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Earthquake Safety and the San Francisco-Oakland Bay Bridge

Bay Bridge cross section
Bay Bridge cross section

Caltrans recently completed an unprecedented aerial engineering feat. During the 2009 Labor Day weekend, the San Francisco-Oakland Bay Bridge was closed to traffic while Caltrans cut away a 300-foot long double deck section of the existing bridge at Yerba Buena Island, and connected a temporary half-mile detour. The entire operation was performed 150 feet in the air. Rerouting the bridge that serves more than 280,000 vehicles per day is just one of the many structural engineering milestones achieved in the nation's most ambitious bridge project to date - building the new San Francisco- Oakland Bay Bridge east span. "The Bay Bridge east span will be the largest Single Tower Self-Anchored Suspension Bridge ever built," says UC San Diego Jacobs School of Engineering Dean Frieder Seible. Seible is a member of the Seismic Safety Peer Review Panel, a group of experts convened by Caltrans to provide advice on the seismic integrity of the Bay Bridge after its partial collapse during the 1989 Loma Prieta Earthquake.

"We found that three-quarters of the existing members of the bridge needed to be replaced, and that was going to be virtually impossible to do under full traffic," says Seible. With that recommendation, the State embarked on the design and construction of the new Bay Bridge, a project which is described as the single largest public works project in California's history.

Caltrans turned to the Jacobs School's Structural Engineering Department to complete all of the testing required to verify new structural systems for the seismic safety of the Bay Bridge. Indeed, several of the new systems built into the bridge were first envisioned by the structural engineering faculty at UCSD.

"One of the key innovations in the Bay Bridge which has come out of UCSD is the concept for the single 525 ft.-tall tower that supports the selfanchored suspension bridge," says Seible.

This main tower is comprised of four separate legs connected by shear link beams. "The links are designed to be sacrificial. They will absorb the energy of the earthquake, preventing damage to the tower legs, and can easily be replaced without disrupting traffic," says Seible. "However, if one of the legs is damaged, the other three can still keep the bridge standing." Testing of the shearlinks was done under the direction of Seible and Professor Chia-Ming Uang.

The new Bay Bridge will eventually replace the current structure, first opened to the public in 1936. It is designed to withstand potential seismic threats from both the nearby San Andreas and Hayward faults, which could be of even greater force than the Loma Prieta quake. Construction is expected to be complete by 2013.

Earthquake Safety and the San Francisco-Oakland Bay Bridge

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