stiffened cylinder under bending moement

In calculations in BS 5500, the pressure vessel code, no parallel axis method is used for stiffener rings. I would refer to that or similar codes to check your method.

corus

Let me state what I think you are asking:
You have a circular cylinder with longitudinal stiffeners.
This cylinder is loaded with axial compression and longitudinal bending moment. Like a beam-column.
If that is the case then the "y" is the distance from the cross-section's center of gravity to the point you are interested in finding the stress. In the expression
stress = My/I, I is for the entire cross-section as stated in the parallel-axis theorem( Ixx = Io +Ad^2).
Assuming this is a symetrical section, then if you want the stress in the shell, then "y" is the shell radius. If you want the stress in the stiffener, then "y" is the distance from the entire section's C.G. to the outer edge of the stiffener.

Thanks warrenw,

What i understand from your answer, is that for symmetrical case (as that of stiffened cylinder), y=Radius of cylinder for thin cylinder and y=distance from center of cylinder to outer edge of flange for stiffner. is it true?

Also, i have 'I' for stiffners at their centroid, should i use it or transform it to cylinder longitudinal axis using parallel axis theorm.

Your "y" values are correct.
As I said above, "I" should be for the entire cross-section
I(entire section)= I(cyl.)+I(stiff.) +Ad^2(cyl.)+Ad^2(stiff)

Thanks again,

in equation
I(entire section)= I(cyl.)+I(stiff.) +Ad^2(cyl.)+Ad^2(stiff)

Do i need to use Ad^2(cyl.), since I(cyld)=(PI/64)*(Dou^4-Din^4), give I at its center.

In your case the cylinder "d"(the distance from the entire section C.G to the cylinder C.G.) = 0 ,so Ad^2(cyl.)=0

Dravid,

I am not sure whether I got you. But to avoid buckling, you provide circumferencial stiffeners which are useless in bending.

For bending, you add longitudinal stiffeners. But these stiffeners don't stop buckling. Correct me if I am wrong.