Pure torsion in a beam is typified by a series of inclined cracks running roughly parallel throughout the length of the member. The cracks basically form a spiral, running up one face of the beam, across the top, down the other side, and back across the bottom to connect with another crack on the first face.
If only an isolated portion of the beam is accessible, a torsion crack may appear similar to a shear crack. However, in torsion, the crack on the far face will be at the opposite angle to that on the front face while in shear the cracks on both faces will be at the same angle.
When the member has been correctly designed, torsion cracks will be controlled by the provision of shear reinforcement in the form of links.
Appropriate guidance is given in the design Eurocodes Eurocode 2; Part 1-1 covers buildings, Part 2 covers bridges and Part 3 covers Liquid retaining and containing structures. These parts of Eurocode 2 have replaced BS 8110, BS 5400 Part 4 and BS 8007.