ACI Minimum Flexural Reinforcement for Structural Slabs and Footings

[pmour]

Link

All references are to ACI 318-05.

It the above link it is clarified that the minimum flexural reinforcement is not controlled by the provisions of ACI 10.5.1 and 10.5.2 as they do not apply to structural slabs and footings of uniform thickness, rather the minimum flexural reinforcement is determined by 10.5.4 as 0.0018bh. For a structural slab having top and bottom reinforcement both of which exihibit flexural tension, this minimum flexural reinforcement of 0.0018bh for each layer would control over the temperature and strinkage reinforcement of 0.0009bh for each layer (0.0018bh/2).

My question: Is the provision of 10.5.3 applicable also?

If not, As minimum would always be 0.0018bh for each layer.

If so, and in the situation where the required As is small enough, the minimum flexural reinforcement determined by 10.5.3 as 4/3 * As,reg could be less than that determined by 7.12 as 0.0009bh for each layer, thus As minimum being 0.0009bh for each layer.

Another way to write it (applies to slabs and footings having top and bottom steel):

If As > 0.0018bh, use As
If As < 0.0018bh < 4/3As, use 0.0018bh
If 0.0009bh < 4/3As < 0.0018bh, use 4/3As
If 4/3As < 0.0009bh, use 0.0009bh

 

[JAE]

I think 10.5.4 is the ONLY section that would apply to structural slabs, mats and footings.

Sections 10.5.1 - 10.5.3 are for elevated beams, joists, etc.

 

[grantstructure]

That linked thread was actually entertaining.

10.5.3 specifically references 10.5.1 and 10.5.2. It would appear, based on the language, to be a stretch to apply it to 10.5.4.

But really, how much lower than .0018 do you want to go?

 

[hokie66]

Grant,
If you were entertained by that one, try this:

thread507-251751

Sorry JAE, couldn't resist.

 

[JAE]

whimper sniff sniff....

 

[grantstructure]

Good Lord. The entertainment value has pretty much vaporized now, hasn't it?

I went to grad school with Robert Frosch. I'm sure he'd love to know he had a walk-on role in that thread.

I've been too distracted (read: cheap) to buy the new 08 code. What does 15.10.4 say? The updated (to 08) PCA handbook is still checking 0.0018 as minimum flexural reinforcing and refers that check to 15.10.4.

 

[hokie66]

15.10.4 is for mat foundations, and refers to 7.12.2 for crack control reinforcement. I am sure a lot of people will be confused by that as well as by all the other instances in the ACI code where flexural reinforcement and crack control reinforcement are commingled.

 

[grantstructure]

Well, y'all did yeoman's work on that other thread.

 

[JAE]

ACI tends to do way too much commingling.

I hear that the 2014 version will be completely re-formatted (all new chapter organization based on members (beam chapter, column chapter, etc.) rather than load effects (shear chapter, bending chapter, etc.)

Perhaps the commingling will be eliminated.

 

[hokie66]

Perhaps, but they have been doing it for a long time, 50 years at least. By the time they stop, I'll probably be commingled with the earth.

 

[TXStructural]

Yes, the next 318 will be completely different. But it is far too early to know for certain how much better or worse it will be. The current work is to organize chapters for each kind of member, as mentioned above. This results in quite a bit of repeating (i.e., more paper or electrons), but hopefully less cross referencing and mixing of T&S steel values and minimum reinforcement values. The new layout will allow each section to say what it means.

However, ACI has been on a trend of referencing rather than repeating, which caused the problem noted above. The idea is that repeating a formula or value means it has to be changed in each place it appears with each change to the code (and the risk of missing one occurrence goes up.) They now are trying to make it simpler to use the code, so hopefully they will recognize that repeating is better than referring, maybe. This example shows how the use of a formula out of context causes problems.

 

[pmour]

Wow, I thought the first link was rough.

I would like thank everyone for the posts. I agree that 10.5.3 is intended to provide an out for very large sections and 10.5.4 provides the minimum tensile steel for flexural members.

I now have a follow-up question. For background, this is a 8 ft x 8 ft x 18 inch foundation for a piece of equipment (electrical circuit breaker). The required rho for this foundation was calculated as 0.00012 on a per foot basis. I would suspect that structural plain concrete would suffice in this application, ACI 318 Chapter 22. In this application, if you wanted to add rebar as a "feel good", would adding 0.0009bh for each layer be sufficient as it would meet T&S requirements, or does the presence of the rebar drive the need to meet the flexural minimum?

Perhaps I just need to battle the issue of "we have always done it this way" and drop in the plain concrete design; with a little fiber.

 

[hokie66]

If this is just an equipment pad and not subject to significant vertical concentrated forces or bending moments, I would tend to just use plain concrete. For that size, I see no need for shrinkage reinforcement. If I were going to reinforce it, I would use more than the minimum. Fiber? Useless.

 

[JoshPlumSE]

I went through this exact code section a few years ago. Here is what I came up with as a means to rationally understand the code provisions (and intent):

1) 10.5.1 is a code provision intended to prevent the type of brittle failure that can occur if the sections cracking moment is greater than it's reinforced capacity. Conceptually, this section applies to all flexural members INCLUDING slabs and footings.

2) Section 10.5.3 softens the requirements of 10.5.1 so that you need only provide a 33% over capacity in the event that the cracking moment is greater than the reinforced section capacity.

3) Section 10.5.4 is a specific section for slabs and footings to ensure that the section 10.5.3 does not allow the total reinforcement in these slabs to be less than the temp / shrinkage requirements of chapter 7. The key interepretation here is that this section refers to the TOTAL reinforcing. Others have interpreted this to be the min reinforcement on one side of the slab, but that just seems overly conservative to me.

 

[JAE]

JoshPlum,
I agree that 7.12 can seem overly conservative with thick footings. However, looking through the thread referenced above (hokie's Nov. 28th reference) we concluded that it is meant for the tension sides of the "flexural member" and you can't divide by two and put half on top and half below.

This was backed up by numerous references, text books, etc.

I'm not sure I agree that footings and slabs also must meet 10.5.1 when you trace through the text of 10.5 but I have to concede that I've always applied 10.5.1 to them.

 

[TXStructural]

If the design for the footing works as plain concrete, there is no code requirement to have any amount of reinforcement, and it is permitted to have any amount between zero and the maximum reinforcement permitted elsewhere in the code (i.e., spacing to assure that there is enough concrete to bond the bars into to the concrete mass.)

You are required to adhere to cover requirements in ACI 318 7.7 (per ACI 318 R22.1.2).

If you feel the need to have reinforcement in the pad/slab/footing, it would be best to place at least the minimums for T&S in each direction on the exposed side or at mid-depth. Keep in mind that the width of cracks will be proportionate to the depth of the cover above the cracks in most instances - deeper placement of bars results in wider cracks.

 

[JoshPlumSE]

JAE -

What you concluded based on reading a previous thread is irrelevant to what I concluded based on my previous investigation of the code section. My conclusion is based on interpreting the code in a way that makes logical sense when following first principles. I realize that the code language in this section obfuscates the issue and allows you to come away with an alternate interpretation. I just don't agree with that interpretation.

To come away with the alternative interpretation you have to say that the logic of section 10.5.1 (preventing brittle failure when the reinforced capacity of the cracked section is less than the cracking moment) is NOT valid for footings. Or, that this type of brittle failure is okay for footings and slabs. That just doesn't seem logical to me. Do footings and slabs experience flexure? If so, then where in the code does it say that these cannot be considered flexural members? And, if they are not considered flexural members then NONE of the provisions of 10.5 ( titled Minimum Reinforcement of Flexural Members) would apply. So, the fact that they're directly included in 10.5.4 means that the code DOES consider them to be flexural members. Therefore, section 10.5.1 and 10.5.3 would also apply. And, 10.5.4 is logically included to prevent section 10.5.3 from allowing a total reinforcement less than the requirements of 7.12 would require.

The whole code section is very poorly worded and should be scrapped and re-written. But, my interpretation remains the best logical interpretation based on 1st principles of concrete behavior. I will acknowledge that the alternative interpretation may be the better LEGAL interpretation based on the poor wording of that section. But, I don't think that the wording is so strong that it would not allow my interpretation.

 

[JAE]

It is a messy section. I don't totally disagree with your interpretation (if you read my post above I do say that I include 10.5.1 for slabs/footings)

It just seems strange that in 10.5.4 they didn't say this (red letters are my addition):

10.5.4 - [red]In addition to the requirements of section 10.5.1[/red], for structural slabs and footings of uniform thickness, A_s,min in the direction....

 

[rapt]

7.12.2.1 commentary suggests that the face the reinforcement is placed at must be decided based on the specific conditions for the member being designed.

My interpretation of this comment is that

Logically, the reinforcement is only placed at a face if that face is in tension. If the stresses are purely from restraint to shrinkage, and the resulting stress is even over the depth of the section, then the reinforcement should be distributed between the top and bottom faces.

If the member is in flexure, with tension only at one face, then the reinforcement must all be nplaced at the tension face.

Which gets us back to .0018 at the tension face for flexural members and half at each face for shrinkage and temperature reinforcement.

One addition to that is that the crack control provided by .0009 at each face in a restrained member will be abyssmal and the cracking would be severe. You probably need 3 times that amount of reinforcement in a restrained member to get good crack control.

 

[pmour]

I have attached a section from Wang and Salmon's Reinforced Concrete Design, 1998 which provided a good reference on the topic. From this, I agree with JAE that

10.5.1 applies to rectangular sections and T-sections having slab in compression
10.5.2 applies to T-sections having slab in tension
10.5.3 is an excape clause for 10.5.1 and 10.5.2, but not 10.5.3
10.5.4 applies to structural slabs and footings of uniform thickness.

I believe the key is the use of the word "except" within the language of 10.5.1.

To summarize,

Reiterating above, I agree that 10.5.3 does not apply to structural slabs and footings of uniform thickness.

Further, from Hokie66's comments, I will be completing future designs either utilizing the flexural minimum of 10.5.4 or via plain concrete design.

If rebar is requesting by the client, it will either be provided in accordance with 10.5.4, or as a single layer each direction as the minimum for T&S on the exposed side or at mid-depth as commented by TXStructural above.

As to JAE's statement "I'm not sure I agree that footings and slabs also must meet 10.5.1 when you trace through the text of 10.5 but I have to concede that I've always applied 10.5.1 to them." Base on the interpretation above, I also believe footings and slabs need not meet 10.5.1. Although, this does not preclude the use of this value, as there may be situations that favor this direction (i.e. a combined footing).

I would like to thank everyone for their input; it is highly valued.