Baseplate with overturning on SLAB ON GRADE 5

[zstructural]

How to I determine if an existing slab on grade is sufficient to support a baseplate which has an overturning moment?

 

[Ron]

Well, likely, it is not capable. You can start, assuming the baseplate is fully capable of transferring the moment to the slab, with resolving the moment into a force couple and checking the pullout of the anchor bolts. The column will try to rotate on the baseplate, thus pulling one set of anchor bolts up and compressing the concrete around the set on the opposite side.

In actuality, the failure mode is more complex than that, but that will suffice for a check.

Unless you have a very low moment, or a very thick slab, it probably won't work.

In most cases, we design such connections as pinned connections, not moment connections.

 

[smb4050]

Based on what you wrote, I see a light structural column and base plate. Slab thickness 4" to 6" thick, and the slab is unreinforced with width and breath in excess of 2 or 3 times base plate or 4 or 5 times slab thickness. Based on this, I would check the anchors in tension as the governing failue mode. Different ways to look at it depending on rigidity of the base plate and slab. Could be plain unreinforced structural concrete.

 

[ToadJones]

Also, you may want to look at punching shear.
I agree with Ron, unless it is a small column/moment it probably wont work.

 

[zstructural]

To be more specific, I have a baseplate for a robot, which has a fairly good size moment. The slab is 8" thick, with rebar top and bottom. ACI 360r-10 doesn't address this condition, from what I can see.

Checking the anchors is pretty straight forward. But, I'm wondering how to analyze the slab itself. Is there a way to make it analagous to checking a spread footing for the same situation? Or, could I check it as a cantilever, similar to how you would check the baseplate?

Thanks so much for your input.

 

[hokie66]

Since the slab is 8" thick and reinforced top and bottom, it is a spread footing. The trick is in deciding what plan dimensions to use. I would just determine how much of the slab is required to resist the overturning, ignore the slab beyond that size, then check the slab strength.

The problem with that approach, for a slab on grade, is that often the reinforcement is not where it is supposed to be.

 

[vandede427]

I look at slab-on-grades as a beam on an elastic foundation.

 

[zstructural]

vandede427 - can you expand on that? I'm not familiar elastic foundations. Is there a reference that would explain the design process for that? Or is it something you can explain?

Thanks

 

[ToadJones]

Beam on an elastic foundation merely refers to the fact that your slab can be treated as a beam which is resting on a subgrade which is not infinitely stiff. In reality it has some spring constant associated with it; or a subgrade modulus.

 

[zstructural]

How/where do I find the design process for designing a beam on an elastic foundation?

 

[ToadJones]

I'm not sure that a B.O.E.F analysis is necessary.
I do them by FEA software in order to get soil bearing pressures.
I think what Hokie is suggesting is to figure out your overturning moments then determine how much of the existing slab would be required (plan area) in order to balance that overturning moment with some reasonable factor of safety. From there, you will essentially have a rectangular spread footing that can be checked for strength....most likely cantilever bending of the slab.
I am not sure that stresses in plates or slabs are quite that straight forward.

 

[zstructural]

Thanks.

I'm going to do the analysis like you described - find my "equivalent footing" area based on overturning, and check that for bending, as a cantilever from the resulting soil pressure. Does anyone see any problems with this?

 

[ToadJones]

If you wind up with a huge area of slab needed to resist overturning, then yes, I guess I would have a hard time accepting it as a logical approach.

 

[zstructural]

If the slab area is huge, it has to be that much stronger, as the cantilever is huge.

As it turned out, my slab was just undersized to cantilever the 4', which was the dimension required to resist overturning.

If anyone has any other comments/suggestions, I'd be happy to hear them. Especially if you think I'm doing something wrong.

Thanks!

 

[ToadJones]

Well, if the logic went that a huge area of slab was better then it implies that a piece of the slab a few hundred feet away is effective in helping resist overturning....not very logical.
The key is determining how much of the slab is actually seeing forces from the base plate.

 

[zstructural]

Is there FEA software that would work well for this application?

 

[vandede427]

use this spreadsheet. it was written by the master.

 

[zstructural]

That's terrific, Vandede. Thanks!

 

[ToadJones]

zstructural-
My apologies....I forgot about that spreadsheet....I think every engineering office in the world now has downloaded those.
As with any spreadsheet..read and use it carefully!!!!

 

[CTW]

It's going to be difficult to arrive at a definitive answer for this problem. As you're probably becoming aware, there are multiple ways to check this based on assumptions that are probably not that accurate nor represent actual conditions. For instance, hokie66 offered a reasonable approach which you have checked. So you ended up with a 4 ft cantilever but the slab was just undersized. Is it close enough to call it good and move on? But you checked a cantilever which in reality you do not have. So you may very well have enough capacity in the slab to handle it as the pressure will spread out more than what was assumed.

While the BOEF is another good method, it requires some determination of how much slab is being engaged. Another assumption that may or may not represent actual conditions.

Also the steel is probably not where it is supposed to be unless it has been field located, and the slab is probably not 8" thick (could be more, could be less).

Ron offered the method that I would have started with; the anchor bolts. Depending on the tensile capacity required, the 8" slab may not be enough.