Seismic response of submerged internal systems in base-isolated structures

Abstract

Base-isolation of a primary structure has shown remarkable performance in seismic response attenuation of an 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 the submerged secondary system when base-isolation is going to be applied to a primary structure with submerged secondary system. This thesis analyzes dynamics of the submerged secondary system on the base-isolated primary structure and illustrates how much the base-isolation can aggravate the seismic response of the secondary system when no alteration is made to the secondary system. It is shown that additional treatments, e.g. appropriate control of fluid gap between the secondary system and the primary structure, may be helpful in reducing the adverse effects. Closer gap within the limit of no impact turns out to yield smaller seismic responses for most natural frequency range of submerged system in base-fixed structure. But the optimal gap for the minimum peak response of the system in base-isolated structure is much dependent on the natural frequency of the system by resonance effect. It is shown that analysis considering only the initial gap condition, in case of large displacement relative to the fluid gap, may lead to less conservative results than the actual one, because change of system dynamic characteristics at each time step of integration can be considerable enough to affect the response. In addition, a concept of double base isolation, e. g. LRB base isolation of the submerged secondary system on a base-isolated primary structure, is introduced as more positive s...