RHS 120×80×5 vs CF RHS 120×80×5
RHS 120×80×5 is an RHS Hot-finished section from the UK Blue Book (SCI P363); CF RHS 120×80×5 is the closest equivalent in the UK Blue Book (SCI P363). RHS 120×80×5 carries more bending: 74.6 cm³ of plastic modulus against 72.4 cm³, 3.0% more. CF RHS 120×80×5 is the lighter of the two at 14.4 kg/m against 14.7 kg/m, a saving of 2.1% on steel weight. Both are about 120 mm deep, so either fits the same construction zone.
| Property | RHS 120×80×5 RHS Hot-finished | CF RHS 120×80×5 RHS Cold-formed | Difference |
|---|---|---|---|
| Mass | 14.7 kg/m | 14.4 kg/m | +2.1% |
| Depth | 120 mm | 120 mm | same |
| Width | 80 mm | 80 mm | same |
| Web thickness | 0 mm | 0 mm | same |
| Flange thickness | 0 mm | 0 mm | same |
| Area | 18.7 cm² | 18.4 cm² | +1.6% |
| Second moment, major (Iy) | 365 cm⁴ | 353 cm⁴ | +3.4% |
| Plastic modulus, major (Wpl,y) | 74.6 cm³ | 72.4 cm³ | +3.0% |
| Elastic modulus, major (Wel,y) | 60.9 cm³ | 58.9 cm³ | +3.4% |
| Second moment, minor (Iz) | 193 cm⁴ | 188 cm⁴ | +2.7% |
| Plastic modulus, minor (Wpl,z) | 56.1 cm³ | 54.7 cm³ | +2.6% |
| Radius of gyration, minor (iz) | 3.21 cm | 3.2 cm | +0.3% |
| Torsion constant (It) | 401 cm⁴ | 402 cm⁴ | -0.2% |
RHS 120×80×5 to BS EN 10210-2:2006; CF RHS 120×80×5 to BS EN 10219-2:2006. Difference is expressed relative to CF RHS 120×80×5.
How to choose between them
On stiffness, RHS 120×80×5 has the larger second moment of area: 365 cm⁴ against 353 cm⁴, a difference of 3.4%. 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 (5.1 against 5.0 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, RHS 120×80×5 is the stronger section and CF RHS 120×80×5 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.