by Robert Darragh and I.M. Idriss
Darragh, Robert, and I.M. Idriss (1997). Shear-Wave Velocities and Design Response Spectra - An Examination Using Strong-Motion Data from the Gilroy Array: Preliminary Results. SMIP97
Seminar on Utilization of Strong-Motion Data, p. 71 - 88.
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Borcherdt (1994) proposed that the short- and mid-period amplification factors used to scale the estimate of
site-dependent response spectra could be calculated as a continuous function of shear-wave velocity averaged in
the upper 30 m for various input ground-motion levels. This proposal appears to be an improvement for estimating
design response spectra over site classifications defined by soil descriptions (e.g., SEAOC, 1988) or by ranges of
average shear-wave velocity (e.g., NEHRP, 1991).
However, it is not clear that the accuracy available in current measurements of shear-wave velocity is sufficient to
support their use directly in design calculations of motion. For example, at Gilroy #2 there are differences of
about 300 m/sec in the shear-wave velocity measurements in the upper 30 m (EPRI, 1993). In this paper, we analyze
the effect of variations of shear-wave velocity in the upper 30 m for design applications including the effects of
nonlinear soil response using an equivalent-linear site response formulation (Schnabel et al., 1972). The analysis
uses the extensive geotechnical site characterization and shear-wave velocity measurements at Gilroy #2, a stiff
soil site that has been characterized to a depth of 240 m (EPRI, 1993). Response spectral accelerations from the
recorded strong motions are compared to calculated values from the equivalent-linear analyses with several shear-wave
velocity profiles in the top 30 m.
The preliminary analyses suggest that the Borcherdt (1994) methodology works well at this stiff-soil site for design
levels of motion near 0.4 g, appropriate for many parts of California, even though there is a difference of 60% in the
measured average of the shear-wave velocities in the upper 30 m.