Steel Column (AS 4100:2020)

How it calculates

The Steel Column (AS 4100:2020) calculator designs hot-rolled steel columns and posts using limit state design to AS 4100:2020. It runs a first-order elastic analysis per Clause 4.4.2 to determine demands, then checks capacity for each limit state.

Structural analysis

The calculator performs FEA on the column as a beam-column, resolving axial forces, bending 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. The member is assumed to be of uniform cross-section along its full length. Interaction limit states are conservatively evaluated at the maximum individual demand in each span regardless of load case.

Compression section capacity

The nominal section capacity Ns sets the upper limit on member capacity in compression and feeds the combined actions checks. It is not reported as a separate utilization.

Ns = kf * An * fy

Where the net area An is at least Ag * fy / (0.85 * fu), the gross area Ag is used instead. The form factor kf is the ratio of effective to gross area. It accounts for local buckling of slender plate elements, using effective flange and web widths, or an effective diameter for CHS.

Member buckling capacity

Compression member capacity is checked as:

utilization = N / (phi * Nc) ≤ 1.0*

where phi = 0.90 and Nc = alphac * Ns, not more than Ns. The member slenderness reduction factor alphac depends on the modified slenderness lambda_n and the member section constant alphab. Nc is calculated separately about the X and Y axes, using each axis's effective length and radius of gyration, and the lower value governs. Mono-symmetric sections such as channels are also checked for flexural-torsional buckling, using the AS 4600 method that AS 4100 refers to.

Flexural capacity

Moment member capacity (lateral-torsional buckling) is checked as:

utilization = |M| / (phi * Mb) ≤ 1.0*

Mb is the section moment capacity Ms multiplied by the moment modification factor alpham and the slenderness reduction factor alphas. It is not taken above Ms. Segments with full lateral restraint use Ms directly. Flange and web compactness classifications (compact, non-compact, slender) set the effective section modulus used for Ms. About the minor axis, the section moment capacity is used, and a biaxial bending check combines both axes.

Shear

Shear capacity is checked about each axis with phi = 0.90. At supports where the moment demand exceeds 0.75 of the factored section moment capacity, the shear capacity is reduced for shear-moment interaction.

Combined axial and bending

Combined axial compression and biaxial bending is checked against both section capacity and member capacity. Both major- and minor-axis moment demands are included. Moments are amplified before the interaction check. The braced amplification factor is calculated, and a sway amplification factor you enter applies to sway members and cantilevers. Where the braced factor exceeds 1.4, the calculator flags that a second-order analysis is needed outside Calcs.com.

Deflection

Lateral deflection about each axis is checked under short-term, long-term, and imposed load combinations. Each limit is the lesser of a span ratio and an absolute value you set. Axial extension is checked in the same way. Reported deflections do not include second-order effects.

Outputs

Results are displayed as color-coded utilization ratios for each limit state with AS 4100:2020 clause references. Section properties, capacity factors, governing demands, and design actions are tabulated for straightforward report documentation.