that the Blackw^ell's 
Island Bridge might, sooner or later, share the fate of the Quebec Bridge. 

For the purpose of obtaining an unbiased opinion as to the true con- 
dition of the Blackwell's Island Bridge, the Commissioner of the Depart- 
ment of Bridges of New York City appointed two experts to examine the 
design and construction of this bridge. 

Owing to the technical nature of their reports and a lack of clear under- 
standing of the significance of the assumptions upon which the computations 
were based, the statements and conclusions contained therein have led to many 
serious misconceptions in the public mind. They have been misunderstood 
and misinterpreted by engineers who are not experts in bridge design, have 
been used by a small section of the local engineering press as a basis for the 
unjust assumption that the early designers of the bridge, as well as those 
who followed, blundered seriously, and foreign technical journals have taken 
the opportunity for the abuse and wholesale condemnation of American 
practice in general, and the judgment of American engineers in particular. 

The pubUc confidence, which was disturbed by the Quebec failure and 
by the unwarranted comparison of that bridge with the Blackwell's Island 
Bridge structure, has certainly not been restored by the reports of the city's 
experts on the latter bridge. In fact, the strength of the Blackwell's Island 
Bridge has now become a question of such serious and far-reaching impor- 
tance, affecting not only the confidence of the public in engineering works, 
but the professional standing of American engineers, that it is assuredly 
proper and advisable for the contractors to make available their knowledge 
of the situation; and the undersigned, at the request of The Pennsylvania 
Steel Co., as conveyed in your letter of December 22nd, 1908, have reviewed 
the report of your Chief Engineer covering this subject. 

7 



8 REPORT OF COMMISSION 

We have carefully examined this report, dated November 27th, 1908, 
together with all data and information furnished in connection therewith, 
including the specifications, contract, strain sheets, reports of experts, etc., 
etc., and substantially endorse the arguments, conclusions and recommen- 
dations therein set forth. 

Our report should, in our judgment, be an answer to the unjust and 
disturbing criticisms that have appeared in the public prints rather than 
a resume of the entire subject; the structure has been so overloaded mathe- 
matically that the confidence of the public has been shaken, and it is only 
by an appeal to common sense, rather than to technicalities, that this lost 
confidence can be regained. 

No question has been raised as to the sincerity of the contractors in exe- 
cuting the work; and the quality of the material, character of workman- 
ship and adherence to approved plans have been endorsed by the city's 
experts. It is our understanding, after careful study, that the contract gave 
free powers to the Bridge Department in general design and engineering 
changes, and does not presuppose engineering knowledge on the part of 
the contractor further than is necessary for the proper execution in detail 
of the general orders of the Department. 

The entire argument can well be based on the consideration of this ques- 
tion: Were the original specifications, and the subsequent modifications made 
by the Bridge Department, of such a nature that there can be any engineer- 
ing doubt as to the safety and usefulness of this structure as a public high- 
way? 

A bridge is a highway and is not designed primarili/ to carry so many 
pounds per linear foot, but to accommodate so much traffic, and from an 
estimated weight of such traffic the design is perfected. For a railroad bridge 
such weights are easily determined, but for a roadway structure, the deter- 
mination of live load weights is largely a matter of good judgment. 

No material difference of opinion can arise as to the proper loading to 
be assumed in the designing of minor parts, i. e., hangers, floor-beams, stringers, 
etc.; such loading would cover but small areas and a maximum density of 
traffic could easily be conceived, but for the main trusses in a structure 
of any magnitude, the application of live loads of maximum density ("con- 
gested") over any extended areas would preclude all possibility of motion, 
a condition that would destroy the usefulness of the structure as a vehicle 
*of traffic, a condition so absurd that it would not be tolerated in a city street. 

The measure of usefulness of any public highway is the amount of traffic 
that can be safely and expeditiously handled over same, and any police 
regulation made for the proper handling of such traffic is a necessity for the 
benefit of the traveling public and does not, in itself, lessen confidence in 
the structure. 

Traffic rules, requiring the surface cars to keep to certain clear distances 
apart are no hardship; quite the reverse, in allowing for greater freedom 
of motion, they are in the interest of increased traffic and consequently in- 
creased capacity. On elevated railroad tracks, proper spacing of trains is 



REPORT OF COMMISSION 9 

a measure of safety, and the greater the interval of space, the greater could 
be the permissible speed; block signal and automatic devices now in use 
absolutely prevent electric trains from encroaching on the space deemed 
necessary for their prompt and safe handling. 

Live Load. Professor Burr, in his report on this bridge, states: 

"Proper provision for various classes of loading for a structure 
of such magnitude, designed to carry an extraordinary volume of 
traffic, with the corresponding working stresses, is largely a matter 
of judgment." 

In this connection, we wish to call attention to the investigations of a 
Commission appointed in 1903 by the Mayor of New York City to examine 
and pass upon the plans of the Manhattan Bridge, which was then designed 
as a suspension bridge with stiffened eye-bar cables. While the report of 
this Commission refers wholly to the Manhattan Bridge, the recommenda- 
tions as to the live loads for which it should be designed have been applied 
to the Blackwell's Island structure, which was intended to carry the same 
kind of traffic. This Commission, consisting of Messrs. George S. Morison, 
C. C. Schneider, Henry W. Hodge, Mansfield Merriman and Theodore Cooper, 
made a very thorough investigation of the subject and recommended that 
the Manhattan Bridge should be designed, in so far as the main members 
are concerned, to carry a "maximum working" load of 8,000 pounds per 
Unear foot, and that a so-called "congested" load of 16,000 pounds be used 
in proportioning the hangers. Quoting from this report, in regard to further 
use of the "congested" load, 

"We consider that the bridge should be so proportioned that 
■ with a congested load of 16,000 pounds per Unear foot, covering the 
whole bridge, combined with dead load and wind pressure, no stresses 
should be produced anywhere reaching the elastic limit of the material 
or impairing the stability of the anchorages. We consider that the 
working load of 8,000 pounds per linear foot should be used in design- 
ing the main members of the structure. The congested load should 
be used in proportioning the hangers." 

Any fair interpretation of this report would indicate that the intention 
of the Commission was to recommend the use of the "congested" load for 
proportioning the hangers (using a working-unit stress) and as an extreme 
test of the cables and anchorages; and that the "working" load be used in 
proportioning the main members of the structure. The above quotation 
appears (in part) in the report of one of the city's experts, but the omission 
of the words "or impairing the stability of the anchorages" and the omission 
of the last two sentences quoted above, admits of an entirely different con- 
struction being placed on the recommendation. 

Independent investigations which we have made confirm the estimate 
of the Manhattan Bridge Commission as well as that of the city's experts, 
fixing the maximum load on any extended area of roadway or footwalk at 



10 REPORT OF COMMISSION 

50 pounds per square foot. With the increased weights of elevated and 
surface cars, cited in the experts' reports, we would obtain as a "congested" 
load, 15,955 pounds per linear foot, made up as follows: 

4 elevated 8-car trains, at 1,810 pounds 7,240 per lineal foot 

4 trolley tracks, at 1,460 pounds 5,840 per lineal foot 

35.5 feet roadway, at 50 pounds per square foot 1,775 per lineal foot 

22 feet footwalk, at 50 pounds per square foot 1,100 per lineal foot 

15,955 

Such a load, with roadway and footwalk crowded, trolley cars bumper to 
bumper, and elevated tracks completely covered, is an impossibility unless 
special and extraordinary means were taken to produce it, and the term 
"congested" appUed to such a loading in connection with the computations 
of stresses in main members is misleading; a "theoretical test load" or 
"extraordinary load" would be terms more applicable. 

A maximum working load is much more complex of analysis than a "con- 
gested" load, and is a matter on which the judgment of engineers may be 
expected to differ. With a "congested" load provided for, the possibility 
of failure from collapse is eliminated, but to provide for such a load at ordi- 
nary working units would be an unwarranted extravagance, hence the econom- 
ical necessity for determining a working load and providing for such loading 
with working units. The working load of 8,000 pounds per linear foot of 
bridge recommended by the Manhattan Bridge Commission and used in the 
computations of the Blackwell's Island structure by the Bridge Department 
could be analyzed as made up of 20 pounds per square foot on the footwalks 
(an uncomfortable walking crowd), 30 pounds per square foot on the road- 
way (equivalent to a semi-congestion of average vehicles), 1,945 pounds 
per linear foot on four trolley tracks (equal to heaviest loaded cars spaced 
two car-lengths apart) and 4,550 pounds per linear foot as an equivalent 
load for the elevated trains on four tracks, trains spaced about 1,000 feet 
apart; such loading can be conceived, but it is doubtful whether these weights 
would be reached once in a period of years. 

Dead Load. Among the first criticisms appearing in the public prints 
was the assertion that the dead load had been enormously increased with- 
out a corresponding increase in size of the main carrying members. This 
assertion, — one that would appeal directly to the fears of the public, for 
whose benefit this bridge has been constructed, appeared in one of the lead- 
ing daily papers, and was answered at the time by the Bridge Department, 
but it can be better answered now by a frank discussion of the entire dead- 
weight problem. 

The weight of steel now in place, plus an estimated weight of the addi- 
tional steel required to complete the overhanging footwalks, amounts to 
106,650,000 pounds. The same items were assumed, in the calculations 
of 1904, at 103,100,000 pounds. The assumptions as to these items were 
therefore exceeded but 3^ per cent. 

The uniform loading (other than steel) covering the weight of track 



REPORT OF COMMISSION 11 

for both elevated and surface cars, hand-rails, paving for roadway and foot- 
walks, pipes, etc., assunaed in the 1904 calculations, was 5,109 pounds per 
foot, aggregating 19,030,000 pounds. This weight was estimated from such 
plans of the structure as were perfected at that time; but the Bridge Depart- 
ment in 1907 changed the plans as to roadway, increasing the weight of pav- 
ing, inside trolley rails and hand-rails, making the uniform load (other than 
steel) 6,968 pounds per linear foot, or an increase of 1,859 pounds for these 
items. This was an admitted error, and has been partially remedied by reduc- 
ing the weight of paving, etc., on the river spans to the extent of 1,168 pounds 
per foot, no change being made on the anchor and island spans, as the added 
dead weight on these spans reheved the maximum stresses to some extent. 
The total dead weight of the structure, with four elevated tracks and 
overhanging footwalks, would then be about 130,700,000 pounds against 
an assumed (1904) weight of about 122,130,000 pounds,— an increase of 6^ 
per cent. The dead load stre.sses, however, are not increased to the above 
extent, as the distribution of increased weights is not uniform, a large pro- 
portion of increase in steel having gone to the towers and anchorages. 

The design of this structure, a cantilever bridge without a suspended 
connecting span, gives a continuity not found in the ordinary cantilever, 
inasmuch as a load on any part of the bridge affects the stresses in the entire 
structure from end to end. A strict interpretation of the specifications requires 
the loads to be placed in such positions as to give the greatest stress on any 
member of the structure. The Bridge Department, in preparing the strain 
sheets from which the bridge was built, did not strictly follow this clause, 
and applied the loads both "working" and "congested" in one continuous 
stretch, this stretch of any length covering one or more of the subdivisions 
of the bridge or the entire length of the structure, but with no unloaded 
gaps. The city's experts, in making their analysis of the structure, inter- 
preted the specifications literally, and obtained stresses (the mathematical 
accuracy of which we do not question) alarmingly high, when considering 
the "congested" load together with the increased weight of pavement. 

A reasonable and proper distribution of assumed live load on a struc- 
ture of this type and magnitude is again a matter of engineering judgment. 
The adopted method of the Bridge Department was well within their 
rights, especially as regards the so-called "congested" load, and it is our 
judgment that such placing of the loads would cover all possible contin- 
gencies liable to arise. 

The alarmingly high stresses obtained by the experts, as stated before, 
were arrived at by a strict interpretation of the printed specifications by 
placing the "congested" load of 16,000 pounds per linear foot on certain 
fixed portions of the bridge with fixed lengths of gaps in which there could 
be no load whatever; a method that might well be described as the placing 
of impossible loads in an impossible manner. 

Professor Burr's method of calculation of stresses produced by the ele- 
vated railroad trains, spacing eight car trains in position to give maximum 
stress, but not less than 1,000 feet apart, is rational, and we fully endorse 



12 REPORT OF COMMISSION . 

his method; but, ■T;vhen more than one track is treated in this manner, some 
concession should be made either in unit stresses or weight of trains for 
economic reasons; the possibihty that two or more trains of 410 feet length 
(eight cars) fully loaded, occupying exact spaces on one track, should be 
provided for; that this same loading and spacing could occur on a second 
track at the same instant of time is only conceivable, but that all four tracks 
should be loaded in this exact manner is well nigh impossible, and places 
such loading immediately in the category of "congested" loads to be pro- 
vided for by a higher unit. 

According to Professor Burr's conclusions, "a controlled traffic on the 
four trolley lines of the lower deck and on two elevated railways of the 
upper deck, carrying the heaviest cars of their classes now in use in the 
City of New York, together with a vehicular traffiic on the roadway, and 
two loaded sidewalks, may be permitted without exceeding the specified 
unit stresses for the regular live load and dead load and without exceeding the 
safe limits of stresses for such a structure." This conclusion was based on 
reducing the dead load by a considerable amount. 

We have made an investigation, using Professor Burr's method of dis- 
tribution of train load on four elevated tracks, together with 8,000 pounds 
per linear foot of bridge, and find that the stresses produced by this extreme 
load practically agree with those specified for the congested load, and are, 
therefore, well within the limits of safety. 

Considering the character of the structure and assumed loads, the unit 
stresses specified and used in the computations were conservative; a dis- 
tinction should be made, how^ever, between unit stresses for "working" 
loads and "congested" loads. The City's experts recommend, with one excep- 
tion, the unit stresses for working loads fixed by the specifications, the excep- 
tion being a slightly higher unit for steel in compression, due to change in 
reduction formula. One of the experts, after listing "working" unit stresses 
substantially in accord with the specifications, stated that these stresses 
"are the limit of safety for the direct stresses from the sum of the live and 
dead loads, as the secondary and snow load stresses heretofore referred 
to will add to these stresses." The secondary stresses are small, especially 
in the tension members, where the higher units are specified, and we believe 
that a snow load may be safely neglected when considering working or "con- 
gested" loads; it would, therefore, seem that the term "Umit of safety," 
as applied to such working stresses, was unfortunate and tending to cause 
unnecessary alarm. The "limit of safety" would, in a theoretically perfect 
structure, be just under the elastic limit of the material; secondary stresses 
and imperfect distribution of stresses should be allowed for, and we believe 
that sufficient allowance was made for such factors in the specifications, in 
fixing on the unit stresses to be used in connection with the "congested" 
load. 



REPORT OF COMMISSION , 13 



CONCLUSIONS 



(1) We are of the opinion that the live loads provided for in the original 
specifications, with the subsequent modifications made by the Bridge Depart- 
ment, both as to weights and distribution of same, are sufl&cient for the traffic 
the bridge is intended to carry, and cover all possible contingencies. 

(2) That the unit working stresses specified are in accordance with good 
practice, and the limiting stresses for extreme conditions of loading are well 
within safe limits. 

(3) That the actual weight of steel superstructure practically agrees with 
the estimated weight used in calculating the stresses, within the usual allow- 
ance permitted in bridge work. ^ 

(4) That the superstructure, as built, conforms to the specifications and 
designs approved by the Bridge Department. 

(5) That the bridge, as now constructed, with provision for two elevated 
tracks is entirely safe to carry all traffic which can possibly come upon it 
under present conditions, without any other restrictions than those necessary to 
regulate such traffic (see cut, page 58). 

(6) That, for conditions of traffic, i. e., the weight of vehicles, surface and 
elevated cars, as now existing, the bridge would also be safe to carry all the 
lines of traffic contemplated in the final design of the bridge subject to ordinary 
traffic regulations (see cut, page 59). 

Respectfully submitted, 

Charles Macdonald, 
C. C. Schneider, 
H. R. Leonard, 

J. E. Greiner. 

March 8th, 1909 



Report on Blackwell's Island Bridge 



THE PENNSYLVANIA STEEL COMPANY 

BRIDGE AND CONSTRUCTION DEPARTMENT 
Thomas Earle 

Superintendent 
F. C. KuNZ 

Chief Engineer 

Steelton, Pa., November 27th, 1908 

Mr. J. V. W. Reynders, Vice-President. 

Dear Sir: The Reports of the Consulting Engineers, Prof." William H, 
Burr and Messrs. Boiler & Hodge, appointed June 9th by the Commissioner 
of the Department of Bridges of the City of New York, in accordance with 
the resolution of the Board of Estimate and Apportionment, dated June 5th, 
to examine "the design and structure" of the Queensboro Bridge, formerly 
called Blackwell's Island Bridge, are now pubUc property. Coming from such 
an eminent source, the conclusions of the reports have been accepted without 
question, and much unfavorable criticism of the design of the bridge followed. 
It would, no doubt, interest the profession to know that there are engineers, 
not responsible for the design, who do not agree with all the conclusions of 
the reports. It would seem that the experts were not in possession of all data 
concerning the design,- which would not be surprising, considering the fact 
that the construction of this bridge was directed by three different adminis- 
trations of the Department of Bridges. Furthermore, while the reports are 
fair, they are based on assumptions different from those originally made. 

Professor Burr remarks at the beginning of his criticism: Permissible 

Unit Stresses 

"Proper provision for various classes of loading for a structure of Depend on 
such magnitude, designed to carry an extraordinary volume of traffic Live Load 
with corresponding working stresses, is largely a matter of judgment." 

Here lies the whole difficulty. After the necessary assumptions have been 
made, the rest is a mechanical procedure, and the bridge will be declared 
"unsafe in some members and wastefuUy proportioned in others," or "safe 
and uniformly proportioned," depending on these assumptions; the moment 
they are changed, the verdict may be totally reversed. It is safe to state 
that, the assumptions being a matter of judgment, in other words, one on which 
engineers may honestly hold different opinions, no two engineers, working 
independently, would be likely to recommend the same maximum live loads 
and corresponding unit stresses for a structure of the magnitude of the Black- 
well's Island Bridge, with its two independent floors, accommodating four- 
teen lines of traffic of four different kinds, viz.: four trolley tracks, four 
elevated railroad tracks, four rows of wagons on the roadway and two 

15 



16 CHIEF ENGINEER'S REPORT 

promenades for pedestrians. It is true that there were specifications; but 
all specifications, even for ordinary bridge work, can be interpreted in dif- 
ferent ways by different engineers. 

The original specifications for the Blackwell's Island Bridge, written in 
1903, prescribed a "regular" live load of 6,300 pounds and a "congested" 
live load of 12,600 pounds per linear foot of bridge, which in April, 1904, were 
changed to 8,000 pounds and 16,000 pounds respectively, when the Depart- 
ment of Bridges decided to add two elevated railroad tracks, making the 
traffic facilities equal to those of the Manhattan Bridge. These live loads 
were adopted from a report submitted to the Department of Bridges early in 
1903, by a commission of bridge experts appointed to examine and pass upon 
a design of the Manhattan Bridge with eye-bar chains. The specifications 
for both bridges having been written at the same time, it would have been 
surprising had the specified loads not been the same. 

The following is the paragraph of the report on the Manhattan Bridge 
concerning the live loads for truss members: 

"The maximum congested load which could possibly be brought 
upon this bridge would consist of a continuous train of rapid transit 
cars on each of the four elevated tracks, of a continuous line of trolley 
cars on each of the four trolley tracks; of a crowd of heavy tearns on the 
roadway and of a crowd of people on the footway. The heaviest rapid 
transit train w^hich may run over this bridge is that adopted by the 
Interborough Company for the subway; such a train, in which two- 
thirds of the cars are motor cars (corresponding to the practice of the 
Manhattan Railway), with 120 passengers on each car, is estimated 
to have a possible weight of 1,700 pounds per linear foot and an 
extreme axle load of 26,000 pounds. The estimated maximum weight 
of a continuous line of trolley cars is 1,000 pounds per linear foot. The 
estimated greatest possible congested weight per linear foot on this 
bridge would then be as follows: 

4 rapid transit trains, at 1,700 pounds 6,800 pounds 

4 lines of trolley cars, at 1,000 pounds 4,000 pounds 

35.5 feet of roadway, at 100 pounds per square foot 3,550 pounds 

22 feet of footway, at 75 pounds per square foot 1,650 pounds 

Total 16,000 pounds 

" This is a possible load which could never occur unless special pains 
were taken to produce it. The conditions which would block the tracks 
with continuous lines of cars in one direction would probably prevent 
cars entering the bridge from the other direction. The weight on the 
sidewa ks would correspond to about twelve people per linear foot, 
or something like 35,000 people on the bridge. The congestion of the 
roadway would be such that teams could not move. Under these con- 
ditions we consider that one-half of this amount. may be taken as a 



CHIEF ENGINEER'S REPORT 17 

maximum working load; this would make the working load 8,000 pounds 
per linear foot, ivhich is three times that provided in the plans of the 
Brooklyn Bridge and 40 per cent, greater than that taken for the Williams- 
burg Bridge.^ 

"The provision of 2,000 pounds per linear foot for wind pressure 
proposed by the Commissioner is ample. 

"In calculating the effect of temperature, provision has been made 
for an extreme variation of 110 degrees Fahrenheit, which we consider 
sufficient. 

" We consider that the bridge should be so proportioned that with 
the congested load of 16,000 pounds per linear foot, covering the whole 
bridge, combined with dead load and wind pressure, no stresses would 
be produced anywhere reaching the elastic limit of the material or im- 
pairing the stability of the anchorages. In other words, it should not 
be possible for such extraordinary congested load to do any permanent 
injury to the bridge. 

" We consider that the working load of 8,000 pounds per linear foot 
should be used in designing the main members of the structure." 

[The itahcs are ours. — F. C. K.] 

The Commission on the Manhattan Bridge did not specify any unit stresses The Specified 
under the action of the two kinds of live load except that "such extraordinary ^^'^ stresses 
congested load should not do any permanent injury to the bridge." The 
specifications for the Blackwell's Island Bridge, prescribed for tension of truss 
members 20,000 pounds per square inch for "regular'" or working, and 24,000 
pounds per square inch for "congested" or extraordinary live load, with more 
than the usual reduction for compression, viz., 20,000-90 l/r and 24,000-100 l/r 
respectively. [Messrs. Boiler & Hodge and Professor Burr, in their reports, 
reduce their permissible unit stress for compression according to formula 
20,000-50 l/r; many engineers allow for compression up to "I over r" equal 
40 and even 50, nearly or exactly the same unit stress as for tension.] Pro- 
fessor Burr calls these stresses "safe and satisfactory in the light of knowledge 
and precedent available when they were drawn" (in 1903) and, since the 
amount of the "regular" and the "congested" live load was also specified 
and actually used in the calculations, it would only remain to explain how this 
live load was placed to obtain the greatest stresses. 

The specifications stated that the live load should be "placed so as to Distribution 
give the greatest strain in each part of the structure." This is the usual °conthiuous" 
wording used in specifications for bridge work. For ordinary trusses, the live or "Discon- 
load causing the greatest strain or stress in each truss member covers one 
continuous, i. e., uninterrupted stretch. However, in arches, most types of 
suspension bridges, like the Brooklyn and the Manhattan Bridges, swing 
bridges or trusses with several supports, like the Blackwell's Island Bridge, 
the live load for the greatest theoretical stress in some members would have 

♦Brooklyn Bridge 2,600 pounds, Williamsburg Bridge 5,700 pounds for the cables and 4,500 pounds 
for the stiffening trusses, all figures per linear foot of bridge. 



tinuous" 



18 



CHIEF ENGINEER'S REPORT 




Comp, 



to cover certain disconlinuous, i. e., isolated stretches, different for different 
groups of members, with absolutely no live load between these stretches 
and on the other parts of the bridge. Any live load outside of these isolated 
stretches would reduce the greatest stress in the truss members. 

To illustrate this, three influence lines are shown in the accompanying 
figure; the first for the stress in a diagonal of a swing bridge on three sup- 
ports, the second for the stress in a top chord member near the center of 
an arch with two hinges, and the third for the stress in a top chord member 
of the island span of the Blackwell's Island Bridge. A live load in the stretch 

ab or cd will cause tension in the 
diagonal or the top-chord members 
respectively, and a live load in the 
stretch be will cause compression in 
these members. The absolute great- 
est tension, therefore, would be 
caused by ab and cd loaded and be 
unloaded. 

The usual method, however, is to 
assume a. continuous, i.e., uninterrupted 
stretch, loaded as follows: 
If area B is smaller than area C, then 
load from a to d; if area B is greater 
than area C, then load from a to b; 
in other words, choose that uninter- 
rupted loaded stretch (ab or ad) which- 
ever gives the greatest stress. This 
method was followed in the design of 
the Blackwell's Island Bridge. It is 
the usual method, even in single-track 
railroad bridges, where one train may 
cover cd and another ab. The stresses 
in the arch bridge of 840 feet span at the Niagara Falls were calculated in a 
similar manner. For the Washington arch bridge in New York City, the 
stresses were determined for a live load covering the whole span (510 feet) 
and a live load covering the stretch from abutment to the center of span 
only, and the greater stresses considered. 




Tens/or? 
Compression 




Compress/on 



Live Load 
Stresses 



In March, 1904, the contractor was furnished with the "Loading Key" 
(dated January 26th, 1904), prepared by the Department of Bridges. (See 
paper by Mr. R. C. Strachan, Assistant Engineer of the Department of Bridges, 
published in the Proceedings of the Engineers' Club of Brooklyn, 1905; 
also in " Engineering News," February 16th, 1905, in which the analytical 
method of computation is described; also paper by Mr. F. H. Cilley, in the 
Transactions of the American Society of Civil Engineers, 1904, describing 
the graphical method used for checking the live load stresses.) This "Load- 
ing Key" gives the influence on the stresses in the different truss members 



CHIEF ENGINEER'S REPORT 19 

of a live load of 1,000 pounds per foot per truss, covering different adjoining 
stretches of the bridge. 

The following is quoted from Mr. Strachan's paper: 

"The calculations were made by the writer for the Department 
of Bridges of the City of New York during the years 1902 and 1903, 
and have more recently been revised to conform to altered data. 
"The subsequent addition of two elevated railroad tracks, giving 
a total "congested" live load of 8,000 pounds, or "working load" 
of 4,000 pounds per foot per truss, made necessary a revision of both 
dead and live load stresses; and the results herein given are based upon 
the latter load. 

"The connection of the cantilever arms at mid-channel causes 
a load on any subdivision of the bridge to produce stresses in all the 
others; the effect, however, upon the east truss of a load on the west 
truss, or vice versa, is generally small, as will appear when the results 
are examined. Moreover, the simultaneous and complete stoppage 
of all the many lines of traffic in such a way as to give full live load 
on two widely separated subdivisions (divisions 2 and 7, for exam- 
ple), and nowhere else, is practically out of the question, as is the 
assumption of live load covering any short isolated portion of the 
bridge. 

"Stresses for seven positions of live load were therefore calcu- 
lated, the load in each case covering one of the seven subdivisions; 
and the maxima, both tensile and compressive, obtained by com- 
bining these for a continuous load extending over one or more sub- 
divisions, are the governing stresses for the main members of the 
truss. 

" In the absence of any similar truss with which to compare, weights 
were obtained by successive approximation, after the study of a 
sufficient number of members of each class had made possible a reli- 
able estimate of the percentages for details. Those stresses, due to 
either dead or live load, which are unaffected by the action of the 
rockers, were obtained graphically and checked by analytical methods, 
more especially to detect errors in scaling, since the proper closing 
of the graphic diagrams constitute a check upon the graphical work 
itself. The results of the action of the rockers were ascertained ana- 
lytically. 

" We thus have for each position of load what may be called the 
partial stresses. The greatest tension and compression from a load 
extending over one, two or more continuous subdivisions, obtained 
by the algebraic addition of the partial stresses for each member, 
are then tabulated." 

The following diagram explains the continuous and discontinuous load- 
ing for a few members. 



CHIEF ENGINEER'S REPORT 21 

It may be of interest to mention that, in the discussion which followed 
the reading of this paper, and in which Messrs. Nichols and R. S. Buck par- 
ticipated, and the correspondence sent in by Messrs. Duryea, Mayer, Hodge 
and Moisseiff, no mention was made of the difference of the effect of a live 
load iii' continuous stretches as compared with that of a live load in discon- 
tinuous stretches. 

Mr. Henry W. Hodge writes in his discussion: 

"I have taken great pleasure in reading Mr. Strachan's paper 
on the "Computation of Stresses in the Blackwell's Island Bridge," 
and his method of arriving at these stresses in the rocker members 
connecting the cantilever arms is as simple and accurate as any method 
can be which is based on deflection of framed structures. 

"As far as the computation of stresses in this structure is con- 
cerned, there is little discussion possible; but, as to the wider ques- 
tion of the advisability of constructing a cantilever structure on 
this semi-continuous design, engineers' opinions vary considerably." 

Mr. Joseph Mayer writes: 

"The method given, which is based on that used in the accurate 
calculation of drawbridge stresses is, I believe, the only accurate 
method available. I believe the stresses and sections required in the 
absence of rockers will give good preliminary data for calculating 
the deflections produced by given forces at the rockers. Your paper 
is so faultless that it does not lend itself to much discussion." 

Mr. Edwin Duryea, Jr., referring in his letter to the assumption as to 
"probable" maximum live load, mentions that a "more rational method" 
of estimating probable maximum load had been used by Mr. Joseph Mayer 
in designing a railway bridge across the North River. In looking this up 
in the Transactions of the American Society of Civil Engineers, Vol. 48, 
1902, p. 375, we find: 

"For the purpose of comparing the merits of various types for 
a bridge across the Hudson, the writer, therefore, will use a bridge 
having twelve tracks, of which two are for freight trains, two for 
long-distance passenger trains, six for trains of the elevated and 
underground roads of New York City and two for surface cars. 

"Such a bridge has sufficient capacity for the business it can secure 
in the near future. The distant future may bring other competing 
bridges or tunnels, therefore it need not be considered. 

"The moving load would consist of two freight trains, each 1,000 
feet long, weighing 3,000 pounds per linear foot; two long-distance 
passenger trains, each 1,000 feet long and weighing 1,500 pounds 
per linear foot; six r£[pid transit electric trains, each 500 feet long 
and weighing 1,200 pounds per linear foot. 

"The surface cars, on the two tracks provided for them, should 



22 CHIEF ENGINEER'S REPORT 

run at a speed of at least 15 miles per hour. They would, therefore, 
be twice as far apart as on the street approach; a distance of 100 
feet from center to center of cars would be closer than practicable. 
This distance, with cars weighing 40,000 pounds, gives a load of 400 
pounds per linear foot of track. 

"For the calculation of the cables, anchorages and the towers 
above the floor level, this load is equivalent to 8,421 pounds per linear 
foot of bridge, covering the whole length of the main span. The writer, 
therefore, will assume a moving load of 8,500 pounds per linear foot 
of bridge for these calculations. 

"For the calculation of the stiffening trusses, the loads on the 
surface-car tracks may be neglected, as they are nearly uniformly dis- 
tributed over the length of the bridge. The stresses produced by the 
trains 1,000 feet long and those produced by the trains 500 feet long 
would have to be calculated separately and then added, if the exact 
stresses corresponding to these loads are wanted. No equivalent 
load of one length will give the same stresses in every member of the 
stiffening trusses as the two loads of different lengths." 

[The italics are ours. — F. C. K.] 

The working live load of 8,500 pounds per linear foot, representing ten 
railway tracks, two of which for heavy freight and two trolley lines, is therefore 
about the same as the 8,000 pounds working or "regular" specified in 1903 
for the Blackwell's Island Bridge, based on a weight of rapid transit trains 
of 1,700 pounds per linear foot, in place of Mr. Mayer's 1,200, and of trolley 
cars weighing 43,000 pounds each, in place of Mr. Mayer's 40,000. (The 
reports of Messrs. Boiler & Hodge and Professor Burr give 1,810 pounds 
per linear foot and 62,000 pounds respectively, which is an increase of 50 
per cent, in seven years.) It is also interesting to note that Mr. Mayer, for 
a bridge of 2,800 feet span, assumes on each of the ten railway tracks only 
one train of 1,000 feet or 500 feet length respectively, and disregards the 
possibility of partial "bunching" of trolley cars on the two tracks provided 
for them. 

The method of probabilities mentioned by Mr. Duryea as having been 
used by Mr. Mayer to determine the probable frequency of the concurrence 
of trains with the maximum load on two, three, four, five, six, etc., tracks, 
assuming the trains running on schedule time with certain constant veloci- 
ties, could not be employed here, as even the most elaborate assumptions 
as regards speed and time of the rapid transit trains on the four tracks will 
be often overthrown by delays, accidents, etc., and the traffic on the trolley 
tracks, roadway and promenades is beyond the reach of a mathematical 
expression of its probability. 

The following is quoted from "Engineering News" concerning Mr. 0. 
. F. Nichols' (at that time Chief Engineer of the Department of Bridges) 
discussion of Mr. Strachan's paper: 



CHIEF ENGINEER'S REPORT 23 

"Speaking then in some detail of the width and traffic capacity 
of the structure, Mr. Nichols expressed the opinion that the roadway 
as described is far too narrow to accommodate the street traffic which 
will come upon it. He expects that the roadway will be overcrowded 
as soon as the bridge is opened. The provision of two lines of ele- 
vated railway as originally made would also have proved a serious 
error, but the four tracks now provided will suffice for some years. 
One or two details of the structural design were also discussed by 
the speaker." ("Engineering News," 1905, Vol. 53, p. 178.) 

In this connection, it may also be of interest to quote from Mr. 0. S. 
Morison's paper (see Transactions of the American Society of Civil Engi- 
neers, Vol. 36, 1896) regarding the assumed live load for a suspension bridge 
of 3,100 feet span over the Hudson River in the City of New York, even if 
the paper was written twelve years ago: 

"The bridge has been designed to carry a total load of 25 tons, 
or 50,000 pounds, per lineal foot. The design has then been devel- 
oped and the dead weight calculated, and the result is a balance 
for the live load of 11,000 pounds per foot over the entire structure. 
As the width of the structure is 92 feet between the stiffening trusses, 
this corresponds to about 120 pounds per square foot of floor. If 
this space were to be occupied by eight railroad tracks, it would 
amount to 1,375 founds per lineal foot per track, which exceeds the 
weight of any passenger train. It would amount in the aggregate on 
a length of 3,100 feet to 34,110,000 pounds — equivalent to eight 
freight trains 1,400 feet long, each weighing 3,000 pounds per lineal 
foot. It is probable that the requirements of any location where a 
bridge of this magnitude would be considered would be satisfied by 
four railroad tracks adapted to a heavy class of traffic and four rapid 
transit tracks to be operated by electric cars or short trains of a char- 
acter which would require only a floor stiffener to secure the necessary 
rigidity. Therefore, in proportioning the stiffening truss, the varia- 
ble load has been taken on the basis of 12,000 pounds per lineal foot, 
corresponding to a load of 3,000 pounds per foot on each of the rail- 
road tracks, with no provision for unequal weight on the rapid transit 
tracks, or to 1,500 pounds per Uneal foot on all eight of the tracks. 
These provisions correspond to four maximum freight trains or eight 
maximum passenger trains." 

[The italics are ours. — F. C. K.] 

The original estimate of the contract of the Blackwell's Island Bridge, 
with two elevated railroad tracks, made in 1903, was about 86,000,000 
pounds; the addition of two elevated railroad tracks to the capacity of the 
bridge, with changes and additions made by the Department of Bridges 
in 1904, as well as more specific knowledge in regard to the probable weight 
of details, indicated that, with the Uve load in continuous stretches, the 



24. CHIEF ENGINEER'S REPORT 

total steel weight would run up to about 100,000,000 pounds, with a great- 
est cross-section of the bottom chord of the trusses equal to about 1,100 
square inches, while, for the live load in discontinuous stretches, the total 
steel weight would be about 10 per cent, and the chords about 25 per cent, 
heavier, with all the accompanying difficulties of excessive thickness of 
material, inadequate space for the lacing of bottom chord and for the pack- 
ing on the pins, etc. 

The question of continuity or discontinuity of the live load, therefore, 
involved a question of an increase of the steel weight of the structure of about 
10,000,000 pounds, at a corresponding additional cost of approximately 
$700,000. 

The adoption of the continuous live load by the Department of Bridges 
was based on the unlikelihood that the fourteen lines of live load, consisting 
of four kinds of traffic, i. e., rapid transit trains, trolley cars, wagons and 
pedestrians, on two independent floors, would be distributed simultaneously 
in the following manner, viz.: 

First — Each kind of traffic up to its assumed "regular" or "congested" 
maximum per linear foot ; 

Second — Each separate kind of traffic in two or three isolated stretches 
only, these stretches being in certain exact distances from each other and 
from the ends of the bridge ; 

Third — These stretches and distances to be alike, i. e., in the same loca- 
tion for all the fourteen lines of traffic on the two floors ; 

Fourth — Absolutely no other live load on the bridge. 

Such conditions of loading could scarcely be produced, even as an experi- 
ment, since trains, cars, wagons and horses each cover certain definite spaces. 

The Department of Bridges, therefore, adopted in 1904 the following 
conditions for designing the truss members, Nos. 1 and 2 providing for con- 
tinuous live loads, while Nos. 3 and 4 take account of extraordinary con- 
ditions that might arise from discontinuous loading, viz.: 

(1) The "regular" five load of 8,000 pounds per finear foot of 
bridge in one continuous stretch for each member, over such a portion 
of the bridge as to cause maximum stress, and using 20,000 pounds 
per square inch in tension (reduced for compression) for structural 
and 30,000 pounds per square inch in tension for nickel-steel as per- 
missible without wind acting, and increased 20 per cent, with wind 
acting. 

(2) The "congested" five load of 16,000 pounds per linear foot 
of bridge in one continuous stretch for each member, over such a por- 
tion of the bridge as to cause maximum stress without wind, using 
24,000 pounds per square inch in tension (reduced for compression) 
for structural and 39,000 pounds per square inch in tension for nickel- 
steel as permissible. 

Of these two conditions, the one causing the greater cross section in 
each truss member should be taken. 



CHIEF ENGINEER'S REPORT 



25 



(3) If the "congested" live load of 1G,000 pounds per linear foot 
of bridge in discontinuous stretches (without wind acting) causes 
an absolute maximum compression greater than the dead load ten- 
sion in a member, such member to be made of a section able to with- 
stand the resulting compression. 

(4) Furthermore, in order to be sure that no overstressed con- 
dition could result from the "regular" live load of 8,000 pounds 
per linear foot of bridge, in discontinuous stretches causing absolute 
maximum stress without wind, the unit stresses are to be analyzed 
for this condition. 

For this last condition, the greatest unit stress in tension, with two excep- 
tions, would not be greater than 20,700 pounds, and in compression 18,800 
pounds for l/r=27 for structural steel and 34,100 pounds in tension for 
nickel-steel, the exceptions being the verticals U59-L59 and U73-L73, which 
are stressed in tension to 22,900 (see plate No. 5). 

The combination of live load with the greatest wind pressure was con- 
sidered for condition (1) only, and the unit stresses increased by the usual 
amount of 20 per cent., since it is hardly possible that the bridge will be 
crowded on the roadway and the promenades in stormy weather. A con- 
currence of a great wind pressure with conditions (2), (3) or (4) is practi- 
cally impossible. 

The wind pressure was assumed at 2,000 pounds per linear foot of bridge, 
representing about 35 to 40 pounds per square foot, which would be caused 
by a velocity of wind of 100 miles per hour; and it is clear that in the same 
degree as the wind pressure increases, the live load on the roadway, prome- 
nades, etc., naturally decreases, which is distinctive of highway bridges 
as compared with railroad bridges, since, in the latter, the maximum live 
loads will pass irrespective of the velocity of the wind. 

Attached plates Nos. 1 and 2 show the position of the live load in con- 
tinuous stretches as assumed by the Department of Bridges for the design, and 
plates Nos. 3 and 4 the live load in discontinuous stretches as assumed by 
the experts for the report. Heavy full lines indicate loading for greatest ten- 
sion; dotted Unes, loading for greatest compression in each member. 

To illustrate: For the top and bottom chord, in fact for most truss mem- 
bers of the Island span, where, according to the reports of the experts, the 
highest stiiesses occur, the continuous "congested" loading covers the distance 
between piers Nos. 1 and 4; i. e., both river spans and the Island span, a 
total length of 2,796 feet, while for the discontinuous loading the "congested" 
load would cover the distance of 1,182 feet between piers Nos. 1 and 2, no 
live load whatever for a distance of 630 feet between piers Nos. 2 and 3, 
and again, the "congested" live load for a distance of 984 feet between 
piers Nos. 3 and 4; whether loading is continuous or discontinuous, both 
anchor spans have to be absolutely free of any live load. The possibility 
that the "congested" Uve load of 16,000 pounds per linear foot of bridge, 
representing fourteen lines of traffic of four different kinds on two inde- 



Distribution 
of Live Load 
for the Great- 
est Stresses in 
the Island 
Span (Com- 
pare Table 
No. 7) 



26 CHIEF ENGINEER'S REPORT 

pendent floors of uniform construction from end to end of the bridge, will 
cover 1,182 feet, is indeed very small; that another bunching of traffic on all 
fourteen lines on the two floors precisely in the same location for a length 
of 984 feet will occur, is still smaller; but what can be the possibility that 
these two loaded stretches will be 630 feet apart with no live load on the 
two floors between them, a distribution which even for the working live load 
of a single-track railroad bridge would be exceptional? 

It would not be surprising if some engineers, differing honestly in their 
judgment from that of the designers, would reverse the question and claim 
that the "congested" five load of 16,000 pounds per Unear foot of bridge, 
even in a continuous stretch of 2,796 feet, representing a distribution of 
cars, wagons, pedestrians, as shown on plate No. 7, from piers Nos. 1 to 4, 
and no other live load on the two floors from these piers to the abutments, 
is a practical impossibility; that therefore the Island span has been waste- 
fully designed by the Department of Bridges and that the engineers of the 
contractor must have known this and should have protested even if the bridge 
was paid for by the pound. 

In order to represent on plate No. 7 the "congested" load of 16,000 pounds 
per linear foot of bridge, it was necessary to place: 

First — On each of the four trolley tracks an uninterrupted line of trolley 
cars of 1,000 pounds per linear foot of track ; 

Second — On each of the four rapid transit tracks, an uninterrupted 
line of trains of 1,700 pounds per linear foot of track ; 

Third — On the two promenades, a crowd of people weighing 75 pounds 
per square foot, and 

Fourth — On the roadway (35^ feet wide) four uninterrupted lines of 
the heaviest electric trucks known to us, of 7 feet by 18 feet net area and 
weighing loaded 18,000 pounds, all assumed carrying their maximum load 
and placed two feet apart in the clear. It is questionable whether two feet 
clear space is sufficient, but this would be the only possibility to reach a live 
load of 100 pounds per square foot on the roadway. The heaviest electric 
trucks used in New York City (20,000 pounds, with 8 feet by 20 feet net 
area) and the heaviest coal wagons (19,000 pounds, drawn by three horses 
of 1,800 pounds each, and 9 feet 3 inches by 25 feet total net area) would 
give considerably smaller load per linear foot of bridge, being not only lighter 
per square foot, but also wider, so that the roadway between the trolley 
tracks could accommodate only three lines. Considering further that in the 
great lengths of live load necessary to cause maximum stresses in the main 
members of the trusses, there will be different kinds of vehicles weighing 
considerably less (the heaviest touring car weighs 4,800 pounds, with a net 
area of 5 feet 6 inches by 13 feet, which gives 43 pounds per square foot, 
assuming two feet clear space), especially since the bridge is, for its greater 
part, on a grade of 3^ per cent., and that there will be many large floor spaces- 
empty on account of irregular dimensions of the vehicles, it would seem 
that a live load of even 50 pounds per square foot of roadway could hardly 
be reached. 



CHIEF ENGINEER'S REPORT 27 

The "regular" live load of the specifications (8,000 pounds per linear 
foot of bridge, represented by 37.5 pounds per square foot on the promenades, 
50 pounds per square foot on the roadway, 4X500=2,000 pounds per linear 
foot of bridge for the trolley tracks, and 4X850=3,400 pounds per linear 
foot of bridge for the rapid transit tracks) takes into account only one-half 
of the density of the "congested" live load (shown for most members of 
the Island span on plate No. 7), simply assuming a greater space between 
the small units represented by pedestrians, wagons, trolley cars and the 
trains. This requires no further explanation except for the rapid transit 
trains. 

In the "congested" live load the average weight of a train is assumed influence of 
at 1,700 pounds per linear foot; since of this amount about three-fourths xr^ains^iT"^^' 
is for the cars and one-fourth for the passengers, it is clear that a smaller stresses 
number of passengers reduces the weight of the trains only slightly, so that 
their share of 6,800 pounds in the "congested" live load of 16,000 pounds 
per linear foot of bridge cannot be reduced materially for their share in the 
"regular" live load of 8,000 pounds per Unear foot of bridge. However, 
there are other more important considerations which make it permissible 
to assume that the stresses due to the trains in the "congested" live load of 
16,000 pounds per linear foot of bridge can be reduced one-half to repre- 
sent those due to the trains in the "regular" live load, viz.: 

First — The trains are limited to a certain length (express trains 
about 400 feet long, with eight cars, and local trains about 250 feet 
long, with five cars), so that they produce in most members only part 
of the stress which would be produced by the lengths required for 
the maximum stress, these lengths being, as a rule, considerably 
greater. 

Second — The trains following on the same track have to main- 
tain, on account of safety of operation, a clear interval between them 
of not less than 2,400 feet (for the stresses in the Island span, for 
instance, no two trains can therefore be at the same time on the same 
track between piers Nos. 1 and 4). 

Third — The probability that all the trains on the four tracks 
will pass simultaneously through the danger zones where they con- 
tribute their maximum share to the stress in a truss member is very 
small, as a rule they will be more uniformly distributed over the 
bridge. 

Should it, however, happen that, through some accident, two trains on 
the same track would follow each other in a shorter clear distance than 2,400 
feet (for the greatest stress in the Island span about 1,300 feet clear), the 
probability that each of the other three tracks will be loaded by two trains 
following each other in the same clear distance, that these two groups, each 
of four trains, will pass at the same time through their respective danger 
zones (for the Island span, four trains in the middle of the one river span. 



28 CHIEF ENGINEER'S REPORT 

and four trains in the middle of the other river span), and at the same time 
that the other three kinds of traffic of "regular" density weighing 8,000 
minus 3,400=4,600 pounds per linear foot of bridge will cover even the 
continuous lengths necessary to produce the maximum stresses, is so remote 
that this combination of the train loads with the other loading should be 
considered an extreme condition, and unit stresses allowed somewhere between 
the permissible unit stress for the "regular" and for the "congested" live 
load. As a matter of fact, this condition would cause a unit stress in tension 
in the vertical U59-L59 of approximately 21,200 pounds and in the top 
chord U65-U69 (nickel-steel) of approximately 33,500 pounds. 

The examples in the following table show the influence on the stresses 
due to a load of 4X1,700=6,800 pounds per Hnear foot of bridge of vari- 
ous length and location. (6,800 pounds is the assumed train load on the 
four tracks per foot of bridge.) 




8 


5 


^ 


1 




i^ 


^ 


5 




•0 


1 


1 




1 


1 


1 


1 


1- 


1 


1 
^ 


1 


^ 


i 


^ 


^ 






i?5 


5i 


1 


<5i 

1 


1 


? 


1 

1- 


^ 
? 


1 


1 


K§ 


1 


^ 


1 


?J 


5! 


§ 


12 




SJ 
•n 




1 


1 

1- 


1 


15 


1 


5 
;? 
^ 


1 


S 
^ 



'VJ 



wi 



LJ 






L-^ 



»<) 






I 



I 



s 






1 

5? 



§l| 



^^^ 

^ 



I 



I 



I 



^^ 



30 CHIEF ENGINEER'S REPORT 

The stresses from the 410-ft. trains are only approximate; the influence 
lines were not given us by the Department of Bridges, the "Loading Key" 
indicating only the totals for certain stretches of the live load. 

stresses in These are the two members in the truss which, for the "congested" 
TT..« 7^i^^^^^l discontinuous live load, make the most unfavorable showing, since they 

U59-L59 and • i j 

U73-L73 get the greatest tension for the same condition of loading as the top chord 
in the Island span. Their greatest unit stress in tension for dead and maxi- 
mum live load is (see table No. 5): 

for the "regular" continuous live load, 18,600 pounds; 
for the "regular" discontinuous Uve load, 22,900 pounds; 
for the "congested" continuous Uve load, 25,600 pounds.; 
and would be for the "congested" discontinuous live load, 34,200 pounds. 

If we assume a position of Uve load which is more probable than the 
"regular" discontinuous loading, viz., 4,000 pounds per linear foot of bridge 
covering the bridge from end to end, and additional 4,000 pounds per Unear 
foot in discontinuous stretches, we get a total unit stress of 20,300 pounds. 

If we assume a load of 8,000 pounds per linear foot, covering the bridge 
from end to end, and additional 8,000 pounds in discontinuous stretches, 
merely to show the effect of a milder form of a discontinuous "congested" 
load, we get 29,000 pounds per square inch. 

To judge these stresses, it should be considered that verticals have no 
bending stresses from own weight and that the secondary stresses of these 
verticals, due to the vertical deformation of the truss, are practically zero, 
since they are pin connected not only at top and bottom, but also at the 
middle, where the horizontal strut connects. 

Even in designing these verticals for condition (2) ("congested" con- 
tinuous) as this gave the greater stress, their packing was so difficult that 
their upper half had to be made flared at the top in order to minimize the 
great bending moment of the pin caused by the kink in the outUne of the 
top chord, which necessitated a pin joint in their center; and their bottom, 
consisting of three ribs, each five inches thick, had to be made of nickel-steel 
to provide for the necessary area through and back of the pin hole, in spite 
of the fact that the bottom chord in the Island span is ten inches wider than 
in the rest of the bridge. 

Difference in The effect of the discontinuous live load on the stresses compared with 

Effect of Dis- ^)^^^ Qf ^^Q continuous live load is different according to the location of the 

and Continu- truss member. In the Island span it is the greatest, in other members the 

ous Live Load difference is zero or approximately zero, as for instance in nearly all the 

Uniform members of the cantilever arms, in the bottom chord L58-L59 of the Island 

span, etc. This, and the fact that in every bridge, especially of long span, 

there are certain members which cannot be reduced in their section in the 

same proportion as the stresses decrease, account for the non-uniformity 

of the unit stresses, and, therefore, apparent waste of material, deduced 



CHIEF ENGINEER'S REPORT 31 

by a certain Engineering paper from the reports of the experts, who considered 
a discontinuous "congested" live load. 

As mentioned before, the condition (3), page 25, that is, the "con- Provision 

gested" discontinuous live load, has been considered for reversal of tensile *or Reversal 
. . of Stress in 

stresses. If m a riveted member or an eye-bar in tension the elastic limit Tension 

or yield point should be slightly exceeded, nothing more serious than an Members 
imperceptible permanent set would occur which would even decrease (not 
disappear) if a certain time would elapse before a repetition of this stress, 
since the yield point rises with an excessive stress. [It may be of interest 
to mention in this connection that a well-known mechanical engineer pro- 
poses to stress above the yield point every eye-bar in full size before using 
it in the structure in order to raise its yield point.] In making a full-size eye- 
bar test, the yield point is reached within the first minute, while it may take 
half an hour of increasing pulling to reach the ultimate strength and break 
the bar. If, however, an eye-bar were only slightly compressed, the bridge 
might fall. For this reason, some top chords in the middle of the river spans 
and at the ends of the anchor arms and some diagonals were made riveted 
members; and since they had to have a certain "I over r" to be stiff, their 
unit stress is naturally low and their section, therefore, seemingly excessive. 
Taking a practically impossible load, like the "congested" discontinuous 
live load for reversal of tensile stresses, but not for determining the cross 
section of truss members, is in line with modern specifications for railroad 
bridges, one of the foremost railroads in the country using for a working 
load of Cooper's E50 a unit stress in tension of 16,000 pounds (consider- 
ing impact), but also providing for an extraordinary live load of Cooper's 
ElOO, allowing in this case twice the unit stress for the working load, there- 
fore, for tension, 32,000 pounds per square inch (elastic limit), in order to 
reach those truss members whose stresses could be reversed by a consid- 
erable future increase of live load and other unforeseen circumstances. 

In the Transactions of the American Society of Civil Engineers, Vol 
42, p. 547, Mr. H. S. Prichard, discussing the provision for a future increase 
in live load of 100 per cent., states: 

"In a good modern specification the unit stresses allowed are so 
low that, except for the counterstresses, a bridge designed and built 
in accordance therewith could reasonably be expected to be able to 
carry without serious injury a live load at least twice as great as the 
live load specified." 

In the Blackwell's Island Bridge, the impact of the live load need not impact 
be considered; coming from three different kinds of traffic (rapid transit. Negligible 
trolleys and wagons) it cannot accumulate, and what little may result is 
absorbed or diffused by the solid floor and the heavy floor construction 
before reaching the heavy trusses; and, furthermore, the more the bridge 
is crowded, the less motion of the live load is possible. 



32 



CHIEF ENGINEER'S REPORT 



But, assuming, for the sake of argument, that the Blackwell's Island 
Bridge would carry eight tracks of a steam railway with wooden ties instead 
of with a heavy, solid floor, then the impact, according to the specifications 
of the American Railway Engineering and Maintenance of Way Associa- 
tion, would be for the top and bottom chords of the Island span (also for the 
previously mentioned verticals U59-L59 and U73-L73) : 

300 300 



with continuous load 



with discontinuous load 



8 X 2,796 .+ 300 22,668 
300 300 



= 0.013 = 1.3 per cent, 
= 0.017 = 1.7 per cent, 



8 X 2,166 + 300 ~ 17,628 
since the loaded length has to be taken equal to the total length of single 
track; in fact, the percentage could even be reduced, since the number of 
tracks should be increased to twelve, counting also the four lines of wagons 
on the roadway. 

Mr. Prichard, in his paper "Proportioning of Steel Railway Bridge Mem- 
bers" (Proceedings of the Engineers' Society of Western Pennsylvania, 1907, 
also "Engineering News," September 19th, 1907), reduces the impact to 
zero for spans of 1,000 feet for one track, of 700 feet for four tracks, and of 
300 feet for eight tracks. 



Snow Load 
Negligible 



To consider a snow load did not seem necessary. Assuming a thickness 
of compact snow of 12 inches, or of ice and slush of four inches, representing 
about 15 pounds per square foot, a larger amount may safely be deducted 
from the "congested" live load of 100 pounds per square foot (represent- 
ing a sohd mass of people unable to move) or the "regular" hve load of 50 
pounds per square foot (representing a crowd hardly able to move) of the 
roadway or the promenades. To combine the absolute maxima of stresses 
caused by a discontinuous "congested" live load representing 14 lines of 
traffic with those caused by a discontinuous greatest wind load covering the 
same discontinuous stretches and no others and do probably the same thing 
with a snow load would be a reductio ad absurdum. 



Temperature From a uniform change of temperature the trusses of the Blackwell's 

stresses Island Bridge receive stresses only on account of the action of the rocker 

®^'^' ® posts in the middle of the river spans. When the two adjoining cantilever 

arms expand or contract, these rockers assume an inclined position, which 

results in the lowering of the one and the lifting of the other cantilever arm, 

causing insignificant stresses. 

By order of the Department of Bridges, early in 1904, the originally 
intended pin at the bottom of each tower post was replaced by a flat bear- 
ing; consequently, the towers receive bending stresses from any horizontal 
deflection of their top. From a change of temperature, the top of tower 
U75-L75 only is deflected, since this is the only tower where the trusses 
are able to slide, being rigidly connected to the piers at the other three towers. 
For a change of +60° the deflection at the top is 3| inches and the stress 
at the bottom of tower 3,200 pounds per square inch in the extreme fiber 



CHIEF ENGINEER'S REPORT 33 

of the section, while the actual stress for the dead and the "regular" con- 
tinuous live load is 15,200 pounds per square inch. The total extreme fiber 
stress, therefore, is only 18,400 pounds per square inch, which is well below 
20,000 pounds per square inch permissible for a uniformly distributed direct 

stress. 

To illustrate the effect of the "regular" and the "congested" live load Effect of ^_ 
of the Blackwell's Island Bridge on an actually used bridge of similar mag- and "Con- 
nitude, the writer derived some figures from a publication by Professor gested" Live 
Melan concerning the stiffening trusses of the Williamsburg Bridge. (Hand- Trusses of 
buch der Ingenieur-wissenschaften, Arch and Suspension Bridges.) This Williamsburg; 
bridge has four trolley tracks, only two rapid transit tracks (similarly to " ^® 
the original plans for the Blackwell's Island Bridge), but has two roadways 
20 feet each, two foot-walks 10 feet 6 inches each and two "bicycle" paths 
7 feet each; it has, therefore, 4 feet 6 inches wider roadways, and 13 feet 
wider promenades than the Blackwell's Island Bridge. This would give a 
"congested" load of 16,000—3,400+450+950=14,000 pounds per linear 
foot of bridge and half of this, that is, 7,000 pounds per linear foot of bridge, 
as "regular" load; but, assuming only those originally specified for the 
Blackwell's Island Bridge (with two rapid transit tracks), namely, 12,600 
and 6,300 respectively, which is 10 per cent, less, we get for the unit stresses 
in the stiffening trusses the following maxima in pounds: 

Top Chord 

Tension Compression 

For "regular" live load alone 21,600 20,800 

For "regular" live load and temperature 24,600 23,600 

For "regular" live load, temperature and wind 32,200 30,200 

For "congested" live load alone 43,200 41,600 

For "congested" live load and temperature 46,200 44,400 

For "congested" live load, temperature and wind ....52,000 50,000 

Bottom Chord 

For "regular" live load alone 25,600 19,800 

For "regular" live load and temperature 28,200 21,400 

For "regular" live load, temperature and wind 36,000 27,400 

For "congested" live load alone 51,200 39,600 

For "congested" live load and temperature 53,800 40,700 

For "congested" live load, temperature and wind 59,400 46,600 

To caiise these stresses, the live load has to extend for only about 1,100 
feet in a continuous stretch from the pier, and may be of any length beyond 
the pier, a condition which is certainly far more possible than even the con- 
tinuous live load mentioned above for the island span of the Blackwell's 
Island Bridge. These figures do not take into account any snow load what- 
ever. 

To treat the Williamsburg Bridge as rigorously as the Blackwell's Island 
Bridge, it should be considered that a suspension bridge has to be so ad- 
justed that, for no live load on it, the ends of the stiffening trusses have no 
reaction and that it is very probable that this condition is not fulfilled at 



34 CHIEF ENGINEER'S REPORT 

the present time in the WilUamsburg Bridge, which would add some more 
thousand pounds to the chord stresses in the stiffening trusses. 

To compare these figures with those for the Blackwell's Island Bridge 
it should also be kept in mind that in a stiffening truss temperature stresses 
are positively sure, like dead load stresses in ordinary trusses, since the con- 
dition of no reaction of the stiffening trusses can be accomplished only for 
a certain temperature, the stiffening trusses being deflected and, therefore, 
stressed for any other temperature. 

, Up to September 1908, the two elevated tracks on the Williamsburg 
Bridge were not in use, but the bridge had been open to the other traffic 
since February 1905; taking a "regular" live load of only 6,300^1,700^ 
4,600 and a "congested" load of 9,200, the above-given unit stresses would 
be reduced by only about one-fourth, would therefore be in maximum: 

for the "regular" live load, 29,000 pounds in tension, and 23,000 

pounds in compression, and 

for the "congested" live load, 44,000 pounds in tension, and 35,000 

pounds in compression, 
considerably greater, therefore, than the corresponding values even for 
the whole traffic of 8,000 pounds per linear foot of bridge as "regular" live 
load, or of 16,000 pounds per linear foot of bridge as "congested" five load 
in discontinuous stretches for the Blackwell's Island Bridge. 

As a matter of fact, the stiffening trusses of the Williamsburg Bridge 
were proportioned for a load of 4,500 pounds per linear foot of bridge, this 
to represent the whole traffic including the two elevated railroad tracks; 
no "congested" or extraordinary load was considered. 

Dead Loads The statement has been made that the dead load stresses of the Black- 
Compared well's Island Bridge were considerably underestimated. This is only partly 

with Subse- correct. The designers realized from the beginning that the dead load of 
AssumpUons ^ highway bridge, especially of one of this magnitude, is of the greatest 
importance. For this reason, the floor-beams were curved in their top chord, 
to follow the crowning of the paving in order to reduce its weight to a mini- 
mum. There are city bridges in existence for which the weight of the paving 
was assumed for the design with 75 pounds per square foot, while it actually 
amounts to 150 pounds, and even more, eating up all allowance for live load. 
The Department estimated the weight of the paving material for the lower 
floor, concrete at 40 pounds, wood blocks at 20 pounds, and for the prome- 
nades at 20 pounds per square foot. 

The original estimate of the steel weight, made by the Department of 
Bridges, was 86,000,000 pounds. Beginning early in 1904, the Department 
ordered changes and additions made, such as changing the tracks of the 
upper floor from longitudinal ties with bulb angles to wooden cross ties, 
addition of two elevated railroad tracks, reduction of pressure of the tower 
bases on the masonry, replacing the pin-bearing of the bottom of the towers 
by a flat bearing, additional sway bracing, bottom and top laterals, top 
struts, bracing between stringers, secondary verticals from the middle pins 



CHIEF ENGINEER'S REPORT 35 

to the top chord, etc. (See "Report of the Commissioner of the Department 
of Bridges, 1904," printed in 1905, pages 31, 32, 33, 34), all this increas- 
ing the steel weight to about 95,000,000 pounds, with a total dead load of 
116,000,000 pounds. New dead load stress sheets were made by the Depart- 
ment, and at their suggestion, also, by the contractor subject to their ap- 
proval, in order to hasten the time of the ordering of the material. To these 
dead load stresses, the live load stresses calculated by the Department for 
the new live load (with four elevated railroad tracks) were added, the sec- 
tions of the truss members determined and all section sheets which the con- 
tractor prepared for use in his own drawing-room were submitted to and 
finally approved by the Department with certain changes. 

Shop drawings for the Island span were now made by the contractor 
and approved by the Department with changes, and the material ordered. 
In the meantime, new dead load concentrations and new stress sheets were 
worked out, finished about November, 1904, based on a new estimate of 
the steel weight of 100,750,000 pounds, giving with the originally prescribed 
weight of paving, pipes, etc., a total dead load of 122,130,000 pounds. As 
far as possible, the old stress and section sheets were then revised. To change 
the material for the Island span, at that time in the process of manufac- 
ture in the Bridge Shop, was impossible, and it is here that some dead load 
stresses figured with these new dead load concentrations slightly overrun 
the older figures. The actual shipping weight of the bridge proper (deduct- 
ing test eye-bars and other test material) is about 105,150,000 pounds, which, 
of course, includes all changes and additions made after November, 1904, 
as reinforcing certain trolley stringers for elevated railroad trains, strength- 
ening of the anchorages, etc., giving a total dead load of 125,680,000 pounds. 
It should be stated in this connection that certain changes in the design, 
which caused a considerable addition to the steel weight, did not affect the 
dead load stresses, as strengthening of the anchorages, increase in weight 
of tower bases and towers, in the weight of the box girders between trusses 
at the piers, in the weight of the shoes on top of the towers, etc. 

About a year ago, after the whole bridge material had been fabricated, 
the writer, for his own satisfaction, had new stress sheets prepared, using 
the dead load concentrations of November, 1904, and the stresses from 
the continuous "regular" Uve load given us by the Department of Bridges, 
considering also the fiber stresses due to the own weight of the truss members, 
and satisfied his own mind that the bridge was able to carry the specified 
traffic. The dead load concentrations of November, 1904 were considered 
sufficiently accurate, as they were based on a total dead load of 122,130,000 
pounds, while the actual dead load using the shipped steel weight was only 
3 per cent, higher, that is, 125,680,000. 

In the meantime, the Department changed the plans for the paving. 
As mentioned above, the original paving, consisting of concrete and wooden 
blocks, was intended to be as light as possible and to extend only between 
the curbs for 35^ feet; the tee-rails for the inside trolley tracks were to be 
5 inches high and their top about 6 inches above top of stringer; the buckle 



36 CHIEF ENGINEER'S REPORT 

plates between the rails merely covered with asphalt and the rails free. The 
Department now decided to extend the paving over the whole roadway 
between the trusses (53 feet) with heavy cast-iron curbs and of much greater 
thickness, using 7-inch grooved rails with their tops nearly 12 inches above 
the top of the stringer, and imbedded entirely in concrete. If ordinary con- 
crete and not cinder concrete be used, this makes the paving about 1,341 
pounds per linear foot heavier than originally intended and assumed in the 
calculations, increasing the total dead load to 132,300,000 pounds, which 
figure was used by the experts in their calculations of the dead load stresses. 
The change in the paving was an after-thought, not concerning the con- 
tractor, since at that time the bridge was completely manufactured, and, 
in its greatest part, erected. This, and the fact that the additional paving, 
being of uniform weight over the whole bridge, forms a larger percentage 
of the dead load near the ends of the cantilever arms, where its influence 
on the stresses is the greatest, accounts for the great difference in the dead 
load stresses as used for the design of the bridge and as calculated by Pro- 
fessor Burr and Messrs. Boiler & Hodge. 

The writer understands that the Department of Bridges recalculated 
the dead load stresses after arranging for heavier paving, and that they got 
the same excessive figures as the experts, but that they had not come to any 
conclusion pending a recalculation of the live load stresses when the experts 
were invited to make a report. 

Although it does not seem within the scope of this report, it may never- 
theless be useful to mention, for the information of those critics of the Black- 
well's Island Bridge who are not experts in bridge engineering, that the 
excellence of the design of different bridges cannot be judged by the propor- 
tion of the steel weight necessary to carry a pound of live load. This depends 
for bridges of similar character almost entirely on the length of the span. 
For a very short span, one pound of steel may suffice to carry one hundred 
pounds of live load, and for a very long span the reverse may be the case. 
The proportion of steel weight to the assumed working live load is for the 
following bridges approximately: 

Greatest 
Span Proportion 

Thebes bridge 671' 1.0 

Monongahela bridge 812' 1.0 

Memphis bridge (one track only) 790' 1.7 

Blackweli's Island bridge (heavy solid floor) 1,182' 3.4 

Quebec bridge 1,800' 4.3 

Firth of Forth bridge 1,710' 4.7 

The Blackweli's Island Bridge carries a heavy solid floor, the others only 
wooden ties. If the former had had a fight wooden floor, possibly 2.5 pounds 
of steel instead of 3.4 pounds would have been sufficient to carry a pound of 
live load. 



CHIEF ENGINEER'S REPORT 37 

The report of Messrs. Boiler & Hodge states: Reverse ajid 

Secondary 
"We have made no additions for reverse stresses, as the speci- stresses 

fications state that the sections are to be computed for the stress 
requiring the greatest area, so that the unit stresses we have shown 
are the direct stresses from dead and live loads without any addi- 
tions for reverse stresses, snow, wind, impact, or secondary stresses." 

The effect of snow, wind and impact has been mentioned before; there 
remains now to discuss reverse and secondary stresses. 

In most specifications for railroad bridges written in the last ten years, 
the effect of a reversal of stress is only considered if the reversal occurs in 
immediate succession, as "counter stresses in web members or chords in 
continuous trusses" (this is the usual wording) under the passage of the 
same train; in highway bridges, it is as a rule disregarded. It is hard to con- 
ceive how a reversal of stress could happen in the Blackwell's Island Bridge 
in immediate succession with the different kinds of live load. 

The report of Messrs. Boiler & Hodge states: 

"The secondary stresses due to distortion of the true figures of 
the trusses by the live load are quite considerable, as the vertical 
deflection of the point L37 is 18-j^ inches, and of the point L91, 14-j^q 
inches for a live load of 3,000 pounds per 1 linear foot of truss. 

" We have made a careful analytical computation of the hori- 
zontal movement of the point U17 (and other similar points over the 
main piers) caused by this distortion. " 

To avoid possible misunderstanding, it may be well to point out that 
the horizontal movement of point U17 (top of tower) is not caused by the 
vertical deflection of point U37 (where the cantilever arms join), but merely 
by one contributing cause to this vertical deflection, namely the deformation 
of the anchor arm., the other more important cause contributing to the 
vertical deflection of point U37 being the deformation of the cantilever itself 
which does not affect the horizontal movement of the top of the tower. 

The effect of secondary stresses, which lately has been given its deserved 
prominence, should, however, not be over-estimated. In the case of rolled 
or riveted girders, in bending, we allow the same extreme fiber stresses as for 
axial stresses, since the chords are concentrated as in an articulated truss, 
being kept apart by the web. In bending of a compact section, however, 
like a pin, we allow up to 50 per cent, more stress, which is not only based 
on experience with pins but also on theory, or rather the non-fulfilment of 
one assumption of the theory of bending, viz., that the fibers deform inde- 
pendently. The fiber stresses not being uniformly distributed over the whole 
section, only the outer rows having the maximum stress, the less-stressed 
adjoining fibers must hold in position, that is, relieve, the overstressed fibers. 
The assumption that a cross section remains a plane after bending is, in 
reality, not strictly fulfilled, even for stresses within the elastic limit, and 



38 CHIEF ENGINEER'S REPORT 

the beam is actually stronger than would seem from the theory of bending. 
The allowable bending stress should be derived from bending tests, but the 
results would vary with the shape of the cross section. 

The secondary stresses caused by the own weight of the members and 
by the distortion of the truss are bending stresses, and as such could be 
allowed for the compact sections of the truss members of the Blackwell's 
Island Bridge at least 20 per cent, higher units than for axial tension. Cooper's 
standard specifications (and others) take this into account, specifying "if 
the fiber strain resulting from the weight only of any member exceeds 10 per 
cent, of the allowed unit strain on such member, such excess must be con- 
sidered in proportioning the area." Most specifications disregard these 
stresses entirely. 

The original specifications (1903) did not contain a clause to consider 
these fiber stresses. In April, 1904, the Department decided to take them 
into account, this to the writer's recollection having been done in order to 
be on the safe side in proportioning the truss members liberally, as it was 
realized at that time that the actual dead load stresses may easily overrun. 

The Quebec The writer is not aware that the failure of the Quebec Bridge caused 
and the ^ny "suspicion" (as stated in a technical paper a few weeks ago) of the 
Blackwell's safety of the Blackwell's Island Bridge. He does not think that such was 
the case until the Royal Canadian Commission, which was appointed "to 
inquire into the cause of the collapse of the Quebec Bridge," made the follow- 
ing published statements: 

"By reference to the table, it will be seen that the specified stresses 
for the Quebec Bridge, under working conditions, are in advance 
of current practice, and we believe they are without precedent in 
the history of bridge-engineering. Under extreme conditions, the 
Quebec Bridge stresses are in general harmony with those permitted 
in the Blackwell's Island Bridge." (Page 148 of their report.) 

And, comparing the bottom chords of the two bridges (page 140): 

"The development of the detail plans of the Blackwell's Island 
Bridge was contemporaneous with that of the Quebec Bridge plans; 
the Quebec designers had not access to the Blackwell's Island plans. 
In fairness to the Quebec Bridge designers, however, it should be 
pointed out that in the Blackwell's Island Bridge the proportions of 
many of the details are much more nearly in accord with Quebec 
Bridge practice than are those of the earlier bridges, although the 
principles of the designs are very different." 

Now, the Blackwell's Island Bridge was not on trial and, being unfinished, 
a comparison of unit stresses allowed for the sum of the dead and imaginary 
live loads did not prove anything as far as the Quebec failure was concerned, 
merely creating a false impression. It would not even have proven anything 
if the Blackwell's Island Bridge had been in use, since the Quebec Bridge 
was a steam railway bridge, designed to carry two tracks of comparatively 



CHIEF ENGINEER'S REPORT 39 

light loading, and neglecting entirely any live load on the two roadways 
of 17 feet width each, while the Blackwell's Island Bridge was designed 
to carry eight tracks for electric traffic, which were assumed loaded with a 
heavy load in addition to a heavy load on the roadways and promenades, a 
total live load unprecedented for any highway bridge, even in New York City. 

The Commission compares the six largest cantilever bridges as to the 
live loads and permissible unit stresses assumed when they were designed. 
It would have been more interesting to have indicated the greatest live 
loads actually used at the present time on the four which are in use. 

The Forth Bridge (designer Sir Benjamin Baker) is designed for two 
tracks, each loaded with approximately Cooper's E22 loading, and a com- 
pression allowed of 17,000 pounds per square inch, with no reduction for 
buckling, and 16,350 pounds per square inch for tension; no impact is con- 
sidered. The actual live load on this bridge has certainly increased well 
beyond the original assumptions, and may have to be doubled and tripled 
in the near future. (In this country, the Atchison, Topeka & Santa Fe use 
Cooper's E66, while the New York, New Haven, the Carolina, Clinchfield 
& Ohio, the Lake Erie & Western, the Crescent line, the Tidewater R. R., 
specify a working live load of Cooper's EGO.) 

The Monongahela Bridge (designers Messrs. Boiler & Hodge) is designed 
for two tracks, each for E45 working load, and considering 100 per cent, 
impact to the live load, allows 21,000 pounds per square inch compression, 
with no reduction for l/r smaller than 40, and 22,000 pounds per square inch 
for tension. 

The Thebes Bridge (designer Mr. R. Modjeski) is designed for two tracks, 
each for a working load of a little less than E50, and, considering 100 per 
cent, impact to the live load, allows 21,000 pounds per square inch compres- 
sion, with no reduction for l/r smaller than about 45, and a tension of 20,000 
pounds per square inch. 

The Memphis Bridge (designer Mr. G. S. Morison) is designed for one 
track for a loading of E40, allowing a compression of 14,000 pounds per square 
inch, with no reduction for l/r smaller than 45, and no consideration of impact; 
and a tension of 20,000 pounds per square inch, adding an impact of 100 
per cent, to the live load. 

The Quebec Bridge was designed for two tracks, carrying 

(1) A working load for certain members hardly equal to E30, with a 
permissible unit tension and compression reaching 21,200 pounds per square 
inch (f. i. in the compression chords) with impact considered by means of 
a "minimum over maximum" formula, but no reduction for buckling made 
for l/r smaller than 50 (no reduction, therefore, for the compression chords) ; or, 

(2) An extraordinary load of 50 per cent, more, therefore, about E45, 
with a permissible unit stress of 24,000 pounds per square inch in tension, 
and also in compression for the chords and main diagonals, and of 24,000 — 
100 l/r in compression for the posts. (A snow load of 1,600 pounds per linear 
foot of bridge was added to the specifications in June, 1905, after the stress 
sheets and some shop drawings were approved.) 



40 CHIEF ENGINEER'S REPORT 

In order to compare the "safety" of these five bridges, we would not only 
have to compare the unit stresses, the assumed live loads, the loaded length 
causing maximum stresses, the assumed impacts, etc., but also investigate 
how closely the assumed live loads approach the present everyday work- 
ing loads, and the possibiUty of a future increase in the latter for which, 
with the exception of the Quebec Bridge, no provision was made, no extra- 
ordinary load having been specified with corresponding increase in unit 
stresses. 

The comparison of the design of these structures, all of them being rail- 
road bridges, is therefore, not a simple matter; how much more complex 
to compare them with the Blackwell's Island Bridge, which was designed 
for a h'ghway traffic of unprecedented complexity. 

In a double-track railroad bridge, the trusses are stressed close to the 
permissible maximum every time two trains meet on the br dge, and yet 
most engineers allow higher unit stresses than for single-track bridges, or 
reduce the five load per track. 

Sir Benjamin Baker, in a paper on "Working Stress of Iron and Steel," 
read before the American Society of Mechanical Engineers in 1886, remarks 
justly (see Railroad Gazette, February 18th, 1887): "A machine or bridge 
can only be well proportioned by carefully considering the special condi- 
tions of the case, in the light of experimental data and past experience. A 
string of formulae will not make an engineer." 

In the report of the Royal Commission on the Quebec Bridge, page 161, 
are given the specifications revised by Mr. Theodore Cooper in 1904 as fol- 
lows: 

First case.— "The maximum strains produced by the following 
live loads and wind shall be used for proportioning all members of 
the trusses or towers: 

/'(I) A continuous train of any length, weighing 3,000 pounds 
per foot of track, moving in either direction on each track. 

" (2) A train 900 feet long, consisting of two E33 engines, followed 
by a load of 3,300 pounds per linear foot, upon each railroad track 
and moving in either direction. 

" (3) A train 550 feet long, consisting of one E40 engine, followed 
by 4,000 pounds per linear foot of track, on each track. 

" (4) For the suspended span a lateral wind force of 700 pounds 
per linear foot of the top chord and 1,700 pounds per linear foot of 
the lower chord, one-half of which shall be used for lateral and diag- 
onal bracing. 

"For the cantilever and anchor arms, a lateral force of 500 pounds 
on the top chord and 1,000 pounds on the lower chord, per Unear foot, 
in addition to the wind force on the suspended span, shall be con- 
sidered. 

"Only one-third of this maximum wind force need be considered 
in proportioning the chords. It shall be considered as a live load. 



CHIEF ENGINEER'S REPORT 41 

Unless this increases the strains due to the Uve and dead loads only 
more than 25 per cent, the sections need not be increased." 

Second case. — On page 162, provision for a future increase of 50 
per cent, in the train loads is called for. 

Mr. C. C. Schneider, who was appointed on behalf of the Canadian Gov- 
ernment "to inquire into and pass upon the sufficiency of the present design 
of the Quebec Bridge," independently of the Royal Commissioners, states 
as follows: 

"The first case will be called hereafter the working load, and 
the second case the extreme load. The strains produced by the work- 
ing load, which is by no means excessive, should leave a reasonable 
margin for safety. The strains produced by the extreme loads should 
remain within the elastic limit of the material." 
On page 156, Mr. Schneider reports as follows: 

"The extreme unit strains within which in the writer's judgment 
the structure may be considered to be able to sustain the loads pro- 
vided for in the specifications, are: 

(1) For the dead and live loads combined with the snow load: 
For tension, 21,000 pounds per square inch of net section; for com- 
pression, 21,000 — 90 l/r per square inch of gross section. 

" (2) For the extreme provisions of one and one-half times the 
live load, dead and snow loads, combined with one-third of the wind 
strains: For tension, 24,000 pounds per square inch of net section, 
for compression, 24,000 — 100 l/r per square inch of gross section." 

From this follows that Mr. Schneider considered for the Quebec Bridge 
a live load (from less than E30 to less than E40 for the different truss mem- 
bers) which in all probability would have occurred every time two trains 
had met on the center span as "working load," and a live load 50 per cent, 
higher as "extreme load," and for these loads together with the dead and 
specified snow loads he assumes, but "does not advocate," unit stresses not 
exceeding 21,000 and 24,000 pounds respectively. These live loads cannot 
be compared with those of the Blackwell's Island Bridge, that is 8,000 pounds 
per linear foot of bridge as "regular" and 16,000 pounds per linear foot 
of bridge as "congested." 

It should also be remembered that the Quebec Bridge was built to carry 
besides the double-track steam railway also "a roadway on each side 17 
feet wide in the clear, suitable for ordinary highway traffic with one electric 
railway track on each roadway," and that for the roadways absolutely no 
allowance was made in the live load for the trusses, originally not even for 
a snow load, so that at least the effect of a snow load should be added to the 
originally specified unit stresses before a comparison is made. In the Black- 
well's Island Bridge a snow load can safely be deducted from the assumed 
uniformly distributed live load on the roadways and footwalks, since it is 
only reasonable to assume that the live load will be reduced by that amount. 



42 CHIEF ENGINEER'S REPORT 

Considering the probabilities of the different live loads specified for the 
Quebec Bridge and the Blackwell's Island Bridge, it would seem more correct 
to compare the effect of the "extreme" load plus the snow load on the Que- 
bec Bridge with the effect of the "regular" load of 8,000 pounds per linear 
foot of bridge, in continuous stretches on the Blackwell's Island Bridge. 
This would give, for instance, for the bottom chords a greatest unit stress 
in compression of 30,100 pounds for the Quebec Bridge, to be compared 
with 17,000 pounds for the Blackwell's Island Bridge. 

The following greatest unit stresses in compression for the bottom chords 
may be of interest: 

Quebec Bridge 

For "extreme" condition, after completion . . . .30,100 (24,000 specified) 
For "working" condition, after completion . . . .24,600 (20,500 specified) 
For dead load alone after completion 17,500 

Blackwell's Island Bridge 

With a live load of 16,000 pounds per linear foot 

after completion 22,100 (21,300 specified) 

With a live load of 8,000 pounds per linear foot 

after completion 17,000 (17,600 specified) 

For dead load alone after completion 11,900 

To illustrate further Mr. Schneider's ideas of the difference between 
"working" and "extreme" load, the following taken from his closing dis- 
cussion of his paper on "The Structural Design of Buildings" (Transactions 
of the American Society of Civil Engineers, 1905) may be of interest: 

"Some of the discussors have discovered that it is possible to obtain 
greater loads than those specified. The writer wishes to state that 
he was fully aware of all these possibilities of loading, and, also, has 
carefully studied all the literature on the subject of experiments on 
the weights of crowds of people, . . . and has given all these facts 
due consideration in determining the live loads to be specified for build- 
ings. . . . The writer, in determining upon the live loads of build- 
ings of various kinds, as stated before, has been guided by what is 
now considered the best practice in bridge-building. It is not a ques- 
tion of how much load of any kind it is possible to pile on a square 
foot of floor area. The question is to make the buildings absolutely 
safe without wasting large quantities of material in places where it is 
not needed. A structure should be proportioned for a working load 
with the ordinary unit strains, and provision made for a congested 
load with unit strains well within the elastic limit, so that the 
structure may yet be safe under such extraordinary conditions of load- 
ing. The working load should be the probable maximum load which 
may be reasonably expected to occur, while the congested load is a 
load which is improbable, but within the reach of possibility." — 



CHIEF ENGINEER'S REPORT 43 

"Railroad bridges are now generally designed for a possible future 
increase in the weight of locomotives and rolling stock, but no engi- 
neer would think of designing a long-span, double-track bridge under 
the assumption that both tracks will be entirely covered with the 
heaviest type of locomotives, or cars carrying heavy ordnance and 
using the ordinary unit strain for that condition. 

"The writer desires to emphasize the fact that such unusual and 
extraordinary conditions of loading have been considered in his speci- 
fications, in assuming that it is rational and unquestionably good 
practice to allow a unit strain of 25 per cent, in excess of the ordinary 
working strain, or 20,000 pounds per square inch for congested loads 
and 50 per cent, in excess, or 24,000 pounds per square inch for very 
extreme cases. 

"There are railroad bridges in existence now, some members 
of which are strained to 24,000 pounds per square inch, including 
impact, almost every time a train passes." 

The statement has been made that, in the last years, the permissible High Unit 
unit stresses used in the design of bridges have been increased; and "high" Desfm^of" 
permissible unit stresses and our "ignorance" in regard to the proper design Compression 
of compression members were widely criticized ever since the Quebec Bridge ®™ *" 
failed. 

The writer, based on his own investigations, is of the opinion that if the 
design of some of the older bridges as described in the Transactions of the 
American Society of Civil Engineers and other publications would be ana- 
lyzed, especially with the actual and not the assumed dead loads and the 
assumed light live loads properly considered, the statement concerning 
unit stresses would appear in a somewhat different light. The impression 
was probably caused by a few specifications allowing high unit stresses 
for a practically impossible excessive live load in order to provide amply 
for counterstresses in tension members which might be caused by a possi- 
ble increase of the ordinary working load and other unforeseen circumstances, 
without increasing unduly the costs of the bridge. This precaution may be 
of great value (as mentioned before) while the additional weight is only 
small. 

Sir Benjamin Baker in the above-named paper (in 1886) states as follows: 

"The writer has availed himself of the opportunity afforded by 
the large use of special plant and machinery at the Forth Bridge 
works to note the influence of varying stresses on full-sized riveted 
steel girders. These observations are still in progress and can be 
but very briefly referred to herein. In one instance the lever of a large 
plate-bending press is of box-girder section, built up of eight 4 x 4 x f 
inch angle bars, 13 x | inch web plates, and two 17 x ^ x,f inch flanges. 
The span is 15 feet 8 inches, and the ordinary daily working stress 
on the metal is 43,000 pounds, and occasionally 57,000 pounds per 



44 CHIEF ENGINEER'S REPORT 

square inch. Many thousand applications of this stress have been 
made, and the beam has taken a permanent set of seven-eighths of 
an inch, but so far is otherwise intact." — 

"As regards the important question of the proper working stress 
on iron and steel, the writer's experience leads him to believe that 
both the old-fashioned government regulations, giving the same 
limiting stress for all kinds of loading, and the modern formula, based 
chiefly on Wohler's experiments, fail to meet the just requirements 
of the practical engineer. It is, in many cases, a great economical 
advantage and convenience to have reference not merely to the vari- 
ation of stress, but also to the probable number of applications. For 
example, the writer knows the bending press box girder lever, previ- 
ously referred to, will last its time, although the working stress is 
about two-thirds of the ultimate strength of the material; and it 
would have been a mere waste of money to make it four times as 
strong, and so give it the factor of safety of six, usual and proper 
enough for a structure such as the Elevated Railway of New York, 
where a practical infinite number of repetitions of stress have to be 
provided for." — 

"The Conway Tubular Bridge, which has carried the heavy traf- 
fic of the London & Northwestern Railway for the past thirty-six 
years, is 412 feet in span, and under its own weight the tensile stress 
is 13,000 pounds per square inch. With ordinary trains, the stress 
is 17,000 pounds, and, if covered with the heaviest engines in use 
on the line, 20,000 pounds per square inch. The ultimate strength 
of the riveted structure is about 42,000 pounds per square inch. No 
indications of weakness have developed during the thirty-six years' 
working nor anything to suggest that the factor of safety of, say 
2 to 2^, is unduly low." — 

"Twenty years ago, being uncontrolled by government regula- 
tions, the writer adopted a working stress of 16,000 pounds per square 
inch* on many large iron girders carrying a heavy dead load, although 
at that time a departure from the usual 11,200 pounds per square 
inch was regarded with suspicion. The results of modern research 
have, however, now given the engineer a free hand, and the British 
five tons per square inch and the Continental six kilos per square 
millimeter have ceased to be regarded with superstitious reverence." 

Professor Engesser, in his well-known book on secondary stresses, pub- 
lished in 1893, writes as follows: 

"For repeated stresses, a rupture of the material takes place 
for stresses below the ultimate strength, depending on the range 
of the stress. This ultimate for repeated stresses is the lower, the 
greater the range, and has its lowest value for alternate tension and 

♦This refers to working live load without impact and to iron, not steel. The writer was unable to find 
any heavier bridge built by Mr. Baker at that time than the Mersey bridge, built 1869, near Liverpool, a 
span of 305 feet with a weight of steel probably only three-fourths of the assumed live load. 



CHIEF ENGINEER'S REPORT 45 

equal compression. The ultimate for repeated stresses is for stresses 
of the same sign greater than the elastic limit, for stresses of oppo- 
site signs smaller than the elastic limit. It should be remembered, 
however, that the tests made by Wohler and Bauschinger were made 
with stresses repeated rapidly in immediate succession, while the 
conditions in a bridge member are entirely different. — The permis- 
sible unit stress should remain therefore for the ordinary working 
load below the ultimate for repeated stresses, while for extraordinary 
conditions it may exceed the ultimate for repeated stresses without 
danger to the structure" — 
in other words, for extraordinary conditions, occurring only a few times, 
ii ever, during the lifetime of the bridge, it could for stresses of the same 
sign without danger even exceed the elastic limit. 

The statement concerning our ignorance in regard to the design of com- 
pression members has to be taken with reservation. Bridge Engineering is 
not an exact science. Any engineer who prepares a design and follows it through 
the drawing-room and shop will admit that there are a hundred and one ques- 
tions where his "string of formulae" proves insufficient and he has to rely 
on his judgment, consciously and unconsciously derived from past experi- 
ence. The lacing of compression members is a detail and, like other details, 
can only partly be analyzed theoretically. The connection of floor-beams 
to the posts, particularly of floor-beams cut out for the pins, in fact, all riveted 
connections, including splices, especially such with several ribs and many 
rivets, are more complex matters than the lacing of a column. But, after 
all, is our knowledge of tension members as complete as pretended? We 
have tested eye-bars and single wires or small ropes in full size, but these 
are not tension members, merely parts of a tension member, and about 
the distribution of stress over the cross-section of a cable, for instance, we 
have only approximate ideas. 

Table No. 5 gives unit stresses of those main members of the trusses Appended 
which, according to the reports of the experts, show most unfavorably. The 
unit stresses due to the dead load were taken not from the stress sheets used 
in the design, but, to avoid criticism, were derived from the dead load stresses 
of the experts, reducing the stresses by an amount due to the reduction 
of the paving, pipes, railings, to their originally assumed weight. Six sets 
of unit stresses due to three different conditions of the "congested" and 
of the "regular" Uve load were compiled from the "Loading Key" and com- 
bined with the stresses from dead load, as follows: 

(1) "Congested" live load in continuous stretches; 

(2) "Congested" live load in discontinuous stretches; 

(3) One-half of the "congested" live load over the whole bridge 
(from end to end) and one-half in discontinuous stretches; 

(4) "Regular" live load in continuous stretches; 

(5) "Regular" live load in discontinuous stretches; 

(6) One-half of "regular" live load over the whole bridge (from end 
to end) and one-half in discontinuous stretches. 



stress Sheet 
Table No. 5 



46 CHIEF ENGINEER'S REPORT 

Recommen- Based on these figures and the foregoing discussion on the probability 
of the different Uve loads, the writer recommends the following: 

(1) The paving should be reduced to its originally intended weight on 
the whole bridge, or, which may be even more effective for its final purpose 
of reducing certain stresses, reduce the paving only on the river spans, leav- 
ing on the Island span and the two anchor arms the heavy paving, as the 
writer suggested to you about three months ago, before the reports of the 
experts were known.* 

(2) With the paving altered in this way, the bridge is safe for the 
intended traffic, viz.: two promenades of 11 feet each, a roadway of 35^ 
feet, four trolley tracks of 1,000 pounds per linear foot, and four elevated 
railroad tracks of 1,700 pounds per linear foot. 

(3) The experts state that, since the bridge was designed, the weight 
of trolleys and rapid transit trains increased, the former from 1,000 to 1,460 
and the latter from 1,700 to 1,810 pounds per linear foot. This may or may 
not be only a passing phase in the development of the rolling stock of elec- 
tric traffic. In the next few years, before any rapid transit traffic will cross 
the Blackwell's Island Bridge, many changes in the rolling stock may occur, 
the cars may get longer, or the character of the traffic may change (moving 
seat platforms, etc.), and the calculations will have to be revised for the 
new conditions. 

(4) To remove the stringers designed to carry two elevated railroad 
tracks is not necessary, and not advisable, as they now support the foot- 
walks and new ones would have to be provided to replace them, and, as 
nobody can tell whether they may not be of use within the next ten years, 
should the weight or character of rapid transit traffic change. 

(5) A thorough investigation of the actual traffic on the existing bridges 
in New York City should be made by the Engineering staff of the Depart- 
ment of Bridges (not by laymen) in order to prove conclusively that the 
traffic needs no police regulations, beyond those customary in the case of 
ordinary city streets on which "congestion" or "bunching" of traffic to the 
extent of 50 pounds per square foot over any great area would not be toler- 
ated by the police, and to establish again that sense of proportion which, in 
this whole controversy, seems to have been lost. 

In taking instantaneous photographs of the traffic of the Williamsburg, 
the Brooklyn and other bridges, possibly also on crowded streets (from 
an upper story) and approximating from them the live load over a certain 
length, a fair estimate of the weight per square foot at different hours and 
its maximum could be established, while the usual assumptions of 50, 75 
or 100 pounds per square foot, derived from experiments in buildings with 
stationary live loads, are for floors of long bridges hardly better than guess- 
work. Very truly yours, 

F. C. KuNZ, Chief Engineer. 

*See Supplement, page 47 



CHIEF ENGINEER'S REPORT 47 



Supplement to the Report 

Steelton, Pa., December 28th, 1908. 

Since this report has been written, the Department of Bridges has pre- 
pared new drawings for the paving of the roadway, which is now under 
construction. The change in the paving consists in a reduction of weight 
on the river spans to approximately that originally assumed, leaving the 
heavy paving on the Island and anchor spans. 

Table No. 6 shows the stresses of the truss members, based on the final 
paving, corresponding to table No. 5, based on the original paving. 

The stresses given in the report refer to the original paving and are changed 
as follows for the final paving: 

Stress for Stress for 

Original Paving Final Paving 

Page 2.5, twelfth hne from top 20,700 21,100 

Page 25, twelfth line from top 18,800 18,900 

Page 2.5, thirteenth line from top 34,100 34,200 

Page 2.5, fifteenth line from top 22,900 22,700 

Page 28, sixth line from bottom 21,200 21,000 

Page 28, fifth line from bottom 33,500 33,600 

Page 30, ninth line from top 18,600 18,400 

Page 30, tenth line from top 22.900 22,700 

Page 30, eleventh line from top 25,600 25,400 

Page 30, twelfth line from top 34,200 34.000 

Page 30, sixteenth line from top 20,.300 20,100 

Page 30, twentieth line from top 29,000 28,800 

Page 42, seventeenth line from top 22,100 22.200 

Page 42, nineteenth line from top 17,000 17.100 

Page 42, twentieth line from top 11,900 12,100 

F. C. K. 



APPENDIX A 

EXTRACTS FROM REPORTS OF EXPERTS 

"The provisions of the specifications, both for the chemical and physical Extracts from 
requirements of the material, are in accordance with the best practice of ^e j^epo^ of 
the present day, and entirely satisfactory. I have as far as possible examined William H. 
the manner of inspection, both in the mill and shop, and I have scrutinized ^^^ 
carefully a mass of records of experimental data, established by tests of 
both specimens and full-size eye-bars in the course of mill inspection, with 
satisfactory results. All of this class of evidence goes to show that the material 
put into the bridge was of excellent quality and fully (met) the requirements 
of the specifications." 

"The character of shop work is evidenced by the condition of the manu- 
factured members in the bridge. These are largely open to ocular inspec- 
tion, and I have many times been on the structure for the purpose of exam- 
ining the results of the shop work. I believe it to be fully up to the require- 
ments of the specifications and generally in accordance with the best prac- 
tice of the present time. The four sub-diagonal posts C56-L57 and L107- 
C108 on both trusses near the main piers on Blackwell's Island were a little 
twisted when put in place, but this difficulty was corrected by riveting cover 
plates on the tops of the posts. These posts are not main truss members 
and the question of the safety or stability of the latter is in no way affected 
by them." 

"Many sections have been carefully calipered and the weights of many 
members have been computed in order to determine whether the actual 
dimensions of pieces as placed in the structure correspond to the require- 
ments of the specifications, contract and shop plans. The results of these 
examinations have been entirely satisfactory. The actual sections of the 
members are generally found a little full, a margin of 2^ per cent being allowed 
by specifications in accordance with common practice." 

"I have made a careful examination of the entire structure as far as 
possible, with a view to determining whether evidence of distress of any 
members exist, such as permanent distortion, loose rivets, or other evidences 
of overloading, misfitting, or any other circumstances which might indicate 
over-stressing. I have not found such evidence. All parts of the structure 
appear to be in satisfactory condition. There are minor or small variations 
from aUgnment, which are usually observed in large, completed bridge 
work in place, but nothing whatever to indicate any inherent weakness 
or unsatisfactory condition." 

"In addition to this inspection, I have had accurate surveys made of 
the entire structure to determine the alignment of trusses and elevation of 

49 



50 



EXTRACTS FROM REPORTS OF EXPERTS 



lower chord points in July and as late as October 26, the past month. These 
surveys indicate that the alignment of the trusses is entirely satisfactory, 
and that the deflections vertically are only those caused by the dead weight 
of the structure at the two dates stated, the dead weight at the latter date 
being considerably more than at the former in consequence of the large 
amount of floor material put in plaCe between the periods named." 

"Adverse comment has been made on the heavy compression lower 
chords of this bridge and their design therefore has been scrutinized with 
great care. . . . No criticisms of these lower chord compression members 
would probably have been made except for the failure of chord sections of 
somewhat similar general shape of section in the Quebec Bridge. The simi- 
larity, however, lies only in the general form of section of the component 
parts. The spacing details of the Blackwell's Island chord sections, consist- 
ing of heavy lattice bars, batten and tie plates, and transverse diaphragms, 
are relatively far heavier, stiffer and stronger than corresponding details 
in the Quebec trusses; indeed, in the latter, there were no transverse dia- 
phragms such as are found in the Blackwell's Island Bridge. There is, there- 
fore, little or no similarity as to the unit-carrying capacity of the compres- 
sion chord members in the two bridges." 

Professor "(Second) Both the shop and mill inspection were efficiently performed. 
Conclusions resulting in securing excellent quality of material and the fabrication of 
truss members of good quality and accurate dimensions. , 

"(Third) The various members of the structure possess the full sections 
required by the unit stresses and the working plans, and the shipping weights 
correspond correctly to those sections as well as to the computed weights. 

"(Fourth) The erection was successfully and satisfactorily performed, 
leaving the trusses in correct alignment and elevation." 



Extracts from "We have examined the detailed reports of the mill inspectors on this 
Messrs. Boiler ^^^aterial, and find that they show the metal fulfilled the above specifications." 
. & Hodge "In addition to figuring the stresses on all members, we have had a large 
number of the actual bridge members measured and calipered in the field, 
and we find that they agree with the sections we took from the shop draw- 
ings and used in these calculations, which sections we show in detail on 
sheets 8 and 9." 

"We have also computed the weight of a number of members from the 
shop drawings and find that such weights agree with the shipping weights 
on the invoices, showing that the scale weights used for the dead load are 
correct." 

"We have not carefully examined all the details of this structure, but 
we have checked the end connections of such members as are most heavily 
stressed, and find them equal in strength to those members." 

"We have carefully considered the form and details of the lower chord, 
as this feature has been criticized in the public journals, and the impression 



EXTRACTS FROM REPORTS OF EXPERTS 



51 



has been given that the lower chords in this structure are weaker than those 
of the Quebec Bridge, which, in our opinion, is not the case." 

"We have made a careful examination of the bridge as now completed, 
and find no evidence of loose rivets or buckling of members, or other indi- 
cations of overloading, but there are four sub-diagonal posts, C56-L57 on 
both trusses, and L107-C108 on both trusses, which had a "wind" in them 
during erection, and this was corrected by riveting a cover plate on the 
top of each post. It will be noticed that both of these members are "sub- 
diagonals," which in no way affect the main stresses and are only for the 
support of one local panel load, and the stress sheet shows that they will 
never be subject to heavy stresses, so they are evidently safe and the "wind" 
was probably due to a bend in the shop or during erection, or to a slight over- 
run in length." 



"(Second) That the steel manufactured for this structure is first-class Messrs. 
bridge material and in accordance with the specifications. Hodee' 

"(Third) That the workmanship of this structure is first-class and in Conclusions 
accordance with the requirements of the specifications. 

"(Fourth) That the erection and field riveting of the structure appears 
to have been done in a first-class manner. 

"(Fifth) That the actual sections of the various members agree with the 
sections ordered on the working drawings and shown on our sheets Nos. 
8 and 9, and that the shipping weights are correct." 



APPENDIX B 

EXTRACT OF SPECIFICATIONS FOR THE STEEL SUPERSTRUCTURE 
OF THE BLACKWELL'S ISLAND BRIDGE 



All steel shall be made by the open-hearth process, and shall fulfil the 



Manufacture 
and 
Requirements following requirements: 



CHEMICAL REQUIREMENTS 





NICKEL STEEL 










Phosphorus P C. Max 


Sulphur 
P C Max 


Nicke, 
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STRUCTURAL STEEL 


Plate Shapes Bars and Pins 


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05 
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54 



EXTRACT OF SPECIFICATIONS 55 

Tensile tests of structural steel, showing an ultimate strength within Allowable 
4,000 pounds of that desired will be considered satisfactory. strength^ '° 



LOADS 

The bridge shall be proportioned to carry in addition to its own weight Live Loads 
and that of the floor a live load either uniform or concentrated, or both 
as specified below, placed so as to give the greatest strain in each part of the 
structure. 

For the main members of the trusses and the towers: 

(a) A load of 8,000 pounds per linear foot of bridge as "regular," or 
(6) 16,000 pounds per linear foot of bridge as "congested" traffic. 
For the secondary members of the trusses, the floor-beams and the floor 
system: 

(c) On each elevated railroad track a load of 52 tons on four axles, 
6+10+6 feet apart (the motor ends of two motor cars of the Inter- 
borough Rapid Transit Co.). 

(d) On each street-car track, either a load of 26 tons on two axles 
10 feet apart, or a load of 1,800 pounds per linear foot of track. 

(e) On any part of the roadway a load of 24 tons on two axles 
10 feet apart and 5 feet gauge (assumed to occupy a width of 12 feet 
and a length of 30 feet), and upon the remaining portion of the floor 
a load of 100 pounds per square foot. 

(/) On the footwalks a load of 100 pounds per square foot. 

The wind pressure shall be assumed as a moving load acting in either ^ffiui^ 



direction horizontally Avith 2,000 pounds per linear foot. 



PROPORTIONING OF PARTS 



Pressure 



For the main sections of members of the trusses and towers, no material Least 

Thick] 
of Steel 



shall be used less than one-half an inch thick; all other material shall not ^*^ ®^^ 



be less than three-eighths of an an inch thick. 

All steel work shall be so proportioned that the maximum strains from Permissible 
dead load and live load, or dead load and wind, shall not cause greater unit ^°'* strains 
strains than the following: 



{a)~For Nickel Steel in Eye-bars and Pins — 

Tension 

Shear on pins 

Bearing on diameter of pins 

Bending on outer fibre of pins 



(b) For Structural Steel in Main Members of Trusses, 
Towers and Bracing — 

Tension 

Compression 

Shear on shop rivets, bolts and pins 



Bearing on diameter of shop rivets, bolts and ] 
pins ) 

Bending on outer fiber of pins 



For Dead Load and 

Regular Live Load 

or for Dead Load 

and Wind 



For Dead Load and 
Congested Live Load 



Pounds per square inch 



30,000 
20,000 
40,000 
40,000 

20,000 

20,000-90-* 
r 

13,000 
25,000 
25,000 



39,000 
24,000 
48,000 
48,000 

24,000 

24,000-100-* 
r 

16,000 
30,000 
30,000 



(c) For Structural Steel in Secondary Members of Trusses — 

Tension in subverticals (hangers) 

Compression in subdiagonals 

Shear on shop rivets and bolts 

Bearing on diameter of shop rivets and bolts 



(d) For Structural Steel in Floor System of Roadway and Footway 
and in All Floor Beams — 



Tension chords 

Shear on shop rivets, bolts and web-plate net section . 
Bearings on shop rivets and bolts 



(e) For Structural Steel in Floor System {including brackets) for 
Railroad and Trolley Tracks — 



Tension chords 

Shear on shop rivets, bolts and web-plate net section 
Bearing on shop rivets and bolts 



Pounds per square inch 



18,000 

18,000-80-* 
' r 

12,000 
24,000 

15,000 
10,000 
20,000 

10,000 

7.000 

14,000 



* Where 1 = length and r^ least radius of gyration, both in inches. 



56 



EXTRACT OF SPECIFICATIONS 57 

Members subject to reversals of strain shall be proportioned for each Reversals of 

kind of strain, and the section shall be determined by the strain requiring ^*''*'° 
the greater net area. 

For combined strains, due to dead load, regular live load and wind, the Combined 

■unit strains given above may be increased 20 per cent. strains 



CONSTRUCTION 

Provision shall be made for a free expansion and contraction of all parts, Temperature 
•corresponding to a variation in temperature of 110° Fahrenheit. 

The deflection of the spans from dead load shall be taken out by correct- Camber 
ing the length of each truss member. 




Cross Section 

BLACKWLLLS ISLAND BRIDGE. 

with 
Two "Rapid. Transit RK. TracKs 



58 




Cross Section 

blackwe.ll:s island bridqe. 

wrth 

Four Rapid Transit R.R. TracKs 



59 



TABLE 1 

Diagram of "continuous" Live 
Load for Chords 



TABLE 2 

Diagram of "continuous" Live 
Load for Web members 



TABLE 3 

Diagram of ""discontinuous" Live 
Load for Chords 



lb 



TABLE 4 

Diagram of "discontinuous" Live 
Load for Web members 



TABLE 5 



Unit Stresses for various conditions 
of loading with original paving 



\'ty 










MANHATTAN ANCHOR ARM 












Ar. 


as 

::: 

■ j'i 

is 




I(^U. 1 ii,n,=.«d 


'^^. 


„^|N_. 


, 




Moic 
3U,0 

138.3 


1 ISLA 
+2M+2T.7+I6. 


-i: 


1 '-'^ I Usii iaa^a 

ANTILEVER ARM. E 

' r, : 1 1 
*M|il:S|lS,1|*qi'? 


11 

AST 

'i. 

V. 


MO.(lh in.2 
-ia.o -is,3 




1 


8 











MANHATTAN CANTILEVER ARM 
















ISLAND 


CANTILEVER 


ARM WEST 












. _ 1 


KM 


U..l«^ 


ToUklLfflid 


SSi *S?t 


Af=i 


"m" 


Li.« L«<l 1 Toul L<Md 


P.miu«U> 






o™. »., 


s:.St:s' 


..,=:,.. 


CbnioUd 




"* lanm 


».i 


iK:-.rSl?iS 


.^:;:..'iir.j{J^|.,^:^.!:;r::tT!!i:' 

HAW POSTS i 
QUEENS ANCHOR ARM 


Rwilai 


as. 

4-24.0 

+3»!o 
+ 30.0 

-20.0 

-aio 

% 

-ISO 
-10-R 

-aoo 


'SOS 

-isii 

+ 30.0 
+30-0 




C30-L31 


108.1 


8SU 
30.0 




* 


Q 


UEENS CANl 
lUIH 


i:ii.izii: 

OHORD 




-18.0 




1 




i 

.10.. 
-...0 


*Mi 


-1 


*i>>.l|i.29.<) 

rtU.? ^27:3 
♦45.0' ^24.0 

-4fi.ol -lo.e 

Ii 


-14-0 




U 9S-U ttT 


|83o!o 


+I5.0l + ^;s 11 1 


'::':': 


11:! 

i 


-It. 
-10.2 


1 Z!-'Z 


6ie.( 


408.0 -f2O.0 

i;:!i:; 

- oil 

l7<!o +H^ 

- 8.1 


ilii 


+18.0 

-10.11 


KAIH 

- so; - ^1 - so 


-ao.3 -aj.B -20,1 

-21 .41 -22 -10^-. 

-206 -a* -auj 


« 


UIOS.LU 


s 


»~~ 






















VE aTRBasES 


*" 




IN UNITS OP 1.0 















ISLAND SPAN 














._ 




IJn Loa.1 1 


IV,. 


To«l l.o«.l 


o'lXl 






IbnC-M _ 


>.„>., 1 


las' 


ll»I. 




LOa-Lflf 


1 

388.1 


ii! 

S08.3 


\ itJt 

', 
' 


sir, ikI „'.::..! sir 

';; ■,:|;:; ,, 






SI 


GREATEST STRESSES FOR VARIOUS 


MEMBERS 


1 


QuwM 




iWul.i 


MuilMf UtrM II«nt*i Slii 


-iTJlT',!!!! 


U>mU( 


i™. .■.„„, .,™ ».„w ..«. 


"; 


im- 


Si "-^1!^ l»;J".M"Lm-^'« 


1 


zi!' 


1 


3;E:ii 



o 



BLACKWELL'S ISLAND BRIDGE 



THE PENNSYLVANIA STEEL CO. 



TABLE 6 

Unit Stresses for various conditions 
of loading with final paving 



MANHATTAN ANCHOR ARM 






MANHATTAN CANTILEVER ARM 



ISLAND CANTILEVER ARM. EAST 






-9io!+20.ol 












ISLAND CANTILEVER ARM WEST 






ISLAND SPAN 



QUEENS CANTILEVER ARM 



QUEENS ANCHOR ARN 






' Ktmhtr 


:S:J KStS S'o 


tToWMd*tK^. 

'"'" 1"- 


U«»l 






Sftg|lii: 



r j ME 



FOR VARIOUS MEMBERS 






o 



BLACKWELL'S ISLAND BRIDGE 



Mb*. I S^aMlbt. 



I STRESSES ARE ( 



1^ 



TABLE 7 

Diagram Showing Congested 
Loading 



i.i-»J i- 



mmmmmmsm 



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