Dear Sir,
Yes, uplifting on the tank does affect the design of the roof structure and wether to to anchor down the tank or not.
Refer to ASCE 7-16 and API 650 ed13.
API 650 does not allow the attaching of the roof plates to the roof support members
and explicitly forbidden for frangible roofs and allow for For Appendix F tanks.
Is this an Appendix F tank ?
When you look to the ( Figure F-1—Appendix F Decision Tree ) where is your position ?
If there is no internal pressure, you will follow basic design requirements.
If the roof plates are not attached to the supporting rafters ( in your case ), The wind uplift will not affect the design of rafters..
Let me explicitly explain = if the roof plates are not attached to the supporting rafters, the wind suction pressure will not affect the design of rafters.
I do not know your wind code, and cone roof angle . If positive pressure develops (may be one side of the roof ), the positive pressure shall be accounted for the design of rafters.
You did not answer the questions (Is this an Appendix F tank ? When you look to the ( Figure F-1—Appendix F Decision Tree ) where is your position ? Is there internal pressure? )
O.K. this is Appendix F tank with frangible roof ..What is the internal pressure ? what is your position at Figure F-1—Appendix F Decision Tree ?
Are you sure with this speed ? that means 280 kph. (i could not see )??
I could not get your point for using the dome roof tank coeff.. If this is a frangible roof, the rules are mentioned at ( 5.10.2.6 Frangible Roof).
Regarding wind suction , (the roof plating supported freely on rafters , this also mandatory req. for frangible roof ) , the wind suction pressure will be resisted by roof plates weight, shell wt, and (if applicable ) anchors. Not by rafters and supporting columns.
