UC 203×203×86 vs W 8X58
UC 203×203×86 is an UC section from the UK Blue Book (SCI P363); W 8X58 is the closest equivalent in the American AISC Shapes Database. They are close to interchangeable in bending: 977 cm³ against 979.95 cm³ of plastic modulus, a difference of 0.3%. They weigh essentially the same, 86.1 kg/m and 86.3 kg/m. Both are about 222 mm deep, so either fits the same construction zone.
| Property | UC 203×203×86 UC | W 8X58 W | Difference |
|---|---|---|---|
| Mass | 86.1 kg/m | 86.3 kg/m | -0.2% |
| Depth | 222.2 mm | 222.3 mm | same |
| Width | 209.1 mm | 208.8 mm | +0.1% |
| Web thickness | 12.7 mm | 13 mm | -2.3% |
| Flange thickness | 20.5 mm | 20.6 mm | -0.5% |
| Area | 110 cm² | 110.3 cm² | -0.3% |
| Second moment, major (Iy) | 9,450 cm⁴ | 9,490 cm⁴ | -0.4% |
| Plastic modulus, major (Wpl,y) | 977 cm³ | 979.95 cm³ | -0.3% |
| Elastic modulus, major (Wel,y) | 850 cm³ | 852.13 cm³ | -0.2% |
| Second moment, minor (Iz) | 3,130 cm⁴ | 3,126 cm⁴ | +0.1% |
| Plastic modulus, minor (Wpl,z) | 456 cm³ | 457.2 cm³ | -0.3% |
| Radius of gyration, minor (iz) | 5.34 cm | 5.33 cm | +0.2% |
| Torsion constant (It) | 137 cm⁴ | 138.6 cm⁴ | -1.2% |
UC 203×203×86 to BS EN 10365:2017; W 8X58 to AISC Shapes Database v16.0. Difference is expressed relative to W 8X58.
How to choose between them
On stiffness, W 8X58 has the larger second moment of area: 9,490 cm⁴ against 9,450 cm⁴, a difference of 0.4%. 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 (11.3 against 11.4 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. UC sections are specified to BS EN 10365:2017 and W to AISC Shapes Database v16.0; 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 a marginal amount of weight.
In short, these two are genuine equivalents. Pick whichever your supplier stocks and the rest of the frame already uses.