Vibration damping.
Vibration damping.
The main cause of vibration in cables/tension members is vortex shedding.
Vortex shedding occurs when wind flows past a cable causing vortices to form on the downwind side (as seen in the above clouds). These vortices are alternating low pressure zones. As the wind flows past a cable it attempts to fill these low pressure areas, and this subtle shift in wind direction encourages the cable to move in this new direction. As the cable moves it builds up kinetic energy and when the frequency of vortex shedding aligns with the cables natural frequency, vibration sets in.
Cable vibration is a complex phenomenon and can occur in any cable configuration, no matter the structure or cable material. Any uniform shaped cable held under tension in an environment exposed to wind flow is susceptible.
Vibration assessment can only be made once a system is structurally active. It is not possible to declare any cable system free of vibration as numerous factors must be taken into account, all of which differ from one structure to another. Factors such as cable tension, superstructure stiffness, interface design, consistency of shape along cable length, wind speed, density and alternate sources of energy all modify the likelihood and extent of vibration.
Think of it like a swing.
Pushing a person in time with the natural interval of the swing (its natural frequency) makes it go higher and higher (increasing energy). Pushing the swing at faster or slower speeds to its natural frequency reduces the energy, resulting in a loss in swinging speed and height.
If vibration does occur, supplementary damping is required.
1. The simplest method is to create a disturbance on the cable to prevent the formation of vortices
A widely used solution is to add a spiral, known as a helical strake, to disrupt wind flow (as seen on many chimneys)
Adding an elliptical flap, able to align with the wind direction, has also proven to be an effective solution
2. The cable itself can be designed to reduce or remove vortices and to encourage ‘out-of-sync’ frequencies
Varying the cross-section shape along the cable length (CL ELLIPSE) has proven be effective
Easing load will alter the frequencies and reduce the likelihood of vibration
3. An external passive ‘energy dissipation’ damping device (CL DAMPER by Carbo-Link)
When a cable begins to vibrate, the damper will move with the cable and dissipate any energy leading to vibration
The CL DAMPER has been successfully installed and proven on a number of CL SOLID cable projects
CL DAMPER.
In collaboration with ETH Zurich, a range of relative displacements were explored to optimize damper geometries & mass
Damping efficiency is independent of the acceleration and mode of cable vibration
The damper does not impede free movement of any cable or affect structural integrity
The damper can be easily installed and adjusted at any time
INSTALLATION.
Identify the vibrating cable/s
A corresponding damper will be produced to suit the cable weight and diameter
The damper is delivered fully assembled. Simply split into two halves, clamp around the cable and fasten
Install in a region where the level of displacement/acceleration is high. Around 1.5m away from an interfac
It may be necessary to move the damper up or down the cable by 1-2m to improve overall effectiveness
