Seismic Soil-Structure-Interaction Analysis Method for Shallow Foundations

Abstract eng:
This paper introduces a new approach for seismic Soil-Structure Interaction (SSI) analysis of shallow foundations using computationally efficient yet practically accurate models. The study is inspired by the increasing interests amongst engineers and researchers in the nonlinear cyclic or rocking behavior of shallow foundation. Furthermore as the industry moves toward the performance based seismic design, the accurate modelling of the inelastic cyclic or rocking behaviour of shallow foundation has attracted considerable attention of researchers. There are two main characteristics of a shallow foundation that need to be considered in a numerical model: inelastic cyclic behaviour of near field soil and frequency-dependent characteristic of soil. One of the rigorous and computationally expensive approaches which can capture both characteristics is to use a sophisticated constitutive model of soil materials, and developing a fine mesh of soil-foundation system using the finite element method. However, due to the extremely large modelling efforts and computational time, the FEM approach is not a feasible option in a routine engineering practice. To overcome the limitation, researchers have proposed macro-elements. In the macro elements, the inelastic cyclic behavior of soil-foundation interaction is captured at the lumped node where geometric nonlinearity such as uplift and sliding of the foundation as well as material nonlinearity are modelled simultaneously. The macro element’s inelastic behaviour is defined by a plasticity law where its stiffness is updated at each computational step using the generalized force-displacement relationship with the bounding surface hypo-plastic model. Macro-element is able to capture the inelastic behavior of the soil foundation system at the vicinity of the foundation. As the element replaces soil and foundation with hypo-plastic spring model, the soil mass and damping properties are missing in this element, which results in inability to capture wave propagation from the foundation to the structure. Soil exhibits frequency-dependent characteristic of stiffness and energy dissipation. Inclusion of the dynamic stiffness is pivotal in order to extend the application of this element to the domain of dynamic loading condition. In the current practices, the soil-foundation system is often modelled with elastic springs and dampers for simplicity which are inherently frequency-independent. In reality, soil exhibits frequency dependent characteristic at different frequencies of excitation. In this paper, it is proposed to integrate a macro element which can capture the inelastic cyclic behaviour of soilfoundation system with a recursive parameter model which can model the frequency-dependent dynamic stiffness in time domain. By the integration of these two models, it can approximately capture the inelastic SSI effect of the soil foundation system and the frequency dependency of soil simultaneously. First part of the paper presents the introduction of the macro-element and the second part of the paper introduces the recursive parameter model. Then, the paper illustrates the combination of the two models using schematic diagrams and equations. The verification of the proposed method is provided with FEM model.

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