Abstract: Three types of flexural-torsional buckling of beams in industrial buildings are introduced, they are girders supporting main beams, beams with purlins on their top flange and beams with fly braces between the lower flange of the steel beam and the purlin. Their elastic buckling moments are given, the differences between them are pointed out, special emphasis is put on the purlin-fly brace and pointing out that it is a type of elastic torsional spring depending on the bending stiffness of the purlins, the buckling moment is directly related to the stiffness of the purlins and thus the traditional concept of lateral braced length is waived.