Hollow sections are available from 20×20 to 300×300 mm square and from 20×40 to 400×200 mm rectangular; wall thickness varies between 1.00 and 12.00 mm.
To make the right choice within that wide range, three questions have to be answered: what load will the member carry, what span will it cross, and in what environment will it stand?
Size ranges
| Square section | 20×20 – 300×300 mm |
|---|---|
| Rectangular section | 20×40 – 400×200 mm |
| Wall thickness | 1.00 – 12.00 mm |
| Galvanised square | 20×20 – 100×100 mm (wall 1.00 – 3.00 mm) |
| Length | 6,000 / 12,000 mm |
Outside dimension or wall thickness?
There are two ways of increasing load capacity: enlarging the outside dimension or thickening the wall. Enlarging the outside dimension is usually more efficient; as material moves away from the centroid, the second moment of area rises rapidly.
But if the outside dimension is fixed by architectural or assembly constraints, thickening the wall is the only option.
Calculating weight
The weight per metre of a hollow section is approximated by the cross-sectional area × 7.85 g/cm³ (the density of steel). Because of the corner radii the actual weight falls a little below the theoretical figure.
When ordering, be clear whether you work in metres or in tonnes; conversion errors between the two units are a common problem.
Galvanised for outdoors
For work that will stay outdoors — greenhouses, fencing, railings and solar mounting structures — galvanised hollow section is preferred.
The size range on the galvanised product is narrower (20×20 – 100×100 mm, wall 1.00–3.00 mm); if a larger section is needed, black section plus subsequent galvanising is considered.