DYNAMIC SOIL-STRUCTURE INTERACTION FOR BRIDGES

Abstract

Soil-Structure Interaction is a phenomenon in which the response of structure subjected to earthquake ground motion is influenced by the foundation soil. Bridges are susceptible to relatively more severe SSI effects during earthquakes compared to buildings due to their spatial extent, varying soil conditions at different supports and possible inconsistency in the seismic input. In addition to the aforementioned issues, soil being a heterogeneous, anisotropic material, increases the complexity. During earthquakes, it is imperative for a bridge to perform well, hence it is necessary to carry out a detailed study before designing a bridge. In this study, the substructure of an existing bridge in Udaipur, Tripura is considered. To show the effect of soil on the structure, four types of soils are considered viz, loose sand, dense sand, dry stiff clay and saturated stiff clay. The system is modelled as a three-dimensional finite element continuum model. To keep the computation time and size to feasible limits, only the substructure with the surrounding soil is modelled, and the mass of the superstructure is assumed to be lumped over the pier cap. The modelling and analysis is carried out in ANSYS (Mechanical APDL), which is a multi-purpose finite element software. Modal analysis, Equivalent static analysis, Harmonic analysis and Time-history analysis are carried out on the system and the response in the form of - - maximum displacement is noted at the top of pier cap and at the ground level. Also, assuming soil as a linear material is a far-fetched assumption and may lead to erroneous - results. So a comparative study is performed, where the soil is assumed to be linear in one case and non-linear in the other. The results demonstrate the effect of soil flexibility and non-linearity and clearly emphasize the importance of soil-structure interaction. This study and its results evidently express the need for considering soil-structure interaction while designing bridges.