FARHAD BEHNAMFAR - Structural Engineering Department, International Institute of Earthquake Engineering and Seismology, P. ۰. Box ۱۹۳۹۵/۳۹۱۳, Tehran, Iran
YOSHIHIRO SUGIMURA - Department of Architecture and Building Engineering, Tohoku University, Sendai ۹۸۰-۷۷, Japan

studied. These two components are caused by the input motion of the building's foundation and soil flexibility under seismic loading. The main purpose is making it clear that to what degree the rotational component of vibration of foundation of a tall building can contribute to the total response. Also, how this contribution changes with the governing type of the waves in the ground excitation. Analytical results of this study show that the ratio of the part of the total lateral displacement of the structure caused by rocking to the part caused by drift is strongly a function of type of the earthquake wave, soil flexibility and number of the stories. It increases with flexibility of soil and height of the structure up to a certain number of stories. Also, under shear waves the amplification factor of the lateral motion is always greater than that of the Rayleigh waves for the accelerograrns considered