Soil-structure interaction analysis of bridges using time domain approximation

Abstract

The bridge responses obtained under the assumption of fixed supports, in which the soil-structure interaction effects are not taken into account, may be seriously erroneous. Thus the interaction effects have to be considered in the seismic analysis and design of bridges, particularly long-span bridges with multiple supports. In this study, a simplified substructure method is presented for bridge-foundation-soil interaction analysis using time domain approximation. At first frequency-dependent impedance functions are evaluated for the bridge foundations on layered soil media, which are modeled by finite and infinite elements. Then the frequency-independent stiffness and damping of the foundation are taken from the impedance at the first natural frequency of the bridge on fixed supports at each direction. Then the results are incorporated into the finite element model of the bridge, which can be easily analyzed in time domain. Verification of the present method is carried out in a pier-foundation system, and numerical analysis is conducted on a long span cable-stayed bridge. Several conclusions are drawn from verification and example study as follows: (1) From the verification study, it has been found that the proposed method gives very reasonable results for SSI analysis. (2) The conventional response spectrum method may give unconservative results. Thus the SSI effects must be considered for safe design of bridges. (3) The free-field motion may be used instead of the scattered motion for the SSI analysis. (4) The amplification of ground motion in soft soil medium greatly affects the magnitude of the effective earthquake forces and member forces of the structure. (5) Pylon responses at softer soil are larger. (6) The interaction effects are more significant in the longitudinal direction that in the transverse direction due to support conditions of bridges.