Slender Underground Storage Tank (Wet Well for Lift Station) 2

[FootNMouth]

I am designing an 8'Ø x 26' deep x 10" thick wet well. I have studied the PCA publication "Circular Concrete Tanks Without Prestressing" however the H^2/Dt values provided in the charts don't come close to meeting my calculated value (89.3).

Is there another resource or is my only option to use FEM?

 

[SlideRuleEra]

Are you calculating external pressure of soil and possibly ground water on an empty tank?
Or, internal pressure of the liquid in a full tank?
Or both cases?

http://www.SlideRuleEra.net

http://www.VacuumTubeEra.net

 

[thaidavid40]

Not many folks in these parts (North Alabama) use cast-in-place wet wells like this. Most everyone uses precast/prestressed wet wells, up to about 12' in diameter. Only larger sizes or odd configurations end up cast-in-place.
Dave

Thaidavid

 

[fel3]

We design quite a few sewer and storm drainage lift stations, mostly using the wet well-only configuration. I have two such sewer lift stations in design right now and a third just bid earlier this week.

Our most common configuration uses a wet well consisting of 96-inch-diameter ASTM C76 reinforced concrete pipe stacked vertically, which is the largest that Rinker makes locally. We sometimes design smaller lift stations. For 72-inch and 84-inch, we use the C76 RCP. For smaller nuisance water lift stations, we may only need a 48-inch wet well, and for those we use regular precast manhole sections.

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"Is it the only lesson of history that mankind is unteachable?"
--Winston S. Churchill

 

[JedClampett]

If you're out of range using the PCA document, just check it using the hoop stresses. All the PCA document does is factor in the effect of the bottom supporting slab.
It's a good learning exercise, but I suspect that a contractor will ask to use precast manhole or pipe sections.

 

[FootNMouth]

The AHJ does not allow RCP for wet wells over 6'Ø.

I have spoken with a few contractors in the area who have done it this way albeit for much larger tanks. They cast the sections in a starter pit and then sink them by excavating out the interior. This is still referred to as the caisson method.

The load conditions are as follow ( I have also attached a diagram illustrating these conditions):
1) Hang up forces when sinking (AHJ requires longitudinal reinforcement in the wall to be designed for up to 1/2 the weight of the walls)
2) caisson wall to full depth with no bottom slab and interior soil excavated. This will induce the larges ring compression forces.
3) empty tank with bottom slab in place full external soil pressure (will induce ring compression forces and bending moments)
4) empty tank with bottom and top slab in place, full external soil pressure
5) full tank with top and bottom slab in place neglecting external soil pressure (will induce ring tension forces and bending moments)

Jed: I agree that I can just use the hoop stresses assuming a free top and base however how would I calculate the moments to determine the required vertical reinforcement?

 

[JStephen]

Roark's Formulas for Stress and Strain includes shell equations for a shell with end moments, etc. I don't recall if they have varying pressure with depth or not- if not, assuming uniform internal pressure would probably be close enough. I would think minimum rebar areas would control rather than any strength requirement.