Eccentric discharge buckling of a very slender silo

Abstract

Metal silos used to store granular solids often take the form of a cylindrical shell with an aspect ratio in the range 2 < H/D < 6. It has long been recognized that the most serious load case for silos is the condition of eccentric discharge of its stored solid, and more failures have occurred under this condition than any other. Two of the chief reasons for this high failure rate are the difficulty in characterizing the pressure distribution caused by eccentric solids flow, and the difficulty in understanding the pattern of stresses that develops in a shall wall under such unsymmetrical pressure regimes. The nonsymmetric behavior of a shell structure under such a loading condition is not at all well described in the voluminous shell structures literature, and only a few studies have explored the mechanics leading to high local stresses which in turn lead to buckling failure under eccentric discharge. In this study, the pressures caused by eccentric discharge are characterized using the new rules of the European Standard EN 1991-4 [5] that defines the Actions in Silos and Tanks. Using this new improved description of unsymmetrical pressures, it is now possible to perform relatively realistic calculations relating to this common but complicated shell buckling condition. The calculations described here are part of a wider study believed to be the first of its kind and are undertaken using geometrically and materially nonlinear analyses in accordance with the European Standard EN 1993-1-6 [6] on Strength and Stability of Shells. The paper explores the structural behavior leading to buckling during eccentric discharge, including the critical effects of changes of geometry and imperfection sensitivity.