Cables of cable-stayed bridges are susceptible to vibrations of large ampli-tudes due to their low eigendamping. In order to prevent cables from large vibration amplitudes, Empa and Maurer Söhne of Munich install magnetor-heological fluid dampers between bridge deck and cables. The damper force is feedback controlled depending on the actual vibration state. The adaptive cable damping system is installed on the „Franjo Tudjman Bridge“ nearby Dubrovnik where it increases the cable damping by approximately a factor of 10.

Cables of cable-stayed bridges are mainly composed of steal. Hence, they exibit fairly low eigendamping in the order of 0.6%-0.9% logarithmic decrement. For this reason, and due to their length of up to 500 m, such cables can vibrate with mid span amplitudes of approximately 1-2 m. This happened during heavy spring storms in 2005 and 2006 on the “Franjo Tudjman Bridge“, and  consequently the bridge had to be closed. Moreover, some cables were partially damaged.

In summer 2006, scientists at Empa and Maurer Söhne of Munich installed feedback controlled magnetorheological fluid dampers on all cables of the “Franjo Tudjman Bridge“, except the shortest three cables. The controller optimally adjusts the damper force according to the actual cable vibrations which are measured by a displacement sensor that is mounted directly on the damper. The field measurements demonstrated that the feedback con-trolled dampers increased the cable damping by up to 9% logarithmic dec-rement. Thus, rain-wind induced cable vibrations are no longer a problem considering that a 4% logarithmic decrement is assumed to be sufficient in such cases.

The adaptive cable damping system of Empa and Maurer will be installed on the cables of the “Sutong Bridge“ over the Yangtse River in China. This bridge has a free span of 1080 m and therefore cables of between 150 and 540 m in length. When it will be finished in 2007, it will be the largest cable-stayed bridge in the world. In January 2007, Hans Distl of Maurer Söhne and Felix Weber of Empa demonstrated the working behaviour of the feedback controlled magnetorheological fluid dampers to the bridge owner. The tests have been performed on a 228 m long test cable which exactly corresponds to stay no.10 of the “Sutong Bridge”. The measurements showed that the simulated damping values could be verified.