COMPARISON OF 1D EQUIVALENT-LINEAR AND NONLINEAR SEISMIC SITE RESPONSES IN DEEP SEDIMENT LAYERS

Abstract eng:
A reasonable description of soil hysteretic constitutive relation has a significant impact on the evaluation of design ground motion parameters. These parameters play an important role in predicting seismic damage and taking further effective anti-seismic measures. In this study, based on the 200m-deep-borehole profiles in Suzhou region (China), the 1-D seismic site responses are compared by two analysis methods. One is an equivalent linear analysis (ELA) modeling in frequency domain, the other is a nonlinear analysis (NLA) using a multi-degree-of-freedom, lumped mass model in term of the modified Matasovic constitutive modeling in time domain. The effects of bedrock motion characteristics (amplitude, frequency content and duration) on the predictions from the two site response analyses in deep sediment layers have also been analyzed. Using the results from a broad range of bedrock motions, we can find the difference between the two site response analyses in different strain levels. Specifically, the surface peak ground acceleration (PGA) calculated by ELA increases monotonically as the PGA of bedrock motion increases. However, the surface PGA calculated by NLA first increases rapidly and then decreases gradually or keeps constant. Moreover, the surface acceleration response spectra (Sa) for short periods computed using ELA and NLA are significantly different. The high-frequency contents of bedrock motions exhibit obvious filtering in ELA while the high-frequency contents in NLA first magnify and then attenuate as the PGA of bedrock motion increases. In addition, for the medium or strong bedrock motions, the surface ground motion duration is not only related to bedrock motion characteristics, but also closely related to the cyclic behavior of nonlinear soil medium. The influence of bedrock motion intensity on the duration prolongation of surface ground motion can be reasonably evaluated using NLA.

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