API 650 - addenda december 2005 , in it sparagraph 3.2.1-f defines TWO Wind forces acting SIMULTANEOUSLY on the tank:
One horizontal with a pressure of 18 lbf/ft2
and the OTHER, AN UP LIFT one ( new!!!) of 30 lb/ft2.

If this is true, in my opinion the overturning moment due to wind is really hudge,
yoy will end up almost for every case with anchor bolts to take care of this wind moment.
The wind moment could be apprixmated by:
Mw= 18*D*H + 0.0.785*(D^2) *(0.5*D)

for instance for a tank 60 ft diametere, you will get (from this up-lift pressure alone) a force equal to and the additional moment
= 644-E04 lb-ft..

I am compyling answers/ reactions and think to send an enquiry to API.

Meanwhile.. I am wondering..
My question..is How to calculate the overturning moment..

TANK 2.55 was released in October of 2005. So this version does not support Addendum 4, which is erroneously dated Dec 2005. People are just now receiving this addendum now, in May. I don't even have my copy yet.

Version 2.55 will not support Addendum 4, that will be left to the next version.

Since I have not reviewed this Addendum, I am not in a position to comment on this change.

Sorry forgot to include this famous UP KIFT pressure
Mw= 18*D*H psf + 0.785*(D^2) *(0.5*D)* 30 psf

D and H in ft , Mw in lf-ft
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Thanks Richard , but I would be glad to hear users opinions..