Slab on Grade Design 1

[kunz82414]

I am civil/environmental engineer, but need to design a slab on grade for a large steel tank.

I expect 2122psf and just need to get started with calcs to back up my slab thickness and possible inclusion of steel. Either rebar or Welded Wire material. I plan to use 3000 psi concrete.

I found a rule of thumb table saying about a 9" slab, but I would prefer to use some calculations to back this up.

Thank you, any help would be appreciated.

 

[BigH]

You indicated it was a large steel tank. How big? Elevated? or flush plate on ground? If the latter, I am not sure why you want to put it on a concrete slab anyway . . .

 

[Ron]

I agree with BigH...if flat bottom steel tank, why put it on a slab unless perhaps you need some type of environmental containment. Otherwise a ring wall would work fine.

On to your question, assuming it is a flat bottom steel tank...

Shear at the perimeter of the tank will likely control the thickness design. You will also need to check bending in the center of the area, which can be done by a strip method. I assume by your post that you are not using structural analysis software, so just keep it simple with the analysis. Use ACI 318 (or its equivalent in your area) for load factoring; however, your reinforcement will be minimal.

If the tank is up on legs, then each point load can be handled as if it were a pavement loading, using typical pavement design techniques. Check punching shear at the legs as well.

 

[BigH]

OOOHHH Ron - why a concrete ring wall?? I'm not sure I'll ever be convinced it is an absolute necessity. We put far too many tanks on 1.5 m of crushed stone tank pad . . .

 

[Ron]

We typically use a ring wall because of erosion issues. Other than that, a crushed stone base is fine.

 

[kunz82414]

Thanks for all the posts, this is helpful. I was just told that this tank has no floor and has to seal to a concrete slab with anchor bolts. This seems very strange and hard to believe.

It is 32' diamter Bentonite Slurry tank. I don't think containment is much of an issue as is the anchor bolts and it being put on a level slab. The embedment for the 24 anchor bolts is 10" so the perimeter of the tank has to be on at least 10 inches of concrete. The manufacturer's drawing shows the tank setting on a 1/2" layer of fiberboard between it and the concrete founation. This just seems overly complicated to me.

 

[Ron]

If your anchor bolt embedment is 10", you need a thickness greater than 10". Also, the 1/2" fiberboard is not a great idea. It will deteriorate. Specify a very flat slab and use a butyl rubber or asphalt gasket. Ram-Nek is one that combines butyl rubber and asphalt in a pre-formed strip that can be used for this.

 

[kunz82414]

OK, thanks for all the help. I am on a remote location and really wish I had my concrete book right now. I have been getting one concrete question after another and it is not something I deal with alot.

The latest one is that we have a 30,000 lb forklift that will be moving pallets on and off a concrete slab for the next 3 years. I need to make a cheap pad, but want to do some calcs to back up my pad thickness. That works out to a live load of about 110 psi. Any ideas, standards to reference? I don't think a factor of safety is a bigg issue with this.

Thanks

 

[Ron]

That becomes a pavement analysis problem. Download a copy of ELSYM5, a public domain elastic layer analysis program. It's an old DOS program, but it works very well and is as accurate as anything out there today. If you can't find that, download EverFE or EverStress, both free from the Washington Department of Transportation.

Analyze your pavement section and compute the lateral stress at the bottom of the concrete layer. If that lateral stress is below 50 percent of your modulus of rupture (flexural strength of the concrete) then you're fine. Adjust the thickness until you get to that point.

For most concrete that is not specifically designed for flexural strength, the modulus of rupture will be 10 to 15 percent of the compressive strength.