Steel Beam Flexural Capacity (AISC 360 Ch. F, Compact + Braced)
Whether a compact, laterally-braced steel W-shape carries the moment: Mn = Mp = Fy x Zx, with the ASD allowable Mn/1.67 and the LRFD design 0.90 Mn. A W18x50 in A992 reaches 421 kip-ft nominal, 252 allowable, 379 design - the numbers a detailer reads off AISC Manual Table 3-2. The shape (Zx), not the grade, sizes the beam: the same steel in a section a fifth the weight carries barely a third of the moment. Compact + braced plastic plateau only (no LTB or slender-element reductions). A design aid, not a substitute for the engineer of record.
Formula and source
Mp = Fy x Zx (kip-in); Mn = Mp / 12 (kip-ft); Ma = Mn / 1.67 (ASD); phi_b Mn = 0.90 Mn (LRFD); util = Mu / Ma.
AISC 360-22 Chapter F (F2.1) and §B3.1 / §B3.2, by name; Zx is read from the AISC Steel Construction Manual for the actual shape.
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