Strain-based approach to local buckling of steel sections subjected to fire

Local buckling under fire conditions needs to be considered in the context of a wider range of cross-sectional slenderness than in ambient temperatures design. This is due to the distinctly non-linear material behaviour of steel at elevated temperatures, and the large strains required for increasing cross-sectional capacity due to plastification. Stress-based design models, which are commonly used to explain local buckling at ambient temperatures, are not ideal for describing local buckling behaviour under fire conditions. Therefore a new strain-based approach has been developed which uses effective widths for stiffened and unstiffened elements at elevated temperatures. The resulting strain-based formulations avoid the use of section classes for fire design, and take into consideration the plastification effects, plastic stress distribution, and strain-dependent non-linear material behaviour of steel at high temperatures. The use of these formulations extends the application range of current state-of-the-art models for local buckling and non-linear material behaviour. They allow us to consider the decreasing branch of the load-carrying behaviour required during fires, in order to analyse sections with non-uniform temperature distribution. For unstiffened elements in compression at elevated temperatures, the load-carrying behaviour in the buckling and post-buckling ranges was analysed using temperature-dependent second-order linear elastic theory, taking into consideration initial imperfections and yield line theory, which allowed us to formulate a novel strength curve for these elements at elevated temperatures. In a parametric study, the resistance to compression and bending of stiffened and unstiffened elements during fire was calculated. The proposed design approach accords with results produced using the finite element approach. (c) 2005 Elsevier Ltd. All rights reserved.