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Slender structures can oscillate excessively by wind-loading at a velocity which follows from the structures resonance frequency and the diameter ( the so-called Von Karman Vortex Shedding ). Large movements start if the product of mass and damping is small. This can be eliminated by adding a damper system. Compared to helical strakes it is estimated that a saving of 35% can be achieved on the foundation cost alone, because of drag difference. Four systems have been developed. The Roll-Box damper is a cylindrical box with a diameter of about 0.60 m and a height of 0.50 m. Inside is a moving weight of 20 to 150 kg. The box can be fixed to a vibrating structure. The effect will be an increase of the damping to 10% of the critical damping. Generally this is sufficient to prevent serious vibrations
The Roll-Ring damper is a ring around a chimney with a weight of 30 to 160 kg. The ring can move over minimal three bearings. The effect of this ring is an increase of the damping of the chimney to about 10% of the critical damping. This will prevent cross-wind vibrations.
The Hydampsystem has a moving mass between 300 to 8000 kg hanging on suspension bars with a length between 0.20 and 2.500 m depending on the frequency of the structure that has to be damped. The mass moves independently from the chimney. In the space between chimney and weight hydraulic shock absorbers are installed to absorb the energy. Concequently vibrations will not accur.
The Box-Damper has a moving weight of 300 to 1000 kg hanging on suspension bars. The weight moves independently from the chimney. The weight inclusive the suspension bars and the hydraulic shock absorbers are installed into a closed Box. Therefore all the moving parts will be protected from any influences caused by the environmental circumstances.
All four damper types have been used frequently and are relatively inexpensive. They can be mounted to every vibrating structure. The condition is: the movements must occur within a limited frequency range.
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