Behaviour and design of structural steel cross-sections in fire

Abstract

At both room temperature and elevated temperatures, the cross-sectional load-carrying capacity of structural steel members is limited by the effects of local buckling. The strength and stiffness of steel also reduce with temperature and the stress-strain relationship becomes increasingly nonlinear. Current structural fire design codes utilise the concept of cross-section classification and the effective width method in line with the corresponding steel design rules at ambient temperature for the design of steel sections at elevated temperatures; this approach artificially separates structural steel cross-sections into discrete behavioural classes and does not reflect the inherent continuous relationship between the cross-section resistance and its local slenderness. Moreover, the utilization of two different design yield strengths results in further discontinuities in the predicted fire resistances at the boundary between non-slender and slender cross-sections. Existing steel design rules at ambient temperature are also based on an assumed elastic, perfectly plastic stress-strain response, which does not well represent the actual nonlinear stress-strain response of steels at elevated temperatures. A series of experiments to investigate the structural response and failure of cross-sections at room temperature is presented. A deformation-based design approach known as the continuous strength method (CSM), which provides an alternative treatment to the conventional concept of cross-section classification and enables a rational exploitation of strain hardening, is then developed for the design of steel cross-sections at elevated temperatures. Predictions of resistance using the CSM for compression and bending, as well as new proposals for combined loading, have been compared with experimental and numerical results and those obtained using EN 1993-1-2 and AISC 360-16; the CSM predictions are shown to offer higher levels of accuracy and reliability than the current design methods.