Seismic Response of Unanchored Liquid Storage Tanks

Unanchored liquid storage tanks under strong seismic loading may exhibit uplifting of their bottom plate, with significant effects on the dynamic behavior and the structural integrity of the tank. In the present paper, base uplifting mechanics is examined numerically through a two-step methodology: (a) a detailed finite element shell model of the tank for incremental static analysis, capable of describing the state of stress and deformation at different levels of loading and (b) a simplified modeling of the tank as a spring-mass system for dynamic analysis, enhanced by a nonlinear spring at its base to account for the effects of uplifting. Three cylindrical liquid storage tanks of different aspect ratios are modeled and examined both as anchored and unanchored. The results are aimed at possible revisions in the relevant seismic design provisions of EN 1998-4 and API 650.