Response spectra of a building are made with a SDOF system taking into account a first mode shape, even though higher modes may affect on the dynamic responses of a high-rise building. A soft soil layer under a building also affects on the responses of a building. In this study, seismic responses of a MDOF system were investigated to examine the effects of higher modes on the response of a tall building by comparing them with those of a SDOF system including the kinematic interaction effect. Study was performed using a pseudo 3D finite element program with seven bedrock earthquake records downloaded from the PEER database. Effects of higher modes on the seismic responses of a tall building were investigated for base shear force and base moment of a MDOF system including story shear forces and story moments. Study results show that higher modes of a MDOF system contribute to a reduction of base shear force up to 1/4-1/5 of KBC and base moment. The effect of higher modes is more significant on the base shear force than on the base moment. Maximum story shear force and moment occurred at the top part of a building rather than at a base in the cases of tall buildings differently from short buildings, and higher modes of a tall building affected on the base forces making them almost constant at the base. A soft soil layer also affects some on the base shear force of a high-rise building independently on the soft soil type, but a soft soil effect is prominent on the base moment.

1. Introduction
 2. Model of a Structure-Foundation-Soil System
 3. Comparison of Elastic Responses of SDOF andMDOF systems
 4. Comparison of Response Spectra of SDOF andMDOF systems
 5. Comparison of Base Shear Forces and Momentsof SDOF and MDOF systems
 6. Comparison of Story Shear and Story Momentof MDOF system
 7. Conclusions
 / Acknowledgement /
 / REFERENCES /

• Kim, Yong-Seok(Department of Architectural Engineering, Mokpo National University) | 김용석 Corresponding author