RHS 180×100×12.5 vs RHS 175×125×12.7
RHS 180×100×12.5 is an RHS Hot-finished section from the UK Blue Book (SCI P363); RHS 175×125×12.7 is the closest equivalent in the European (Continental) range. They are close to interchangeable in bending: 344 cm³ against 340 cm³ of plastic modulus, a difference of 1.2%. RHS 180×100×12.5 is the lighter of the two at 48.7 kg/m against 52.4 kg/m, a saving of 7.1% on steel weight. RHS 180×100×12.5 is deeper, 180 mm against 175 mm, which matters where the floor zone is tight.
| Property | RHS 180×100×12.5 RHS Hot-finished | RHS 175×125×12.7 RHS Cold-formed | Difference |
|---|---|---|---|
| Mass | 48.7 kg/m | 52.4 kg/m | -7.1% |
| 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 | 62.1 cm² | 62.8 cm² | -1.1% |
| Second moment, major (Iy) | 2,390 cm⁴ | 2,286 cm⁴ | +4.5% |
| Plastic modulus, major (Wpl,y) | 344 cm³ | 340 cm³ | +1.2% |
| Elastic modulus, major (Wel,y) | 265 cm³ | 261 cm³ | +1.5% |
| Second moment, minor (Iz) | 908 cm⁴ | 1,356 cm⁴ | -33.0% |
| Plastic modulus, minor (Wpl,z) | 223 cm³ | 270 cm³ | -17.4% |
| Radius of gyration, minor (iz) | 3.82 cm | 4.65 cm | -17.8% |
| Torsion constant (It) | 2,190 cm⁴ | 3,129 cm⁴ | -30.0% |
RHS 180×100×12.5 to BS EN 10210-2:2006; RHS 175×125×12.7 to EN 10219 · Continental Steel. Difference is expressed relative to RHS 175×125×12.7.
How to choose between them
On stiffness, RHS 180×100×12.5 has the larger second moment of area: 2,390 cm⁴ against 2,286 cm⁴, a difference of 4.5%. 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 180×100×12.5 is the more efficient section: 7.1 against 6.5 cm³ per kg/m, 8.9% 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 175×125×12.7 has 270 cm³ of minor-axis plastic modulus against 223 cm³, 17.4% 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 7.1% weight.
In short, RHS 180×100×12.5 is the stronger section and RHS 180×100×12.5 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.