RHS 180×100×7.1 vs RHS 175×125×6.4
RHS 180×100×7.1 is an RHS Hot-finished section from the UK Blue Book (SCI P363); RHS 175×125×6.4 is the closest equivalent in the European (Continental) range. RHS 180×100×7.1 carries more bending: 215 cm³ of plastic modulus against 207 cm³, 3.9% more. RHS 175×125×6.4 is the lighter of the two at 28.3 kg/m against 29.2 kg/m, a saving of 3.2% on steel weight. RHS 180×100×7.1 is deeper, 180 mm against 175 mm, which matters where the floor zone is tight.
| Property | RHS 180×100×7.1 RHS Hot-finished | RHS 175×125×6.4 RHS Cold-formed | Difference |
|---|---|---|---|
| Mass | 29.2 kg/m | 28.3 kg/m | +3.2% |
| Depth | 180 mm | 175 mm | +2.9% |
| Width | 100 mm | 125 mm | -20.0% |
| Web thickness | 0 mm | 0 mm | same |
| Flange thickness | 0 mm | 0 mm | same |
| Area | 37.2 cm² | 35.4 cm² | +5.1% |
| Second moment, major (Iy) | 1,560 cm⁴ | 1,487 cm⁴ | +4.9% |
| Plastic modulus, major (Wpl,y) | 215 cm³ | 207 cm³ | +3.9% |
| Elastic modulus, major (Wel,y) | 173 cm³ | 170 cm³ | +1.8% |
| Second moment, minor (Iz) | 613 cm⁴ | 886 cm⁴ | -30.8% |
| Plastic modulus, minor (Wpl,z) | 142 cm³ | 164 cm³ | -13.4% |
| Radius of gyration, minor (iz) | 4.06 cm | 5.01 cm | -19.0% |
| Torsion constant (It) | 1,410 cm⁴ | 1,832 cm⁴ | -23.0% |
RHS 180×100×7.1 to BS EN 10210-2:2006; RHS 175×125×6.4 to EN 10219 · Continental Steel. Difference is expressed relative to RHS 175×125×6.4.
How to choose between them
On stiffness, RHS 180×100×7.1 has the larger second moment of area: 1,560 cm⁴ against 1,487 cm⁴, a difference of 4.9%. 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, the two are equally efficient (7.4 against 7.3 cm³ per kg/m), so on a pure steel-tonnage basis there is nothing to choose between them. The decision comes down to availability, connection detailing and what the rest of the frame uses.
The two diverge much more about the minor axis: RHS 175×125×6.4 has 164 cm³ of minor-axis plastic modulus against 142 cm³, 13.4% 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.
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 3.2% weight.
In short, RHS 180×100×7.1 is the stronger section and RHS 175×125×6.4 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.