Post-fire behaviour of Q960 ultra-high strength steel welded I-sections under major-axis combined loading

This paper investigates the local buckling behaviour and ultimate capacity of post-fire Q960 ultra-high strength steel welded I-sections under major-axis combined loading, through extensive experiments and numerical simulations. The physical tests included thermal exposure protocols (i.e. heating, soaking, cooling of specimens), post-fire tensile coupon tests, initial local geometric imperfection measurements and 15 eccentrically-loaded stub column tests. The numerical simulations involved the development and validation of finite element models, followed by parametric analyses to generate a total of 764 numerical models of varying cross-section dimensions and loading combinations. Given the absence of post-fire design standards for high strength steel structures, the European, American, and Australian ambient-temperature design rules were evaluated for their applicability to post-fire Q960 ultra-high strength steel welded I-sections under major-axis combined loading, based on post-fire material properties. The assessment results reveal that (i) all design standards yield conservative ultimate capacity predictions, especially for those after exposure to temperatures beyond 800 °C, due to the fact that stress–strain curves of Q960 ultra-high strength steel after exposure to elevated temperatures beyond 800 °C exhibit strong strain hardening characteristic, (ii) Eurocode leads to relatively accurate capacity predictions for compact (i.e. Class 1 and 2) and slender (i.e. Class 4) sections, due to its accurate design interaction curve and more accurate effective width method, respectively and (iii) American specification results in more precise capacity predictions for non-compact (i.e. Class 3) sections, owing principally to its more precise design interaction curve of bi-linear shape. An improved design approach modifying the interaction curves specified in the European standard was proposed to enhance design accuracy.