Menu
Contact Us Search
Organization Title

Shear-Wave Velocities and Design Response Spectra - An Examination Using Strong-Motion Data from the Gilroy Array: Preliminary Results

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.

Click on the link below for the full text:

PDF (Adobe PDF (SIZE 456 KB)) - Requires Adobe Acrobat Reader or other PDF viewer.


Abstract

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.