Slab Buckling 2

[akastud]

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?

 

[ishvaaag]

The slab will be supported on beams solidary to columns. You may quite easily become satisfied of that the problem won't occur by making a 3D analysis of all including some initial deformation of say the actual expected defelection value for both floors and beams. The compression will make the deflections bigger but likely not to the extent of ruining the structure. It is unlikely the compression passed to the beams require individualized decreases of their Young's modulus to model material nonlinearity.

In any case, this analysis will show you where to modify your structure if you are unhappy with what you see.

 

[akastud]

ishvaaag,

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?

 

[ishvaaag]

Certainly if no restraint against buckling upwards only the weight of the slab plus any cohesioned soil will be stabilizing the slab against buckling upwards. I have never known of any buckling of slabs of this cause, but in our zone we practice reinforced slabs on the ground 6 inches and thicker.

 

[akastud]

We have a 4" slab with WWF, that is why I am a little curious on how to verify it, perhaps I will specify a 6" slab with some heavier reinforcing just to feel a little better.

 

[dbuzz]

Perhaps you should treat the slab as a wall element for the purposes of a design check for the in-plane load.

 

[strucuturesareus]

The lateral force of the soil on the side wall or retaining wall must be resisted by contributions from the slab on grade, sliding of the footing and possibly passive pressure of the soil against the footing. I generally do NOT include passive pressure since the amount of movement required to mobilize the passive pressure would probably result in failure of one of the other force components ... so it's not compatible with resistance from the slab on grade or sliding of the footing.
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!

 

[BryanStein]

Great post by Structuresareus. Just one oversight needs to be mentioned.

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.