Theory
Structures vibration have different origins
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wind and vortex
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earthquake
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walking of pedestrians on a bridge or footbridge
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machine excitation..
Wind exited structure
At a given wind speed vortex are generating around the structure. When the critical wind speed is obtained the structure is a subject to resonance phenomenon. The intensity of the phenomenon is bigger when the structures damping is small.
Case of circular section
Vortex downstream of cylinder leads to high vibration when the frequency of the vortex is near one of the frequency modes of the cylinder.
There is a generation of alternative area in pressure or under pressure.. in the crosswind direction. As a result the cylinder starts vibrating in the crosswind direction.
Example of a vortex downstream of a cylinder.
Case of a bdridge beam
Vortex downstream of a bridge
Proposed solutions
First Solution: Adding a Set of Helical Strakes on a Stack
Avantages
- The helical strakes are breaking the vortex. This solution could be adopted on cylinder structure if the Scruton number is bigger than 8.
- In some rare cases they provide the most economical solution.
Disavantages
- Double the wind load on the upper 13 of the stack, increasing stack weight significantly. Nearly always more expensive than a vibration damper.
- Complex geometry make fabrication difficult. Can interfere with other attachments ( ie. ladders, platforms, etc.).
- Foundation and Anchor Bolts usually increase in size.
- Do not work in the case of interfering stacks. Some codes do not allow their use for Scruton Numbers less than 8 (which cover most stacks with vibration issues).
Conclusion:
Only of interest for small stack with small diameters. In all other cases a solution with a damper proves to be more economical.
Second Solution: Installing a Vibration Damper
Placing a damper is an elegant modern solution, which has no disadvantage. This solution is always cheaper than the solution with strakes, except on rare occasions when the stack is small and with a small diameter.
Different types of damper could be used. There is no universal damper :each one has its advantages and disadvantages. On the following pagesyou can see different types of dampers.On bridges, the phenomenon ismore complicated than on cylindrical slender structures and horizontaldampers, vertical dampers and sometimes rotation dampers could berequested.
A vibration damper absorbs the energy due to the vibration and to make the vibration phenomena disappear.
There is no universal damper to solve all vibration problems. Case by case taking into account local conditions (acceleration, amplitude, frequency, space.) will advise you of a given type of damper. dampers are used to suppress horizontal or vertical vibrations. Torsion vibration problems may