Hi fellow engineers,

Both EN 14015 and API 650 give guidelines for fuel storage tank design for small internal pressure. In section 10.5.2 of EN 14015 and annex F-4 of API 650 is mentioned the minimal compression area on which the fixed roof rests.

For a known material and design conditions, this area is only function of the tank diameter and cone roof angle.

EN 14015
A=(50 * pc * r^2)/(sc * tantheta) [NBN EN 14015 : 2004 10.5.2 (p.58), equation #15]
Where
pc : internal pressure
r : radius of tank
sc : allowable compressive stress
theta : cone roof angle

API 650
A=(200 * D^2 * (Pi - (0.00127DLR / D^2) )) / (FY*tantheta) [API 650 12 th edition F.5.1 (p.F-5) ]
Where
D : Tank diameter
Pi : internal pressure
DLR : Weight of roof
FY : Yield strength
theta : cone roof angle

Now this gives the minimal compression area the roof needs for support and it says that "aditionnal compression area shall be provided by thickening the roof or shell plates". However, if I want to verify that criteria, I need another equivalence to calculate the actual compression area of my design. This doesn't seem to be mentioned in both standards anywhere.

Is the actual compression area the cross section of the shell ? Is it the cross section of the roof ? Is it the area on which the roof sits ? If so, how can I calculate that area since it theoretically sits on a edge (null area) ?

N.B. : See link for a visualization of my design. (2 images)
http://s1189.photobucket.com/user/Virgule99/media/tank_001.png.html?sort=3&o=1