So confused with slab design
So confused with slab design
(OP)
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.






RE: So confused with slab design
RE: So confused with slab design
RE: So confused with slab design
RE: So confused with slab design
RE: So confused with slab design
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.
RE: So confused with slab design
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.
RE: So confused with slab design
RE: So confused with slab design
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.)
RE: So confused with slab design
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.
RE: So confused with slab design
RE: So confused with slab design
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.
RE: So confused with slab design
RE: So confused with slab design
Section 10.5 is dealing with minimum required As 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.
RE: So confused with slab design
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.
RE: So confused with slab design
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.
RE: So confused with slab design
I agree 100%. I believe he stated he did not have negative bending in this location.
RE: So confused with slab design
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".
RE: So confused with slab design
Why do we need rebar across the stress line if the analysis shows no reinforcement is required?
RE: So confused with slab design
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.
RE: So confused with slab design
If there is no negative moment at a section, then top steel is not required...consistent with what StructuralEIT stated.
If there IS negative moment, then you need some level of rebar - either that calculated or the minimum amount as required by ACI section 10.5.
RE: So confused with slab design