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Shock Absorber for Bridges

TO survive an earthquake, bridges have to act a little like a tree. Give a little but not too much. Bend but don't break. Michael Constantinou of the National Center for Earthquake Engineering Research at the State University of New York here has developed a new system that does just that. It allows the deck of a bridge to move - dissipating more energy with less deck movement than other similar systems.

"It's a giant shock absorber," Dr. Constantinou says. A key advantage is that existing bridges can be retrofitted with the device, he adds.

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Bridges have devices that accommodate normal deck movements caused by expansion, contraction, and traffic (a large truck coming to a sudden stop, for example). Constantinou's system doesn't interfere with these devices. A friction mechanism keeps the system rigid during these small movements.

If an earthquake hits, however, the big ground movements overcome the system's internal friction. The deck begins to slide back and forth on special Teflon bearings. Eight-inch springs ensure that the deck travels no more than about 4 inches in any direction. And the columns, which could topple almost immediately on a traditional bridge system, are protected by other sets of low-friction bearings that allow the deck to move without transferring its force to the columns.

Laboratory experiments show that the device dampens the earthquake's force on the bridge by 80 percent or more. A local manufacturer of bridge-bearings, Watson Bowman Acme, is marketing the device.

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