WHY CRANKING IS PROVIDED

Crank bar 

Bent up bars and cranks are provided in the slab and other structural members. Bars bent near the supports at an angle of 45°. In shallow beam, the bent angle maybe 30° (where the effective depth < 1.5 breadth).

         In the picture, you can see how the bent up bars are provided in the slab. The slab is supported at two ends. The maximum tensile stress (positive moments (sagging)) acting in the middle of the slab and the maximum compressive stress (negative moments (hogging)) acting at both ends of the support. So bottom steel is required at the mid-span and top steel resists negative moments at the supports. A bent-up bar called a crank bar is provided to make RCC slab safe from compressive stresses. When bent up bars are provided, the strength and deformation capacity of slabs with bent up bars compared to slabs without bent up bars is sufficiently increased.

So crank bars are generally provided

1- To resist the negative bending moment (hogging).

2- To resist shear force which is greater at supports.