API 650 storage tank roof type selection based on tank diameters

[Mech2325]

Hi everyone,

Please tell a thumb rule for the selection of a roof type (self supported, rafter supported, rafter+column and dome) based on the diameter of a storage tank being designed in accordance with API 650

As i have read, the storage tank guide book by Bob Long gives the following criteria for the the roof selection of the tanks based on diameters

For tanks upto 8 metres in diameter: Self supporting cone roof

For tanks upto 15 m in diameter: Rafter supported roof

For tanks greater than 15 m: Go for rafters+columns or truss structure

But i have also seen that if, for example you increase the angle for a self supporting cone roof, say between 15 to 19 degrees, you can have the roof to pass as a self supporting structure, even for a 12 metres diameter tank

Please if you guys can share anything, you are welcome to do so.