Influence of the cross-section shape on the lateral torsional buckling capacity

The bifurcation-load of a bending girder, the ideal lateral torsional buckling moment M-KI is of great importance for the verification of stability. The solutions in the literature exist almost exclusively for double-symmetrical cross sections. The whole problem considering the lateral torsional buckling is quite complex as the analytical solutions for determining the lateral torsional buckling strength exist in closed-form only for the most simple cases, e.g. for the case of a simply supported double symmetric beam of a constant cross-section under uniform moment. However, for most cases, in order to obtain the buckling load, numerical or approximate solutions are required. This article examines the problem of elastic lateral torsional bucking of simply supported, monosymmetric I and T-beams under two different transverse load cases. Correct and approximate Euro Code 3 (EC3) approaches for obtaining elastic lateral torsional buckling capacities of monosymmetric I and T-beams were investigated for each load case. Solutions were obtained in terms of the easily evaluated degree of beam monosymmetry, beta f, beam parameter, (K) over bar, and monosymmetry parameter, r(2). The results obtained are graphically presented and compared. It was found that approximation formulae for r(2) given by Kitipornchai and Trahair are much more accurate compared to the correct formulae than the approximation formulae proposed by EC3. T-beams are considered as a special case of monosymmetric I cross sections. In conclusion, the authors recommend utilisation of different types of cross sections for different types of loading and length of the beams. The result can be significant savings in material as well as increased stability of the structure regarding lateral torsional buckling of monosymmetric cross section girders.