Coupling of Soil-Structure Interaction and Nonlinear Response of Liquefiable Soils

Abstract eng:
The current seismic design philosophy is based on nonlinear behavior of structures where the foundation soil is often simplified by a modification of the input acceleration depending on the expected site effects. When taking into account nonlinear soil behavior, the isolation of structure and soil is no longer valid and soil-structure interaction has to be taken into account. Additionally, when the soil is loose and saturated, more robust models that take into account pore pressure generation are needed. Findings presented in this work illustrate the importance of accounting for both added soil nonlinearity due to seismic liquefaction and for soil-structure interaction. Some models can account for the nonlinear behavior of soils neglecting the effect of pore-pressure distribution and migration, as if the soil was fully drained. However, in most cases, this condition is not valid and there is a need for nonlinear models capable of taking into account the coupling of excess pore pressure (𝛥𝑝𝑤 ) and soil deformation (CPD). In order to highlight the importance of these models, a CPD analysis is compared to a mechanical-equivalent fully drained decoupled (DPD) analysis of a 2D finite element model subjected to a wide range of earthquake motions. Additionally, to assess the effect of CPD combined with SSI, three systems were tested: two nonlinear structures and a model without a structure to use as reference. The differences between the analyses on different engineering demand parameters are evaluated. The effect of the structure on the soil can be observed on the slightly increase on pore-pressure generation throughout the deposit and on the differences in the acceleration amplification at surface, especially when SSI effects are important. With respect to the structure, its coseismic relative settlement with respect to free field can be half if DPD is used. Additionally, concerning the maximum inter-story-drift (ISD), the DPD analysis will underestimate the response for 75% of the motions tested. The combined effect of input ground motion, nonlinear soil and nonlinear structure behavior makes the task of predicting the seismic performance very difficult. Hence for more realistic models both SSI and CPD effects should be taken into account.

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