sme75
Structural
- Feb 6, 2003
- 40
I am working on reinforcing a roof to shell compression ring of a tank for an internal pressure greater than the original design pressure. This has led me into a little research of the allowable stresses for a radially loaded circular ring. Using the equation in section F.4.1, I have found that this is based on a factor of saftey of 1.6 applied to an assumed yield stress of 32,000 psi. It seems to me that the allowable stress has been arbitrarily selected without regard for the stiffness of the ring. The only property of the ring used in the equation is the area. This then gives the same allowable pressure for a ring rolled either way (hard or easy) and attached to the shell. Another source I have develops an allowable stress based on the stiffness of the ring. This gives me an allowable stress of 3.2 ksi which is much less than the 20 ksi assumed in API. Am I missing a limitation in the API code or does API assume that the roof helps to resist buckling of the ring? How do they justify such a large compression stress?
