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)?

 

[csd72]

These things are usually based on the elastic response of the soil and the plain concrete flexure of the slab.

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.

 

[CJJS]

Yes. I didn't think the load would be an issue. I just need to prove it. Do you know of any reference?

 

[DaveAtkins]

PCA publishes a book on the design of slabs on grade, with a chapter dedicated to this very subject.

DaveAtkins

 

[Taro]

Another option is the Westergaard equations for concentrated loads on unreinforced slabs.

 

[JAE]

Here's the PCA book:

PCA Slab on Grade Book

 

[JrStructuralEng]

Even at a conservative soil bearing pressure. You need less than 1ft^2 bearing area. Just throw the point load on a 12"x12"x1/2" Thick steel plate and your good to go. The plate wouldn't even need to be that wide if you consider the 45 degree load path through the 4" slab. I would feel confident that an 8"x8" would work as well.

Considering the fact that this point load will likely work even without a steel plate. Add one in, and your off to the races.

 

[DaveAtkins]

JrStructualEng,

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

 

[abusementpark]

I would think if you can get geotech to give the modulus of subgrade reaction, you could see what kind of tensile stress you get from a point load using Westergaard's equation.

 

[renko]

I tend to agree with DaveAtkins - 'The point load spreads across an area of slab which is a function of the soil subgrade modulus and the thickness of the slab'

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.

 

[CJJS]

There is no geotech on this project. What is a conservative modulus of subgrade reaction for medium clay?

 

[JAE]

75 pci

 

[JAE]

Actually, check out this chart - use the K30 number.

 

[CJJS]

Thanks JAE.

 

[fredPE]

You would be amazed at how many contractors simply pour elevated slabs, with shoring posts bearing on slabs on grades, without even thinking about the capacity of the slab. 8,000 or 10,000 lb loads 6' on center each way, and they don't even bat an eye. They might put a wood shim down to get uniform bearing, sometimes.

 

[AlmostPE]

When I did this calc I just pretend I had a 4" footing that is like 10'x10' and just do the regular calc for footing.

Not almost anymore!

 

[CJJS]

AlmostPE,
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.

 

[AlmostPE]

I assumed it it distributed to 10x10 area and check for punching shear and bending. The bigger the area you assume, the smaller the pressure, but the moment arm will get bigger (for bending calc). We were trying to find out how much load we can put on a basement slab. Sometimes homeowner/architect change their mind AFTER the slab has been poured. If it is not a very much load, then I just tell them to bear it on the slab, if not I tell them to cut the slab and pour thicker footing. I hope I am making any sense.

Not almost anymore!

 

[CJJS]

I am not sure you can assume that the larger the area, the smaller the pressure. The pressure distribution is going to depend on the stiffness/rigidity of the footing. That is why I asked what soil distribution you used beneath the footing.

 

[AlmostPE]

Ok, lets say you have 1 unit dummy point load on 10 by 10 foot slab. The pressure is .01 unitload/sf.

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!