UB 686×254×140 vs S 24X106
UB 686×254×140 is an UB section from the UK Blue Book (SCI P363); S 24X106 is the closest equivalent in the American AISC Shapes Database. They are close to interchangeable in bending: 4560 cm³ against 4572 cm³ of plastic modulus, a difference of 0.3%. UB 686×254×140 is the lighter of the two at 140.1 kg/m against 157.7 kg/m, a saving of 11.2% on steel weight. UB 686×254×140 is deeper, 683.5 mm against 622.3 mm, which matters where the floor zone is tight.
| Property | UB 686×254×140 UB | S 24X106 S | Difference |
|---|---|---|---|
| Mass | 140.1 kg/m | 157.7 kg/m | -11.2% |
| Depth | 683.5 mm | 622.3 mm | +9.8% |
| Width | 253.7 mm | 199.9 mm | +26.9% |
| Web thickness | 12.4 mm | 15.7 mm | -21.0% |
| Flange thickness | 19 mm | 27.7 mm | -31.4% |
| Area | 178 cm² | 200.6 cm² | -11.3% |
| Second moment, major (Iy) | 136,000 cm⁴ | 122,372 cm⁴ | +11.1% |
| Plastic modulus, major (Wpl,y) | 4,560 cm³ | 4,572 cm³ | -0.3% |
| Elastic modulus, major (Wel,y) | 3,990 cm³ | 3,932.9 cm³ | +1.5% |
| Second moment, minor (Iz) | 5,180 cm⁴ | 3,197 cm⁴ | +62.0% |
| Plastic modulus, minor (Wpl,z) | 638 cm³ | 547.33 cm³ | +16.6% |
| Radius of gyration, minor (iz) | 5.39 cm | 3.99 cm | +35.1% |
| Torsion constant (It) | 169 cm⁴ | 420.4 cm⁴ | -59.8% |
UB 686×254×140 to BS EN 10365:2017; S 24X106 to AISC Shapes Database v16.0. Difference is expressed relative to S 24X106.
How to choose between them
On stiffness, UB 686×254×140 has the larger second moment of area: 136,000 cm⁴ against 122,372 cm⁴, a difference of 11.1%. 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 686×254×140 is the more efficient section: 32.5 against 29.0 cm³ per kg/m, 12.3% 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 686×254×140 has 638 cm³ of minor-axis plastic modulus against 547.33 cm³, 16.6% 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 11.2% weight.
In short, S 24X106 is the stronger section and UB 686×254×140 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.