Health Monitoring of the Viennese Reichsbrücke
Wind, earthquakes, road traffic and other influences represents the loading of a bridge during its lifetime. To permanent assess the condition and the safety of the Reichsbrücke, arsenal research has installed in cooperation with MA 29 and Aplica Mess- und Prüftechnik a permanent monitoring system consisting of accelerometers and temperature sensors. The main target is to monitor the condition of the bridge, detect cracks and other damages in an early stage, to provide the serviceability and the safety of the bridge.
By using the tool of health monitoring, it is possible to evaluate the actual reliability (bearing capacity, serviceability and durability) of the structure and furthermore to permanent record the structural changes over time. Thus, the condition of the structure is accurately monitored and future developments can be forecasted. In particular, this information can be taken as a base for decisions for ongoing sustainments and give answers for the following questions:
- Are there damages on the structure?
- If yes, is it possible to localize them?
- What is the amount of the damage, are immediate/urgent measures necessary?
- Is the structure able to continue its function with adequate safety?
To give an answer on those questions, the records of the vibration characteristics of the structure, the so called modal parameters like natural frequencies, mode shapes and damping values, which were evaluated with special sensors, can be used. From these date, a representative situation of the actual structure can be derived. Health monitoring and permanent assessment of Reichsbrücke has been given to commission by the municipal department MA 29 (Dep. for bridge- and soil engineering) to the business unit of transport routes engineering of arsenal research.
Health Monitoring >> Application
The maintenance and monitoring of bridges plays in the economic point of view an important role. Reliable monitoring of the structures is for this reason a main task for the future.
To assess a structure, it is nowadays state of the art to perform a reference measurement with an additional fiinite element model, where the results of the measurement will fit to the numerical model. This reference mesurement as well as the FE-Model are the base for ongoing measurements, where changes in the dynamic properties can be detected. For a continuing monitoring of the structure, it is possible to do this by permanent or periodic assessment.
- Periodic assessment: The measurements to detect the dynamic characteristic are made in pre defined constant time intervals. The positions of the sensors are hereby adjusted to the sensor position of the reference measurement.
- Permanent assessment: Here, the sensors are fix installed in the structure and are recording the vibration data continuously.
Health Monitoring >> Problem formulation
In terms of the supposed life span of the existing structural inventory, there will be enormous costs for maintenance and costs for retrofitting to keep the structure on the necessary safety level. In most cases, it will be necessary to perform in additionally to the visual inspections, a target oriented structural monitoring, to give a quantitative database of objective test results, for future decisions on arrangements.
Austria has a dispositional ability of approximate 9.5 billion € of 12.000 bridges with a total area of 7.4 million m² under traffic, for example. The annual costs for sustainment ranges from 10 to 15 € per m², of trafficable bridge area. Next to strategic costs due to utilization restrictions during the retrofitting works, the costs for reconditioning in bridge engineering are approximate 0.8 % to 2.5 % of the costs of its new construction.
With regard to the necessary investigation decisions for sustentation of structures, there is still the open question of remaining lifetime, actual state of serviceability and load bearing capacity. The remaining lifetime of a structure is mainly dependent on the appropriate service works, which was carried out during the time. Due to the less money, which is often available for the sustainment, this should be used to get the biggest advantage. The choice of planning the sustainment and maintenance methods are comparable to an optimizing task, where the parameters on the one hand side is the fulfilment of the safety/reliability, on the another hand side the costs in accordance to the lifetime is.