The Torsional Strength of Structural Members

In structural engineering many members are subjected to some torque, although this is only occasionally taken into account. The,exact determination of the torsional properties of structural sections is thought to be too complicated for the design office although empirical expressions have been used. For design purposes the torsiond properties required are
(a) the stiffness of the member, and
(b) the torque to produce specified stress conditions.

W.B. Dobie