Structural slabs-on-grade

[jdgengineer]

Question for residential projects related to structural slab-on-grades. Often times we are designing slabs as structural slabs even though they are on grade (either due to void form requirements, or backspanning of backfill, etc.). Typically, these slabs will be between 6-12" thick. For a 6-8" slab they are often #4-#5 bars @ 12" oc centered in slab. For 10-12" slabs we typically go with a double mat of #5 @ 12" oc. The homes are high end so price of the reinforcing is not critical. The performance of the slab assembly is most critical.

In all of these structural slabs we would typically not allow control joints to avoid compromising the span capacity of the slabs. With the double mat I am not concerned with cracking as we have a tight reinforcing near the top of the slab. However, with the 8" slab potentially there is 3 3/8" cover at the top (assuming all is put in perfectly). Without the control joint I'm a little nervous about cracking. Typically the slabs will be covered with tile or other finish after the concrete has cured.

Any thoughts? Typically, my default is to just go with the 10" minimum due to cracking, but I'd like to start thinking of the 8" with single mat if it is feasible.

 

[JAE]

I have never used control joints in structural slabs.

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[Teguci]

With an 8" slab how do you position the rebar?

If you don't need positive moment capacity, then bring you rebar up to your minimum cover. For spans over voids, you can model the slab as cantilever, rotational pin at midspan and cantilever.

 

[theshearstud]

I have never seen control joints in a structural slab either. But take a look at ACI 224R_01 (Control of Cracking of Concrete Structures). See last sentence of 3.5.2 for recommended reinforcement percentage of gross concrete for concrete slabs. = 0.6% The standard also says that the code minimum-reinforcement percentage "does not normally control cracks to within generally acceptable design limits".

 

[Guastavino]

ACI 360R-10 recommends at least 0.5% of steel to eliminate control joints. It will crack, but that should keep the crack widths small. This reference is pretty helpful if you don't already have it.

 

[jdgengineer]

Yes, sorry if I was unclear. We do not use control joints in structural slabs. I haven't used an 8" slab before but am tempted to with a single mat in the center of the slab. This means that the steel would be 3 3/8" from the top of the slab. My concern would be with cracking. I am familiar with ACI 360R-10, but I believe it recommends the steel to be 1.5-2" from top surface or at 1/3 of the slab depth from the top. I suppose this would indicate that an 8" slab with a single mat is not a good idea.

So what do you all do for structural slabs?

single mat centered in slab for 6" slab?
skip 8" slab (as double mat would basically be on top of each other)
then move to 10"+ with double mat?

 

[KootK]

I go straight to 10" and two mats. Usually these are small areas and one way spanning slabs where an effective depth of 4" won't allow you to span too far.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[jdgengineer]

Thanks KootK only concern is he trickle down effect. In my case here the slabs are supported by liers and gradebeam. More concrete leads to more gradebeam reinforcing and deeper / more closely spaced piers.

I can get a 6" slab with #5@12 to span 11' (assuming at least 2 span). The end result between the 6" slab and the 10" slab start to be close when you account for the additional load to support the 10" slab.

I've also typically gone straight to 10" double mat but just looking at alternatives. Seems like 6" may work in some instances. My main hesitation is cracking without the joints and only a single mat of steel

 

[hokie66]

Conflicting requirements. I would never design a 6" or 8" continuous structural slab without both top and bottom reinforcement. But for shrinkage cracking, the reinforcement can be evenly distributed within the thickness, which would allow centrally located bars.

 

[jdgengineer]

Thanks Hokie. I appreciate the response. I have always used 10" slab with double mat. But playing devil's advocate, aside from potential cracking (which you didn't see too concerned about) what is your concern with the 6" slab? In the instance I am considering the slab is spanning ~ 8'-0". It works on paper by a mile. I know to engage the tension steel there may be some cracking in the slab as well. In my case the slab will be covered with bluestone which could hide some cracks but itself would be fairly sensitive to cracking.