Study on the Newly Developed 3-Dimensional Base Isolation System Using a Velocity Reduction Mechanism

Due to the Hyogo-ken Nambu Earthquake in 1995 and the huge earthquakes which are considered to occur in near future, the raise in the seismic design criteria in the horizontal and vertical excitations has been investigated. According to these trends, many base isolation structures have been developed and constructed. However, the most of these structures are limited to the horizontal base isolation. Therefore, the development of the effective 3-dimentional base isolation system becomes more and more important. The conventional 3-dimentional base isolation systems proposed up to now are insufficient in reducing the vertical natural frequency, otherwise are very complex due to facilitation of the active components. In this study, the dynamic characteristics of a high performance 3-dimentional base isolation system newly developed by the authors are reported. This system employs a velocity reduction system using a gear mechanics which can reduce the vertical natural frequency less than 1Hz while that of the conventional system is around 3Hz. And also this employs friction dampers to suppress the large displacement. Further, this system is facilitated with a rocking-suppression system which can effectively suppress the rocking motion inevitable for 3-dimentional base isolation. The response acceleration, displacement and rocking motion are evaluated by numerical simulations in varying the friction force, the predominant frequency of seismic input waves, the eccentricity of the upper structural mass, etc.. As the results, it is shown that the base isolation system developed here has very effective base isolation characteristics and the rocking suppression effects. And the optimization of the structural parameters is also discussed.