RHS 200×100×7.1 vs CF RHS 200×100×8
RHS 200×100×7.1 is an RHS Hot-finished section from the UK Blue Book (SCI P363); CF RHS 200×100×8 is the closest equivalent in the UK Blue Book (SCI P363). CF RHS 200×100×8 carries more bending: 267 cm³ of plastic modulus against 254 cm³, 4.9% more. RHS 200×100×7.1 is the lighter of the two at 31.4 kg/m against 33.9 kg/m, a saving of 7.4% on steel weight. Both are about 200 mm deep, so either fits the same construction zone.
| Property | RHS 200×100×7.1 RHS Hot-finished | CF RHS 200×100×8 RHS Cold-formed | Difference |
|---|---|---|---|
| Mass | 31.4 kg/m | 33.9 kg/m | -7.4% |
| Depth | 200 mm | 200 mm | same |
| Width | 100 mm | 100 mm | same |
| Web thickness | 0 mm | 0 mm | same |
| Flange thickness | 0 mm | 0 mm | same |
| Area | 40 cm² | 43.2 cm² | -7.4% |
| Second moment, major (Iy) | 2,020 cm⁴ | 2,090 cm⁴ | -3.3% |
| Plastic modulus, major (Wpl,y) | 254 cm³ | 267 cm³ | -4.9% |
| Elastic modulus, major (Wel,y) | 202 cm³ | 209 cm³ | -3.3% |
| Second moment, minor (Iz) | 674 cm⁴ | 705 cm⁴ | -4.4% |
| Plastic modulus, minor (Wpl,z) | 155 cm³ | 165 cm³ | -6.1% |
| Radius of gyration, minor (iz) | 4.1 cm | 4.04 cm | +1.5% |
| Torsion constant (It) | 1,630 cm⁴ | 1,810 cm⁴ | -9.9% |
RHS 200×100×7.1 to BS EN 10210-2:2006; CF RHS 200×100×8 to BS EN 10219-2:2006. Difference is expressed relative to CF RHS 200×100×8.
How to choose between them
On stiffness, CF RHS 200×100×8 has the larger second moment of area: 2,090 cm⁴ against 2,020 cm⁴, a difference of 3.3%. 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 200×100×7.1 is the more efficient section: 8.1 against 7.9 cm³ per kg/m, 2.7% 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 200×100×8 is the stronger section and RHS 200×100×7.1 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.