How to place minimum steel in a rectangular footing? 14

[pob11646]

I have got a question about the placement of minimum temperature and shrinkage steel, say for a 35" thick rectangular footing 14 feet long and 5 feet wide. Let's say, that all I need for my footing is minimum temperature and shrinkage steel.

Hence, Ast = 0.0018bh = 0.0018 * (14*12) in * 35 in = 10.58 in2.

Say, I am using #7 bars, thus, I will need 10.58 in2/0.60 in2 = 18 # 7 bars.

Method 1: Or, say I place bars in the top and bottom layers. For my bottom layer, do I need to place 14 #7 parallel to the short side, and 4 # 7 parallel to the long side. And do the same for the top layer.

Or, Method 2: I still place bars in both the top and bottom layer. For the reinforcment parallel to the do I just need to place a total of 18 #7 in both the top and bottom layers, say 9#7 in the bottom layer, and 9#7 in the top layer.

For Method 2, the Ast required parallel to the long side will be 0.0018 * (5*12) in * 35 in = 3.78 in2, or 7 # 7 bars. Do I place a total of 7#7 bars in both the top and bottom layers, say 4#7 in the bottom layer, and 3#7 in the top layer.

Method 3. Not to confuse matters, but can I place reinforcement in the bottom layer alone, say a total of 18# 7 bars, say 14#7 parallel to the short side, and 4#7 parallel to the long side.

Please advise whether Method 1 or Method 2 is more appropriate. And is Method 3 practical?

 

[hokie66]

Yes, BA, that is for shrinkage and temperature reinforcement normal to the principal reinforcement. The provisions for minimum flexural reinforcement are given in Section 911 of the 1963 code, and have remained essentially unchanged through the years. That was the first code to introduce ultimate strength design and the one I learned first. Professor Richard Barker knew it back to front, and he led us through all these provisions.

 

[Lion06]

StructGen-

We are all making our case through the logic and language of the actual code. You seem to be making your case through Dr. Ghosh's email.

Further, I would say that your idea that compression reinforcement can be considered "flexural" reinforcement and, therefore, counts toward As,min doesn't hold water in this case. For the small amount of tension steel that we are talking about, the compression block will be very small. Any top steel, with appropriate cover, will likely NOT be in the compression zone and would need to be 1.) discounted in the tension steel calcs, since it will be close to the NA and essentially worthless, hence rendering it unaccountable for in As,min; or 2.) counted in the tension steel calcs using a strain compatibility analysis, but then it's not on the tension face, and doesn't meet the requirement in 10.5.1 for steel on "the tension face". The language of 10.5.1 still applies to footings and slabs, it's just that the amount of steel is controlled by 10.5.4.

 

[StructGen]

JAE, thanks for detail insight, lets go one by one.

1) I suggest you should go through miecz posts above, he has given a great explanation of the flow of thought inside the code and how section 10.5 may have been developed.

2 & 3) I would have accepted your theory if code have not explicitly called out As,min = min. area of flex reinf. I believe if it would be as per your assumption code would have narrated this notation as "min. are of tensile reinf." What I mean is that this argument works both ways.

4) I believe you are looking at Pre-2005 code, 318-O2 used to have this syntex, 318-05 10.5.4 states "For structural slabs and footings of uniform thickness, As,min in the direction of span shall be the same as that required by 7.12..." and yes the "amount" is still there in commentary but why you assume it is tensile reinf. It is the total amount in the direction of span.

Again all these issues can be argued for ever, as StrEit said it is not black and white, my point is if something is not written clear enough what would you do? you will go to the source. That is what I did, I contacted ACI-318 commitee member. What I dont understand is why it is so hard for some people on this board to accept that they were wrong, why on earth one thinks that his interpretation of code over rules the explanation provided by ACI 318 Commitee member.

 

[BAretired]

hokie,

The provisions of ACI 318-63 Article 911 "Minimum reinforcement of flexural members" does not apply to slabs of uniform thickness. For beams, the minimum percentage of tensile reinforcement is 200/fy or 1.33 times that required to resist the moment as you stated in an earlier post.

A footing 35" x 5' x 14' is more like a beam than a slab, so I would be inclined to reinforce it in accordance with the recommendations for beams, not slabs. However, I would probably use nominal transverse steel as the footing is eminently capable of resisting those moments without reinforcement.

The OP should be aware that, if the columns are some distance from the ends of the footing or if they are stiff enough to develop significant rotational restraint, the footing will have two points of contraflexure and will need to be reinforced both top and bottom.

BA

 

[hokie66]

We all make mistakes, and it is just possible that Dr Ghosh's language in this case is imprecise. I suggest that if you want a definitive answer, you should copy this entire thread and forward it to him as evidence of how practicing engineers interpret the provisions of Section 10.5. You might even invite him to join Eng-Tips and our discussion.

 

[StructGen]

Str.EIT, Yes I am making my case on the basis of ACI-Commitee members provided explanation, is that incorrect?
I am not agruing about the contribution of top steel for flexure, my whole argument is that As,min = total area of steel in the direction of span.
Let me question this why you assume that language of 10.5.1 applies to 10.5.4, if this would be the case ACI have placed a genralzied statement right under 10.5.
Now lets look at section 10.5 heading "Minimum reinforcement of flexural members". I have accepted your opinion if the heading would be "minimum flexural reinforcement"

 

[JAE]

1. What may have been developed in various person's speculation is meaningless. What the codes says is fact. My post above is based solely on what the code says.

2 & 3. I don't agree. I think you are just wrong to bring compression steel into this. It doesn't apply, doesn't make sense, and doesn't fit in the section's overall theme and concern - per the commentary.

4. I'm looking at 318-05. I've also looked at 08. I quoted the text AND the commentary. It says "amount". I assume it is tensile reinforcement because THE WHOLE SECTION IS ABOUT TENSILE REINFORCEMENT.

Finally, your reference to Dr. Ghosh really defeats your argument. In your own letter to him you note that the 318 states the following:

"ACI 318-02
10.5.4 “For structural slabs and footings of uniform thickness the minimum area of [red]tensile[/red]
reinforcement in the direction of span shall be same as that required by 7.12….”

ACI 318-05/08
10.5.4 “For structural slabs and footings of uniform thickness, As,min in the direction of span shall
be the same as that required by 7.12….”

And Dr. Ghosh then states this:

[green]I wouldn't want you to read too much into the change in the wording of Section 10.5.4 between ACI 318-02
and ACI 318-05. For ACI 318-05, it was decided to use notation in place of verbiage, whenever that is practicable.[/green]

So he's telling you that nothing changed substatially between 02 and 05, just wording to be consistent with the use of variables. So he reinforces what 318-02 states, that it is all about tensile reinforcing.

I believe that Dr. Ghosh didn't understand your question. I think he thought you were suggesting that 0.0018bh might have to be placed "per face". He says you don't have to put 0.0018bh per face. (his quote is: "it is not 0.0018bh per face").

Finally, he refers to 15.10.4 commentary where it says,"Min. reinf. steel MAY be distributed near the top or bottom of the section, or MAY be allocated between the two faces of the section as deemed appropriate for specific conditions....".

The language states clearly that it should be placed as appropriate for specific conditions. Why would there be specific conditions that affect where the 7.12 rebar goes? I mean, according to you it can go anywhere. So this section clearly indicates that specific conditions can affect WHERE the reinforcing should go. To me, that clearly means in tensile areas since combined footings and mats usually have varying positive and negative moment regions.

 

[JAE]

StrucGen, I would second the idea of again communicating with ACI - specifically the persons who wrote section 10.5.4 or reviewed it in the recent code editions. I don't know whether Dr. Ghosh is the right person. How did you come to choose him to communicate with?

I've also done a few preliminary calcs on this and found that 0.0018bh does indeed meet or exceed the value of Mcr for 7 and 8 inch thick slabs.

b = 12"
d = 7"
f'c = 4 ksi
fy = 60 ksi

0.0018bh = 0.173 in^2
#4 @ 13" o.c. = 0.185 in^2/ft of slab

fr = 7.5 x sqrt(f'c) = .474 ksi
Ig = 512 in^4
y = 4"
[φ] = 1.0

Mcr = frIg / y = 60.7 in-kips

As calculated to resist 60.7 in-kips = 0.147 in^2/ft of slab

Therefore, 0.0018bh provides greater than Mcr.

 

[JAE]

The chairman of the "Flexure and Axial Loads; Beams, Slabs, and Columns" committee is Robert Frosch. I'll post a question to him on the ACI site.

 

[StructGen]

JAE, You are only reading what supports your opinion from Ghosh's email, he is very clear, I am amazed how come you come up with this argument that he might not have understood my question. Read again I am copying it below for you.

"0.0018bh is the total amount of reinforcement you provide in each orthogonal direction of a structural slab or footing - it is not 0.0018bh per face. This becomes abundantly clear if you refer to the new commentary we have placed under Section R15.10.4, which applies specifically to combined footings and mats. Section 7.12 gives you shrinkage and temperature reinforcement. There should be no expectation that this reinforcement would prevent sudden flexural failure. I do not believe that ACI 318 has tried to produce such an impression. Section 10.5.4 essentially says that if you have provided the minimum shrinkage and temperature reinforcement required by Section 7.12, you shall be deemed to have satisfied the minimum flexural reinforcement requirement of Section 10.5."

 

[JAE]

Heh - "it is not 0.0018bh per face". Obvious to me that he thought you were suggesting 0.0036bh total.

When he then states that there should be no expectation that it prevent sudden flexural failure, I just don't see it with the calcs I did. It clearly CAN exceed Mcr.

I've posted a technical question to ACI and I'll see what they respond with. I'll also talk to a few other profs I know who sit on ACI committees, specifically Robert Frosch's committee.

 

[StructGen]

JAE, I would appreciate if you pose this question to the chairman of ACI Committee 318-0D, I am hopefull that Mr Frosch will enlighten us.

 

[hokie66]

Dr Ghosh says "I do not believe...", which means just that this is his opinion. He did not write it, as it has been there since 1963. He may agree with you, although I am not sure of that based on his response to your enquiry. If he does, it will be interesting to hear the opinion of Robert Frosch.

 

[StructGen]

JAE, If you are posting your question to ACI technical advisery (free service for members) please keep in mind that your question will never reach to Mr. Frosch, please do check the disclaimer of that service, it clearly states that "opinion expressed by the technical staff is not the official position of ACI" and you know how these tech. support works, couple of grad students from Northweastren or University of Chicago doing there internship in skokie illinois.

 

[JAE]

Check out the attached...this is from a recently updated concrete text book by MacGregor & Wight. MacGregor is on the ACI Subcommittee. It is an example problem for a structural slab design and refers to 10.5.4.

Note that clearly the use of 7.12 reinforcement is applied on both top and bottom surfaces of the slab for, respectively, negative and positive moment regions.

This textbook, at least, verifies that the 7.12 reinforcement is a minimum TENSILE reinforcement requirement.

 

[Lion06]

Also, I don't think there's any discrepancy between the wording of As or As,min. 10.5.1 says As provided shall never be less than As,min. Then in 10.5.1 it gives the basic equation for As,min. Then 10.5.2, 10.5.3, and 10.5.4 gives exceptions to the equation in 10.5.1 (i.e. exceptions to the quantity of As,min, not exceptions to the presence of As,min).

 

[miecz]

Wow! When I read Dr. Ghosh's email, I thought it was pretty clear, and that it supported what I believed before I read it. After reading other posts, I guess this is a case of reading what you already believed.

I have to take issue with my previous post being "meaningless." Article 10.5 is not clear. The committee that wrote Article 10.5 could have written it differently. I think it is beneficial to speculate as to why it is worded the way it is. Did they word it that way because:

A. The research on flexural behavior of slabs coincidentally came up with the same amount of minimal tensile reinforcing as is required for temperature/shrinkage of the entire section, so they decided to save ink and paper by referring to section 7.12?

B. The research on flexural behavior of slabs found a causal relationship between the amount of minimal tensile reinforcing as is required for temperature/shrinkage of the entire section, so they decided to point out the relationship by referring to Article 7.12?

C. The committee writng Article 10.5 found no relationship between the amount of minimal tensile reinforcing as is required for temperature/shrinkage of the entire section, but they wanted to tell designers that Article 7.12 also needed to be checked?

These are the only three scenarios that I can think of, and "C" seems a lot more plausible to me than "A" or "B."

Finally, as some have brought up other references, allow me to bring up ACI 350. This code is very similar to ACI318, but is specifically geared toward tank walls, i.e. structural slabs. The wording is ACI350 is slightly different than ACI318. It is more precise. I think ACI350 clearly has T/S steel checked in addition to minimal steel required for flexure.

 

[StructGen]

JAE, let me give you a better one from Macgragor, take a look at the attachment, here he clearly spell out what he bleives. But I dont think he is correct reason is this simply dont work, please take a look at the calculation I posted above, think for a moment ACI dont write code for a specific concrete strength or size of section. Code clause has to be applicable universally unless a limit is defined. In this case it just dont work for any thing above 5000 psi, even for 4000psi you have to rig up the Phi-factor in your calcs.

 

[JAE]

StructGen,

I read your attachment and it states exactly what we've been suggesting all along - that the As,min from 10.5.4 is a minimum tensile reinforcement.

I did a comparison for slabs with f'c = 4000 psi and 6000 psi using phi = 0.9. See attached. What this shows is that the 7.12 steel very closely compares with the cracking moment when placed near the tensile side of the slab.

One thing to note here too is that Mcr isn't necessarily a good measure of prevention of catastrophic, sudden failure as all concrete essentially cracks under load at very low moments. What this shows is that 7.12 steel, even up at f'c=6000 psi, gets you within a narrow range of the cracking moment.

I'm also surprised that you were critical of others for questioning Ghosh's opinion and directly above you question MacGregor's.

 

[StructGen]

JAE, Let me explain this contradiction, yes I was critical of questioning Ghosh's statement becuase none of the question came up with any logical backing or calculation. Forum participants were giving there own opinions, well its not the matter of vanilla vs strawberry vs choclate.

I have added 4000psi conc in my calc and I am attaching it for you, slabs all over united states are done with min. 5000psi conc. and conc strength for slabs go up to 8000 psi. only metal decks are poured with 4000psi.

Why I think Macgragor is incorrect becuase his statement dont reflect the fact, when I crunch the numbers it dont work. Again if the intention of ACI is what Macgragor is writing in his book then phi-Mn should be greater then Mcr by a margin of 1.1 to 1.5 for all concrete strengths. But this is not the fact, it just dont work. Either Macgragor is incorrect or he lives in the world of 2000psi where he get a 1.5 ratio of phi-Mn/Mcr for 0.0018bh