抄録
During intense seismic motion, the shell-to-bottom joint of an unanchored oil storage tank can lift due to an overturning moment. To explore this, we carried out a nonlinear 3D-FEM dynamic analysis to quantify the uplift displacement on an 110, 000m3 oil storage tank when subjected to the maximum ground acceleration (293gal) of the “Far-Off-Sanriku Earthquake” (1994) . A 3D model of fully-coupled shell-liquid which can properly take into consideration the contact condition between the bottom plate and its foundation, large deflection effect of the uplifted bottom plate, and dynamic uplifting behavior, was used for this analysis. The maximum uplift displacement of the tank shell was calculated to be 59mm.
A nonlinear time history analysis using a mechanical model of mass spring system was also carried out, and showed that maximum uplift value was 56mm. The max. uplift of the mechanical model analysis is slightly smaller than that of 3D-shell dynamic analysis, because the shell deformation in the former analysis was not considered.
After the earthquake, it was confirmed by the breaking of the waterproof seal covering the outer edge of the bottom annular plate that the partial uplift of the east-side shell plate occurred during earthquake. The uplift displacement was estimated to be about 60mm by crack initiation strain of the coating material on the annular plate inner surface. Therefore the calculated value (59mm) of nonlinear 3D-FEM dynamic analysis was judged to be more proper than those calculated by other analysis methods, such as 3D-shell static analysis shown in the 1st report.
In order to calculate proper uplift displacement from the viewpoint of relatively economical design method, an effective procedure is proposed which combines time history analysis of mass spring system and mode correction factor obtained by 3D-shell static analysis.
