A Simplified Approach for Analysis of MDOF Systems Considering Soil - Structure Coupling

It is common to design structures on the fundamental assumption that the base is fixed and that the ground motion experienced by its base is the same as the free-field ground motion. For structures supported on soft soil, the base motion experienced by the structure is generally different from the free-field motion, as it could include possible sliding and rocking motion of the flexible medium and dissipation of vibration energy through the soil in the form of damping. The seismic response of a structure at a particular location depends on the characteristics of ground motion at source, and; modified mode shapes and vibration periods because of soil structure coupling. Thus, the ground motion that reaches the foundation can be significantly different from that at bedrock level and there is a possibility of response amplification when vibration period of the structure is close to that of the input ground motion modified by local soil conditions.
 Soil Structure Coupling (SSC) is a process wherein the structure and its supporting soil medium interact with one another to influence the behaviour of both. SSC normally leads to an increase in natural period of the structure, increased damping and an increase in lateral displacements of the structure. The effect of SSC is commonly ignored, because it is often a valid assumption. However, such effects can, in particular circumstances, be detrimental to the structure. The paper presents a simplified procedure for finding out the response of three-storey structure due to the effect of SSC. A simple practical method to assess the likely impact of SSC on the response of a MDOF structure is presented, which can help the researchers understand the detailed behaviour of soil and structure during seismic events. Detailed step-by-step procedure is presented by solving a numerical problem.