Derivation of DSM-type resistance functions for in-plane global buckling of steel beam-columns

Abstract

This paper presents the derivation of a new design formulation for the representation of the buckling strength of steel beam-columns, which follows the format and basic principles of novel and increasingly popular international design methods, such as the Direct Strength Method DSM (Schafer, 2008)– used predominantly in North America for the design of cold-formed steel members – and the General Method GM – included in the Eurocode EN 1993-1-1 (EN 1993-1-1, 2005) section 6.3.4 as an alternative way of designing generic steel members and structural systems. The paper focuses on the in-plane buckling strength of double-symmetric hot-rolled, tubular and welded sections, with compact sections; this focus on an otherwise well-understood problem allows for a clearer focus on the key aspects which need to be accounted for in a DSM/GM type representation of beam-column strength. In particular, a generalized definition of slenderness (in line with the DSM philosophy) and a generalized imperfection term, which accounts for the ratio between bending moments and axial forces in the beam column, are used to obtain an Ayrton-Perry (Ayrton and Perry, 1886; Rondal and Maquoi, 1979) type design formulation for beam-column in-plane global buckling. In the paper, the key components that need addressing in a DSM – as well as any other – beam-column design approach are highlighted, namely: i. the influence of the relative ratio between bending and compression loading, ii. the effect of non-uniform bending moment diagrams, iii. the deterioration of the achievable plastic cross-sectional utilization due to loss of rigidity by yielding in slender members and iv. the interaction between buckling modes, in this case local and global buckling. The paper proposes a coherent, innovative design formulation which accounts for all of these effects and compares the outcome of the new strength predictions with numerical (non-linear FEM) and traditional Eurocode results.