So confused with slab design 3

[cityboy]

Hi, I am doing slab design. I have a #4@12" mat for bottom reinforcement and no mat for top. If i follow the SAFE output, I will have no rebar for some stress lines. Qell, some senior engineers in my office said that i should provide rebar to cover every stress line for my slab. However, some senior engineers told me i didn't need to do that, just put rebar wherever needed and i also see some drawings which don't have rebar to cover every stress line in this office. I tried to find some info in the ACI 05 code, but couldn't find anything related to that. I am so confused now.

 

[AggieYank]

You, and not your program, need to figure it out. If it is a simple span slab, you'd in theory only need reinforcing steel at the bottom. If it's a multi-span slab, you'd need reinforcing steel over the support as well. If you design a single span slab as "fixed" at the ends, you'd have reinforcing steel at the top at the ends. I'd recommend not doing that, because you then have to design the wall / beam that the slab sits on for the end moment that developes. For a single span slab with "pinned" ends, I still add some hook bars at the top of the slab to prevent cracking, as the reality is that it is a fixed connection.

 

[cityboy]

Ok, maybe I didn't explain my question clearly. Now, I attached a pic. My question is that if the analysis shows that I don't need rebar to cover the stress line between column 2 and 4, do I need to provide minimum reinforcement at that location? I mean for the mid strip, I already have rebar for the column strip.

 

[Lion06]

Yes. ACI is pretty clear that, at a minimum, you must provide enough reinforcement for temperature and shrinkage.

 

[cityboy]

StructuralEIT: Yes, ACI requires to provide enough reinforcement for temperature and shrinkage. Here, since I already have a mat for the BOTTOM reinforcement. Do I still need provide enough TOP reinforcement for temperature and shrinkage?

 

[Lion06]

Oh, I didn't quite follow. So you have #4@12" on the bottom everywhere and a top mat only where needed?
That is fine. As long as the #4@12" satisfies T & S then you are fine. Adding the top rebar where it isn't needed will only cause congestion when it isn't needed.

 

[csd72]

cityboy,

You are discovering something about engineering that they dont teach you at uni - engineers have differing opinions on a lot of things.

Much of your confusion is due to the fact that you have asked a number of people and got conflicting opinions. Best to ask only one or two people, and get them to explain their opinion, ask lots of intelligent questions and then back it up with some research of your own.

You need to detail the slab in accordance with you analysis - if you have relied on a negative moment over the supports, then you have to reinforce it for this.

 

[oneintheeye]

sorry on by phone so couldn't see your sketch clearly. if theres no need for rebar in top would still provide some mesh just to help with cracking (depending how deep it is) ni one will thank you if cracks develop

 

[JAE]

Go review Section 10.5 of ACI 318. It expressly states that "at every section of a flexural member where tensile reinforcement is required by analysis..." and then goes on to state what the minimum required As is.

Section 10.5.4 is for slabs and footings and refers you back to 7.12, which is temp and shrinkage reinforcing.

If your analysis shows that you have tensile stress in concrete, and you are not designing that concrete as Plain Concrete per chapter 22, then you have a member where tensile reinforcement is required and you need to meet 10.5 (and thus 7.12.)

 

[cityboy]

StructuralEIT: Yes, I have #4@12" mat on the bottom everywhere . So I need to provide reinforcement for the top slab since there is not mat everywhere.

csd72: The reason I asked a number of people because when I work for different projects, the project/senior engineers have different requirement. You know I have to follow their instructions. Same office, different standard, that's why I am confused.

herewegothen: Additional rebar will help to prevent cracking. But the design will not be economic if we put too much rebar where not needed.

 

[cityboy]

JAE: The section 10.5 states the minimum reinforcement for temperature and shrinkage. I agree we need that reinforcement. My question is that if I already have a #4@12" mat everywhere at the BOTTOM of slab, do I still need to provide minimum reinforcement for temperature and shrinkage at the TOP of slab if analysis shows I don't need reinforcement for tension there?

 

[Lion06]

No.
As long as you have the minimum for T & S covered with the bottom reinforcement and your analysis says you have no negative moment (no tension in teh top of the slab) then you do not need reinforcement in the top of teh slab.
I might change this opinion if you have a very thick slab (maybe 8" - 10") to help with curling, but under normal circumstances, you are fine with the bottom reinforcement.

 

[cityboy]

StructuralEIT: thanks, your answer is so helpful. I will remove some top bars.

 

[JAE]

StructuralEIT - I disagree with you.

Section 10.5 is dealing with minimum required [red]As[/red] at sections where tensile stresses occur.

In cityboy's situation, he has tensile stresses in a negative moment region between the two columns. Despite the fact that there is bottom reinforcing taking the positive moment out in the span, you still need to respond to the negative moment region between the columns.

Section 10.5, then, provides for ANY section that has tensile stress. It is NOT specifying temp and shrinkage reinforcing.

Section 10.5.4. states that As(min) shall be the same as that required by 7.12.

As(min) is NOT temp/shrinkage reinforcing. They are two different things and this section is requiring the engineer to include a minimum amount of steel in tensile stress areas of flexural members. ACI simply uses 7.12 to quantify the amount of steel.

 

[Lion06]

JAE-
He stated very clearly that he DOES NOT have tensile stresses at this location. His question was whether he needed to provide T & S steel at the top even though it is not required for stresses.
He is providing the As in the bottom (where the tensile stresses ocur, he simply wanted to know if he needed at the top as well.

 

[AggieYank]

JAE, you're talking about minimum steel required for concrete in tension, while StructuralEIT is talking about minimum shrinkage steel.

And I agree with both of you. If the concrete section is in negative moment (top of concrete slab in tension), you need minimum steel for tension. And, if the bottom steel in both directions required for strength is greater than the minimum shrinkage steel, then you don't need additional shrinkage steel.

 

[Lion06]

Aggieyank-
I agree 100%. I believe he stated he did not have negative bending in this location.

 

[JAE]

From his picture, it was pretty apparent to me that there would be some level of negative moment across the line between the two columns..at least in using some logic in the span behavior of the slab within that irregular area.

I would agree that if there is no tensile stresses then the bottom steel works for temp/shrinkage. But the OP referred to "stress lines" and even showed a "stress line" on the diagram so I assumed there was tension.

I don't think the OP stated it "very clearly".

 

[cityboy]

JAE
Why do we need rebar across the stress line if the analysis shows no reinforcement is required?

 

[AggieYank]

cityboy. Use some of your own judgement, and stop relying on a computer program that spits out results based on quite possibly bad assumptions.

If that line represents a support for the slab, such as a beam below, then there will be some amount of negative moment over that line, and you will want to use reinforcing steel.