Plastic buckling of unanchored liquid storage tanks under dynamic loads.

Abstract

A theoretical investigation of the plastic collapse of unanchored cylindrical ground-supported liquid storage tanks under dynamic loads is conducted to seek essential improvements in the design of such tanks to resist earthquakes. The study is carried out in two phases: (a) an axial force-shortening relation is determined for all possible values of the geometric and material parameters of the cylindrical shells, (b) a simple mechanical model is presented that can be used to design unanchored liquid storage tanks for collapse under dynamic loads. It is shown that for moderate deflection theory, the behavior of a long pressurized elastic-perfectly plastic cylinder with radially restrained ends under axial compression depends on only two parameters: the ratio of elastic buckling stress to yield stress and the ratio of nominal circumferential stress to yield stress. This makes it feasible to present results covering almost all conceivable values of these two parameters. Equations fitting these results are developed. Their application in the simplified analysis of unanchored tanks under static lateral loads is illustrated by an example. This indicates that the lateral load capacity of such tanks can be substantially larger than the lateral load at which plastic buckling (also referred to as elephant foot bulging) of the tank wall initiated. In addition, a simple mechanical model that represents key phenomena is developed to study the behavior of unanchored tanks under dynamic loads where the results of the parametric analysis mentioned above can also be used. This model determines when the plastic buckling starts and produces the accumulation of damage during an earthquake. To show the applicability and the effectiveness of this model, results are obtained and compared with real damage that occured to two tanks during the San Juan Earthquake. This comparison reveals that this model can predict the failure of tall tanks due to elephant foot bulging, thus confirming the reliability and the sufficient accuracy of this work as a powerful tool for estimating the dynamic response of storage tanks.