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Monitoring of Cable-supported Bridges
Cable-supported bridges, from suspension bridges to cable-stayed bridges, became very popular with the invention of very high-strength steel which made it possible to support longer spans linking even nearby islands. The elegance of these type of structures, compared with the monstrous steel truss type structures, makes it the design choice in most feasibility or tendering competitions. Coupled with the straight forward construction procedure established over the years, there are now hundreds of these bridges spread all over the world.
The construction of a cable-supported bridge is a huge investment for any country and so monitoring and maintenance of these structures is normally included in the project package before construction.
Different causes of damage exist, not only the usual destructive events such as earthquakes or wind-induced vibrations, but also damage and deterioration due to aging, exposure to deleterious chemical substances, fatigue, etc. The cables, most especially, are in danger of rusting which may not be evident using the naked eye because the cable strands are normally covered with polyethylene for corrosion protection. Thus, a more effective form of monitoring is to consider changes in the natural frequencies of the cable as an indicator of damage. However, in continuous monitoring, embedded sensors are normally placed on the bridge decks or towers and very seldom on the cables. At the same time, attaching ordinary sensors to the cables during monitoring is not only time-consuming but also a difficult task.
Here, in the Bridge and Structure laboratory in cooperation with Keisoku Research, we've developed a monitoring system that is fast, safe and accurate. By combining a single-point laser doppler vibrometer (LDV) with a total station (TS) we are able to measure ambient vibration of cables from a safe distance from the bridge. Once the target measurement points have been identified by the total station, the measurement becomes automatic and any number of measurements can be made. This will also enhance safety when monitoring bridge cables after a strong earthquake especially when there is fear of possible collapse.