Over Head Water Tank 3

[WiQi]

Hi
I have to design a OHWT of capacity 60,000 gal (imperial). shape is octagonal on six RC columns height of the base of tank is 60',dia at 60 Elev. 23',dia at 60+5 Elev. is 37' and with this dia it goes straight up for 9',top is coned with an height of 9'. soil BC is 2tsf.how to start the design, what forces/factors will be most critical.

Regards

 

[racookpe1978]

The most critical part of your design is safely holding 60,000 gals of water 60 feet high up in the air above people and parts and things for long periods of time.

you've started with a few values - that's good enough for preliminary structural estimating.
But it is not enough.
Wind (adds loads and forces), seismic (adds loads and moments and inertia), corrosion (reduces metal thickness against all forces), sag & overturning moments (as foundations fail, eliminates symetry, increases point loads) -> Get a basic design, look up your local requirements, then apply the local additions to the "theoretically perfect" structure analysis.

 

[JAE]

The most crucial part of this design is having a qualified engineer design it. Sounds like you need a lot of help and information on how to proceed.
I would suggest that this should come from a more experienced engineer - do you have one in your company?
Not sure we can do that sufficiently on Eng-Tips.



Check out Eng-Tips Forum's Policies here:
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[EngineeringEric]

Would one account for a slight differential settlement? In example, a 1/4" of deflection at the outside edge of the base foundation would result in a large amount of side sway and therefore a leaning Tower of Pisa loading condition with no counter weight.

 

[CELinOttawa]

My understanding is that these structures often have legs-a-plenty in order to be able to sustain the full loss of bearing under a foot and still stand without issues.

I had seen a code for building them once; I'm going to have to put my thinking cap on...

 

[WiQi]

Thanx All of you for responding

@JAE: i have seniors with me at my company. here i posted it to have second opinion, to have latest issues/research/problems that might encounter on the way.Also opening this question may benefit other freshers.

in my company practice is to use SAP2000 , any other software which can be used or should be used to have more realistic results.

Regards

 

[CELinOttawa]

AWWA D100.

 

[WiQi]

AWWA D100 is for steel tanks

 

[darthsoilsguy2]

Why the limit to octaganol? must it be concrete? if steel is on the table..... sharp edges are very inefficient for weld configuration capacity and base material geometry. There is a whole world of proprietary design/build steel tanks out there with market pricing, years of operational performance in multiple climates and wind zones, and optimized constructibility. Why not define the operational and dimensional constraints and let the fabricator design it? That will get you the best price with the best specialized people on it. You will still need tank-experienced Structural, Geotech, and someone who can hold their own with the coatings on-board.

I'll throw out some America-based companies to google.... and this is an incomplete list.....
Phoenix (formerly Universal), Caldwell Tank, Pittsburg Tank, i know there's more

Here is an example of a 60,000 tank.

http://www.phoenixtank.com/wp-content/uploads/2013/02/60-MG-MC.pdf

 

[CELinOttawa]

Apologies; I had thought you were talking steel tank on concrete columns.

Frankly concrete tanks tend to have very specialised requirements when they get exceptionally large. Also, why elevate with concrete? Seems crazy to choose a brittle material that comes with significant dead load. I can understand using RC columns, but not the tank. If you are designing a steel tank on RC columns, the majority of AWWA D100 will still apply.

 

[WiQi]

thanx @darthsoilsguy2 and @CELinOttawa ,thanx, no apologies sir

in our area most of housing societies has the practice of concrete OHWT so that its concrete.

i will consider the option of steel tank on RC columns.

 

[minorchord2000]

Please do not forget the forces due to seismic sloshing. These forces may be substantial.

 

[WiQi]

thanx @minochord how to apply these forces in SAP2000.

 

[CELinOttawa]

Seismic sloshing forces result in a distributed load on the wall of the tank which cycles from side to side. You can use an equivalent side load (just like we use an equivalent base shear when designing a building's lateral load system).

Figure out the the worst case force from sloshing, apply it to the inside surface of your tank and work through the load path all the way to the foundation. Remember that the classic load case of dead plus eq plus reduced live should be full live load as it is a storage application.

 

[JStephen]

For steel elevated tanks per AWWA D100, seismic forces are considered, but seismic sloshing is not. (For ground storage tanks, sloshing is considered, but the effect is a net reduction in the seismic loading.)

Construction and design of elevated steel tanks is a fairly specialized business. If this tank is being built in an area where elevated concrete tanks are the norm, I would expect the same to hold true for them, and it would be prudent to work with designers and constructors of similar tanks.

Serviceability and leakproofing can be major issues, more so with an elevated tank where everyone can see it.