Steel Member (Design Only - LRFD, AISC 360-16)

How it calculates

The Steel Member (Design Only - LRFD, AISC 360-16) calculator takes factored design forces from analysis and checks a selected US steel section against all member limit states defined in AISC 360-16.

Section classification

Plate elements of the cross-section - flanges and web - are classified as compact, non-compact, or slender using the limiting width-to-thickness ratios from AISC 360-16 Table B4.1. Classification affects which capacity equations apply for flexure (Chapter F) and compression (Chapter E). Slender elements trigger effective area reductions and modified LTB provisions.

Flexural capacity and lateral-torsional buckling

For doubly-symmetric I-shaped members, the nominal flexural strength Mn follows Chapter F. In the plastic range (Lb ≤ Lp), Mn = Mp. Between Lp and Lr, LTB reduces Mn linearly. Beyond Lr, the elastic LTB equation governs. The calculator computes Lp and Lr from the section's torsional and warping properties. The Cb factor for non-uniform moment is calculated from the moment diagram, increasing capacity where the moment is lower along the unbraced length.

For HSS, pipes, channels, and other section types, the applicable Chapter F sub-provisions apply automatically, including flange local buckling and web local buckling reductions for non-compact and slender sections.

Compression and column buckling

Chapter E compression checks cover flexural buckling about both axes, torsional buckling, and flexural-torsional buckling as applicable to the section type. The critical stress Fcr is determined from the effective slenderness ratio KL/r. The calculator uses:

φcPn = φc × Fcr × Ag (for compact sections)

For slender elements, Fcr is computed using the effective area Aeff reduced by the effective width method per Section E7.

Combined axial and bending interaction

The Chapter H interaction equations are the primary pass/fail output:

For high axial load (Pr/Pc ≥ 0.2):

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

For low axial load (Pr/Pc < 0.2):

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

Where Pr is the required axial strength, Pc the available axial strength, Mrx and Mry the required flexural strengths, and Mcx and Mcy the available flexural strengths including LTB reductions. Both ratios are shown with full code references.

P-delta effects

The calculator applies a first-order moment amplification factor to account for P-little delta effects within the member, consistent with the assumption that the member is part of a braced frame. Engineers designing sway frames should verify that second-order effects are addressed at the frame level before using this calculator.

Shear

Chapter G shear checks determine the nominal shear strength Vn from the shear area and the web shear coefficient Cv. For compact webs Cv = 1.0 and the full plastic shear capacity is available. For slender webs, tension-field action may be considered if applicable.