I am Desigining a flat roof tank of 5m diameter, how to design the top curb angle for the roof
plate load at top, as per API-650 self supported cone and dome roof procedure is there, but for Flat roof
how to do?
API 650 does not dictate the design procedure for flat roof tanks. 5.10.2.8 permits any roof design the purchaser allows. 5.10.3.1 dictates the use of AISC 360 for the design of roof components. There are many resources for the design of such structures such as Design of Welded Structures by Blodgett and Pressure Vessel Design Handbook by Bednar.
For a supported-cone roof (minimum slope 1:16, not "flat"), there is no top angle design, just comply with the minimum size requirements based on diameter.