Frequency Dependent Effects of Soil-Structure Interaction on Inelastic Behavior of Superstructures

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Title: Frequency Dependent Effects of Soil-Structure Interaction on Inelastic Behavior of Superstructures

Author(s): R. Gash, E. Esmaeilzadeh Seylabi, and E. Taciroglu

Publication: Special Publication

Volume: 316

Issue:

Appears on pages(s): 95-112

Keywords: soil-structure interaction, substructure analysis, foundation impedance function, rational approximation, discrete-time filter.

Date: 4/1/2017

Abstract:
Performance-based seismic design of structures calls for many analyses and therefore becomes computationally expensive in large-scale soil-structure interaction (SSI) problems where the superstructure and its surrounding semi-infinite soil must be considered together. The substructure method is an attempt to reduce this computational cost through substituting the far-field elastic soil with a set of impedance functions, which are generally nonlinear functions of frequency (depending on soil heterogeneity and foundation geometry). This in turn results in integro-differential equations if one wants to take nonlinear frequency dependence of the impedances into account exactly in the time domain. In practice, representative linear functions—i.e., constant stiffness and damping coefficients, are used to avoid this complexity. However, the accuracy of this simplifying approach in predicting the responses of structures, especially when considering inelastic behavior, is not well understood. To address this knowledge gap, herein the inelastic response of superstructures subject to SSI effects is studied. Rational functions are used to approximate nonlinear impedance functions. They are represented efficiently as digital filters in the time domain and are solved along with the superstructure’s equations of motion to obtain response histories when subjected to select ground motions. These results are compared to those obtained both assuming a fixed base (neglecting SSI effects) and using linear impedance functions. The effects of inertial SSI are observed as the difference between the fixed base and substructure responses. Additionally, potential inaccuracies induced through the use of representative linear functions are identified and discussed. The work concludes by offering ductility maps, which graphically depict the effects of inertial SSI on inelastic structural systems, as an example of a practical application of the filter method.