RHS 80×40×3.6 vs RHS 75×38×4.5
RHS 80×40×3.6 is an RHS Hot-finished section from the UK Blue Book (SCI P363); RHS 75×38×4.5 is the closest equivalent in the European (Continental) range. They are close to interchangeable in bending: 20 cm³ against 19.9 cm³ of plastic modulus, a difference of 0.5%. RHS 80×40×3.6 is the lighter of the two at 6.27 kg/m against 6.94 kg/m, a saving of 9.7% on steel weight. RHS 80×40×3.6 is deeper, 80 mm against 75 mm, which matters where the floor zone is tight.
| Property | RHS 80×40×3.6 RHS Hot-finished | RHS 75×38×4.5 RHS Cold-formed | Difference |
|---|---|---|---|
| Mass | 6.3 kg/m | 6.9 kg/m | -9.7% |
| Depth | 80 mm | 75 mm | +6.7% |
| Width | 40 mm | 38 mm | +5.3% |
| Web thickness | 0 mm | 0 mm | same |
| Flange thickness | 0 mm | 0 mm | same |
| Area | 8 cm² | 8.8 cm² | -9.7% |
| Second moment, major (Iy) | 62.8 cm⁴ | 57.1 cm⁴ | +10.0% |
| Plastic modulus, major (Wpl,y) | 20 cm³ | 19.9 cm³ | +0.5% |
| Elastic modulus, major (Wel,y) | 15.7 cm³ | 15.2 cm³ | +3.3% |
| Second moment, minor (Iz) | 20.6 cm⁴ | 19.2 cm⁴ | +7.3% |
| Plastic modulus, minor (Wpl,z) | 12.1 cm³ | 12.3 cm³ | -1.6% |
| Radius of gyration, minor (iz) | 1.61 cm | 1.47 cm | +9.5% |
| Torsion constant (It) | 50.8 cm⁴ | 50 cm⁴ | +1.6% |
RHS 80×40×3.6 to BS EN 10210-2:2006; RHS 75×38×4.5 to EN 10219 · Continental Steel. Difference is expressed relative to RHS 75×38×4.5.
How to choose between them
On stiffness, RHS 80×40×3.6 has the larger second moment of area: 62.8 cm⁴ against 57.1 cm⁴, a difference of 10.0%. This matters more often than the strength comparison, because on normal floor spans deflection rather than bending capacity sets the beam size - so if the two are close on modulus but apart on inertia, the stiffer one is usually the better answer even when it is not the stronger one.
Measured as bending capacity per kilogram of steel, RHS 80×40×3.6 is the more efficient section: 3.2 against 2.9 cm³ per kg/m, 11.2% better. Over a floor of repeated beams that difference is real tonnage, though it is often outweighed by whichever section the fabricator can actually source.
These come from different catalogues, so treat the swap as a design change rather than a like-for-like substitution. RHS Hot-finished sections are specified to BS EN 10210-2:2006 and RHS Cold-formed to EN 10219 · Continental Steel; the steel grades, tolerances and available lengths differ, connection details and bolt gauges are set out differently, and the two are rarely both stocked in the same market. Check availability before changing a section on a drawing to save 9.7% weight.
In short, RHS 80×40×3.6 is the stronger section and RHS 80×40×3.6 is the lighter one; where those are the same section, it is the clear choice, and where they are not, decide on whether your design is governed by capacity or by tonnage.