Fire resistance of slender section beams - Subjected to lateral torsional buckling

The paper shows experimental and numerical research into slender cross-section beams in case of fire. The topic is very important because little investigation has been made and little experimental data has been collected so far. In the framework of the RFCS project FIDESC4 - Fire Design of Steel Members with Welded or Hot-rolled Class 4 Cross-sections, several simple supported beams were tested at elevated temperature at the Czech Technical University in Prague.
Currently, Eurocode 3 contains a number of simple rules for design of slender Class 4 cross-sections which, based on recent numerical simulations, proved themselves to be over-conservative. Through refining these rules, material savings could be achieved, which would lead to greater competitiveness of the steel structures. This is being covered in existing research, but is not published in this paper, which is limited to lateral torsional buckling behaviour only.
Determination of the bending resistance for members subjected to lateral torsional buckling of Class 1-3 cross-sections at elevated temperature is based on the same principles as the design at room temperature, according to EC 3 part 1-1 [1]. However, it differs in using only one imperfection factor for all types of cross-sections. The informative Annex E of the standard (EC 3 part 1-2 [2]) recommends using the design formulas for slender (Class 4) sections as well. But there is a restriction of the critical temperature value and a different reduction of yield strength is used (0.2 % proof strength for Class 4 instead of 2.0 % proof strength for stockier Class 1-3 sections). For the non-uniform members (variable section height), a limited design procedure is given in EC 3 part 1-1 [1]. This is applicable for room temperature only. The possibility of using these rules for fire design is not yet confirmed.