Slab-on-Grade for Equipment/Small Storage Tanks

[CleanWater]

Hi Everyone-

I'm designing a concrete slab-on-grade for an upcoming project where we will support several pieces of equipment and small (under 20K gallons) tanks with many different support configurations. The project will be active for about a year, at which point we'll demob the equipment, tanks, etc. Because the project is temporary, we want to make the design and construction as simple as possible, and are looking at a plain concrete slab (with steel for crack control only at this point). I'm using the ACI/PCA/Army Corps guidance documents for slab on grade thickness design, shrinkage/temperature steel, etc. I will likely thicken the edges at a minimum to protect against erosion at the edge of the slab in addition to providing capacity at the edges (if necessary). We will also add a layer of good sub base under the slab, probably 6-12 inches of dense graded crushed stone.

I have a couple of questions and any clarification or advice would be greatly appreciated:

1) At what distance can a post load be considered an interior load vs. an edge loading condition?

2) Several of the tanks are supported by steel channels or beams near the ends of each tank. They measure approximately 128"X8". Can that be considered a post load (if using the PCA approach) and simply use the 1,024 in2 as the bearing area? If so the charts are out. Is there a better approach to that loading condition?

3) Are the loads used in guidance documents factored loads?

Thanks!

CW

 

[SlideRuleEra]

CW - IMHO, you are on the right track to design the slab to "make the design and construction as simple as possible". However, I believe that your approach strays from that goal:

I totally agree that a good sub base is important, in fact probably more important than the concrete design. Don't skimp on that, 12" (or more) sounds really good.

You mention using plain concrete but then having rebar for crack control. If you have rebar, then the slab is no longer plain concrete. Consider using fibers, instead of temperature steel, for crack control. With a top-notch sub base and suitable joints you probably really can use true plain concrete - no rebar, no fibers. You did not mention if the slab will be demolished after 1 year, but if it is, not having rebar in it will be a BIG advantage.

Unless erosion is a really big problem, I suggest eliminating the thickened edges. This complicates (extra expense) the installation and grading of the sub base and forming of the slab. Also, unless extra deep excavation is made, the all-important sub grade will be thinner under the thickened areas.

Keeping the slab at a constant thickness will make the whole design & construction process much "cleaner". and predictable. Ok, you may have to use a little thicker concrete, but increasing slab thickness (a little) is usually cost effective. The only real "extra" cost is the concrete itself - no change to the sub grade, little, if any, additional forming, minimal extra labor to handle the added concrete, and no change in the concrete finishing.

Whatever you decide, be sure to wet-cure the concrete (REALLY wet cure it, not just in name only) for 7 days.

Concerning your other questions: One arbitrary way to decide if a post loading is internal or edge is to look at punching shear. If the punching shear critical perimeter is a "reasonable" distance from the edge of the slab... that's an interior post. The definition of "reasonable" is open to debate, perhaps:
reasonable = slab thickness

On you other question, is a contact area 128" x 8" a post? No, not to me. Forget the charts, I really don't think they would add much accuracy to your calculations anyway. You don't really know and can not determine how the slab will be loaded anyway - that will depend on field conditions during the slab's use. Make assumptions for the heaviest loads, be sure to consider that there could be impact when equipment is moved around. Then designing a conservative slab (with a constant thickness, plain concrete slab, with an excellent sub base) should be straightforward.

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[TLHS]

For a loading like that, I'd probably just take the bearing pressure, check it against concrete bearing, project it down through the concrete and subgrade to the ground, check the bearing pressure there. If it's a small pressure, I'd call it a day and then just pick a reasonable slab thickness and subgrade. Honestly, your loads are probably small and the slab thickness isn't going to be all that important compared to the workmanship and subgrade.

If I wasn't happy with the pressures, then it's time to start playing with the thickness to make myself more comfortable, or reinforce and treat it as a beam spreading load onto the ground.

 

[CleanWater]

SlideRuleEra, TLHS,

Thanks for your responses and advice. I will definitely make sure the site preparation is the top priority. Also thanks for mentioning the fibers for crack control. I used the term "plain" to mean concrete with only temperature and shrinkage steel (probably a bad habit). I looked into steel fibers and I think I may go in that direction. It will definitely save time and eliminate any worries that the mesh actually ends up where it should.

The load of each piece of equipment is pretty well distributed with skids and the highest load is about 500 psf (small flat bottom water tank), so the slab will not need to be very thick at all, we'll go with 6 inches (we don't go any thinner than 6" on a construction site). We have much larger equipment that weighs much more overall (gross weight), but it is spread out even more.

Thanks for your input, it was very helpful.

CW