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

How it calculates

The Steel Member (Design Only - ASD, AISC 360-16) calculator takes ASD (service-level) design forces from analysis and evaluates all member capacity limit states using allowable strength design safety factors per AISC 360-16.

ASD safety factors

In ASD, available strengths are divided by a safety factor Omega (typically 1.67 for tension yielding, 1.67 for flexure, 1.67 for compression). The demand-to-capacity ratio is expressed as Pa/Pc (axial), Ma/Mc (flexure), or Va/Vc (shear), where each allowable value equals the nominal strength divided by the appropriate Omega factor. All interaction equations use these ASD available strengths.

Section classification

Plate element slenderness ratios are compared to the limiting values from AISC 360-16 Table B4.1. Compact, non-compact, and slender classifications are applied automatically. Slender elements trigger effective area reductions for compression and modified flexural equations for bending.

Flexural capacity and lateral-torsional buckling

Chapter F provisions determine the nominal flexural strength Mn. For doubly-symmetric I-shapes, the plastic moment Mp is the upper limit. Lateral-torsional buckling reduces Mn when Lb exceeds Lp. The calculator computes:

• Lb ≤ Lp: no LTB reduction, Mn = Mp
• Lp < Lb ≤ Lr: linear LTB interpolation
• Lb > Lr: elastic LTB governs

The Cb factor for non-uniform moment is calculated from the moment diagram along the unbraced length, increasing allowable moment where demand is lower. HSS, pipe, channel, and angle sections use the applicable sub-sections of Chapter F.

Compression and column buckling

Chapter E compression checks cover flexural buckling about both axes and torsional or flexural-torsional buckling as required by section type. The slenderness ratio KL/r is evaluated for each axis and the critical stress Fcr is used to compute the nominal compression strength Pn. The ASD allowable compression is:

Pc = Pn / Omega_c (where Omega_c = 1.67)

For slender sections, an effective area Aeff accounts for local buckling.

Combined actions interaction

Chapter H ASD interaction equations form the core output. For high axial ratio (Pa/Pc ≥ 0.2):

interaction ratio = Pa/Pc + (8/9)(Max/Mcx + May/Mcy) ≤ 1.0

For low axial ratio (Pa/Pc < 0.2):

interaction ratio = Pa/(2Pc) + (Max/Mcx + May/Mcy) ≤ 1.0

Each term breaks down the axial and bending contributions, making it easy to see which drives the design. The governing equation is identified and all required and available strengths are shown alongside the code clause.

P-delta and stability

A first-order moment amplification factor is applied within the member for P-delta effects, consistent with the assumption of a braced frame. Frame-level second-order effects should be verified separately before using this calculator for sway-sensitive structures.

Shear

Chapter G shear provisions determine the allowable shear force. The web shear coefficient Cv is used to compute Vn, and the ASD allowable shear is Vc = Vn / Omega_v (Omega_v = 1.67). For compact webs Cv = 1.0.