Lateral-torsional buckling of unrestrained steel beams under fire conditions: improvement of EC3 proposal

Abstract

The final draft of the EN version of part 1.1 of Eurocode 3 has introduced significant changes in the evaluation of the lateral-torsional buckling resistance of unrestrained beams at room temperature that reduce the over-conservative approach of ENV 1993-1-1 in the case of non-uniform bending.Numerical modelling of the lateral-torsional buckling of steel beams at elevated temperature has shown that the beam design curve from prEN 1993-1-2 is over-conservative for loadings other than uniform bending.In line with the safety format of the lateral-torsional buckling code provisions for cold design, an alternative proposal for rolled sections or equivalent welded sections subjected to fire is presented in this paper, that addresses the issue of the influence of the loading type on the resistance of the beam, achieving better agreement with the real behaviour while maintaining safety.

Parametric analysis of the lateral–torsional buckling resistance of steel beams in case of fire

“…6 has already been studied previously [8]. As shown in this figure, for the case of non-uniform bending, i.e.…”

“…To take into account the moment distribution between the lateral restraints of members, the authors have already proposed [8] a modified reduction factor given by where and k c is a correction factor according to Table 4, established by numerical adjustment to match as closely as possible a representative sample of finite element numerical results. The accuracy of this new proposal is shown in the next section.…”

“…For the sake of clarity in the discussions, Table 1 shows the effects that are considered or not in EN 1993-1-1, EN 1993-1-2, Vila Real et al [8], and the new proposal presented in this paper.…”

“…The influence of non-uniform bending moment distributions in the fire condition has been analysed by the authors [8] who showed that the beneficial effect resulting from reduced plastic zones connected with variable bending along the beam [9] should also be considered at elevated temperature. If not, the design is much too conservative for the case of non-uniform bending diagrams [14,15].…”

Experimental bending tests of partially encased beams at elevated temperatures

“…Vila Real et al [18] proposed a new formulae to be used in fire conditions, using the same approach for lateral torsional buckling design, taking into account the moment distribution between the lateral restraints. The reduction factor should be modified to χ LT;fi;mod , according to Eq.…”