Sensors allow long term control of bridge performance
Over the last few decades there has been a considerable increase in the construction of a number of civil engineering structures, such as bridges, tunnels and pipelines.
In line with this growth, the evolution and advancements in terms of materials and construction technologies, as well as usage requirements for these structures, has dictated the need to improve inspection techniques. Bridges are exposed to load effects and weather impacts that lead to their deterioration, so routine inspection and load testing is needed to obtain estimates on bridge response and to support making decisions on appropriate maintenance activities. This will ensure their structural integrity and safety.
MEASURING LONG TERM PERFORMANCE
With this in mind, a recent trend in bridge management is the utilisation of sensors embedded in the bridge structure, monitoring the long-term performance under various loads and environmental impacts. From this data, meaningful information regarding the bridges reliability and performance can be extracted, which is particularly useful with regards to the growing adoption of the prefabrication technology in bridge construction. The basic instruments used for these measurements are straingauges that are attached directly to the structure of the bridge or as sensing elements in the force sensors. Offering the advantage of high long-term stability, this was a particularly relevant feature when it came to the construction of the new Zezelj Bridge in Serbia.
Currently under construction, the new Zezelj Bridge replaces the original structure, which was built in 1961 but was destroyed in 1999 following multiple direct hits during the NATO bombings. Consisting of steel arches, the new bridge is 474m long and has two railway tracks that act as an international corridor. When it came to specifying measurement technology for the safe construction of the new railway bridge, The Department for Civil Engineering and Geodesy in Novi Sad, Serbia, in cooperation with TRC PRO (The Technical Research Centre) turned to HBM.
RELIABLE SENSOR TECHNOLOGY
Since it is a vital transportation line for the : Balkan states and EU, the project is funded by the European Union and comprises of a distributed multichannel configuration for permanent observation of the critical bridge sections. These include, experimental stress analysis, force measurement, vertical displacements, longitudinal displacements, inclinations, accelerations, frequency and sampling parameters, and temperature measurement in the bridge structure.
Installing a total of 472 sensors, which are designed to be operational during the whole lifetime of the bridge, the project comprises of 328 LY41 series strain gauges from HBM, which are attached in 14 sections to monitor the experimental stress analysis of the bridge. Offering good temperature compensation, this is a key factor as the structure is faced with several weather and temperature conditions. In addition to this, the project also consists of 8o force transducers for force monitoring in hangers, 12 displacement transducers in four moveable support sections (for horizontal displacement), 32 bi-axial inclinometers (for bi-axial inclination and vertical displacement from acceleration measurements with embedded tri-axial accelerometers), and temperature sensors in 20 structural sections, as well as one section of the central pier zone.
ACHIEVING AN OVERALL SOLUTION
To achieve fast, reliable measurements with easy set-up, real-time calculations and diagnostics information, the PMX measurement and control system from HBM is used to perform measurement and data acquisition tasks. As an additional protection against dust and EMI, the PMX data acquisition unit is mounted in a cabinet, which offers a cost-effective solution and also includes the power supply. All signals are sampled with 19.2kHz, which guarantees a high measurement-bandwidth and evaluation of the measurement signals. All adjustments to the PMX system are handled via the standard Ethernetinterface and the internal PMX web server. This solution offers the great benefit that adjustments can be done directly at the installation point or via the Ethernet network at the control room or even remotely if additional WiFi is available. This means that each engineer is able to have a real-time view of the application and the status of the test. An integrated operator-control also prevents unsecure operations.
The behaviour of railway and traffic loads require the most complex monitoring system due to the varying type and intensity of loads and also because a bridge's structural safety is important in everyday life. With this in mind, structural health monitoring is paramount. Performed via a set of activities - which includes observation, data acquisition, transfer and analysis of data acquired by long time measurement during the bridge's exploitation - the ultimate goal for monitoring bridges structural health is to form a database for tracking the behaviour of the bridges structure in order to avoid any potential deterioration in the bridge's safety and performance.
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