Intermediate windgirders required for cone roof tanks?

[IFRs]

API 650 11th edition (and previous editions):

5.9 TOP AND INTERMEDIATE STIFFENING RINGS

5.9.1 General
An open-top tank shall be provided with stiffening rings to maintain roundness when the tank is subjected to wind loads.

Question: Where does API require wind girders for closed-top (cone roof) tanks?

The only reference I can find is in Note 2 on page 5-58 which says "This formula is intended to cover tanks with either open tops or closed tops and is based on the following factors...", which implies that cone roof tanks should be evaluated for wind induced shell buckling. Howver, this is digging pretty deep and could easily be overlooked since it is buried deep within 5.9.6

 

[JoeTank]

IFR,
The Code criteria for the top stiffener of an open top tank is intended to minimize distortions or flexing under wind loads. The Code design criteria for intermediate stiffeners is somewhat different in that it is also based upon spacing requirements to reduce the potential for shell buckling between the stiffeners. As to whether the rules for intermediate stiffeners are for both cone roof and open top tanks, the answer is yes. The footnotes that explain the design basis even mentions that the a 5 psf external pressure allowance is included to account for drag (suction) forces for an open top tank or for the typical vacuum set pressures for vacuum relief devices use on fixed roof tanks. I have never understood that the rules should not be applied to cone or dome roof tanks.

Joe Tank

 

[IFRs]

Mr Tank - I never understood that the rules should not apply to cone roof tanks but then again I'm always open to learning new things and people keep asking me to help them read the code. A person reading API 650 would easily find that intermediate siffening rings apply to open top tanks. However it is not as obvious that closed top tanks require the same analysis. Usually API is clear on what is intended and deliberately silent when not intending to include a topic. In this case open-top tanks are specifically referred to but closed-top tanks are not mentioned except in obscure, somewhat sideways references. I have always included intermediate stiffeners in closed-top tank designs, yet I can't quite quote API to justify the expense. Thanks for your confirmation that I have not been too far off base all these years!

 

[IFRs]

Another way to ask the question is:

Why is section 5.9.1 written as it is if all tanks are intended to be evaluated the same way? Why single out open-top tanks?

Presumably because open-top tanks are special.

 

[carthago]

I believe one has to make a difference between 2 things:

1/- Primary wind girders ( for open tanks alone), this ring keeps the shap of the upper edge of the shell.

2/- Secondary or intermediate wind girders ( for both= open or with roofs)..These rings are a bit similar to the one used for ASME VIII for cylinder under external pressure where one uses or calculates the maximum length of a stable shell,ect..

This is clear in Old BS or the new EN standards for storage tanks , where you have exemples for calculation..
You will notice that they both used the same figures.

 

[IFRs]

Here is the answer:

API 650 section 5.6.1.4 says to check all shells for wind buckling and then sends you to 5.9.7 to do so, which conveniently skips over 5.9.1 which was the source of the question.

 

[JoeTank]

Mr. IFR,
It sounds as if we agree that cone roof (and dome roof) tank shell are to be checked for the need of intermediate wind stiffeners. I think your point about API-650 not being clear enough about that is worth discussing. However, the real discussion should be between you and API. Why don't you make your case to API and ask that they clarify the point. They are reaaly nice folk and respond fairly well to requests like this.

Joe Tank