Slab on Grade for Tank Foundation 1

[buckrub]

I have a client who wishes to set a 8,500 gallon diesel fuel storage tank on top of a slab on grade. The tank is saddle supported, with two saddles at each end of the tank, 8" wide by 8.5 ft long. The tank is 26' long. What method should be followed to design the reinforcing steel for the slab on grade? It is a double wall tank, so no perimeter secondary containment is required. I do not have design software to analyze this, and was curious if there is a simple method to ensure I don't undersize or oversize the steel. I've looked at designing this as a one way slab, but due to the weight of the tank and fuel (tank is 21,500 lbs empty) I'm looking at some absurd steel. Pad will be 9" thick due to pavement grading around it.

Also, since it is a slab on grade, would the primary concern be flexural or shear? I've seen some slabs designed by others and the flexural steel seems light in the bottom of the slab, and heavy it the top of the slab. Tank MFG is of no help. Any link to design procedures would be helpful also.

 

[spats]

I assume you want a slab foundation for some reason, or you could just put a rectangular footing under each saddle.

You're almost correct saying it's a one-way slab/beam, but it's really a two way design like a spread footing; it's just that largest bending stresses occur in the long direction. Your problem with "absurd steel" is because you're trying to make it only 9" thick; that's not enough. I assume you are aware that there is a maximum amount of steel you can put in a singly reinforced concrete member (you don't count compression steel unless it's laterally tied). The max should be 0.75 times rho balanced, so you don't have a concrete compression failure. You'll need top steel as well because of the upward bending between saddles. You also need to check plain concrete shear (no stirrups), but it shouldn't control if you get the slab thick enough so that the steel is reasonable.

 

[gwynn]

If the saddles are near the end of the slab, you can expect to see much higher amounts of steel in the top of the slab than in the bottom, as bearing pressures will cause higher negative moments in the middle of the slab than there will be positve under the saddles.

As Spats said, I would expect to see a thicker slab here. If depth possible is limitted, you could also try extending the slab well past the saddles. Placing the saddles at the fifth points of the slab length will give you a significant reduction in the steel required due to reduced moments.