Point load on unreinforced slab on grade
Point load on unreinforced slab on grade
(OP)
I have to check if a 4" thick, unreinforced slab on grade will handle 1700 lb. concentrated load. My first instinct is to determine the required bearing area based on the soil bearing capacity. Then, analyze that area of concrete as a column footing. Is this approach correct? Or should I be using some other approach (I recall some Westergaard methods of analyzing point load on slab on grades)?






RE: Point load on unreinforced slab on grade
You need to find a publication on the design of industrial floors as such things are done very differently to what you describe.
That said, I doubt your 1700lb load will be an issue, but it does need to be checked.
RE: Point load on unreinforced slab on grade
RE: Point load on unreinforced slab on grade
DaveAtkins
RE: Point load on unreinforced slab on grade
RE: Point load on unreinforced slab on grade
PCA Slab on Grade Book
RE: Point load on unreinforced slab on grade
Considering the fact that this point load will likely work even without a steel plate. Add one in, and your off to the races.
RE: Point load on unreinforced slab on grade
It isn't so simple. The point load spreads across an area of slab which is a function of the soil subgrade modulus and the thickness of the slab. Also, punching shear is an issue.
DaveAtkins
RE: Point load on unreinforced slab on grade
RE: Point load on unreinforced slab on grade
If you want to avoid spreading the load, and only need 1ft^2 bearing area as suggested by JrStructuralEng, then why not cut out this area of slab and pour new concrete in the area with a sealed gap around.
RE: Point load on unreinforced slab on grade
RE: Point load on unreinforced slab on grade
RE: Point load on unreinforced slab on grade
RE: Point load on unreinforced slab on grade
RE: Point load on unreinforced slab on grade
RE: Point load on unreinforced slab on grade
Not almost anymore!
RE: Point load on unreinforced slab on grade
What soil distribution do you use beneath your footing? I wouldn't think a 4" thick 10' X 10' footing would be rigid enough to provide a uniform bearing pressure.
RE: Point load on unreinforced slab on grade
Not almost anymore!
RE: Point load on unreinforced slab on grade
RE: Point load on unreinforced slab on grade
Then I do a fixed cantilever beam that is 5 foot long with .1 unitload/ft distributed load and look at the bending. The moment is .01(5^2)/2 = .125 ft.unit load.
So if you assume smaller area, then your pressure will be higher but it will yield the same moment at the fixed end.
This is just something I did and I didnt get it from a text book or something. I felt comfortable with my approach and I think it is pretty conservative. I hope I am making sense.
Not almost anymore!
RE: Point load on unreinforced slab on grade
RE: Point load on unreinforced slab on grade
RE: Point load on unreinforced slab on grade
"It isn't so simple. The point load spreads across an area of slab which is a function of the soil subgrade modulus and the thickness of the slab. Also, punching shear is an issue."
I'm not sure I agree.
If you put a rigid 1ft^2 piece of steel, under a point load that only needs 1ft^2 total bearing area, assuming you have good contact between slab and soil (which I am assuming is the case)you would be placing no bending into the slab. In essence you neglect the slab. You have a 12"x12" steel footing which happens to have conrete in between it and the soil.
RE: Point load on unreinforced slab on grade
Nope - your example doesn't consider that the soil is not infinitely rigid. Your example would be correct if it was that rigid but no soil is. All soil will deflect downward under load to varying degrees.
So with your 12x12 inch plate, the load would initially travel directly to the underlying 12x12 soil, as you suggest, but then the soil MUST deflect under that load and since the surrounding soil is not loaded, and is not deflecting, you have a resulting warp in your slab and, via Hooke's Law, you have moment in the slab.
RE: Point load on unreinforced slab on grade
I would like to see that chapter in the PCA book, but I don't want to spend $50 if it is not worth having. Does anyone know if its a good book? Any better ones?
RE: Point load on unreinforced slab on grade
Never, but never question engineer's judgement