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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


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