Torsion stress in code stress equation ASME B31.3

The ASME B31.3 is including Torsion stress component St in SL and SE formulas

Since other stress components Sa and Sb are longitudinal stress. The St shall be in longitudinal direction also

As normal, when talking about Torsion stress I understand it's a shear stress in circumferential direction.

However, the way I can imagine this shear stress in longitudinal direction is base on 3D stress state at a point, where the shear stress in circumferential direction is equal to another shear stress in longitudinal direction

Is it reasonable explanation to include St in the code stress equations of SL and SE?

3D stress state based on maximum shear stress theory is the basis of this formula.

Warm Regards,

For the thin-walled cylinder, the radial component is negligible, thus the stress evaluation is based on longitudinal and hoop direction, and using Mohr's circle the equation can be derived.

Got it, thanks for comments