API 650 Annex F.4.2 - Maximum design pressure limited by uplift 1

[SGlinvad]

Hi, I have a question about calculating the maximum design pressure for unanchored tanks limited by uplift at the base of the shell. in API 650 Annex F.4.2 the equation is given by:

Pmax = (0.000849*DL_s/D²) + (0.00127*D_LR/D²) - (0.00153*M_w/D³)

My problem is that I'm not able to figure out how to calculate M_w (the wind moment in Nm). I have tried to find the answer in API 650, but I don't understand their descriptions. Can anyone help me? The wind moment is the only value I'm missing in my calculations.

Best regards, Soren

 

[fegenbush]

The wind velocity pressure is given in 5.2.1.k (page 5-10 in the 12th Edition), as is the wind uplift pressure. Follow the Free Body Diagram shown in Figure 5.27 (page 5-94) to determine the wind moment Mw.

 

[SGlinvad]

Hi, thanks for the feedback.

Yes I have found the wind pressure and velocity in 5.2.1.k. But isn't there a formula for the wind moment? I can't seem to find the Free Body Diagram that you mention?
Figure 5.27 (page 5-94) only shows the Overturning Check for Uanchored Tanks. I am not sure I understand what you mean.

 

[fegenbush]

The circle in the lower right-hand corner of the figure is the point about which the moments should be calculated in order to determine Mw.

Wind horizontal force = Wind pressure multiplied by vertical projected area
Wind horizontal moment arm = Half of the tank height

Wind vertical force (uplift) = Uplift pressure multiplied by horizontal projected area
Wind vertical moment arm = Half of the tank diameter

Mw = (Pws)(D)(H)(H/2)+(Pwr)(pi*D^2/4)(D/2)

 

[mariog123]

fegenbush has already answered, however I attach a paper with derivation of that formula, hoping you can find useful the sketch (which, of course, it is based on API 650 Figure).

 

[michek]

Hi,
I have a question about calculating the maximum design pressure for unanchored tankse a question on Annex F. The roof tank is supported by column.
In F.4.1 Dlr equals to the amount of the nominal weight of roof plate and rafters supported by the roof-to-shell junction. The other one being supported by columns.
Am i right?

And i also have to check roof-to-shell junction with forces from roof struture.

Best regards