Assuming regular slab-on-grade, no prestressing etc.
As = (F*L*w)/(2*fs)
As = Area of reinforcement (sq. in.) per foot width of slab
F = Friction Factor (dependent on base and subbase - "typical" is 1.5)
L = Distance between joints (ft)
w = Dead weight of slab (psf)
fs = Allowable stress in the rebar (psi)
Might want to consider factors such as loads, application etc. to tweak this as necessary.
For a non-structural slab on grade- Zero would be the minimum. Just space your joints properly.
Un-reinforced slabs are very common for residential and office type applications. Wouldn't recommend it for industrial or warehouse
ACI 360 "Guide to the Design of Slabs on Ground" is the reference you need! Generally it is most economical to obtain the necessary strength by increasing the thickness of a plain slab than it is to reinforce for strength. As far as T&S, control joints at the correct spacing, is the most economical solution. If you can use control joints, then the new recommendations range from .15% to .5% of the gross area of concrete . The formula jreit refers to is the old Subgrade Drag Theory. It has been removed from the new ACI 360.
We did many warehouse/commercial buildings with un-reniforced slabs. Proper sub grade prep and concrete mix and placement/saw cuts was critical, but many are still uncracked toay. Mesh/rebar for a 100K sf warehouse is expensive, and generally does not help, since it isnt in the slab correctly usually.
The subgrade drag equation was not included in ACI 360-06. I didn't catch it until I just got ACI 360-10. The only guidance in -10 is the recommendation of .5% reinforcement to eliminate control joints. ACI 350-06 has a table based on joint spacing but this reference is for environmental structures. It also has the same upper percentage of 0.5% in joints are not used. I haven't seen any commentary explaining why it was removed.
Hello, I'm practicing in Europe, and I have a question about foundation slab design. Let's say we have a slab 20 x 30 meters, 300 mm thick, and designed with top and bottom mesh to resist building loads (let's say steel or wood). Commonly we don't specify any joints or sawcuts. And it works. Small cracks form but are not an issue. I want to understand american practice with many control joints and sawcuts I'm reading about.
ACI states you need to use .5% reinforcement if there are no joints - for the example slab that would be 15 cm^2 of reinforcement (sum of top and bottom?) which quite a lot. Most of the designs don't have such an amount of reinforcement.
In my area contractors deal with crack control by dividing slab with more pours, using special concrete mixes, even pouring slab in two layers and adding micro reinforcement.
I would love to read about your experience.
