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Monitoring of Cable-supported Bridges
Background
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.
Monitoring System
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.
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| Fig.1 Tatara Bridge with New Monitoring System |
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