| HOME | | | EXTRA |
| WORLD CLASS RAIL VEHICLE ENGINEERING CONSULTANCY |
|
BRIDGE CATASTROPHIES A chapter from the recent book "Bridges" by Judith Dupré. ------------------------------------------------------------------------------ In an 1889 paper on American railroad bridges, respected engineer Theodore Cooper (1839-1919) documented the loads exerted by trains, concluding with a discussion of bridge failure. He called for the careful calculation of stresses, material testing, and inspection, but added that a bridge's stability was more reliant on the engineer's instincts than "merely upon a theory of stresses." His words would come back to haunt him eighteen years later, when he played a role in the most famous bridge collapse in history.
By the end of the nineteenth century, with the expansion of the railroad. bridge collapes were noted with increasing regularity and hysteria. Well qualified scholars still debate how many bridges failed - some say one in four; others, one in four thousand - though without adequate documentation, the actual numbers cannot be ascertained. Many collapses were attibutable to causes other than structural inadequacies, including train derailment, fire and flooding.
The truss designer, Ithiel Town's slogan, "Build it by the mile and cut it off by the yard" describes the attitude and methodology of the many bridge companies competing to build more and longer railroad bridges. Though this climate fostered the development of an ecomomical and practical style of metal truss bridge, it was also rife with corruption - skimping on materials and on the review of the bridge's technical design was not uncommon. Bridges designed by highly esteemed engineers also collapsed, however, indicating the pervasive role of human error in failure.
Robert Stephenson's Dee Bridge (1845) across the River Dee in England - the longest metal truss built to that date - provides the first instance of a metal bridge collapse that might have been avoided if information known at the time had been made available to engineers. Success with earlier truss bridges led to the progressive increase in their span length without considering the additional structural needs that the extra length imposed. The bridge girders buckled under these unconsidered stresses and fell on May 24, 1847, claiming five lives.
On December 28, 1879, Thomas Bouch's Tay Bridge over Scotland's Firth of Tay went down in a gale with the loss of seventy five souls. When completed in 1876 the rail ridge had been hailed as an "engineering triumph" and earned a knighthood for Boucher. Later found to be fatally complacent, Boucher had relied on outdated data that neglected the dynamic forces of violent gusts.
In the 1907 collapse of the Quebec Bridge over the St. Lawrence River in Canada, an ill-fated combination of technical error, corporate parsimony, miscommunication and professional hubris caused the deaths of seventy five workmen. The Quebec Bridge Company had hired Theodore Cooper of New York to consult on the Pennsylvania-based Phoenix Bridge Company's cantilever design, which Cooper had selected as the "best and cheapest" of those submitted; as is often the case, Quebec wanted to spend as little as possible. Cooper increased the length of the bridge's main span from 1,600 to 1,800 feet (488 to 549 meters) without recalculation of stresses. When asked about the soundness of his work, Cooper replied, "There is nobody competent to criticize us."
By August 1907 when the south arm of the bridge had been cantilevered out about 600 feet (183 meters), the discovery that the steel chords were bent was followed by three weeks of debate about when they were bent and not why. Cooper. who never visited the site when the superstructure was under construction, sent a telegram on August 27 ordering the work stopped until the bent chord issue was resolved. The crucial telegram was ignored, either undelivered or unread and work continued until August 29, when the bridge collapsed from its faulty design. Ironically, its replacement would also collapse in 1916, taking eleven lives. The Quebec disaster influenced the development of suspension spans for long distances. Yet the suspension bridge over Tacoma Narrows collapsed in 1940 because the designer failed to consider aerodynamic forces. In 1970 two bridges of a new type, the steel box girder, went own, one in Wales and the other in Australia. A 1977 report by Paul Sibly and Alastair Walker notes a thirty-year cycle of bridge failures and their shared characteristics. Each of the bridges involved was a new type of bridge (trussed girder, truss, cantilever, suspension, and box girder) that was being developed in response to the collapse of an earlier type of bridge. Engineer and author Henry Petroski extends Sibly and Walker's findings by thirty years and theorises that the next collapse might involve a cable-stayed design, the newest type of bridge. While human error will always be a variable in bridge design, improvements in other areas - more reliable materials, expanded technical knowledge, wind testing, computer technology, and the growing recognition that failures, having the most to teach about successful design, should be documented and shared - have taken some of the uncetainty out of bridge engineering. ------------------------------------------------------------------------------ |