Buckling of steel plates at elevated temperatures: Theory of perfect plates vs finite element analysis

The local buckling capacity of fire exposed thin-walled steel cross sections is affected by the reduction in strength and stiffness due to elevated temperatures and the amplitude of the initial local imperfections. Several researchers have proposed design methods to calculate the capacity of the plates (i.e. web and flanges) that compose these steel members at elevated temperatures, but they used different shapes of steel plates (sides ratio a/b) and different amplitudes of local imperfections. This variability in hypotheses happens because there is no clear provision defining the numerical modeling procedure for fire design of steel plates in the codes (European or US). According to the theory of perfect plates, the critical load depends of the shape of the rectangular plate (e.g. the sides ratio a/b) and the corresponding buckling mode (number of half waves), the boundary and the loading conditions. This paper reviews the existing code provisions and compares the existing design models and their assumptions for thin-walled steel cross sections. Elements of the theory of perfect plates are presented. Parametric finite element analyses are then conducted on isolated steel plates at elevated temperatures to investigate the effect of the plate shape (a/b ratio) and imperfections (amplitude and number of half wave lengths). From the analysis, the governing parameter will be estimated (a/b vs imperfections) for simulation of isolated flanges and webs. Finally, recommendations for the numerical modeling of steel plates at elevated temperatures are proposed.