RHS 180×100×8 vs RHS 175×100×9.5
RHS 180×100×8 is an RHS Hot-finished section from the UK Blue Book (SCI P363); RHS 175×100×9.5 is the closest equivalent in the European (Continental) range. RHS 175×100×9.5 carries more bending: 246 cm³ of plastic modulus against 239 cm³, 2.8% more. RHS 180×100×8 is the lighter of the two at 32.6 kg/m against 37.1 kg/m, a saving of 12.1% on steel weight. RHS 180×100×8 is deeper, 180 mm against 175 mm, which matters where the floor zone is tight.
| Property | RHS 180×100×8 RHS Hot-finished | RHS 175×100×9.5 RHS Cold-formed | Difference |
|---|---|---|---|
| Mass | 32.6 kg/m | 37.1 kg/m | -12.1% |
| Depth | 180 mm | 175 mm | +2.9% |
| Width | 100 mm | 100 mm | same |
| Web thickness | 0 mm | 0 mm | same |
| Flange thickness | 0 mm | 0 mm | same |
| Area | 41.6 cm² | 45.5 cm² | -8.6% |
| Second moment, major (Iy) | 1,710 cm⁴ | 1,673 cm⁴ | +2.2% |
| Plastic modulus, major (Wpl,y) | 239 cm³ | 246 cm³ | -2.8% |
| Elastic modulus, major (Wel,y) | 190 cm³ | 191 cm³ | -0.5% |
| Second moment, minor (Iz) | 671 cm⁴ | 694 cm⁴ | -3.3% |
| Plastic modulus, minor (Wpl,z) | 157 cm³ | 166 cm³ | -5.4% |
| Radius of gyration, minor (iz) | 4.02 cm | 3.9 cm | +3.1% |
| Torsion constant (It) | 1,560 cm⁴ | 1,719 cm⁴ | -9.2% |
RHS 180×100×8 to BS EN 10210-2:2006; RHS 175×100×9.5 to EN 10219 · Continental Steel. Difference is expressed relative to RHS 175×100×9.5.
How to choose between them
On stiffness, RHS 180×100×8 has the larger second moment of area: 1,710 cm⁴ against 1,673 cm⁴, a difference of 2.2%. 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 180×100×8 is the more efficient section: 7.3 against 6.6 cm³ per kg/m, 10.6% 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.
These come from different catalogues, so treat the swap as a design change rather than a like-for-like substitution. RHS Hot-finished sections are specified to BS EN 10210-2:2006 and RHS Cold-formed to EN 10219 · Continental Steel; the steel grades, tolerances and available lengths differ, connection details and bolt gauges are set out differently, and the two are rarely both stocked in the same market. Check availability before changing a section on a drawing to save 12.1% weight.
In short, RHS 175×100×9.5 is the stronger section and RHS 180×100×8 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.