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?
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