Helical strakes on stack

[Biss73]

Hi,
We are designing a stack which the bottom half is in a building while the rest is exposed to the wind. According to ASME STS-1, we need to install helical strakes which are supposed to cover 1/3 of the stack. Does anybody know if it needs to cover 1/3 of the part exposed to the wind or 1/3 of the whole stack?

Details:
- Ø54”
- Total height: 78’
- Height of building: 43’

Thanks,
Serge

 

[dhengr]

I’m not sure exactly what ASME STS-1 says, about the 1/3 of height, but you would just apply the strakes on the stack above the roof. They are intended to break up the wind flow around the stack and interrupt vortex shedding which can cause dynamic problems.

 

[Biss73]

Thanks,
I understand that you only put helical strakes on the section above the roof, but does the fact that the lower part of the stack is protected from the wind enables you to have shorter strakes? Or does the fact that the stack is not guided at the junction with the roof does not influence the length of the strakes?

Thanks,
Serge

 

[rb1957]

that'd be the length of (the stack protected by) the strakes.

i'd apply the 1/3rd to the exposed stack

 

[Biss73]

Thanks rb1957, but could you tell me what are you basing this on?
Serge

 

[rb1957]

common sense ... ie i don't know your codes or specs

 

[ash9144]

Since the roof provides no support I would think it would be 1/3 of total height since your bending moment would be the same.

 

[rb1957]

isn't the idea to control vortex shedding ? don't see the linkage between strakes and bending moment.

the code assumes the stack is completely exposed (yes?) but this stack is partially unloaded. i guess you could do a research project to figure out the effects of wind loading applied to part of the stack.

with something like 2/3rds of the stack total height buried (= unloaded by wind) in the building, applying 1/3rd total height seems excessive. just my opinion.

 

[jtseng123]

1/3 of the section exposed to wind is good enough. Nothing to do with bending moment. As long as wind broken away, so stack will not dance with wind, you are safe.