Study on the Seismic Load Coupling Effects Due to Horizontal and Vertical Excitation for the Newly Developed 3-Dimensional Base Isolation System

Abstract

Recently, many huge earthquakes occurred in many places, such as Hyogo-ken Nambu earthquake in 1995, Kocaeli earthquake and Chi-Chi earthquake in 1999 and Niigata Chuetsu earthquake in 2004 etc., and huge earthquakes are also considered to occur in near future. Therefore, many horizontal isolation structures have been proposed and constructed. In addition to the horizontal isolation, 3-dimensional isolation systems are under development as well. For these reasons, the authors also have developed a new 3-dimensional base isolation system and evaluated the effectiveness of the system. This isolation system employs sliding pads and laminated rubber bearings for horizontal isolation as some other isolation systems do. Thus, the vibration response of the upper structure depends on the friction force between sliding pads and the floor. In the actual seismic events, these sliding type isolation systems suffer from both horizontal and vertical seismic excitations simultaneously. For these reasons, the friction force in the horizontal direction has two components; one depends on the self weight and the other depends on the vertical seismic response. The latter will change time-dependently. Therefore, it can be said that the coupling effects between the horizontal and vertical seismic excitations become predominant. In this study, the coupling effects of the horizontal and vertical seismic excitations are investigated analytically as for the rocking motion, response displacement and response acceleration for the various seismic wave inputs. The effects of the phase difference between the horizontal and vertical seismic input and the amplitude of the vertical excitation are clarified and the design guide lines for the sliding type base isolation systems exposed to the horizontal and vertical simultaneous seismic excitations are proposed.