Ali Rezaei.Tabrizi - MSCE, Senior Earthquake Engineer of Earthquake Resistant Consultants (ERC), Tehran, Iran
Fariborz Nateghi-A - PhD, P.E.; Professor of International Institute of Earthquake Engineering and Seismology (IIEES), Tehran, Iran
Farhad BEHNAM FAR - Ph.D, Assistant Professor of Isfahan University of technology, Isfahan, Iran

In Classical methods of structural Analysis, it is assumed that the motion at the foundation level of structures is equal to the ground free field motion. For the structures constructed on the soft soil, a rocking component caused by the support flexibility on the horizontal motion of the foundation is added which can be important for tall buildings. Presence of the soil layers changes frequency content of the ground motion and thus filters the high frequencies. The effect of soil layer on earthquake waves can appear in the form of dynamic amplification or de-amplification in different frequencies. Numerous studies have shown that the interaction between adjacent buildings can result in changes in nonlinear dynamic response of structures, damage and performance level, depending on the dynamic specifications of structures involved and the frequency content of the input motion. To study these effects, finite element method is used for the analytical investigations and total soilfoundation- structure system is modeled all together. For modeling purposes and in order to realize the effects of the adjacent buildings on the dynamic response, two buildings namely; 15 story and 30 story tall buildings which separated by distance of 1/4 and 1/8 of the width of the foundation and located on hard and soft soil profiles were considered. It was concluded that in the case where soil and structure's period were near to each other, the interaction of adjacent structures on increasing the nonlinear responses (displacement and interstory drift) and structural damage indexes were noticeable, therefore not negligible. Whereas, in the case where periods are distant from each other, interaction of adjacent buildings has a decreasing effect on damage indexes and the nonlinear responses, therefore negligible. Theoretical, analytical and results obtained in this study will be presented in this paper.