API-650 Uplift Load Cases Table 5.21

[MikeG7]

Hi All
In the Table 5.21, for uplift load cases, it is written for W1 and W2 definition: -
W1 = is the corroded weight of the roof plates plus the corroded weight of the shell and any other corroded permanent attachments acting on the shell
W2 = is the corroded weight of the shell and any corroded permanent attachments acting on the shell including the portion of the roof plates and framing acting on the shell

Is there a difference?
W1, W2 includes roof plates corroded
W1, W2 includes shell plates corroded
W1, W2 includes permanent shell attachments
W2 includes roof framing whereas W1 does not. This is the only difference I notice.

W1 is used for pressure load cases
W2 is used for wind load cases

In my view, all dead load (from roof, shell, attachments and framing) counteracts wind.

Maybe I miss some of the finer points here that can be pointed out.

Michael

 

[JStephen]

For a self-supporting roof (no columns), they should be the same.
For a column-and-rafter-supported roof, W1 will include the total roof plate weight.
Whereas W2 will include only the portion of the roof plate supported by the shell.
The assumption is that if you pressurize a supported cone roof, once the pressure exceeds the roof plate weight, the entire roof plate can be figured to resist pressure uplift.
Whereas for an overturning moment applied to the shell, only the portion normally supported by the shell is resisting uplift on the shell.

 

[HTURKAK]

API 650 does not encourage the attachment of roof plates to interior supporting frame members. If the purchaser approves continuously attaching the roof to cone supporting members , the dead weight of framing will also resist to wind OT.



You forgot to add W3 = dead load of the shell using as-built thicknesses and any dead load other than roof plate acting on the shell using as-built thicknesses .

Which is used for failure and frangibility check.

 

[MikeG7]

I should just follow the code, but I find a nagging question in my head about the reasons behind it all!
You make the important distinction about the portion supported by the shell in W2 term. With a column-supported roof, only a portion of the roof weight is supported by the shell and the rest of the roof plate weight goes to ground via the columns.
But at the end of the day, it is all mass - whether it is supported on the shell or supported on the columns it is still weight. The weight still has the effect of resisting uplift? What am I missing?

 

[HTURKAK]

The base plate is very flexible. When uplift starts, only a small portion of the base plate detaches from the ground. That is, the columns and the roof portion supported by columns will not mobilize to resist to OT.

If you are looking for a FS say 1.5 against OT, literally you are looking for the tank or some portion of the tank will not detach from the ground and the base plate ,interior columns together with roof loads will not mobilize.

Although the following picture is for resisting forces due to earthquake OT moment, still good to get the concept..

 

[JStephen]

Weight that is supported on the columns does not resist uplift of the shell.
Where overturning is due to seismic or wind forces on the shell itself, that distinction comes in.

 

[MikeG7]

Alright, I get the distinction now and the reason for it being the flexible base. Thank you both HTURKAK and JStephen. It has been interesting & insightful.