Design response spectra were developed for the Washington State Department of Transportation (WSDOT) for nine soil groups representative of deposits that can be found in Washington State. These response spectra differ from the spectra developed for and adopted by the Applied Technology Council (ATC). Unlike the ATC spectra, the WSDOT spectra are based on the subduction zone setting that dominates Pacific Northwest seismicity. Both sets of spectra are based on numerical studies using SHAKE. The new response spectra were developed using new dynamic moduli curves for cohesive soils which have been accepted as more representative of the properties of the soils in the region. SHAKE tends to attenuate high frequencies. SHAKE based analysis compared well with analysis based on a nonlinear finite element code. The basis of most recent ATC attenuation maps is outlined, and found to be based on similar assumptions and methodology as the original ATC attenuation maps and not compatible with Washington State seismicity.
Cover title: Seismic Response Spectra-- Refinement.