Steel Column (AS 4100:1998)

How it calculates

The Steel Column (AS 4100:1998) calculator designs hot-rolled steel columns and posts using limit state design per AS 4100:1998 (R2016). It runs a first-order elastic analysis per Clause 4.4.2 to determine demands, then applies AS 4100:1998 capacity equations for all relevant limit states.

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 based on the maximum individual demands in each span regardless of load case.

Axial section capacity

Section capacity in compression is checked per AS 4100:1998 Section 6:

utilization = N / (phi * Ns) ≤ 1.0*

where phi = 0.90 and Ns = kf * An * fy. The form factor kf accounts for local buckling of slender plate elements in the cross-section.

Member buckling capacity

Compression member buckling capacity is checked per AS 4100:1998 Section 6:

utilization = N / (phi * Nc) ≤ 1.0*

Nc is determined from the member slenderness reduction factor alphac, which depends on the modified slenderness lambda_n and the member section constant alphab from Table 6.3.3. Major- and minor-axis effective lengths are entered separately to capture the governing buckling axis.

Flexural capacity

Moment member capacity (lateral-torsional buckling) is checked per AS 4100:1998 Section 5:

utilization = |M| / (phi * Mb) ≤ 1.0*

Mb is determined from the reference buckling moment Ms and the slenderness reduction factor alphas. Flange and web compactness classifications govern whether the full plastic, compact, or slender section capacity applies.

Combined axial and bending

Combined axial compression and biaxial bending is checked per AS 4100:1998 Section 8 interaction equations. The interaction checks account for amplified moments where first-order analysis is used for design.

Outputs

Results are displayed as colour-coded utilization ratios for each limit state with AS 4100:1998 clause references. Section properties, governing demands, and capacity factors are tabulated for report documentation.