Cold-Formed Section Design Theory - Z-Purlins & Lipped Channels
Cold-formed sections - Z-purlins, lipped and plain channels - are thin-walled members rolled from strip at ambient temperature, used as light secondary members for roofs, walls and cladding.
📐Worked example coming soonA worked example is being prepared once the EC3 engine covers these sections.Worked example →Thin walls and local buckling
Because the material is thin (often 1–4 mm), the flat plate elements buckle locally well before the whole section yields. Design (EN 1993-1-3) therefore uses an effective cross-section - the buckled parts are discounted - rather than the gross section used for hot-rolled steel.
Lips and distortional buckling
The small return "lips" on lipped channels and Z-purlins stiffen the flange edges, raising the local buckling stress. But cold-formed sections can also fail by distortional buckling, where the flange-plus-lip rotates about the web - a mode unique to thin-walled members that has no equivalent in hot-rolled design.
Purlins as secondary members
Z and C purlins span between primary frames to support roof sheeting. The sheeting usually provides lateral and torsional restraint to the purlin (the "restrained beam" case), and continuity over supports plus sleeving/overlapping are used to increase efficiency. Z-sections nest, which makes them convenient for continuous and overlapped systems.
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