Rebar sizes for middle bars in deep beams - the theory 1

[ChrispyBacon]

In deep beams there is a code requirement to have no bar further than 300mm from a restrained bar. As far as I know, this is a crack prevention thing. So, for example, on a 600mm deep beam, you will need one middle bar.
From the detailing code, I gather that the bar diameter is determined by the beam width. But if the purpose of the bar is to prevent cracks, why does the bar increase in size with the diameter? It is assumed to not be working, so what effect does the width have? Or is it more of a practical consideration?

As a note, I work in South Africa using the SANS codes (South African National Standards). Most concrete is based on British codes of practice, but the theory should be universal.

 

[hokie66]

Crack control reinforcement does not prevent cracks, it controls them. When a crack forms, the bar controls the width of the crack by providing a tensile, or clamping, force. Therefore the bar is "working".

 

[ChrispyBacon]

Let me try clarify:
The steel is 'working' in the sense that it stops crack propagation. However, it is assumed to not be working in the sense that it is assumed it is not working against bending in the beam.

So the question is why would you need a thicker bar for a 500mm (+/- 20") wide beam, as opposed to a 250mm (+/- 10") wide beam with a smaller diameter bar to prevent cracking? Or is there another reason for it?

I don't like giving the answer "because the code says so", and my knowledge in concrete cracking is not able to explain this. Yet.

 

[hokie66]

You have a good point, if that is what your code says. As I am not familiar with the SANS codes, I can't say, but apparently the theory is not universal.

ACI318 and the Australian Standard AS3600 treat this reinforcement as "skin" or "side face" reinforcement, and I think the amount required is independent of the beam width. The ACI code only requires these bars in the tension half depth of beams, and states that the size of the bars is not so important. In practice, I think most engineers would provide the bars over the full height rather than switching back and forth.

 

[ChrispyBacon]

Thanks Hokie, that reference to the other codes definitely helps. I was looking this up as skin reinforcing and size does not seem to be a governing factor in design. Perhaps then it's a detailing question for more practical consideration rather than design.
Much appreciated.