Resistance to LTB of steel bridge girders

Lateral torsional buckling (LTB), a complex phenomenon of instability, occurs when a beam is solicited by a bending force. In the area of steel and composite bridges girders which are characterised by slender plate girders, few experimental and theoretical studies exist in order to evaluate their structural security particularly with respect to LTB. Therefore, bridge girder resistance models refer to those existing for beams destined for buildings, by applying nevertheless a high level of conservatism. Steel bridge girders are three-dimensional structures, the behaviour of which is influenced by a number of parameters such as: cross bracing, variable geometry of cross sections, loading effects or even aspects related to manufacturing and to material. In fact, the manufacturing procedure of plate girders consists of flame cutting and then welding thin web to thick steel sheets. This results in geometric imperfections on the elements as well as in residual stresses inside the material, the effect of which on the behaviour of the structure is not to be neglected. This paper presents the actual results of experimental and theoretical studies carried out on the subject of LTB of steel bridge girders.