Point load on unreinforced slab on grade 2

[CJJS]

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)?

 

[CJJS]

I understand what you are doing. I would be careful in assuming the pressure distribution is uniform though. Especially on a 10ft. x 10ft. area for a 4" thick footing.

 

[twinnell]

See ACI 360R-06. It has three methods to determine thickness required.

 

[JrStructuralEng]

Dave,

"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.

 

[JAE]

JrStructuralEng,

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.

 

[JrStructuralEng]

Jae, your explanation makes sense... but is considering slab deflections in house construction common? There would be deflections in every slab, in every house and almost every slab for the past 80 years has no reinforcing. Everytime there is a telepost on a pad footing, the slab is cast over top. The soil beneath the footing will deflect as you mentioned, and because the slab is cast over the footing you will get bending in the slab (not to the same extent, but there would be some)

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?

 

[COEngineeer]

PCA note book is definately worth the money. It has so many examples for all chapters on ACI318.

Never, but never question engineer's judgement