Stress in the Uplifted Shell-to-Bottom Joint of a Large-Sized Oil Storage Tank under Seismic Loading

摘要

The shell- to- bottom joint of unanchored oil storage tank will uplift due to overturning moment applied to the tank against strong horizontal seismic excitation, and excessive plastic strain cyclically occurs in this joint. Three large tanks spilled their contents over the port due to the failure caused by uplifting force applied to the tank shell in the 1978 Miyagiken-oki earthquake. This paper presents the systematic evaluation for structural integrity of a large- sized oil storage tank against strong earthquake. In the first method of this study, the nonlinear seismic response analysis taking the uplifting force applied to the tank shell into consideration is carried out, and useful information on the relation between the seismic character and the uplift displacement is derived. The second method is the elastic-plastic, large displacement finite element stress analysis of the shell-to-bottom joint under uplifting load. The relation between the peak strain at the toe of fillet weld of this joint and the seismic character is obtained from the 1st and 2nd method. Crack initiation and crack propagation behavior under cyclic bending load is investigated on the specimens with fillet weld joint by the experimental test for low cycle fatigue. The relation between the peak strain and the fatigue life at the toe of fillet weld of this joint is finally obtained. Consequently, the tank's integrity can be evaluated on the viewpoint of the seismic character, the uplift displacement of the shell-to-bottom joint, the peak strain and the fatigue life at the toe of weld of this joint.