WT 3X7.5 vs ST 3X6.25
WT 3X7.5 is an WT section from the American AISC Shapes Database; ST 3X6.25 is the closest equivalent in the American AISC Shapes Database. They are close to interchangeable in bending: 16.879 cm³ against 16.551 cm³ of plastic modulus, a difference of 2.0%. ST 3X6.25 is the lighter of the two at 9.3 kg/m against 11.2 kg/m, a saving of 20.4% on steel weight. Both are about 76 mm deep, so either fits the same construction zone.
| Property | WT 3X7.5 WT | ST 3X6.25 ST | Difference |
|---|---|---|---|
| Mass | 11.2 kg/m | 9.3 kg/m | +20.4% |
| Depth | 76.2 mm | 76.2 mm | same |
| Width | 152.1 mm | 84.6 mm | +79.8% |
| Web thickness | 5.8 mm | 5.9 mm | -1.7% |
| Flange thickness | 6.6 mm | 9.1 mm | -27.5% |
| Area | 14.3 cm² | 11.8 cm² | +21.2% |
| Second moment, major (Iy) | 58.69 cm⁴ | 52.45 cm⁴ | +11.9% |
| Plastic modulus, major (Wpl,y) | 16.879 cm³ | 16.551 cm³ | +2.0% |
| Elastic modulus, major (Wel,y) | 9.455 cm³ | 8.964 cm³ | +5.5% |
| Second moment, minor (Iz) | 194 cm⁴ | 37.5 cm⁴ | +417.3% |
| Plastic modulus, minor (Wpl,z) | 38.837 cm³ | 15.24 cm³ | +154.8% |
| Radius of gyration, minor (iz) | 3.68 cm | 1.78 cm | +106.7% |
| Torsion constant (It) | 2.1 cm⁴ | 3.45 cm⁴ | -39.1% |
WT 3X7.5 to AISC Shapes Database v16.0; ST 3X6.25 to AISC Shapes Database v16.0. Difference is expressed relative to ST 3X6.25.
How to choose between them
On stiffness, WT 3X7.5 has the larger second moment of area: 58.69 cm⁴ against 52.45 cm⁴, a difference of 11.9%. 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 3X6.25 is the more efficient section: 1.8 against 1.5 cm³ per kg/m, 15.3% 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 3X7.5 has 38.837 cm³ of minor-axis plastic modulus against 15.24 cm³, 154.8% 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, WT 3X7.5 is the stronger section and ST 3X6.25 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.