Earthquake input for finite element analysis of soil-structure interaction on rigid bedrock

When using the finite element method to analyze seismic soil-structure interaction (SSI) problem on rigid bedrock, several methods have been developed to input earthquake on the lateral boundary of finite element model. These boundary conditions include the free boundary, the viscous-spring (VS) boundary, the tied degrees of freedom (TDOF) boundary and the free-field loading combined viscous-spring (FFL-VS) boundary. Their accuracy and location should be investigated. In this paper, the formulations and ABAQUS implementations of these boundaries are given. The accuracy properties of these boundaries are then compared by numerical examples including the free-field and SSI problems. The comparison studies indicate that the free and VS boundaries failed to reproduce the free-field and SSI responses when a relatively small size SSI model is employed. The TDOF and FFL-VS boundaries can simulate the exact free-field response and have good performances in seismic SSI analyses. The FFL-VS boundary is more accurate than the TDOF boundary. The appropriate locations for the TDOF and FFL-VS boundaries depend on the SSI model. A simplified approach is developed for appropriate boundary location. This approach is applicable to the FFL-VS boundary but not to the TDOF boundary due to the former more accurate than the latter.