Technical guide
Same steel, different shape. A rectangular hollow section puts more material where the bending moment is, but only if the load comes from one predictable direction.
For the same weight per metre, an RHS with a 2:1 depth-to-width ratio has roughly 1.6–1.9× the bending stiffness about its strong axis compared with an SHS of the same weight. That is why beams are deep and narrow.
| Member | Best shape | Why |
|---|---|---|
| Simply supported beam | RHS (deep, narrow) | Material far from the neutral axis |
| Column | SHS | Equal stiffness both ways, no weak axis |
| Truss chord | RHS | Bending in the vertical plane only |
| Gate or guard frame | SHS | Loads from any direction, torsion common |
| Headroom-limited lintel | RHS laid flat or deep | Depth is free, height is not |
RHS wider than about 200 mm and thinner than 4 mm tends to distort during welding at the corners — flats that wide need either a thicker wall or a stiffener. Our practical range runs 20×40 to 300×500 mm with 1.5–16 mm wall; the full matrix is in the catalogue.
For galvanising, an RHS needs a drain hole at each end of the longest dimension and a vent at the opposite corner — a long flat section can trap air.
Only if you brace the weak axis. An unbraced RHS column buckles about its narrow axis long before the strong one.
Per tonne they are similar; per metre of finished beam the RHS is usually cheaper because you need less steel.
We roll 20×40 through 300×500 mm; the common structural sizes are 40×80, 50×100, 60×120 and 100×200.