Current practice usually neglects the effects of soil-structure interaction (SSI) in the seismic analysis and design of bridges. This work attempts to assess the significance of SSI on the seismic response of isolated multi span bridges.

The soil medium has been analyzed by applying different soil densities in order to consider the soil stiffness. The accent of the study has been given to the soil structure interaction effects and the results are analyzed accordingly. The soil medium has been taken into consideration as a four layered infill as dense and loose medium. The bridge structure is taken to be an RC structure. The boundaries in the soil medium are considered as infinite elements in order to absorb the radiating waves.

The formulation of infinite elements is the same as for the finite elements in addition to the mapping of the domain. Based on the isoparametric concept, the infinite element in global coordinate is mapped onto an element in local coordinate system. In the formulation of the infinite element, only the positive direction extends to infinity thus allowing the waves to propagate outside of the soil medium.

Related comparisons are done with references and experimental results in which considerably acceptable results are obtained. The newly proposed methodology efficiently models both the interaction of soil and bridge structures and simultaneously the far field of soil model in which the wave reflections are softened. The case study chosen in this work considers different strength of soil models on which the bridge structural response is analyzed in detail.