In this study, earthquake response analysis is carried out for multi-storey buildings, treating the foundation to be fixed. Both symmetric and un-symmetric plan of equal area are considered for the study. The analysis is repeated by replacing fixed supports with different soil conditions like cohesive and cohesion-less soil. Soil structure interaction effects due to earthquake excitation are observed. Four storey building models with total height of 17.5m were subjected to El Centro earthquake excitation, to obtain the response under different soil conditions. Time based responses are compared for structural members.

Time History analysis of both symmetrical and un-symmetrical buildings founded on different soil is carried out considering soil-structure effect. Un-symmetrical buildings were highly affected compared to symmetric building. The following conclusions were arrived

• Percentage increase for joint displacement for unsymmetrical building found on cohesion-less soil was 13.8% and 68.6%, compared to cohesive soil and hard strata.
• Beam end forces are enhanced by 42.2% and 98.6%, compared to cohesive and hard strata.
• Beam stresses are increased by 78.2% and 98.9%,compared to cohesive soil and fixed support condition.
• Time based results also show similar increment of 46.7% for displacement, compared to symmetrical building.
• Sway in unsymmetrical building is accompanied by twisting or torsional effect due to mass irregularity. The effect is again reinforced due to cohesion less soil.

The time based joint displacements, beam end forces and beam stresses shows enhanced values for un-symmetrical buildings found in cohesion less soil. It is concluded that un-symmetrical buildings should be constructed with symmetrical shear walls for preventing twisting effect. For buildings to be constructed on soft soil, the soil must be stabilized and design of foundation should be carried out accordingly.