ACI 318-08, Section 10.5.4 interpretation 5

[cliff234]

ACI 318-08, Section 10.5 gives minimum flexural reinforcing steel requirements for beams and slabs. Section 10.5.4 states that As,min shall be no less than required per 7.12.2.1 (temperature reinforcing). For years I have been confused by the wording of 10.5.4. If I have top AND bottom steel in a thick slab or footing then I have always assumed that I could put half in the top and half in the bottom and that I did not need to have FLEXURAL reinforcing steel equal to at least 0.0018bh. (Sections 10.5.1, 10.5.2 and 10.5.3 provide additional requirements for minimum flexural reinforcing steel.)

My interpretation is that when I have top and bottom steel, then half of the temperature steel can be in the top and half in the bottom. If we only have bottom reinforcing steel then we need at least 0.0018bh in the bottom.

Has anyone else ever wondered about the wording of 10.5.4? Is my interpretation correct? The purpose of 10.5.4 is solely to insure that there will always be at least temperature reinforcing in slabs and footing. The purpose (in my opinion) is not to mandate that flexural reinforcing will never be less than 0.0018.

Does anyone disagree with that interpretation? It can make a big difference for mats or foundations with top and bottom flexural reinforcing.

Thanks!

 

[PUEngineer]

CRSI 2008 design handbook section 13-23 has the following to say (the language comes from the pile cap section):

"...The requirements for minimum areas of flexural reinforcement (ACI 10.5 and 7.12) are satisfied by the following conservative interpretation, where As is the calculated area of reinforcement required for flexure (ACI 7.12 and 10.5):

1 - if As > 0.0033bd, use As (where 200/fy=0.0033)
2 - if As < 0.0033bd < 4/3As, use 0.0033bd
3 - if 0.0018bD < 4/3As < 0.0033bd, use 4/3As
4 - if 4/3As < 0.0018bD < 0.0033bd, use 0.0018bD"

I think until the issue is cleared up, it's an interpretation. With the conservative one noted about and I think it follows the interpretation of JAE and Hokie.

 

[hokie66]

PUEngineer,
Thanks for that. That coincides with my interpretation, but I don't agree that it is conservative, just that it is correct, and actually quite simple. If the 0.0018bD did not result from referring back to an irrelevant clause, we wouldn't be having this discusssion.

JoshPlum,
I do and have done a lot of industrial facilities, as well as mat foundations, and have never had a conniption fit about the minimum steel requirements. To the contrary, industrial design is usually more conservative than the 'minimum' standards.

 

[JoshPlumSE]

Hokie -

One of my old colleagues worked on a 9 ft thick mat (controlled by punching shear) to support a large vertical vessel. Temperature / Shrinkage reinforcement was far greater than that required for flexure. But, in your interpretation, it would be reasonable to put 0.0018*b*h on one side of the mat?

I think in order to get a rebar spacing that was constructable, he went with 3or 4 layers equally spaced through the thickness of the mat for his temp/shrinkage reinforcement through the thickness of the mat.

Aside from the practical limitations of doing this for thick mats, my issue has to do with understanding the intent of this code provision. Since it is so clear to you perhaps you can explain the rational of this code section for me. Specifically, what is the behavior that we're trying to achieve with 10.5.4 that isn't already achieve by enforcing 10.5.1 and 10.5.3? And, why is that behavior unique for slabs?

I'm not arguing with you here. I understand how you can read the code with that interpretation. And, I'm willing to concede that it may be the best interpretation based on the legal language of the code. However, I need to understand the theory behind the code requirement. After all, anyone can read a recipe.... But, it takes a lot more understanding to become a master chef. And, that understanding is what I'm searching for.

 

[Hokie93]

The sections in question are quite clear to me. My interpretation is that for structural slabs and footings of uniform thickness, section 10.5.4 provides the minimum required area of flexural (tension) reinforcement (0.0018bD for grade 60 reinforcing steel). In other words, I believe that section 10.5.4 supersedes the requirements of section 10.5.1 for structural slabs and footings of uniform thickness so one only need to apply section 10.5.4 for such members. Section 10.5.3 permits an alternative only to sections 10.5.1 and 10.5.2. I have heard engineers disagree about whether 0.0018bD represents an adequate minimum amount of reinforcing steel in footings, as opposed to 0.0033bd (for grade 60). I personally typically use an amount of reinforcing steel in footings closer to 0.0033bd than 0.0018bD. "Design of Concrete Structures" (14th edition) by Nilson, Darwin, and Dolan has a nice discussion of this subject (as it relates to footings) in Chapter 16. "Reinforced Concrete, Mechanics and Design" (4th edition) by MacGregor and Wight also supports the concept that the minimum reinforcing requirement in section 10.5.1 does not apply to footings.

 

[JAE]

The long post below here is what I sent ACI on the previous thread. I'm reproducing it here for everyone's enjoyment (place sarcasm icon here).

[blue]My question to ACI was this:
I have been discussing an issue related to ACI 318-05, section 10.5 with a number of other engineers on an engineering forum website. The issue that has many of us confused is as follows:

Section 10.5.1 through 10.5.4 presents minimum areas of reinforcement for flexural members. 10.5.1 provides a formula for As,min for tensile reinforcement.

10.5.2 provides for T-beams with flanges in tension.

10.5.3 provides an out for very large sections, using the 1/3 greater than As(calc).

So all of these are very clear. No problems.

However, we come to 10.5.4 and it states that for "structural slabs and footings of uniform thickness" we are told that As,min is the AMOUNT (commentary) required by 7.12.

The confusion we have here can be resolved into two different interpretations:

Interpretation 1.
For structural slabs and footings - 10.5.4 points you to section 7.12 and you calculate 0.0018bh and insert that into your structural slab or footing in any place within the thickness you wish. 7.12 doesn't require positioning of the min. steel in the tension area. You can put it in the middle, in the top, split it between faces, etc. Also, since 10.5.4 refers to 7.12, we don't need to meet 10.5.1-3 at all for slabs and footings. In addition, the 318-08 commentary for 15.10.4 seems to imply that the 7.12 reinforcing can be place anywhere within the section.

Interpretation 2
Section 10.5.4 is located within section 10.5 which is clearly dealing with flexural reinforcement minimums in tension. Therefore, 10.5.4 is referring to 7.12 to give us an AMOUNT of As,min, and this should be placed in position where tension occurs. It should never be split between faces, placed in the center of the section, or especially placed in the compression area of the section.
In addition, section R15.10.4 states that this 7.12 reinforcement should be placed "as deemed appropriate for specific conditions" - therefore, the only specific condition that would sway me where to place the reinforcement would be where tensile stresses occur.

This section has produced numerous lines of communication on our website and I know it is a very confusing section of 318. I would ask if you could clear this up for us.

1. Does 10.5.4 imply that 7.12 is simply an AMOUNT of reinforcement and that it still should be placed in tension areas?

2. Does 10.5.4 then negate the use of 10.5.1-3 for structural slabs and footings or should structural slabs and footings also meet those sections in addition to 7.12?

3. Should the 10.5.4 reinforcement from 7.12 be a second check on As,min. In other words, do we check 10.5.1-3 for a structural slab and THEN also check to see if it meets 7.12?

Thanks for your help.[/blue]

Here is ACI's response:

[red]ACI publishes codes, specifications, and reports for the concrete user. This message is in response to your technical question/inquiry.

ACI 318 is a consensus document and thus language is debated until all committee members can come to an agreement. This may not lead to the most concise set of requirements as many voices are trying to relay their thoughts.

Question 1. Attached is a copy from MacGregor and Wight book addressing this issue. see the attachment below

Question 2. Section 10.5.4 directly sets the minimum requirement for a subset of flexural members, structural slabs and footings of uniform thickness.

Question 3. See answer to Question 2.

Sincerely,
Technical Staff [/red]

[blue]So it appears that they refer to MacGregor's book and in it he clearly indicates that the 7.12 steel is a tensile reinforcing and should therefore be placed in the tensile area of the footing or slab.
They also indicate that 10.5.4 "directly" specifies the min. steel for slabs and footings in response to my question on whether slabs and footings also need to meet 10.5.1, 10.5.2, etc.[/blue]

(NOTE - the document that ACI sent is attached to this post.)

 

[JAE]

And attached to this post is an example from a MacGregor book with a structural slab example where he clearly shows that the total 0.0018bD amount goes all on the tensile side.

 

[hokie66]

Josh,
Not sure that I have time at the moment to address your question, but I certainly was not implying that flexural steel always controls. Restraint shrinkage steel (direct tension reinforcement) in mat foundations is often required in a greater quantity than the entire amount of flexural steel. All the more reason for the two requirements to be separated.

 

[hokie66]

Josh,
Back to your last post.

In the case of your colleague's 9' thick mat, that is a mass concrete situation, and the shrinkage and thermal effects are of a different nature than in most of our routine structural elements. I have never been closely involved in the design of such a thick mat, but have seen drawings. Thermal stresses are the biggest issue, and they have to be dealt with in three directions, not just two as in thin elements. The vertical steel in these mats is not just for support of the layers of horizontal steel.

As to explaining the rationale of the code as it pertains to minimum reinforcement in slabs, I would have to do some research, but may not have good access to the required information. These sections of ACI318 are identical in effect, if not in precise wording, to provisions in ACI318-63, so have remained unchanged for at least 50 years. Perhaps that is why I am so convinced of my interpretation...that is the way I was taught by Dr Richard Barker at Va Tech, so it must be true. But Dick Barker is no longer with us, and many of the members of ACI Committee 318 from that era are probably not around either, or don't remember. This is one Code problem which can't be blamed on new age academics or engineers, except as a sin of omission.

 

[cliff234]

I guess I asked a good question.

In any case I am sticking to my original viewpoint. I'm meeting 10.5.3. I'm interpreting 10.5.4 to mean that I need to have temperature reinforcing within the total depth of the slab. I'll put half in the top and half in the bottom, and in doing so 10.5.3 will govern my design. Futhermore my slab is a 4' deep combined footing. The structure is SDC "B" which means that if I wanted to, I could have designed the footing as a plain concrete footing with no reinforcing steel - but I did not. I'll be conservative and use flexural reinforcing steel. It sounds as if there will continue to be differing opinions on 10.5.4 until ACI clarifies it. I respect everyone's opinion and I thank everyone for providing their opinion. I learned something. I questioned my original interpretation for a while as I read the various points and counterpoints, but I came away being comfortable with my original interpretation. I still wish 10.5.4 was not ambiguous. As a structural engineer, I like clarity.

 

[JAE]

I'm interpreting 10.5.4 to mean that I need to have temperature reinforcing within the total depth of the slab. I'll put half in the top and half in the bottom...

Well, I think you are totally wrong there. All of the 10.5.4 reinf. should go on the tension side of the flexural element.

 

[hokie66]

JAE is correct, and there is no ambiguity, just misinterpretation. T&S reinforcement from 7.12 or elsewhere can be distributed throughout the section, but all 10.5 reinforcement, no matter which subparagraph controls, must be on the tension face.

 

[JoshPlumSE]

Hokie -

You said, "there is not ambiguity, just misinterpretation". Uh, sounds like you saying that anyone who thinks there is ambiguity here is an incommpetennt idiot? Or, maybe I'm just being thinned skinned....

Regardless, I have a couple of questions about your previous couple of posts.

Question 1: In one of your previous posts, you said the following:

"Restraint shrinkage steel (direct tension reinforcement) in mat foundations is often required in a greater quantity than the entire amount of flexural."

I dont understand that comment. How is this possible when 0.0018b*h is approximately equal to 0.0018b*d? Also, where do you get 0.0018b*d from. The code never says that to my knowledge. 10.5.4 only says As_min = the requirements from 7.12. If I follow your interpretation, that would mean As_min = 0.0018b*h.


Question 2:
In another previous post, you indicated that my 9 ft thick mat should not be referred to as a slab and should instead be referred to as "Mass Concrete". I don't necessarily disagree with you there. But, it's not so black and white to me. Where do you draw the line between a mat slab foundation and a "Mass concrete" situation? Is there a slab width / thickness ratio below which it has to be considered a slab? I've never worked on a 9 ft think mat / slab before. But, I'ved worked on a few 5 to 6 ft think mats. And, many 3 to 4 ft thick mats.

At some point, I would think there would be an intersection between the two provisions where the reinforcement would be approximately the same whether you called it a mass concrete situation or a mat slab.

 

[JoshPlumSE]

JAE -

Thanks for that ACI response. Thanks also for those snippets from Macgregor's book. The 2nd snippet comes closest to my goal of understanding the theory behind why this provision is there. I appreciate your effort.

Macgregor's book states the following:
This amount of steel should provide a moment capacity between 1.1 and 1.5 times the flexural cracking moment and hence should be enough to prevent sudden failures at the onset of cracking.

Though it certainly doesn't explain everything and some ambiguity still remains.... at least to me. Specifically, I don't see why we should not be allowed to use 4/3*As_required as we are in 10.5.3 for other flexural members. Is there some reason why it is more important to avoid brittle cracking failures for slabs than for other flexural members? In my thinking slab foundations have a good bit more load re-distribution ability than other flexural members. Therefore, the failure would be more ductile than for beams which are allowed the 10.5.3 liberalization.

 

[JAE]

JoshPlum - I think I'm pretty convinced that 10.5.4 uses the AMOUNT of steel in 7.12 (0.0018bh - or 0.0018bD in hokie's terms) and it should be placed 100% on the tension side - not split into halves.

- but the question that was never resolved in my mind was whether a slab or footing, using 10.5.4, would also use the other parts of 10.5, including the 4/3As(req'd).

 

[cliff234]

I emailed ACI this morning and asked them to clarify 10.5.4. I noted that there is disagreement between engineers as to the intent of 10.5.4. Someone correctly pointed out that there is no ambiguity in the current wording - and they are correct! I just think that the current wording is clearly telling me to do something that is unnecessary. I offered ACI two suggestions for rewording 10.5.4. One suggestion if the intent of 10.5.4 is to provide T/S steel, and a second suggestion if the intent of 10.5.4 is to provide flexural steel equal to 0.0018bh on the tension side(s) of the member.

 

[hokie66]

JoshPlum,
I meant no insult, but I just don't see the ambiguity. Chapter 10 is for strength, and 7.12 is for serviceability cracking. Two different criteria. The 7.12 T&S reinforcement is stated in the Commentary to be "required at right angles to the principle reinforcement to mimimize cracking...". I don't know why the 10.5.4 cross-referencing occurred, and wish that it had not.

Your Question 1.: My statement that restraint shrinkage steel often is required in greater quantities than flexural steel is correct. For example, in basement slabs which resist uplift from water pressure and must be watertight, the amount of crack control reinforcement is typically around 0.006Ag, while the flexural reinforcement can be more or less that amount, depending on the spans and uplift loading.

Your Question 2.: I didn't mean to imply that a thick mat is not a slab, but was instead suggesting that with mass concrete elements, the reinforcement requirements are often different due to early age thermal effects. These problems can be dealt with to some extent by concreting techniques, e.g. ice in the mixing water. There is a lot of information available about mass concrete structure, but I think most engineers start to be concerned when the thickness of a concrete slab exceeds about a metre.

 

[hokie66]

Back to the original question, two authoritative sources have been given in this thread:
1. PUEngineer's post of 26 April 1358 give CRSI's inequality decision tree.
2. JAE's post of 26 April 1903 links to an example in the MacGregor book.

These sources agree.