OSMS 94-30

OSMS 94-30

"Interaction at Separation Joints of a Concrete Bridge During 1992 Earthquakes in California"

by P.K. Malhotra, M.J. Huang and A.F. Shakal

Malhotra, P.K., M.J. Huang and A.F. Shakal (1994). Interaction at Separation Joints of a Concrete Bridge During 1992 Earthquakes in California. Proceedings Volume 1 of Earthquake Awareness and Mitigation Across the Nation. Fifth U.S. National Conference on Earthquake Engineering, July 10-14, 1994, Chicago, Illinois, p. 347-356.

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A multi-span, curved, concrete box-girder bridge has been extensively instrumented by the California Strong Motion Instrumentation Program (CSMIP) in cooperation with the California Department of Transportation (Caltrans). On June 28, 1992, the bridge was shaken by the magnitude 7.5 Landers and magnitude 6.6 Big Bear earthquakes in Southern California. The epicenters of these earthquakes were 50 and 29 miles (81 and 46 km) from the bridge, respectively. All thirty-four strong-motion sensors installed on the bridge recorded its response to these earthquakes and provided an insightful set of response data. A striking aspect of the response is the presence of intermittent sharp spikes in nearly all of the acceleration records from sensors on the deck of the bridge. Among these the highest spike was 0.80g for the Landers and 1.00g for the Big Bear earthquake. The peak ground acceleration at the bridge site was only about 0.10g for both these earthquakes. With the aid of visual examination and simple analysis it is deduced that: (i) the spikes were caused by forces generated at separation joints between adjacent bridge segments by impacts and stretching of the cable restrainers; and (ii) the forces of impacts and cable stretching are directly proportional to the size of the spikes and can be estimated by the use of a simple formula.