Cold-formed steel design (AS 4600)

CivilKit Studio checks cold-formed C and Z purlins and girts to AS/NZS 4600 using the Direct Strength Method (DSM) - automatically, the moment you assign a Lysaght section and solve.

When to use this

Cold-formed sections are the thin, roll-formed steel profiles used for roof purlins and wall girts - the C (Cee) and Z (Zed) shapes, as opposed to the thick hot-rolled beams and columns covered by the AS 4100 design checks. They are too thin to design with the AS 4100 rules: their slender walls can buckle locally well before the steel yields, so a different standard - AS/NZS 4600 - and a different method - the Direct Strength Method - apply. Studio recognises a cold-formed section and switches to AS 4600 for you; you do not pick the code by hand.

Assigning a Lysaght C or Z section

1. Open the member's inspector and choose Browse sections.
2. Pick from the Cold-Formed Channel (C) or Cold-Formed Zed (Z) families - the LYSAGHT purlin/girt range, designated like C15015 or Z20019 (depth + thickness). See the section library for how the browser works.
3. Solve (S). The member is now checked to AS 4600, not AS 4100.

How Studio knows it is cold-formed

You do not flag anything. A section is routed to AS/NZS 4600 automatically when it is a cold-formed catalogue entry - recognised by its family or by a Lysaght designation (a C or Z followed by five digits). Everything else (universal beams, channels, hollow sections) stays on AS 4100.

The AS 4600 check and what is reported

After you solve, read the results in the Design tab of the results panel, or select a member to see Result · AS/NZS 4600 design (DSM) in its inspector. When every steel member in the model is cold-formed, the tab itself is labelled AS/NZS 4600; in a mixed model, hot-rolled members are checked to AS 4100 and cold-formed members to AS 4600 side by side in the same table.

Output	Meaning
Class / DSM mode	The governing buckling mode for that member, shown as 4600·local, 4600·dist or 4600·global - what limits the section
φNc	Design compression capacity, kN (0.85 × the lowest of the three compression modes)
φMb	Design bending (moment) capacity, kN·m (0.90 × the lowest of the three bending modes)
N*	Design compression from the analysis, kN
M*	Design moment from the analysis, kN·m
Utilisation	The worst demand/capacity ratio, including the Cl 3.5 combined axial-plus-bending interaction
Status	OK (≤ 1) or OVER (> 1)

The demands N* and M* come straight from the analysis; the capacities come from the validated AS 4600 engine. As with AS 4100, you can filter to failures only and group by section to design to the worst member of each profile.

The DSM mode breakdown (local / distortional / global)

The Direct Strength Method works out a section's capacity from three ways a thin section can fail, then takes the weakest. For a non-engineer, picture a purlin under load:

• Local buckling - the flat faces (web, flange) ripple into small waves while the corners stay put. A thin, wide flat buckles first.
• Distortional buckling - the whole lipped flange rotates outward about the flange-to-web corner, like a hinge opening. This is unique to lipped cold-formed shapes and often governs purlins.
• Global buckling - the member as a whole bows sideways or twists over its length (column buckling, or lateral-torsional buckling in bending).

Studio computes all three for both compression and bending and reports the governing mode in the class column, so you can see at a glance whether a member is limited by a thin flat (local), a flapping lip (distortional) or its length and restraint (global). Open show the working on the governing member to read the clause-referenced DSM trace - the elastic buckling load for each mode (Nol, Nod, Noc and the bending equivalents) and the strength-curve capacity it produces, against Cl 7.2.1 (compression) and Cl 7.2.2 (bending). This working also flows into the PDF report.

Verified against Lysaght and CUFSM

The DSM capacities are cross-checked against Lysaght's published AS/NZS 4600 section capacities. The distortional result is additionally sanity-checked against the kind of invariants a CUFSM finite-strip analysis guarantees - for example, that distortional buckling only applies to lipped sections and that its buckling wavelength lands in the physically expected band. Live CUFSM runs remain the intended future gold-standard check.

The flange-restraint input

Purlins and girts are usually screwed to roof or wall sheeting, and that sheeting holds the compression flange straight - which removes lateral-torsional (global) buckling and lets the section carry more.

Tick Flange restrained (sheeting) in the member inspector when the compression flange is continuously restrained by fixed cladding. With it on, Studio suppresses the lateral-torsional mode and φMb rises towards the section capacity. Leave it off (the default) for an unrestrained span, which is conservative.

Be honest about restraint

Only tick this when the sheeting really is fixed to the compression flange for the full span. A wrongly ticked box overstates the bending capacity. Note also that the effective length otherwise defaults to the member length - no intermediate restraints (bridging/fly-bracing) are modelled yet.

Auto-sizing cold-formed members

Auto-size works on cold-formed members too: it searches the Lysaght C/Z range and picks the lightest section that keeps each member's AS 4600 utilisation within target, using the same DSM capacities (including the governing mode) as the on-screen check. Run it the same way as for hot-rolled steel.

The manual cold-formed calculator

Separately from the catalogue workflow above, the inspector carries a standalone Cold-formed steel (AS 4600) panel where you can type a C, Z or hollow section's dimensions (depth, flange, thickness, lip, yield) and read a quick capacity. This panel uses the simpler effective-width method (φNc = 0.85·fy·Aeff, φMs = 0.9·fy·Zeff) rather than the full DSM, and is handy for a one-off section that is not in the catalogue. For a designed member, prefer assigning a real Lysaght section and reading the DSM check.

Scope and assumptions (read before professional use)

This is an early, transparent slice - treat it as a first-pass check:

• Australia only. The cold-formed catalogue is the LYSAGHT Zeds & Cees range (16 Cees + 16 Zeds, G450 steel, fy = 450 MPa), from the June 2017 user guide. There is no US/EU cold-formed set yet, and the international steel checks do not cover cold-formed sections.
• Indicative section data. The published gross properties are cross-checked against independent calculators to within about 0.5%, but every value must be confirmed against a current manufacturer catalogue before professional use.
• Effective length defaults to the member's length - no intermediate restraints (bridging, fly-bracing) are modelled. φMb / φNc are conservative if the member is braced between ends.
• Combined actions use the AS/NZS 4600 Cl 3.5 linear N-M interaction.

Verify before professional use

Always confirm the section data, steel grade, restraints (including the flange sheeting tick), bearing/web-crippling detail and the governing load combination against AS/NZS 4600 and a current Lysaght catalogue before relying on these numbers.