Code References:
ACI 318-14, PCI Handbook 8th Edition
Definition:
For the purposes of this discussion, torsion is defined as a moment about the centerline of a horizontal member. For design, the horizontal member is idealized as a tube, and the center portion of the solid beam can conservatively be neglected.
Critical sections:
For horizontal simply supported members, 9.4.4.3 has defined the critical section for torsion to be a distance d from the face of the support for non-prestressed members and h/2 for prestressed members. If a concentrated load is present between the critical section and the face of the support, then the face of support should become the critical section. The user must make an affirmative response to move the critical section away from the support face. The critical section for non-simply supported members is the face of support (9.4.4.2).