Lateral-torsional Buckling
When a simple beam is loaded in flexure, the top side is in compression, and the bottom side is in tension. When a slender member is subjected to an axial force, failure takes place due to bending or torsion rather than direct compression of the material. If the beam is not supported in the lateral direction (i.e., perpendicular to the plane of bending), and the flexural load increases to a critical limit, the beam will fail due to lateral buckling of the compression flange. In wide-flange sections, if the compression flange buckles laterally, the cross section will also twist in torsion, resulting in a failure mode known as lateral-torsional buckling.
Read more about this topic: Buckling