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# circular walls with earth fill inside

## circular walls with earth fill inside

(OP)
While designing the walls and foundations of circular steel tanks (@ ground level) how to take care of earth ptressure . Whether the walls are to be designed for hoop
tension due to earth pressure or as cantilevers ? I feel that upto certain diameter of wall the hoop tension predominates. After that the wall should behave as cantilevers.
I need some advice where this bifurcation occurs?

### RE: circular walls with earth fill inside

Let's say the contents of the steel tank it is cohesionless sand. Conservatively you can use then at rest earth pressure (if the radius is big enough). Otherwise some silo effect happens, and it depends on the radius and material to establish a push law.

Once you get the pressure law for thin shell normally a membrane would be assumed for hoop tension, and you can analyze it in FEM. For such thin shells either you let the bottom of the sheel slip or you will have plastifications/bucklings at the foundation insert. You see to let slip something that does not weighs much to contain something that does is almost impossible task, since the sheet risks to be lifted by bottom spill. So some degree of indeterminacy at the bottom is likely to happen, but better indeterminacy than ruin.

Thinking the thin sheet (even corrugated) shells taking by themselves cantilever moment I see anomalous, and maybe even as insets between standing column cantilevers.

### RE: circular walls with earth fill inside

With a circular tank the hoop stress is capable of taking the earth pressure for any diameter, as large as you want, so I don't see why you shouldn't count on it.
What changes with the type (cohesion) of filling is only how high the pressure is to be taken when calculating the hoop stress. If you consider the filling as a fluid you are of course on the safe side.
Bending stresses at wall to bottom attachment are normally not accounted for for steel tanks.
Moreover no FEM is required for such tanks: in the site below, under Vessels->API Std.650 you find a sheet for calculating wall thicknesses of fluid containing circular steel tanks.

prex
motori@xcalcsREMOVE.com
http://www.xcalcs.com
Online tools for structural design

### RE: circular walls with earth fill inside

I agree with PREX.  In looking at flatness, based on normal applications, I don't see how you could have a diameter large enough to discard the restraining hoop tension.  Moreover this is little more than work from Mechanics of Materials perhaps supplemented with Roark's Formulas for Stress and Strain.  Or the spreadsheets noted by PREX.

I would, for overall stability, look at the tank similarly to that used with retaining walls.  But even then the height of the tank would have to exceed that of the diameter by quite a bit.

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