Let is try to built up a sort of check list ..kind of screening...
Of course feel free to add extra points....
1/-Your tank is 60 m diam.
2/-How is your roof slope ?? (1/6 = 9 degree.. or something similar)
It should appear after calcs that the deflection would not allow to water to accumulate (ponding)
3/How long is your rafter ?
probably longer than 30 meter ...this could be built up out of several standard lengths...
4/-What are your loads ( on the rafter) , if I remember from API 650 it is = 25 psf live load (LD)+ 20 psf dead load (DL) of the roof + own weight of all beams
the 25 and 20 psf will give a trapezoidal distribution on the rafter ( small near centerline..)
5/-What is the distance between two adjacent rafters on the shell side ?
According to API 650 , this shall not exceed (2*3.14 * foot )= 1. 9 meter.
6/-How are the reactions transmitted to the tank walls?
( rafters connected to a brackets , which are welded to the inner wall of the shell or something equivalent)
Do to the roof slope , you will get also radial loads on the walls
7/-Do you have headers ( a ring of beam between centerline and tank edge ( at say OD/4 to support the subrafters?
You might need them , because you can not have all rafters all running from center to tank wall
8/-Do you have subrafters ?
This is because you want to keep those 1.9 m distance...
9/-How are rafters supported near the tank centerline ?? ( on a ring ? on a column ?)
The ring will take the horizontal reactions (again because of the slope) ...
I am trying to do something on roof and trying to exchange info..on another site ( ast-forum) but apprently all the guys are on leave...(lol)