The objective of cranked bars is to provide reinforcements at the bottom in the central point and reinforcements at the top adjacent to the edges both employing the same bars.
Beams generally have maximum sagging bending moment at mid span and maximum hogging moment at supports. Hence the area of bottom reinforcement bars is maximum at mid span and reduces towards supports. To resist the hogging moment at supports, maximum steel is required at top at supports.
To gain economic advantages, part of the bottom reinforcement bars is compressed at a specific distance from the support.
Usually, cranked bars are bent up at 45 degrees. These bars also facilitate to withstand shear force, in the bent up portion. Shear force becomes extreme at supports and here optimal shear resistance is necessary.
Benefits of providing crank bars in slabs
1.To withstand Hogging (Negative Bending Moment) at supports.
2.To get rid of the risk of the failure of the slabs.
3.To experience the shear force. The shear force is always larger at the sections of supports.
4.The strength of slab is raised significantly with crank bars.v