Global instability of cold-formed steel elliptical hollow section members under combined compression and biaxial bending

The present study numerically investigated the global instability of cold-formed (CF) steel elliptical hollow section (EHS) members under combined compression and biaxial bending. A finite element (FE) model for CF tubular members under combined compression and biaxial bending was first developed, in which the strength enhancement, geometric imperfection and residual stresses due to roll-forming were incorporated. The FE model was validated against the existing test results. The validated FE model was employed to carry out a parametric study for CF steel EHS members, which covers an extensive range of cross-section geometries, material grades (including both high strength and normal grade steels), member slenderness, loading eccentricities and loading angles. Based on the generated FE data, the application of the current interaction design formulae for steel members under combined loading in EN 1993-1-1, together with the codified slenderness limits for circular hollow section (CHS) under combined loading, were assessed for the design of CF steel EHS members under combined compression and biaxial bending. It can be found that the design formulae in EN 1993-1-1, together with the slenderness limits for CHS, can give very conservative predictions for CF steel EHS members under combined loading. In addition, the influences of different parameters, including the loading eccentricity, loading angle, loading level and the reduction factor for lateral-torsional buckling, on the member behavior were also examined. Finally, new design recommendations are proposed for the design of CF steel EHS members under combined compression and biaxial bending.