Lateral-Torsional Buckling

Lateral-torsional buckling (LTB) is the instability failure of a beam bent about its major axis in which the compression flange, behaving like a column with the slender minor axis, buckles sideways and the section twists, so the member fails at a moment well below its in-plane plastic capacity. It governs the design of long, laterally unrestrained beams with slender or open (I) sections, where minor-axis and torsional stiffness are low compared with major-axis bending strength.

IS 800 Cl. 8.2.2 handles it by reducing the bending strength to a design bending strength using a bending stress reduction factor (χLT) that depends on the non-dimensional slenderness — a function of the elastic critical moment, which in turn depends on the effective (unrestrained) length, section properties and the moment gradient. The practical control is lateral restraint: properly restraining the compression flange (by the slab, secondary beams or bracing) shortens the effective length, raises capacity and is usually far more economical than upsizing the section.