Slab-On-Grade Cracking Under Post Loads

[Lion06]

I have an existing reinforced slab-on-grade that we are going to post off of. The slab is 6" thick and reinforced with rebar, not mesh.

The post loads aren't huge (they vary from 2kips - 4kips), but I am concerned about cracking in the slab. I've looked through ACI 360 (It's an old one, it was copyrighted in 1992), and I want make sure I'm looking at this correctly.

There are two tables (one for wheel loading and one for post loading) that, when you learn how to read them, give you a required slab thickness. This is based on unreinforced concrete, using the modulus of rupture of the concrete with a Safety Factor. The table for the post loads is somewhat limited in that it doesn't list contact areas greater than 80 sq. in. and it doesn't give modifications for a subgrade modulus other 50 pci.

Using these tables (with the unreinforced assumption) makes sense since we're trying to prevent cracking, and the steel contributes little to the actual cracking capacity. I believe that the acual load carrying capacity is increased by the rebar, but that the cracking load would be the same (or very close to the same).

My first question is am I going down the right road here or am I barking up the wrong tree?

My second question is if there is any more recent (and modern) design/analysis procedure by which to determine the adequacy of a slab-on-grade to take point loads without cracking?

 

[slickdeals]

Did you try looking in the army reference manual for slab on grade with heavy loads?

http://www.army.mil/usapa/eng/DR_pubs/dr_a/pdf/tm5_809_12.pdf

 

[Ron]

SEIT...in this case you'd be concerned about two parameters....first, punching shear, which is not likely a problem. The other is bending as you would get for a wheel load. If you do an elastic layer analysis of your slab, assuming a load of 4 kips and a contact area of 16 square inches, a 6-inch thickness, and a compressive strength of 3000 psi (elastic modulus of 3500 ksi), the lateral stress at the bottom of the slab should be less than the modulus of rupture of the concrete (assume 500 psi for 3000 psi concrete with normal size aggregate...less for smaller aggregate)

 

[msquared48]

You'd better work in creep here too, as the 4K is likely a long term load, also affecting the bending.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[PEinc]

StructuralEIT, attached is a PDF of a MathCAD file for designing an unreinforced concrete slab with a column load.

Also, you never called me about the micropile testing in a previous thread. I am in eastern PA also.

www.PeirceEngineering.com

 

[Lion06]

Pe-
I thought you were being nice, but didn't realize you were serious. If the offer's still there I'll call on monday.

 

[PEinc]

Call after 9 AM. I sleep late!

http://www.PeirceEngineering.com

 

[BAretired]

If you treat the slab as unreinforced, the methods given so far are appropriate. If the slab reinforcement is located in the middle of the slab it will help considerably. If the reinforcement is lying in the sand below the slab, it won't help at all.

If you don't have confidence in the location of the reinforcement, assume it is not there.

BA

 

[OHIOMatt]

The most effective means of analysis of this type of problem that I have fond is the Westergaard Method. It basically boils down to modeling the slab supported by a bed of compression only springs. The "springs" are the soil, and the spring constant would be approximatley the modulous of subgarde reaction. If this is not know, use a relatively conservative value such as 50 to 75 pci.

You will find that the slab will have surprising capacity with forces acting towards the soil. I have modeled slabs in this method and also tested the results.

You will likely find that the slab as described will support the loads you are looking for.

Uplift loading is a different animal and the description above is not relavent for this.

 

[Lion06]

I'm not really worried about slab CAPACITY. I'm comfortable doing the strength checks. What I'm not so comfortable with is how to assess the slab cracking as a result of the post loads.

I think I'm going to start with a 6" slab, assume a base plate size, and back out what the maximum permissible load is using the table in ACI 360. Then I'll reduce the post spacing to stay under that load.

Does that seem reasonable?

 

[KootK]

A couple of months ago, I toured some warehouse slabs (long story) in town. The slab thicknesses were in the 6-10" range and loaded by racks with point loads in the 2-8 kip range. I saw numerous cracks that seemed to originate at the rack post locations.

I guess what I'm getting at is this: I think that your posts will result in some slab cracking. It might be prudent to focus your efforts on managing owner expectations rather trying to stay crackless.

 

[ToadJones]

Maybe cut the slab out and pour a new nice little isolated, reinforced pad for the post with some expansion material between old and new.

 

[OHIOMatt]

If you use unreinforced concrete properties in the FEM analysis, you will be able to see where the cracks occur. The posts will induce cracking on the bottom of the slab first, which will not be visible. As the load increases, you would eventually get cracking in a radial pattern away from the post.

Keep in mind, that the concrete is already cracked, they may just not be visible.

The Westergaard Method is specifically for cracking of a plate on an elastic medium.