Code Reference
StructuralAISC 360-22

Section F1/Flexural Member Design

AISC 360-22 Chapter F covers beam design including lateral-torsional buckling, local buckling, and yielding limit states.

What this section requires

The nominal flexural strength of a beam is the lowest value obtained from the applicable limit states: yielding (plastic moment Mp for compact sections), lateral-torsional buckling (LTB), flange local buckling (FLB), and web local buckling (WLB). For doubly symmetric compact I-shaped members with adequate lateral bracing, the full plastic moment Mp = Fy x Zx controls. When the unbraced length exceeds the limiting length Lp, lateral-torsional buckling reduces the available moment capacity.

Why this section exists

Steel beams can fail by several mechanisms depending on their proportions and bracing. A compact, well-braced beam reaches its full plastic moment before any instability occurs. A slender or unbraced beam can buckle laterally (twist and deflect sideways) at a moment well below the plastic moment. The design provisions check all applicable limit states and use the lowest value to ensure the beam is safe regardless of the failure mode.

What plan reviewers look for

Plan reviewers check beam sizes on the framing plan against the structural calculations. They verify the unbraced length of the compression flange (the distance between lateral bracing points) and confirm that lateral-torsional buckling has been checked for long unbraced spans. They also check beam deflection against the limits in IBC Table 1604.3.

Common violations

Unbraced length not accounted for
The beam design assumes continuous lateral bracing (full Mp) when the compression flange is unbraced over a significant length. Without intermediate bracing from floor deck, joists, or bracing members, the LTB limit state reduces the available moment.
Deflection not checked
The beam is sized for strength but the live load deflection exceeds L/360 or the total load deflection exceeds L/240 per IBC Table 1604.3. Deflection often controls for long-span beams.
Non-compact section used without reduction
A wide-flange section with slender flanges or web is used at its compact section capacity without applying the reduction for local buckling per Sections F3 or F4.
Compliance tip
Include a beam schedule showing member size, span, unbraced length, moment demand, and moment capacity (noting which limit state controls). Check deflection for both live load and total load. Where the compression flange is unbraced, document the bracing points and the LTB capacity.

Related AISC 360 requirements

Chapter E covers compression member design. Chapter H covers combined axial and flexural loads (beam-columns). Section J10.1 covers concentrated forces on beam flanges and webs at support points and load application points.

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Related sections

E1Compression Member DesignAISC 360-22J2.2Welded ConnectionsAISC 360-22J3.1Bolted ConnectionsAISC 360-22