Slab Buckling
Slab Buckling
(OP)
I have a question for everyone. I use enercalc for several of my calculations, and when I do a retaining wall, one of the options is "Slab present to resist all sliding forces". I am doing a basement wall where there is about 3000 plf of compression being resisted by the slab (theoretically 4500 for my 1.5 factor of safety) for each wall on either side of the basement. Well, it is real nice to click that little button and be satisfied with a relatively small footing (relatively big according to the contractor, but much smaller than it would be if I didn't have the slab). Anyway, now I have about 6000# of compression on a 4" slab of concrete about 30' long, (assumed 1' wide). Seems to me KL/r is a little high. How do I verify that my slab will not buckle?






RE: Slab Buckling
In any case, this analysis will show you where to modify your structure if you are unhappy with what you see.
RE: Slab Buckling
I have gotten the load at the upper floor level into the floor diaphragm, and I am not so concerned about that, I was discussing the slab on grade at the floor of the basement, although you have given me the idea to treat it as a diaphragm as opposed to a compression member. Anyone else have any other thoughts?
RE: Slab Buckling
RE: Slab Buckling
RE: Slab Buckling
RE: Slab Buckling
Subtract the sliding resistance of the footing from the total lateral force and the remaining must go into the slab on grade.
The slab on grade is resisted in buckling by the soil it rests upon and by gravity. If we consider that 2% lateral force is required to resist buckling - and we assume that your slab is flat so there isn't any initial out-of-straightness, then the slab on grade can resist at least its own weight divided by .02 ... assuming continuous lateral support by gravity.
In fact, the slab on grade can resist considerably more. If you figure the slab on grade can span between lateral support points at least some distance without buckling ... let's just GUESS it's 5 feet (because I'm too lazy to figure it out at this moment ... hey, it's Christmas holidays!) ... then you need a lateral restraint force of at least 2% of the axial load at no more than 10 ft centres. Since the mass of the slab on grade is available to employ, you've got 10 ft of slab on grade to provide the force ... so 5 ft of 4" sog is 250 pounds. Thus 250/.02 is 12,500 lb. Well, that should do.
Now, I personally think that 2% is inadequate since it envisages a perfect world, which doesn't exist in the reality of our design world ... so I use 5% ... so the slab on grade (if 5 ft were correct) has about 5000 lb. of resistance capacity.
Does this make sense?
Of course, if the slab on grade slides away when a 5000 lb force is applied ... it wont be much good to you!
Cheers!
RE: Slab Buckling
Any downward loading on the end walls should be tranfered into a footing and not into the slab unless you design the slab for this end load and resulting moment. If the downward load from the end walls is transfered into the slab, then the possibility of slab buckling just increased significantly because the soil will be pushing up on the slab to resist the downward loads inherent from the end walls.