Study of Buckling in Steel Silos under Eccentric Discharge Flows of Stored Solids

Abstract

The most serious loading condition for slender thin-walled metal silos has long been recognized to be the condition of discharge, with eccentric discharge causing more catastrophic failures than any other. Two key reasons for this high failure rate are the difficulties in characterizing the pressure distribution caused by eccentric solids flow, and in understanding the associated unsymmetrical stresses in the silo wall. Few studies have addressed either the linear elastic behavior of such a silo or its buckling failure under eccentric discharge.In this study, the eccentric discharge pressures are characterized using the new rules of the European Standard EN 1991-4 on Silos and Tanks. This novel description of unsymmetrical pressures permits a study of the structural behavior leading to buckling during eccentric discharge, including the critical effects of change of geometry and imperfection sensitivity, to be undertaken using geometrically and materially nonlinear computational analyses. The mechanics of the behavior are found to be quite complicated. A silo which is safe under axisymmetric loading is found to be susceptible to catastrophic stability failure under eccentric discharge.

Buckling of very slender metal silos under eccentric discharge

“…In the present study, as in the earlier study of [30], the unsymmetrical pressures caused by eccentric discharge are investigated using the rules of the relatively recent EN 1991-4 [8], based on a simplified version of the theory of Rotter [21,22]. This theory proposes a distribution for the pressures resulting from a parallel-sided circular flow channel forming against the wall, shown This simple model for the pressures associated with eccentric discharge is reasonably valid for solids that form steep-sided flow channels, which are packed more densely than the critical state, or with potential to develop cohesion [25,33].…”

“…[2,15]), though these attempts have not been successful at reproducing even the form of experimental silo pressure observations [28]. Unfortunately, codified representations of patch loads differ considerably from one standard to another [30], mainly due to the lack of calibration using the pressures arising during eccentric discharge. The effect of patch loads has been found to be very detrimental in linear bifurcation analyses, but their effect on geometrically nonlinear bifurcation loads seems to be quite small [31,32].…”

“…This theory proposes a distribution for the pressures resulting from a parallel-sided circular flow channel forming against the wall, shown This simple model for the pressures associated with eccentric discharge is reasonably valid for solids that form steep-sided flow channels, which are packed more densely than the critical state, or with potential to develop cohesion [25,33]. In the previous study [30], the silo was designed to store wheat, which certainly does not flow in this manner, so the calculations presented there must be understood to relate to a densely packed solid that happens to have properties similar to those of wheat. In this paper, the stored solid is, instead, assumed to be cement, which is much more likely to develop a flow pattern in this form [16].…”

“…This made the wall just procedure, though for a silo of a lower aspect ratio which additionally included a uniform wall thickness design, was used in the previous study [30]. …”

“…The failure mode in such slender thin-walled silos is buckling due to local axial compressive stresses, which are induced by unsymmetrical normal pressures against the wall. In an earlier study [30], the phenomenon of buckling in a moderately slender perfect silo was explored. The basic mechanics were described, but it was not possible to determine how significant the effects of slenderness and imperfection sensitivity might be.…”