Curb Angle Design for API+650 tanks

Ref to API 650 2010 edition : Sec: 5.1.5.9 Roof and Top-Angle Joints :

Table gives you the minimum TA angle sizes, just select the closest size for your requirement. Then go and refer to section: 5.9.6 : Table 5-20a—(SI) Section Moduli (cm3) of Stiffening-Ring Sections on Tank Shells. You can also calculate the required section modulus for design wind speeds to rrive at the size for the Top Angle.