Concrete slab on slab on grade
Concrete slab on slab on grade
(OP)
I'm checking an existing concrete slab on grade for new equipment loads. The existing 8" slab was poured on top of an older 6" slab-on-grade; only friction connects the two.
I will assume that the two slabs do not act together. How does the lower slab (that is directly on grade) effect the bending in the upper slab? Is there a way I can adjust the modulus of subgrade reaction to account for this?
The load from the equipment is about 30 kips per leg. The 4 legs are spaced about 7.25 ft apart in a square.
Thanks for the help.
I will assume that the two slabs do not act together. How does the lower slab (that is directly on grade) effect the bending in the upper slab? Is there a way I can adjust the modulus of subgrade reaction to account for this?
The load from the equipment is about 30 kips per leg. The 4 legs are spaced about 7.25 ft apart in a square.
Thanks for the help.






RE: Concrete slab on slab on grade
What is it used for? If its had a significant amount of forklift traffic, you could count no some degree of improved compaction. You could also add some vertical dowels to tie the two together for a composite section...
What kind of equipment- is settlement going to be a crucial issue?
RE: Concrete slab on slab on grade
BA
RE: Concrete slab on slab on grade
Is some type of load testing out of the question? Seems like you have a lot of slab, and if you have a good subbase you may be fine, but may be difficult to prove on paper.
RE: Concrete slab on slab on grade
RE: Concrete slab on slab on grade
RE: Concrete slab on slab on grade
As for the two slabs acting as one...
In my opinion, I would assume the two are connected due to shear friction provided that the two slabs extend "far enough" beyond the edge of the applied loadings. My reasoning would be that shear flow between the two pieces in a purely gravity aspect would be exceptionally low and that this would be able to be resisted by the friction.
RE: Concrete slab on slab on grade
Considering the slab as a pavement section, the loads are discrete and I would check for lateral stress in both directions as the bottom of the slab and check for shear at the leg base plates. Assuming the baseplates are at least 12"x12", shear is probably not an issue.
Overall soil pressure is low.
RE: Concrete slab on slab on grade
RE: Concrete slab on slab on grade
Even if you are using a beam analogy, the slab separation is close to the neutral axis of a plain section.
RE: Concrete slab on slab on grade
What we have decided to do: Since the top slab is sloped, we have to pour some concrete anyway to level it for the equipment. So we are going to pour this new concrete about 5" or 6" thick and dowel it into the existing slab, to give us about 13-14" of slab.
We know nothing about the condition of the original slab (poured in 1968), so I'm just going to ignore its structural contribution.
I used PCA and WRI charts to come up with a slab thickness.
Does this sound reasonable?
RE: Concrete slab on slab on grade
RE: Concrete slab on slab on grade