Bridging the Gap Between Structural and Geotechnical Engineers in SSI for Performance-Based Design

Abstract

Performance-based design (PBD) involves designing structures to achieve specified performance targets under specified levels of seismic hazard, and requires the analysis of the entire soil-structure system. Dynamic soil-structure interaction (SSI) encompasses linear and nonlinear response features of the structure and foundation soil. Consequently, both structural and geotechnical expertise are needed. Most available computational tools, however, provide either elaborate models for the structure with simplified soil representation, or detailed geotechnical models with simplified structural idealization. This paper discusses SSI pertinent to the analysis of soil-pile-structure interaction (SPSI) problems that are of direct ­concern to PBD of structures. The paper also presents a model developed with considerations from both geotechnical and structural engineering, thus, facilitating the effective incorporation of SSI in PBD. The model is developed within the ­framework of Beam-­on-Nonlinear-Winkler- Foundation (BNWF) approach and can be easily integrated into commercially-available nonlinear structural analysis software. It accounts for soil nonlinearity and discontinuity conditions at pile/soil interface; cyclic degradation of stiffness and strength due to variable-amplitude loading; and reduction in radiation damping with increased nonlinearity. The model is implemented in a readily available structural package, and hence allows structural engineers to properly account for SSI effects when performing PBD. The model was used to analyze the performance of piled foundations reported in the literature. The results showed that soil cave-in and recompression reduces the pile maximum bending moment and its depth from the ground surface, and increases the hysteretic damping of the soil-pile system, therefore enhancing the performance of the SPSI system. This could be particularly beneficial for situations where plastic hinges develop belowground level. In addition, some important features of pile response to cyclic loading are discussed.