RHS 100×60×7.1 vs RHS 100×75×6
RHS 100×60×7.1 is an RHS Hot-finished section from the UK Blue Book (SCI P363); RHS 100×75×6 is the closest equivalent in the European (Continental) range. RHS 100×60×7.1 carries more bending: 62.9 cm³ of plastic modulus against 61 cm³, 3.1% more. RHS 100×75×6 is the lighter of the two at 15.1 kg/m against 15.8 kg/m, a saving of 4.6% on steel weight. Both are about 100 mm deep, so either fits the same construction zone.
| Property | RHS 100×60×7.1 RHS Hot-finished | RHS 100×75×6 RHS Cold-formed | Difference |
|---|---|---|---|
| Mass | 15.8 kg/m | 15.1 kg/m | +4.6% |
| Depth | 100 mm | 100 mm | same |
| Width | 60 mm | 75 mm | -20.0% |
| Web thickness | 0 mm | 0 mm | same |
| Flange thickness | 0 mm | 0 mm | same |
| Area | 20.2 cm² | 18.6 cm² | +8.6% |
| Second moment, major (Iy) | 244 cm⁴ | 245 cm⁴ | -0.4% |
| Plastic modulus, major (Wpl,y) | 62.9 cm³ | 61 cm³ | +3.1% |
| Elastic modulus, major (Wel,y) | 48.8 cm³ | 49 cm³ | -0.4% |
| Second moment, minor (Iz) | 106 cm⁴ | 156 cm⁴ | -32.1% |
| Plastic modulus, minor (Wpl,z) | 43.2 cm³ | 49.9 cm³ | -13.4% |
| Radius of gyration, minor (iz) | 2.29 cm | 2.89 cm | -20.8% |
| Torsion constant (It) | 245 cm⁴ | 320 cm⁴ | -23.4% |
RHS 100×60×7.1 to BS EN 10210-2:2006; RHS 100×75×6 to EN 10219 · Continental Steel. Difference is expressed relative to RHS 100×75×6.
How to choose between them
On stiffness, RHS 100×75×6 has the larger second moment of area: 245 cm⁴ against 244 cm⁴, a difference of 0.4%. 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 (4.0 against 4.0 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 100×75×6 has 49.9 cm³ of minor-axis plastic modulus against 43.2 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 4.6% weight.
In short, RHS 100×60×7.1 is the stronger section and RHS 100×75×6 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.