Beams and lateral-torsional buckling

A steel I-beam is efficient in bending about its strong axis, but that efficiency has a weakness. The compression flange behaves like a strut that wants to buckle sideways. Because it is joined to the tension flange through the web, it cannot simply bow out; instead the whole section moves sideways and twists together. This is lateral-torsional buckling, and it can govern long, unbraced beams well below their full plastic moment.

If the compression flange is held sideways at close intervals, by a concrete slab, purlins or bracing, the beam is laterally restrained. Its strength is then the plastic or elastic moment of the section, depending on section class: plastic and compact sections reach Zp fy / γm0, semi-compact sections only Ze fy / γm0. Shear in the web is checked separately, and high shear reduces moment capacity.

For laterally unsupported beams, the elastic critical moment Mcr depends on the unbraced length, the section's lateral stiffness, torsional and warping constants, the moment distribution and where the load is applied. Loads applied on the top flange are worse than loads at the shear centre. The code converts Mcr to a design strength through a slenderness and imperfection factor, in the same style as column curves. Practical cures are bracing the compression flange, choosing wider flanges, or using box sections. Deflection and web bearing under concentrated loads complete the checks.