In all the cases of Earthquake excitation, it has been assumed that the earthquake motions are introduced as specified quantities at the structural support points, In effect, these important displacements are assumed to depend only on the earthquake generation and wavel mechanism and are not influenced by the response of the structure. In actual fact, the structure and the soil on which it is founded form a combined dynamic response mechanism, and there may be significant feed back from the structure into the soil layer. Hence, the extent to which the structural response may alter the characteristics of earthquake motions observed at the foundation level depends on the relative mass and stiffness properties of soil as well as the structure. Thus the physical properties of the foundation medium forms an important factor in the intensity of the earthquake response of structures supported on it. Because of these reasons the soil layer will have influence on the structural response to earthquakes.