The Washington State Department of Transportation adopted recommendations for seismic response spectra to replace the AASHTO guidelines. The replacement spectra were based on input and soil amplification representative of the geologic conditions of the Pacific Northwest. A deep subduction zone earthquake was used as the source event rather than a shallow strike-slip earthquake typical of that used in the development of the AASHTO guidelines. Soil data from 123 boring logs from actual bridge sites in Washington State were processed, and nine soil groups representative of the soil conditions in the region were identified, based on similarities of standard penetration test data. Soil amplification spectra were derived for the nine soil groups. These were compared with spectra provided by current guidelines and similar works. They were also correlated with damage from previous earthquakes in the area.
Abstract