API 560 Stack Strake procedure

[PSSC]

This my first post, so be gentle.

I have completed the API 560 procedure for stack calculations.
This is the first time I have done this calc that adding strakes is unavoidable.
After adding the strakes, and increasing the shell thickness a little, I rechecked the stack for Vc1 > 60 mph, using the Diameter Average of the stack + strakes.
It checks out with the strakes.
Am I done?
I did recheck the strength of the stack to handle the added area in wind load.

One more question
When checking the vibration in the eq. f=0.5587*((E*I*G)/(W*H^4))^1/2 (H.12), is the "I" at the base, or the average of the top 1/3 of the stack?
The Diameter Average in eq H.11 is for the top 1/3 of the stack, I wasn't sure if this applies to the "I" also.
In this case, it won't make much of a difference as I have the same diameter for the entire length of the stack.
But I could see that in a multiple diameter stack it could make a big difference.

Thanks in advance.

 

[JStephen]

I would think "I" would be the value at the bottom or the averaged value going up, not the value at the top.