AISC 360-22
AISC 360-22 Chapter F | Flexural Strength of Steel Beams
Steel beams fail in bending either by local buckling of their flanges/web, yielding across the whole section, or by twisting sideways (Lateral-Torsional Buckling). The unbraced length determines which failure mode controls.
Governing Formula
Variables
Nominal flexural strength
kip-in or kN-m
Lateral-torsional buckling modification factor
unitless
Plastic bending moment (Fy * Zx)
kip-in or kN-m
Length between points braced against lateral displacement
in or mm
Limiting laterally unbraced length for full plastic bending capacity
in or mm
Quick Example
1
Determine capacity zones
Calculate Lp and Lr for the wide flange section. These boundaries separate yielding, inelastic LTB, and elastic LTB.
2
Compare bracing length
If unbraced length Lb ≤ Lp, the beam fully yields.
3
Account for moment gradient
If Lb falls between Lp and Lr, use the inelastic LTB equation and multiply by the Cb moment gradient factor.
When does this govern?
Lateral-torsional buckling governs for most open-section roof beams and purlins that lack continuous deck bracing.
Common Mistake
Automatically assuming Cb = 1.0 (conservative) instead of calculating it, leading to significantly oversized beams.