WT 7X21.5 vs Tee 171×178×26
WT 7X21.5 is an WT section from the American AISC Shapes Database; Tee 171×178×26 is the closest equivalent in the UK Blue Book (SCI P363). WT 7X21.5 carries more bending: 115.53 cm³ of plastic modulus against 113 cm³, 2.2% more. Tee 171×178×26 is the lighter of the two at 25.5 kg/m against 32 kg/m, a saving of 25.5% on steel weight. Tee 171×178×26 is deeper, 177.4 mm against 173.5 mm, which matters where the floor zone is tight.
| Property | WT 7X21.5 WT | Tee 171×178×26 Tees (from UB) | Difference |
|---|---|---|---|
| Mass | 32 kg/m | 25.5 kg/m | +25.5% |
| Depth | 173.5 mm | 177.4 mm | -2.2% |
| Width | 203.2 mm | 171.5 mm | +18.5% |
| Web thickness | 7.7 mm | 7.4 mm | +4.1% |
| Flange thickness | 13.5 mm | 11.5 mm | +17.4% |
| Area | 40.7 cm² | 32.4 cm² | +25.6% |
| Second moment, major (Iy) | 911.5 cm⁴ | 882 cm⁴ | +3.3% |
| Plastic modulus, major (Wpl,y) | 115.53 cm³ | 113 cm³ | +2.2% |
| Elastic modulus, major (Wel,y) | 65.221 cm³ | 63.9 cm³ | +2.1% |
| Second moment, minor (Iz) | 940.7 cm⁴ | 484 cm⁴ | +94.4% |
| Plastic modulus, minor (Wpl,z) | 141.58 cm³ | 87.1 cm³ | +62.5% |
| Radius of gyration, minor (iz) | 4.8 cm | 3.86 cm | +24.4% |
| Torsion constant (It) | 21.73 cm⁴ | 11.9 cm⁴ | +82.6% |
WT 7X21.5 to AISC Shapes Database v16.0; Tee 171×178×26 to BS EN 10365:2017. Difference is expressed relative to Tee 171×178×26.
How to choose between them
On stiffness, WT 7X21.5 has the larger second moment of area: 911.5 cm⁴ against 882 cm⁴, a difference of 3.3%. 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, Tee 171×178×26 is the more efficient section: 4.4 against 3.6 cm³ per kg/m, 18.5% 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: WT 7X21.5 has 141.58 cm³ of minor-axis plastic modulus against 87.1 cm³, 62.5% 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. WT sections are specified to AISC Shapes Database v16.0 and Tees (from UB) to BS EN 10365:2017; 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 25.5% weight.
In short, WT 7X21.5 is the stronger section and Tee 171×178×26 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.