ACI Minimum Spread Footing Reinforcement

[Norby_acn]

Thank you in advance for any help on this issue.

I have an unusual situation that has come up. I am designing the footings for a modular structure. The frost depth is 33" and the contractor wants to pour solid concrete from the top of grade down to the bottom of the footing. This is fine, easy to design for. The loads are tiny, in the 10-25 kip range. But the ACI minimum shrinkage and temperature reinforcing of 0.0018 for a footing this thick is way way waaaaaay beyond what is structurally required for strength. I can justify through calculation not needing any reinforcement and the footing being just plain concrete.

So my question is, is there a portion of the ACI code which gives an allowance for using less than chapter 10 minimum S&T (ACI 318-11)? Maybe there is an allowance for if I can empirically prove that the concrete is significantly over-strength or something of that nature. What I'm really after is a portion of the code or an exception which would justify me only throwing a few #4 bars in the bottom rather than loading it up with a ton of bars to meet that S&T minimum.

Any help is really appreciated!

 

[JAE]

So my question is, is there a portion of the ACI code which gives an allowance for using less than chapter 10 minimum S&T (ACI 318-11)?

Yes - you already referred to it - the Plain Concrete section.
In a sense, this allows you to "design" for plain concrete and then throw in some nominal reinforcement just to "hold the concrete together".

Having said that, if you have the potential for these grade beams (or large footings) to experience some significant bending (say expansive clays, liquifaction, etc.) then the Chapter 10 reinforcement would be a good idea.

And to make things worse, the 0.0018Ag applies to each tension surface...wherever bending occurs, you need 0.0018Ag on that tension side. You can't split the steel 50-50 top and bottom as this is a Chapter 10 minimum flexural steel requirement, not a T&S requirement.



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[MotorCity]

Note that plain concrete does not necessarily mean concrete without ANY reinforcing, but rather it means concrete with less than the minimum reinforcement.

So if you ONLY provide S&T reinforcement, it is considered plain concrete.

 

[JoshPlumSE]

At some point you may have to use some engineering judgment at deviate from the code requirements slightly. When I first graduated I did a lot of large pedestals on a mat foundation. The pedestals were for really large vertical vessels. We ran into the following code issues:
1) UBC said explicitly that we could not design "plain concrete foundations" for the seismic area we were working in.
2) There was no way we could provide temperature / shrinkage steel for these short, but very wide pedestals.
3) Our design procedure pretended like these were reinforced pedestals when we were designing the pedestal, even though we were no where near meeting min steel requirements.

Now, in reality, it was the anchor rods (which usually went down into the Mat slab) that really transferred the overturning forces. So, our final designs could be viewed more as providing a minimum spacing of bars in the pedestal face than as having a real strength demand. However, there was nothing we could really do to get around this "gap" in the code. We were somewhere between plain and reinforced concrete and the code just never considered the possibility.

 

[Agent666]

I did a few thick pads (1.2m thick or so) for large transformers in switch yards in Australia in my early days (15+ years ago), they used to have some provisions (and may well still have) that limited the concrete depth you considered to be something like 250mm from the free face for applying temp and shrinkage provisions. So if you had 500mm thickness, you considered the whole thing, but for 1.2m you considered 500mm thickness, considering each face separately at 250mm. This was the provisions from older version of AS3600, someone might be able to confirm if its still there in later versions and what it is based on, following on from this there was some further equations based on D:-

EDIT:-

found my commentary which explains the thinking behind the limit:-