Is it allowed to mix rebar diameters in flexural members? 1

[TarikHKJ]

Hi,

Is it allowed to mix rebar diameters in flexural members like beams and ribs in a ribbed slab?

What is the implications of that? Wouldn't the smaller diameter bars yield before the larger ones under loading making them useless?

Thank you!

 

[Agent666]

Yes, strain is the same, so stress is the same, force in each bar size is what will be different (due to different areas) assuming bars at the same effective depth.

 

[TehMightyEngineer]

Seconded Agent666. Some caveats; extreme bar size differences can cause some minor issues (development length for the larger bar, depth to the centroid is different for the bars, etc.).

Biggest issue is the contractor is much more likely to screw up the bar placement. Thus, keep the layout simple and repetitive so that they have the best shot at getting it right the first time.

Ian Riley, PE, SE
Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries

https://www.facebook.com/AmericanConcrete/

 

[TarikHKJ]

Yes bars at the same effective depth,
I'm sorry I'm not that familiar with the theory of how stress can be distributed to each bar mm2 evenly so they will yield or fail together,

Can you please elaborate on that its very interesting, I imagine the concrete bond is the conduit for the stress distribution.

Sorry for the lack of scientific language.

 

[YuleMsee]

I don't know if it's a requiremwnt or good engineering practice, I ensure there is rebar size symmetry in my beams.
E.g 2T20 and 1T16 in the middle or 2T25 at the edges and 2T20 in the middle. Also I avoid very large differences, e.g I would mix T25 and T16

 

[JAE]

I'm sorry I'm not that familiar with the theory of how stress can be distributed to each bar mm2 evenly so they will yield or fail together,

If you have two bars of different area - embedded at the same depth in the beam - and the beam undergoes flexure....the two bars will stretch together essentially the same amount such that the strain in each is the same. If strain is the same for two bars of the same material, then the stress is the same naturally.

There's nothing you need to "do" to achieve this other than putting them at the same depth.

But if the stress is the same in each, but they have different areas, then the force in each will be different (F = Stress x Area).



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[TarikHKJ]

@Jea Thank you for the clarification,

 

[rapt]

Australian Code and Eurocode both indicate an effect of very different bar sizes in crack control. Nothing dangerous, but the very small bars should be ignored.

 

[Teguci]

For constructability and inspection, do not use consecutive sized bars. Always skip at least a size so that the different bars can be visually differentiated.

 

[TarikHKJ]

Teguci great point