Structural engineering criteria for high-strength cold-formed galvanized purlins under TS EN 1993-1-3: thickness selection, span limits, snow loads, and wind uplift resistance.
Cold-formed galvanized C and Z purlins carrying insulated roof panels and profiled steel sheets are paramount structural elements determining industrial building safety and steelwork cost efficiency. Under TS EN 1993-1-3 and Eurocode 3 standards, structural purlin design requires comprehensive verification of steel yield strength (S320GD / S350GD), base metal thickness, deflection limits (L/200 to L/300), and continuity effects under downward gravity and upward wind suction combinations.
1. Structural Systems: Simply Supported vs. Continuous Overlapped Purlins
Standard portal frame spans typically range between 6.00m and 8.00m. C-sections are generally deployed as single-span simply supported beams, providing ease of alignment and installation. In contrast, Z-sections feature asymmetrical flanges allowing nesting (overlapping) over frame rafters. This overlap transforms the purlin system into a fully continuous multi-span beam, reducing peak mid-span bending moments and increasing structural load capacity by 25% to 35% using identical steel weight.
2. Gravity Snow Load & Upward Wind Suction Combinations
Two dominant load combinations govern purlin sizing: Downward Gravity: Dead load (purlin + roof panel) + ground snow load + live maintenance loads. The top flange experiences compression and receives continuous lateral restraint from screw-fixed sandwich panels. Upward Wind Suction (Uplift): Severe aerodynamic uplift puts the bottom purlin flange into compression. Because the bottom flange is unrestrained by cladding screws, lateral torsional buckling may occur unless sag rods and anti-sag struts are engineered at mid-span.
3. High-Strength Structural Steel: S320GD+Z vs. S350GD+Z
Cold-formed purlins must never be rolled from commercial grade mild steel. SerPan Metal manufactures purlins strictly from certified structural steel grades S320GD+Z (320 MPa yield strength) or S350GD+Z (350 MPa yield strength) with hot-dip zinc coating weights from Z140 to Z275 g/m² per EN 10346, ensuring over 30 years of corrosion integrity.
Comprehensive Engineering Comparison Matrix
| Purlin Profile Section | Gauge (mm) | Weight (kg/m) | 4.50 m Span Capacity (kg/m²) | 6.00 m Span Capacity (kg/m²) | 7.50 m Span Capacity (kg/m²) |
|---|---|---|---|---|---|
| C / Z 150 / 50 / 15 | 1.50 mm | 3.12 kg/m | 195 kg/m² | 110 kg/m² (Light duty only) | Insufficient (Not recommended) |
| C / Z 200 / 60 / 20 | 2.00 mm | 5.25 kg/m | 340 kg/m² | 190 kg/m² (Standard Industrial) | 115 kg/m² |
| C / Z 200 / 60 / 20 | 2.50 mm | 6.52 kg/m | 430 kg/m² | 245 kg/m² (High Snow / Rooftop Solar) | 155 kg/m² |
| C / Z 250 / 65 / 20 | 2.50 mm | 7.60 kg/m | 560 kg/m² | 320 kg/m² | 205 kg/m² (Wide Span Frames) |
Verdict: For 6.00m portal frame spacing, overlapped continuous Z 200 / 2.00 mm provides the optimum structural balance. Where rooftop solar PV or heavy snow zones exist, specify 2.50 mm thickness or step up to Z 250.
Calculate Live with Your Project Dimensions
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Galvanized C / Z / Sigma Purlin Span & Deflection Calculator
Specify your purlin span between trusses, purlin spacing and profile section to calculate deflection limits (L/200, L/300), safe capacity, and required steel tonnage per EN 1993-1-3.
Z200 / 2.00 mm (L = 6.00 m)
Frequently Asked Questions
Can bolt holes be pre-punched during production? ▼
Yes. SerPan Metal purlins are manufactured on fully automated CNC roll-forming lines with custom pre-punched slotted or round bolt holes and exact custom lengths to eliminate site drilling.