Structural slabs-on-grade
Structural slabs-on-grade
(OP)
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.
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.






RE: Structural slabs-on-grade
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RE: Structural slabs-on-grade
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.
RE: Structural slabs-on-grade
RE: Structural slabs-on-grade
RE: Structural slabs-on-grade
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?
RE: Structural slabs-on-grade
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.
RE: Structural slabs-on-grade
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
RE: Structural slabs-on-grade
RE: Structural slabs-on-grade