RHS 250×100×8 vs CF RHS 250×150×6.3
RHS 250×100×8 is an RHS Hot-finished section from the UK Blue Book (SCI P363); CF RHS 250×150×6.3 is the closest equivalent in the UK Blue Book (SCI P363). RHS 250×100×8 carries more bending: 404 cm³ of plastic modulus against 391 cm³, 3.3% more. CF RHS 250×150×6.3 is the lighter of the two at 37.2 kg/m against 41.4 kg/m, a saving of 11.3% on steel weight. Both are about 250 mm deep, so either fits the same construction zone.
| Property | RHS 250×100×8 RHS Hot-finished | CF RHS 250×150×6.3 RHS Cold-formed | Difference |
|---|---|---|---|
| Mass | 41.4 kg/m | 37.2 kg/m | +11.3% |
| Depth | 250 mm | 250 mm | same |
| Width | 100 mm | 150 mm | -33.3% |
| Web thickness | 0 mm | 0 mm | same |
| Flange thickness | 0 mm | 0 mm | same |
| Area | 52.8 cm² | 47.4 cm² | +11.4% |
| Second moment, major (Iy) | 3,940 cm⁴ | 4,000 cm⁴ | -1.5% |
| Plastic modulus, major (Wpl,y) | 404 cm³ | 391 cm³ | +3.3% |
| Elastic modulus, major (Wel,y) | 315 cm³ | 320 cm³ | -1.6% |
| Second moment, minor (Iz) | 909 cm⁴ | 1,830 cm⁴ | -50.3% |
| Plastic modulus, minor (Wpl,z) | 209 cm³ | 276 cm³ | -24.3% |
| Radius of gyration, minor (iz) | 4.15 cm | 6.2 cm | -33.1% |
| Torsion constant (It) | 2,430 cm⁴ | 4,080 cm⁴ | -40.4% |
RHS 250×100×8 to BS EN 10210-2:2006; CF RHS 250×150×6.3 to BS EN 10219-2:2006. Difference is expressed relative to CF RHS 250×150×6.3.
How to choose between them
On stiffness, CF RHS 250×150×6.3 has the larger second moment of area: 4,000 cm⁴ against 3,940 cm⁴, a difference of 1.5%. 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 250×150×6.3 is the more efficient section: 10.5 against 9.8 cm³ per kg/m, 7.2% 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 250×150×6.3 has 276 cm³ of minor-axis plastic modulus against 209 cm³, 24.3% 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, RHS 250×100×8 is the stronger section and CF RHS 250×150×6.3 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.