RHS 120×60×8.8 vs CF RHS 120×80×8
RHS 120×60×8.8 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). CF RHS 120×80×8 carries more bending: 102 cm³ of plastic modulus against 99.6 cm³, 2.4% more. They weigh essentially the same, 21.8 kg/m and 21.4 kg/m. Both are about 120 mm deep, so either fits the same construction zone.
| Property | RHS 120×60×8.8 RHS Hot-finished | CF RHS 120×80×8 RHS Cold-formed | Difference |
|---|---|---|---|
| Mass | 21.8 kg/m | 21.4 kg/m | +1.9% |
| Depth | 120 mm | 120 mm | same |
| Width | 60 mm | 80 mm | -25.0% |
| Web thickness | 0 mm | 0 mm | same |
| Flange thickness | 0 mm | 0 mm | same |
| Area | 27.8 cm² | 27.2 cm² | +2.2% |
| Second moment, major (Iy) | 452 cm⁴ | 476 cm⁴ | -5.0% |
| Plastic modulus, major (Wpl,y) | 99.6 cm³ | 102 cm³ | -2.4% |
| Elastic modulus, major (Wel,y) | 75.3 cm³ | 79.3 cm³ | -5.0% |
| Second moment, minor (Iz) | 142 cm⁴ | 252 cm⁴ | -43.7% |
| Plastic modulus, minor (Wpl,z) | 59.2 cm³ | 76.9 cm³ | -23.0% |
| Radius of gyration, minor (iz) | 2.27 cm | 3.04 cm | -25.3% |
| Torsion constant (It) | 366 cm⁴ | 584 cm⁴ | -37.3% |
RHS 120×60×8.8 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, CF RHS 120×80×8 has the larger second moment of area: 476 cm⁴ against 452 cm⁴, a difference of 5.0%. 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, CF RHS 120×80×8 is the more efficient section: 4.8 against 4.6 cm³ per kg/m, 4.1% 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.
The two diverge much more about the minor axis: CF RHS 120×80×8 has 76.9 cm³ of minor-axis plastic modulus against 59.2 cm³, 23.0% more. If the member sees any minor-axis bending, or if it is unrestrained over a long length and lateral-torsional buckling governs, that gap decides the comparison even though the major-axis numbers looked equivalent.
In short, CF RHS 120×80×8 is the stronger section and CF RHS 120×80×8 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.