A method for the numerical derivation of plastic collapse loads

Two key reference loads: (i) the plastic collapse load and (ii) the elastic buckling load are commonly used to determine the slenderness and hence the resistance of structural steel elements in international design standards. Utilising numerical methods, the plastic collapse loads are typically obtained through a Materially Nonlinear Analysis (MNA) based on small displacement theory (i.e. a first order plastic analysis). However, such analyses can often yield ambiguous or even spurious results due to, for example, the load-deformation paths not reaching a peak value or reaching a peak value but only after unrealistically large deformations, resulting in misleading predictions of plastic collapse loads and mechanisms. In this paper, a standardised means of determining plastic collapse loads from numerical MNA based on attaining a tangent stiffness of 1% of the initial slope of the load-deformation curve is presented. Furthermore, for analyses that terminate prematurely, an extrapolation technique to predict the full load-deformation paths and hence estimate the plastic collapse load is proposed. The accuracy and practicality of the proposed approach over existing methods is illustrated for a wide range of structural scenarios, with an emphasis on structural elements under concentrated transverse forces.