Guidelines for Selecting the Freeboard in Ground Storage Tanks 4

[WissamH]

Hi,

Are there any guidelines or design specifications to follow when selecting freeboard distance in potable water ground storage tanks?

Thank you

 

[LittleInch]

Not really.

Enough so it doesn't slop over the edge when the wind blows.

So 300mm?
Maybe 200 for a small tank.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[WissamH]

Dear LittleInch, thank you for your answer. The tank is actually not subject to wind action because it is covered with a concrete slab.

 

[LittleInch]

Make it 25mm then.

Or whatever height you need to flow enough water down your overflow without it coming over the top.

Totally up to you.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[WissamH]

Thank you LittleInch.

 

[JStephen]

The only specific guideline I'm aware of is to allow room for seismic sloshing without damaging the roof. This can be several feet in high-seismic areas, or negligible amount in a lot of low-seismic areas.
Take into account tolerances in level measurements, levelness of the tank, how long it takes controls/valve to activate, flow rates, etc.
Also consider the consequences if it overflows- maybe no big deal, maybe major hassle.

 

[WissamH]

Thank you for your kind response JStephen.

 

[fel3]

AWWA D100 (welded steel water storage tanks) and AWWA D103 (bolted steel water storage tanks) include a procedure for estimating the sloshing wave height and provide criteria for determining the required freeboard.

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

 

[LittleInch]

It turns out this is a concrete? rectangular tank 21m x 30m x 3m deep buried in the ground by the sounds of it....

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[WissamH]

Dear fel3, the storage tanks I am working on are reinforced concrete tanks. I would appreciate it if you could refer me to any AWWA guidelines for concrete storage tanks.

 

[WissamH]

LittleInch, thank you for your help posting the tank dimensions and material type.

 

[fel3]

WissamH....

My apologies. I had missed your statement about the concrete cover slab.

AWWA has two concrete tank standards:
AWWA D110-13(R18), "Wire- and Strand-Wound, Circular Prestressed Concrete Water Tanks"
AWWA D115-20, "Tendon-Prestressed Concrete Water Tanks"

D100 is obviously for circular tanks and it covers the tanks offered by DN Tanks (and possibly by others, but I don't know any other vendor than DN Tanks). D115 doesn't have "circular" in its title and the abstract for the current version doesn't mention shapes. The 1995 version was only for circular tanks, but the 2006 version explains that it was expanded to also cover other shapes (see

https://engage.awwa.org/PersonifyEbusiness/Bookstore/Product-Details/productId/18999).

So, if you have a rectangular prestressed tank, you want D115.

Unfortunately, I have never used either standard and I don't have ready access to them. All of my water storage tank projects used steel tanks, mostly welded with a few bolted. However, I was able to read the Tables of Contents for both standards on the AWWA website and found that both standards include a section on minimum freeboard. I don't know what those sections look like, but I suspect that they are similar to if not identical to the sections in the D100 and D103. I have both D100 and D103, and their treatment of the sloshing wave height and freeboard calculations are identical (except for one typo in D100).

AWWA does not appear to have a standard for conventionally reinforced concrete tanks. So, for a circular tank, the design could be based on the Portland Cement Association publication "Circular Concrete Tanks Without Prestressing (

http://members.cement.org/EBiz55/ProductCatalog/Product.aspx?ID=345).

I have used this publication for the structural design of several circular clarifiers for wastewater treatment plants, but that was quite a few years ago.

For a rectangular tank, you might consider PCA's "Rectangular Concrete Tanks" (

http://members.cement.org/EBiz55/ProductCatalog/Product.aspx?ID=2104).

However, I recall reading on this website years ago a post by one of the long-time structural engineers that the earlier version of this publication has errors in the moment coefficient tables and he recommended using "Moments and Reactions for Rectangular Plates" by the US Bureau of Reclamation (

https://www.usbr.gov/tsc/techreferences/hydraulics_lab/pubs/EM/EM27.pdf).

I have both the old and current PCA documents and the USBR document, but I haven't compared them in any detail.

One other PCA document that you may want to see is "Design of Liquid-Containing Concrete Structures for Earthquake Forces." The PCA website is showing it as not available, but a search for the title shows many possible sources. I have this publication, but I have never used it. It has a short section on freeboard based on 1994 to 2000 building codes, so that part (at least) is out of date.

I hope this helps. There are almost certainly other resources that I haven't listed, but this should get you going.



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

 

[WissamH]

Dear fel3,

I sincerely appreciate the time you dedicated to providing me with such a thorough response. Your insights have proven to be immensely valuable and have greatly aided me in my current situation. Thank you for your invaluable assistance.

Warm wishes,

 

[JStephen]

See 15.7.6.1.2 in ASCE 7-16, which includes rectangular tanks.

 

[WissamH]

Hi JStephen, Thank you so much. This looks to have the answer. Unfortunately, I don't have access to the document.

 

[fel3]

WissamH…

Here is the text that JStephen referred to, but from ASCE 7-22. This is the same general procedure that AWWA D100 and D103 uses.

Fred

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

 

[WissamH]

Dear fel3,
Thank you so much for sharing the document. It is super useful.