RHS 300×150×10 vs RHS 300×200×9
RHS 300×150×10 is an RHS Hot-finished section from the UK Blue Book (SCI P363); RHS 300×200×9 is the closest equivalent in the European (Continental) range. RHS 300×200×9 carries more bending: 840 cm³ of plastic modulus against 811 cm³, 3.5% more. They weigh essentially the same, 66.7 kg/m and 66.5 kg/m. Both are about 300 mm deep, so either fits the same construction zone.
| Property | RHS 300×150×10 RHS Hot-finished | RHS 300×200×9 RHS Cold-formed | Difference |
|---|---|---|---|
| Mass | 66.7 kg/m | 66.5 kg/m | +0.3% |
| Depth | 300 mm | 300 mm | same |
| Width | 150 mm | 200 mm | -25.0% |
| Web thickness | 0 mm | 0 mm | same |
| Flange thickness | 0 mm | 0 mm | same |
| Area | 84.9 cm² | 84 cm² | +1.1% |
| Second moment, major (Iy) | 9,720 cm⁴ | 10,370 cm⁴ | -6.3% |
| Plastic modulus, major (Wpl,y) | 811 cm³ | 840 cm³ | -3.5% |
| Elastic modulus, major (Wel,y) | 648 cm³ | 691 cm³ | -6.2% |
| Second moment, minor (Iz) | 3,250 cm⁴ | 5,561 cm⁴ | -41.6% |
| Plastic modulus, minor (Wpl,z) | 496 cm³ | 637 cm³ | -22.1% |
| Radius of gyration, minor (iz) | 6.18 cm | 8.14 cm | -24.1% |
| Torsion constant (It) | 7,840 cm⁴ | 11,820 cm⁴ | -33.7% |
RHS 300×150×10 to BS EN 10210-2:2006; RHS 300×200×9 to EN 10219 · Continental Steel. Difference is expressed relative to RHS 300×200×9.
How to choose between them
On stiffness, RHS 300×200×9 has the larger second moment of area: 10,370 cm⁴ against 9,720 cm⁴, a difference of 6.3%. 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 300×200×9 is the more efficient section: 12.6 against 12.2 cm³ per kg/m, 3.7% 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 300×200×9 has 637 cm³ of minor-axis plastic modulus against 496 cm³, 22.1% 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 a marginal amount of weight.
In short, RHS 300×200×9 is the stronger section and RHS 300×200×9 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.