API 650 Section 5.11.2 (Wind Load Unanchored Tanks)

[xema77]

Can some one please advise me as to how tb (required thickness of the bottom plate under shell that is used to resist wind overturning) is calculated in API 650 section 5.11.2.

Thanks

 

[IFRs]

It is not calculated in 5.11.2. It is specified by the tank purchaser subject to API 650 minimums in section 5.4 and calculated thicknesses in other chapters like 5.5.2 and 5.5.3.

 

[xema77]

Thank you IFRs. I was confused by the definition of tb in 5.11.2. I thought that you had to calculate tb based on the overturning wind moment, but know that I read it again I see it is the bottom thickness used to resist the wind moment.

 

[xema77]

I guess that I still have some confusion in calculating the resisting moment Mf. 5.11.2 indicates that wl shall be the lesser of 0.9HD or 59tb(FbyH)^.5. I have the following questions.

1) When calculating wl based on 59tb(FbyH)^.5 the units are N/m, I assume that this is multiplied by the tank circumference to get the liquid weight. Please confirm.

2) The wl calculation using 0.9HD yields a value that is about 50 times less than in (1) above and the units would be in m^2? Can anyone explain this for me please.

Thanks

 

[svdeun]

Hi,


I'm struggeling with the same problem as you xema77.
1) yes indeed ,if you multiplie the tank circumference and wl, you get the liquid weight.
(I assume that because, in API 650 edition 10 ad4,in formula E-23, wa is also computed for the tank circumference)


2) did you have already found an answer on this question?

 

[JStephen]

The 0.9HD is in US units, not metric. Compare to the equations in App. E.

 

[svdeun]

Thanks for your answer, but could you explain why the units from the first formula are N/m and the second one are m^2? Or have they considered this difference in the constants 59 and 140.8( calculated value for 0.9 in metric units)?

 

[JStephen]

I haven't checked those specific equations, but generally, the equations in API-650 are not dimensionally consistent. That is, the constant term has units that are not shown. For example, the 0.9HD would give you units of area, but it is actually lbs/ft.

 

[IFRs]

I usually break out the unit conversions, arrive at a new constant outside, repeat for SI units and verify the results using the equations without units. Sure, it is a pain but then I see if both SI and US equations yield the same results and begin to understand the equations better. Often the constant outside goes away or becomes Blair's constant or something similar.