ACI 318-08, Section 10.5.4 interpretation
ACI 318-08, Section 10.5.4 interpretation
(OP)
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!
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!






RE: ACI 318-08, Section 10.5.4 interpretation
RE: ACI 318-08, Section 10.5.4 interpretation
You're not alone. This is a terribly worded section of code that makes little to no sense for structural foundation slabs and footings. There have been a number of threads about this section of code in the past. They haven't added any more clarity to the issue...at least not for me.
The majority of the folks on this forum will argue similar to TXStructural. But, that's based on the As wording currently in the code. They will not be able to explain why slabs needed to be treated so special that the 10.5.3 liberaization is not sufficient. This code requirement has the effect of saying that using a top and bottom layer of reinforcing in a foundation slab is undesirable for some reason... but provides not explanation as to why. This is especially problematic because most folks who regularly work with thick foundation slabs come to the opposite conclusion.
Now, it may very well be that for thin elevated slabs, it is better and more efficient to bend your bars so that all temp/shrinkage reinforcement is on the tension side of the member. Elevated slabs are very different from foundation slabs. So, my guess this is a case where the commercial building guys on the code committee have be so focused on making this code provision work for elevated slabs that they forgot that their provisions also apply to other types of slabs as well.
RE: ACI 318-08, Section 10.5.4 interpretation
I understand what you are saying, but doesn't Section 10.5.1 and 10.5.3 take care of that? (i.e., insuring that when very little steel is required for strength, there will be a baseline minimum amount of flexural reinforcing steel provided to insure ductile behavior and guard against sudden rupture.) The wording in the commentary seems to bear out that the temperature reinforcing steel required per 10.5.4 is required purely as temperature steel (and the intent of 10.5.4 is not for the purpose of insuring ductility because 10.5.1 and 10.5.3 insure ductility).
RE: ACI 318-08, Section 10.5.4 interpretation
I am glad that I'm not the only one confused. Yes, elevated slabs are usually working much harder and 10.5.4 will seldom govern. Mat slabs and combined footings are another story. I am convincing myself that my interpretation is correct. As long as I meet 10.5.1 and 10.5.3 I will have ductile behavior and (in my opinion) 10.5.4 is there solely to insure that there will be sufficient temperature reinforcing steel and that the temperature steel computation may include all of the steel (top and bottom).
RE: ACI 318-08, Section 10.5.4 interpretation
It would appear that this reinforcement can be distributed IF the moments can redistribute. If the slab or footing is such that failure of the reinforcement at any section can result in loss of function (such as in a simple span condition), the minimum is required to be in the critical tension section.
RE: ACI 318-08, Section 10.5.4 interpretation
thread507-251751: How to place minimum steel in a rectangular footing?
It is unfortunate that ACI has commingled the flexural requirement and the shrinkage requirement. The requirements for bending reinforcement and direct tension reinforcement are completely different, but for some reasons, the minimum flexural requirement for steel on the tension face defaults to the same amount as required for T&S distributed reinforcement. This has been the case for at least 50 years in ACI318, and still causes confusion.
RE: ACI 318-08, Section 10.5.4 interpretation
I don't agree with using 1/2 top and 1/2 bottom if you are designing the footing for flexure and placing steel there for that flexure (again - see the referenced thread).
RE: ACI 318-08, Section 10.5.4 interpretation
That thread doesn't look quite right to me. It seems to be more related to whether you can count T/S steel in two perpendicular directions. I think there are better threads for this topic. But, am somehow forgetting how to directly link to a previous thread..... Used to know how to do that. Must be getting old.
RE: ACI 318-08, Section 10.5.4 interpretation
RE: ACI 318-08, Section 10.5.4 interpretation
Please don't put flexural steel on the face which is in compression. Steel intended to limit crack widths due to direct tension can be distributed in the section, but flexural steel has to be on the correct face. I don't think there is a "50/50 split of opinion" on this, it may just be that the confused people are more vocal.
RE: ACI 318-08, Section 10.5.4 interpretation
Of course not. I won't put the flexural steel on the wrong face of the flexural member. My original question concerned my confusion about 10.5.4 and whether that section required that I needed to put 0.0018bd T/S steel on both the top and bottom of deep foundation elements. I will be proceeding as follows:
Compute the required flexural steel for strength (As1).
Compute the minimum required amount of flexural reinforcing steel to insure against sudden rupture per Section 10.5.1. (As2)
Compute 4/3 x As1
Provide flexural reinforcing steel equal to the greater of 4/3 x As1 or As2
Compute the required amount of temperature reinforcing steel (AsTemp).
If the member has flexural reinforcing steel on one face only, then provide the greater of 4/3 x As1, As2 or AsTemp
If the member has top and bottom reinforcing steel, then provide flexural reinforcing steel on the tension side of the flexural member equal to the greater of 4/3 x As1, As2 or AsTemp/2.
Perform this calculation for both the top and bottom of the flexural member.
The above is my interpretation of section 10.5. My reading of the commentary convinced me of this interpretation (albeit indirectly). The reinforcing steel required per 10.5.1 guards against failure due to sudden rupture. Section 10.5.4 has nothing to do with the prevention of sudden rupture. It is required solely to provide restraint against T/S cracking - and that was my confusion. I think ACI 318 needs to clarify this with one sentence in the commentary. I am going to email ACI and offer this suggestion.
RE: ACI 318-08, Section 10.5.4 interpretation
10.5.4 minimum reinforcement is .0018 bD, not bd. This is a quantity of reinforcement to be used as minimum flexural steel, and the .0018 is not multiplied by bd.
Temperature and shrinkage steel is for direct tension crack control, and is on the whole cross-section, not on each face.
Clear as mud?
RE: ACI 318-08, Section 10.5.4 interpretation
The D term (I think) refers to the total height (thickness) of the footing). I think that was the distinction being made between bD and bd.
The 10.5.4 is simply a method ACI uses to require 0.0018bD MINIMUM FLEXURAL REINFORCEMENT in footings. It has nothing to do with Temp/Shrinkage.
ACI for some STUPID reason decided to out-reference footing min. flexural reinforcement to chapter 7 instead of just stating directly that it is 0.0018bD.
From the thread hokie referenced there was still left some confusion on whether footings need to ALSO meet 10.5.1 and 10.5.3 in addition to 10.5.4. I always use all of them for footings but if you read the code precisely, it does tend to suggest that footings ONLY need to meet 10.5.4 - which is the Ch. 7 0.0018bD value.
hokie66 - correct me if I mis-interpreted your bD intent.
RE: ACI 318-08, Section 10.5.4 interpretation
Your explanation is better than mine, but we agree. And I agree that 10.5.1, 10.5.3, and 10.5.4 all apply to footings.
Pad footings in general don't really need restraint shrinkage reinforcement due to their size. And another thing...unless footings need bars in the top due to flexure, top bars are undesirable because they often result in reflected cracking due to plastic settlement.
RE: ACI 318-08, Section 10.5.4 interpretation
Likewise the commentary to section 10.5.1 notes that equation 10-3 is intended to provide a minimum amount of reinforcing steel to prevent sudden rupture when the computed amount of flexural reinforcing steel is so small that the flexural strength of the plain concrete slab based on the modulus of rupture is greater than the flexural strength computed with the tiny amount of reinforcing steel. In fact section 10.5.1 states that As,min must never be less than 200bd/fy. For grade 60 reinforcing steel that equates to 0.0033bd which would typically be much more than 0.0018bh.
The commentary to section 10.5.3 seems to say (and I am paraphrasing), "...well, we suppose if you put in 4/3 more reinforcing than you calculated that you needed for strength, well then that extra reinforcement will be enough to insure against that sudden rupture that we were so worried about in section 10.5.1".
RE: ACI 318-08, Section 10.5.4 interpretation
As to 10.5.3, I wouldn't put it quite like that as it is based on more than "we suppose", but your interpretation is correct for design purposes. Personally, I would never use that provision for beams, but rather only for slabs.
RE: ACI 318-08, Section 10.5.4 interpretation
The whole section 10.5 is about flexural minimum reinforcement - not T&S.
RE: ACI 318-08, Section 10.5.4 interpretation
This is just speculation. But, I question whether the code committee has any more agreement on the issue than we do. Those who specialize in thick foundations for industrial facilities would have a conniption fit if it were "clarified" per Hokie's interpretation. Therefore, they are likely demanding to see test data which supports this interpretation.
Those who argue per Hokie are likely only thinking of elevated slabs where this extra bit of safety may be needed or desirable. Therefore, they are unwilling to clarify it the other way. However, they may lack sufficient research to justify their interpretation. In addition, they can't justify it based on first principals either. Therefore, a crappy piece of code stays this way for code revision after code revision.
The only hope they have to change this is to squeak in an "editorial" change that doesn't need to be approved by the same consensus procedure. That's likely how the 10.5.4 got the term As,min added to it.
Code updates are not always easy or pretty.... And, the results are frequently imperfect. As engineers, we will always have to use engineering judgmennt and experience to interpret the code. The key is to understand the principals behind the code provisions. If you do that, then you can make a good interpretation. Even if you don't come to the same conclusion as everyone else.
RE: ACI 318-08, Section 10.5.4 interpretation
The way codes evolve, there are frequently shortcuts taken to reduce the need to coordinate changes among sections every cycle. We often try to avoid repeating material already elsewhere in the code.
I am also hoping to get an advance copy of the revised 318-xx language related to this subject, so we can better understand the provision.
RE: ACI 318-08, Section 10.5.4 interpretation
"...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.
RE: ACI 318-08, Section 10.5.4 interpretation
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.
RE: ACI 318-08, Section 10.5.4 interpretation
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.
RE: ACI 318-08, Section 10.5.4 interpretation
RE: ACI 318-08, Section 10.5.4 interpretation
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.
Here is ACI's response:
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
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.
(NOTE - the document that ACI sent is attached to this post.)
RE: ACI 318-08, Section 10.5.4 interpretation
RE: ACI 318-08, Section 10.5.4 interpretation
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.
RE: ACI 318-08, Section 10.5.4 interpretation
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.
RE: ACI 318-08, Section 10.5.4 interpretation
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.
RE: ACI 318-08, Section 10.5.4 interpretation
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.
RE: ACI 318-08, Section 10.5.4 interpretation
RE: ACI 318-08, Section 10.5.4 interpretation
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.
RE: ACI 318-08, Section 10.5.4 interpretation
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.
RE: ACI 318-08, Section 10.5.4 interpretation
- 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).
RE: ACI 318-08, Section 10.5.4 interpretation
RE: ACI 318-08, Section 10.5.4 interpretation
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.
RE: ACI 318-08, Section 10.5.4 interpretation
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.