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Baseplate with overturning on SLAB ON GRADE
5

Baseplate with overturning on SLAB ON GRADE

Baseplate with overturning on SLAB ON GRADE

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

RE: Baseplate with overturning on SLAB ON GRADE

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.

RE: Baseplate with overturning on SLAB ON GRADE

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.

RE: Baseplate with overturning on SLAB ON GRADE

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

RE: Baseplate with overturning on SLAB ON GRADE

(OP)
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.

RE: Baseplate with overturning on SLAB ON GRADE

2
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.

 

RE: Baseplate with overturning on SLAB ON GRADE

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

RE: Baseplate with overturning on SLAB ON GRADE

(OP)
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

RE: Baseplate with overturning on SLAB ON GRADE

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.  

RE: Baseplate with overturning on SLAB ON GRADE

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

RE: Baseplate with overturning on SLAB ON GRADE

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.  

RE: Baseplate with overturning on SLAB ON GRADE

(OP)
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?

RE: Baseplate with overturning on SLAB ON GRADE

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.  

RE: Baseplate with overturning on SLAB ON GRADE

(OP)
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!

RE: Baseplate with overturning on SLAB ON GRADE

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.  

RE: Baseplate with overturning on SLAB ON GRADE

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

RE: Baseplate with overturning on SLAB ON GRADE

(OP)
That's terrific, Vandede.  Thanks!

RE: Baseplate with overturning on SLAB ON GRADE

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!!!!

RE: Baseplate with overturning on SLAB ON GRADE

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.

RE: Baseplate with overturning on SLAB ON GRADE

I basically wanted to say what CTW is saying, but I couldn't find a way to type it quickly morning

RE: Baseplate with overturning on SLAB ON GRADE

(OP)
Thanks CTW.  So, are we saying that the method I used ("equivalent footing" method, if you will) is conservative?  It seems like we're all in agreement that there is no definitive solution to my particular condition (overturning moment on a slab on grade).  I'm still a little confused at what people think is the best estimated method.  Equivalent footing?  BOEF?  FEA software?  Other?  I'm okay with neglecting the fact that the steel might not be exactly in the right place, etc., as I consider the safety factors to account for that.  I just need a method of analysis to hang my hat on.  Conservative is okay.

Thanks again for any input you might have.

RE: Baseplate with overturning on SLAB ON GRADE

I think the "equivalent footing" may not be conservative.  

RE: Baseplate with overturning on SLAB ON GRADE

if it comes to it, maybe you could install a larger grouted sole or bearing plate underneath the robot base. This might not be desirable for the robot though as it will change the height of the machine.  

RE: Baseplate with overturning on SLAB ON GRADE

(OP)
Thanks, Toad.  Yeah, I'm considering increasing the size of the baseplate.  But, I'm still faced with having to analyze the slab somehow, no matter how big the baseplate is.

Let's say I can make the baseplate as big as I want it.  What's the best method to analyze the slab?  Does a gigantic base plate simplify the slab design any?  Would that make the equivalent footing method more logical?

RE: Baseplate with overturning on SLAB ON GRADE

For a uniaxial moment, I'd try the BOEF spreadsheet and see what you come up with.  

RE: Baseplate with overturning on SLAB ON GRADE

ToadJones is correct that your method may not be conservative.   

RE: Baseplate with overturning on SLAB ON GRADE

Toad or CTW,
Perhaps you can explain why you think my method is not conservative.  It is just a spread footing, with slab on ground connected around the sides, but ignored in the analysis.

RE: Baseplate with overturning on SLAB ON GRADE

hokie66-
I'm glad you posted this question as it kept me in check.  I should have re-read my initial post and slowed down a bit before I responded with "may not be conservative".  I don't know where my brain was on that post as that's certainly not what I think.  It's been a long week and I was tired when I responded.  

So let me formally retract my last post stating the "equivalent footing" method "may not be conservative".

zstructural-
Yes, I was stating (in a round about way) that hokie66's "equivalent footing" method is conservative.  What I was trying to point out was that your 4 ft cantilever may be sufficient afterall.  You stated that it was "just undersized".  Your analysis is based on a cantilever or single spread footing which doesn't actually exist, which isn't a bad thing.  You instead have much more slab surrounding this cantilever.  So the behavior is not a true cantilever and the moment used in the analysis may not be as high.  Therefore the slab may be adequate.  It would be a judgement call at that point but I think you can justify calling it good if your analyis results are not too far over the allowable.

I liked Ron's approach and would have started with that since it may be difficult developing enough tensile capacity from the 8" slab.  But it may be fine depending on the number of bolts, type of post-installed anchor and the tension required.  

If that checked out, I would have gone to hokie66's method to check the slab capacity.

There's no defined method that I'm aware of for this situation.  It's going to take being confident in your assumptions, making sure those assumptions are realistic, being comfortable with your analysis method and results, and using your engineering judgement.  

We don't know what actual moment or reiforcing size are used in your analysis, so only you can determine if what you've done is conservative.  As long as you feel there is a sufficient safety factor to cover any unknowns, then you can probably hang your hat on your decision and methods used to arrive at your conclusion.

It's a neat problem.  Keep us updated on what you decide and what methods you used in your final analysis.

 

RE: Baseplate with overturning on SLAB ON GRADE

when I say it "may not be conservative" I actually am wondering myself....not saying that it definitively is not conservative.
My problem is this....this problem is more along the lines of plate theory and plate stresses then a straight up cantilever bending moment. The stresses at or near the baseplate may be much higher since the slab that we are treating as a cantilever is not a true cantilever and in reality cannot rotate like a cantilever due to the constraint of the surrounding slab.
It would seem to me that the greatest stress concentrations would be very near the base plate leading me to believe that a giant "equivalent footing" may not be the best representation.
Also, the thinner the slab, the shorter the distance from the base plate I would feel comfortable using in checking overturning. Certainly if you bolted a basketball hoop or the like to your 100'x20'driveway you wouldn't count your entire driveway in checking overturning, would you? Bad and cheesy example, I know.  
I don't know this for sure; just voicing concerns I might have.  

RE: Baseplate with overturning on SLAB ON GRADE

(OP)
Thank you everybody for your input.  This has helped me, if for no other reason than to bounce it off of you, and know I'm not way out in left field - and hear that the equivalent footing method is logical (if used in a logical manner).

I feel comfortable using a large enough area to resist overturning, as long as that area checks out for the cantilever.  Toad, to your basketball hoop analogy, I would say if your driveway was 24" thick with reinforcement, you could use a pretty large area to prevent overturning.  In other words, as long as I can get the cantilever to check out, that distance/area would be okay to use in resisting the overturning.  That's my thought process.  Let me know if you disagree.

I'm ending up making the plate a little bigger so the numbers look a little better on paper, and I'm not relying too much on my engineering judgement.  That way if someone screwed up the rebar 30 years ago, or something, and the slab starts cracking, I can numerically prove that what I designed, given my parameters.

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