available second moment of area

[ausengg]

thread794-187769

Im just doing some calculations for stiffening rings for a cylinderical pressure vessel under external pressure. Refering to UG-29 can someone help me for the formula for finding "available moment of inertia, I" in the definition given. The stiffened ring is a rectangular cross section. if I apply bh^3/12 on this 40*8 cross section it comes out to be 1706.6mm^4 which i get a feeling is not the value used for "available moment of interia".
thanks for help

 

[TomBarsh]

This paragraph defines "I" as "available moment of inertia of the stiffening ring cross-section about its neutral axis parallel to the axis of the shell"

The formula you use for moment of inertia of the flat bar about its own neutral axis is correct but is your orientation correct? If your flat bar 40x8 is oriented with the 40 mm side perpendicular to the shell plate surface (the "strong" way) then b = 8 and h = 40, giving a much greater moment of inertia.

 

[ausengg]

thanks for your response tom."parallel to the axis of the shell" makes it clear to find the appropriate orientation to have max possible moment of inertia.