Uplift force in storage tank, API 650 addendum..

[carthago]

I though about sending an equiry to API..Probably I would do it, if my confusion remains..but If someone understand this , please let me know.
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Refering to API-650 (Addendum December 2005), paragraph 3.2.1-f - I guess this is the last version of API 650.

In calculating the wind load on an API 650 storage tank with say, Diameter D and Height H.
My understanding is that there is a horizontal load = Fh , applied at (H/2) and there is a UPLIFT load applied at roof center , this is a vertical upward =Fv

The total overturning moment would be:
Mw = Fh*(H/2) + F_uplift *(D/2) with:

Fh = p_horizontal * H*D
F_uplift = p_uplift * ( 3.14 :4*) *D^2

Is this correct ?
Of course one can include the cone.., but I am talking about the formulation.

First question: Do we have always to take this UPLIFT verical force into account ??
I have seen many design were this was omitted...
In fact people have added (if ever they did) anchor bolts for some other reasons!!

If answer to this 1st question is yes , than we always end up will (almost every time) end up with anchor bolts (
Anchor bolt tensile stress = (4*Mw / d ) - (Tank Weight) > zero ).

Second question : Is my understanding of thiese factors correct), i.e:
p_horizontal = 18(lbf/ft2)* V/120) ^2 = 0.86(kPa)*(V/190)^2

Uplift:
p_vertical = 30(lbf/ft2)*(V/120) ^2= 1.44(kPa)*(V/190)^2

This means that for a tank 20 m diameter F _uplift = 314 * 1440 = 452 kilo newton !!! ..huge

Third question: API-650 ( Addendum December 2005), paragraph 3.2.1-f-2 states that up-lift pressure on roof = p_vertical = 30 ( lbf/ft2)* (V/120) ^2 in [lbf/ ft] = 1.44 (kPa) * (V/190)^2
shall be LESS THAN ( 1.6 * P_internal as ),
P_internal as given in F.4.1

A/-Is see an inconsisty here !! how can we relate the wind effect to the design pressure ??
Does this mean that one, has to Derivate/ calculate The internal pressure from the uplift pressure ?? And then check the detail and its section as per appendix F.
I think API has the to add this as an additional check to appendix F..

B/-What if there is no internal pressure.. which uplift pressure do we have to take ?

 

[JStephen]

I think your concerns are well-founded. This is a major change in the loading on tanks. Whether it is justified, or whether it will be changed, I can't say.

It does appear that you are calculating the moment correctly. You mention about including the cone, but note that Addendum 4 doesn't require you to include the projected roof area in the horizontal force calculation.

First Question- as the standard is now written, you always have to take that upward force into account. This is a distinct change, and people have not done it previously because the tank codes didn't require it previously.

Second Question- the english pressures are correct, haven't check the metric. Keep in mind that in a lot of cases, the design wind velocity will be less than 120.

I think the limitation on pressure is actually there to simplify things. The way I read it, if there is no internal design pressure, you can use a standard roof, you don't have to design the roof itself for the wind uplift pressure. And then the wind uplift pressure used in the overturning check need not exceed the strength of the as-built roof.