Boundary reaction method for nonlinear analysis of soil-structure interaction under earthquake loads

This paper presents a boundary reaction method (BRM) for nonlinear time domain analysis of soil-structure interaction (SSI) under incident seismic waves. The BRM is a hybrid frequency-time domain method, but it removes global iterations between frequency and time domain analyses commonly required in the hybrid approach, so that it operates as a two-step uncoupled method. Specifically, the nonlinear SSI system is represented as a simple summation of two substructures as follows: (I) wave scattering substructure subjected to incident seismic waves to calculate boundary reaction forces on the fixed interface boundary between a finite nonlinear structure-soil body and an unbounded linear domain; and (II) wave radiation substructure subjected to the boundary reaction forces in which the nonlinearities can be considered. The nonlinear responses in the structure-soil body can be obtained by solving the radiation problem in the time domain using a general-purpose nonlinear finite element code that can simulate absorbing boundary conditions, while the boundary reaction forces can be easily calculated by solving the linear scattering problem by means of a standard frequency domain SSI code. The BRM is verified by comparing the numerical results obtained by the proposed BRM and the conventional frequency-domain SSI analysis for an equivalent linear SSI system. Finally, the BRM is applied to the nonlinear time-domain seismic analysis of a base-isolated nuclear power plant structure supported by a layered soil medium. The numerical results showed that the proposed method is very effective for nonlinear time-domain SSI analyses of nonlinear structure-soil system subjected to earthquake loadings.