Influence of SSI on the Stiffness of Bridge Systems Founded on Caissons

This paper presents a parametric study that identifies the relevance of soil-structure interaction (SSI) on the static stiffness matrix of bridge piers founded on caissons. Bridge piers and caissons with different geometric features and foundation soils characterized by low or high shear wave velocity are encompassed. A discrete model for the foundation-soil system (lumped-spring approach) and a single-degree-of-freedom (SDOF) scheme for the superstructure are adopted to analyze the coupled system of pier, caisson, and soil. The SSI is introduced in the structural model by concentrated springs and masses taking into account the embedment of the caisson. For each case of the parametric study, the ratio between the overall stiffness of the soil-caisson-pier system and the stiffness of the pier in its fixed-base configuration is evaluated by defining the geometrical and mechanical parameters that play the primary role. Last, a closed-form equation combining the key parameters is developed for providing a unique parameter that allows engineers to quickly assess the relevance of SSI in bridge pier design before performing cumbersome and time-consuming interaction analyses.