UB 305×127×48 vs S 12X35
UB 305×127×48 is an UB section from the UK Blue Book (SCI P363); S 12X35 is the closest equivalent in the American AISC Shapes Database. S 12X35 carries more bending: 730.86 cm³ of plastic modulus against 711 cm³, 2.7% more. UB 305×127×48 is the lighter of the two at 48.1 kg/m against 52.1 kg/m, a saving of 7.7% on steel weight. UB 305×127×48 is deeper, 311 mm against 304.8 mm, which matters where the floor zone is tight.
| Property | UB 305×127×48 UB | S 12X35 S | Difference |
|---|---|---|---|
| Mass | 48.1 kg/m | 52.1 kg/m | -7.7% |
| Depth | 311 mm | 304.8 mm | +2.0% |
| Width | 125.3 mm | 129 mm | -2.9% |
| Web thickness | 9 mm | 10.9 mm | -17.4% |
| Flange thickness | 14 mm | 13.8 mm | +1.4% |
| Area | 61.3 cm² | 65.8 cm² | -6.8% |
| Second moment, major (Iy) | 9,570 cm⁴ | 9,490 cm⁴ | +0.8% |
| Plastic modulus, major (Wpl,y) | 711 cm³ | 730.86 cm³ | -2.7% |
| Elastic modulus, major (Wel,y) | 616 cm³ | 624.35 cm³ | -1.3% |
| Second moment, minor (Iz) | 461 cm⁴ | 409.6 cm⁴ | +12.5% |
| Plastic modulus, minor (Wpl,z) | 116 cm³ | 111.43 cm³ | +4.1% |
| Radius of gyration, minor (iz) | 2.74 cm | 2.49 cm | +10.0% |
| Torsion constant (It) | 31.8 cm⁴ | 43.7 cm⁴ | -27.2% |
UB 305×127×48 to BS EN 10365:2017; S 12X35 to AISC Shapes Database v16.0. Difference is expressed relative to S 12X35.
How to choose between them
On stiffness, UB 305×127×48 has the larger second moment of area: 9,570 cm⁴ against 9,490 cm⁴, a difference of 0.8%. 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, UB 305×127×48 is the more efficient section: 14.8 against 14.0 cm³ per kg/m, 5.4% 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. UB sections are specified to BS EN 10365:2017 and S 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 7.7% weight.
In short, S 12X35 is the stronger section and UB 305×127×48 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.