RESPONSE OF THE DUMBARTON BRIDGE IN THE LOMA PRIETA EARTHQUAKE

The Dumbarton bridge was the only major San Francisco Bay area bridge instrumented with strong motion accelerometers at the time of the October 17, 1989, Loma Prieta earthquake. The 7300 ft (2225m) long bridge has two approach structures with prestressed concrete spans and a main channel structure of twin steel box girders. The strong motion acceleration records, which include the ground motion near the west end of the bridge and twenty-five channels over eight spans, are valuable data for improving the engineering understanding of the earthquake response of long, multiple-span bridges on soft clay soils. Evaluation of the recorded motion of the bridge in the Loma Prieta earthquake shows that the deck experienced a significant amplification of the ground motion recorded at the free-field. A detailed three-dimensional model of the Dumbarton bridge was developed to represent the forty-three spans, the piers, and the pile foundations. The conclusions about the response of the bridge in the Loma Prieta earthquake cannot be generalized to the case of a major Bay area earthquake, which may be of larger magnitude, longer duration, and closer to the bridge.