Testing, numerical modeling and design of Q960 ultra-high strength steel welded I-section beam–columns after fire exposure

This paper investigated the structural behavior and residual resistances of post-fire Q960 ultra-high strength steel (UHSS) welded I-section beam–columns under combined compression and major-axis bending through comprehensive experiments and numerical simulations. The experimental program consisted of specimen heating, soaking and cooling, post-fire tensile coupon tests, measurements of initial geometric imperfection and 14 beam–column tests. The numerical simulations included developing and validating finite element models, followed by parametric studies which generated 740 numerical models covering varied cross-sectional dimensions, member lengths and loading combinations. Due to the lack of design standards for high strength steel structures after fire exposure, the applicability of design rules in European, American and Australian standards was assessed for Q960 UHSS welded I-section beam–columns after exposure to elevated temperatures, utilizing post-fire material properties. The evaluation revealed that: (i) three design standards provide overall conservative resistance predictions, particularly for slender I-section members, (ii) the American specification yields more accurate (i.e. less conservative) and consistent resistance predictions for nonslender sections than those predicted by other two standards, due to the adoption of bi-linear shape of interaction curve and accurate bending end point, whereas the Eurocode leads to the most accurate resistance predictions for slender sections among three codes, due to the more precise adoption of effective width method. Finally, an improved design approach adopting the format of the EC3 design interaction curves, but with more accurate compression and bending end points determined from the Direct Strength Method (DSM) and rational buckling reduction factors, was proposed to enhance design accuracy.