Concrete slab on grade with column loads

[jeffhed]

There are many threads on this issue and I have read through as many as I can find but still feel like I am missing something. ACI 318 does not control slab on grade design. ACI 360R-10 Chapter 12 states that structural slabs on ground supporting building code loads need to meet the strength requirements of ACI 318. I feel I am missing something in how the ACI 318 strength checks are applied. The project I am working on has a really thick existing slab with small loads, but I ran into some large discrepancies while running calculations to back it up. The existing slab is 7" thick. Column loads are D = 664 lbs, S = 3889 lbs, and wind = +/- 2015 lbs (total design load is D+0.75*S+0.75*0.6*W = 4488 lbs). Subgrade modulus = 120 pci, column is 7x7 w/ 49 sq. in contact area, and f'c = 4000 psi. When I use ACI 360 with a FS = 2 at an interior location, the slab is ok. There is reinforcing in the slab but it is unknown what the reinforcing is so I am neglecting it. I then decided to go to the plain concrete section of ACI to make the actual strength checks. So first I factored my loads Pu = 1.2*D+0.5*W+1.6*S = 8129 lbs. ACI 360 uses the modulus of rupture of the concrete to assign an allowable bending stress. So for comparison I just removed Sm from the equation Mn = 5*lambda*fc^0.5*Sm which would give me the strength design allowable stress for comparison. My strength design allowable stress is then 0.6*5*1*(4000)^0.5 = 189 psi. Allowable stress from ACI 360 is 7.5*sqrt(4000)/2 = 237 psi which is 25% higher than the strength design allowable stress. My LRFD design load is 81% higher than my ASD loads. Add that on top of the fact that I have to use a slab that is 2" thinner because the concrete is cast against earth it seems like once I have to use ACI 318, slab thicknesses are going to have to be much thicker. In the slab example I gave above, under ACI 360 my flexural stress is 138 psi, less than the 237 psi and the slab is ok. Under ACI 318, my flexural stress in the slab is 433 psi which when compared to 189 psi calculated above is not remotely close to working. What am I missing here on going from ACI 360 to ACI 318 for a slab with structural loads?

 

[dik]

See attached by Alex T...

Dik

 

[jeffhed]

Thank you, Dik. I have a copy of this spreadsheet that I have modified, and it is what I used to get my output in my original post. However, I don't see how this satisfies the requirement that the ACI 318 strength checks be performed when a slab supports building loads. Also, it still doesn't help me on why the allowable stresses are so different when I try to apply the ACI 318 strength checks.

 

[JAE]

ACI 318 limits "footings" as having a depth, d, no less than 6".
With a 7" slab, and 3" cover, you'd not be able to make that work other than as a plain concrete design - so using that is correct.

The possible reason for the lack of strength using 318 plain concrete methods is that 318 is presumed to be for significant building structures, supporting floors, roofs, etc.
That is in direct contrast to 360 where a rack post load on a floor slab isn't as critical.

So perhaps more critical = more conservatism.

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[jeffhed]

JAE,
I wondered that maybe they were more conservative for that reason also. But when I can support a 7800 lb storage rack load on a 7" slab and in order to support a 4488 ASD load (8129 LRFD) according to ACI 318 I need a 10" thick slab, something doesn't seem right. In order to support a load that is 43% less than the storage rack load I need a slab that is 43% thicker. I am wondering if others have run into this same issue, because I am really surprised that the numbers are so much different. Designing floor slabs on grade by Ringo and Anderson specifies a factor of safety for column loads on a slab of 1.7. The factor of safety I used with ACI 360 calculations was 2, so I guess I just expected them to come out fairly close. The driving factor between the differences is the ACI 318 requirement to reduce the concrete thickness by 2". This on top of the allowable flexural stress of 189 psi in comparison to the allowable flexural stress in ACI 360 of 237 psi which is about a 20% difference. In some threads others have mentioned they use the Ringo and Anderson book to determine allowable loads on slabs, others have mentioned they use TM 5-809-12. Am I just over complicating it by looking at the unreinforced section of ACI 318?

 

[DTGT2002]

I wonder if we are fighting semantics here - is the definition of "structural slabs on ground" plain to you? To me, it can be a difficult definition. I think it means a slab on ground capable of spanning between distinct supports without reliance on bearing on the soil (or other base materials) between supports.

318 is well focused on protecting the reinforcement and where plain concrete is used, redundancy is low enough to demand a more conservative approach - but I would think a situation like this is precisely why 318 would not want to speak to SOG.



I think the sog definition in Chapter 2.1 where it functionally limits slab-on-ground to slab, supported by ground, whose main purpose is to support the applied loads by bearing on the ground.

Do you believe your path to ground is direct to the underlying soil?

 

[jeffhed]

DTGT2002,
I don't know that it is 100% clear to me. But I have always looked at a structural slab on ground as a slab that has to transmit building type loads to the soil (by building loads I mean mezzanine column loads, storage rack loads, auto lift loads, etc.) not necessarily a slab that can span without support from the underlying soil. Although, that could just be another situation where a structural slab on ground would be needed if there were weak soils, etc. And my load path is direct to the underlying soil as the slab was poured directly on the soil. As far as 318 not wanting to speak directly to slabs on grade, section 12.1 of ACI 360 specifically states that slabs on ground supporting building code loads should be designed in accordance with ACI 318. It does also state that where the soil is used to form the slab and the slab should resist the loads by spanning to piles, piers or other foundation elements, that those slabs should also be designed in accordance with ACI 318. Once I ran numbers according to ACI 318 and saw the discrepancy between 318 and 360, I began to wonder if something was off in how I was approaching it.