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OSMS 90-03

by M.J. Huang, T.Q. Cao, U.R. Vetter and A.F. Shakal

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Strong shaking was recorded during the 5.5 ML Upland earthquake of February 28, 1990 in the base-isolated San Bernardino County Law and Justice Center (Ref. 1). The building is located in Rancho Cucamonga, approximately 12 km from the earthquake epicenter. This report presents results of the digitization and processing performed on the record from this building.

The Law and Justice Center is instrumented with 16 accelerometers in the building and three accelerometers about 330 feet from the building to record free-field ground motion. The locations of the accelerometers are shown schematically in Figure 1. The accelerometers are connected to two centrally-located recorders. Sensors 1 to 13 are connected to one recorder and 14 to 19 are connected to the second. Strong-motion records were also obtained from this building during the 1985 Redlands earthquake (Ref. 2), the 1986 Palm Springs earthquake (Ref. 3), and the 1987 Whittier earthquake (Ref. 4). Although these records were of low amplitude, they have been digitized and the processed data are available from CSMIP.

The level of shaking during the 1990 Upland earthquake was significantly greater than that recorded during the previous events. The peak horizontal acceleration at the free-field site was 0.26 g. This is almost twice the amplitude at the foundation level. Figure 2 shows the accelerations in the transverse direction from the sensors located at the center of the building. The peak acceleration recorded at the foundation level (below the isolators) was 0.14 g. The peak acceleration values recorded by the three transverse sensors in the basement (above the isolators) range from 0.05 to 0.08 g. The peak motion at the roof was 0.16 g.

Comparison of the records above and below the isolators shows that high-frequency horizontal motions were filtered out by the isolator, which was also observed in the records from previous earthquakes. The period of the structure vibration during this event was near 0.75 second; this is longer than the 0.6 second period present in the other low-amplitude records. The differences in the horizontal motions at different levels in the structure can also be compared in the response spectra in Figure 3. They show a reduction at high frequency as well as amplification at the structural period.