UC 305×305×137 vs H-300x300x20x21
UC 305×305×137 is an UC section from the UK Blue Book (SCI P363); H-300x300x20x21 is the closest equivalent in the European (Continental) range. They are close to interchangeable in bending: 2300 cm³ against 2295 cm³ of plastic modulus, a difference of 0.2%. UC 305×305×137 is the lighter of the two at 136.9 kg/m against 147 kg/m, a saving of 6.9% on steel weight. UC 305×305×137 is deeper, 320.5 mm against 312 mm, which matters where the floor zone is tight.
| Property | UC 305×305×137 UC | H-300x300x20x21 MB & MC | Difference |
|---|---|---|---|
| Mass | 136.9 kg/m | 147 kg/m | -6.9% |
| Depth | 320.5 mm | 312 mm | +2.7% |
| Width | 309.2 mm | 310 mm | -0.3% |
| Web thickness | 13.8 mm | 20 mm | -31.0% |
| Flange thickness | 21.7 mm | 21 mm | +3.3% |
| Area | 174 cm² | 187 cm² | -7.0% |
| Second moment, major (Iy) | 32,800 cm⁴ | 31,370 cm⁴ | +4.6% |
| Plastic modulus, major (Wpl,y) | 2,300 cm³ | 2,295 cm³ | +0.2% |
| Elastic modulus, major (Wel,y) | 2,050 cm³ | 2,011 cm³ | +1.9% |
| Second moment, minor (Iz) | 10,700 cm⁴ | 10,450 cm⁴ | +2.4% |
| Plastic modulus, minor (Wpl,z) | 1,050 cm³ | 1,040 cm³ | +1.0% |
| Radius of gyration, minor (iz) | 7.83 cm | 7.48 cm | +4.7% |
| Torsion constant (It) | 249 cm⁴ | 298 cm⁴ | -16.4% |
UC 305×305×137 to BS EN 10365:2017; H-300x300x20x21 to Continental Steel. Difference is expressed relative to H-300x300x20x21.
How to choose between them
On stiffness, UC 305×305×137 has the larger second moment of area: 32,800 cm⁴ against 31,370 cm⁴, a difference of 4.6%. 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 305×305×137 is the more efficient section: 16.8 against 15.6 cm³ per kg/m, 7.6% 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. UC 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 6.9% weight.
In short, UC 305×305×137 is the stronger section and UC 305×305×137 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.