Analysis & loading · Sections & mechanics
Torsion of thin-walled closed sections (Bredt) calculator
Shear flow q = T/(2Ao), wall shear stresses and torsion constant J = 4Ao²/∮ds/t for a rectangular box.
Standard: Bredt–Batho thin-walled torsion
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Inputs in the worked example
The live calculator starts with these values; change any of them and every result, check and diagram updates instantly.
| Input | Value | Unit |
|---|---|---|
| Centreline width | 300 | mm |
| Centreline depth | 500 | mm |
| Flange wall thickness | 12 | mm |
| Web wall thickness | 8 | mm |
| Torque | 60 | kNm |
| G | 80000 | MPa |
Worked example — results
| Result | Value | Unit |
|---|---|---|
| Enclosed area | 150,000 | mm² |
| Shear flow T/(2Ao) | 200.0 | N/mm |
| Flange shear stress | 16.67 | MPa |
| Web shear stress | 25.00 | MPa |
| J = 4Ao²/Σ(s/t) | 514,285,714 | mm⁴ |
| Twist per metre T/(GJ) | 0.001 | rad/m |
- ✓ Max shear ≤ 0.6·250
Method, formulas and assumptions
- Enclosed area = 300*500 = 1.500e+5 mm²
- Shear flow T/(2Ao) = 60*1E6/(2*1.500e+5) = 200 N/mm
- Flange shear stress = 200/12 = 16.667 MPa
- Web shear stress = 200/8 = 25 MPa
- J = 4Ao²/Σ(s/t) = 4*1.500e+5^2/(2*300/12+2*500/8) = 5.143e+8 mm⁴
- Twist per metre T/(GJ) = 60*1E6/(80000*5.143e+8)*1000 = 0.0014583 rad/m
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For preliminary design and education. Check every result against the current standard and your own engineering judgement before use.