Seismic Foundation Stiffness for Bridges

This paper presents a simple procedure to develop stiffness matrix for bridge foundations under seismic loading conditions. The methodology is based on the results of a pilot program for the Washington State Department of Transportation in which a design manual was developed to assist the bridge engineers for seismic design of bridge foundations for typical bridges in the state. The proposed procedure consists of a simple approach, which combines the results of a dynamic soil-structure interaction finite element program SASSI with the results of SHAKE, LPILE and APILE to develop stiffness matrices of pile group/shaft foundations, which are more realistic. The results are presented in the form of design charts, which show the variation of horizontal, vertical, rocking, torsional, and cross coupling stiffness components with the deflection. The results of our analyses show that the cross coupling terms are important for single shafts but can be ignored for the pile groups. In addition, we found that the contribution of the pile caps in the lateral stiffness of pile foundation can be as high as 75 percent of the total stiffness.