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 a s the primary structure in many studies. When, however, a secondary sy stem is submerged and hence subject to hydrodynamic effects, its seism ic response can be aggravated by the base-isolation of the primary str ucture because the fundamental frequency of the primary structure coul d be so lowered as to be close to the resonant frequency of the second ary system. Therefore, a careful evaluation should be made of the pote ntial seismic response jump of submerged secondary systems when a base -isolation is to be applied to their primary structure. This paper des cribes the dynamic analysis of submerged secondary systems on base-iso lated primary structures and illustrates how much the base-isolation c an aggravate the seismic response of the secondary systems when no alt eration 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 reducin g 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.