A simplified approach is described for determining the extent of plastic buckling of the tank wall near the base (elephant’s foot bulging) for ground-supported unanchored liquid storage tanks subjected to seismic overturning moments. The method is applied to a 100-ft diameter and 40-ft high tank with a roof. Results show that the ultimate seismic overturning moment that can be withstood by the tank is 31 percent higher than the overturning moment at which elephant’s foot bulging begins. The reason for this is redistribution of vertical compressive stresses in the tank wall: initially these vertical compressive stresses are concentrated over a small contact length. However, as the elephant’s foot bulge develops, the size of the contact length increases, resulting in a more favorable distribution of stresses.

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