RHS 120×80×3.6 vs RHS 125×50×4.6
RHS 120×80×3.6 is an RHS Hot-finished section from the UK Blue Book (SCI P363); RHS 125×50×4.6 is the closest equivalent in the European (Continental) range. They are close to interchangeable in bending: 55.6 cm³ against 55.2 cm³ of plastic modulus, a difference of 0.7%. RHS 120×80×3.6 is the lighter of the two at 10.8 kg/m against 11.7 kg/m, a saving of 7.7% on steel weight. RHS 125×50×4.6 is deeper, 125 mm against 120 mm, which matters where the floor zone is tight.
| Property | RHS 120×80×3.6 RHS Hot-finished | RHS 125×50×4.6 RHS Cold-formed | Difference |
|---|---|---|---|
| Mass | 10.8 kg/m | 11.7 kg/m | -7.7% |
| Depth | 120 mm | 125 mm | -4.0% |
| Width | 80 mm | 50 mm | +60.0% |
| Web thickness | 0 mm | 0 mm | same |
| Flange thickness | 0 mm | 0 mm | same |
| Area | 13.7 cm² | 14.7 cm² | -6.8% |
| Second moment, major (Iy) | 276 cm⁴ | 265 cm⁴ | +4.2% |
| Plastic modulus, major (Wpl,y) | 55.6 cm³ | 55.2 cm³ | +0.7% |
| Elastic modulus, major (Wel,y) | 46 cm³ | 42.4 cm³ | +8.5% |
| Second moment, minor (Iz) | 147 cm⁴ | 61.5 cm⁴ | +139.0% |
| Plastic modulus, minor (Wpl,z) | 42 cm³ | 28.6 cm³ | +46.9% |
| Radius of gyration, minor (iz) | 3.27 cm | 2.04 cm | +60.3% |
| Torsion constant (It) | 301 cm⁴ | 170 cm⁴ | +77.1% |
RHS 120×80×3.6 to BS EN 10210-2:2006; RHS 125×50×4.6 to EN 10219 · Continental Steel. Difference is expressed relative to RHS 125×50×4.6.
How to choose between them
On stiffness, RHS 120×80×3.6 has the larger second moment of area: 276 cm⁴ against 265 cm⁴, a difference of 4.2%. 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 120×80×3.6 is the more efficient section: 5.1 against 4.7 cm³ per kg/m, 9.1% 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 120×80×3.6 has 42 cm³ of minor-axis plastic modulus against 28.6 cm³, 46.9% 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 7.7% weight.
In short, RHS 120×80×3.6 is the stronger section and RHS 120×80×3.6 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.