Difference between Main Bars and Distribution Bars in Slab

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When we look at any reinforcement detailing, we able to see there may be two sizes of bars used in the slab.

We know that is main reinforcement bar and distribution reinforcement bar. But wait there is more than just the name.

Let’s Discuss !!!

Bending Moment on Slab

We have studied that the load act on the slab will be high at the centre of the slab (on bottom) as shown in the pic.

Load on slab

Let’s take an example,

Place a thin plastic pad at the support of four ends. 

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.

You may see the deflection high in the shorter span and low in longer span. The support which is very near experience high tension than the far one.

Function of Main Reinforcement Bars

In order to transfer that bending moment developed at the bottom of the slab, Main reinforcement bar is to provide at the shorter span direction. The purpose of the main bar is to transfer the bending load developed at the bottom of the slab to the beams.

Slab reinforcement

That’s why 

  • we place main reinforcement bars at the bottom of the slab
  • Also, use slightly stronger (higher) dimension bars
  • we place main bars at the shorter direction

We discussed the difference between one way and two way slabs.

In one way slab, the slab is supported at two parallel sides where main reinforcement will be placed.

In two way slabs, the span will be supported at four ends. So there won’t be any difference in bar size. Because each side will have to transfer the same amount of stress evenly.

Main bars and distribution bars - Section Drawing

Function of Main Reinforcement Bars

On other hand, Distribution bars are used to resist the shear stress, cracks developed in the longer span.

Please refer the post why crank rod is provided in a slab? for better understanding

That’s why 

  • we place the distribution bars perpendicularly on top of the main bars.
  • We use lesser dimension since it is only to resist the cracks developed due to shear stress on top of the slab.
  • we place them at the longer span direction

Difference Between Main Bars and Distribution Bars

  • Main reinforcement normally used at the bottom of the slab. Distribution bars placed on top of the main bar.
  • Main Reinforcement bars are placed in shorter direction and Distribution bars placed in longer span direction
  • Main Reinforcement Bars are used to transfer the bending moment developed at the bottom of the slab. Distribution Bars are used to hold the slabs on either way and to resist the cracks and shear stress developed at the top.

Happy Learning 🙂

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