Seismic response of submerged secondary systems in base-isolated structures

Base-isolation of a primary structure has performed remarkably in the seismic response attenuation of the in-air secondary system, as well as the primary structure in many studies. When, however, a secondary system is submerged and hence subject to hydrodynamic effects, its seismic response can be aggravated by the base-isolation of the primary structure because the fundamental frequency of the primary structure could be so lowered as to be close to the resonant frequency of the secondary system. Therefore, a careful evaluation should be made of the potential seismic response jump of submerged secondary systems when a base-isolation is to be applied to their primary structure. This paper describes the dynamic analysis of submerged secondary systems on base-isolated primary structures and illustrates how much the base-isolation can aggravate the seismic response of the secondary systems when no alteration is made to the secondary systems. It is shown that additional treatments, such as the appropriate control of the fluid gap between a secondary system and the primary structure, may be helpful in reducing adverse effects. In addition, a concept of double base isolation, e.g. the LRB base isolation of the submerged secondary system on a base-isolated primary structure, is introduced as another solution for the case where the fluid gap control is limited. (C) 1997 Elsevier Science Ltd.