Strain-based approach to local buckling of steel sections subjected to fire: Fid-Bau Portal

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

Knobloch, Markus / Fontana, Mario

AbstractLocal 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.