Waster Water Concrete Bottom Slab Design

[Berniedog]

I need help. I have been asked to perform a preliminary design for a concrete slab design for a waste water tank. The sides are steel (CST Storage tank). I know ACI 350 gives the shrinkage and temperature steel requirements for the slab but how do you determine how thick the slab should be? The tank is 61' in diameter and has 19.26 of head. Allowable soil bearing pressure is 3500 psf.

Thanks

Berniedog

 

[JedClampett]

I've done a lot of wastewater tanks. First of all, if you don't define an abbreviation (CST?), you're going to lose us.
Steel tanks in wastewater are kind of rare. Coating them is a major headache.
Next, does the tank have a steel bottom? Or is it attached to a steel angle, bent to the diameter of the tank? They're completely differnt designs and details.

 

[Berniedog]

Jed:

CST is a manufacturer of steel storage tanks (

http://www.cstindustries.com/Applications/Above-Ground-Storage/).

There is no steel plate on the bottom. Along the outside ring they embed a steel plate with seal strips to make everything watertight.

THanks

 

[JedClampett]

Outside of the embedded ring, the concrete slab acts basically as a membrane. There's liquid over it, soil underneath it and it just pasees the load through. We use a lot of 8 inch slabs, with #5's at 12 inches for that kind of design. The sections will need radial construction joints (CJ), probably four. Thicken the concrete at the CJ'a so that waterstop can be installed. Note that the bar layout gets tricky in a round tank.
At the ring, I would make sure the slab can carry the weight of the water plus the tank above it. To fit everything in, you'll need at least 12 inches. Add circumferentail reinforcing at the ring to take the hoop stresses, even though the embedment detail could probably handle them on it's own.

 

[Berniedog]

Jed:

You would not place the #5's @ 12" o/c. in a radial type layout would you?

 

[JedClampett]

Ah, the eternal question.
You can place them orthoganally (#5's @ 12 inches EW). But that makes the laps at the CJ's a pain. Or you can place them radially and circumferentially. The circumferential bars are easy. But the radial bars need to have their number and length adjusted to keep the spacing reasonable. Or you can chicken out and just define a maximum and minimum spacing.

 

[Berniedog]

Jed:

Thanks. I am a chicken.