UB 203×133×25 vs S 8X18.4
UB 203×133×25 is an UB section from the UK Blue Book (SCI P363); S 8X18.4 is the closest equivalent in the American AISC Shapes Database. S 8X18.4 carries more bending: 270.39 cm³ of plastic modulus against 258 cm³, 4.6% more. UB 203×133×25 is the lighter of the two at 25.1 kg/m against 27.4 kg/m, a saving of 8.4% on steel weight. Both are about 203 mm deep, so either fits the same construction zone.
| Property | UB 203×133×25 UB | S 8X18.4 S | Difference |
|---|---|---|---|
| Mass | 25.1 kg/m | 27.4 kg/m | -8.4% |
| Depth | 203.2 mm | 203.2 mm | same |
| Width | 133.2 mm | 101.6 mm | +31.1% |
| Web thickness | 5.7 mm | 6.9 mm | -17.4% |
| Flange thickness | 7.8 mm | 10.8 mm | -27.8% |
| Area | 32 cm² | 34.8 cm² | -8.0% |
| Second moment, major (Iy) | 2,340 cm⁴ | 2,393 cm⁴ | -2.2% |
| Plastic modulus, major (Wpl,y) | 258 cm³ | 270.39 cm³ | -4.6% |
| Elastic modulus, major (Wel,y) | 230 cm³ | 235.97 cm³ | -2.5% |
| Second moment, minor (Iz) | 308 cm⁴ | 153.6 cm⁴ | +100.5% |
| Plastic modulus, minor (Wpl,z) | 70.9 cm³ | 52.111 cm³ | +36.1% |
| Radius of gyration, minor (iz) | 3.1 cm | 2.1 cm | +47.6% |
| Torsion constant (It) | 5.96 cm⁴ | 13.94 cm⁴ | -57.2% |
UB 203×133×25 to BS EN 10365:2017; S 8X18.4 to AISC Shapes Database v16.0. Difference is expressed relative to S 8X18.4.
How to choose between them
On stiffness, S 8X18.4 has the larger second moment of area: 2,393 cm⁴ against 2,340 cm⁴, a difference of 2.2%. 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 203×133×25 is the more efficient section: 10.3 against 9.9 cm³ per kg/m, 4.2% 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 203×133×25 has 70.9 cm³ of minor-axis plastic modulus against 52.111 cm³, 36.1% 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 8.4% weight.
In short, S 8X18.4 is the stronger section and UB 203×133×25 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.