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!

 

[TXStructural]

The minimum for flexural reinforcement is to avoid yield or rupture of the steel when the concrete cracks, maintaining performance of a reinforced section rather than acting as unreinforced. In the case of flexure, the transfer of load from concrete to steel in a reinforced section requires the minimum to be in the tension region. This is demonstrated by the use of the term "As" ("the area of nonprestressed longitudinal tension reinforcement...".) The reference to the temperature steel minimum is a convenient way to give a number without repeating a formula or value, but unnecessarily confuses the purpose and location the the reinforcement.

 

[JoshPlumSE]

Cliff -

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.

 

[cliff234]

Thanks for responding TXStructural.

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).

 

[cliff234]

JoshPlum,

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).

 

[TXStructural]

The first two paragraphs of commentary on 10.5 seem to explain this. The minimum quantities are tensile reinforcement requirements. I checked the latest edition of the PCA notes and they simply regurgitate the ACI commentary.

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.

 

[hokie66]

As Josh said, this has been debated here before, lots of times. Just for your reading enjoyment, this is probably the longest thread.

thread507-251751

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.

 

[JAE]

I'd agree with looking at that thread that hokie66 referenced.

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).

 

[JoshPlumSE]

Hokie -

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.

 

[cliff234]

What an amazing resource this website is. As several of you suggested I did a search and found many previous posts on this topic. If anyone is interested, do a search of "10.5.4". There seems to be a 50/50 split of opinion regarding whether the 10.5.4 bars must be placed in the tension face or whether it can be split top and bottom - which frustrates me. ACI must know about this confusion. Why not at least clarify the intent of 10.5.4 with one sentence in the commentary? That said, I read several convincing arguements in favor of permitting the T/S reinforcing steel to be split, which is the direction that I was leaning towards.

 

[hokie66]

cliff234,
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.

 

[cliff234]

Hokie66,

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.

 

[hokie66]

10.5.1 is the general minimum, but is not required if 10.5.3 is met, for slabs and footings. So in your analysis above, you would be using the greater of 4/3 x As1 or AsTemp as minimum reinforcement for slabs and footings, except in the case where 4/3 As1 exceeds As2, and then you would use the greater of As1 or As2.

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?

 

[JAE]

Can I chime in on a couple of points hokie66 just mentioned directly above?

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.

 

[hokie66]

JAE,
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.

 

[cliff234]

Not to belabor the point, but I thought 10.5.4 was related to shrinkage. The commentary to 10.5.4 says, "The minimum reinforcement required for slabs should be equal to the same amount as that required by 7.12.2.1 for shrinkage and temperature reinforcement."

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".

 

[hokie66]

The point that JAE and I were trying to make is that 10.5.4 is NOT related to shrinkage. Chapter 7 of ACI318 deals with serviceability issues (one of which is shrinkage cracking), while Chaper 10 addresses strength. The minimum amounts of reinforcing required for both purposes being the same is coincidental, and the two requirements being commingled just because the values are similar is just poor writing.

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.

 

[JAE]

cliff - the key word in the commentary is AMOUNT. 10.5.4 refers to Chapter 7 to define an amount of minimum flexural reinforcement.
The whole section 10.5 is about flexural minimum reinforcement - not T&S.

 

[JoshPlumSE]

Why doesn't ACI just clarify this?

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.

 

[TXStructural]

I have put a query out to a member of 318B (the reinforcement subcommittee of 318) for a bit of clarification, and will let you know when I get a reply.

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.