BASE-ISOLATION DESIGN PRACTICE IN JAPAN: INTRODUCTION TO THE POST-KOBE APPROACH

Japan has twenty years of experience in designing and constructing base-isolated building structures. Construction has increased significantly since the 1995 Hyogoken-Nanbu (Kobe) earthquake, having reached over 150 annual construction projects. Many new developments and refinements have been made in the material, device, design, and construction of these structures. This paper summarises recent design and construction of base-isolated building structures in Japan, including statistical data with respect to the common usage as well as the number of new projects. It is notable that the size, height, and fundamental natural period of new base-isolated buildings increase steadily with time, indicating that base-isolation in Japan is reaching maturity. Base-isolators and dampers commonly adopted in Japan are also introduced, with emphasis on recent design efforts to enlarge the natural period of base-isolated structures and reduce the lateral forces induced in the superstructure. Basic design procedures are presented, including determination of design earthquake forces, modelling of base-isolation layers, modelling of the superstructure, selection of ground motions, time-history analyses, and performance criteria. A mandated peer-review system, unique for design of base-isolated structures, is also noted. Several characteristic issues in the design of base-isolated structures are discussed: Variation of base-isolation material properties, applications to high-rise buildings, effects of vertical ground motions, and response when subjected to near-fault ground motions.