Engineering & Design
Cold-Formed Steel Design for Purlins and Girts
Cold-formed steel design is the engineering of thin steel members shaped at room temperature, such as Z-purlins, C-girts, eave struts and roof or wall panels. Because these sections are thin, their design must account for local, distortional and lateral-torsional buckling, and for the restraint provided by the cladding.
Key facts
- Cold-formed members are roll-formed from coil, often with a metallic coating applied at the mill.
- Thin walls make buckling, not material yield, the main design concern.
- Lapped Z-purlins can act as continuous beams across frames, reducing deflection and steel weight.
- Through-fastened panels usually restrain purlins better than standing seam roofs.
- Separate standards govern cold-formed members in each code family.
What makes cold-formed steel different
Hot-rolled and built-up members have thick plates that usually yield before they buckle. Cold-formed members are thin, so their flat elements can buckle locally at stresses below yield, and the whole cross-section can distort or twist. Design methods therefore calculate an effective section or use direct strength approaches that account for these buckling modes.
Cold forming also raises the strength of the steel at the corners, and high-strength coil is commonly used. The result is a light, efficient member, but one that depends on correct bracing, connections and fastening to reach its design capacity.
Why purlin laps and continuity matter
Z-purlins nest into one another, so they can be lapped over the frames and bolted together. A lapped purlin line behaves as a continuous beam, sharing moment between spans and reducing midspan deflection. The length of the lap, the bolt pattern and the purlin thickness at the lap are all part of the design.
C-sections do not nest in the same way and are usually used as simple spans or with separate splice details. End bays often carry more moment than interior bays, so purlins in end bays may be heavier or more closely spaced.
How cladding and bracing restrain purlins and girts
Under gravity load the compression flange of a purlin wants to move sideways, and under wind uplift the free flange is in compression. Through-fastened roof sheets screwed directly into the top flange provide significant lateral and rotational restraint. Standing seam roofs attach with clips that allow thermal movement, so they provide less restraint and the purlins usually need additional bracing.
Sag rods, bridging, purlin braces and anchorage to the frames or eave struts complete the restraint system. Removing or omitting these elements on site can reduce purlin capacity significantly, so they must be installed as shown on the erection drawings.
How the four code families design cold-formed members
In the United States, AISI S100, the North American Specification for the Design of Cold-Formed Steel Structural Members, is the referenced standard, with AISI S240 covering cold-formed steel structural framing. Canada uses CSA S136, which is the Canadian edition of the same North American specification with Canadian provisions.
Australia and New Zealand use AS/NZS 4600, Cold-Formed Steel Structures. Europe uses EN 1993-1-3, which covers cold-formed members and sheeting with supplementary rules, alongside the relevant National Annex. All four allow design by calculation and recognize design assisted by testing for situations where calculation alone is not sufficient.
Connections and practical limits
Cold-formed members are connected with bolts, self-drilling screws or clips. The thin material limits bearing and pull-out capacity, so fastener type, spacing and edge distance are designed rather than chosen on site.
Corrosion protection is especially important because a small loss of thickness is a large share of a thin section. All purlin and girt sizes, laps and bracing are confirmed by the project engineer to the governing code.
Points to confirm with your engineer
- Roof type, since through-fastened and standing seam roofs restrain purlins differently.
- Purlin and girt spacing limits from the cladding supplier.
- Lap lengths, bolt patterns and end bay requirements.
- Bracing, sag rods and anchorage locations.
- Coating class of the coil for the site environment.
- Any hung loads that will be attached to purlins.
Frequently Asked Questions
Why are Z-purlins preferred on many metal building roofs?
Z-sections nest when lapped, so they form continuous lines over the frames. This continuity reduces deflection and steel weight compared with simple spans of equivalent depth.
Can sag rods or bridging be left out to save time?
No. Bracing is part of the design capacity of thin members. Omitting it can cause purlins or girts to twist and lose strength under load.
Does a standing seam roof change purlin design?
Yes. Standing seam clips allow the roof to move thermally and give less restraint than screwed sheets, so purlins are usually checked with reduced restraint and often need additional bracing.
Engineering content is general guidance. Every World Steel building is designed by qualified engineers to the code and loads of the project.
