UB 305×165×46 vs S 12X35
UB 305×165×46 is an UB section from the UK Blue Book (SCI P363); S 12X35 is the closest equivalent in the American AISC Shapes Database. They are close to interchangeable in bending: 720 cm³ against 730.86 cm³ of plastic modulus, a difference of 1.5%. UB 305×165×46 is the lighter of the two at 46.1 kg/m against 52.1 kg/m, a saving of 11.5% on steel weight. Both are about 306 mm deep, so either fits the same construction zone.
| Property | UB 305×165×46 UB | S 12X35 S | Difference |
|---|---|---|---|
| Mass | 46.1 kg/m | 52.1 kg/m | -11.5% |
| Depth | 306.6 mm | 304.8 mm | +0.6% |
| Width | 165.7 mm | 129 mm | +28.4% |
| Web thickness | 6.7 mm | 10.9 mm | -38.5% |
| Flange thickness | 11.8 mm | 13.8 mm | -14.5% |
| Area | 58.7 cm² | 65.8 cm² | -10.8% |
| Second moment, major (Iy) | 9,900 cm⁴ | 9,490 cm⁴ | +4.3% |
| Plastic modulus, major (Wpl,y) | 720 cm³ | 730.86 cm³ | -1.5% |
| Elastic modulus, major (Wel,y) | 646 cm³ | 624.35 cm³ | +3.5% |
| Second moment, minor (Iz) | 896 cm⁴ | 409.6 cm⁴ | +118.7% |
| Plastic modulus, minor (Wpl,z) | 166 cm³ | 111.43 cm³ | +49.0% |
| Radius of gyration, minor (iz) | 3.9 cm | 2.49 cm | +56.6% |
| Torsion constant (It) | 22.2 cm⁴ | 43.7 cm⁴ | -49.2% |
UB 305×165×46 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 305×165×46 has the larger second moment of area: 9,900 cm⁴ against 9,490 cm⁴, a difference of 4.3%. 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 305×165×46 is the more efficient section: 15.6 against 14.0 cm³ per kg/m, 11.3% 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.
The two diverge much more about the minor axis: UB 305×165×46 has 166 cm³ of minor-axis plastic modulus against 111.43 cm³, 49.0% more. If the member sees any minor-axis bending, or if it is unrestrained over a long length and lateral-torsional buckling governs, that gap decides the comparison even though the major-axis numbers looked equivalent.
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 11.5% weight.
In short, S 12X35 is the stronger section and UB 305×165×46 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.