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]

hokie66, Compression steel As' also works for flexure and is also the flexural steel, so the total flexural steel in a section is the sum of top and bottom.
So you believe that CRSI which is an independent organization is more credible then ACI-commitee member? what is the reason behind your conclusion?
For all practical purposes 5000 psi is the least strength one can use. See attached comparison at no point 0.0018bh provides a capacity comparable to Phi-Mcr.


StrEIT, where did you read this in my statement that there is no minimum requirment for footing and slabs, I believe it is exactly the same as other flexural members + temprature and shrinkage.
Do you know why 10.5.4 is applied only to combined footings, becuase slabs useually have more reinforcement than 0.0018 and isolated footings dont have top reinforcement.


You guys answere one simple question and I will call this debate off, if 10.5.4 is the minimum flexural steel why it is not applicable to beams which are the primary flexural members. dont tell me that I have over sized the section, since 10.5.3 can result a number lower then 10.5.4

 

[Lion06]

StructGen-

So you are saying that 10.5.1 (and eq. (10-3)) applies to footings? That seems to be what miecz was saying, too. I've never read it that way, and this is the first time I'm hearing of someone else reading it that way. It's certainly possible, and that would make your position more realistic. Every engineer, program that I've used, and even PCA Notes has used the 0.0018 as a min.

 

[hokie66]

StructGen,

I actually gave an answer to your "one simple question" in the post before yours.

Why do you insist that "For all practical purposes 5000 psi is the least strength one can use"? What's wrong with 4000?

I just think you are in the minority in this, and I hope all the designs I have done for 40 years using various versions of ACI318 and AS3600 have not been wrong.

I respect the ACI code writers, and I don't think there is a big issue with the way 10.5 is written. I would prefer a clear division of the provisions for flexural steel from those for T&S steel. Maybe it will happen next time.

 

[StructGen]

I dont mind being in minority as long as I am correct, what was that post exactly?

 

[StructGen]

StructuralEIT, please check CRSI book they use 10.5.1, 5.3 and 5.4 for min. reinf. in a footing, I dont completely agree with them because again they use 0.0018bh on each face.

 

[JAE]

I think the key here is that The SUBJECT (i.e. the noun) of 10.5.4 is....ready.....A[sub]s[/sub]

A[sub]s[/sub] is the flexural reinforcing that goes on the tension side of the member.

So 10.5.4 is saying that the value of A[sub]s[/sub] (on the tension side) is equal to the value given in 7.12.

hokie mentioned this earlier

As,min is defined in Chapter 2 as "minimum area of flexural reinforcement".

 

[StructGen]

Again it is flexural reinforcement, where did you get this idea that all flexural reinforcement should go on tension side, compression reinforcement is also flexural steel. Flexural steel = Top + Bott.

 

[hokie66]

So do you count both tensile and compression reinforcement in complying with 10.5.1 for minimum beam reinforcement? I hope not.

 

[StructGen]

For sure not, because 10.5.1 says "at every section of a flexural member where tensile reinforcement is required....." I am bound to use Ast per 10.5.1 for Tensile reinforcement. I like your sense of humor.

 

[hokie66]

Thanks, but in this instance I wasn't trying to be funny. My point is that both 10.5.1 and 10.5.4 specify values for As,min, and the definition of As,min does not change from clause to clause.

 

[JAE]

Sure, "flexural reinforcing" can be considered as compression reinforcing - but compression rebar has its own set of limits totally unrelated to what this section is covering. I think you are stretching a bit to use compression rebaar as an argument in this matter.

 

[StructGen]

Valid point, defination of As,min dont change from clause to clause, that is why ACI 318 uses two different notations for different clauses.
10.5.1 uses notation of "As" which is defined in Section 2.1 "As = Area of non-prestressed longitudnal tension reinforcement"
While 10.5.4 uses notation of "As,min" which is "minimum area of flexural reinforcement."

Thanks for bringing up this point I think we have nailed this issue.

 

[hokie66]

Look again. As,min is used in 10.5.1 and also 10.5.2.

 

[StructGen]

Read the clause 10.5.1 and 10.5.3 not the eq 10-3,

 

[hokie66]

It is clear to me in 10.5.1, 10.5.2, and 10.5.4 that As=As,min, but the values of As,min and thus As are different in the three clauses. Clause 10.5.3 is for beams where 10.5.1 and 10.5.2 don't have to be complied with.

If As and As,min are not interchangeable in Section 10.5, how do you explain the use of As,min instead of As in 10.5.2?

 

[StructGen]

Exactly how this is clear to you that As=As,min while code explicitly calls it out as a different notation and further provides the defination of this notation in chapter 2.

10.5.2 is an special case, I will answere your question when you explain me what is your reason to assume As = As,min

 

[hokie66]

The whole section 10.5 is about minimum flexural reinforcement. As,min is the value of this minimum, and varies with the three clauses, which are all "special cases". As JAE said above, you are stretching the interpretation of As,min to include compression reinforcement. He was too kind. I would say you are dead wrong.

 

[JAE]

Here's another thing to consider, StructGen. (I was once in your camp but after a few rehashes with co-workers and on Eng-Tips here I changed my mind on this issue. I hope you will consider this)

1. The whole section 10.5 is dealing with flexural [red]tension[/red] reinforcement. Read the first paragraph of the commentary of 10.5 (section R10.5) There it lays out the reason for the whole section - [blue]fear of sudden tensile failure of a section in flexure[/blue]. In fact, the word "tension/tensile" occurs five times throughout this section of commentary.

2. The variable [blue]A[sub]s[/sub][/blue] is defined in Chapter 2 as "tensile reinforcement". A'[sub]s[/sub] is the compression reinforcement. So section 10.5, creating a minimum amount of A[sub]s[/sub], is limiting tension reinforcement. It does not say A'[sub]s,min[/sub]. It is not limiting compression reinforcement at all. This isn't even a slightly remote element of this section.

3. Thus, the value A[sub]s,min[/sub] is a minimum value of tension reinforcement.

4. Section 10.5.4 states that the minimum amount of tension reinforcement (i.e. a limit on A[sub]s[/sub]) is "the same as that required by 7.12". The commentary, just to the right of it states it a bit differently: [red]The minimum reinforcement required for slabs should be equal to the same AMOUNT as that required by 7.12.[/red]. So the key word here is AMOUNT. It is not saying you go to 7.12 and simply provide temp/shrinkage steel wherever the heck you want. It is saying, precisely, that you use the AMOUNT of steel indicated by 7.12 as a minimum value for tensile reinforcement.

While this section has created multiple threads, discussions, and arguments, if you read through the logical language of the text, it is pretty clear what is required.

 

[BAretired]

Here is an excerpt from ACI 318-63.

BA

 

[StructGen]

hokie, With all due respect you are not logical at all to defend you position, calling me dead wrong is not going to strengthen your point of view. I understand it is difficult to accept that one was incorrect, I am quoting clauses and notations from the ACI and you are coming with what? You have left no room for further discussion with you. Thanks