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CSMIP 00-06

"Verification of Response Spectral Shapes and Anchor Points for Different Site Categories in Building Design Codes"

by W. Silva and G. Toro

March 2000, 48 pp.

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The dramatic increase in strong ground motion recordings over the last several years has provided both the impetus and opportunity to empirically examine the seismic design criteria in both the UBC 94 and NEHRP 94 code provisions. In this project both spectral design shapes and the usefulness of two spectral anchors are investigated using a comprehensive strong motion database and updated empirical attenuation relations. For the shapes, comparisons with statistical shapes with mean magnitudes in the range of 6.6 to 6.9 to design shapes suggest that both the UBC 94 and NEHRP 94 design spectra provide enveloping criteria (except for site D at short periods) including cases for sites within 10 km of the fault rupture surface. For the NEHRP 94 design spectra, comparison of the Fa and Fv factors to those implied by a recently developed empirical attenuation relation suggest that the NEHRP 94 Fa factors may reflect too little nonlinearity while the Fv factors may show too much nonlinearity. Comparisons of the code shapes to the results from probabilistic seismic hazard analyses indicate that the fixed UBC 94 shape has a moderate tendency to under-predict amplitudes for T ≥ 1 see in places like San Francisco where large (M > 7) earthquakes dominate the hazard at these periods. The more flexible NEHRP 94 shape avoids this problem, but requires the appropriate specification of two anchoring points.