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?
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Baseplate with overturning on SLAB ON GRADE
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Baseplate with overturning on SLAB ON GRADEBaseplate 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?
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RE: Baseplate with overturning on SLAB ON GRADE
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
RE: Baseplate with overturning on SLAB ON GRADE
I agree with Ron, unless it is a small column/moment it probably wont work.
RE: Baseplate with overturning on SLAB ON GRADE
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
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
RE: Baseplate with overturning on SLAB ON GRADE
Thanks
RE: Baseplate with overturning on SLAB ON GRADE
RE: Baseplate with overturning on SLAB ON GRADE
RE: Baseplate with overturning on SLAB ON GRADE
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
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
RE: Baseplate with overturning on SLAB ON GRADE
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
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
RE: Baseplate with overturning on SLAB ON GRADE
RE: Baseplate with overturning on SLAB ON GRADE
RE: Baseplate with overturning on SLAB ON GRADE
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
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
RE: Baseplate with overturning on SLAB ON GRADE
Thanks again for any input you might have.
RE: Baseplate with overturning on SLAB ON GRADE
RE: Baseplate with overturning on SLAB ON GRADE
RE: Baseplate with overturning on SLAB ON GRADE
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
RE: Baseplate with overturning on SLAB ON GRADE
RE: Baseplate with overturning on SLAB ON GRADE
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
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
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
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.