|
by W. Silva and G. Toro
March 2000, 48 pp.
Click on the link below for the full text:
Abstract
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.
|
