Bars are bent near the supports at an angle of 45°. The angle bent may also be 30° in shallow beams where the (effective depth< 1.5 breadth).
Crank bars or bent up bars are provided in a slab due to the following reasons
1- To resist positive bending moment or sagging BM at the mid-span. This is done by the bottom reinforcement provided in a slab.
2- To resist negative bending moment or hogging BM at the support of the slab. This is done by the top reinforcement provided in a slab.
3- To resist the shear force which is greater at the supports.
4- To reduce the risk of a brittle failure of the slab- column connection.
5- To reduce the amount of steel used.
6- For the economization of materials.
The image shows how the bent up bars are provided in the slab.
The slab is supported at two ends. The maximum tensile stress is in the middle of the slab and the maximum compressive stress is at both ends.
To make RCC slab safe from compressive stresses we place reinforcement in form of bent up bars.
When bent up bars provide the strength and deformation capacity of slabs with bent up bars compared to slabs without bent up bars is sufficiently increased.