The contribution of SSI in bridges is found more significant when supporting soil is soft. The role of soft soils can contribute to unexpected seismic demands to the bridge structure. First, a soft soil profile can modify the frequency content of the seismic wave such that the bridge base motion can increase the structural responses. And second, the dynamic parameters of the bridge structure can be modified by the presence of the surrounding soil moving the full soil-structure system to a region of sensitive response.