Introduction to Elasticity/Torsion of thin walled closed sections
Torsion of thin-walled closed sections 
The Prandtl stress function can be approximated as a linear function between and on the two adjacent boundaries.
The local shear stress is, therefore,
where is the parameterizing coordinate of the boundary curve of the cross-section and is the local wall thickness.
The value of can determined using
where is the area enclosed by the mean line between the inner and outer boundary.
The torque is approximately