WT 8X22.5 vs ST 7.5X21.45
WT 8X22.5 is an WT section from the American AISC Shapes Database; ST 7.5X21.45 is the closest equivalent in the American AISC Shapes Database. They are close to interchangeable in bending: 176.98 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. WT 8X22.5 is deeper, 205 mm against 190.5 mm, which matters where the floor zone is tight.
| Property | WT 8X22.5 WT | ST 7.5X21.45 ST | Difference |
|---|---|---|---|
| Mass | 33.5 kg/m | 31.9 kg/m | +5.0% |
| Depth | 205 mm | 190.5 mm | +7.6% |
| Width | 178.8 mm | 139.7 mm | +28.0% |
| Web thickness | 8.8 mm | 10.4 mm | -15.4% |
| Flange thickness | 14.4 mm | 15.8 mm | -8.9% |
| Area | 42.8 cm² | 40.6 cm² | +5.4% |
| Second moment, major (Iy) | 1,573 cm⁴ | 1,369 cm⁴ | +14.9% |
| Plastic modulus, major (Wpl,y) | 176.98 cm³ | 176.98 cm³ | same |
| Elastic modulus, major (Wel,y) | 99.961 cm³ | 98.159 cm³ | +1.8% |
| Second moment, minor (Iz) | 682.6 cm⁴ | 296.8 cm⁴ | +130.0% |
| Plastic modulus, minor (Wpl,z) | 118.31 cm³ | 74.397 cm³ | +59.0% |
| Radius of gyration, minor (iz) | 3.99 cm | 2.69 cm | +48.3% |
| Torsion constant (It) | 23.1 cm⁴ | 31.84 cm⁴ | -27.4% |
WT 8X22.5 to AISC Shapes Database v16.0; 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, WT 8X22.5 has the larger second moment of area: 1,573 cm⁴ against 1,369 cm⁴, a difference of 14.9%. 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 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: WT 8X22.5 has 118.31 cm³ of minor-axis plastic modulus against 74.397 cm³, 59.0% 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.
In short, ST 7.5X21.45 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.