How do you analyze torsion in non-circular cross-sections?

Non-circular sections experience warping during torsion, invalidating simple torsion theory. For rectangular sections: τmax = T/(αbt²) and θ = TL/(βbt³G), where α and β are geometric constants. For thin-walled open sections like I-beams, torsional rigidity is very low (sum of bt³/3 terms). Closed thin-walled sections (tubes) use Bredt's formula: τ = T/(2At) where A is enclosed area. Warping restraint adds complexity.