RHS 120×80×7.1 vs CF RHS 120×80×8
RHS 120×80×7.1 is an RHS Hot-finished section from the UK Blue Book (SCI P363); CF RHS 120×80×8 is the closest equivalent in the UK Blue Book (SCI P363). They are close to interchangeable in bending: 100 cm³ against 102 cm³ of plastic modulus, a difference of 2.0%. RHS 120×80×7.1 is the lighter of the two at 20.3 kg/m against 21.4 kg/m, a saving of 5.1% on steel weight. Both are about 120 mm deep, so either fits the same construction zone.
| Property | RHS 120×80×7.1 RHS Hot-finished | CF RHS 120×80×8 RHS Cold-formed | Difference |
|---|---|---|---|
| Mass | 20.3 kg/m | 21.4 kg/m | -5.1% |
| 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 | 25.8 cm² | 27.2 cm² | -5.1% |
| Second moment, major (Iy) | 482 cm⁴ | 476 cm⁴ | +1.3% |
| Plastic modulus, major (Wpl,y) | 100 cm³ | 102 cm³ | -2.0% |
| Elastic modulus, major (Wel,y) | 80.3 cm³ | 79.3 cm³ | +1.3% |
| Second moment, minor (Iz) | 251 cm⁴ | 252 cm⁴ | -0.4% |
| Plastic modulus, minor (Wpl,z) | 75.2 cm³ | 76.9 cm³ | -2.2% |
| Radius of gyration, minor (iz) | 3.12 cm | 3.04 cm | +2.6% |
| Torsion constant (It) | 535 cm⁴ | 584 cm⁴ | -8.4% |
RHS 120×80×7.1 to BS EN 10210-2:2006; CF RHS 120×80×8 to BS EN 10219-2:2006. Difference is expressed relative to CF RHS 120×80×8.
How to choose between them
On stiffness, RHS 120×80×7.1 has the larger second moment of area: 482 cm⁴ against 476 cm⁴, a difference of 1.3%. 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 120×80×7.1 is the more efficient section: 4.9 against 4.8 cm³ per kg/m, 3.4% 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 120×80×8 is the stronger section and RHS 120×80×7.1 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.