Australian Cold-Formed Steel — AS/NZS 4600 Design Guide

Complete reference for AS/NZS 4600:2018 Cold-Formed Steel Structures — the joint Australian and New Zealand cold-formed steel design standard. Covers the effective width method for local buckling, distortional buckling, CFS section properties for C-and Z-section purlins and girts, screw connection capacity, welded connections, and a design example for a C-section purlin under gravity load.

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AS/NZS 4600 — Australian Cold-Formed Steel Standard

AS/NZS 4600:2018, jointly published by Standards Australia and Standards New Zealand, governs the design of cold-formed steel structural members. It is closely harmonised with the North American AISI S100:2016 and shares the same fundamental effective width methodology. The standard applies to structural members with thicknesses up to 12.7 mm — beyond this thickness, AS 4100 governs.

Cold-formed steel (CFS) differs fundamentally from hot-rolled AS 4100 steel design:

Cold-formed steel is widely used in Australian construction for:

• Roof purlins and side rails (C and Z sections)
• Steel wall frames in residential and commercial buildings
• Steel roof trusses in light industrial buildings
• Metal building secondary framing
• Steel decking for composite slabs (AS 2327)
• Storage racking and shelving systems

Effective Width Method (AS/NZS 4600 Clause 2.2)

Cold-formed steel sections have thin walls that buckle locally at stresses below the yield point. The effective width method accounts for this by reducing the width of slender compression elements to an effective width that, when stressed to the yield strength, carries the same load as the actual element in its post-buckled range.

Effective Width for Stiffened Elements (Clause 2.2.1.2)

For stiffened compression elements with both longitudinal edges supported (webs, flanges with edge stiffeners):

b = w                       when lambda ≤ 0.673
b = ρ × w                   when lambda > 0.673

where:
  ρ = (1 - 0.22 / lambda) / lambda
  lambda = (1.052 / √k) × (w/t) × √(f* / E)

  w   = flat width of element (mm)
  t   = base steel thickness (mm)
  f*  = design compressive stress (MPa)
  k   = buckling coefficient (k = 4.0 for stiffened elements)
  E   = Young's modulus (200,000 MPa)

For a C-section web 150 mm wide, t = 1.5 mm, fy = 550 MPa, f* = fy: lambda = (1.052 / √4.0) × (150/1.5) × √(550/200,000) = 0.526 × 100 × 0.0524 = 2.76

Since lambda = 2.76 > 0.673: ρ = (1 - 0.22/2.76) / 2.76 = (1 - 0.080) / 2.76 = 0.920 / 2.76 = 0.333 b = 0.333 × 150 = 50.0 mm

The web effective width is only 50 mm out of 150 mm — the section is in the highly slender range. This is typical of CFS sections where the full cross-section is rarely fully effective under pure compression.

Effective Width for Unstiffened Elements (Clause 2.2.1.3)

For unstiffened elements (one longitudinal edge free, such as the outstanding leg of a C-section lip):

k = 0.425 for unstiffened elements in uniform compression
lambda = (1.052 / √0.425) × (w/t) × √(f* / E)
ρ = (1 - 0.22 / lambda) / lambda     when lambda > 0.673

The lip of a C-section in compression is an unstiffened element. For a 20 mm lip (flat width 17 mm), t = 1.5 mm, fy = 550 MPa: lambda = (1.052/√0.425) × (17/1.5) × √(550/200,000) = 1.614 × 11.33 × 0.0524 = 0.958

Since 0.958 > 0.673: ρ = (1 - 0.22/0.958) / 0.958 = (1 - 0.230) / 0.958 = 0.770 / 0.958 = 0.804 b = 0.804 × 17 = 13.7 mm

Distortional Buckling (AS/NZS 4600 Clause 2.3)

Distortional buckling is a unique CFS failure mode where the flange and lip assembly rotates about the flange-web junction. It is distinct from local buckling and flexural-torsional buckling. AS/NZS 4600 Clause 2.3 provides the Direct Strength Method (DSM) for distortional buckling:

phi-Nd = phi × Ae × fy

where:
  phi = 0.85
  Ae  = effective area accounting for distortional buckling

The nominal distortional buckling stress f_d is calculated using the finite strip method (programs like CUFSM or THIN-WALL) or using simplified DSM equations:

f_d = (lambda-d^(-0.6)) × fy    (empirical DSM expression)

where lambda-d = √(fy / f_cr,d)
      f_cr,d = elastic distortional buckling stress

For typical Australian C-section purlins (C20015, C25019, C30025), distortional buckling governs for intermediate span lengths where the section is neither fully braced against lateral buckling nor so short that local buckling dominates. The distortional buckling capacity is typically 60-80% of the section capacity for unrestrained flanges.

Simplified Distortional Buckling Check

For standard C-sections, AS/NZS 4600 permits a simplified distortional check based on the flange and lip geometry:

• A stiffener lip depth of at least 5t is required for the section to be considered fully effective against distortional buckling
• Lip depth of 5-8t provides partial restraint
• Lip depth greater than 10t provides full restraint (local buckling governs instead)

Australian C-sections typically have 15-25 mm lips (approx. 10-15t), making them fully effective against distortional buckling for standard applications.

CFS Section Properties — Australian Sections

Australian cold-formed steel sections are typically roll-formed from G550 or G500 steel (AS 1397) with zinc/aluminium-zinc coating. Common section types include:

C-Section Purlins

Z-Section Purlins

Z-sections are lipped sections with unequal top and bottom flange widths, designed for nesting and lapping at supports:

Z-sections are preferred for continuous span purlin systems because the nesting geometry allows simple lapped connections at supports, providing semi-continuity.

Screw Connection Capacity (AS/NZS 4600 Clause 5.4)

Screw connections are the primary fastening method for CFS. AS/NZS 4600 Clause 5.4 provides design capacities for self-drilling and self-tapping screws:

Screw Shear Capacity

phi-Vs = phi × min(Vs, Vu, Vb, Vc)

where:
  phi = 0.65 (capacity factor for screw connections)
  Vs  = screw shear capacity per manufacturer data
  Vu  = ply bearing capacity = 3.2 × t × d × fu
  Vb  = tilting capacity (thin ply — screw tilts in connected sheet)
  Vc  = end distance capacity = t × e × fu

Screw Tension Capacity

phi-Nt = phi × min(N_thread, N_pullover)

Where:

• N_thread is the screw thread stripping capacity
• N_pullover is the sheet pull-over capacity (the screw head pulls through the sheet)

For typical Australian CFS connections (G550 steel, 12-14 gauge hex head screws):

• 12g screw in 1.5 mm sheet: phi-Vs ≈ 2.5-3.5 kN per screw
• 14g screw in 2.4 mm sheet: phi-Vs ≈ 5.0-7.0 kN per screw
• Minimum edge distance: 3 × screw diameter
• Minimum spacing: 3 × screw diameter (within a group)

Screw Spacing Requirements

AS/NZS 4600 minimum spacing requirements:

Diaphragm Action in CFS Construction

Cold-formed steel roof and floor diaphragms provide lateral load resistance through shear action of the steel decking acting as a deep beam. AS/NZS 4600 Clause 5.5 provides design provisions for steel diaphragms:

Diaphragm shear strength depends on:

1. Deck profile — trapezoidal or re-entrant profiles provide different shear capacities
2. Fastener pattern — screw spacing at sheet edges and at supports
3. Sheet thickness — heavier gauge provides higher shear capacity
4. Span direction — strong direction (perp to ribs) vs weak direction (parallel to ribs)

For typical Australian purlin-supported roofs with 0.7 mm trapezoidal deck:

• Fastened at every purlin line: phi-Vd ≈ 8-15 kN/m shear capacity
• Fastened at every second purlin: phi-Vd ≈ 4-8 kN/m
• Side lap screws at 300 mm spacing: adds 30-50% capacity

Diaphragm action is essential for stability of CFS-framed buildings and often eliminates the need for separate bracing systems for wind loads in low-rise construction.

Purlin Design Example — C-Section Under Gravity

Problem: Design a C-section roof purlin at 1200 mm spacing, spanning 6.0 m between rafters, under wind uplift (dominant) and gravity load.

Design Parameters:

• Span: L = 6.0 m (simple span, lapped at ends)
• Purlin spacing: S = 1.2 m
• Roof pitch: 10° (1:6)
• Dead load: 0.15 kPa (roof sheeting + insulation + services)
• Live load: 0.25 kPa (maintenance, AS 1170.1)
• Wind uplift: Wu = -1.2 kPa (ultimate, AS 1170.2, Region A)
• Steel: G550 to AS 1397 (fy = 550 MPa, fu = 550 MPa)
• Section: C25024 (d = 250 mm, bf = 75 mm, lip = 22 mm, t = 2.4 mm, m = 9.0 kg/m)
• Lateral restraint: Full (sheeting connected to top flange)

Step 1 — Ultimate Loads: Gravity (1.2G + 1.5Q): w = 1.2 × (0.15 + 0.09) + 1.5 × 0.25 = 0.29 + 0.38 = 0.67 kPa × 1.2 m = 0.80 kN/m

Wind uplift (0.9G + Wu): w = 0.9 × (0.15 + 0.09) + (-1.2) = 0.22 - 1.2 = -0.98 kPa × 1.2 m = -1.18 kN/m (Uplift governs by a wide margin.)

Step 2 — Section Properties (C25024, G550): Using effective width method:

• Full web depth: 250 mm (effective portion varies with stress gradient)
• Compression flange: fully effective at 75 mm (w/t = 31.25, moderate slenderness)
• Lip: 22 mm, effective width ≈ 17 mm (after lip buckling reduction)
• Effective section modulus Zex ≈ 52.8 × 10³ mm³ (typical for C25024 in bending)

Step 3 — Moment Check (Uplift): M* = w × L² / 8 = 1.18 × 6.0² / 8 = 1.18 × 36 / 8 = 5.31 kN·m

phi-Ms = phi × fy × Ze = 0.85 × 550 × 52.8 × 10³ / 10⁶ = 24.7 kN·m

5.31 ≤ 24.7 → OK (21% utilisation)

Step 4 — Shear Check: V* = w × L / 2 = 1.18 × 6.0 / 2 = 3.54 kN

phi-Vs = phi × 0.64 × fy × d × t / √3 = 0.85 × 0.64 × 550 × 250 × 2.4 / √3 / 1000 = 0.85 × 0.64 × 550 × 250 × 2.4 / 1732 = 0.85 × 0.64 × 550 × 250 × 0.001385 = 103.6 kN

3.54 ≤ 103.6 → OK (3% utilisation)

Step 5 — Deflection Check (Serviceability): Unfactored wind: w_ser = 1.0 kPa (peak gust) × 1.2 m = 1.2 kN/m Delta = 5 × 1.2 × 6000⁴ / (384 × 200,000 × Ix × 10³) Ix ≈ 5.28 × 10⁶ mm⁴ (gross, typical for C25024) Delta = 5 × 1.2 × 1.296 × 10¹⁵ / (384 × 200,000 × 5.28 × 10⁶) = 7.78 × 10¹⁵ / (4.05 × 10¹⁴) = 19.2 mm

L/300 = 20 mm → 19.2 ≤ 20 → OK

Step 6 — Screw Connection: Purlin-to-rafter connection: 2-14g screws per side phi-Vs per screw ≈ 6.0 kN (G550, 2.4 mm) Total capacity: 2 × 6.0 = 12.0 kN Required: V* = 3.54 kN → OK

Result: C25024 purlin at 1.2 m spacing, 6.0 m span, G550 steel is adequate for AS/NZS 4600. Screw connections with 2-14g screws per purlin-to-rafter connection.

Educational reference only. Verify against AS 4100 and relevant standards. Results are PRELIMINARY — NOT FOR CONSTRUCTION.