In concrete silos, which are relatively thick walled but weakly reinforced, it is the pressure normal to the wall which dominates the design, and wall bending is spread quite widely. Steel silos generally have much thinner walls, and most failures are caused by buckling. The compressive stresses which lead to buckling failures arise
firstly from solids friction against the wall, but also from unsymmetrical pressures during discharge, and from wind pressures when empty or accidental partial vacuum. The buckling strength is often very sensitive to fabrication imperfections, which often cannot be known with certainty at the design stage. Naturally, silo built in a factory environment using an established repeated procedure has different imperfections from a one-off site constructed structure, and this difference affects both the strength and reliability of the construction.