FRP box purlins 30x60, 40x80, 50x100 and 60x120 are the Santiago sizes SF-60, SF-80, SF-100 and SF-120, with 3 – 3.5 mm walls and 0.98 – 2.36 kg/m. The two large sizes, SF-160 and SF-220, are double-cell boxes 55 mm wide and 160 or 220 mm tall, for rafter spacing of 4 – 6.5 m.
How are FRP purlin sizes written?
An FRP box purlin is described by height, width and wall thickness in millimetres. SF-100 is 100 mm tall, 50 mm wide with a 3.5 mm wall, written 100 × 50 × 3.5; many people call it a 50x100 purlin. It is always installed with the 100 mm side vertical.
Dimensions and weight of each size
| Common name | Code | Box (mm) | Cells | Weight (kg/m) | 6 m length | Ix (cm⁴) |
|---|---|---|---|---|---|---|
| 30x60 | SF-60 | 60 × 30 × 3 | 1 | 0.98 | about 5.9 kg | 22.5 |
| 40x80 | SF-80 | 80 × 40 × 3 | 1 | 1.33 | about 8 kg | 55.9 |
| 50x100 | SF-100 | 100 × 50 × 3.5 | 1 | 1.95 | about 11.7 kg | 128.4 |
| 60x120 | SF-120 | 120 × 60 × 3.5 | 1 | 2.36 | about 14.2 kg | 226.7 |
| 55x160 | SF-160 | 160 × 55 × 4 | 2 | 3.60 | about 21.6 kg | 501.9 |
| 55x220 | SF-220 | 220 × 55 × 4 | 2 | 4.53 | about 27 kg | 1,148.5 |
Ix (second moment of area) is calculated by Santiago FRP for the nominal box; the larger it is, the less the purlin sags.
Why does a taller box span so much further?
Bending stiffness grows roughly with the cube of the height. SF-220 weighs only about 1.3 times as much as SF-160 but has about 2.3 times its Ix. That is also why FRP purlins are always installed upright.
How far can each size span?
Maximum rafter spacing for plastic roof sheets, purlins at 660 mm, deflection L/200, by basic wind pressure W0 of the Vietnamese wind zones:
| Code | 0.65 kPa (I) | 0.95 kPa (II) | 1.25 kPa (III) | 1.55 kPa (IV) | 1.85 kPa (V) |
|---|---|---|---|---|---|
| SF-60 (30x60) | 1.9 m | 1.6 m | 1.4 m | 1.3 m | 1.2 m |
| SF-80 (40x80) | 2.5 m | 2.2 m | 2.0 m | 1.8 m | 1.7 m |
| SF-100 (50x100) | 3.4 m | 2.9 m | 2.6 m | 2.4 m | 2.2 m |
| SF-120 (60x120) | 4.0 m | 3.5 m | 3.2 m | 2.9 m | 2.7 m |
| SF-160 (55x160) | 5.2 m | 4.6 m | 4.1 m | 3.8 m | 3.6 m |
| SF-220 (55x220) | 6.8 m | 6.1 m | 5.5 m | 5.0 m | 4.7 m |
For a site outside Vietnam, send the design wind pressure and roof type and Santiago FRP will check the size.
Is a 50x100 FRP purlin the same as a 50x100 steel box?
They share outside dimensions but are not interchangeable one for one. FRP is less stiff, so its walls are thicker (3.5 mm against about 1.4 mm for a typical steel box) and its maximum span is different. Compare them on span, not on size: FRP vs steel vs timber.
Frequently asked questions
Which code is the 50x100 FRP purlin?
Santiago SF-100: 100 mm tall, 50 mm wide, 3.5 mm wall, 1.95 kg/m. At 0.95 kPa basic wind pressure it spans up to 2.9 m with purlins at 660 mm.
How long are FRP purlins?
Santiago FRP cuts each purlin to the rafter spacing of the project and pre-drills it, so lengths vary by roof. Joints always sit on a rafter.
How heavy is a 6 m FRP purlin?
About 5.9 kg for SF-60, 11.7 kg for SF-100, 14.2 kg for SF-120 and 27 kg for SF-220.