Tee 305×305×75 vs ST 12X40
Tee 305×305×75 is an Tees (from UB) section from the UK Blue Book (SCI P363); ST 12X40 is the closest equivalent in the American AISC Shapes Database. ST 12X40 carries more bending: 550.61 cm³ of plastic modulus against 538 cm³, 2.3% more. ST 12X40 is the lighter of the two at 59.5 kg/m against 74.6 kg/m, a saving of 25.4% on steel weight. Both are about 305 mm deep, so either fits the same construction zone.
| Property | Tee 305×305×75 Tees (from UB) | ST 12X40 ST | Difference |
|---|---|---|---|
| Mass | 74.6 kg/m | 59.5 kg/m | +25.4% |
| Depth | 306.1 mm | 304.8 mm | +0.4% |
| Width | 304.8 mm | 177.8 mm | +71.4% |
| Web thickness | 11.8 mm | 12.7 mm | -7.1% |
| Flange thickness | 19.7 mm | 22.1 mm | -10.9% |
| Area | 95 cm² | 75.5 cm² | +25.8% |
| Second moment, major (Iy) | 7,410 cm⁴ | 6,743 cm⁴ | +9.9% |
| Plastic modulus, major (Wpl,y) | 538 cm³ | 550.61 cm³ | -2.3% |
| Elastic modulus, major (Wel,y) | 307 cm³ | 304.8 cm³ | +0.7% |
| Second moment, minor (Iz) | 4,650 cm⁴ | 874.1 cm⁴ | +432.0% |
| Plastic modulus, minor (Wpl,z) | 469 cm³ | 170.43 cm³ | +175.2% |
| Radius of gyration, minor (iz) | 7 cm | 3.4 cm | +105.9% |
| Torsion constant (It) | 99.8 cm⁴ | 101.6 cm⁴ | -1.8% |
Tee 305×305×75 to BS EN 10365:2017; ST 12X40 to AISC Shapes Database v16.0. Difference is expressed relative to ST 12X40.
How to choose between them
On stiffness, Tee 305×305×75 has the larger second moment of area: 7,410 cm⁴ against 6,743 cm⁴, a difference of 9.9%. 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, ST 12X40 is the more efficient section: 9.3 against 7.2 cm³ per kg/m, 22.1% 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: Tee 305×305×75 has 469 cm³ of minor-axis plastic modulus against 170.43 cm³, 175.2% 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. Tees (from UB) sections are specified to BS EN 10365:2017 and ST 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 25.4% weight.
In short, ST 12X40 is the stronger section and ST 12X40 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.