Two simplified models are presented for determining the hydrodynamic loads developed during the seismic response of tall unanchored liquid storage tanks. In both models the general procedure starts by first solving the fluid response problem in closed form. This eliminates all but the sloshing unknowns associated with the hydrodynamic problem. In the first model, the geometrical discretization is performed by applying the assumed mode method. In the second model, the system parameters are identified by requiring the same base loads when the real system and the model are subjected to the same base motion. During the formulation process, special attention is paid to developing a simple but sufficiently accurate analytical modeling of the mechanism associated with the separation of the base plate of the tank from its foundation, which occurs during strong ground motion. Soil flexibility and liquid sloshing effects are also modeled. The accuracy of both models is verified by comparison with experimental data. In addition, this comparison shows the applicability of these models for determining the seismic loads in tall unanchored tanks.

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