Reduction of Seismic Acceleration Parameters for Temporary Bridge Design

No prevailing method is currently available for reducing the seismic response acceleration parameters PGA, S_S, and S₁ from the probabilistic seismic hazard values used for permanent bridge design to levels suitable for temporary bridge design. A method of spectral reduction using spectral reduction factors was proposed to reduce the previously published spectral response acceleration parameters, the peak ground acceleration (PGA), the short-period response acceleration coefficient (S_s), and the long-period response acceleration coefficient (S₁), from the 1,000-year return period used for permanent bridge design to a return period suitable for temporary bridge design. The proposed spectral reduction factors are the ratios of the return period used for the seismic design of a permanent bridge to the return period used for the seismic design of a temporary bridge. Spectral ratios were obtained for each of the three seismic response acceleration coefficients for 100 locations around the United States. The examination of the short- and long-period response acceleration coefficients are presented in this paper. As a result of this examination, two spectral reduction factors for the seismic design of temporary bridges were proposed: one spectral reduction factor, of 2.5, to reduce each PGA, S_S, and S₁ parameter for the western United States, and one spectral reduction factor, of 3.75, to reduce each PGA, S_S, and S₁ parameter for the central and eastern United States.