What is lateral-torsional buckling and how do designers prevent it in steel girders?

• A. Lateral-torsional buckling is the instability of unrestrained compression flanges under bending; prevention includes adequate lateral bracing, stiffeners, proper cross-section selection, and controlling unbraced length.
• B. Lateral-torsional buckling occurs when a beam is fully restrained; prevention is to remove bracing.
• C. It is not a concern for steel girders.
• D. It is prevented by increasing the amount of concrete cover.

Answer: (A)

Lateral-torsional buckling is the instability that occurs when a beam bending about its strong axis has its compression flange inadequately restrained laterally, so the flange can move sideways and the section can twist as it buckles. This mode lowers the beam’s ultimate bending capacity and tends to occur when the unbraced length of the compression flange is long or the section is slender in torsion. Designers prevent it by providing adequate lateral bracing to keep the compression flange from moving laterally, adding stiffeners along the flange to resist local buckling and maintain alignment, and choosing cross-sections with higher torsional stiffness or reducing slenderness so the member can resist twisting. It’s not addressed by concrete cover, and removing bracing would worsen the problem, not prevent it.