Circular Concrete Tanks Expansion Issues

[MrEngineerUS]

I'm currently using the PCA manual to analyze the capacity of an existing tank per ACI 350. The client would like to expand the capacity of the current tank as much as is possible by adding height to the walls. My plan is to determine the actual capacity of the current tank and give them a reasonable estimate on how much higher they can make the walls.

One issue I'm curious about is the effect of introducing a horizontal joint at the height of the original tank. Typically, these tanks have bars at a fairly close spacing and the new bars would need to be drilled and grouted into the existing tank. Does anyone see this being an issue? I also expect that a waterstop would need to be installed at this location as well.

Are there other concerns that I should consider with a tank expansion of this type?

 

[JedClampett]

I'd worry about the hoop capacity of the circumferential steel at the top and near the bottom, plus the constructibility issues you mention. Note that you don't want to stress the bars more than about 20 ksi using working stress. I think you're going to find it doesn't cost out.
When they do this with steel tanks they lift the whole thing and add to the bottom.

 

[racookpe1978]

A concrete tank wall would be much more difficult to lift evenly without distortion or cracking (less forgiving of side stress, strain, bowing, etc. while lifting) than a steel tank wall.

How high is the present tank? Wall Th? Wall extra height?

I'd consider it, but think about the second wall (if floor and foundation are ok) as a second poured wall sitting on the first, but with a short inner section inside the lower wall and a sealing gasket/membrane between the two. Leakage or movement of the lower wall vs the upper wall would be stopped by the gasket, and the added circumferential reinforcement just mentioned can be accommodated in the thicker joint section.

 

[hokie66]

racookpe,
But with your solution, Jed's concern about the hoop capacity at the bottom of the existing tank would still be applicable. The ring tension is directly proportional to the depth of water.

 

[TehMightyEngineer]

I've seen some big concrete tanks fail after a long service life, the force of the water released was enough to pickup a shipping container, carry it a fair distance, and slam it into a building. Easily could have been fatal but thankfully happened when there was nobody around the tank when it let go. When these things fail they do not go quietly.

I've done plenty of clamshell tanks like you're considering but only in buried structures. I'd be very hesitant to do an above ground tank in the fashion you're describing with any joints. I wouldn't balk at it completely but I'd be very careful about pushing the envelope.

Professional and Structural Engineer (ME, NH)
American Concrete Industries

http://www.americanconcrete.com

 

[KootK]

Racooke's gasket method appeaks to me. It's hard to imagine how you'd get drill and epoxy bars a) close enough to the surface to control cracking and b) far enough from the surface to prevent spalling during install. If the hoop stress at the bottom of the tank gives you grief, maybe you could do something clever like external post tensioning.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[racookpe1978]

If the bottom of the existing wall becomes limiting, then easiest to add circumferential external tension cables wrapping around the current concrete. Just need to make sure the added external tension doesn't collapse the existing wall when the tank is empty. Older silos would have external cables 4-6 inches apart, increasing with height to 12- 18 inches apart.

Note! I haven't done the math, haven't seen the original poster's dimensions or reserve strength.

 

[MrEngineerUS]

Thanks for all the comments! To answer a few questions: it's a 29-ft diameter tank (Side question: This would be considered a "small diameter tank" per PCA, correct? Anyone know the cut-off diameter for "small tanks"?). The 10-inch thick walls are 14.5-ft tall with 8-ft being below the groundline. The client would like to add 5-ft to the wall height and the tank will be used to store sludge. There will be no top to the tank and the slab is 12-inches thick and integral with the walls at the base.

If the solution becomes much more complicated than simply adding wall height then the client will likely scrap the tank. My impression is they don't want to sink much money into the expansion. Lifting the tank would be out of the question. Tension cables may still be in play.

KootK, I was also concerned about epoxying two mats of steel in a 10-inch wall without sacrificing water-tightness (induced cracking and spalling). Another concern is I'm not sure the tank even satisfies the basic requirement per ACI 350 of 12-inch walls for heights greater than or equal to 10-ft. Granted that is for new tanks (or at least those designed after 2006) but would they essentially have to bring this tank "up to code"?

Thanks again!

 

[KootK]

Another concern is I'm not sure the tank even satisfies the basic requirement per ACI 350 of 12-inch walls for heights greater than or equal to 10-ft. Granted that is for new tanks (or at least those designed after 2006) but would they essentially have to bring this tank "up to code"?

I'm afraid that I can't help with that one. Hopefully someone more knowledgeable about the industry can enlighten us.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[BUGGAR]

Have you checked with manufacturers of fiberglass or carbon fiber for reinforcing the existing concrete?
There are (used to be) tank manufacturers that would wrap an existing concrete with prestressing strand then shoot gunnite, but I believe that will be too expensive.

Maybe not related, but if you're storing sludge, will you have to add containment? Is a new tank the better way to go?

 

[JedClampett]

My initial answer was in regards to a much larger tank and a taller addition. It's likely for a 29 foot diameter tank 14.5 feet tall, that the minimum reinforcing, whatever that is, is enough for a 5 ft. increase.
I'd add a steel tee and (5/16 in) plate bent to a 29 foot diameter, all welded together, to the top. Attach with epoxy or expansion anchors, probably stainless. I can think of several ways to seal it, none of which are perfect. One of which is to smash a gasket into the concrete at the concrete tee interface. Than caulk with a good quality (two part polyurethane) caulk in the space. Coat the interior with a good epoxy coating.
This should be pretty cheap and easy to install.

 

[MrEngineerUS]

Jed Clampett, I'm having difficulty visualizing your steel tee solution. Could you send a quick sketch of what your are thinking? I'm assuming this is to tie the added wall height to the original perimeter wall. If it's what I'm thinking it seems reasonable, in theory.

One other related question, does anyone know how exactly to use Table A-20, Design Aid for Bending Moment Reinforcing, in the PCA manual? I understand the instructions shown on the table but not sure what the column header is supposed to represent for each column (numbers ranging from .000 to .009) and why you would pick one column over another?

Thanks!

 

[TLHS]

I haven't really thought this through, but what if instead of pouring on top of the existing wall, you pour behind it. Add thickness to the lower wall to add strength, and then just keep going up.


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You can use some part of the strength in the existing wall, and you don't have the same type of construction joint. You have an exposed joint, but you can put something in there to block part of it out and then seal it properly.

 

[jayrod12]

I've never done tank design, by TLHS' idea was my first thought.

 

[MrEngineerUS]

Well, per my analysis. It's looking like the current tank is adequate in all respects except for tension. Not near the base but further up the wall where the hoop steel was reduced for the current design. In that area, with the wall height increase, the hoop steel becomes significantly overstressed. I've relayed some of the ideas on here as options to increase the capacity of the tank but I don't expect they will want to mess with the cost. Thanks for the help, all!