Seismic Response of Isolated Elevated Water Tanks

Abstract

The paper presents the results of an analytical investigation of the seismic response of isolated elevated water tanks. A discrete three-degree-of-freedom model of the isolated structure is presented that includes the isolation system, tower structure, and sloshing fluid. Fluid-structure interaction is modeled using the mechanical analogy proposed by Housner. The model captures the salient features of the system response, yet is still amenable to closed-form solution. The natural frequencies and mode shapes are determined, and a response spectrum analysis is conducted. Results are investigated for the full range of tank capacities and height-to-diameter ratios found in practice and for the full range of fluid elevations. Results of the isolated elevated tank are compared to the corresponding fixed-base tank design and indicate that seismic isolation is effective in reducing the tower drift, base shear, overturning moment, and tank wall pressure for the full range of tank capacities. Isolation is most effective for the smallest capacity tank; however, in this case, a single mode solution for the isolated structure is not adequate, as the fluid motion contributes significantly to the overall structural response. Isolation does, however, increase the relative convective fluid displacement in the tank.