Flexural behaviour and strengths of press-braked S960 ultra-high strength steel channel section beams

A thorough experimental and numerical study of the flexural behaviour and strengths of press-braked S960 ultra-high strength steel (UHSS) channel section beams bent about the minor principal axes is reported in this paper. The experimental study was conducted on eight different UHSS plain channel sections, and included measurements on the material flat and corner properties and initial local geometric imperfections of the beam specimens as well as 20 four-point bending tests performed about the minor principal axes in both the 'u' and 'n' orientations. A complementary numerical investigation was then conducted, where finite element (FE) models were firstly developed and validated against the experimental results, followed by parametric studies carried out to acquire further numerical data over a broader range of cross-section dimensions. It is worth noting that the existing design codes for steel structures, as established in Europe, America and Australia/New Zealand, are only applicable to those with material grades up to S690 (or S700 for Eurocode) and cannot be directly used for S960 UHSS structural members. In the present study, the applicability of the codified design provisions and formulations for flexural members to the examined S960 UHSS channel section beams was evaluated, based on the ultimate moments derived from structural testing and numerical modelling. The quantitative evaluation results generally revealed that the current European code provides overall consistent and precise flexural strength predictions for Class 1 and Class 2 S960 UHSS channel sections in minor-axis bending, but leads to a high level of inaccuracy (scatter and conservatism) for the design of their Class 3 and Class 4 counterparts, whilst the American specification and Australian/New Zealand standard result in scattered and excessively underestimated design flexural strengths, except for the cases of slender S960 UHSS channel section beams in 'u'-orientation bending.