WT 12X58.5 vs ST 12X45
WT 12X58.5 is an WT section from the American AISC Shapes Database; ST 12X45 is the closest equivalent in the American AISC Shapes Database. ST 12X45 carries more bending: 673.51 cm³ of plastic modulus against 642.37 cm³, 4.6% more. ST 12X45 is the lighter of the two at 67 kg/m against 87.1 kg/m, a saving of 30.0% on steel weight. Both are about 306 mm deep, so either fits the same construction zone.
| Property | WT 12X58.5 WT | ST 12X45 ST | Difference |
|---|---|---|---|
| Mass | 87.1 kg/m | 67 kg/m | +30.0% |
| Depth | 307.3 mm | 304.8 mm | +0.8% |
| Width | 325.1 mm | 181.1 mm | +79.5% |
| Web thickness | 14 mm | 15.9 mm | -11.9% |
| Flange thickness | 21.6 mm | 22.1 mm | -2.3% |
| Area | 111 cm² | 85.2 cm² | +30.3% |
| Second moment, major (Iy) | 8,824 cm⁴ | 7,908 cm⁴ | +11.6% |
| Plastic modulus, major (Wpl,y) | 642.37 cm³ | 673.51 cm³ | -4.6% |
| Elastic modulus, major (Wel,y) | 365.43 cm³ | 370.35 cm³ | -1.3% |
| Second moment, minor (Iz) | 6,202 cm⁴ | 928.2 cm⁴ | +568.2% |
| Plastic modulus, minor (Wpl,z) | 585.02 cm³ | 183.54 cm³ | +218.7% |
| Radius of gyration, minor (iz) | 7.47 cm | 3.3 cm | +126.4% |
| Torsion constant (It) | 139.4 cm⁴ | 125.3 cm⁴ | +11.3% |
WT 12X58.5 to AISC Shapes Database v16.0; ST 12X45 to AISC Shapes Database v16.0. Difference is expressed relative to ST 12X45.
How to choose between them
On stiffness, WT 12X58.5 has the larger second moment of area: 8,824 cm⁴ against 7,908 cm⁴, a difference of 11.6%. 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 7.4 cm³ per kg/m, 26.6% 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 12X58.5 has 585.02 cm³ of minor-axis plastic modulus against 183.54 cm³, 218.7% 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 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.