Effects of soil-structure interaction on longitudinal seismic response of MSSS bridges

This paper presents the results of a comprehensive study on the effects of soil-structure interaction (SSI) on longitudinal seismic response of existing bridges. The Federal Highway Administration's guidelines for a footing foundation on a semi-infinite elastic halfspace are used to determine translational and rotational stiffnesses at the base of bridge abutments and piers. Similarly, stiffness and strength of abutment backfill soil are determined based on FHWA's procedures. Various stiffnesses at the abutments are then lumped (condensed) into one translational spring at the point of impact between the abutment and the deck. Translational springs at the abutments are bilinear with their yield strength in compression determined based on the Mononobe-Okabe method. In tension, it is equal to the friction force at the footing. Among the parameters considered is the case of a damaged back wall, where it is assumed that due to shear failure at the juncture of the back wall and the breast wall, the abutment strength and stiffness, as well as mobilized abutment mass, have changed. Results indicate that SSI has a significant effect on the seismic response in the longitudinal direction. Abutment strength is the most critical parameter. Impact force, deck sliding, and SSI affect all plastic rotations at the base of columns. Thus, it is important that analytical models used in seismic evaluation of bridge systems explicitly consider SSI.