UC 305×305×118 vs HP 12X84
UC 305×305×118 is an UC section from the UK Blue Book (SCI P363); HP 12X84 is the closest equivalent in the American AISC Shapes Database. They are close to interchangeable in bending: 1960 cm³ against 1966.4 cm³ of plastic modulus, a difference of 0.3%. UC 305×305×118 is the lighter of the two at 117.9 kg/m against 125 kg/m, a saving of 5.7% on steel weight. Both are about 313 mm deep, so either fits the same construction zone.
| Property | UC 305×305×118 UC | HP 12X84 HP | Difference |
|---|---|---|---|
| Mass | 117.9 kg/m | 125 kg/m | -5.7% |
| Depth | 314.5 mm | 312.4 mm | +0.7% |
| Width | 307.4 mm | 312.4 mm | -1.6% |
| Web thickness | 12 mm | 17.4 mm | -31.0% |
| Flange thickness | 18.7 mm | 17.4 mm | +7.5% |
| Area | 150 cm² | 158.7 cm² | -5.5% |
| Second moment, major (Iy) | 27,700 cm⁴ | 27,055 cm⁴ | +2.4% |
| Plastic modulus, major (Wpl,y) | 1,960 cm³ | 1,966.4 cm³ | -0.3% |
| Elastic modulus, major (Wel,y) | 1,760 cm³ | 1,737 cm³ | +1.3% |
| Second moment, minor (Iz) | 9,060 cm⁴ | 8,866 cm⁴ | +2.2% |
| Plastic modulus, minor (Wpl,z) | 895 cm³ | 871.79 cm³ | +2.7% |
| Radius of gyration, minor (iz) | 7.77 cm | 7.47 cm | +4.0% |
| Torsion constant (It) | 161 cm⁴ | 176.5 cm⁴ | -8.8% |
UC 305×305×118 to BS EN 10365:2017; HP 12X84 to AISC Shapes Database v16.0. Difference is expressed relative to HP 12X84.
How to choose between them
On stiffness, UC 305×305×118 has the larger second moment of area: 27,700 cm⁴ against 27,055 cm⁴, a difference of 2.4%. 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 of steel, UC 305×305×118 is the more efficient section: 16.6 against 15.7 cm³ per kg/m, 5.7% 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 HP 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 5.7% weight.
In short, HP 12X84 is the stronger section and UC 305×305×118 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.