# Why crank bar is used in slab- bent up bar

Why crank bar is used in slab- bent up bar,Why do we use crank in one way and two way slab,let us discuss about very important topic why crank bar is used in slab and what is one way and two way slab.

When load is applied on middle of slab its shortest Span bar that is main bar experience positive bending moment towards downward on simply supported beam that is known as sagging and simply supported beam experience negative bending moment in upward direction simultaneously on slab that is Hogging.

Both sagging and hogging moment on beam and slab create it very much tension in slab that’s might be leads to failure of slab, to counter and balance this compressive force created on slab crank is used. Crank is also known as bent up bar

## Why do we use crank in one way & two way slab

We know that slab reinforcement design is used in two category

1) one way slab

2) two way slab

### what is one way slab

In reinforcement design of slab if crank or bent up bar used in only one direction of slab is known as one way slab. In one way slab ,slab are supported by beam on the two opposite sides only, and crank is provided in shorter direction of slab.

If ratio of longer span and shorter span is equal or greater than 2 to then we apply one way slab means crank is used in only one direction on the top of main bar which is bent with angle of 45 degree or 30 degree.

### what is two way slab

In reinforcement design of slab if crank or bent up bar used in two direction of slab is known as two way slab. In two way slab ,slab are supported by beam on all four sides and crank is provided in shorter direction and longer span both of slab.

If ratio of longer span and shorter span is less than 2 then we apply two way slab means crank is used in both direction on the top of main bar which is bent with angle of 45 degree or 30 degree.

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### reinforcement (rebar) in slab

1) length of crank is minimum about 300 mm which is approx 1:10 of main bar and distribution bar

2) if we use Steel bar of 10 and 12 dia as main bar at the bottom of slab Centre to centre spacing will be about 6 inches and if we use 8 mm dia of Steel bar as main bar then Centre to Centre spacing should be 4 inches

3) if we use Steel bar of 10 and 12 dia as distribution bar at the top of slab Centre to centre spacing will be about 9 to12 inches and if we use 8 mm dia of Steel bar as distribution bar then Centre to Centre spacing should be 7 to 9 inches.

### Why do we use crank in slab

1) crank is used to resist and counter the positive that is sagging and negative bending moment that is hogging created on slab and beam when load is applied on slab.

2) crank is used to resist the attack of shear stress and Crack developed in longer span

3) crank is used to increase the strength of slab that’s increase long life and durability of slab

4) crank is used to counter the risk of failure of slab to fall