Why Crank Rod is Provided in the Slab?

You may notice difference shape of bent up bars and cranks are provided in the slab and other structural members.

Do you know the reason? Let us explain.

We have seen what is the difference between main bars and distribution bars. If you are familiar with that this one is easy peasy.

Let’s begin.

Load on slab

Take an example,

Place a thin plastic pad at four supports at four corners.

Now give pressure with your fingers at the top of the pad. You see the deflection under the bottom centre of the pad and a raise at the top of the support as shown in the above pic.

What actually happens is, the bottom of the pad experience a sagging (deflection) moment and the supports experience a hogging (raising) moment. The bottom of the slab (reinforcement) at centre and the top of the slab at supports experiencing high tension at the same time.

So in order to compensate these tensions, we have to provide reinforcement at the bottom (slab centre) and at the top (near the supports ).

Main bars and distribution bars - Section Drawing

To reduce the wastage and for economic purpose, we are using a single reinforcement which supports the bottom and cranked over the top of the supports in that critical tension area (may be L/4 or L/5).

Hope this helps. Happy Learning.