Bridges Dance With Earthquakes

Julie Kalista
Online Editor

The problem seems to be that bridge towers are usually anchored to expensive foundations that transmit the full force of the earthquake to the entire bridge.

Researchers at the University of Buffalo are standing this traditional design on its head by designing bridge towers that respond to ground motions by literally jumping off the ground.

UB engineers developed a design 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.

Researchers used devices to control structure's uplift. The dampers, husteric or viscous, were inserted at the base of the towers to let them rock while absorbing part of the earthquake's energy and controlling the uplift.

Testing with the dampers was conducted on a model truss tower, 20 ft. high and weighing 9 tons. The tests were performed on a six-degrees-of-freedom shake table in UB's Structural Engineering and Earthquake Simulation Laboratory (SEESL).

Tests on the shake table subjected truss towers to ground motions equal to that of the 1994 Northridge earthquake. The tower legs lifted about 2 in. in the air for less than 1 sec. and, in some cases, the legs lifted nearly 4-in.

"All of the tests were successful," said one researcher. "The dampers reduced the amount of uplift and impact velocity." In addition to construction cost savings, the design also saves money if seismic retrofitting needs to be done.