Steel Column (LRFD, AISC 360-16)

How it calculates

The Steel Column (LRFD, AISC 360-16) calculator designs hot-rolled steel columns and posts using Load and Resistance Factor Design. It runs a first-order structural analysis to determine demands, then applies AISC 360-16 capacity equations for all relevant limit states.

Structural analysis

The calculator performs FEA on the column as a beam-column, resolving axial forces, moments, and deflections under applied loads. End conditions (pinned, fixed, roller) are specified at each support. Concentrated axial loads and distributed lateral loads can be applied at any height. A first-order moment amplification factor accounts for P-delta (P-little delta) effects, and the member is assumed to be part of a braced frame.

Axial compression capacity

Nominal compressive strength is determined per AISC 360-16 Chapter E:

utilization = Pu / (phi_c * Pn) ≤ 1.0

where phi_c = 0.90. The nominal strength Pn is governed by flexural buckling about the weak axis for most standard shapes, using the effective slenderness ratio KL/r and the critical stress Fcr. For singly-symmetric and unsymmetric sections, flexural-torsional buckling is also checked. Slender element reduction factors (Q) are applied where local buckling governs the section capacity.

Flexural capacity

When moments are present from eccentricity or lateral loads, flexural strength is checked per AISC 360-16 Chapter F:

utilization = |Mu| / (phi_b * Mn) ≤ 1.0

where phi_b = 0.90. The lateral-torsional buckling limit state governs for unbraced lengths exceeding Lp. Compact, non-compact, and slender web and flange classifications are evaluated per Table B4.1b.

Combined axial compression and bending

For members carrying both axial compression and bending the AISC H1-1 interaction equations are applied:

• For Pu / phi_c Pn ≥ 0.2: (Pu / phi_c Pn) + (8/9) * (Mux / phi_b Mnx + Muy / phi_b Mny) ≤ 1.0
• For Pu / phi_c Pn < 0.2: (Pu / 2 phi_c Pn) + (Mux / phi_b Mnx + Muy / phi_b Mny) ≤ 1.0

Both strong- and weak-axis moment demands are included, allowing the full biaxial bending case to be assessed.

Axial deformation

Axial shortening under the design load is calculated and reported alongside the capacity checks, allowing serviceability limits to be verified where column shortening is relevant to the structural system.

Outputs

Results are displayed as traffic-light utilization ratios for each limit state. Code clause references are shown for every check. The governing utilization ratio, critical load combination, and section properties are summarised at the top of the results panel.