Earthquake resistant design of offshore building structures

A proposed earthquake resistant design system for offshore building structures in Japan is introduced. Offshore building structures are defined as the fixed type offshore structures which are constantly utilized for the purpose of habitation, duty, work, operation, gathering, recreation, sightseeing, etc. In the first section, basic principles for the design of offshore building structures are introduced considering safety of human life, maintenance of building functions, and protection of property. In the second section, the seismicity in and around the Japan Islands is determined by utilizing the past earthquake data and recent researches concerning seismicity, and a seismic macrozoning map in Japan is herein revised. In the third section, the earthquake resistant design system based on the basic principles for the design of offshore building structures and the seismicity in Japan is proposed. This system consists of three kinds of earthquake resistant design methods according to the fundamental natural period of the structure: seismic coefficient design, modified seismic coefficient design, and dynamic analysis. The choice of these methods to be used is entrusted to the designer of the structure. In each earthquake resistant design method, two levels of earthquake activity which the structure may be subjected to are considered: strong earthquake design level and severe earthquake design level. The strong earthquake design level requires that the structure be adequately seized for strength and stiffness to maintain the maximum level of earthquake activity which may be expected during the life of the structure. The severe earthquake design level requires that the structure have sufficient ultimate strength or ductility to prevent the collapse of the structure during a maximum credible earthquake. In the final section, the need for seismic instrumentation to acquire ground motion on underwater ground during a strong earthquake is emphasized to establish more efficient earthquake resistant design for offshore building structues as a conclusion.