Structural capacity of a leaning vertical tank

[shaneelliss]

I work at a plant where we have a round wood stave tank, 55' diameter, that has settled about 12" on one side, putting a bit of a tilt to the tank. I am wondering how I figure out at what point this becomes a safety issue. How do I determine what angle the tank can be sitting at without the tank collapsing to one side? The height of the tank is 15'.

 

[WARose]

I think it’s a situation where you have to just walk it through the design process: first, model the situation and then see what the loads are and then check it from top to bottom. I know that a bit vague but I don’t know enough specifics about your problem to go deeper.

 

[shaneelliss]

Thank you warose for taking time to respond, but you are right that your answer is a bit vague. I have never designed a tank, but other than the diameter, the height, and the tilt, what other specifics are required? The tank is filled with a heavy salt brine that is about 75-80 pcf. But I really don't want someone to tell me the answer, I would love it if someone could walk me through the process on how to determine where the tipping point is. I am feeling a quite lost on where to begin.

It seems for a standard tank, you would calculate the radial pressure on the tank based on the fluid pressure on the wall, but when it is tilted, I know instinctively that it could be a problem, but can't come up with a calculation process to figure out when it becomes a problem.

 

[Worldtraveller]

Not really knowing much about the actual design, I would think the supports are probably the most critical, and less the tank itself?

1. Figure out the change in CG (per degree or something to make it easier on yourself later if it tilts more) as a first step.
A simple free body should give the increased load on the side it's tilting to.
2. Check loads into your supports. I would suspect (again, depending on the actual arrangement) that there might be bending there that either increased or wasn't there before, especially if it's a truss structure.

After that, it becomes highly dependent on the structure: Crippling in flanges an issue? Column buckling when combined with the bending loads, etc.

 

[JStephen]

On a conventional steel tank, no problem. The minor tilt is similar to seismic loading (which there are standardized methods for working through all that).

The problem is the wooden/hoop construction. I would assume you'd develop some vertical shear between staves, but don't know of anyway to evaluate that.
Another consideration would be the bottom pulling loose from the shell. You can calculate uplift (probably none) by treating the tank shell like a big circular beam, and sigma=MC/I +/- P/A. Once again, the problem is, how much uplift is permissible? And then there's the condition of that connection, how od you evaluate that?

 

[WARose]

Yeah, I agree with WorldTraveller: your critical points are likely to be the foundation and the tank's anchorage to it. The thing about something like that is: it can snowball over time. Once it tilts to one side it just keeps getting worse. Depending on what your calculations show, you may want to recommend some sort of fix (like maybe micro piles; if the existing mat can take that sort of load locally).

 

[shaneelliss]

Thanks. I guess I should mention that the tank is sitting on a concrete mat foundation and not elevated on supports. So it is just the stability of the tank itself I am worried about right now. I am working on a solution to stabilize the settlement issue, but that is another topic.

 

[SlideRuleEra]

shaneelliss - A few more questions before you go further... is the top of the tank open? That is, as the tank tilts Will need the weight of the liquid spills out the top, reducing the volume of liquid as the tilt increases?

or, is the tank top sealed? Then the volume of liquid in the tank, no matter what it is, remains constant regardless of tilt?

Also, is the tank securely fastened to the slab? If it is, the tank / slab can be consider as a unit. Dimensions of the slab (all of them) will be needed if the connection is secure.

tank, too.

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[SlideRuleEra]

Sorry, my first sentence is garbled. Should read:

is the top of the tank open? That is, as the tank tilts will liquid spills out the top, reducing the volume of liquid as the tilt increases?

Last question got garbled, too. Should be:

Empty weight of the tank will be needed, too.

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[dhengr]

Shaneelliss:
I agree that the real issue is the conc. foundation mat and its settlement. This settlement has likely been going on gradually since the tank was first filled. Does the conc. mat showing any cracking or differential movement or deflection, or is it rotating as a rigid mat, in one solid piece? I’d set up a dozen points on the top of the conc. mat, near the tank shell, and shoot elevations on these points, and monitor them regularly. You want to see if you can get a feel for how it is moving and how quickly. My experience on these kinds of things has been that someone notices the problem, and all of a sudden it becomes a major issue. When, in fact, it has been going on for 40 years, and really caused no problems. The settlement should be brought under control, but it is not likely a failure mechanism tomorrow. I would set up a monitoring program while you really determine what to do. Do you see any relative longitudinal movement btwn. staves, at their joints? Do you see any relative movement btwn. the staves and the tension hoops, or in the hoop hardware? Do you see any distress in the hoops or the staves? Any new leakage in the tank? Is the tank shell (wall?) out of plumb by about the same amount as that mat appears to be sloping? The shell should be out of plumb by 3-4" in one direction, over its height. Is the conc. mat still a fairly consistent plane surface, despite its slope? Do you see any lifting (elevation change) in surrounding soils or slabs? You will probably want to get a GeoTech guy involved for their soil testing and input on a solution.

A 55' dia. wood stave brine tank, 15' high, on a rigid conc. mat, with brine at about 75 or 80 pcf, that’s 1.2 kips/sq.ft., plus. A 1' settlement on one side (a 2% slope, about 1̊) does not sound like an imminent failure situation to me. You would see cracking in the conc. mat; you would see relative longitudinal movement (a shearing movement, a racking of the slats w.r.t. each other) btwn. the staves.

Mini-piles, mud jacking, pressure grouting might all be fixes. You want to control the long term settlement of the soil under the tank loading. You probably won’t actually level the conc. mat and tank unless you can empty it.