damping of steel stacks 1

[3dboy]

I am designing a steel stack for a client and SMACNA recommends an annular damping pad between the base plate and the footing. They claim that this will damp out any vibrations due to vortex shedding etc. I'm not sure I buy this idea. In the past I've used strakes, large sections, multiple brace points (to change the fundamental frequencies) etc. to avoid vortex shedding effects. All of the literature I've read on the topic suggests these solutions as well as others (such as tuned mass damping) It just seems a little fishy that a little damping pad is going to solve all of the woes due to dynamic vibrations when the vast majority of the literature gives other solutions.

 

[VoyageofDiscovery]

Hi 3dboy,

I believe this may be because, they have found fatigue cracking in the area of the base of the stack.

Regards

VOD

 

[warrenw]

3dboy;
I just came across this and thought it might be of some help to you. I have no experience with this method, like you I have alwayed used helical strakes.

ASME STS-1-2000 Steel Stacks
Section 5.3.2 Damping Methods
"Preformed Fabric Pads. The control of damping in a stack is obtained by installing a preformed fabric pad at the base of the stack. --- This will require the addition of a preformed fabric pad(washer) and steel backing plate beneath each anchor bolt nut. This method has been demonstrated to provide a damping value up to approximately 0.03."

 

[3dboy]

Thanks warrenw,

Incidentally I looked into it further and contacted a company that designs and provides these and they run about the same as strakes

 

[LaylaZ]

Hi guys,
I am new to stacks. My boss gave me a serie of 3 stacks, that we want to brace to reduce vibrations. Could you please shed some light on that subject. Provide a method if possible or recommend any references. Your answer is appreciated.
Layla Zelnick.

 

[3dboy]

Wow! I remember the first one I did. Lots of research etc. Stacks look pretty simple but are amazing complex little pieces of work. The long and the short of the design process is to calculate the fundamental natural frequencies and modes of the stack. this is based on a dynamic analysis. I have used a basic SA package to do this (Like Risa 3d or Ram Advanse) You then calculate frequency at which vortex shedding, ovalling, etc occurs for the design wind speed. Resonance occurs when the excitation frequency (due to vortex shedding) equals the natural frequency of the stack. So if the natural frequency of the stack is higher than the vortex shedding frequency at the design wind speed you are good. If not, you will have resonance occur at some wind speed below your design wind speed. Bracing your stack changes the natural frequency of the stack (by making the stack stiffer and the frequencies higher) and is a good solution. You need to model how stiff you want your support to be so that you get the right frequency and then design your brace to have the appropriate stiffness.