A Response Spectrum Method for Surface Strata Site Amplification

Abstract eng:
Many seismic codes have suggested that the response spectrum should be specified at bedrock level, and then, the calculated free field response spectrum from bedrock will incorporate local site effects. This paper proposes a simple and accurate procedure to obtain the free field response spectrum of a layered soil profile from a specified bedrock response spectrum, taking into account the nonlinear behavior of soil. In this procedure, the soil profile is modeled as a multi-degreeof-freedom system with a rigid base, by replacing radiation damping by equivalent material damping. Using the developed model, the nonlinear behavior of soil is evaluated by classical model analysis with bedrock response spectrum directly. Finally, the free field response spectrum is obtained directly from the input bedrock response spectrum by using a method developed for calculating the floor response spectrum. To demonstrate the validity and usefulness of this approach, eight examples are presented. The free field response spectra of these eight soil profiles are obtained using our proposed method, and are then compared with the results obtained using the SHAKE program. The results of our proposed procedure are found to be accurate.

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