| Summary: | Cantilever construction techniques and reduced deck bending moments have made cable stayed bridges popular for long spans. Recently; however, large amplitude wind induced cable vibration problems have occurred on several of these long span bridges. One of these bridges had several of its longest cables fitted with discrete dampers near the lower end cable socket. Although effective, these discrete dampers, which resemble large shock absorbers, are obvious and very unappealing.
Some of these large cable oscillations are caused by eddy shedding but others appear to be created by some other means of excitation. Past research suggests that wind induced support motion combined with low damping in the cables may produce local vibration problems. Many authors have proposed closed form and analytical solutions to the dynamic response of a cable with sag. Due to a lack of measured values, estimates of cable damping have been used in these analyses.
Although seme scale model tests of a cable stayed bridge have been done, no actual full scale tests have been performed. With the Annacis cable stayed bridge being constructed in the Vancouver area a unique opportunity arose to perform full scale dynamic tests of the cables as well as the entire bridge.
Through a series of tests of cables of varying length, the natural frequency and damping value of each cable was determined. A parameter study was undertaken in order to determine the nature of the damping present.
Each of the cables supporting the Annacis bridge is fitted with a split neo-prene ring in the cable anchorage assembly at each end. Hidden from view, this neoprene ring was designed to seal the top socket and provide a discrete damper at each end of the cable. Tests done before and after these dampers were in place determined their effectiveness.
The possibility of large cable motions being caused by wind induced support motion as suggested by the previous literature was investigated. Tests were carried out to determine if cable motions were harmonically related to the motion of the towers or deck.
Finally a simple pendulum was used to harmonically excite the completed bridge in several of its lower modes. This test provided a check on the natural frequencies of these modes as well as providing full bridge damping values from the free vibration response. === Applied Science, Faculty of === Civil Engineering, Department of === Graduate
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