SHS 100×100×7.1 vs CF SHS 100×100×8
SHS 100×100×7.1 is an SHS Hot-finished section from the UK Blue Book (SCI P363); CF SHS 100×100×8 is the closest equivalent in the UK Blue Book (SCI P363). CF SHS 100×100×8 carries more bending: 91.1 cm³ of plastic modulus against 89.2 cm³, 2.1% more. SHS 100×100×7.1 is the lighter of the two at 20.3 kg/m against 21.4 kg/m, a saving of 5.1% on steel weight. Both are about 100 mm deep, so either fits the same construction zone.
| Property | SHS 100×100×7.1 SHS Hot-finished | CF SHS 100×100×8 SHS Cold-formed | Difference |
|---|---|---|---|
| Mass | 20.3 kg/m | 21.4 kg/m | -5.1% |
| Depth | 100 mm | 100 mm | same |
| Width | 100 mm | 100 mm | same |
| Web thickness | 0 mm | 0 mm | same |
| Flange thickness | 0 mm | 0 mm | same |
| Area | 25.8 cm² | 27.2 cm² | -5.1% |
| Second moment, major (Iy) | 367 cm⁴ | 366 cm⁴ | +0.3% |
| Plastic modulus, major (Wpl,y) | 89.2 cm³ | 91.1 cm³ | -2.1% |
| Elastic modulus, major (Wel,y) | 73.4 cm³ | 73.2 cm³ | +0.3% |
| Second moment, minor (Iz) | 367 cm⁴ | 366 cm⁴ | +0.3% |
| Plastic modulus, minor (Wpl,z) | 89.2 cm³ | 91.1 cm³ | -2.1% |
| Radius of gyration, minor (iz) | 3.77 cm | 3.67 cm | +2.7% |
| Torsion constant (It) | 589 cm⁴ | 645 cm⁴ | -8.7% |
SHS 100×100×7.1 to BS EN 10210-2:2006; CF SHS 100×100×8 to BS EN 10219-2:2006. Difference is expressed relative to CF SHS 100×100×8.
How to choose between them
On stiffness, SHS 100×100×7.1 has the larger second moment of area: 367 cm⁴ against 366 cm⁴, a difference of 0.3%. 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, SHS 100×100×7.1 is the more efficient section: 4.4 against 4.3 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.
In short, CF SHS 100×100×8 is the stronger section and SHS 100×100×7.1 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.