NUMERICAL AND EXPERIMENTAL INVESTIGATION OF BASE PLATE INTEGRITY IN UNANCHORED LIQUID STORAGE TANKS

Abstract eng:
Ground-supported unanchored liquid-storage cylindrical tanks, when subjected to strong seismic loading may exhibit uplifting of their bottom plate, which has significant effects on their dynamic behavior and strength. Those effects mainly concern: (a) the increase of axial (meridional) compression at the tank base, resulting in premature buckling in the form of elephant’s foot and (b) the significant plastic deformation at the vicinity of the welded connection between the tank shell and the bottom plate that may cause failure of the welded connection due to fracture and fatigue. The present study focuses on base uplifting mechanics and tank performance with respect to the shell/plate welded connection through numerical and experimental tools. First, a numerical two-step methodology is presented: (1) a simplified modeling of the tank as a springmass system for dynamic analysis, enhanced by a nonlinear spring at its base to account for the effects of uplifting, (2) 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, in order to obtain the overturning moment-rocking angle relationship to use in the simplified model. Two cylindrical liquid storage tanks of different aspect ratios are modeled and analyzed in terms of local performance of the welded connection. The results are aimed at better understanding of tank uplifting mechanics and motivating possible amendments in existing seismic design provisions. In addition to the above numerical investigation, experiments are performed, so that the strength of the end-plate/tank-shell connection is examined under severe cyclic loading that may cause low-cycle fatigue. The experiments are employed to define an appropriate fatigue curve of the welded connection, for use in the numerical investigation.

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