Behaviour of axially compressed angles and built-up steel members at elevated temperature

Angles and built-up steel sections are widely employed in structures as members in bracing systems or in truss structures and are mainly designed to withstand axial loads. Torsional and flexural-torsional buckling might affect angles and built-up members subjected to compressive stresses. Great interest has been shown by researchers relative to the instability of steel elements in fire, but there is a lack of studies on the buckling behaviour of angles and built-up steel members in compression and this topic is not explicitly treated in the current version of Eurocode EN 1993-1-2. In order to provide new insights, a comprehensive numerical investigation of the behaviour of concentrically compressed angles, tee and cruciform steel members at elevated temperature was performed. A parametric study was carried out on Class 1 to 3 profiles with three different steel grades, namely S235, S275, S355, subjected to uniform temperature distribution. More than 41000 geometrically and materially nonlinear imperfect analyses (GMNIA) were performed on columns with different length and temperature by means of 3D beam and shell elements. Results showed that the buckling curve given in EN 1993-1-2 provides unconservative predictions for a range of slenderness of practical interest. To better predict the behaviour of the investigated steel members, an improved buckling curve was proposed, which allows for safer predictions, as also confirmed by statistical investigation.