These devices (see Photo) are clamped to the road-support cables running down from the main suspension cables of the Severn Bridge, which links England and Wales. Each of the curiously shaped masses is about 20 centimetres long. What are they for and why are they shaped so strangely, positioned at seemingly random angles and fixed where they are?
鈥� These objects are called Stockbridge dampers. They dampen wind-induced vibrations of the cables, reducing cable fatigue. As wind passes across the cables, they shed vortices which cause them to vibrate at low amplitudes 鈥� in the millimetre range 鈥� but at high frequency, around 60 hertz.FIG-mg19626273.000-1_250
The oddly shaped masses are called dogbones, for an obvious reason. They are tuned so that they flap when the cable starts to vibrate. The flapping of the dogbones dissipates the energy of the vibrating cable. They are also used on high-tension power lines and for large road-sign support spans.
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鈥淭he dampers are tuned so that they flap and dissipate energy when the cable starts to vibrate鈥�
Quinn Smithwick, Cambridge, Massachusetts, US
鈥� The dampers counterbalance the vibration of the cables by flexing and twisting, preventing the cables from shaking violently. They are shaped and arranged to maximise their effectiveness, depending on the cable. Note that the pairs of damper assemblies in the picture are positioned at 90 degrees to one another around the axis of the cable. This arrangement takes care of the vibration whatever the direction of the wind.
Nobby Miura, Senior Engineer, Kinectrics, Toronto, Canada
鈥� These devices are variations on what are generally called 鈥渢uned mass dampers鈥� or 鈥渄ynamic vibration absorbers鈥�. The Stockbridge dampers that are shown here were invented in 1926.
They have their own natural frequency which is tuned to the frequency of the vibrating bridge cable, ensuring that vibrations of the cable are transferred to the absorber, which vibrates instead. It can be thought of as a device to suck energy out of the cable. The dampers depicted are particularly ugly; most are more symmetrical.
尝辞苍诲辞苍鈥檚 (still known as the Wobbly bridge) is fitted with tuned mass dampers to stop its infamous wobbling.
鈥湷⒋遣曰宕遣遭�檚 Millennium bridge is fitted with tuned mass dampers to stop its infamous wobbling鈥�
To see how they work, a simple experiment can be carried out at home using a wine glass and a paper clip (for instructions, visit ).
Hugh Hunt, Cambridge, UK