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OSMS 93-08

by R. Darragh, T. Cao, M. Huang and A. Shakal

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Important sets of records were obtained from the I10/215 Interchange bridge southwest of San Bernardino during the Landers and Big Bear earthquakes of 28 June 1992. The bridge instrumented is the connecting structure between I10 from Los Angeles and I215 toward San Bernardino. The bridge, about 2540 feet long and 90 feet high near the center, was seismically strengthened by Caltrans in 1991 and instrumented with 34 sensors by the California Strong Motion Instrumentation Program (CSMIP). The sensor location sheet shows the locations of the 34 sensors installed on the bridge. The locations of the sensors were selected to achieve specific instrumentation objectives (Huang and Shakal, 1993). In addition, a nearby reference ground-response station (San Bernardino - E & Hospitality) located approximately 400 to 450 m east of the bridge also recorded both earthquakes.

During the Landers and Big Bear earthquakes, the peak horizontal ground accelerations at the nearby reference ground-response station were 0.09 and 0.10 g, respectively. Higher accelerations, 0.81 and 1.02 g, were recorded on the bridge deck during the Landers and Big Bear earthquakes, respectively. The many spikes on some of the records are due to interaction of the bridge superstructure across the hinges (Malhotra and others, 1993). These are the first significant records from this type of bridge in California.

Records from all CSMIP stations are presented in the Landers and Big Bear data report on the earthquakes (Shakal and others, 1992; Huang and others, 1992). Digitization and processing of these records started after the data reports were completed and is continuing. The preliminary results of digitizing and processing for 6 ground-response stations that recorded the Landers earthquake were included in the first release report (CSMIP, 1992). The results of digitizing and processing 8 stations, including 6 ground-response stations and 2 buildings were included in the second release report (Darragh and others, 1992). The results of digitizing and processing 10 ground-response stations were included in the third release report (Darragh and others, 1993). This report supersedes an earlier report (CSMIP, 1993) by including the processing of the reference ground-response station data from both earthquakes and the bridge data from the Big Bear earthquake. Further details on accelerogram digitizing and processing will be presented in the forthcoming final CSMIP processed data report.