I am designing a ring beam foundati 5

[Soldierson]

I am designing a ring beam foundation for a steel waste water storage tank, 32' diameter X 9.5' tall, about 56,000 gallons. The problem is it will be supported by about 10 feet of fill over bedrock. I dont want to pier a ring beam as this could cause problems with the tank floor and I dont want to use a piered mat foundation as it would mean a lot of expense. My question is how much settlement/differential settlement can a steel tank take? I have done some research and one guide indicated a deflection ratio (delta/length) of .008 which equates to about 3". Does this does seem reasonable?

Secondly, what is the best way to calculate the stresses in the ring beam for a given settlement?

Third, my elastic half space analysis doesnt seem to give realistic potential settlement for 10' of lean gravelly clay/clayey gravel fill (only about 0.2" for bearing of 600psf?). Even compacted fill could settle an inch or more. Any advice?
Thanks

 

[JoelTXCive]

My advice is to offload all that future liability onto a geotech.

For the tanks we do, I ask the mechanical/process engineers to give me a desired max settlement number.

Then, I just tell the geotechs we need that number; or we need recommended soil improvements to get us there.

 

[oldestguy]

What you seem to understand is the differential settlement should be small. That seems likely with your description of the planed work. Make that compacted fill area efficiently wide so there are no "edge weakness effects" on the tank.

 

[HTURKAK]

32' diameter X 9.5' tall is a small tank and the bed rock 10 feet below..

Consulting with geotech is the best option as JoelTXCive pointed out..

IMO, stripping the top soil+ compaction of the sub grade + provide 12 in. compacted gravel will be O.K. Provide ring pad foundation with dimensions in the range of h 16 in. and width could be 24 in .

You may look one of the corrugated steel tank supplier's typical foundation detail.

The codes are silent for the total allowable settlement and in general there are limits for plumbness ( 1/100 ).

 

[BigH]

Do not know where you are located but your ring foundation should be located below frost depth. That would likely be 3 to 4 ft in the norther USA and more as you go up north into Canada. That would reduce the thickness of the fill beneath beneath the tank foundation. I would presume that the tank itself would be at grade except, of course the rim would be on your beam. You might wish to camber the centre of the tank a bit to offset the settlement under the centre.

Also you mentioned nothing about the fill. Engineered fill? Common uncompacted fill? These are questions to consider. Another point is to use flexible connections of piping into the tank to the tank.

Get a geotech involved.

Now for my take on tank foundations and this goes against many engineers. Why do you need a concrete ring foundation? I had worked on many large steel storage tanks - refineries and the like in Canada. It was non-seismic zone and none of the tanks were founded on concrete ring foundations. All were founded directly on a granular pad extending about 10 to 15 feet beyond the edge of the tank. These tanks were up to 150 ft in diameter. We never had any problems. I knew Dr. Nitin Som in Calcutta (recently, sadly, he just passed) - a super good person and very well respected on soft soil engineering in India - he had done, too, many many tanks and none or very few had ring foundations.

A case history of years agao - Dames and Moore were involved with a tank in, if I remember correctly, Maryland. There was very poor ground and settlements were estimated to be about 24 inches. Of course, this was engineered design and construction. However, the tank settled seomthing like 36 inches. The tank was still fully serviceable but the client got nervous and removed the tank, drove many many piles and reset the tank. Sued Dames and Moore - but have no idea the outcome. The main thing on tanks, I think, is that they remain serviceable.

 

[1503-44]

The ring is used to distribute tank roof (esp for a non-floating roof scenario) and wall loads over a base area sized so that those equivalent bearing pressures are equal to the fluid pressure under the rest of the tank floor. Differential settlements are minimized when all static bearing pressure distributions are equal.

 

[Soldierson]

Thanks so much for the replies. All contain good information. Thanks for the feedback. I wish the geotech had been involved but this is a private sector project, no requirements were made of the geotech, maybe an oversight by others, but I will add a qualifying statement to the design. It looks more like a risk reduction task than a structural design!

 

[fel3]

Soldierson...

I have always designed my own ringwalls, but some engineers punt that to the tank fabricator's structural engineer. That's what several engineers at my last firm did, until I showed them how to design their own. I like to design my own because the ringwall size has some effect on site design.

You might want to consider this approach and include the requirement that the tank fabricator get a geotech investigation and report that covers the information needed for tank foundation design (some borings, allowable bearing pressure, differential settlement estimate, seismic parameters, and probably some things I am forgetting at the moment). I'm sure another civil or geotech here can recommend what specifically you need. Otherwise, check AWWA M42 or PIP STE03020 and see if these documents provide enough information.

Fred

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

 

[IFRs]

With proper fill inside the ringwall and hoop bars in the ringwall you should not have much differential settlement from the inside of the tank to the shell. As far as the tank settling unevely, that's not a big deal for the tank, especially one this short but it could be a problem for the piping, so perhaps look into that? However, I agree with BigH and am of the opinion that for this short a tank a concrete ringwall is not needed since the load per square foot is pretty small in the grand scheme of many soils. I'd spend my money on characterising the soils under the tank footprint and modifiying those as needed, then use a gravel pad, perhaps using API 650 Annex B as a guide (see below).

 

[Ron]

Ring walls are primarily for confinement of the interior soils and essentially create a sand column. The ringwall only needs temperature reinforcement as the lateral stresses are generally small. Since you are in Texas, I would not consider the frost depth to be significant, though check per BigH's recommendation.

 

[HTURKAK]

Dear Soldierson, the subject tank has small diameter and height.. and the tank for storage of sewage water or could be farm water distribution system for the swine..

I will suggest you to consult with one of the corrugated tank manufacturer..they have typical details.

The ringwall only needs temperature reinforcement as the lateral stresses are generally small[/b]. Since you are in Texas, I would not consider the frost depth to be significant, though check per BigH's recommendation.]

The hoop reinforcement for the ring wall is primary reinforcement and calculated for the active thrust loading of the confined soil and fill interior of the ring foundation...
The following doc. is useful for tank foundation design.

https://www.pip.org/docs/default-so...5a262f789edff00008ddc6a.pdf?sfvrsn=1880ca9e_0