Some Implications of Strong-Motion Data From the 1994 Northridge Earthquake

by A. F. Shakal, M. J. Huang and R. B. Darragh

Shakal, A. F., M. J. Huang and R. B. Darragh (1994). Some Implications of Strong-Motion Data From the 1994 Northridge Earthquake. SMIP94 Seminar on Seismological and Engineering Implications of Recent Strong-Motion Data, p. 1 - 20.

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Some of the highest acceleration ever recorded at structural and ground response sites occurred in the Northridge earthquake. These accelerations are greater than most existing attenuation models would have predicted. The thrust mechanism of this event as well as its location under a metropolitan area may have contributed to the number of high acceleration recordings. Although the accelerations are high, the correspondence between measured acceleration and damage requires further study, since some sites with high acceleration experienced only moderate damage. Some vertical accelerations were larger than the horizontal, but in general this event fits the pattern observed in previous earthquakes. Strong-motion records processed to date show significant differences in acceleration and velocity waveforms and amplitudes across the San Fernando Valley.

Analysis of processed data from four buildings in the San Fernando Valley indicate that the stiff, short-period building experienced large forces and relatively low story drift during the Northridge earthquake. On the other hand, three moment frame buildings (periods between 1 and 3 seconds) experienced large drifts. The two non-ductile concrete moment frame buildings suffered column cracking and other damage. For this earthquake, accelerations did not always amplify from base to roof for flexible structures like these three buildings, but the displacements were always larger at the roof. The records from a base-isolated building indicate that high- frequency motion was reduced significantly by the isolators, which only deflected 3.5 cm. The records from a parking structure show important features of the seismic response of this type of structure.