The Earthquake Engineering Online Archive

Response of the Dumbarton Bridge in the Loma Prieta earthquake

Fenves, Gregory L.; Filippou, Filip C.; Sze, David T.

UCB/EERC-92/02, Earthquake Engineering Research Center, University of California, Berkeley, 1992-01, 170 pages (710.7/F35/1992)

The Dumbarton Bridge was the only major San Francisco Bay Area bridge instrumented with strong motion accelerometers at the time of the Oct. 17, 1989, Loma Prieta earthquake. The 7300-ft-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 25 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. The bridge has a large number of vibration modes closely spaced at frequencies less than 2 Hz (periods greater than 0.5 sec). The fundamental vibration frequency of the instrumented spans is 0.52 Hz (vibration period of 1.9 sec), which is associated with longitudinal vibration of the main channel structure. The transverse mode of the approach structure has significant energy at 0.62 Hz (1.6 sec). A detailed three-dimensional model of the bridge was developed to represent the 43 spans, the piers, and the pile foundations. The input ground motion for the dynamic analysis of the bridge is based on the free-field ground motion recorded at a site approximately 1.2 km north of the west approach to the bridge and on a separate one-dimensional site response analysis. The computed motions of the model for four different input motions are compared with the recorded motion of the bridge in the Loma Prieta earthquake. Results are presented.

Available online: http://nisee.berkeley.edu/documents/EERC/EERC-92-02.pdf (13 MB)