Slab-On-Grade Cracking Under Post Loads
Slab-On-Grade Cracking Under Post Loads
(OP)
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?
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?






RE: Slab-On-Grade Cracking Under Post Loads
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RE: Slab-On-Grade Cracking Under Post Loads
RE: Slab-On-Grade Cracking Under Post Loads
Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask
RE: Slab-On-Grade Cracking Under Post Loads
Also, you never called me about the micropile testing in a previous thread. I am in eastern PA also.
www.PeirceEngineering.com
RE: Slab-On-Grade Cracking Under Post Loads
I thought you were being nice, but didn't realize you were serious. If the offer's still there I'll call on monday.
RE: Slab-On-Grade Cracking Under Post Loads
www.PeirceEngineering.com
RE: Slab-On-Grade Cracking Under Post Loads
If you don't have confidence in the location of the reinforcement, assume it is not there.
BA
RE: Slab-On-Grade Cracking Under Post Loads
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.
RE: Slab-On-Grade Cracking Under 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?
RE: Slab-On-Grade Cracking Under Post Loads
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.
RE: Slab-On-Grade Cracking Under Post Loads
RE: Slab-On-Grade Cracking Under Post Loads
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.