UB 406×178×85 vs H-400x200x10x18
UB 406×178×85 is an UB section from the UK Blue Book (SCI P363); H-400x200x10x18 is the closest equivalent in the European (Continental) range. H-400x200x10x18 carries more bending: 1815 cm³ of plastic modulus against 1730 cm³, 4.7% more. UB 406×178×85 is the lighter of the two at 85.3 kg/m against 88.2 kg/m, a saving of 3.3% on steel weight. Both are about 414 mm deep, so either fits the same construction zone.
| Property | UB 406×178×85 UB | H-400x200x10x18 MB & MC | Difference |
|---|---|---|---|
| Mass | 85.3 kg/m | 88.2 kg/m | -3.3% |
| Depth | 417.2 mm | 410 mm | +1.8% |
| Width | 181.9 mm | 202 mm | -10.0% |
| Web thickness | 10.9 mm | 10 mm | +9.0% |
| Flange thickness | 18.2 mm | 18 mm | +1.1% |
| Area | 109 cm² | 112 cm² | -2.7% |
| Second moment, major (Iy) | 31,700 cm⁴ | 33,060 cm⁴ | -4.1% |
| Plastic modulus, major (Wpl,y) | 1,730 cm³ | 1,815 cm³ | -4.7% |
| Elastic modulus, major (Wel,y) | 1,520 cm³ | 1,612 cm³ | -5.7% |
| Second moment, minor (Iz) | 1,830 cm⁴ | 2,478 cm⁴ | -26.2% |
| Plastic modulus, minor (Wpl,z) | 313 cm³ | 378 cm³ | -17.2% |
| Radius of gyration, minor (iz) | 4.11 cm | 4.7 cm | -12.6% |
| Torsion constant (It) | 93 cm⁴ | 101 cm⁴ | -7.9% |
UB 406×178×85 to BS EN 10365:2017; H-400x200x10x18 to Continental Steel. Difference is expressed relative to H-400x200x10x18.
How to choose between them
On stiffness, H-400x200x10x18 has the larger second moment of area: 33,060 cm⁴ against 31,700 cm⁴, a difference of 4.1%. 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 (20.3 against 20.6 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.
The two diverge much more about the minor axis: H-400x200x10x18 has 378 cm³ of minor-axis plastic modulus against 313 cm³, 17.2% 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. UB sections are specified to BS EN 10365:2017 and MB & MC to 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 3.3% weight.
In short, H-400x200x10x18 is the stronger section and UB 406×178×85 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.