Scope of AS/NZS 4600:2018

AS/NZS 4600 covers the design of structural members cold-formed from carbon and low-alloy steel sheet, strip, plate, or bar, with thicknesses not exceeding 25 mm. The standard applies to structural members used in buildings and other structures, including purlins, girts, wall studs, floor joists, roof trusses, and portal frames.

The standard uses the limit states design (LSD) philosophy consistent with AS 4100, but the capacity equations differ fundamentally because of the thin-walled behaviour.

Key Differences Between AS 4100 and AS/NZS 4600

Aspect AS 4100 (Hot-Rolled) AS/NZS 4600 (Cold-Formed)
Local buckling Section classification (Table 5.2) Effective width method (Clause 2.2)
Distortional buckling Not addressed Explicit design check (Clause 3.3)
Post-buckling reserve None for slender sections Accounted via effective width
Torsional buckling Rarely governs for UB/UC Frequently governs for channels and Z's
Yield strength increase Not applicable Cold-work increase per Clause 2.1.3
Section thickness range > 3 mm (generally) 0.5 - 25 mm
Tolerances AS/NZS 3679.1 AS/NZS 4600 Appendix A

Common Cold-Formed Section Types in Australian Practice

Lipped Channel (C-Section)

Used for purlins, girts, wall studs, and floor joists. The lip stiffens the flange against local buckling and increases the distortional buckling resistance. Standard Australian sizes range from C75 to C350 with thicknesses from 0.75 mm to 3.0 mm.

Key design considerations:

Z-Section (Zed)

Most commonly used for roof purlins in Australian metal building systems. Z-sections exhibit monosymmetry about the minor principal axis, which affects the lateral-torsional buckling behaviour. The principal axes are rotated relative to the geometric axes.

Key design considerations:

Top Hat Section

Used for battens, furring channels, and light-gauge framing. The outward-turned top flanges provide a nailing surface for sheathing.

Hollow Flange Beam (HFB) / LiteSteel Beam (LSB)

Australian innovation: cold-formed sections with hollow rectangular flanges connected by a slender web. Combines the efficiency of cold-forming with improved torsional stiffness. Design guide: AS/NZS 4600 with specific provisions for hollow flanges.

Effective Width Method — Clause 2.2

When a thin plate element in compression buckles locally, it does not lose all load-carrying capacity. The post-buckling reserve strength is captured by the effective width concept: after local buckling, the central portion of the plate sheds load to the stiffer edge regions, and the plate behaves as though only the effective width is available.

The effective width b_e for a stiffened element (supported along both longitudinal edges) under uniform compression is:

For lambda <= 0.673: b_e = b (full width effective, no local buckling)

For lambda > 0.673: b_e = rho x b

where rho = (1 - 0.22/lambda) / lambda <= 1.0

and lambda = sqrt(f* / f_ol), where f* is the design stress in the element and f_ol is the elastic local buckling stress.

The elastic local buckling stress is:

f_ol = k x pi^2 x E / (12(1-nu^2)) x (t/b)^2

where k is the plate buckling coefficient (k = 4.0 for a stiffened element, k = 0.425 for an unstiffened element, k = 1.277 for a lip-stiffened flange).

Distortional Buckling — Clause 3.3

Distortional buckling is a cross-section distortion mode where the flange-lip assembly rotates about the flange-web junction. Unlike local buckling, which involves plate bending without junction translation, distortional buckling involves both plate bending and movement of the flange-web or flange-lip junction.

The distortional buckling capacity is:

For lambda_d <= 0.561: M_bd = M_y (full capacity, no distortional buckling)

For lambda_d > 0.561: M_bd = M_y x (1 - 0.25/lambda_d) / lambda_d

where lambda_d = sqrt(M_y / M_od), and M_od is the elastic distortional buckling moment.

Distortional buckling is particularly important for:

Cold-Work Strength Increase — Clause 2.1.3

Cold-forming increases the yield strength of the steel in the corners (bends) and, to a lesser extent, in the flat portions of the section. The average increased yield strength f_ya may be used in design:

f_ya = f_yb + (n x C x (f_uv - f_yb) x A_corners) / A_total

where:

For typical Australian cold-formed steel with Grade 300 or Grade 350 base metal, the strength increase is 5-15% depending on the number of corners and the section geometry.

Worked Example: Lipped Channel Beam in Bending

Problem: A C200-15 lipped channel in Grade 350 steel (f_yb = 350 MPa, f_uv = 480 MPa) spans 4.5 m as a simply supported roof purlin. The section has dimensions: web depth d = 200 mm, flange width b_f = 75 mm, lip length b_l = 15 mm, thickness t = 1.5 mm. Inside bend radius r_i = 2.0 mm. Check the section moment capacity including local and distortional buckling.

Given section properties (computed):

Solution:

Step 1: Cold-work strength increase

C = 0.75 (r_i/t = 2.0/1.5 = 1.33, from Table 2.1.3) A_corners ~ 4 x pi/2 x (r_i + t/2) x t = 4 x 1.571 x (2.0 + 0.75) x 1.5 = 25.9 mm^2 A_total ~ 575 mm^2 (from manufacturer tables)

f_ya = 350 + (4 x 0.75 x (480 - 350) x 25.9) / 575 = 350 + (4 x 0.75 x 130 x 25.9) / 575 = 350 + 17.6 = 367.6 MPa

Step 2: Flange effective width (local buckling check)

Flange is a stiffened element (supported at web and at lip). Buckling coefficient k = 4.0 for the stiffened compression flange.

Elastic local buckling stress: f_ol = 4.0 x pi^2 x 200000 / (12(1 - 0.3^2)) x (1.5/75)^2 = 4.0 x 1.806 x 10^5 x 0.000400 = 289 MPa

Design stress f* = f_ya = 367.6 MPa

lambda = sqrt(367.6 / 289) = sqrt(1.272) = 1.128 > 0.673

rho = (1 - 0.22/1.128) / 1.128 = 0.805 / 1.128 = 0.714

Effective flange width: b_e = 0.714 x 75 = 53.5 mm

Step 3: Effective section modulus

The ineffective portion of the compression flange (75 - 53.5 = 21.5 mm) is deducted from the section. Recalculating the section properties with the reduced flange gives:

Z_eff ~ 0.88 x Z_x = 0.88 x 47.8 x 10^3 = 42.1 x 10^3 mm^3

Step 4: Distortional buckling check

For a lipped channel, the elastic distortional buckling stress f_od must be calculated using a rational buckling analysis (e.g., finite strip method, or the simplified method in AS/NZS 4600 Appendix D). For this C200-15 section, assume f_od = 310 MPa from finite strip analysis.

lambda_d = sqrt(367.6 / 310) = 1.089 > 0.561

Capacity reduction = (1 - 0.25/1.089) / 1.089 = 0.769

M_bd = 367.6 x 42.1 x 10^3 x 0.769 x 10^(-6) = 11.9 kNm

Step 5: Design moment capacity

phi = 0.90 (bending, cold-formed)

phi M_bd = 0.90 x 11.9 = 10.7 kNm

The capacity is governed by the interaction of local buckling (reducing Z to Z_eff = 0.88 Z) and distortional buckling (further reducing to 0.769 of the local-buckling capacity).

Result: Design moment capacity phi M_bd = 10.7 kNm. Both local and distortional buckling are active limit states.

Frequently Asked Questions

What is the main standard for cold-formed steel in Australia?

AS/NZS 4600:2018 (Cold-formed steel structures) is the primary standard for cold-formed steel design in Australia and New Zealand. It covers structural members cold-formed from steel sheet, strip, plate, or bar up to 25 mm thickness. Hot-rolled steel members are governed by AS 4100, not AS/NZS 4600, even if they are subsequently cold-formed into a different shape.

What is the difference between local buckling and distortional buckling in cold-formed sections?

Local buckling involves plate bending of individual elements (flanges, webs) without movement of the junctions between elements. It is governed by the plate width-to-thickness ratio and is analogous to flange/web local buckling in hot-rolled sections. Distortional buckling involves rotation of the flange-lip assembly about the flange-web junction, with translation of the junction. It occurs at a longer half-wavelength than local buckling and is unique to cold-formed sections with edge stiffeners.

How does the effective width method differ from AS 4100 section classification?

AS 4100 classifies the entire section as compact, non-compact, or slender, and selects a capacity formula accordingly. AS/NZS 4600 uses the effective width method, where each plate element is individually reduced to its effective width based on the design stress in that element. The section properties are then recomputed using the reduced (effective) section. This accounts for the progressive nature of local buckling in thin-walled sections and captures the post-buckling strength reserve.

Does cold-forming increase the steel yield strength?

Yes. The cold-working during section forming increases the yield strength of the steel, particularly in the corner regions (bends), through strain hardening. AS/NZS 4600 Clause 2.1.3 permits the designer to use an average increased yield strength f_ya that accounts for this cold-work effect. The increase is typically 5-15% for standard C and Z sections, and can be up to 25% for sections with many bends (e.g., top-hat sections). The increased yield can be used for all capacity checks except where the increased yield is locally reduced by welding.

What bridging is required for cold-formed Z-section purlins?

Z-section purlins require intermediate bridging to restrain lateral-torsional buckling. Standard practice in Australian metal building construction is to provide bridging lines at 1/3 and 2/3 of the span for purlins up to 9 m, and at 1/4, 1/2, and 3/4 for purlins longer than 9 m. The bridging must be designed to resist 2% of the compression flange force per AS/NZS 4600 Clause 4.4 as a minimum lateral restraint force. Bridging members are typically cold-formed channels or angles bolted to the purlin webs.

Educational reference only. All design values must be verified against the current edition of AS/NZS 4600:2018 and the project specification. This information does not constitute professional engineering advice. Always consult a qualified structural engineer for design decisions.

Disclaimer: This content is for educational purposes only. Results must be verified by a licensed professional engineer. Steel Calculator provides preliminary design tools — NOT a substitute for professional engineering judgment.