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| Bad Vibrations: The ancient craft of bridge design still holds surprises |
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In the middle of rush hour on Aug. 1, at 6:04 p.m., traffic zoomed across the westbound span of the I-35 Mississippi River bridge in Minneapolis. By 6:05, the 40-year-old structure had buckled and broken, dumping most of the bridge into the river and killing 13 people. Though it came as a shock, this was in retrospect an accident waiting to happen, experts say. The Minneapolis bridge had been poorly maintained, with cracks in its iron arches that had been patched up over the years. And the bridge's design lacked redundancy. "This is a classic example of how ... a single failure can lead to a collapse," says Spiro Pollalis, a bridge designer who teaches at Harvard University. "At the time [the bridge was built], it was considered an acceptable risk," he adds. "Now we try to be more careful." Whether because of obsolete design or disrepair, thousands more U.S. bridges are similarly at risk, according to the American Society of Civil Engineers' 2005 "Report Card for America's Infrastructure." Yet the majority of bridges built in the 1950s and 1960s are still holding up, even though they typically carry much more traffic than they were designed to handle. With increasingly sophisticated computer tools and wind tunnel tests, and more-detailed understanding of steel, concrete, and other materials, engineers have a better grasp than ever of how bridges work. But some recently built bridges have surprised their designers by showing disturbing and unexpected vibrations. Every new bridge that's different from those that have been built before—with a longer span, say, or a novel design—represents a leap into the unknown. "Until you build structures, they really are like scientific hypotheses," says Henry Petroski, a civil engineer and historian at Duke University in Durham, N.C. "If it's never been done before, no matter how many theoretical supports you have, the proof is only in building it." ... Click here to read the rest of the article on the Science News website. |
23 November 2007, on the cover of