Cold-Formed Purlin Systems in Australian Construction

Australian steel-framed buildings use cold-formed purlins as secondary members spanning between portal frame rafters (roof) or columns (wall girts). The purlin transfers roof cladding loads (dead, live, wind) to the primary steel frame. The dominant systems are:

Cold-formed sections are manufactured from coil steel to AS 1397 with a minimum yield stress of 450-550 MPa. The cold-working at corner bends increases the effective yield stress by approximately 10-15% relative to the virgin coil — this strength increase can be used in design per AS/NZS 4600 Clause 2.2.1.2 provided the section satisfies the prequalified geometry limits.

Australian Steel Grades for Cold-Formed Purlins

Important: G550 steel (f_y = f_u = 550 MPa) has no strain-hardening capacity. Section capacity for G550 members is controlled by local buckling, not yield. The design yield stress for sections with slender plate elements must be reduced per AS/NZS 4600 Clause 2.2.1.2.

For hot-rolled PFC channels used as purlins (HA300/HA350), design is per AS 4100 Clause 5 (not AS/NZS 4600), with mono-symmetric bending provisions per Clause 5.6.

Design Load Cases for Australian Roof Purlins

Purlin design must consider the following load combinations per AS/NZS 1170.0:

Downward (gravity) combinations:

1. 1.35G (dead load only — construction case)
2. 1.2G + 1.5Q (dead + live/imposed)
3. 1.2G + psi_s x Q + W_u (dead + reduced live + wind uplift dominant)

Upward (wind uplift) combinations:

4. 0.9G + W_u (minimum dead + wind uplift — often governs purlin design)
5. 1.2G + W_u + psi_c x Q (dead + wind + reduced live)

Where:

• G = self-weight of cladding + purlin + insulation + services (typically 0.10-0.25 kPa on slope)
• Q = imposed roof live load per AS/NZS 1170.1 Table 3.1 (0.25 kPa for non-trafficable roofs)
• W_u = ultimate wind uplift pressure per AS/NZS 1170.2
• psi_s = 0.0 (roof live load short-term factor — wind case)
• psi_c = 0.4 (roof live load combination factor)

For most Australian non-cyclonic regions, wind uplift governs purlin design for roof slopes less than 15 degrees. The combination 0.9G + W_u typically controls for spans exceeding 6 m in Regions A and B (AS/NZS 1170.2 wind classification).

Section Capacity — Bending per AS/NZS 4600 Clause 3.3.3

The design bending capacity of a cold-formed purlin in the plane of the roof is:

phi_b x M_b = phi_b x Z_e x f_y / 10^6 (kNm)

Where:

• phi_b = 0.90 (bending, cold-formed steel)
• Z_e = effective section modulus at yield (mm^3) — reduced for local buckling of slender elements
• f_y = design yield stress (MPa)

For a fully effective section (no local buckling reduction), Z_e = Z (gross elastic modulus). For sections with slender compression flanges or webs, Z_e < Z per the effective width method of Clause 2.2.1.2.

Typical section properties for Stramit Exacta C and Z purlins (G500):

Note: Capacities assume fully effective sections. For G550 material, check effective section modulus per AS/NZS 4600 Clause 2.2.1.2.

Shear Capacity per AS/NZS 4600 Clause 3.3.4

The design shear capacity of the purlin web:

phi_v x V_v = phi_v x 0.64 x f_y x d_1 x t / 10^3 (kN)

Where:

• phi_v = 0.90 (shear, cold-formed steel)
• d_1 = depth of flat portion of web = D - 2 x (r + t), where r = internal bend radius (typically 1.5t to 2.5t)
• t = base metal thickness (BMT)

For a C20024 purlin (D = 200 mm, r = 2t = 4.8 mm, BMT = 2.4 mm, G500): d_1 = 200 - 2 x (4.8 + 2.4) = 200 - 14.4 = 185.6 mm phi_Vv = 0.90 x 0.64 x 500 x 185.6 x 2.4 / 1000 = 128.3 kN

Shear rarely governs purlin design. Web crippling at supports and combined bending at end spans are the typical controlling limit states.

Web Crippling at Supports — AS/NZS 4600 Clause 3.3.6

Cold-formed purlins are susceptible to web crippling at interior supports and end reactions. For an end support with a single web (unstiffened):

phi_w x R_b = phi_w x C x t^2 x f_y x (1 - 0.20 x r_i / t) x (1 + 0.01 x N / t) / 10^3 (kN)

Where:

• phi_w = 0.75 (web crippling, cold-formed steel)
• C = coefficient from AS/NZS 4600 Table 3.3.6.1 (C = 4 for EOF — End One Flange loading)
• r_i = inside bend radius (mm), typically 1.5t to 2.5t
• N = bearing length (mm), minimum 50 mm for standard purlin cleats

For C20024, G500, N = 50 mm, r_i = 4.8 mm: phi_Rb = 0.75 x 4 x 2.4^2 x 500 x (1 - 0.20 x 4.8/2.4) x (1 + 0.01 x 50/2.4) / 1000 = 0.75 x 4 x 5.76 x 500 x (1 - 0.40) x (1 + 0.208) / 1000 = 0.75 x 4 x 5.76 x 500 x 0.60 x 1.208 / 1000 = 6.25 kN

For an interior support (IOF loading), the coefficient C = 8, approximately doubling the capacity. End supports typically require 50 mm minimum bearing plus a purlin cleat to distribute the reaction.

Combined Bending and Shear — AS/NZS 4600 Clause 3.3.5

When the design shear force V* exceeds 0.6 x phi_Vv, the bending capacity must be reduced:

(M / phi_M_s) + (V / phi_Vv - 0.6)^2 <= 1.0** (tension flange) (M / phi_M_s) + (V / phi_Vv - 0.6) <= 1.0** (compression flange)

In practice, combined bending and shear rarely governs purlin design because maximum moment and maximum shear occur at different locations along the span (mid-span vs support). The exception is lapped Z-sections at the lap termination point, where significant moment and shear coexist.

Deflection Limits per AS/NZS 1170.0 Appendix C

Roof purlin deflections must not impair the serviceability of the roof cladding system:

For a 7.5 m span C20024 purlin under downward load:

• Span/250 = 30 mm maximum
• Span/300 = 25 mm maximum for live load alone

Deflection is calculated using the gross second moment of area I_x (not effective), unless the section is consistently in the post-buckled range under service loads — which is rare for roof purlins.

Mid-span deflection for a simply-supported beam under UDL: delta = 5 x w x L^4 / (384 x E x I_x)

For C20024: I_x = 6.32 x 10^6 mm^4, E = 200,000 MPa For w = 1.5 kN/m (service): delta = 5 x 1.5 x 7500^4 / (384 x 200,000 x 6.32 x 10^6) = 5 x 1.5 x 3.164 x 10^15 / (384 x 200,000 x 6.32 x 10^6) = 23.7 mm > 25 mm — OK for live load, but marginal.

Bridging Configuration per AS/NZS 4600 Clause 3.4

Anti-sag bridging prevents lateral-torsional buckling of the purlin compression flange. The bridging design must satisfy:

Maximum unbridged compression flange length: L_b <= r_y x sqrt(pi^2 x E / f_oc)

Where r_y is the radius of gyration of the compression flange plus one-sixth of the web in compression.

Standard bridging rules for Australian practice:

Bridging rows must be evenly spaced. The first bridging line must be within L_b/2 of each support. Bridging must be positive-connecting — bolted angle or channel sections, not friction-grip clamps. In cyclonic regions C and D (AS/NZS 1170.2), bridging bolts must be M12 minimum with washers both sides of the purlin web.

Worked Example — C20024 Simply-Supported Roof Purlin

Problem: Design a simply-supported C20024 purlin (G500, BMT = 2.4 mm) spanning 7.5 m between portal frame rafters. Roof slope 10 degrees. Purlin spacing 1.2 m. Location: Melbourne (Region A, non-cyclonic). Assess all limit states.

Given:

• C20024 section, G500: f_y = 500 MPa, Z_x = 58.0 x 10^3 mm^3, I_x = 6.32 x 10^6 mm^4
• Span L = 7500 mm, spacing = 1200 mm
• Dead load G: cladding 0.05 kPa + insulation 0.03 kPa + self-weight (6.79 kg/m = 0.067 kN/m) / 1.2 = 0.056 kPa + 0.067/1.2 = 0.056 kPa = 0.136 kPa total
• Live load Q: 0.25 kPa (non-trafficable roof)
• Wind uplift W_u (AS/NZS 1170.2, Region A1, 10 m height, terrain category 3): C_pi = +0.2, C_pe = -0.9 (corner zone); p_u = -1.25 kPa net

Step 1 — Design line loads (UDL on purlin at 1.2 m spacing):

Downward:

• 1.2G + 1.5Q: w = 1.2 x 0.136 + 1.5 x 0.25 = 0.163 + 0.375 = 0.538 kPa x 1.2 m = 0.646 kN/m

Upward (wind uplift governs):

• 0.9G + W_u: w = 0.9 x 0.136 + (-1.25) = 0.122 - 1.25 = -1.128 kPa x 1.2 m = -1.354 kN/m

Step 2 — Maximum bending moment (simply-supported UDL):

Downward: M* = wL^2/8 = 0.646 x 7.5^2 / 8 = 0.646 x 56.25 / 8 = 4.55 kNm

Upward (wind): M* = 1.354 x 7.5^2 / 8 = 9.52 kNm (governs)

Step 3 — Bending capacity check:

phi_M_bx = 26.1 kNm (from table, fully effective) M* / phi_M_bx = 9.52 / 26.1 = 0.365 < 1.0. OK, plenty of capacity.

However, check flange slenderness for effective section. For C20024 flange (76/2 = 38 mm): lambda_e = (38/2.4) x sqrt(500/250) = 15.83 x 1.414 = 22.4. The plate slenderness limit for a fully effective flange in uniform compression is lambda_ey = 25 (Clause 2.2.1.2 Table 2.2.1). Since 22.4 < 25, the flange is fully effective. OK.

Step 4 — Shear capacity check:

Maximum shear V* = wL/2 = 1.354 x 7.5 / 2 = 5.08 kN phi_Vv = 128.3 kN (from shear section above) V* / phi_Vv = 5.08 / 128.3 = 0.040 < 1.0. OK, shear is negligible.

Step 5 — Web crippling at end support:

Reaction R* = 5.08 kN (wind uplift) or 0.646 x 7.5 / 2 = 2.42 kN (downward) phi_Rb = 6.25 kN (from earlier calculation) > 5.08 kN. OK.

Downward reaction is 2.42 kN < 6.25 kN. OK even with 0.9G case.

Step 6 — Deflection check:

Downward (service): w_serv = (G + psi_s x Q) x spacing = (0.136 + 0.7 x 0.25) x 1.2 = 0.311 x 1.2 = 0.373 kN/m delta = 5 x 0.373 x 7500^4 / (384 x 200,000 x 6.32 x 10^6) = 5 x 0.373 x 3.164 x 10^15 / (384 x 200,000 x 6.32 x 10^6) = 5.90 x 10^15 / 4.85 x 10^14 = 12.2 mm Limit = L/300 = 7500/300 = 25 mm. OK.

Wind uplift: w_serv = W_serv x spacing. For serviceability wind (1/25-year return), W_serv ~ 0.75 x W_u = 0.94 kPa. delta = 12.2 x (0.94/0.311) = 36.9 mm. Limit = L/150 = 50 mm. OK.

Step 7 — Bridging requirement:

C200 purlin, span 7.5 m > 5.5 m. One row of bridging required at mid-span (3.75 m from each support). The unbraced length L_b = 3750 mm.

Result: C20024 purlin at 1.2 m spacing. One row of bridging at mid-span. Fully effective section, no capacity reduction. Deflection governs the span limit — at 7.5 m span, downward deflection is 48% of limit. Could extend to 9.0 m with two rows of bridging (L_b = 3000 mm), but check combined bending at lap joints if using lapped Z-sections.

Purlin Connection Design

Purlins connect to rafters via bolted cleats. The standard Australian detail uses:

• Purlin cleat: 75 x 75 x 6 EA (Grade 300) bolted to rafter flange with 2 x M12 8.8/S bolts
• Purlin-to-cleat: 2 x M12 8.8/S bolts through purlin web
• Bridging connection: M12 bolts through bridging angle and purlin web

The cleat design must resist the purlin end reaction plus a nominal axial force of 2.5% of the purlin compression flange force (for stability bracing per AS 4100 Clause 5.6.2).

Cleat bolt shear check (M12 8.8/S, threads in plane): phi_Vf = 0.80 x 0.62 x 830 x 84.3 / 1000 = 34.7 kN per bolt 2 bolts: phi_Vf = 69.4 kN > maximum reaction 5.08 kN. OK by inspection.

Cleat bearing check (6 mm angle, M12 bolt): phi_Vb = 0.90 x 3.2 x 12 x 6 x 440 / 1000 = 91.2 kN > 5.08 kN. OK.

Purlin cleat connections are rarely the governing limit state for standard configurations.

Frequently Asked Questions

When are Z-section purlins preferred over C-section purlins in Australian practice?

Z-sections are preferred for multi-span continuous lapped purlin systems on roof slopes steeper than 10 degrees because their rotated principal axes align more closely with the roof plane, reducing out-of-plane bending and improving the bending efficiency by 15-20% compared to an equivalent-depth C-section. C-sections are preferred for simply-supported single spans under 6 m and for wall girts, where the shear centre eccentricity is less critical and the flat web face simplifies cladding attachment. In Australian industrial sheds, Z200 and Z250 lapped purlins dominate for 6-12 m spans; C200 is standard for 4-6 m portal frame girts. The lap length for Z-sections is typically 10-15% of the span, and the lap bolts must be tensioned to develop the full moment capacity at the lap splice.

What bridging spacing does AS/NZS 4600 require for roof purlins?

AS/NZS 4600 Table 3.4.1 requires anti-sag bridging at a maximum spacing that ensures the purlin compression flange does not buckle laterally between bridged points. For Z150 sections, one row of bridging is required for spans exceeding 4.0 m. For Z200 sections, one row at spans over 5.5 m; Z250 over 7.0 m; Z300 over 9.0 m. Bridging must be positive-connecting (bolted, not friction-clamped) and anchored to a rigid element (rafter, ridge channel, or sag rod system). Each bridging line must be capable of resisting 2.5% of the compression flange force in all purlins it restrains. Missed or loose bridging is the most common purlin failure mechanism observed in Australian cyclonic regions — the unbridged compression flange buckles laterally under uplift, and the purlin rolls over, tearing the cladding fasteners.

How does wind uplift govern purlin design differently from gravity loads?

Under gravity loads, the purlin top flange is in compression and is continuously restrained by the roof cladding (through-fastened or concealed-fixed). The lateral-torsional buckling check uses L_b = bridging spacing, with a moment modification factor alpha_m >= 1.0. Under wind uplift, the BOTTOM flange is in compression and is NOT continuously restrained — it can buckle laterally between bridging points. This is the critical difference. The design bending capacity under uplift is substantially lower than under gravity for the same purlin section because the effective length for LTB is the full bridging spacing (not zero as for the top flange under gravity). Additionally, the stress gradient across the section is reversed, and the section's mono-symmetric properties (beta_x for Z-sections, y_o for C-sections) introduce warping torsion effects that are not present under gravity loading.

What is the minimum purlin lap length for Z-section continuous systems?

For lapped Z-section purlin systems per the Australian Steel Institute (ASI) Design Guide, the minimum lap length at interior supports is 0.10 x span (10%) for double-span systems and 0.15 x span (15%) for continuous multi-span systems. The lap must be bolted with a minimum of two M12 8.8/S bolts per flange (top and bottom), placed within the outer thirds of the lap length. The bolts must be tensioned to snug-tight minimum to ensure the lap transfers the full bending moment between adjacent purlin lengths. At end spans of continuous systems, the lap length at the penultimate support must be increased by 25% to account for the higher moment demand at the first interior support. For a 9.0 m span Z250 purlin in a three-span continuous system: lap at interior support = 0.15 x 9000 = 1350 mm minimum, with 2 x M12 bolts per flange.

Related Pages

• AS/NZS 4600 Cold-Formed Steel Design — Complete Guide
• AS 4100 Beam Design — Bending, Shear & Deflection
• AS 4100 Lateral Torsional Buckling — alpha_m & alpha_s
• AS 4100 Steel Deflection Limits — Span Tables
• AS 4100 PFC Channel Design — Mono-Symmetric Bending
• Wind Load Calculator — AS/NZS 1170.2 Free Tool
• All Australian Steel Design References

Design Resources

Calculator tools

• Steel Beam Deflection Check
• Wind Load Calculator — ASCE 7 & AS/NZS 1170.2
• Beam Capacity Calculator
• Snow Load Calculator — AS/NZS 1170.3

Design guides

• AS 4100 PFC Channel — Mono-Symmetric Bending & LTB
• AS 4100 Lateral Torsional Buckling — alpha_m & alpha_s
• AS 4100 Compact Section Limits — Slenderness Tables
• EN 1993-1-3 Cold-Formed Purlin Design — Eurocode
• CSA S16 Cold-Formed Steel — Canadian Practice

This page is for educational reference only. Purlin design per AS/NZS 4600:2018 and AS 4100:2020. Verify section properties against manufacturer data (Stramit, Lysaght, Fielders). All structural designs must be independently verified and certified by a licensed Professional Engineer. Results are PRELIMINARY — NOT FOR CONSTRUCTION.