A post-tensioned concrete beam of rectangular section 100mm wide and 300mm deep is stressed by a parabolic cable with zero eccentricity at the supports and an eccentricity of 50mm at the centre. The primary purpose of this design is to:

A. Increase the beam's deflection capacity
B. Reduce the beam's overall weight
C. Enhance the beam's tensile strength
D. Minimize the beam's shear stress

The design of a post-tensioned concrete beam with a parabolic cable arrangement is to minimize the beam's shear stress, particularly in the center where bending moments are highest.

Explanation:

The primary purpose of the post-tensioned design mentioned in the question is d) Minimize the beam's shear stress. The parabolic cable arrangement, which has zero eccentricity at the supports and eccentricity at the center, creates a prestressing force that counteracts the bending moments induced by loads. This design is particularly effective at minimizing the shear stress in the beam at the center, where the moments are highest.

The tension in the cable is introduced after the concrete has cured and induces a compression in the bottom fibers of the beam, which is opposite to the tension that would otherwise be caused by bending moments due to applied loads.