Minimum Flexural Reinforcement in Footings and Mat Foundations ACI 318-14 1

[cjg695]

Don't know if this has already been covered, but it appears our long national nightmare is over. ACI 318-14 has finally put to rest the debate hashed out here, here, and here.

In ACI 318-14, Chapter 13 now covers foundations. For one-way shallow foundations, section 13.3.2.1 states that design shall be per the applicable provisions of Chapter 7. Section 7.6.1.1 references Table 7.6.1.1 which has the familiar .0018 x Ag amount for As min. Commentary section R7.6.1.1 now states, "The required area of deformed or welded wire reinforcement used as minimum flexural reinforcement is the same as provided for shrinkage and temperature in 24.4.3.2. However, whereas shrinkage and temperature reinforcement is permitted to be distributed between the two faces of the slab as deemed appropriate for specific conditions, minimum flexural reinforcement should be placed as close as practicable to the face of the concrete in tension due to applied loads".

For two-way and mat foundations, section 13.3.3.1 references Chapter 8 and 13.3.4.4 specifically references 8.6.1.1 for mat foundations. Section 8.6.1.1 references Table 8.6.1.1 which has the same .0018 x Ag for As min. Commentary R8.6.1.1 has the same explanation as given in section R7.6.1.1 above.

Ambiguity has finally been removed - congrats to those who felt it necessary to have .0018 x Ag on the tensile face, you are the winners.

 

[JoshPlumSE]

Yes, all ambiguity has been removed....

Though I imaging the next code cycle will change it again, to allow the other interpretation for foundation slabs greater than a certain thickness.

 

[hokie66]

Let's hope you imagine wrong, JoshPlum. The two requirements, for shrinkage reinforcement and for flexural reinforcement, should be separate.

 

[JoshPlumSE]

Not quite was I was saying: Rather, I expect the next version of the code to allow As_min to be ignored if 4/3*As_required is provided (like we allow for beams). That should be sufficient to prevent non-ductile failure of the thick foundation slabs).

But, then another provision to ensure that overall reinforcement doesn't violate the T/S minimums.

Right now they have "corrected" the problem by forcing foundation slabs to obey the stricter requirements which thin elevated slabs are required to obey.

 

[hokie66]

If we are talking about mat foundations, they are heavily restrained, so 0.0018 Ag is not enough crack prevention reinforcement. In my experience, the amount of reinforcement to adequately control cracks in mats often exceeds the required flexural reinforcement.

 

[JAE]

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[hokie66]

Wipe that grin off your face. This will not continue.

 

[JoshPlumSE]

Hokie -

Please qualify that statement. What is heavily restraining the foundation slab? Because that seems somewhat off base for the industrial foundations that I've worked on.

I don't want to re-hash any arguments here. Just want to understand what you mean by that statement.

 

[hokie66]

Mat foundations, as well as all slabs on ground, are restrained by the surface they are cast on. If it is a mat on piles, the mat is also restrained by the piles. Mat foundations are often at the basement level of buildings and can be below the water table. To control cracking to make these more watertight, it is common to require as much as 0.06 Ag reinforcement. That amount of reinforcement may or may not satisfy the flexural requirement.

 

[JoshPlumSE]

I can see the need for the extra reinforcement.... When the desire is to make them watertight.

Though I still don't know that I would really view them as "heavily restrained" except for the cases where you have basement walls on all sides. If the argument is that the soil provides the restraint, then by that logic, the top surface of the slab certainly could be considered completely unrestrained.

Not trying to be argumentative here. Just trying to get my understanding to align with what you're talking about. I'm only part way there. Still doesn't feel like it applies to the types of industrial structures that I've mostly worked on. But, at least I'm understanding your logic more. Maybe I should add in KootK's signature:
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[hokie66]

The top surface is unrestrained from the soil. The restraint is at the bottom, which leads to the age old debate about where reinforcement should be placed in slabs on grade.

For spread footings of limited dimension, I see no need for "T&S" reinforcement, but they always need flexural reinforcement unless designed as unreinforced.

 

[JAE]

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