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API 620 dome roof- self supporting or with roof structure?

API 620 dome roof- self supporting or with roof structure?

API 620 dome roof- self supporting or with roof structure?

(OP)
Hi all,

Designing an isomerate storage tank based on API-620 (on AME tank) with the following parameters:

Design Pressure: 6.72 psig
Tank Diameter: 50 ft
Tank Height: 39.3696 ft
According to the design calculations, I ended up with the following:
6 shell courses with the following thicknesses in inches( 0.5118,0.5118, 0.4720, 0.4720, 0.3937, 0.6290) Note: the top shell course thickness was increased to meet the minimum roof-to-shell participating area in the compression region
Roof thickness: 0.3937 inches


As far as I (and my senior design supervisor) can tell, API 620 does not specify a roof structure as such for dome roofs and any such supporting member is left to the discretion of the designer. For our design we opted for an external structure ( 28 laterally braced IPN-300 rafters) based on past experiences with similar projects.

The client is unsure about the roof structure as it adds to the cost of the tank but wants to ensure that the structural integrity is maintained. My question, therefore, is this- Is there some design basis that I can refer to in order to reach a deeper an more conclusive analysis to appease the client (other than good engineering practices and past experiences). What do the good people here think is the best way forward from here?

RE: API 620 dome roof- self supporting or with roof structure?

My approach- set the crown radius at the 0.8D to 1.2D range allowed for API-650 umbrella roof tanks, use the API-650 umbrella roof design for preliminary thickness, then check with API-620 using external pressure/dead load/live load in the membrane stress equations.
I would assume that a tank of those dimensions with a roof of that thickness would not require any structure, but it would need to be confirmed.

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