RHS 150×100×7.1 vs RHS 150×100×8
RHS 150×100×7.1 is an RHS Hot-finished section from the UK Blue Book (SCI P363); RHS 150×100×8 is the closest equivalent in the European (Continental) range. RHS 150×100×8 carries more bending: 169 cm³ of plastic modulus against 163 cm³, 3.6% more. RHS 150×100×7.1 is the lighter of the two at 25.9 kg/m against 27.7 kg/m, a saving of 6.5% on steel weight. Both are about 150 mm deep, so either fits the same construction zone.
| Property | RHS 150×100×7.1 RHS Hot-finished | RHS 150×100×8 RHS Cold-formed | Difference |
|---|---|---|---|
| Mass | 25.9 kg/m | 27.7 kg/m | -6.5% |
| 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 | 32.9 cm² | 35.2 cm² | -6.5% |
| Second moment, major (Iy) | 990 cm⁴ | 1,008 cm⁴ | -1.8% |
| Plastic modulus, major (Wpl,y) | 163 cm³ | 169 cm³ | -3.6% |
| Elastic modulus, major (Wel,y) | 132 cm³ | 134 cm³ | -1.5% |
| Second moment, minor (Iz) | 520 cm⁴ | 536 cm⁴ | -3.0% |
| Plastic modulus, minor (Wpl,z) | 122 cm³ | 128 cm³ | -4.7% |
| Radius of gyration, minor (iz) | 3.97 cm | 3.9 cm | +1.8% |
| Torsion constant (It) | 1,090 cm⁴ | 1,206 cm⁴ | -9.6% |
RHS 150×100×7.1 to BS EN 10210-2:2006; RHS 150×100×8 to EN 10219 · Continental Steel. Difference is expressed relative to RHS 150×100×8.
How to choose between them
On stiffness, RHS 150×100×8 has the larger second moment of area: 1,008 cm⁴ against 990 cm⁴, a difference of 1.8%. 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 150×100×7.1 is the more efficient section: 6.3 against 6.1 cm³ per kg/m, 3.2% 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 6.5% weight.
In short, RHS 150×100×8 is the stronger section and RHS 150×100×7.1 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.