CHS 139.7×5.6 vs CHS 139.7×6
CHS 139.7×5.6 is an CHS Hot-finished section from the UK Blue Book (SCI P363); CHS 139.7×6 is the closest equivalent in the European (Continental) range. CHS 139.7×6 carries more bending: 107 cm³ of plastic modulus against 101 cm³, 5.6% more. CHS 139.7×5.6 is the lighter of the two at 18.5 kg/m against 19.78 kg/m, a saving of 6.5% on steel weight. Both are about 140 mm deep, so either fits the same construction zone.
| Property | CHS 139.7×5.6 CHS Hot-finished | CHS 139.7×6 CHS | Difference |
|---|---|---|---|
| Mass | 18.5 kg/m | 19.8 kg/m | -6.5% |
| Depth | 139.7 mm | 139.7 mm | same |
| Width | 139.7 mm | 139.7 mm | same |
| Web thickness | 0 mm | 0 mm | same |
| Flange thickness | 0 mm | 0 mm | same |
| Area | 23.6 cm² | 25.2 cm² | -6.3% |
| Second moment, major (Iy) | 531 cm⁴ | 564 cm⁴ | -5.9% |
| Plastic modulus, major (Wpl,y) | 101 cm³ | 107 cm³ | -5.6% |
| Elastic modulus, major (Wel,y) | 76.1 cm³ | 80.8 cm³ | -5.8% |
| Second moment, minor (Iz) | 531 cm⁴ | 564 cm⁴ | -5.9% |
| Plastic modulus, minor (Wpl,z) | 101 cm³ | 107 cm³ | -5.6% |
| Radius of gyration, minor (iz) | 4.75 cm | 4.73 cm | +0.4% |
| Torsion constant (It) | 1,060 cm⁴ | 1,129 cm⁴ | -6.1% |
CHS 139.7×5.6 to BS EN 10210-2:2006; CHS 139.7×6 to EN 10219 · Continental Steel. Difference is expressed relative to CHS 139.7×6.
How to choose between them
On stiffness, CHS 139.7×6 has the larger second moment of area: 564 cm⁴ against 531 cm⁴, a difference of 5.9%. 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 (5.5 against 5.4 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.
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 6.5% weight.
In short, CHS 139.7×6 is the stronger section and CHS 139.7×5.6 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.