Civil Engineering Reference
In-Depth Information
Tsing Ma Bridge
The Tsing Ma Bridge is a suspension bridge in Hong Kong. An integrated
monitoring system has been installed on the bridge to monitor the condition of
the structure (Wong, 2004). The system includes sensors such as accelerometers,
GPS, strain gauges, anemometers, temperature sensors, and video cameras - a total
of 326 data channels. The system produces about 65 MB of data every hour, and
these data are sent through a fiber-optic network to the base station. This extensive
monitoring system is intended for a variety of purposes related to SHM, including:
(1) measuring loading sources, (2) measuring the system response (e.g., cable forces
and fatigue assessment), and (3) determining system characteristics (e.g., global
dynamic properties).
Among the above applications, the SHM systems for the Humber Bridge and the
National Aquatic Center were designed for temporary use. The SHM system on the
Bill Emerson Memorial Bridge and Tsing Ma Bridge collect data continuously.
11.1.2 Drawbacks of existing SHM systems
Although the previously mentioned wire-based SHM systems have provided
important structural information about the structures being monitored, there
are still shortcomings of these systems.
Currently, SHM systems are only reserved for special structures located at
critical areas. The main reason for the scarcity of the deployment of SHM systems
is the high cost. The cost of a SHM system is mainly associated with sensors, cables,
data acquisition system and installation. For example, the total cost of the
SHM system deployed on the Bill Emerson Memorial Bridge is about US$1.3
million, while the cost of the SHM system on the Tsing Ma Bridge is estimated at
$8 million. In addition, deploying a wire-based SHM system can take months or
even years. This disadvantage can be more obvious if the system only for short-term
purposes.
Due to the system cost, sensors in a wire-based SHM system usually do not have
high spatial density. On the other hand, since structural damage, such as corrosion
and cracking, is a local phenomenon, it is expected that sensors must be deployed in
close proximity to the damage to more accurately capture the response changes.
Therefore, the spatial density of the sensor nodes should be high enough to give
reliable damage information. The relatively low density of SHM system makes
damage detection less effective. For example, in the Tsing Ma Bridge, the number of
accelerometers is only 39. Considering the length of the bridge is over 2 km, such a
sparse distribution of sensors can hardly give an accurate global vibration char-
acteristic for the bridge. Consequently, it is very difficult to detect any possible
corrosions or cracks at an initial stage by examining the accelerations recorded.
One point that should be noted is that most of the current systems do not realize
fully automated monitoring. The large volumes of data are first collected and then
post-processed by experienced civil engineers. Reporting of possible structural
damage automatically in a real-time manner has not been achieved by most of the
current SHM systems.
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