×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

Point load on unreinforced slab on grade

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

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.

RE: Point load on unreinforced slab on grade

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

RE: Point load on unreinforced slab on grade

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

DaveAtkins

RE: Point load on unreinforced slab on grade

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

RE: Point load on unreinforced slab on grade

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.

RE: Point load on unreinforced slab on grade

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

RE: Point load on unreinforced slab on grade

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.

RE: Point load on unreinforced slab on grade

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.

RE: Point load on unreinforced slab on grade

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

RE: Point load on unreinforced slab on grade

75 pci

RE: Point load on unreinforced slab on grade

(OP)
Thanks JAE.  

RE: Point load on unreinforced slab on grade

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.  

RE: Point load on unreinforced slab on grade

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!  smile

RE: Point load on unreinforced slab on grade

(OP)
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.  

RE: Point load on unreinforced slab on grade

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!  smile

RE: Point load on unreinforced slab on grade

(OP)
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.  

RE: Point load on unreinforced slab on grade

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!  smile

RE: Point load on unreinforced slab on grade

(OP)
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.      

RE: Point load on unreinforced slab on grade

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

RE: Point load on unreinforced slab on grade

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.

RE: Point load on unreinforced slab on grade

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.

RE: Point load on unreinforced slab on grade

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?

RE: Point load on unreinforced slab on grade

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

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources