SHS 180×180×8 vs SHS 175×175×9
SHS 180×180×8 is an SHS Hot-finished section from the UK Blue Book (SCI P363); SHS 175×175×9 is the closest equivalent in the European (Continental) range. They are close to interchangeable in bending: 349 cm³ against 348 cm³ of plastic modulus, a difference of 0.3%. SHS 180×180×8 is the lighter of the two at 42.7 kg/m against 45.3 kg/m, a saving of 5.7% on steel weight. SHS 180×180×8 is deeper, 180 mm against 175 mm, which matters where the floor zone is tight.
| Property | SHS 180×180×8 SHS Hot-finished | SHS 175×175×9 SHS Cold-formed | Difference |
|---|---|---|---|
| Mass | 42.7 kg/m | 45.3 kg/m | -5.7% |
| Depth | 180 mm | 175 mm | +2.9% |
| Width | 180 mm | 175 mm | +2.9% |
| Web thickness | 0 mm | 0 mm | same |
| Flange thickness | 0 mm | 0 mm | same |
| Area | 54.4 cm² | 57 cm² | -4.6% |
| Second moment, major (Iy) | 2,660 cm⁴ | 2,546 cm⁴ | +4.5% |
| Plastic modulus, major (Wpl,y) | 349 cm³ | 348 cm³ | +0.3% |
| Elastic modulus, major (Wel,y) | 296 cm³ | 291 cm³ | +1.7% |
| Second moment, minor (Iz) | 2,660 cm⁴ | 2,546 cm⁴ | +4.5% |
| Plastic modulus, minor (Wpl,z) | 349 cm³ | 348 cm³ | +0.3% |
| Radius of gyration, minor (iz) | 7 cm | 6.68 cm | +4.8% |
| Torsion constant (It) | 4,160 cm⁴ | 4,246 cm⁴ | -2.0% |
SHS 180×180×8 to BS EN 10210-2:2006; SHS 175×175×9 to EN 10219 · Continental Steel. Difference is expressed relative to SHS 175×175×9.
How to choose between them
On stiffness, SHS 180×180×8 has the larger second moment of area: 2,660 cm⁴ against 2,546 cm⁴, a difference of 4.5%. 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 180×180×8 is the more efficient section: 8.2 against 7.7 cm³ per kg/m, 6.4% 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. SHS Hot-finished sections are specified to BS EN 10210-2:2006 and SHS 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 5.7% weight.
In short, SHS 180×180×8 is the stronger section and SHS 180×180×8 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.