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AS 4100 (Steel structures): the design code every steel beam sits under

AS 4100 is the Australian Standard for designing structural steelwork: ultimate limit state, member capacity, connections. Engineer-only design; every drawing cites it.

By Dan Waring. Last updated 15 May 2026. Verified 15 May 2026.

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In plain English

AS 4100, Steel structures, is the Australian Standard that sets the design rules for structural steelwork in buildings: how to size a steel member, how to design a connection (bolted, welded, pinned), how to check stability against buckling, and how to verify the structure works under both serviceability (deflection, vibration) and ultimate limit state (capacity, collapse) load combinations. The current edition is AS 4100:2020 (verified 2026-05-15), which superseded AS 4100-1998 plus its amendments. AS 4100 is the steel equivalent of AS 3600 for concrete and AS 1684 / AS 1720 for timber framing.

For a builder, AS 4100 is engineer-only territory: you do not size beams from this standard, your structural engineer does. But every set of structural drawings on a residential job with steel in it cites AS 4100 directly or indirectly, and the connections shown on the drawings are designed under it. The builder’s job is to construct exactly to those drawings.

What it requires

For the engineer designing a steel member or connection, AS 4100 imposes (selectively, this is not exhaustive):

  1. Limit state design. Every member must be checked against ultimate limit state load combinations (factored loads) and serviceability limit state combinations (working loads with deflection limits). Permissible stress design is no longer the standard.
  2. Member capacity calculations. For each member: tension capacity (s.7), compression capacity with buckling (s.6), bending capacity with lateral-torsional buckling (s.5), shear capacity (s.5.11), bearing capacity (s.5.13), and combined-action checks (s.8).
  3. Connection design. Bolted connections under s.9, including bolt class (4.6, 8.8), pretension or snug-tight, bearing or friction type. Welded connections under s.9.6, sized to weld category and parent material strength.
  4. Stability and bracing. Frame stability (s.4), member bracing requirements, sway sensitivity (P-delta check) for portal frames and moment-resisting frames.
  5. Material specification. Members must be supplied to AS/NZS 3679.1 (hot-rolled) or AS/NZS 3679.2 (welded sections); plate to AS/NZS 3678; hollow sections to AS/NZS 1163. Standard grade is 300 MPa minimum yield (Grade 300).

What it doesn’t cover

  • Fabrication and erection workmanship. That sits in AS/NZS 5131 (verified 2026-05-15). AS 4100 sets the design; AS/NZS 5131 sets how the fabricator works the steel.
  • Welding procedure and qualification. That sits in the AS/NZS 1554 series. AS 4100 calls them up; the fabricator’s welders work to them.
  • Cold-formed steel framing. Light-gauge cold-formed sections (residential studs, batten rails) are covered by AS/NZS 4600, not AS 4100. Hot-rolled sections are AS 4100 territory.
  • Concrete-encased composite beams. Composite construction sits under AS/NZS 2327 (Composite structures), which calls up AS 4100 for the steel component but adds the composite-action design.
  • Member sizing tables for builders. AS 4100 is a design code, not a span table. The builder reads the engineer’s mark-up, not the standard.

Practical implications

  • Every steel member on the drawings has been designed to AS 4100. When a drawing calls for “200 UB 25” at a lintel, the engineer has verified bending capacity, deflection, lateral-torsional buckling, end bearings, and connection design under AS 4100. Substituting a different section without engineer’s mark-up is a structural defect that fails frame inspection.
  • Connections matter more than the section. Most steel beam failures are connection failures, not member failures. The bolt class, plate thickness, weld category and edge distance shown on the drawings are not negotiable. Do not improvise a connection on site.
  • Fabricator certification cascades up the chain. Steel for Class 2 (apartment) and bigger projects must be fabricated by an AS/NZS 5131-certified fabricator at the appropriate construction category (CC1, CC2, CC3, CC4). On Class 1a residential the construction category requirement is lighter, but the design under AS 4100 still applies.
  • AS 4100:2020 introduced fatigue updates and brought specification into closer alignment with the NCC 2022 Part B.1 referenced documents list. Older AS 4100-1998 designs are still valid for already-approved work; new work under NCC 2022 uses AS 4100:2020.
  • Cold-formed and hot-rolled are not interchangeable. A residential “steel frame” (cold-formed) is under AS/NZS 4600, not AS 4100. A “structural steel beam” (e.g. 200 UB 25) is under AS 4100. Specifying the wrong section type is a common drafting error.

References

See also

Last updated: 2026-05-15. Verified: 2026-05-15. Quarterly review for currency.

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