The ultimate shear resistance of prestressed concrete sections with web shear cracking but without flexural cracks, is mainly governed by limiting value of principal tensile stress developed in concrete, failure is assumed to take place when principal tension exceeds tensile strength of concrete.

I = 10mm, l=100mm
Slope of cable at support = (4e/l) = (4×100/10×100) = 0.04 radians.

The effect of this shear stress is to induce principal tensile stresses on diagonal planes and in pure shear, strength of concrete is twice that of strength in tension local failures first appear in form of diagonal tension cracks in legions of height shear stress.

In case of structural concrete members subjected to torsion, shear stresses develop depending upon type of cross section and magnitude of torque, shear stresses in association with flexural stresses may give rise to principal tensile stresses, value of which when it exceeds tensile strength of concrete results in development of cracks on surface of member.

The research by Humphery and Zia has shown that by suitably adjusting value of prestressing force torsional resistance can be increased by as much as 2.5 times that for corresponding plain concrete member and for members subjected to pure torsion; concentric prestress is more advantageous than eccentric prestress.

Flexure ? shear cracks are first initiated by flexural cracks in inclined direction and y are developed when combined shear & flexural tensile stresses produce a principal tensile stress exceeding tensile strength of concrete.

The spacing provided for shear reinforcement S_v = (A_sv0.87f_y/0.4b), S_v = spacing of stirrups along length of member, A_sv = cross sectional area of stirrup legs effective in shear, b = breadth (for T,I,L beams it is taken as breadth of rib, b_w).

Vertical prestressing is not generally adopted because length of cables being short, loss of prestress due to anchorage slip is excessively large post tensioning is generally uneconomical for vertical prestressing due to losses of prestress encountered.

When V exceeds 1.8V_c, maximum spacing should be reduced to 0.5d_t, lateral spacing of individual legs of stirrups provided at a cross section should not exceed 0.75d_t, if V > 1.8V_c, maximum spacing is:
(S_v) max = 0.5 d_t.

The behavior of a prestressed concrete member is affected by relative magnitude of internal actions, such as torque, bending moment and shear force, in circular sections and if torsion is small, it has little effect on overall behavior and failure are controlled by eir flexure or shear.