| Specification | SF-120 |
|---|---|
| Box section | 120 × 60 mm, wall 3.5 mm, 1 cell |
| Weight | 2.36 kg/m (about 14.2 kg per 6 m) |
| Max rafter spacing, W0 0.65 kPa (zone I) | 4.0 m |
| W0 0.95 kPa (zone II) | 3.5 m |
| W0 1.25 kPa (zone III) | 3.2 m |
| W0 1.55 kPa (zone IV) | 2.9 m |
| W0 1.85 kPa (zone V) | 2.7 m |
Spans are for plastic roof sheets or plastic roof tiles, purlins spaced at 660 mm, deflection limited to L/200.
When should I choose SF-120 rather than SF-100?
Choose SF-120 when the rafter spacing is above the SF-100 limit for your wind pressure, or for the edge rows of a roof that uses SF-100 in the middle.
- Farms with 3 m rafters at 0.95 – 1.25 kPa wind pressure.
- Eave, corner and gable rows of a roof using SF-100 elsewhere.
- Greenhouses spacing rafters 3.5 – 4 m apart at 0.65 kPa.
- Factories with 6 m frames that add an intermediate rafter, giving 3 m spacing.
Why is SF-120 tall with only a 3.5 mm wall?
FRP is less stiff than steel, so FRP purlins are sized by deflection. For the same amount of material, a taller box is stiffer than a lower box with thicker walls, so SF-120 uses a 120 mm high box with 3.5 mm walls. Sizes compared: FRP purlin sizes.
Does SF-120 need anti-rust paint?
No. Ammonia and humidity rust galvanised steel purlins within a few years in livestock buildings, but FRP contains no metal. A 6 m length weighs about 14.2 kg and is supplied cut and drilled, with 304 stainless fittings. Greenhouse use: purlins for greenhouses.
Frequently asked questions
What are the dimensions of SF-120?
SF-120 is an FRP box section 120 × 60 mm with a 3.5 mm wall, single cell, weighing 2.36 kg per metre.
What rafter spacing can SF-120 span?
With purlins at 660 mm under plastic roof sheets, SF-120 spans up to 4.0 m at 0.65 kPa, 3.5 m at 0.95 kPa, 3.2 m at 1.25 kPa, 2.9 m at 1.55 kPa and 2.7 m at 1.85 kPa basic wind pressure.
Can a greenhouse with 4 m rafters use SF-120?
Only at a basic wind pressure of 0.65 kPa. At 0.95 – 1.25 kPa, 4 m rafters need SF-160.