Plastic collapse of restrained steel silo hoppers

Abstract

Elevated steel silos and tanks commonly consist of a cylindrical shell, a conical hopper and a skirt. Much of the total weight of the stored material is supported by the hopper, which is in biaxial tension and may fail by forming a plastic collapse mechanism. This paper examines the plastic collapse of hoppers which are sufficiently restrained by a ring at the hopper/cylinder junction for the collapse mode to be entirely confined to the hopper. The hopper joints are assumed to be stronger than the shell plate. An elastic-plastic finite-element program is used to study the plastic collapse behaviour of these hoppers. It is found that the plastic collapse mode is usually a local mechanism near the top of the hopper. Collapse strengths are determined for hoppers of both uniform thickness and varying thickness subject to internal pressure with and without frictional shear. Most of the calculations are performed using elastic-plastic small deflection theory, because this leads to well-defined collapse strengths and relates to classical limit analysis. Calculations made using large deflection theory show that the non-linear changes of geometry have a significant stiffening and strengthening effect. The parametric study presented in this paper defines a simple lower bound to the strength of the hopper.