RHS 180×100×4 vs CF RHS 180×100×4
RHS 180×100×4 is an RHS Hot-finished section from the UK Blue Book (SCI P363); CF RHS 180×100×4 is the closest equivalent in the UK Blue Book (SCI P363). They are close to interchangeable in bending: 128 cm³ against 126 cm³ of plastic modulus, a difference of 1.6%. They weigh essentially the same, 16.9 kg/m and 16.8 kg/m. Both are about 180 mm deep, so either fits the same construction zone.
| Property | RHS 180×100×4 RHS Hot-finished | CF RHS 180×100×4 RHS Cold-formed | Difference |
|---|---|---|---|
| Mass | 16.9 kg/m | 16.8 kg/m | +0.6% |
| Depth | 180 mm | 180 mm | same |
| Width | 100 mm | 100 mm | same |
| Web thickness | 0 mm | 0 mm | same |
| Flange thickness | 0 mm | 0 mm | same |
| Area | 21.6 cm² | 21.3 cm² | +1.4% |
| Second moment, major (Iy) | 945 cm⁴ | 926 cm⁴ | +2.1% |
| Plastic modulus, major (Wpl,y) | 128 cm³ | 126 cm³ | +1.6% |
| Elastic modulus, major (Wel,y) | 105 cm³ | 103 cm³ | +1.9% |
| Second moment, minor (Iz) | 379 cm⁴ | 374 cm⁴ | +1.3% |
| Plastic modulus, minor (Wpl,z) | 85.2 cm³ | 84 cm³ | +1.4% |
| Radius of gyration, minor (iz) | 4.19 cm | 4.18 cm | +0.2% |
| Torsion constant (It) | 852 cm⁴ | 854 cm⁴ | -0.2% |
RHS 180×100×4 to BS EN 10210-2:2006; CF RHS 180×100×4 to BS EN 10219-2:2006. Difference is expressed relative to CF RHS 180×100×4.
How to choose between them
On stiffness, RHS 180×100×4 has the larger second moment of area: 945 cm⁴ against 926 cm⁴, a difference of 2.1%. 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, the two are equally efficient (7.6 against 7.5 cm³ per kg/m), so on a pure steel-tonnage basis there is nothing to choose between them. The decision comes down to availability, connection detailing and what the rest of the frame uses.
In short, these two are genuine equivalents. Pick whichever your supplier stocks and the rest of the frame already uses.