Behaviour and design of S960 ultra-high strength steel non-slender welded I-section beam–columns

The structural behaviour of S960 ultra-high strength steel (UHSS) welded I-section beam–columns is studied in the present paper. Nonlinear finite element (FE) models were first created and validated against relevant test results collected from the literature; the models were then used to carry out parametric studies to obtain additional FE data over an extensive spectrum of cross-section dimensions, member lengths and loading combinations. Given the lack of existing design rules for S960 UHSS structures, the codified beam–column interaction curves prescribed in the current European (EC3) and American (AISC 360) design specifications for lower strength steel members, were assessed for their applicability to the studied S960 UHSS non-slender welded I-section members, utilizing the generated numerical data. It was found that EC3 yielded conservative failure load predictions for Class 1 and 2 S960 UHSS welded I-section beam–columns buckling about either principal axis; similar results were found for Class 3 members under combined compression and strong-axis bending, with the mean FE to predicted failure load ratio being 1.29, the corresponding COV being 0.12, but rather conservative strength predictions arose for Class 3 members under combined compression and weak-axis bending, with the mean FE to predicted failure load ratio being 1.50. AISC 360 led to overall accurate and consistent failure load predictions for S960 UHSS non-slender welded I-section beam–columns, but resulted in some unconservative strength predictions for members under combined compression and weak-axis bending. New design proposals, adopting the format of EC3 design interaction curve, but with more accurate compression and bending end points, interaction factors and buckling reduction factors determined from the continuous strength method (CSM), were proposed and shown to lead to improved design accuracy.