How Cold-Formed Channels Are Made
Cold-formed channels are produced by feeding steel strip through a series of rollers at room temperature, which bends the strip into a U, C, or Z profile without heating. The cold working increases the yield strength of the steel slightly, and the process allows very thin walls, typically 1.0 to 4.0 mm, with close dimensional accuracy and long continuous lengths. The sections are then cut to length, punched with holes for fixings, and galvanized or painted, and they are produced to standards such as GB/T 6723 or EN 10162.
U Section vs C Section Geometry
A U section has a flat web and two simple flanges on the same side, similar to a small hot rolled channel but with thinner walls and parallel flanges. A C section has flanges with inward lips at the free edges, which stiffen the flanges and increase the section's resistance to local buckling. The lipped C section is the standard purlin and girt profile in light steel construction, while the plain U section is used for edge trims, framing studs, and support channels where the lips are not required.
Purlin and Girt Applications
In metal building systems, cold-formed C sections are used as roof purlins spanning between portal frame rafters, and as wall girts spanning between columns, supporting the roof and wall cladding. They are also used as studs in light steel framed buildings, floor joists, ceiling battens, and secondary members in mezzanines and carports. The sections are light enough to handle by hand, which speeds erection and reduces the cost of cranes and site labor.
Design Considerations for Cold-Formed Sections
Because the walls are thin, cold-formed sections are designed with local buckling and distortional buckling checks in addition to bending and shear checks, following a cold-formed design code such as EN 1993-1-3 or AISI S100. The designer must also account for holes in the web, which reduce the section capacity, and for the direction of the load relative to the section's principal axes. Purlin spacing, typically 1.2 to 2.0 meters, and the cladding weight and wind load determine the required section depth and thickness.
Coating and Durability
Cold-formed channels are supplied in galvanized steel with coating classes from Z180 to Z275 per EN 10346, or as black sections for painting, and they can also be supplied with pre-painted color coatings for visible applications. The zinc coating protects the thin steel from corrosion, and cut edges and drilled holes should be touched up with zinc-rich paint where the coating is damaged. For aggressive coastal environments, heavier coating classes or additional paint systems are specified.
Frequently Asked Questions
Q: What is the difference between cold-formed and hot-rolled channel?
Cold-formed channel is roll-formed from strip with thin walls and tight tolerances, while hot-rolled channel is rolled from heated billets with thicker walls and larger sizes.
Q: What does the lip on a C section do?
The lip stiffens the flange against local buckling, which lets the section carry higher loads before the flange distorts.
Q: What spacing are purlins installed at?
Purlin spacing is typically 1.2 to 2.0 meters, depending on the cladding span capacity, wind load, and section size.
Q: Can cold-formed channels be welded?
Yes, they weld with standard consumables, but the thin walls require short, controlled welds to avoid burning through, and bolted connections are often preferred.
Q: What coating is standard?
Galvanized coating classes Z180 to Z275 are standard for structural sections, with heavier coatings or paint systems for coastal service.