Steel Member (Design Only - old AS 4100:1998)

How it calculates

The Steel Member (Design Only) calculator to AS 4100:1998 accepts analysis forces directly - axial load, biaxial bending moments, and shear - and checks the selected Australian steel section against all relevant member capacity limit states.

Section classification

Before capacity checks run, the calculator classifies each plate element of the cross-section (flange and web) as compact, non-compact, or slender using the slenderness limits from AS 4100 Table 5.2. The section slenderness determines which effective section properties apply to bending and local buckling checks. The web and flange are always referred to by reference to the X-axis orientation, regardless of the actual bending axis, so the terminology stays consistent across all section orientations.

Bending capacity and lateral-torsional buckling

The nominal section moment capacity Ms is calculated as the lesser of the first yield moment and the plastic moment, scaled by the section form factor. For bending about the X axis (major axis), the calculator determines the member moment capacity Mbx accounting for lateral-torsional buckling. The reference buckled moment Mo is derived from the geometry of the segment between lateral restraints - using the segment length Ls, the twist restraint factor kt, the load height factor kl, and the lateral rotation restraint factor kr (AS 4100 Clause 5.6). The slenderness reduction factor is then applied to give Mbx, with the moment modification factor αm amplifying capacity when the bending moment is non-uniform along the segment.

Compression and flexural buckling

Compression capacity checks follow AS 4100 Section 6. An effective area reduction factor is applied for slender sections. The member slenderness ratio Le/r is computed for both major- and minor-axis buckling using the effective lengths set by the restraint conditions. The section compression capacity is reduced by the member slenderness reduction factor αc (AS 4100 Clause 6.3.3) to give the nominal member capacity Nc. Residual stress category, as assigned by section type, influences the imperfection factor used in αc.

Tension capacity

Tension capacity follows AS 4100 Section 7. The calculator checks the gross section yield capacity and, where holes are declared, the net section fracture capacity. By default the net area is assumed equal to the gross area with the maximum code-permitted holes.

Shear capacity

Web shear capacity follows AS 4100 Section 5.11. For compact webs the capacity is the full plastic shear capacity of the web element. For slender webs the shear buckling reduction applies. Shear capacity is checked independently, and bending-shear and bending-bearing interaction checks are applied when significant shear or bearing coincides with high moment.

Combined actions interaction equations

The critical output is the combined actions check from AS 4100 Section 8. For members carrying both axial compression and bending:

In-plane interaction ratio = N*/φNs + Mx*/φMix ≤ 1.0

Out-of-plane interaction ratio = N*/φNcx + Mx*/φMbx ≤ 1.0

Where N* is the design axial force, Ns the section axial capacity, Ncy the member buckling capacity about the minor axis, and Mix the in-plane member moment capacity. For biaxial bending, an additional interaction check combines moments about both axes at the section level. Each interaction ratio is shown with the governing equation reference, making it straightforward to see which limit state controls and by how much capacity remains.

βm and stability factors

The moment distribution factor βm and the amplification factor δs for compression-bending interaction are entered manually by the engineer, as they depend on the broader frame context and moment diagram shape. The calculator provides clear input fields for these values with code guidance notes.