Tee 305×305×90 vs ST 12X45
Tee 305×305×90 is an Tees (from UB) section from the UK Blue Book (SCI P363); ST 12X45 is the closest equivalent in the American AISC Shapes Database. ST 12X45 carries more bending: 673.51 cm³ of plastic modulus against 656 cm³, 2.6% more. ST 12X45 is the lighter of the two at 67 kg/m against 89.5 kg/m, a saving of 33.6% on steel weight. Both are about 307 mm deep, so either fits the same construction zone.
| Property | Tee 305×305×90 Tees (from UB) | ST 12X45 ST | Difference |
|---|---|---|---|
| Mass | 89.5 kg/m | 67 kg/m | +33.6% |
| Depth | 310 mm | 304.8 mm | +1.7% |
| Width | 307.1 mm | 181.1 mm | +69.6% |
| Web thickness | 14.1 mm | 15.9 mm | -11.3% |
| Flange thickness | 23.6 mm | 22.1 mm | +6.8% |
| Area | 114 cm² | 85.2 cm² | +33.8% |
| Second moment, major (Iy) | 9,040 cm⁴ | 7,908 cm⁴ | +14.3% |
| Plastic modulus, major (Wpl,y) | 656 cm³ | 673.51 cm³ | -2.6% |
| Elastic modulus, major (Wel,y) | 372 cm³ | 370.35 cm³ | +0.4% |
| Second moment, minor (Iz) | 5,700 cm⁴ | 928.2 cm⁴ | +514.1% |
| Plastic modulus, minor (Wpl,z) | 572 cm³ | 183.54 cm³ | +211.6% |
| Radius of gyration, minor (iz) | 7.07 cm | 3.3 cm | +114.2% |
| Torsion constant (It) | 170 cm⁴ | 125.3 cm⁴ | +35.7% |
Tee 305×305×90 to BS EN 10365:2017; ST 12X45 to AISC Shapes Database v16.0. Difference is expressed relative to ST 12X45.
How to choose between them
On stiffness, Tee 305×305×90 has the larger second moment of area: 9,040 cm⁴ against 7,908 cm⁴, a difference of 14.3%. 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, ST 12X45 is the more efficient section: 10.1 against 7.3 cm³ per kg/m, 27.1% 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 305×305×90 has 572 cm³ of minor-axis plastic modulus against 183.54 cm³, 211.6% 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. 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 33.6% weight.
In short, ST 12X45 is the stronger section and ST 12X45 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.