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RE: Foundation for large tanks

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Hello Rajendran,
It depends on the resulting stresses in the bottom and the shell and on economics.  One case I wrote about in STRUCTURE magazine had over three feet of differential settlement between center and ring in some 150 foot tanks.   We coned it up in the middle so that after settlement it still would have slope from center to edge.  However we had to analyze the stresses in the bottom plates and their welds as it moved down.  Some of the stresses changed sign as settlement progressed. This was a little risky but the saving over going to pile support was enormous. 
 In general the more cohesive the soil the more the center goes down relative to the edge.  In purely non-cohesive soil the edge can go down more so you have to have a geotechnical analysis and size the ring wall accordingly.
For total settlement it is usually also an economic trade off.  The more foundation work the less you have to design for movement in the piping connections, draw-off, etc.
For side to side differential it depends on whether you have a floating roof or fixed and I believe that API has that criteria but I do not recall where it is at the moment.  I believe that you have to prepare the foundation to limit that to a very small amount or they have to go back in and relevel the tank. That is why an accurate geotechnical report is important.
Richard Hess, S.E.
-----Original Message-----
From: Padmanabhan Rajendran [mailto:rakamaka(--nospam--at)yahoo.com]
Sent: Monday, October 22, 2007 2:40 PM
To: seaint(--nospam--at)seaint.org
Subject: Foundation for large tanks

A few of the list members serve the oil/gas industry. Large storage tanks, say 150' diameter, are commonly used to store products. Among other things, the design of the tank foundations, depends on the level of tank settlement.

Typically, three types of tank settlement are addressed. Total settlement, differential settlement between the tank center and the circumference and the differential settlement along the circumference. Maximum settlement occurs at the center of the tank.

Problems can arise in the wall or bottom plate of the tank or at the nozzle connections if the settlement is "large". In floating roof tanks, the operation of the roof itself may be affected if the settlement issue is not adequately addressed in the design stage. So, what is a "large" settlement?

I have not come across any prescription for limiting any of the above three in the API codes, which commonly govern tank design for the gas/oil industry. Nevertheless, common sense suggests there must be threshold values to each of the above parameter for settlements.

What has been the experience of the list members? What performance criteria should be included in a typical geotechnical specification associated with developing a foundation design?

Thanks.

Rajendran

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