Bridges rock on with new design

USA Today (Society for the Advancement of Education), Sept, 2007

Bridges that "dance" during earthquakes could be the safest and least expensive to build, retrofit, and repair, according to engineers from the University at Buffalo (N.Y.). The researchers have developed and successfully tested the first seismic design methodology for bridge towers that respond to ground motions by literally jumping a few inches off the ground.

The new methodology allows steel truss towers that support bridge decks to be built or retrofitted at far less expense than conventional approaches, where each leg of a bridge tower is anchored strongly to its footing. "Our approach is unconventional, counterintuitive," admits Michael Bruneau, professor of civil, structural, and environmental engineering. He developed the new design with doctoral candidate Michael Pollino.

"With an earthquake, conventional wisdom dictates that the most important thing is to anchor the bridge tower," explains Bruneau. "The mass wants to overturn, so you have to tie it down." To do that, he points out, the tower must be secured with a very expensive foundation system, which, in turn, subjects it to the full force of the earthquake. "In this scenario, something usually has to yield. Here, we're standing the concept on its head. By letting the tower rock, we're significantly reducing the overturning force."

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The engineers developed a design procedure in which the legs of the truss tower are disconnected from their base and briefly uplifted by a small amount if significant ground motions occur.