SHS 150×150×7.1 vs CF SHS 150×150×8
SHS 150×150×7.1 is an SHS Hot-finished section from the UK Blue Book (SCI P363); CF SHS 150×150×8 is the closest equivalent in the UK Blue Book (SCI P363). CF SHS 150×150×8 carries more bending: 226 cm³ of plastic modulus against 213 cm³, 5.8% more. SHS 150×150×7.1 is the lighter of the two at 31.4 kg/m against 33.9 kg/m, a saving of 7.4% on steel weight. Both are about 150 mm deep, so either fits the same construction zone.
| Property | SHS 150×150×7.1 SHS Hot-finished | CF SHS 150×150×8 SHS Cold-formed | Difference |
|---|---|---|---|
| Mass | 31.4 kg/m | 33.9 kg/m | -7.4% |
| Depth | 150 mm | 150 mm | same |
| Width | 150 mm | 150 mm | same |
| Web thickness | 0 mm | 0 mm | same |
| Flange thickness | 0 mm | 0 mm | same |
| Area | 40 cm² | 43.2 cm² | -7.4% |
| Second moment, major (Iy) | 1,350 cm⁴ | 1,410 cm⁴ | -4.3% |
| Plastic modulus, major (Wpl,y) | 213 cm³ | 226 cm³ | -5.8% |
| Elastic modulus, major (Wel,y) | 180 cm³ | 188 cm³ | -4.3% |
| Second moment, minor (Iz) | 1,350 cm⁴ | 1,410 cm⁴ | -4.3% |
| Plastic modulus, minor (Wpl,z) | 213 cm³ | 226 cm³ | -5.8% |
| Radius of gyration, minor (iz) | 5.81 cm | 5.71 cm | +1.8% |
| Torsion constant (It) | 2,120 cm⁴ | 2,360 cm⁴ | -10.2% |
SHS 150×150×7.1 to BS EN 10210-2:2006; CF SHS 150×150×8 to BS EN 10219-2:2006. Difference is expressed relative to CF SHS 150×150×8.
How to choose between them
On stiffness, CF SHS 150×150×8 has the larger second moment of area: 1,410 cm⁴ against 1,350 cm⁴, a difference of 4.3%. 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, the two are equally efficient (6.8 against 6.7 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.
In short, CF SHS 150×150×8 is the stronger section and SHS 150×150×7.1 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.