The stability of anchored cylindrical steel tanks with a secondary stiffening ring

Abstract

Cylindrical steel tanks are used in most countries to store bulk volumes of liquid products such as oil, water and chemicals. These structures are commonly constructed from thin steel plates. The thin wall makes the tank vulnerable to buckling under external pressure due to wind, or to a partial vacuum when it is empty or partially emptied. A stiffening ring is widely used to enhance the strength of the tank wall against external pressure instability. Recent research by the author has determined stress resultants in order to provide a rational design procedure for the primary stiffening ring in open-top steel tanks considering interaction between the ring and the cylindrical tank shell. In addition to primary stiffening ring, one or more secondary stiffening rings are generally needed to enhance the buckling resistance under external pressure and wind. From this premise, the strength and stiffness requirements for the secondary stiffening ring in anchored cylindrical steel tanks are explored in this new study. Pursuant to this aim, the stress resultants in the isolated secondary stiffening ring subjected to non-uniform wind loading are derived using Vlasov's curved beam theory. Next, a new shell-to-ring stiffness ratio is presented by considering the ratio of the stiffness of the cylindrical shell to that of the secondary stiffening ring. In addition, a parametric study employing finite element analysis is performed to obtain the required strength and stiffness of the secondary stiffening ring in anchored cylindrical steel tanks. The variations in response quantities are expressed as a function of the proposed shell-to-ring stiffness ratio. These findings are finally expressed in terms of simple equations that can be easily used in practical design.