CHS 168.3×5.6 vs CHS 177.8×5
CHS 168.3×5.6 is an CHS Hot-finished section from the UK Blue Book (SCI P363); CHS 177.8×5 is the closest equivalent in the European (Continental) range. They are close to interchangeable in bending: 148 cm³ against 149 cm³ of plastic modulus, a difference of 0.7%. CHS 177.8×5 is the lighter of the two at 21.31 kg/m against 22.5 kg/m, a saving of 5.6% on steel weight. CHS 177.8×5 is deeper, 177.8 mm against 168.3 mm, which matters where the floor zone is tight.
| Property | CHS 168.3×5.6 CHS Hot-finished | CHS 177.8×5 CHS | Difference |
|---|---|---|---|
| Mass | 22.5 kg/m | 21.3 kg/m | +5.6% |
| Depth | 168.3 mm | 177.8 mm | -5.3% |
| Width | 168.3 mm | 177.8 mm | -5.3% |
| Web thickness | 0 mm | 0 mm | same |
| Flange thickness | 0 mm | 0 mm | same |
| Area | 28.6 cm² | 27.1 cm² | +5.5% |
| Second moment, major (Iy) | 948 cm⁴ | 1,014 cm⁴ | -6.5% |
| Plastic modulus, major (Wpl,y) | 148 cm³ | 149 cm³ | -0.7% |
| Elastic modulus, major (Wel,y) | 113 cm³ | 114 cm³ | -0.9% |
| Second moment, minor (Iz) | 948 cm⁴ | 1,014 cm⁴ | -6.5% |
| Plastic modulus, minor (Wpl,z) | 148 cm³ | 149 cm³ | -0.7% |
| Radius of gyration, minor (iz) | 5.76 cm | 6.11 cm | -5.7% |
| Torsion constant (It) | 1,900 cm⁴ | 2,028 cm⁴ | -6.3% |
CHS 168.3×5.6 to BS EN 10210-2:2006; CHS 177.8×5 to EN 10219 · Continental Steel. Difference is expressed relative to CHS 177.8×5.
How to choose between them
On stiffness, CHS 177.8×5 has the larger second moment of area: 1,014 cm⁴ against 948 cm⁴, a difference of 6.5%. 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, CHS 177.8×5 is the more efficient section: 7.0 against 6.6 cm³ per kg/m, 5.9% 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. CHS Hot-finished sections are specified to BS EN 10210-2:2006 and CHS 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 5.6% weight.
In short, CHS 177.8×5 is the stronger section and CHS 177.8×5 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.