CSMIP 96-01

"An Investigation of UBC Serviceability Requirements from Building Responses Recorded During the 1989 Loma Prieta Earthquake"

by C.-M. Uang and A. Maarouf

September 1996, 140 pp.

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The Uniform Building Code (UBC) seismic design procedure does not address the serviceability limit state explicitly. In the light of significant economic losses of building damage in the 1989 Loma Prieta earthquake, the UBC seismic serviceability requirements are examined from the recorded responses of four multistory buildings. An analytical study showed that the intensity of the UBC-implied "moderate" design earthquake for buildings with a fundamental period greater than 0.7 seconds is only one-sixth that of the "severe" design earthquake. The four buildings (one steel building and three reinforced concrete buildings) selected in this study have effective peak ground accelerations similar that of the UBC-implied moderate design earthquake.

Detailed structural responses of each building at peak responses are computed by imposing lateral displacements to a three-dimensional finite element model; a modal superposition technique is used to "recover" lateral displacements at floors that are not instrumented. An analytical study shows that member forces in ductile building systems may exceed member capacities if the structure were to respond elastically during moderate earthquakes. This is confirmed by the structural analyses of a 13-story steel frame; the actual stress ratio may exceed 1.4, and the maximum story drift ratio is about one percent. Because of the large lateral stiffness, not the UBC drift limits, the serviceability performance tends to be satisfactory for the types of reinforced concrete buildings studied.

It is recommended that, in addition to considering the ultimate limit state for sever design earthquakes, the serviceability limit state for moderate design earthquakes be considered explicitly to limit story drifts and member forces in UBC.