Quebec Bridge

What materials and systems were used in the construction of this structure?

The specifications of the Quebec Bridge called for a cantilever structure.  The material for the structure was to be steel. The basic configuration of a cantilever bridge is shown in Figure 1. Suspension and arch bridges were other options considered, provided they came with their own set of specifications. French engineer Gustave Eiffel had considered the bridge type and found that a cantilever design would be superior to either a suspension or an arch bridge for the Quebec site.

The cantilever structure was used first in 1867 (Pearson and Delatte 2006, p. 86). William Middleton gives a clear definition of a cantilever bridge in his book: “cantilever bridge: a bridge form based upon the cantilever principal. In its typical form cantilever arms projecting toward the center of the span from main piers are continuous with and counterbalanced by anchor arms extending between the main piers and anchor pier at each end. A simple span suspended between the two cantilever arms completes the structure. The weight of the suspended span and the cantilever arms is counterbalanced by that of the anchor arms and an anchorage embedded in the anchor pier” (175).

  Figure 1: Structural behavior of a cantilever bridge.

The Quebec Bridge was the longest cantilever structure ever to be attempted during its day (Pearson and Delatte 2006, p. 85).  It would bridge the St. Lawrence River approximately 14 km north of Quebec connecting into the Grand Trunk rail line. The cantilever arms would reach a total distance of 171.5 m.  These arms were to support a suspension span of length 205.7 m.  The bridge would stand 45.7 m above the river. Initially, the design clear span length was 487.7 m (Pearson and Delatte 2006, p. 86).

In May 1900, the clear span length was increased to 548.6 m by Theodore Cooper. Cooper was a famous bridge builder and consultant out of New York City, and he was chosen to be the consulting engineer for the Quebec Bridge.  Cooper had very strong qualifications which is why he was chosen to be on the project.  In his long career, he had written an award-winning paper pioneering the use of steel for railway bridges, and had prepared general specifications for iron and steel bridges (Petroski 1995). His method of accounting for railroad loads on bridge structures became widely used (Middleton 2001, p. 37).  When Cooper increased the clear span length, he stated that this would do three important things; it would eliminate the uncertainty of constructing piers in such deep water, lessen the effects of ice, and shorten the time of construction of the piers (Pearson and Delatte 2006, p. 87).   Although there were sound engineering reasons for this change, it was also true that the lengthening of the span would also make the Quebec Bridge the longest cantilever bridge in the world (Petroski 1995, p. 46; Middleton 2001).

Construction of the bridge officially began on October 2, 1900, after a grand ceremony. The Quebec Bridge Company had enough funds to begin erecting the substructure. The completed piers would stand approximately 8 m above the highest water level. The piers were made of huge granite facing stones with concrete backing (Pearson and Delatte 2006, p. 87). The top 5.8 m of each pier was made of solid granite. The piers were tapered 1 in 12 until they reached the dimensions of 9.1 m by 40.5 m at the top. Each pier rested on a concrete filled caisson that was 14.9 m wide, 7.6 m high, and 45.7 m long, weighing 16.2 MN (1,600 t) (Middleton 2001, pp. 48–50).

What were the causes of the structural failure?

The causes of the failure are best summarized by the Royal Commission Report.  This report states that immediate cause of failure was found to be the buckling of compression chords A9L and A9R. The official report attributed the collapse to a number of reasons (Pearson and Delatte 2006, p. 89).  Below, major reasons of the failure are listed below.  A man named Peter Szlapka is mentioned in the reasons.  He is the chief design engineer for the Phoenix Company; this company was the firm in charge of the construction of the bridge.  The seven major reasons will be listed below (Holgate et al. 1908, pp. 9-10):

1.     “The collapse of the Quebec Bridge resulted from the failure of the lower chords in the anchor arm near the main pier. The failure of these chords was due to their defective design.”

2.     “We do not consider that the specifications for the work were satisfactory or sufficient, the unit stresses in particular being higher than any established by past practice. The specifications were accepted without protest by all interested.”

3.     “A grave error was made in assuming the dead load for the calculations at too low a value and not afterwards revising this assumption. This error was of sufficient magnitude to have required the condemnation of the bridge, even if the details of the lower chords had been of sufficient strength, because, if the bridge had been completed as designed, the actual stresses would have been considerably greater than those permitted by the specifications. This erroneous assumption was made by Mr. Szlapka and accepted by Mr. Cooper, and tended to hasten the disaster.”

4.     “The loss of life on August 29, 1907, might have been prevented by the exercise of better judgment on the part of those in responsible charge of the work for the Quebec Bridge and Railway Company and for the Phoenix Bridge Company.”

5.     “The failure on the part of the Quebec Bridge and Railway Company to appoint an experienced bridge engineer to the position of chief engineer was a mistake. This resulted in a loose and inefficient supervision of all parts of the work on the part of the Quebec Bridge and Railway Company.”

6.     “The work done by the Phoenix Bridge Company in making the detail drawings and in planning and carrying out the erection, and by the Phoenix Iron Company in fabricating the material was good, and the steel used was of good quality. The serious defects were fundamental errors in design.

7.      “The professional knowledge of the present day concerning the action of steel columns under load is not sufficient to enable engineers to economically design such structures as the Quebec Bridge. A bridge of the adopted span that will unquestionably be safe can be built, but in the present state of professional knowledge a considerably larger amount of metal would have to be used than might be required if our knowledge were more exact.” 


What were the consequences of the failure?

It is worth noting and emphasizing the structure collapse did not come as a surprise to some of the engineers on the project.  One young engineer, Norman McLure, was noting deflections in the chords as early as two months before the collapse.  McLure told Cooper, and the men came to the conclusion that these deflections occurred due to an unknown preexisting condition (Pearson and Delatte 2006, pp. 86-87).  The men were not alarmed with the deflections, and considered them to be relatively insignificant.  Deflections continued to increase as the project went on, and Cooper was notified of them.  Two weeks before the collapse, Cooper wired a message to McLure asking how bend had occurred in more chords (Middleton 2001, pp. 72-73).

The members under the highest compression loads continued to buckle, and tensions grew on the worksite.  Cooper was not on the worksite, but the workers there could see the deflections and became very concerned.  McLure did not have the confidence to contradict Cooper because he was a young engineer and Cooper was renowned in the field.  Another inspection took place two days before the collapse, and it was found that the deflections in the A9L and A9R chords had increased by three times as much in the last two weeks (Pearson and Delatte 2006, p. 89).  McLure halted work on the site and notified Cooper that work was halted.  McLure then went straight to New York to seek advice from Cooper (Middleton 2001, pp. 78-79).

The erection foreman resumed worked the next day (the day before the collapse) without McLure’s permission, because he was confident the deflections were not a large concern.  The next day, news of the resumed work reached the Phoenix Company office, and the supervisors met to discuss the plan of action.  The supervisors at the office determined work resuming was a good decision (Pearson and Delatte 2006, p. 89).  That same day, McLure and Cooper wired the office, saying, “Add no more load to the bridge until after due consideration of facts.  McLure will be over at five o’clock.”  Once McLure got the office, he and the supervisors met, and decided to resume their meeting the next morning (Pearson and Delatte 2006, p. 89).

Back at the construction site, at roughly the same time the supervisors in Phoenixville were ending their meeting, the Quebec Bridge collapsed at 5:30 p.m. The thunderous roar of the collapse was heard 10 km away in Quebec (Pearson and Delatte 2006, p. 90).  The entire south half of the bridge, approximately 189 MN of steel, fell into the waters of the St. Lawrence River within 15 seconds.  Eighty-six workers were present on the bridge at the time; only 11 workers on the span survived.
The A9L bottom compression chord, the one already bent, gave way under the increasing weight of the bridge. The load transferred to the opposite A9R chord that also buckled (Pearson and Delatte 2006, pp. 91-92). The piers were the only part of the structure that survived. The wreckage is shown below in Figure 2.

Figure 2: Wreckage of the bridge.

What technical lessons were learned from the failure?

The fall of this massive bridge can be traced back to several technical factors. The top and bottom chords for the anchor and cantilever arms of a bridge were typically designed as straight members. This common practice made the fabrication of these members easier. The bottom chords for the anchor and cantilever arms in the Quebec Bridge were slightly curved, as shown in Figure 3, for aesthetic reasons. This added difficulty to the fabrication of such unusually large members. The curvature also in- creased the secondary stresses on the members, reducing their buckling capacity. According to a letter written to Engineering Record, “As a rule secondary stresses are much more dangerous in compression than in tension members, which seem to have been the first to give way in the Quebec bridge”

Another concern during the erection of the bridge was the joints. The ends of all the chords were shaped to allow for the small deflections that were expected to occur when the chords came under their full dead load. These butt splices were bolted to allow for movement. The splices initially touched only at one end, and would not fully transfer their load until they had deflected enough for full bearing at the splices. At this point, they were to be permanently riveted in place. The result was to be a rigid joint that transferred loads uniformly across its area to ensure only axial loading. Great care had to be taken while working around these joints until they were riveted.

Adding to the design problems, Cooper increased the original allowable stresses for the bridge. He allowed 145 MPa for normal loading and 165 MPa under extreme loading conditions. These were questioned by the bridge engineer for the railways and canals as being unusually high. The new units’ stresses were accepted based solely on Cooper’s reputation.

Figure 3: Quebec Bridge just before collapse.

What ethical lessons were learned from the failure?

Several ethical concerns can be pointed out in this case. The major one is that deformations went unheeded for so long. The engineers on site argued among themselves as to the cause. Al- though the workers who failed to report to work because of the deformations lacked the technical expertise, they seemed to be the only ones who understood what was really happening to the bridge (Middleton 2001, p. 78).  Engineers and others in charge must be open minded to the ideas of the laborers, many of which have years of experience.

Another ethical concern was Cooper’s rejection of an independent engineer to check his work. His decisions were not questioned, even when they seemed to be unusual. An independent consultant may not have allowed the higher than normal design stresses. Some of the other errors such as the underestimated dead loads and the failure to recheck the weight could have been dis- covered before the bridge collapsed. In the end, “Cooper’s engineering expertise became the sole factor that was relied upon for assuring structural integrity of the bridge” (Roddis 1993).

What were the legal ramifications of the failure?

There were no legal ramifications for any of the engineers or foreman.  Cooper and Szlapka, arguably the ones most at fault, walked free.

References

Holgate, H., Derry, J., G. G., and Galbraith, J. (1908) Royal Commission Quebec Bridge Inquiry Rep. l            Sessional Paper No 154, S.E. Dawson, printer to the King, Ottawa.

Middleton, W. D. (2001) Bridge at Quebec, Indiana University Press, Ind.

Pearson, C., and Delatte, N. (2006). “Collapse of the Quebec Bridge, 1907.” J.Perform.Constr.Facil.,              l               20(1), 84-91.

Petroski, H. (1995). Engineers of dreams: Great bridge builders and the spanning of America, Knopf, l       New York.

Roddis, W. M. K. (1993) “Structural failures and engineering ethics.” J. Struct. Eng., 119(5), 1539–1555.