RHS 50×30×3.6 vs RHS 50×25×4
RHS 50×30×3.6 is an RHS Hot-finished section from the UK Blue Book (SCI P363); RHS 50×25×4 is the closest equivalent in the European (Continental) range. RHS 50×30×3.6 carries more bending: 7.94 cm³ of plastic modulus against 7.67 cm³, 3.5% more. They weigh essentially the same, 4.01 kg/m and 4.07 kg/m. Both are about 50 mm deep, so either fits the same construction zone.
| Property | RHS 50×30×3.6 RHS Hot-finished | RHS 50×25×4 RHS | Difference |
|---|---|---|---|
| Mass | 4 kg/m | 4.1 kg/m | -1.5% |
| Depth | 50 mm | 50 mm | same |
| Width | 30 mm | 25 mm | +20.0% |
| Web thickness | 0 mm | 0 mm | same |
| Flange thickness | 0 mm | 0 mm | same |
| Area | 5.1 cm² | 5.2 cm² | -1.7% |
| Second moment, major (Iy) | 15.4 cm⁴ | 14.4 cm⁴ | +6.9% |
| Plastic modulus, major (Wpl,y) | 7.94 cm³ | 7.67 cm³ | +3.5% |
| Elastic modulus, major (Wel,y) | 6.16 cm³ | 5.75 cm³ | +7.1% |
| Second moment, minor (Iz) | 6.67 cm⁴ | 4.48 cm⁴ | +48.9% |
| Plastic modulus, minor (Wpl,z) | 5.46 cm³ | 4.54 cm³ | +20.3% |
| Radius of gyration, minor (iz) | 1.14 cm | 0.929 cm | +22.7% |
| Torsion constant (It) | 15.4 cm⁴ | 11.6 cm⁴ | +32.8% |
RHS 50×30×3.6 to BS EN 10210-2:2006; RHS 50×25×4 to EN 10219 · Continental Steel. Difference is expressed relative to RHS 50×25×4.
How to choose between them
On stiffness, RHS 50×30×3.6 has the larger second moment of area: 15.4 cm⁴ against 14.4 cm⁴, a difference of 6.9%. 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 of steel, RHS 50×30×3.6 is the more efficient section: 2.0 against 1.9 cm³ per kg/m, 5.1% 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 50×30×3.6 has 5.46 cm³ of minor-axis plastic modulus against 4.54 cm³, 20.3% 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.
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 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 a marginal amount of weight.
In short, RHS 50×30×3.6 is the stronger section and RHS 50×30×3.6 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.