Analysis of a Disaster*

Transcription

1 In 1907, one of the worlds largest cantilevered bridge collapsed Analysis of a Disaster* n In 1887 the Quebec Bridge Company was established by an Act of Parliament. n Its purpose was to oversee the construction of a bridge across the St Lawrence River n The bridge was to span across the St. Lawrence River approximate six miles from Quebec City n The bridge was to be approximately 2800 ft with a single 1600 ft single span, 150 ft above the water, 67 ft wide to accommodate 2 railway tracks, two streetcar tracks and two road ways 3 4 n It was recognized that the cost of such a bridge would be very expensive and that costs must be kept to a minimum n Things moved very slowly; indeed, it took eleven years before they started the bidding process In 1897, Theodore Cooper, a very well known bridge designer and builder was interested in assisting the Quebec Bridge company to build this Quebec Bridge Cooper was one of the major players in the Bridge affair and as such a bit of his history is warranted 5 6 1

2 n Born in 1839 Cooper s history n Graduated as a civil engineer from Rensselaer Institute (Polytechnic) in 1858 at the age of 19 n Enlisted in the Navy and then took on a teaching position at the US Naval Academy Cooper s history (cont.) n Left the Navy in 1872 and worked under Capt. James Eads, also a well known bridge builder n Was involved in the design and building of the St. Louis Bridge, also a cantilevered bridge n By 1874, Cooper was very well known and in demand 7 8 Cooper s history (cont.) n He was involved with the Seekonk Bridge in Providence, the Sixth Street Bridge in Pittsburgh and the Second Ave. Bridge in New York n He was most interested in the challenges of the Quebec Bridge as it could be his final masterpiece Collapse of the Quebec City Bridge 9 10 n A cantilever bridge can be considered as a continuous beam which is anchored at its ends to pillars. n It is a common construction practice where interior pillars cannot be used. n Thus weight at the center becomes an issue n In 1897 the Quebec Company had been in touch with the Phoenix Bridge Company in Phoenixville, Penn. n Phoenix had started some preliminary plans for the bridge n When bidding for the Bridge started, the Quebec Company made it well known that they wanted the Phoenix Bridge Company

3 n In 1899, Cooper was asked to meet with the Quebec Bridge Company and asked to review the bids. n It was stressed that cost was a major concern n The Phoenix Bridge Company was in contact with both Cooper and the Quebec Bridge Company during the bidding process n In 1899, Cooper recommends the Phoenix Bridge Company s bid as the best and cheapest. n Cooper denies that any pressure was put on him to choose the Phoenix Bridge Company s bid n Cooper stresses the fiscal consequences of major decisions but never placed (directly) cost before safety. n In May 1, 1900, Cooper lengthens the span of the Bridge from 1600 ft to 1800 ft n Moves the piers back reducing the expense and risk of dealing with ice in the river. n But to keep the increased cost of the extra steel in check, he made some changes to the design specifications that would allow a higher unit stress 15 n Five days later Cooper was appointed the consulting engineer for the Quebec Bridge Company for the remainder of the work. n His recommendations were accepted without question and the work proceeded on some of the supports etc. by the Phoenix Bridge Company based on the original plans n The challenges of the new design were not considered for three years while the Quebec Government made up its mind to provide the actual money 16 n During these three years, the engineering challenges that would accompany the added length sat in limbo; no preliminary testing was done n Finally, in 1903, the Canadian Government approves the cost ( $6.7M) n The initial drawing by Szlapka of Phoenix Bridge Company was approved by Cooper with little changes. The weight, however, was based on the original drawings, not the extended span

4 Collapse of the Quebec City Bridge n Cooper only visited the site three times, the last being in 1903 citing health reasons n In the mean time, Ottawa decides to have the bridge plans reviewed by the chief engineer of the Department of Railways and Canals because the stress levels had been questioned by one of their engineers n Cooper is ticked off big time and offers his resignation, but does not when the Quebec Bridge Company refuses to accept it. n As a compromise a young graduate engineer, Norman McLure was installed at Quebec to serve as his (Cooper s) eyes and ears on the bridge 21 n The super structure was started to be erected in 1904 with the Phoenix Bridge Company supplying the Steel n Since no one with authority was on site, any actions had to be passed on to Cooper who was many miles away n In 1905, shop drawing of the south arm of the bridge was almost completed, a missed opportunity by all to re-compute the loads and verify the modified design 22 n Feb , the Phoenix Bridge Company s materials inspector revealed that the weight was out by some eight million pounds n Cooper decides that this was within engineering tolerances and allowed work to continue. Since the south anchor arm, tower and two panels of the cantilever arm were ready and some of the panels of the anchor arm installed, he decided that they should proceed

5 n By 1907, the effect of the high stresses were starting to show up in the developing structure n Some parts of the bridge were not riveted at the ends even as the span was increasing n McClure informs Cooper that they were having trouble riveting the bottom cord splices and that cantilever arms were bent 25 n The Phoenix Bridge Company insisted that these deflections were normal and that they occurred before they were sent to Quebec n On Aug 27, McClure noticed that the deflection of one of the chords had increased from ¾ inch to 2 ¼ of an inch over a half week n He sent a letter (not a wire) to Cooper which took several days to get there 26 n Aug 27 th, the Phoenix Bridge Company s on site foreman suspends work because of safety concerns n Hoare sees a problem as fear grows by the workers and sends McClure to see Cooper for a decision n McClure arrives the same time as his letter (August 29) n The next morning he resumes work with the blessing of the Quebec Bridge Company (The chief Engineer, Hoare, might have insisted on it) 27 n Cooper wires Phoenixville (not Quebec) not to add any load until all facts had been considered. n Cooper thinks that work on the bridge had come to a stop based 28 on McClure s letter n McClure heads to Phoenixville but forgets to send the wire to Quebec as instructed and so work continued on. n At 5:00 pm, The Phoenix Bridge Company meets with McClure and still maintains the bends occurred at Phoenixville and were still serviceable At 5:30 Pm the same day, the Bridge collapses killing all but 11 of the 86 men on the bridge at that time

6 The bridge was restarted in 1913 and completed in Aftermath n Royal Commission of Inquiry (1908) We are satisfied that no one connected with the work was expecting immediate disaster and we believe that in the case of Mr. Cooper, his opinion was justified. He understood that erection was not proceeding: and without additional load, the bridge might have held out for days They also stated that The failure cannot be attributed directly to any cause other than errors in judgement on the part of these two engineers (Cooper and the original designer for Phoenix, (Szlapka)) A grave error was made in assuming the dead load for the calculations at too low a value.this error was of sufficient magnitude to have required the condemnation of the bridge even if the details of the lower cord had been of sufficient strength It was, in essence the end of Cooper s career