Section F2/Doubly Symmetric Compact I-Shaped Members in Bending
AISC 360-22 Section F2 covers the flexural design of compact doubly symmetric I-shaped beams including the plastic moment capacity and lateral-torsional buckling limit states.
Section F2 applies to doubly symmetric I-shaped members (W-shapes, S-shapes) bent about their major axis with compact flanges and compact webs. Two limit states must be checked: yielding (plastic moment) and lateral-torsional buckling (LTB). The nominal flexural strength for yielding is Mn = Mp = Fy x Zx, where Fy is the yield stress and Zx is the plastic section modulus. For lateral-torsional buckling, the behavior depends on the unbraced length Lb relative to two limiting lengths: Lp (the limiting laterally unbraced length for full plastic moment, approximately equal to 1.76 x ry x sqrt(E/Fy)) and Lr (the limiting unbraced length for inelastic LTB). When Lb is less than or equal to Lp, the full plastic moment Mp governs and LTB does not apply. When Lb is between Lp and Lr, inelastic LTB governs and Mn is interpolated linearly between Mp and 0.7 x Fy x Sx. When Lb exceeds Lr, elastic LTB governs and Mn is calculated from the elastic critical buckling moment Mcr. The Cb factor (moment gradient coefficient) increases the LTB capacity for non-uniform moment diagrams: Cb = 1.0 for uniform moment and Cb up to 2.27 for reverse curvature (double curvature with equal and opposite end moments), significantly increasing the effective capacity in typical loading conditions.
Why this section exists
Steel I-beams are the most common bending members in steel construction. Their efficiency comes from concentrating material in the flanges far from the neutral axis, but this shape makes them susceptible to lateral-torsional buckling: the compression flange tends to buckle sideways (laterally) while the beam rotates (torsion). The unbraced length between lateral bracing points determines whether the beam can develop its full plastic moment or whether LTB reduces the capacity. In most building construction, floor and roof decks provide continuous lateral bracing to the compression flange, allowing the full plastic moment to be used. When the compression flange is unbraced (cantilevers, bottom-flange loading, construction phases), LTB must be checked and can significantly reduce the available capacity.
What plan reviewers look for
Plan reviewers check the structural calculations for the unbraced length Lb of each beam and compare it to Lp and Lr for the selected section. They verify the Cb factor is applied correctly for the actual moment diagram (Cb = 1.0 is conservative but may result in heavier sections). They check that the beam is compact (flange width-to-thickness ratio and web height-to-thickness ratio meet Table B4.1b limits). For beams braced by floor decks, they verify the deck attachment provides adequate lateral bracing. For combined loading (axial plus bending), they check the interaction equation from Chapter H.