RHS 100×50×4 vs RHS 100×60×3.6
RHS 100×50×4 is an RHS Hot-finished section from the UK Blue Book (SCI P363); RHS 100×60×3.6 is the closest equivalent in the European (Continental) range. They are close to interchangeable in bending: 35.2 cm³ against 34.7 cm³ of plastic modulus, a difference of 1.4%. RHS 100×60×3.6 is the lighter of the two at 8.37 kg/m against 8.78 kg/m, a saving of 4.9% on steel weight. Both are about 100 mm deep, so either fits the same construction zone.
| Property | RHS 100×50×4 RHS Hot-finished | RHS 100×60×3.6 RHS Cold-formed | Difference |
|---|---|---|---|
| Mass | 8.8 kg/m | 8.4 kg/m | +4.9% |
| Depth | 100 mm | 100 mm | same |
| Width | 50 mm | 60 mm | -16.7% |
| Web thickness | 0 mm | 0 mm | same |
| Flange thickness | 0 mm | 0 mm | same |
| Area | 11.2 cm² | 10.7 cm² | +4.7% |
| Second moment, major (Iy) | 140 cm⁴ | 140 cm⁴ | same |
| Plastic modulus, major (Wpl,y) | 35.2 cm³ | 34.7 cm³ | +1.4% |
| Elastic modulus, major (Wel,y) | 27.9 cm³ | 28 cm³ | -0.4% |
| Second moment, minor (Iz) | 46.2 cm⁴ | 63.3 cm⁴ | -27.0% |
| Plastic modulus, minor (Wpl,z) | 21.5 cm³ | 24.3 cm³ | -11.5% |
| Radius of gyration, minor (iz) | 2.03 cm | 2.44 cm | -16.8% |
| Torsion constant (It) | 113 cm⁴ | 143 cm⁴ | -21.0% |
RHS 100×50×4 to BS EN 10210-2:2006; RHS 100×60×3.6 to EN 10219 · Continental Steel. Difference is expressed relative to RHS 100×60×3.6.
How to choose between them
On stiffness, RHS 100×60×3.6 has the larger second moment of area: 140 cm⁴ against 140 cm⁴, a difference of 0.0%. 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 100×60×3.6 is the more efficient section: 4.1 against 4.0 cm³ per kg/m, 3.3% 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 100×60×3.6 has 24.3 cm³ of minor-axis plastic modulus against 21.5 cm³, 11.5% 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 4.9% weight.
In short, RHS 100×50×4 is the stronger section and RHS 100×60×3.6 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.