A Method for the Numerical Derivation of Plastic Collapse Loads

Abstract

Two key reference loads: (i) the plastic collapse load and (ii) the elastic buckling load are commonly used todetermine 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 NonlinearAnalysis (MNA) based on small displacement theory (i.e. afirst order plastic analysis). However, such analysescan often yield ambiguous or even spurious results due to, for example, the load-deformation paths not reachinga peak value or reaching a peak value but only after unrealistically large deformations, resulting in misleadingpredictions of plastic collapse loads and mechanisms. In this paper, a standardised means of determining plasticcollapse 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 tech-nique to predict the full load-deformation paths and hence estimate the plastic collapse load is proposed. Theaccuracy and practicality of the proposed approach over existing methods is illustrated for a wide range ofstructural scenarios, with an emphasis on structural elements under concentrated transverse forces.