RHS 180×100×6.3 vs CF RHS 180×80×8
RHS 180×100×6.3 is an RHS Hot-finished section from the UK Blue Book (SCI P363); CF RHS 180×80×8 is the closest equivalent in the UK Blue Book (SCI P363). CF RHS 180×80×8 carries more bending: 198 cm³ of plastic modulus against 194 cm³, 2.0% more. RHS 180×100×6.3 is the lighter of the two at 26.1 kg/m against 28.9 kg/m, a saving of 9.7% on steel weight. Both are about 180 mm deep, so either fits the same construction zone.
| Property | RHS 180×100×6.3 RHS Hot-finished | CF RHS 180×80×8 RHS Cold-formed | Difference |
|---|---|---|---|
| Mass | 26.1 kg/m | 28.9 kg/m | -9.7% |
| Depth | 180 mm | 180 mm | same |
| Width | 100 mm | 80 mm | +25.0% |
| Web thickness | 0 mm | 0 mm | same |
| Flange thickness | 0 mm | 0 mm | same |
| Area | 33.3 cm² | 36.8 cm² | -9.5% |
| Second moment, major (Iy) | 1,410 cm⁴ | 1,360 cm⁴ | +3.7% |
| Plastic modulus, major (Wpl,y) | 194 cm³ | 198 cm³ | -2.0% |
| Elastic modulus, major (Wel,y) | 156 cm³ | 151 cm³ | +3.3% |
| Second moment, minor (Iz) | 557 cm⁴ | 377 cm⁴ | +47.7% |
| Plastic modulus, minor (Wpl,z) | 128 cm³ | 111 cm³ | +15.3% |
| Radius of gyration, minor (iz) | 4.09 cm | 3.2 cm | +27.8% |
| Torsion constant (It) | 1,280 cm⁴ | 1,040 cm⁴ | +23.1% |
RHS 180×100×6.3 to BS EN 10210-2:2006; CF RHS 180×80×8 to BS EN 10219-2:2006. Difference is expressed relative to CF RHS 180×80×8.
How to choose between them
On stiffness, RHS 180×100×6.3 has the larger second moment of area: 1,410 cm⁴ against 1,360 cm⁴, a difference of 3.7%. 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 180×100×6.3 is the more efficient section: 7.4 against 6.9 cm³ per kg/m, 8.5% 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: RHS 180×100×6.3 has 128 cm³ of minor-axis plastic modulus against 111 cm³, 15.3% 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, CF RHS 180×80×8 is the stronger section and RHS 180×100×6.3 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.