UC 305×305×118 vs W 12X79
UC 305×305×118 is an UC section from the UK Blue Book (SCI P363); W 12X79 is the closest equivalent in the American AISC Shapes Database. They are close to interchangeable in bending: 1960 cm³ against 1950.1 cm³ of plastic modulus, a difference of 0.5%. They weigh essentially the same, 117.9 kg/m and 117.6 kg/m. Both are about 315 mm deep, so either fits the same construction zone.
| Property | UC 305×305×118 UC | W 12X79 W | Difference |
|---|---|---|---|
| Mass | 117.9 kg/m | 117.6 kg/m | +0.3% |
| Depth | 314.5 mm | 315 mm | -0.2% |
| Width | 307.4 mm | 307.3 mm | same |
| Web thickness | 12 mm | 11.9 mm | +0.8% |
| Flange thickness | 18.7 mm | 18.7 mm | same |
| Area | 150 cm² | 149.7 cm² | +0.2% |
| Second moment, major (Iy) | 27,700 cm⁴ | 27,555 cm⁴ | +0.5% |
| Plastic modulus, major (Wpl,y) | 1,960 cm³ | 1,950.1 cm³ | +0.5% |
| Elastic modulus, major (Wel,y) | 1,760 cm³ | 1,753.4 cm³ | +0.4% |
| Second moment, minor (Iz) | 9,060 cm⁴ | 8,991 cm⁴ | +0.8% |
| Plastic modulus, minor (Wpl,z) | 895 cm³ | 889.82 cm³ | +0.6% |
| Radius of gyration, minor (iz) | 7.77 cm | 7.75 cm | +0.3% |
| Torsion constant (It) | 161 cm⁴ | 159.8 cm⁴ | +0.8% |
UC 305×305×118 to BS EN 10365:2017; W 12X79 to AISC Shapes Database v16.0. Difference is expressed relative to W 12X79.
How to choose between them
On stiffness, UC 305×305×118 has the larger second moment of area: 27,700 cm⁴ against 27,555 cm⁴, a difference of 0.5%. 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, the two are equally efficient (16.6 against 16.6 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.
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.