RHS 150×100×8 vs RHS 150×100×9
RHS 150×100×8 is an RHS Hot-finished section from the UK Blue Book (SCI P363); RHS 150×100×9 is the closest equivalent in the European (Continental) range. RHS 150×100×9 carries more bending: 185 cm³ of plastic modulus against 180 cm³, 2.7% more. RHS 150×100×8 is the lighter of the two at 28.9 kg/m against 31.4 kg/m, a saving of 8.0% on steel weight. Both are about 150 mm deep, so either fits the same construction zone.
| Property | RHS 150×100×8 RHS Hot-finished | RHS 150×100×9 RHS Cold-formed | Difference |
|---|---|---|---|
| Mass | 28.9 kg/m | 31.4 kg/m | -8.0% |
| Depth | 150 mm | 150 mm | same |
| Width | 100 mm | 100 mm | same |
| Web thickness | 0 mm | 0 mm | same |
| Flange thickness | 0 mm | 0 mm | same |
| Area | 36.8 cm² | 39 cm² | -5.6% |
| Second moment, major (Iy) | 1,090 cm⁴ | 1,089 cm⁴ | +0.1% |
| Plastic modulus, major (Wpl,y) | 180 cm³ | 185 cm³ | -2.7% |
| Elastic modulus, major (Wel,y) | 145 cm³ | 145 cm³ | same |
| Second moment, minor (Iz) | 569 cm⁴ | 577 cm⁴ | -1.4% |
| Plastic modulus, minor (Wpl,z) | 135 cm³ | 140 cm³ | -3.6% |
| Radius of gyration, minor (iz) | 3.94 cm | 3.85 cm | +2.3% |
| Torsion constant (It) | 1,200 cm⁴ | 1,320 cm⁴ | -9.1% |
RHS 150×100×8 to BS EN 10210-2:2006; RHS 150×100×9 to EN 10219 · Continental Steel. Difference is expressed relative to RHS 150×100×9.
How to choose between them
On stiffness, RHS 150×100×8 has the larger second moment of area: 1,090 cm⁴ against 1,089 cm⁴, a difference of 0.1%. 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 150×100×8 is the more efficient section: 6.2 against 5.9 cm³ per kg/m, 5.7% 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.
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 8.0% weight.
In short, RHS 150×100×9 is the stronger section and RHS 150×100×8 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.