Cold roll forming of purlins, girts and panels

What is cold roll formed in a PEB

All secondary members and most cladding of a pre-engineered building are made by cold roll forming: purlins, girts, eave struts, roof and wall sheeting, standing seam panels, trims and flashings. Coil is passed through a series of roll stands, each of which bends the strip a little further until the final profile is reached. Because forming is done at room temperature, the steel keeps its coating and gains some strength at the corners from work hardening.

The purlin line

1. Decoiling and levelling – slit coil of the specified width, grade and coating is uncoiled and flattened.
2. Inline punching – bolt holes for laps and frame connections, sag rod holes and service holes are punched by CNC press units driven from the detailing data.
3. Forming – typically 12–20 roll stands form the web, flanges and lips. Quick-change or automatic size adjustment allows different depths and flange widths on one line; convertible lines produce both Z and C sections.
4. Cut to length – a flying or stop-and-cut shear cuts each member to its exact length; each piece is marked with its piece number.
5. Bundling – members are stacked and strapped by mark for shipping.

Z sections have unequal flanges so that they can nest and be lapped over the frames, giving continuity and lower deflection; C sections are used where laps are not required, for example at openings and endwalls. See cold-formed steel for material and design details.

Panel lines

Roof and wall sheeting lines form trapezoidal or corrugated profiles from pre-painted metallic-coated coil. Rolls must be kept clean and polished so that the paint film is not scratched, and strippable film may be applied for premium colours. Standing seam panels are often formed on site from portable machines when panel lengths exceed transport limits – a 60–100 m single-length roof panel eliminates end laps entirely. Panel length, straightness, rib height and cover width are checked at the start of each run and periodically during production.

Forming coated and aluminium coil. Pre-painted coil must pass the T-bend flexibility requirement of its paint system (ASTM D4145) for the tightest bend radius on the profile; cold coil in winter or old stock can crack at the ribs. Aluminium coil forms easily but scratches and marks more readily, so rolls are cleaned when changing from steel. Curved eave panels and curved roofs are produced from straight panels by crimp-curving or smooth-curving machines, with the minimum radius depending on profile and thickness.

Material and tolerance control

Coil is checked on receipt for grade, base metal thickness and coating mass against the mill certificate – for example ASTM A653 / A1011 in North America, AS 1397 in Australia, EN 10346 in Europe. Key in-process checks are profile depth and flange width, lip length and angle, hole position and length. Tolerances follow the purchaser's specification or the MBMA Metal Building Systems Manual, AS/NZS 4600 and EN 1090-4, which covers execution of cold-formed thin-gauge structural elements and sheeting in Europe. See metallic-coated and pre-painted sheet for coatings.

Galvanized secondary members are cut and punched after coating; small cut edges are protected by the sacrificial action of the zinc and normally need no touch-up in a dry internal environment.

WSC supply

World Steel roll-forms purlins, girts, eave struts and cladding profiles as part of its secondary members and cladding packages, sourcing coated coil from established mills such as NS BlueScope Vietnam, Hoa Sen and Ton Dong A according to the market specification.