The API 650 states that self-supporting cone roofs shall have a minimum thickness of 5 mm (0.1875 in) and a maximum of 12.5 mm (0.5 in) excluding any corrosion allowance.
The design loadings for self-supporting cone roofs are sustained entirely by the roof sheeting itself, without any supporting structure.
The slope of self-supporting roofs shall be within the range of 9.5 to 37 which is (1 : 6 to 1: 1.333).
☐ Radial Rafters
This type of roof is supported by a radial rafter framework composed of structural sections.
As an alternative to providing a structure which is supported only by the tank shell, the column-supported roof introduces a series of vertical supporting columns. These are arranged in a series of circumferential rings around a single center column.
The rings of columns are circumferentially linked by girders which in turn support radial rafters on which the roof plating is laid. It is usual to adopt a shallow conical shape (1 in 16) and in theory there is no limit to the size of the tank roof which can
be constructed in this way and it is reported that a tank of 110 meters in diameter has been built.
☐ Trussed Frame Support
This type of supporting structure takes the form of a series of radial trusses, generally made up from steel angle sections. Between these trusses are circumferentially arranged members providing stability and support for the roof plating.
This type of roof is commonly used within the range of 15 m to 60 m in diameter.
☐ Externally-Framed Roofs
This type of supporting structure consists of a series of radial steel sections. The roof petal plate sections are welded to the underside of the lower flange of each beam.