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?

 

[StructGen]

JAE, also you are trying to go above and beyond to defend your opinion, if Mcr isn't necessarily a good measure of prevention of catastrophic, then you are not agreeing with your own argument.
Let me tell you, I work on commercial buildings, office, high end condos, I have not used 4000 psi from last 5 years. I have used concrete up to 12000 psi and have seen pour breaks of 16000 psi, Macgragors statement is not from our world.
I am attaching a calculation based on Eq 10-3 check this out you always have a constant ratio (for each section) regardless of your concrete strength, this is called min. reinforcement.

 

[JLNJ]

So using a 1/3, 2/3 distribution the 0.0018 reinforcement, the bottom of footing steel could be as little as 0.0006bt? It seems unlikely though I can see how one might read it that way.

Maybe we are splitting hairs here because under real world loading conditions the minimum flexural reinforcement calculated (x 4/3) will almost always be greater than 0.0018bh.

 

[Lion06]

StructGen-

I think you are "not in our world" if you never see f'c<5ksi. Additionally Mcr is NOT a good measure of prevention of catastrophic failure because there is no warning, no ductility. The member cracks, and bang...... failure occurs immediately.

I'll look at your calcs tomorrow.

 

[JLNJ]

...laptop powered out before I completed the thought.

I see where StructGen is coming from with pro rho min argument, but I think the logic in the Code predates the argument he is presenting. For most isolated 3-4 ksi footings history has found that the rho min (as for beams) is overkill. Unless of course your client's last name is Steinbrenner.

New materials and non-traditional uses may mean new thinking for certain code provisions. Maybe this is one of them.

 

[JAE]

StructGen - I don't know of any reference where minimum tensile reinforcement is intended to be above Mcr.

You are basing your opinion on Ghosh's statements (which I think are confused) and on your presumption that Mcr is the limit state for catastrophic failure.

Your statement of using f'c in the high end may be true in the general high office building world you work in but in the vast number of projects in the US, f'c = 3000 psi or f'c = 4000 psi is very often used. f'c = 5000 to 6000 is used for prestressed concrete more times than not.

The more I've looked into this the more I'm convinced that the 7.12 reference is to the AMOUNT of TENSILE reinforcement in a flexural member. I'm convinced. The text books I've looked at all conform to this. The exact English language logic agrees. Nothing you've said has been a convincing argument to me at all so I'm done here.

I'm weary of this back-and-forth thread so until I hear back from ACI and another professor I've contacted, I'm letting this go. We'll just have to agree to disagree. Still with respect to you and everyone on this thread of course.

 

[hokie66]

Footings don't catastrophically collapse because of too little steel. The soil retains the cracked sections until the steel picks up the load. Ductile behaviour then occurs.

Flexural reinforcing in slabs is provided in the direction of the span, which can be either in one direction or two. If a crack forms, the steel in the other direction, whether flexural or T&S, distributes the load in the transverse direction, therefore slabs with minimum reinforcement don't collapse catastrophically either due to too little steel, as ductile behavior occurs due to the redistribution. No "bang" happens. Beams bang, slabs sag.

The argument about varying reinforcement with concrete strength raises another question. If a slab or footing is designed for 3000 psi concrete and the test results show 8000 psi, is the concrete to be rejected? I think not.

Using high strength concrete in slabs is a poor use of the material.

 

[StructGen]

JAE, looks like we lost each other in the middle of discussion, the comparison of Phi-Mn/Mcr is mentioned in the reffrence I posted from Macgragors concrete design book 4th edition.

ACI in R10.5 also explains the same reason of minimum reinforcement. "With a very small amount of tensile reinforcement, the computed moment strength as a reinforced concrete section using cracked section analysis becomes less then that of corresponding unreinforced concrete section computed from its modulus of rupture, faliure in such a case can be sudden"

 

[ali07]

results, who is right??

 

[slickdeals]

When egos run high on a sensitive topic, I don't think we will each a conclusion on it.

 

[BAretired]

It should not be a sensitive topic. And egos should not play a role in it. The argument is not of great importance anyway. Anyone who believes that temperature steel specified in the code is inadequate to prevent sudden catastrophic collapse is free to provide additional reinforcement. If unsure, take the conservative route.

BA

 

[apsix]

It's egos which can make any topic sensitive, regardless of the nature of the topic.

The one thing that is clear is that ACI318 is unclear on this subject.

 

[hokie66]

BA,

For some reason I missed your post of 30 August, 21:13. Section 911(b) of ACI318-63 does cover minimum reinforcement of structural slabs of uniform thickness. It is essentially the same provision which we have now been arguing about. All those years, I thought it was clear, but maybe it isn't.

 

[BAretired]

hokie,

The meaning in ACI318-63 seems clear enough to me. The reason for treating slabs of uniform thickness differently from other flexural members is not clear to me.

I do not have the current ACI318 so cannot comment intelligently on its minimum reinforcement provisions.



BA

 

[hokie66]

BA,

The argument I made 31 Aug 17:49 about slabs vs beams is not mine originally. I read it in a text, but can't put my hands on it now.

 

[BAretired]

It appears that beam depth has considerable influence on the required amount of minimum reinforcement in a beam. Check out this link:

http://www.vbt.bme.hu/phdsymp/2ndphd/proceedings/bruckner.pdf

BA

 

[JAE]

OK - I sent a question to ACI and I got a response I got back from them today.

My question to ACI was this:
[blue]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.[/blue]

Here is ACI's response:

[red]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 [/red]

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.

Hope this helps - sorry for the long post.

 

[SKIAK]

Funny how ACI still, even in the response, never actually uses the word "tensile" but refer you to somebody else who did. Why can't they just say to use that as a check for min tension steel? I never questioned that this was their intent but it's nice to hear it anyways. Thanks JAE.

 

[ali07]

So is the mystery solved?

 

[JAE]

Well, to me having ACI refer to MacGregor's book where he clearly states that 10.5.4 rebar (from 7.12) is TENSILE reinforcement, and MacGregor's examples show the 7.12 steel is placed wherever tension exists whether top or bottom clears it up for me.

 

[enginerding]

So if you put all the T&S steel on the tensile side of a 2' thick footing, no need to worry about T&S cracking on top side?