Circular Concrete Tanks

[rholder98]

Are the charts in PCA's "Design of Circular Concrete Tanks Without Prestressing" valid for negative loading? That is, can you check the empty condition with soil (and liquid, if applicable) on the exterior by simply using a negative load? Is it as easy as a matter of sign convention? The book mentions the need to check all load conditions, but doesn't directly say that the charts work that way--unless, of course, I missed it. Thanks in advance.

 

[JStephen]

External pressure on a cylinder loads the shell in compression. I assume the charts in the book you reference are giving the reinforcing requirements, and those wouldn't be applicable for compressive loading. Those charts may also be based on crack control which wouldn't be an issue in compression.

Thin shells loaded externally will buckle well before any yielding takes place. I doubt this is the case with reinforced concrete walls with uniform loading, but it shouldn't be too hard to check. See Roark's formulas for stress and strain, for example, which includes eleastic buckling of short cylinders. I'm not sure what thickness or stiffness values you'd use for RC, but I wouldn't think it would be hard to show adequacy. Uneven loading around the tank might be more of a problem.

By way of comparison, you might note that the wire-wound concrete tanks are loaded externally by the wire, but still use thinner walls than reinforced concrete normally does.

If you had any significant water pressure on the underside of the floor, you could have problems from uplift.

 

[rholder98]

Never mind. . . . I read on, and found my answer in the design examples.

 

[Ussuri]

And was it a yes or a no???

 

[rholder98]

Oops, sorry. The answer is yes. Ring tensions become ring compressions, and moments are obviously reversed.