Rectangular vessel, combined cross section shell wall+stiffeners

[maxpcc]

Hello all,
I'm having some trouble about the calculation of a stiffener:

I shoul calculate the modulus of inertia of the stiffener; i'm quite confident that I should consider the wall thickness too, in green in the picture, but I cannot find any rules or any method to be sure HOW MUCH area I should consider.


Here an extract from EN13445-3 par.15.6.6

Any suggestion?
TIA!

 

[HTURKAK]

Here is excerpt from BS EN 13445-3:2009 par.15.6.6

If the question is the effective length , you may assume be= 10e for each side ( total max. 20 e)

If the problem is ,Membrane and bending stresses in the transverse section, follow the rules specified at 15.6.5 ...