RHS 150×100×7.1 vs CF RHS 150×100×8
RHS 150×100×7.1 is an RHS Hot-finished section from the UK Blue Book (SCI P363); CF RHS 150×100×8 is the closest equivalent in the UK Blue Book (SCI P363). CF RHS 150×100×8 carries more bending: 169 cm³ of plastic modulus against 163 cm³, 3.6% more. RHS 150×100×7.1 is the lighter of the two at 25.9 kg/m against 27.7 kg/m, a saving of 6.5% on steel weight. Both are about 150 mm deep, so either fits the same construction zone.
| Property | RHS 150×100×7.1 RHS Hot-finished | CF RHS 150×100×8 RHS Cold-formed | Difference |
|---|---|---|---|
| Mass | 25.9 kg/m | 27.7 kg/m | -6.5% |
| Depth | 150 mm | 150 mm | same |
| Width | 100 mm | 100 mm | same |
| Web thickness | 0 mm | 0 mm | same |
| Flange thickness | 0 mm | 0 mm | same |
| Area | 32.9 cm² | 35.2 cm² | -6.5% |
| Second moment, major (Iy) | 990 cm⁴ | 1,010 cm⁴ | -2.0% |
| Plastic modulus, major (Wpl,y) | 163 cm³ | 169 cm³ | -3.6% |
| Elastic modulus, major (Wel,y) | 132 cm³ | 134 cm³ | -1.5% |
| Second moment, minor (Iz) | 520 cm⁴ | 536 cm⁴ | -3.0% |
| Plastic modulus, minor (Wpl,z) | 122 cm³ | 128 cm³ | -4.7% |
| Radius of gyration, minor (iz) | 3.97 cm | 3.9 cm | +1.8% |
| Torsion constant (It) | 1,090 cm⁴ | 1,210 cm⁴ | -9.9% |
RHS 150×100×7.1 to BS EN 10210-2:2006; CF RHS 150×100×8 to BS EN 10219-2:2006. Difference is expressed relative to CF RHS 150×100×8.
How to choose between them
On stiffness, CF RHS 150×100×8 has the larger second moment of area: 1,010 cm⁴ against 990 cm⁴, a difference of 2.0%. 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, RHS 150×100×7.1 is the more efficient section: 6.3 against 6.1 cm³ per kg/m, 3.2% 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.
In short, CF RHS 150×100×8 is the stronger section and RHS 150×100×7.1 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.