Storage Tank - Foundation Recommendations 2

[matrixeric]

Hello All:

I am a structural engineer and have been recently assigned with the job of designing the foundation for a water storage tank (26' dia x 35' high).

From my online/literature research I see that the options for the tank foundation are either a crushed stone ringwall or a concrete ring wall or a reinforced concrete slab.

A question I have to my fellow engineers is: Who makes the call or recommends the type of foundation suitable for the tank? Is it the Geotechnical Engineer or the Structural Engineer?

My hunch is that that the Geotechnical Engineer must advise on the type of foundation (one of the above mentioned types)and shoud not just state in his/her report that a shallow footing is recommended for the structure at the site.

Thank you for your comments/advise/opinion.

 

[JedClampett]

Usually the Geotechnical Engineer gives soil capacity and recommendations for different types of foundations (mat foundation, driven piles, etc.) and the structural engineer picks from the choices. It's hard to get a Geotechnical Engineer, who's just tasked with doing a report, to give specific direction on the type of foundation.

 

[Geo23]

The geotechnical engineer gives the recommendations for foundation types and/or ground improvement (if required) depending on the results of their investigation which usually consists of soil borings (SPT's).

 

[weab]

Matrixeric,

Be aware that posting questions to multiple forums is frowned upon. It seems that you are new, so this is understandable.

 

[BigH]

You forgot the alternative of not using a concrete or granular ringwall but using a granular pad foundation - usually about 10 ft or so wider than the tank - typically in the order of 5 ft or so thick with a thin crusher pad. That's the way I've seen them designed; most seem to think ring walls (API, I gather) - we are all prisoners of our own experience.

 

[matrixeric]

Thank you all for the comments ...
My apologies for posting it on two forums as I wasn't aware of the etiquette and this will not happen again ... all i was trying is to get the opinion of both structural and geotechnical engineers ...Thank you Weab for bringing it to my attention.

 

[fel3]

I'm a civil engineer in California. While I don't practice structural engineering, I have a modest background in the subject and I have designed a fair number of water tanks.

Since I am in a seismically active area, I consider your tank to be very tall for its diameter. We usually shoot for the diameter to be at least 2x the height (my tanks have ranged from 1.3x to over 4.0x). I know that in other areas tall cylinders are used, but around here its not common.

If you're in a seismically active area this tank will be subject to overturning and I'm pretty sure it will need to be anchored (in Zone 4 it definitely would need to be anchored). The only tank I have anchored is the 1.3x aspect ratio tank, and it was anchored to a ringwall, which in turn was anchored to the rock beneath.

Another thing to consider in seismic areas is sloshing of the water. That enters into determining the required freeboard (distance between maximum water level and underside of the roof structure).



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

 

[chicopee]

Check this site

http://civildesignhelp.info/;

foundation details about several types of equipment however look at the PDF drawing details on tanks.

 

[jrm73]

There are several ways to support large above grade tanks. It has been my experience that the tank builders will want to elevate the tank shell by a foot or so above the ground surface. To accomplish, I recommend concrete ring walls. In this case, the tank bottom sits on the ground surface unless you need to remove upper weak soil and replace with stronger, compacted fill and the tank structure sits on a concrete "footing". The ringwall is sized so that the tank bottom and the ring wall have about the same bearing pressure. Of course, you have to make sure the soil can support the pressures.

 

[irawanfirmansyah]

Large tanks normally founded on gravel layer above competent ground surface (you may need to remove the upper soft soil,and replace it with gravel, if necessary). Ring wall is required in order to provide confinement to the gravel, in such that the gravel can support the tank bearing pressure.

 

[BigH]

Irawanfirmansyah - sorry but I disagree. If you are on a gravel pad (full pad under the tank extending some 2 to 4 m beyond the edge of the tank, it is not imperative that a ring wall is required . . . We have put 30 ft high tanks (large diameter) on soils with undrained shear strengths of 50 kPa before.

 

[irawanfirmansyah]

BigH,
I agree that certain meter of extention of gravel beyond the tank edge can replace the ring wall. I believe the extension is not only for distributing the stress induced by the tank in such that it meets the original soil bearing pressure, but also to provide confinement to the stressed gravel.
Regardless of the above, ring wall has an advantage that it can be specifically designed to provide inward radial pressure for balancing certain tank load.