A base spectrum and soil amplification spectra are developed andare intended to replace the seismic response spectrum and site coefficients presented in the aashto guidelines for highway bridge design in washington state. The base spectrum is constructed using available data on ground motion from subduction zone earthquakes similar to those that occur in washington state. These earthquakes generally have larger high-frequency components than shallow-focus earthquakes. Because the existing codes are based primarily on data from shallow-focus earthquakes, the base spectrum developed has a larger high-frequency content than the existing base spectrum. The soil amplification spectra are derived using 123 boring logs from actual bridge sites in washington. Data from the boring logs are correlated to dynamic soil properties, which are used in the computer program shake to find the frequency-dependent amplification properties of the soil profiles. The profiles are grouped by depth and type of soils. Nine groups are identified, and mean amplification spectra are developed for each group. The design spectra are compared with results from other site-dependent studies as well as to the responses of the 1949 and1965 puget sound earthquakes. This paper appears in transportation research record no. 1309, Geotechnical engineering 1991.
Abstract