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by C.-M. Uang and A. Maarouf
September 1996, 140 pp.
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Abstract
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.
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