Cold-formed steel describes sections and panels shaped from thin steel strip at room temperature, most often by roll forming or press braking. The material is not a different alloy from ordinary structural steel; the difference lies in the forming route, the section thickness and the way the finished member is designed. Understanding that distinction is what allows a buyer to specify the right product form, grade and coating for a project.
What Cold-Formed Steel Actually Is
Roll forming feeds a coil of coated or uncoated strip through a series of contoured rolls, each pair adding a small amount of bend, until the strip leaves as a finished C, Z, sigma or hat section. Press braking bends cut sheet in a brake press instead, which suits shorter runs and heavier gauges. Because the strip is worked below its recrystallisation temperature, the operation raises yield strength through work hardening while holding dimensions to tight tolerances.
The sections are thin-walled, so their capacity comes from cross-sectional shape rather than from material thickness. That is why cold-formed design must check local buckling, distortional buckling and effective width, not just the gross section area used for hot-rolled members.
Hot Rolling, Cold Rolling and Cold Forming Compared
Three processes are often confused. Hot rolling takes place above the recrystallisation temperature and produces plate, heavy sheet and structural sections with a mill scale surface. Cold rolling reduces hot-rolled, pickled strip at room temperature to thinner gauges with a smooth surface and tighter tolerances. Cold forming then bends strip or sheet into a section profile without reducing its thickness.
| Product form | Typical thickness | Typical use |
|---|---|---|
| Plate | over 3 mm | structural and heavy fabrication |
| Sheet | 0.4 to 3 mm | panels, enclosures, cladding |
| Strip and coil | 0.1 to 3 mm | press feed, roll forming |
| Cold-formed sections | 0.4 to 3 mm, up to about 6 mm in heavier framing | purlins, studs, framing, racking |
Grades and Standards
Coated strip for cold forming is specified by grade, coating and dimensional standard. Under EN 10346 the structural grades are designated by minimum yield strength, with S250GD, S280GD and S350GD giving 250, 280 and 350 megapascals minimum yield respectively, and the coating written as Z plus coating mass, for example S350GD+Z275. Equivalent coated sheet in North America is covered by ASTM A653/A653M, where structural grades such as SS Grade 40 denote a minimum yield of 275 megapascals, and by ASTM A1003/A1003M for sheet used in cold-formed framing members. Continuously hot-dip zinc-coated sheet and strip is covered in China by GB/T 2518.
| Grade | Minimum yield strength | Typical application |
|---|---|---|
| S250GD+Z275 | 250 MPa | purlins, studs, general framing |
| S280GD+Z275 | 280 MPa | profiled decking, cladding rails |
| S350GD+Z275 | 350 MPa | high-strength purlins, racking, trusses |
| SS Grade 40 (ASTM A653) | 275 MPa | load-bearing panels and studs |
Design is governed separately from the material standard: AISI S100 in North America, EN 1993-1-3 in Europe and GB 50018 in China set out the rules for effective width, buckling, connections and serviceability of cold-formed members. Substitution between national grades must therefore be verified on mechanical properties and chemistry rather than on the grade name alone.
Benefits in Building and Industrial Work
Strength to weight. A thin, high-yield, shaped section carries its load with far less steel mass than a hot-rolled alternative, which reduces framing weight, foundation loads and crane time.
Dimensional accuracy. Roll forming holds close tolerances along the full length of a coil, so members fit, stack and clad without site adjustment.
Speed of construction. Light sections can be handled manually, cut on site and fixed with self-drilling screws, so frames are erected quickly and can be closed in early.
Consistent quality. The process is continuous and machine-controlled, which gives uniform mechanical properties and a repeatable galvanized or painted finish.
Non-combustible and recyclable. Steel framing does not add fuel load in the way timber does and remains fully recyclable at end of life.
The limitations deserve equal attention. Thin walls are sensitive to local buckling and to damage during handling, connections must be designed specifically for thin material rather than copied from hot-rolled practice, and because a small loss of wall thickness is a large loss of capacity, corrosion protection is a structural rather than a cosmetic decision.
Coating, Corrosion Protection and Verification
Because the sections are thin, the coating does most of the durability work. A Z275 coating contains 275 grams of zinc per square metre over both surfaces, about 19 microns per face, and is the usual specification for load-bearing purlins, studs and decking in external or humid service. Aluminium-zinc coatings are used where a combination of barrier and sacrificial protection is required, and organic coatings are added for appearance or for aggressive chemical exposure. Cut ends exposed to weather are normally protected during installation.
Before accepting a delivery, the purchase order should state product form, grade, coating mass, dimensions and standard, and the mill certificate should be checked for yield strength, tensile strength, elongation and coating mass. Dimensional checks cover section depth, flange width, lip length, thickness and squareness, since a small thickness shortfall has a disproportionate effect on load capacity.
Frequently Asked Questions
Q: What thickness counts as cold-formed steel?
Most cold-formed sections are formed from strip or sheet between 0.4 and 3 millimetres thick, while heavier framing members can be shaped from sheet up to about 6 millimetres.
Q: Are cold-rolled steel and cold-formed steel the same thing?
No. Cold rolling reduces strip to a thinner gauge at room temperature, while cold forming bends strip or sheet into a section profile without changing its thickness.
Q: Which grades are used for cold-formed purlins and studs?
The common grades are S250GD, S280GD and S350GD under EN 10346 with zinc coatings such as Z275, and equivalent ASTM A653 or ASTM A1003 sheet grades in North America.
Q: Is cold-formed steel strong enough for structural use?
Yes, when it is designed under the cold-formed rules of AISI S100, EN 1993-1-3 or GB 50018 and protected against corrosion, it is widely used for load-bearing purlins, studs, trusses and decking.
Q: Why is the strength-to-weight ratio of cold-formed steel higher?
The section shape places material where it resists bending most effectively and the cold working raises yield strength, so a thin shaped member can match a much heavier hot-rolled profile.
Q: How is cold-formed steel protected against corrosion?
It is normally supplied with a hot-dip zinc coating such as Z275, and cut edges and site-damaged areas are re-protected with a zinc-rich repair coating after installation.
