Advitam has developed a structural monitoring system to measure and define the behaviour of the Rion-Antirion Bridge during normal operation, strong winds, and earthquakes. The project has inspired a similar monitoring system for the Millau Viaduct in France.
Advitam, a subsidiary of Vinci Construction, develops asset management, civil engineering, and structural monitoring services with a focus on occurrence detection and risk analysis methods. With this unique experience, Advitam was in charge of the design and implementation of a structural monitoring system for the Rion-Antirion Bridge, which spans the Corinth Strait and links the Peloponnese in southern Greece to the Greek mainland. At 2883 metres long, the bridge is frequently exposed to heavy lateral wind forces. It is also located in an area of high seismic activity, with each end on different tectonic plates, resulting in a relative movement of almost two centimetres per year.
The challenge was to design a monitoring system that could operate in this harsh environment while providing a reliable, continuous data stream to record measurements and events that can affect the bridge's structural integrity.
Implementation
The project started three years before the bridge opened and began with a risk analysis to define which elements of the bridge were critical and in need of monitoring. Advitam identified which sensors were required, as well as where they should be placed on the bridge. Required sensors:
* 3D accelerometers on the deck, pylons, stay cables, and on the ground to characterise wind movements and seismic tremors.
* Strain gauges and loadcells on the stay cables and their gussets.
* Displacement sensors on the expansion joints to measure the thermal expansion of the deck.
* Water-level sensors on the pylon bases to detect infiltration.
* Temperature sensors in the deck to detect freezing conditions.
* Linear variable differential transducer (LVDT) sensors on the stay cables to measure movement.
* Loadcells on the restrainers for recalibration in the event of an earthquake.
* Two weather stations to measure wind intensity, direction, air temperature, and relative humidity.
With the addition of a power supply and a lightning conductor control, the required system needed a total of 372 measurement channels. It also had to be capable of acquisition and simultaneous, dynamic processing of multiple signals for the purpose of permanent monitoring, while presenting a user-friendly interface and well-designed output reports. Given the large number of input channels and the adverse operational environment, Advitam chose the National Instruments PXI/SCXI chassis housing National Instruments LabVIEW Real-Time software to perform the task.
The resulting system houses and performs the following tasks with a high degree of reliability:
* Signal conditioning.
* Data acquisition.
* Data processing.
* Program control.
* Data storage.
* Data transmission.
Each of the bridge's four pylons is equipped with an NI PXI-1010 chassis (with eight PXI slots and four SCXI slots) equipped with an NI PXI-8175 controller, SCXI signal conditioning modules linked to the sensors, an NI PXI-6040E module to acquire the data, and an NI PXI-8423/2 module to integrate (in RS485) the data from the weather station. NI LabVIEW Real-Time software runs on each PXI controller to ensure the acquisition and scaling of the measurements, comparing them to fixed or variable thresholds to trigger alarms if necessary. The four systems are linked through a fibre-optic Ethernet network to a control PC installed in the operations building.
Each PXI device continuously acquires data and creates history files that are regularly sent to the control PC. When a threshold goes past its limit, a selective acquisition unit with realtime recording is started on each chassis. A pre-trigger mechanism is also incorporated to identify and record the events that occurred immediately before the alarm.
The interface on the control PC offers different windows to the user, including a synoptic view of the bridge that shows measurement points. The interface also includes automatic and expert function analysis. The computer supports remote access via a modem for viewing data and to redefine monitoring parameters.
For more information contact National Instruments South Africa, +27 (0)800 203 199, [email protected], www.ni.com/southafrica
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