NZS 3404 shares its member moment capacity formula almost verbatim with AS 4100 — both use the identical αs = 0.6[√((Ms/Mo)²+3) − Ms/Mo] reduction factor with its built-in ceiling, a reminder that the Australia/New Zealand joint standards tradition runs deep in steel design.
Every check above runs automatically inside the Structyze Steel Beam module, free to try online — change any input and every downstream number recalculates instantly, with the governing code clause shown next to each result.
The Example We'll Use
- Span 6 m, unbraced length 0.5 m
- Section 400 × 140 × 16 × 10 mm
- Superimposed dead 2 kN/m, live 5 kN/m
- Material AS/NZS 3679.1 Grade 300, fy = 300 MPa
- Deflection limit L/360 (floor, typical)
Working Through It
Factored load w* = max(1.35G, 1.2G+1.5Q) = 10.654 kN/m governs from the live-load-inclusive combination, giving M* = 47.94 kN·m. Section moment capacity Ms = Zpfy ≈ 359.6 kN·m.
The elastic reference moment Mo, built from Iy, J, Iw and the short 0.5 m unbraced length, comes out enormous at ≈ 11,244 kN·m — giving Ms/Mo = 0.032 and αs ≈ 1.020.
With αm = 1.13 for the simply-supported UDL case, Mb = 1.13(1.020)(359.6) ≈ 414.7 kN·m — exceeding Ms, so the Mb ≤ Ms cap governs and φMb = 0.9(359.6) ≈ 323.6 kN·m is the final capacity.
What Governs, and Why
Deflection and bending run within a similar range (utilisation 0.187 vs 0.148), both well ahead of shear's loose 0.049 margin — the identical pattern this series finds for AS 4100 on the same geometry, since NZS 3404's formula set is drawn from the same joint AS/NZS development. With le = 0.5 m, this beam's bending reserve is set entirely by section yielding, not lateral-torsional reduction.
Where Structyze Takes Over
Every check above runs automatically inside the Structyze Steel Beam module, free to try online — change any input and every downstream number recalculates instantly, with the governing code clause shown next to each result.
The free trial covers a curated set of flagship modules including this one. For the full catalogue — RCC, Steel, Composite, Timber, Masonry and more, across 20+ design codes — buy the full Structyze desktop app or see everything it does on the Software page.
Quick FAQ
Why are NZS 3404 and AS 4100 so similar?
Both standards share a common development history under the Australia/New Zealand joint standards framework for structural steel — the member capacity formulas, including the αs lateral-torsional buckling reduction factor, are essentially identical between the two, though each retains its own clause numbering and national-specific provisions elsewhere.
Where would NZS 3404 and AS 4100 actually diverge?
Load combinations (both reference AS/NZS 1170.0, so these typically match too), but seismic design provisions, some connection detailing rules, and material availability/grade conventions can differ meaningfully between the two countries even when the core member-capacity mechanics stay aligned.
Final Thoughts
Seeing NZS 3404 and AS 4100 produce an identical numeric result on the same beam is a useful confirmation of just how closely the two standards' member-capacity provisions track each other.