UC 356×406×235 vs HE 340 M
UC 356×406×235 is an UC section from the UK Blue Book (SCI P363); HE 340 M is the closest equivalent in the European (Continental) range. They are close to interchangeable in bending: 4690 cm³ against 4720 cm³ of plastic modulus, a difference of 0.6%. UC 356×406×235 is the lighter of the two at 235.1 kg/m against 248 kg/m, a saving of 5.2% on steel weight. Both are about 379 mm deep, so either fits the same construction zone.
| Property | UC 356×406×235 UC | HE 340 M HE | Difference |
|---|---|---|---|
| Mass | 235.1 kg/m | 248 kg/m | -5.2% |
| Depth | 381 mm | 377 mm | +1.1% |
| Width | 394.8 mm | 309 mm | +27.8% |
| Web thickness | 18.4 mm | 21 mm | -12.4% |
| Flange thickness | 30.2 mm | 40 mm | -24.5% |
| Area | 299 cm² | 316 cm² | -5.4% |
| Second moment, major (Iy) | 79,100 cm⁴ | 76,400 cm⁴ | +3.5% |
| Plastic modulus, major (Wpl,y) | 4,690 cm³ | 4,720 cm³ | -0.6% |
| Elastic modulus, major (Wel,y) | 4,150 cm³ | 4,050 cm³ | +2.5% |
| Second moment, minor (Iz) | 31,000 cm⁴ | 19,700 cm⁴ | +57.4% |
| Plastic modulus, minor (Wpl,z) | 2,380 cm³ | 1,950 cm³ | +22.1% |
| Radius of gyration, minor (iz) | 10.2 cm | 7.9 cm | +29.1% |
| Torsion constant (It) | 812 cm⁴ | 1,510 cm⁴ | -46.2% |
UC 356×406×235 to BS EN 10365:2017; HE 340 M to EN 10365 · Continental Steel. Difference is expressed relative to HE 340 M.
How to choose between them
On stiffness, UC 356×406×235 has the larger second moment of area: 79,100 cm⁴ against 76,400 cm⁴, a difference of 3.5%. 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, UC 356×406×235 is the more efficient section: 19.9 against 19.0 cm³ per kg/m, 4.8% 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.
The two diverge much more about the minor axis: UC 356×406×235 has 2380 cm³ of minor-axis plastic modulus against 1950 cm³, 22.1% more. If the member sees any minor-axis bending, or if it is unrestrained over a long length and lateral-torsional buckling governs, that gap decides the comparison even though the major-axis numbers looked equivalent.
These come from different catalogues, so treat the swap as a design change rather than a like-for-like substitution. UC sections are specified to BS EN 10365:2017 and HE to EN 10365 · 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 5.2% weight.
In short, HE 340 M is the stronger section and UC 356×406×235 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.