API 650 Roof - Uplift Pressures

[wingvalk]

I have a question regarding the intent/philosophy related to the design of supported cone roofs in API 650.

The specific question is regarding design of the roof for suction (uplift) forces from wind. I see that the roof plate is not normally welded to the supporting structure and I am interested in understanding how the roof carries the loading to the shell. Is it assumed/tested that the roof plate will simply deflect enough to carry the wind loads via membrane stresses to the perimeter? If so, are the any practical diameter limits? Are their concerns with impact loading on the roof structure as the plate vibrates due to variable wind suction pressures?

I am asking these questions as the Engineering Representative for an Owner purchasing the tank. I just want to be sure that we do not have issues with the roof during a wind event.

Any assistance provided would be greatly appreciated.

 

[fegenbush]

If you are concerned about uplift, it is permissible to weld the roof plate to the supporting structure. If you do this, your roof cannot be considered frangible. For many owners, frangibility is important from a safety aspect. The choice must be made as to whether roof frangibility is required or the roof can be supported in the uplift condition

The roof would have to carry the uplift forces via membrane stresses to the perimeter. The slope of the roof aids this.

Small tanks (subjective) are not able to have frangible roofs, so are not negatively affected by having their supporting structures welded to the roof plate. Of course, these tanks often require no supporting structure at all.

I believe the effect of impact loading would be covered by the required live load rating of the roof.

 

[IFRs]

API 650 no longer requires you to consider uplift for supported cone roofs.
Thousands have been in service for decades without failure from wind load.
I think you can rest easy.

 

[carthago]

to IFRs ..and others: so why is this uplift story is still included in the Code ? why it is there anyway ?
Have we ever had a "code case in regarding the up-lift in API650" ..what was the answer of the committee ?

why do not we have this issue of up lift with vessels..etc..

of course many "how' s"

Thanks a lot

take care and thanks

 

[IFRs]

Uplift is still in the code for other types of tank roofs, see 5.2.1.k 1) and 2), 5.11 specifically 5.11.2.2 and 5.12. Section 5.11.2.2 is a recent revision changing unanchored supported cone roof tanks wind overturning calculations.