API 650 - overturning stability of foundation

[D_YU]

I am updating a spreadsheet for API 650 tank foundation design. I've posted a couple questions and thanks to those who have provided your answers.
Here comes the 3rd question.
With regard to foundation stability (E.6.2.3), API 650 requires to check the stability under slab moment against the resistance from TOTAL weight of the tank content (plus weights of tank, foundation and soil), whereas in determining anchorage (self-anchored or not) only the portion of content directly above the effective annular ring is considered. How can it be justified that the content in the middle of the tank contributes to the foundation overturning stability (suppose in case of ring wall foundation the content in the middle doesn't bear on foundation)?
Considering an extreme case, if the tank is NOT self-anchored, how to make sure a tiny foundation is able to hold down the tank?

 

[JStephen]

See the note in the commentary on that section. It sounds like the intent was that the tank also has to meet that criteria, not that that is the only criteria that it must meet.

 

[HTURKAK]

Good question.. Bu when the tank starts uplifting , at least the wt of the product on the effective width of the annular plate will contribute to resist to uplifting ..


Pls look commentary of E6.2.3


EC.6.2.3 Foundation

....A requirement for a mechanically-anchored tank stability check was added. This check assumes that the tank,
product and foundation behave as a rigid body and is over-turning about the toe (i.e., base of the tank). This is not the
actual behavior of the tank system but is a convenient model to use for checking the gross stability of the foundation.
See Figure EC.10. The required factor of safety is 2.0 for this model.



He is like a man building a house, who dug deep and laid the foundation on the rock. And when the flood arose, the stream beat vehemently against that house, and could not shake it, for it was founded on the rock..

Luke 6:48

 

[D_YU]

For mechanically anchored tanks, the weight of foundation and the fill directly on it (and uplift resistance of piles if applicable) should be able to hold the maximum force that could be developed in the anchors. I think API 650 should explicitly include this clause in the standard, in addition to (or replacing) the foundation stability requirement currently in the code.

 

[D_YU]

In the code it uses the entire weight of the product for stability check, but uses only weight on effective width of annular plate for tank self-anchorage check, and ignores any product weight in anchor (number and size) check. While I can understand these are for either ease of design or to be conservative, when it comes to foundation design, the inconsistency produces confusion. API should make it clear, as I replied to a moment ago.

 

[HTURKAK]

Regarding inconsistency , according to your interpretation, do you think that for the foundations without piles , the wt of foundation and soil on the footing SHALL resist to the uplift ?
Do you think that the tank foundation can experience uplift and there would not be contribution of liquid content to resist ?
In this case imo , you need to understand the behavior of the tank during EQ.

I will suggest you to look ,
- to the following doc. By Priestly , ( 38 yrs old but still fresh )
- BS EN 1998-4
- NZSEE: Seismic Design of Storage Tanks

https://bulletin.nzsee.org.nz/index.php/bnzsee/article/view/829

...

He is like a man building a house, who dug deep and laid the foundation on the rock. And when the flood arose, the stream beat vehemently against that house, and could not shake it, for it was founded on the rock..

Luke 6:48