Now, rocking bridges that safely dance through quakes (Re- Issue)

Earthquake engineers at the University of Buffalo and MCEER have successfully tested a rocking bridge design methodology that promises to make bridges dance during quakes, making them more safer as well as less expensive to build, retrofit and repair.

Washington, May 11 : Earthquake engineers at the University of Buffalo and MCEER have successfully tested a 'rocking bridge" design methodology that promises to make bridges 'dance' during quakes, making them more safer as well as less expensive to build, retrofit and repair.

The new methodology allows steel truss towers that support the bridge decks, to respond to ground motions, by literally jumping a few inches off the ground. It also allows for the truss towers to be built or retrofitted at far less expense than conventional approaches, where each leg of a bridge tower is strongly anchored to its footing.

The design recently underwent successful testing on a model truss tower that was 20 feet high and weighed nine tons.

The testing was conducted on a six-degrees-of-freedom shake table in university's Structural Engineering and Earthquake Simulation Laboratory (SEESL), one of the versatile earthquake engineering laboratories, within the university's School of Engineering and Applied Sciences.

According to Michel Bruneau, Ph.D., director of MCEER and UB professor of civil, structural and environmental engineering, who developed the new approach with Michael Pollino, a doctoral candidate in the UB Department of Civil, Structural and Environmental Engineering, their approach is totally "unconventional and counterintuitive".

"With an earthquake, conventional wisdom dictates that the most important thing is to anchor the bridge tower. The mass wants to overturn, so you have to tie it down. To do that, the tower must be anchored with a very expensive foundation system, which in turn, subjects it to the full force of the earthquake," said Dr. Bruneau.

"In this scenario, something usually has to yield. Here, we're standing that concept on its head. By letting the tower rock, we're significantly reducing the overturning force," Dr. Bruneau added.

The UB engineers' design procedure involved disconnecting the legs of the truss tower from their base and briefly uplifting them by a small amount if significant ground motions occurred.

One of the options they evaluated included using specialized devices to control the structure's uplift. The devices, called hysteretic or viscous dampers, were inserted at the base of the towers to allow the tower to rock while absorbing part of the earthquake's energy and helping it to control the amount of uplift to the structure.

During the tests, the experimental truss tower fitted with these devices was subjected to ground motions simulating the 1994 Northridge, California earthquake. Testing also was conducted without any devices attached, as the design procedure was developed to generally address performance both with and without dampers.

According to Dr. Bruneau, "all the tests were successful".

"The damper systems typically reduced the magnitude of uplift and the velocity upon impact, which may be important, in some conditions. Typically, during testing, the tower's legs uplifted nearly two inches in the air for less than a second. For some of the free-rocking cases, the tower legs lifted nearly four inches," he said.

He said the methodology would not allow uplifts to exceed limits considered safe by the design procedure and dictated by the tower design, local conditions, and the need for the tower to return safely to its original position.

"Professional engineers are starting to recognize that it is economical to allow this type of rocking in their designs, as long as the structure remains stable and the speed with which the legs come down is carefully controlled to minimize the forces that develop during the rocking," said Dr. Bruneau.

"In addition to the cost savings in construction, this design also saves money if seismic retrofit needs to be done. It's much easier to fix a tower to enhance its seismic resistance if the crew only has to work at the base, instead of having to climb 60, 80 or 120 feet to strengthen individual members along the height of those towers," he added.

ANI