Plastic buckling strength of T-section transition ringbeams in steel silos and tanks

A T-section ringbeam, consisting of an annular plate and a vertical edge stiffener, is often provided at the intersection between the cylinder and the cone of an elevated storage silo or tank to resist a large circumferential compressive force at the intersection (the transition junction). Out-of-plane buckling, involving periodical twisting deformations of the ringbeam about its inner edge, is a common failure mode for this ringbeam. This paper is aimed at the development of a simple and rational approximation for the plastic out-of-plane buckling strength. As plastic buckling is the result of interaction between elastic buckling and plastic collapse, the present development is based on existing information on these two failure modes and a finite element parametric study on plastic buckling. Finite element results show that the interaction relationship between elastic buckling and plastic collapse, when cast in a suitable dimensionless form, is independent of yield stresses and most geometric ratios. This observation enables the establishment of two design approximations which are lower bound curves to finite element results. The simpler and more conservative approximation is proposed for use in design and its application is demonstrated in an example.