1. Context

To study the load-bearing capacity of a structural element, we first assume that the element is ideally perfect. Its strength is then given either by the cross-sectional strength, determined on the basis of elastic-perfectly plastic behavior, or by the elastic critical load. These two quantities constitute the upper limits of the structural response of industrially-manufactured elements, which is this time affected by the adverse effects of the geometric and structural imperfections inevitably present.

At high strains, the behavior is essentially elastic, and the load-bearing capacity tends towards the value of the elastic critical buckling load for columns, or the elastic critical torsional moment for beams.

In the low-strain range, the effects of instability are more than offset by the material's work-hardening effects, and the load-bearing...