The effect of soil condition on earthquake response has been studied by considering the effects of soil-structure interaction and soil amplification. In this study, idealized Reinforced Concrete buildings (1, 2, 5 and 10-storied) are subjected to ground vibrations recorded during the Northridge earthquake in 1994.

Surface foundations on soils of shear wave velocity c_s = 300 ft/sec, 1000ft/sec and 10000ft/sec (representing soft, medium and hard soil respectively) are used as the substructure. The maximum forces (shear force and bending moment) are compared for different conditions of the foundation and the subsoil.

The results from soil-structure interaction show that the maximum bending moment occurs for the 5-storied building. The soil-structure interaction does not significantly modify the bending moments for any of the structures. This is because the considered foundations are surface foundations and are quite stiff compared to the structures themselves.

However, the maximum forces are significantly affected if the effect of soil amplification is also taken into account. For structures built on soft soils the maximum bending moment occurs for the 2-storied building instead of the 5-storied building. This is because the natural frequency of the soil layer nearly coincides with the natural frequency of the 2-storied building, thereby amplifying the energy content of the earthquake at those frequencies.

Therefore in assessing the possible impact of an earthquake on a particular structure, it is important to know the main energy content of the vibration and the natural frequencies of the structure, and avoiding resonance.