Experimental and numerical investigations of S890 and S960 ultra-high strength steel circular hollow section stub columns after fire exposure

The post-fire material properties and local buckling behaviour of S890 and S960 ultra-high strength steel (UHSS) circular hollow sections have been investigated in this paper, underpinned by testing and numerical modelling programmes. The testing programme included heating, soaking and cooling of specimens, post-fire tensile coupon tests, initial local geometric imperfection measurements as well as stub column tests. The measured post-fire material properties were analysed and adopted to propose a new set of retention factor curves to accurately predict the post-fire residual material properties for S890 and S960 UHSSs. On the basis of the post-fire stub column test results, finite element models were developed and validated and afterwards adopted to carry out parametric analyses. Due to the absence of design provisions for post-fire UHSS circular hollow sections, the applicability of the relevant room temperature design rules given in the current European, American and Australian standards to post-fire S890 and S960 UHSS circular hollow section stub columns was assessed, using post-fire material properties. The assessment results revealed that the codified slenderness limits were conservative for cross-section classification of post-fire S890 and S960 UHSS circular hollow sections. Regarding the post-fire cross-section compression resistances, all three sets of codified design rules were shown to provide relatively conservative predictions, while the level of design accuracy and consistency was higher for slender S890 and S960 UHSS circular hollow sections than the non-slender ones. It was also indicated from the assessment results that the American design standard led to more accurate and consistent compression resistance predictions than the European and Australian design standards.