Concrete slab on slab on grade

[ugandabob]

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.

 

[a2mfk]

What was the original slab and subgrade design, if you know?

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?

 

[BAretired]

Do you know the reinforcement in both existing slabs?

BA

 

[a2mfk]

What I've done with existing SOG before is drill through in a few locations and do hand held CPTs, cheap and effective way of estimating what your bearing conditions are. But far from very thorough and foolproof.

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.

 

[delagina]

i think i read somewhere for slab it will act together due to friction no need for shear connectors.

 

[a2mfk]

delagina- the OP has no idea how the slab was prepared when the second slab was placed.. Would you sign off in that situation?

 

[epitome1170]

I use a PCA document (old... 1976) titled "Slab Thickness Design for Industrial Concrete Floors on Grade" by Robert Packard when I am looking at existing slabs that need more in-depth analysis than what I would typically provide.

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.

 

[Ron]

Since you apparently have no idea of the amount of reinforcing, one must assume there is none. Given that, it makes no difference if the slabs are connected.

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.

 

[a2mfk]

Ron- I don't follow. If you check it as plain concrete in bending, it'd be way better to have a 14" total thickness for your section modulus...? Maybe you are assuming another method. I figured the OP would be using a PCA/Army chart or something similar.

 

[Ron]

a2mfk....elastic layer analysis, not using beam analogy.

Even if you are using a beam analogy, the slab separation is close to the neutral axis of a plain section.

 

[ugandabob]

Thanks for the responses everyone.

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?

 

[Ron]

Probably ultra-conservative, but workable.

 

[a2mfk]

Agreed, 14" of total concrete plus the original slab, you can do whatever you like with that... I still may be tempted to do a few CPTs through the existing slab just in case for some reason there is a weak spot of soil or a void. But with what you are doing it could span quite a bit of bad soil anyway.