RHS 80×40×3.6 vs CF RHS 80×40×4
RHS 80×40×3.6 is an RHS Hot-finished section from the UK Blue Book (SCI P363); CF RHS 80×40×4 is the closest equivalent in the UK Blue Book (SCI P363). CF RHS 80×40×4 carries more bending: 20.9 cm³ of plastic modulus against 20 cm³, 4.3% more. RHS 80×40×3.6 is the lighter of the two at 6.27 kg/m against 6.71 kg/m, a saving of 6.6% on steel weight. Both are about 80 mm deep, so either fits the same construction zone.
| Property | RHS 80×40×3.6 RHS Hot-finished | CF RHS 80×40×4 RHS Cold-formed | Difference |
|---|---|---|---|
| Mass | 6.3 kg/m | 6.7 kg/m | -6.6% |
| Depth | 80 mm | 80 mm | same |
| Width | 40 mm | 40 mm | same |
| Web thickness | 0 mm | 0 mm | same |
| Flange thickness | 0 mm | 0 mm | same |
| Area | 8 cm² | 8.6 cm² | -6.7% |
| Second moment, major (Iy) | 62.8 cm⁴ | 64.8 cm⁴ | -3.1% |
| Plastic modulus, major (Wpl,y) | 20 cm³ | 20.9 cm³ | -4.3% |
| Elastic modulus, major (Wel,y) | 15.7 cm³ | 16.2 cm³ | -3.1% |
| Second moment, minor (Iz) | 20.6 cm⁴ | 21.5 cm⁴ | -4.2% |
| Plastic modulus, minor (Wpl,z) | 12.1 cm³ | 12.8 cm³ | -5.5% |
| Radius of gyration, minor (iz) | 1.61 cm | 1.59 cm | +1.3% |
| Torsion constant (It) | 50.8 cm⁴ | 55.2 cm⁴ | -8.0% |
RHS 80×40×3.6 to BS EN 10210-2:2006; CF RHS 80×40×4 to BS EN 10219-2:2006. Difference is expressed relative to CF RHS 80×40×4.
How to choose between them
On stiffness, CF RHS 80×40×4 has the larger second moment of area: 64.8 cm⁴ against 62.8 cm⁴, a difference of 3.1%. 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 3.1 cm³ per kg/m, 2.4% 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.
In short, CF RHS 80×40×4 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.