Concrete thickness for slabs on grade subjected to uniform loading

[fa2070]

Hello,

What is the rationale for selecting the concrete thickness for a slab on grade subjected to uniform loading ?

The available literature ([red][1][/red] and [red][2][/red]) on slabs on grade design focuses on case studies where bending moments or punching shear on the slab are the driving factors, as is the case with lift truck traffic, concentrated loads (rack posts) or even uniform loads with an aisle, where significant negative bending moments may develop (think of warehouses).

In my particular case, I need to design the floor for water storage tanks where the loading is exclusively uniform. Assuming the underlying soil is uniform and stable (clays ruled out), how do I go about picking the concrete thickness ?
I understand this configures a very simple case (if not the simplest) as no flexural stresses are involved at all. Interestingly, [red][3][/red] references [red][2][/red] as the source for floor design, despite the fact that [red][2][/red] doesn't cover slabs on grade subjected to uniform loads.

Thanks in advance.

References:
[red][1][/red] "Designing Floor Slabs on Grade" by Boyd C. Ringo and Robert B. Anderson
[red][2][/red] "ACI 360R-92 Design of Slabs on Grade"
[red][3][/red] "Recommended Practice for Precast Prestressed Concrete Circular Storage Tanks" by the PCI Committee on Precast, Prestressed Concrete Storage Tanks.

 

[JedClampett]

The floor could theoretically be very thin. But you'll find that you'll need to thicken the slab to about 12 inches at all joints to fit in waterstop. Plus even though concrete is relatively non porous, if a slab is too thin the pressure could push the liquid through it. Or possibly the cover for the reinforcing is not adequate and you'll get corrosion. We use about 8 inches minimum with thickened edges at joints and walls. You might need more than 12 inches to support your walls.

 

[csd72]

There is a minimum thickness formula that I have seen. I think it it based on the concept of a theoretical soft spot and the required thickness for the slab to span over this.

The theory makes sense as you cannot 100% guarantee that the entire soil surface under the floor is of a uniform stable nature - Regardless of how well you spec it.

I will try and find the formula.

csd

 

[fa2070]

Thanks JedClampett and csd72 for the replies.

@ csd72: I will really appreciate if you can find the formula. Some corporate customers request us the calculations of the structures they purchase, and certainly we cannot say "Well, for the floor item, it is common practice to adopt a thickness of xxx inches..."

Having a formula to back us up will look more professional.

Thanks again.

 

[minorchord2000]

You can assume a soft zone over which the slab on grade must span. I usually use 8" minimum for this type of application and assume a sof zone span of 4 to 5 feet. Assume fixed ends and design the reinforcing to span that distance. It must support its own weight (DL) + the superimposed LL from the tanks.

If you have perimeter walls, the top of the slab near the walls will go into tension.