When a simply supported steel “I” beam bends under downward loads the extreme fibres at the top of the beam are in compression. This puts the top flange of the beam into compression, and the bottom flange into tension. The top flange therefore acts as a strut, and may buckle under the compressive force. Generally the web of the steel section will prevent the flange from buckling by deflecting vertically, but the flange can move horizontally by a combination of lateral movement and twisting.