AS2304 Water storage tanks - wall design

[sdz]

AS 2304 - 5.3.2.5 Limit state buckling pressure
The limit state buckling pressure shall be determined by the following equation:
Pf = 0.65 x Pc . . . 5.3.2.5
The value of Pf shall be greater than the wind actions determined in Section 4 of this Standard.

The code is not clear which wind action is to be used for this. It does make a reference in 5.3.2.2 to “More detailed information is given in AWWA D103-97”
Referring to AWWA D103 - 15.1 I find Pw=qz G Cf where for a circular tank with a roof of < 15 degrees Cf=0.6.

From this it appears that applicable wind action is P*=quz Cfig <= Pf = 0.65 x Pc; where Cfig=0.63. I assume from this that the variation of wind pressure around the tank is included in this equation.

If the tank passes this limit then no wind girders are required, including on the top edge. (Noting however that fire tanks have a roof.)

Question: Is this interpretation correct?

It would have been a lot clearer if the code writers had simply said
"P* <= Pf; where P*=quz Cfig"

Side issue; In Figure 5.3.2.3 is “d” the overall dimension or to centrelines? I have assumed it is to centrelines.