RHS 160×80×7.1 vs CF RHS 160×80×8
RHS 160×80×7.1 is an RHS Hot-finished section from the UK Blue Book (SCI P363); CF RHS 160×80×8 is the closest equivalent in the UK Blue Book (SCI P363). CF RHS 160×80×8 carries more bending: 163 cm³ of plastic modulus against 158 cm³, 3.1% more. RHS 160×80×7.1 is the lighter of the two at 24.7 kg/m against 26.4 kg/m, a saving of 6.4% on steel weight. Both are about 160 mm deep, so either fits the same construction zone.
| Property | RHS 160×80×7.1 RHS Hot-finished | CF RHS 160×80×8 RHS Cold-formed | Difference |
|---|---|---|---|
| Mass | 24.7 kg/m | 26.4 kg/m | -6.4% |
| Depth | 160 mm | 160 mm | same |
| Width | 80 mm | 80 mm | same |
| Web thickness | 0 mm | 0 mm | same |
| Flange thickness | 0 mm | 0 mm | same |
| Area | 31.5 cm² | 33.6 cm² | -6.3% |
| Second moment, major (Iy) | 994 cm⁴ | 1,000 cm⁴ | -0.6% |
| Plastic modulus, major (Wpl,y) | 158 cm³ | 163 cm³ | -3.1% |
| Elastic modulus, major (Wel,y) | 124 cm³ | 125 cm³ | -0.8% |
| Second moment, minor (Iz) | 327 cm⁴ | 335 cm⁴ | -2.4% |
| Plastic modulus, minor (Wpl,z) | 95.9 cm³ | 100 cm³ | -4.1% |
| Radius of gyration, minor (iz) | 3.22 cm | 3.16 cm | +1.9% |
| Torsion constant (It) | 804 cm⁴ | 882 cm⁴ | -8.8% |
RHS 160×80×7.1 to BS EN 10210-2:2006; CF RHS 160×80×8 to BS EN 10219-2:2006. Difference is expressed relative to CF RHS 160×80×8.
How to choose between them
On stiffness, CF RHS 160×80×8 has the larger second moment of area: 1,000 cm⁴ against 994 cm⁴, a difference of 0.6%. 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 160×80×7.1 is the more efficient section: 6.4 against 6.2 cm³ per kg/m, 3.6% 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 160×80×8 is the stronger section and RHS 160×80×7.1 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.