RHS 350×250×7.1 vs RHS 350×150×10
RHS 350×250×7.1 is an RHS Hot-finished section from the UK Blue Book (SCI P363); RHS 350×150×10 is the closest equivalent in the European (Continental) range. RHS 350×150×10 carries more bending: 1035 cm³ of plastic modulus against 999 cm³, 3.5% more. RHS 350×250×7.1 is the lighter of the two at 64.9 kg/m against 75 kg/m, a saving of 13.5% on steel weight. Both are about 350 mm deep, so either fits the same construction zone.
| Property | RHS 350×250×7.1 RHS Hot-finished | RHS 350×150×10 RHS | Difference |
|---|---|---|---|
| Mass | 64.9 kg/m | 75 kg/m | -13.5% |
| Depth | 350 mm | 350 mm | same |
| Width | 250 mm | 150 mm | +66.7% |
| Web thickness | 0 mm | 0 mm | same |
| Flange thickness | 0 mm | 0 mm | same |
| Area | 82.6 cm² | 94.9 cm² | -13.0% |
| Second moment, major (Iy) | 14,700 cm⁴ | 14,320 cm⁴ | +2.7% |
| Plastic modulus, major (Wpl,y) | 999 cm³ | 1,035 cm³ | -3.5% |
| Elastic modulus, major (Wel,y) | 843 cm³ | 818 cm³ | +3.1% |
| Second moment, minor (Iz) | 8,800 cm⁴ | 3,737 cm⁴ | +135.5% |
| Plastic modulus, minor (Wpl,z) | 794 cm³ | 566 cm³ | +40.3% |
| Radius of gyration, minor (iz) | 10.3 cm | 6.27 cm | +64.3% |
| Torsion constant (It) | 17,000 cm⁴ | 9,633 cm⁴ | +76.5% |
RHS 350×250×7.1 to BS EN 10210-2:2006; RHS 350×150×10 to EN 10219 · Continental Steel. Difference is expressed relative to RHS 350×150×10.
How to choose between them
On stiffness, RHS 350×250×7.1 has the larger second moment of area: 14,700 cm⁴ against 14,320 cm⁴, a difference of 2.7%. 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 350×250×7.1 is the more efficient section: 15.4 against 13.8 cm³ per kg/m, 11.5% 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 350×250×7.1 has 794 cm³ of minor-axis plastic modulus against 566 cm³, 40.3% 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 13.5% weight.
In short, RHS 350×150×10 is the stronger section and RHS 350×250×7.1 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.