Steel Member (Design Only - LRFD)

How it calculates

The Steel Member (Design Only - LRFD) calculator to AISC 360-22 accepts factored design forces from analysis and runs all LRFD member capacity checks for the selected US steel section.

Section classification

Plate element slenderness ratios (b/t for flanges, h/tw for webs) are compared to the limiting values from AISC 360-22 Table B4.1a (compression) and B4.1b (flexure). Compact, non-compact, and slender classifications are determined automatically for each element. Slender elements trigger effective width reductions for compression and modified flexural strength equations.

Flexural capacity and lateral-torsional buckling

Chapter F provisions govern flexure. For W-shapes and doubly-symmetric I-sections, the plastic moment Mp sets the upper bound. Lateral-torsional buckling reduces capacity when the unbraced length Lb exceeds Lp (onset of LTB). The calculator determines the limit states boundaries:

• Lb ≤ Lp: Mn = Mp (plastic, no LTB reduction)
• Lp < Lb ≤ Lr: linear LTB interpolation
• Lb > Lr: elastic LTB governs, Mn = Fcr × Sx

The Cb factor for moment gradient is computed from the moment diagram, amplifying capacity where the bending moment varies along the unbraced length. For HSS, pipes, channels, and single angles, the corresponding Chapter F sub-sections apply, including flange local buckling and lateral-torsional buckling provisions specific to each section type.

Compression and buckling

Chapter E covers flexural buckling about both principal axes and, for non-symmetric sections, flexural-torsional buckling. The slenderness ratio KL/r is evaluated for each axis. The critical stress Fcr is derived from:

φcPn = 0.9 × Fcr × Ag

For slender element sections, the effective area Aeff accounts for local buckling, and Fcr is computed on the basis of Aeff/Ag. Torsional and flexural-torsional buckling provisions apply for sections with low torsional stiffness such as double angles and tees.

Combined actions interaction

Chapter H interaction equations are the core result. For Pr/Pc ≥ 0.2 (high axial):

interaction ratio = Pr/Pc + (8/9)(Mrx/Mcx + Mry/Mcy) ≤ 1.0

For Pr/Pc < 0.2 (low axial):

interaction ratio = Pr/(2Pc) + (Mrx/Mcx + Mry/Mcy) ≤ 1.0

Each term is shown with its available and required strength values, so the relative contribution of axial and bending demands is immediately visible.

P-delta and stability

A first-order moment amplification factor is used to account for P-little delta effects within the member. The member is assumed to be part of a braced frame. Second-order effects at the frame level should be confirmed before applying this calculator.

Shear

Chapter G shear checks determine Vn from the product of the shear area and the shear coefficient Cv2. For compact webs Cv2 = 1.0. For slender webs the coefficient is reduced and tension-field action may be considered for interior panels.