Tank overturning calculation?

[SJEC]

I am working on a old tank built in 1975 and have problems on tank overturning calculation.

Before asking the question, I search the website and found following discussion.

http://www.eng-tips.com/viewthread.cfm?qid=168184&page=9

However, I still have some questions about it. I appreciate any advice.

1. Per latest standard API 650 API increase the wind uplift pressure from 15 lbs/ft^2 to 30 lbs/ft^2 since 2005.
Should I use the new standard or use the as-build standard?

2. The tanks is structure supported roof tank. During the calculation MDL . My understanding is we should account whole roof weight. However, the software e-tank only use the weight supported on the shell (particular weight supported by the structure), which one is correct?

Thanks

 

[JStephen]

The 15 psf pressure was for projected roof area, meaning area projected into a vertical plane. IE, it was a lateral load on the roof, not an uplift load, by my understanding. In that respect, the uplift pressure is an addition to the standard, not just an adjustment in value used. Note that along with this addition, you're also allowed to use part of the contents to resist overturning.

I guess which code should be used would depend on why you're analyzing it in the first place. But in general, structures are expected to meet the code they were built under, and you don't normally go back and try to retrofit and redesign every tank or building each time a new standard is issued.

Unless you have the original design calculations, there will always be some ambiguity in the design that will make it difficult to tell if it even met the original standard. For example, design wind speed may vary, or it may use wind pressures from UBC, etc. Seems like the older standards allowed you to just check the wind pressure and report to the owner instead of actually designing for a particular wind load.

On the roof weight issue, the answer depends. I think properly, if the wind uplift pressure exceeds the weight of the roof plate, then the entire roof plate plus the structure weight supported by the shell would be applicable. However, this is not how API defines the terms, so it depends on what you're trying to show with the calculations- stability or code compliance.

 

[SJEC]

Thanks for your advice JStephen.
Although most of answer is depended on personal experience.
I appreciate your advice.