Under twisting or torsional moments, a member develops normal (warping) and shear stresses. The ACI 318 Building Code assumes that no torsion is resisted by concrete and the entire nominal torsional strength is provided by reinforcement.
The reinforcement required for torsion must be added to that required for shear, moment, and axial force.
Torsional design may be based on
where ƒpc = compressive stress in concrete (after allowance for all prestress losses) at centroid of cross section resisting externally applied loads or at junction of web and flange when the centroid lies within the flange, psi. (In a composite member, ƒpc is resultant compressive stress at centroid of composite section, or at junction of web and flange when centroid lies within the flange, due to both prestress and moments resisted by precast members acting alone.)
For T-beam construction, where stirrup reinforcement is required for torsion, it may be more practical to neglect the area and perimeter of the overhanging flanges than to provide reinforcement for them.
In statically indeterminate prestressed and non-prestressed structures, where the torsional moment, Tu, in a member is not required to maintain equilibrium, design may be based upon reduced torsional cracking moments equal to four times the values given in Eqs. (9.43) and (9.44). When taking advantage of redistribution of torsional moments, the end moments of continuous members may be reduced likewise and the positive moments increased.
To reduce unsightly cracking and prevent crushing of surface concrete, the size of a solid cross-section is limited such that