Steel Member (Design Only)

How it calculates

The Steel Member (Design Only) calculator to AS 4100:2020 accepts analysis forces directly and evaluates all member capacity limit states for the selected Australian steel section. It is designed for design-only use: engineers provide forces from their own analysis model or from a linked analysis calculator in Calcs.com.

Section classification

Plate elements - flanges and webs - are classified using the element slenderness ratios and the compact and non-compact limits from AS 4100 Table 5.2. Compact sections can reach the full plastic moment. Non-compact sections are limited by first yield with an effective section modulus. Slender sections use an effective section with reduced plate areas. The "flange" and "web" terminology always refers to the element orientation if the member were bent about its X-axis, regardless of the actual bending direction.

Bending capacity and lateral-torsional buckling

Section moment capacity Ms is the lesser of the yield moment and plastic moment, scaled by the section form factor kf. Member moment capacity Mbx for major-axis bending accounts for lateral-torsional buckling between restraint points. The reference buckling moment Mo is computed from:

• The segment length Ls between lateral restraints
• Twist restraint factor kt (accounts for end connection stiffness)
• Load height factor kl (accounts for load application point relative to shear centre)
• Lateral rotation restraint factor kr (accounts for restraint against lateral rotation at ends)

The slenderness reduction factor αs is then applied to give the member moment capacity Mbx, modified by the moment modification factor αm for non-uniform bending diagrams. For minor-axis bending, lateral-torsional buckling does not apply and the section capacity governs directly.

Compression and flexural buckling

Section 6 compression checks apply. An effective area reduction factor kf is applied for sections with slender plate elements. The member compression capacity Nc is derived from the section compression capacity Ns reduced by the member slenderness reduction factor αc (Clause 6.3.3). The slenderness ratio is computed for both major and minor axis using the effective lengths:

αc depends on the section slenderness and the member slenderness ratio λn = (Le/r) × sqrt(kf × fy / 250)

Residual stress category (hot-rolled, welded, etc.) influences the imperfection factor used in αc.

Tension capacity

Section 7 tension checks cover the gross section yield capacity (Ag × fy) and, where holes or reduced net sections are present, the net section fracture capacity (0.85 × kt × An × fu). By default the net area equals the gross area assuming maximum permissible holes.

Shear capacity

Section 5.11 shear checks apply. For compact webs (dp/tw ≤ 82/sqrt(fy/250)) the full plastic shear capacity governs. For slender webs, shear buckling reduces the capacity using the shear buckling coefficient. Bending-shear interaction and bending-bearing interaction checks are applied when significant shear or bearing coincides with high moment.

Combined actions interaction equations

The critical check is from AS 4100 Section 8. For members with combined compression and bending:

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

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

Where N* is design axial force, Ns is section axial capacity, Ncy is the minor-axis buckling capacity, Mix is the in-plane member moment capacity, and Mbx is the lateral-torsional buckling member moment capacity. The βm distribution factor and δs amplification factor are entered manually by the engineer based on the moment diagram shape and frame context. Both interaction ratios are shown with full code references.