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]

miecz,

If .0018bt is not intended as the lower limit on flexural reinforcement in a slab, then what is?

 

[StructGen]

Guys one last time,

Try to understand the intention of code, it expects you to provide minimum reinforcement as per eq 10-3 (0.0033bh) if your design flexure is low you can provide 1.33As-req and can get away with the minimum reiforcement requirement.
Take a pause at this moment.
Code wants you to check your temprature and shrinkage reinforcement. 10.5.4 forces you to check your As-min in the direction of design vs 7.12.
Take a deep breath, think about it, if code's intention is to check flexure minimum in 10.5.4 it will not reffer you to 7.12, it will simply provide you the limit. The way it does for every thing else.

Next why they added 15.10.4, slabs are between 6" to 12" in most cases. typical bottom mesh reinforcement is almost alway more then 0.0018bh, it the footing which can go as deep as 12 feet (for a 700-ft high hurricane zone building). 0.0018bh is a huge number for this big raft that is why it is clearly stated in ACI 318-08 15.10.4

I am amused of the comments, "I dont know why they added 15.10.4" well they added it becuase they feel that need to clarify this issue.
Next is "I dont agree with Ghosh's interperatation", he is not interpreting it, he is telling me what he wrote in ACI 318 and what he wants us to read out of those lines.
another one is "I have'nt read any thing from Ghosh's email to sway my opinion"
What are we trying to establish here we can not have a reasonable discussion?

 

[hokie66]

15.10.4 just tells you that T&S reinforcement is required in mat footings, etc. It does not address the flexural requirement. The two requirements may be the same, but not necessarily.

Again as I said before, it is illogical for the code to refer back to the T&S section for flexural reinforcement. On that we agree. But it does.

As to the intention of ACI, it has been that way for a long time, possibly prior to Dr Ghosh's involvement. I pulled out the 1978 CRSI Handbook, in which slab designs are tabulated based on ACI318-77. The minimum flexural reinforcement used in the tables was 0.0018bt. So at least 32 years of confusion, maybe more. I don't have my older versions ready at hand.

 

[Lion06]

I don't know, I'm still not seeing it. It is in the code section titled "MINIMUM FLEXURAL STEEL". That seems pretty clear to me. The fact that they refer back to the T&S QUANTITY............. the actual code section is in "MINIMUM FLEXURAL REINFORCEMENT".
Additionally, 15.10.4 only applies to COMBINED footings, not slabs or isolated spread footings.
I think it also mentions that the AS,min needs to be placed near the tension face. If AS,min is per 7.12 (0.0018bh), and it needs to be placed near the tension face, doesn't that mean that you can't split it? I just can't imagine that there is no lower bound limit for reinforcement for a reinforced footing.
The biggest question I would have at this point is why are slabs and footings even included in section 10.5 if they have no minimum flexural requirements.

 

[Lion06]

I'm reading it over and over, and I'm not doubting Dr. Ghosh's intent, but his explanation is not the way the text reads. I'll go through my thought process one more time, please tell me where I'm going astray.

Section 10.5.1 ('08) says, "At every section of a flexural member where tensile reinforcement is required by analysis, except as provided in 10.5.2, 10.5.3, and 10.5.4, As provided shall not be less than that given by ......"
I think we'll agree that a footing is a flexural member. This, to me, says that any section that doesn't work as plain concrete needs to have a minimum amount of tensile reinforcement. Do you agree with that statement?

That minimum amount, EXCEPT as provided in 10.5.2, 10.5.3, and 10.5.4 shall be equation (10-3). The exception provided in 10.5.2 is for a T-beam with a flange in tension. The exception provided in 10.5.3 is for a very large section where eq. (10-3) would be excessive. These are both for tensile steel requirements. Do you agree with this?

The exception provided in 10.5.4 is for slabs and footings of uniform thickness, and As,min (which is clearly defined as the minimum amount of tensile reinforcement of a flexural member) is the same as that required by 7.12 - which is 0.0018bh. If just meeting the requirements of 7.12 satisfy 10.5.4, then there are no minimum flexural reinforcement requirements as 7.12 has no requirement about where the steel needs to be placed. It could be placed all on the compression face and, by your reasoning, meet the requirements of "minimum reinforcement of flexural members".

Like I said, I don't doubt his obvious intent after reading the response you posted, but the text does not read that way. I'm also not sure I understand the reasoning for essentially having no desire for minimum tensile steel in footings. The response posted makes that pretty clear. Any section that has 0.0009bh (assuming you split the steel between top and bottom) tensile steel will NOT be a reinforced section (because the plain section will have greater capacity - even with the lower phi factor).

 

[BENDOG]

PCA MATS program treats minimum tensile steel as 0.0018bh. If you split this 0.0009bh top and bottom you may provide less steel than required by design. I proved this to co-workers who used to split top and bottom. I modeled a mat where the program said min steel controlled. But calculating As req'd based on Mu was greater than 0.0009bh. So according to PCA MATS minimum tensile steel is 0.0018bh.

 

[miecz]

hokie66

Ignoring 10.5.2 for the moment, I believe the lower limit of reinforcement is covered by 10.5.1 and 10.5.3,and that the T/S provisions of 7.12 must be met, but not at the tension face, at the whole section. This has been my position since I joined this debate, in 2007. See thread507-185223 and thread167-225434.

That may not be what ACI Article 10.5 says, but it is what I believe is the intent of the code writers.

 

[StructGen]

StrEIT, I agree with you untill you assume that As-min in 10.5.4 is the miniumum amount of tensile steel. No it is not the minimum amount of tensile steel. It is Area of steel in the direction of design, let me quote it from ACI "“For structural slabs and footing of uniform thickness, As-min in the direction of span shall be same as that required by 7.12.2.1 ……"
The flexure minimum reinforcement requirements are coverd by 10.5.1, 5.2 & 5.3. While 5.4 ensures that T&S requirments are met. Now here comes your argument why this clause is placed under 10.5 which is flexure minimum reinforcement. Simple reason, it is an additional check so that you dont end up providing less reinforcement then T&S requirement and yes it can be placed at the compression face if you meet the requirments of 10.5.1 & 5.3 for tension face.
Since you reffered to plain concrete, lets look at this issue from a different prespective, you are allowed to design plain concrete but code penalize you with higher Phi, have you ever thought why? to ensure that faliure never happens, same is for the compression controled section. Lets get back to minimum steel, you have to provide min. steel as per Eq 10-3 to ensure a ductile faliure, if you dont want to do that, code forces you to increase your reinforcement 33%. just to ensure that the member will never see faliure (under the intended design loads). Do you see a trend established here?

Now lets take on 0.0018bh, it does not ensure ductile behaviour, it never provide a capacity which is greater gross cross-section capacity Then why on earth it should be flexure minimum reinforcement. Further assuming if it is Flexure min. reinforcement that why its not applied to beams, why beams are exempt of this clause?

Yes I agree with you that text is not black and white that is why I asked this question from the commitee member. Let me tell you some thing, if you don't trust 318-Commitee member then should not trust the code it self becuase these are the guys who write the code not Aliens from another galaxy.

 

[hokie66]

StructGen,

As,min is defined in Chapter 2 as "minimum area of flexural reinforcement". I don't see how you can consider steel on the compression face to be flexural reinforcement, or all the steel in the direction of the span to be counted as As,min.

Dr Ghosh didn't write this clause, so it is not a matter of trusting him or anyone else. It is just a matter of what the language says, and to me it is crystal clear.

The same provision which allows slabs and footings to have a lower percentage as minimum flexural reinforcement has been there for a long time. I am not sure how long. CRSI has over the years done a good job of producing design handbooks which run in tandem with the Code, so I attach a certain amount of credence to their interpretations. However, the CRSI design tables were only for lower strength concretes, and as Rapt said in the earlier thread which miecz referenced, the requirement for strength to be greater than cracking moment is met by .0018bt steel only up to f'c of about 4500 psi.

 

[Lion06]

Structgen-

There are plenty of footing sections that have a greater reinforced moment capacity than cracking moment using 0.0018bh. Run the numbers. I did it for a 16" thick footing with f'c=4ksi (assuming d=12.5") and Mn/Mcr was about 1.05.
I don't see how you make the case that a clause that is physically located in the section titled "MINIMUM FLEXURAL REINFORCEMENT" isn't actually minimum flexural reinforcement.

As for 10.5.3 - Unless you have a ridiculously oversized section, it still provides more steel than 0.0018bh. You would need a req'd rho (in the beam sense, rho=As/(bd)) of less than 0.0015 to require less steel than that required by 0.0018bh. That is less than half of the min. per eq. (10-3).

 

[Lion06]

StructGen-

What you are saying, essentially, is that there is no minimum flexural steel requirements for slab or footings. Do you believe that to be true?

 

[miecz]

If you were a code writer who wanted the minimum tensile steel to be .0018bh, why would you write "the same as required by 7.12." You'd have to be an idiot to write it that way. First of all, it would be so much easier to simply say that the minimum tensile steel shall be .0018bh. Secondly, you would avoid any ambiguity, as 7.12 is for tension and compression steel. Thirdly, you would avoid having to change the code in the event that Article 7,12 changed. It makes no sense.

 

[Lion06]

miecz-
You have a point, but I still can't get past the title of the section of the code that 10.5.4 is located in. To have it located there and have it NOT be the steel that is referenced in the title makes even less sense.

 

[miecz]

Here's how it makes sense to me. I imagine that I'm writing Article 10.5. So I write all of it but 10.5.4 and then I say to myself, "but I don't want anyone to forget to check T/S requirements." So I tack on 10.5.4.

I think this way because I used to write design procedures for production engineers. Writers have some unwritten cardinal rules. Never link to an independent Article that may change, forcing you to rewrite your Article, or worse... Try to be as clear as possible. Try to be simple to read. Article 10.5.4, as you read it, violates all that.

 

[Lion06]

You are saying that footings have no minimum flexural steel requirements? That is the net result of your position.

 

[miecz]

StructuralEIT

As I read ACI, for slabs with large bending, 10.5.1 provides a fixed minimum, and for slabs with little bending, 10.5.3 provides a variable minimum. In the case of slabs with little bending, that minimum may be smaller than the cracking moment. But it is still much larger than the applied moment.

 

[Lion06]

10.5.1 doesn't apply to slabs or footings, right?

 

[miecz]

The way I read it, 10.5.1 applys to slabs. 10.5.4 does not override 10.5.1 for slabs. I know some peole read it that way, but I don't. Again, I read 10.5.4 as a reminder to check T/S for slabs with small bending moments.

If 10.5.4 overrides 10.5.1 for slabs, then, for footings with larger moments, the minimum steel is .0018bh, which, I think, is too small.

 

[eit09]

The way we read it is Section 10.5 is for beams and not foundations. When code says Min Reinf. of Flesural members, the word members, to me, is the same as saying beams. 10.5.4 says Slabs and footings (which are foundations not members or beams). So after reading these posts I would change the word member in 10.5 heading to beam and delete section 10.5.4.

 

[hokie66]

The requirement for ultimate capacity in bending to exceed the cracking moment means a lot more in beams than it does in slabs and footings. If a footing cracks, then what happens? It stays where it is, doesn't fall down. If a slab cracks, what happens? The crack propagates far enough for redistribution to occur to a greater area of the slab. So again, it doesn't fall.