JSCE Journal of Earthquake Engineering INVESTIGATION OF THE EFFECT OF KOBE EARTHQUAKE ON A THREE-DIMENSIONAL SOIL-STRUCTURE SYSTEM Kayoko HASHIMOTO 1 and Nawawi CHOUW 2 1 Graduate Student, Graduate School of Natural Science and Technology, Okayama University, Okayama 700-8530, Japan, kayoko@poo.civil.okayama-u.ac.jp 2 Dr.-Ing., Associate Professor, Faculty of Environmental Science and Technology, Okayama University, Okayama 700-8530, Japan, chouw@cc.okayama-u.ac.jp The study addresses the relationship between the characteristics of near-source earthquakes and the system frame structure with multiple foundations and subsoil. The ground excitations are the ground accelerations of the 1995 Kobe earthquake at 22 locations. The investigation of the epicentral distance effect reveals that the contributions of the time coincidence of the peak responses and the vertical ground motions to the structural response should be considered. The soil-structure interaction can reduce and also amplify peak responses and response spectrum values in certain frequency range. Key words: Near-source earthquakes, Kobe earthquake, epicentral distance, time coincidence, vertical ground motion, soil-structure interaction, peak, frequency 1. INTRODUCTION In near-source regions the ground can experience strong movement not only in the horizontal directions but also in the vertical direction. The vertical ground motions can have larger amplitude and higher frequency vibration than the horizontal ones. Far from the source, the vertical ground motions are in general smaller than the horizontal ground motions. Up to now many design regulations are still based on the knowledge of the far-source earthquake, so that the effect of the vertical ground motion may be neglected. The effect of the vertical ground motions is studied by some researchers (e.g. Kusunoki et al. 1) ). Investigations on the simultaneous ground excitations on the structural responses are often limited to the two-dimensional problems. Nakamura et al. 2) showed that the vertical ground motion does not have a strong effect on the response. The reason is that the largest vertical and horizontal ground accelerations usually do not occur at the same time due to the difference in the velocities of the compressive and shear waves in the soil. However, Chouw 3) showed that the time coincidence of the subsequent peaks of the ground motions could strongly amplify the structural responses. Three-dimensional investigations are still very limited. Takabatake and Nonaka 4) confirmed the significance of a strong vertical ground excitation. Their investigation, however, did not include the influence of the soil. Some researchers study the influence of the vertical ground motion in the shaking table tests, e.g. Kawai and Hirasawa 5) showed that in the horizontal shaking test the structural damage occurs gradually. However, in horizontal and vertical shaking test it occurs immediately an at many locations. The effect of the soil on the structural responses is often studied as two-dimensional problems. Fukasawa et al. 6) paid attention to the influence of a natural frequency of the structure including subsoil. However, their soil system is limited to the vicinity of the foundation only. Asai et al. 7) showed the relationship between the first natural periods of the structure and soil. However, they did not indicate the relationship between the characteristic of the ground motions and the structural response. Zheng et al. 8) showed the effect of the slenderness ratio of 1