Tee 178×203×34 vs ST 7.5X21.45
Tee 178×203×34 is an Tees (from UB) section from the UK Blue Book (SCI P363); ST 7.5X21.45 is the closest equivalent in the American AISC Shapes Database. They are close to interchangeable in bending: 177 cm³ against 176.98 cm³ of plastic modulus, a difference of 0.0%. ST 7.5X21.45 is the lighter of the two at 31.9 kg/m against 33.5 kg/m, a saving of 5.0% on steel weight. Tee 178×203×34 is deeper, 204.6 mm against 190.5 mm, which matters where the floor zone is tight.
| Property | Tee 178×203×34 Tees (from UB) | ST 7.5X21.45 ST | Difference |
|---|---|---|---|
| Mass | 33.5 kg/m | 31.9 kg/m | +5.0% |
| Depth | 204.6 mm | 190.5 mm | +7.4% |
| Width | 178.8 mm | 139.7 mm | +28.0% |
| Web thickness | 8.8 mm | 10.4 mm | -15.4% |
| Flange thickness | 14.3 mm | 15.8 mm | -9.5% |
| Area | 42.8 cm² | 40.6 cm² | +5.4% |
| Second moment, major (Iy) | 1,570 cm⁴ | 1,369 cm⁴ | +14.7% |
| Plastic modulus, major (Wpl,y) | 177 cm³ | 176.98 cm³ | same |
| Elastic modulus, major (Wel,y) | 100 cm³ | 98.159 cm³ | +1.9% |
| Second moment, minor (Iz) | 682 cm⁴ | 296.8 cm⁴ | +129.8% |
| Plastic modulus, minor (Wpl,z) | 118 cm³ | 74.397 cm³ | +58.6% |
| Radius of gyration, minor (iz) | 3.99 cm | 2.69 cm | +48.3% |
| Torsion constant (It) | 23 cm⁴ | 31.84 cm⁴ | -27.8% |
Tee 178×203×34 to BS EN 10365:2017; ST 7.5X21.45 to AISC Shapes Database v16.0. Difference is expressed relative to ST 7.5X21.45.
How to choose between them
On stiffness, Tee 178×203×34 has the larger second moment of area: 1,570 cm⁴ against 1,369 cm⁴, a difference of 14.7%. 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 7.5X21.45 is the more efficient section: 5.5 against 5.3 cm³ per kg/m, 4.8% 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 178×203×34 has 118 cm³ of minor-axis plastic modulus against 74.397 cm³, 58.6% 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 5.0% weight.
In short, Tee 178×203×34 is the stronger section and ST 7.5X21.45 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.