Elastic local buckling of three-flanged cross-sections

In current structural steel design specifications, the local buckling of cross-sections is typically treated on an element-by-element basis, with the boundary conditions along the adjoined longitudinal edges of the individual plates assumed to be simply-supported. In reality, cross-sections buckle locally as a whole and the individual plate elements interact. As a result, the boundary conditions along the adjoined longitudinal edges of the critical isolated plate (i.e. that with the lowest elastic local buckling stress) lie between lower and upper bounds of simply-supported and fixed, respectively. Based on this concept, explicit formulae to predict the elastic local buckling stress of full cross-sections of common profiles, including I-sections, have recently been developed (Gardner et al., 2019) [1]. In the present paper, the formulae for single I-sections set out in Gardner et al. (2019) [1] are extended to cover the case of three-flanged cross-sections that arise in longitudinally-stiffened plate girders and in the haunch and apex regions of portal frames. The geometry and loading of the studied cross-sections are assumed to remain constant along the member length, i.e. the influence of tapering and moment gradients on local buckling are not considered herein, but has been evaluated in parallel work (Quan et al., 2020) [2]. The proposed formulae are calibrated against results from finite strip analysis performed using CUFSM v4.05 (Li and Schafer, 2010) [3] on a range of three-flanged sections, and provide predictions of elastic local buckling stresses that are typically within 5% of the numerically obtained values. •Elastic local buckling behaviour of three-flanged cross-sections has been investigated.•Formulae for calculating the elastic local buckling stress of three-flanged cross-sections have been developed.•Accuracy of predictions using the developed formulae has been confirmed against results obtained from finite strip analysis.•The predictions using the developed formulae have also been compared with results calculated on an element-by-element basis.