Polar moment of inertia vs torsion constant for welds

[CQT3]

Hello all, I am a junior engineer and I am wondering if anyone can answer the following question. From my understanding, we use the polar moment of inertia when find the torsion stress in welds but we use the torsion constant when finding the torsion stress in members. Is this correct? If it is, can anyone explain why we don't use the torsion constant for welds as well and vice versa? Any clarification or guidance would be appreciated. Thanks!

 

[DaveAtkins]

Welds do not see torsion. A weld group will resist torsion by a combination of transverse and longitudinal shear stress in the individual welds which make up the group. That is why you use the polar moment of inertia of the weld group to determine the shear force in each weld.

DaveAtkins