Ad-hoc evaluation of bridges with optical camera tachymetry
The Federal Highway Research Institute (Bundesanstalt für Straßenwesen) has assessed 51,608 of the 140,000 domestic bridges in Germany and awarded a condition rating of inadequate or worse in 2,360 cases (as of 2018). Bridge structures must be inspected for their state of health at regular intervals. Monitoring is mainly carried out by acceleration sensors.
An innovative optical method is being investigated as part of the project. The modular, camera-based precision total station MoDiTa enables non-contact and high-frequency measurement of natural targets on the structure. Even longer image series can be taken at several hundred hertz. In this way, bridge deformations are recorded at high frequency.
We see the strengths of the optical system in the fact that the measuring system can be used ad hoc on a structure. Static or dynamic changes can be recorded reproducibly. While typical systems of Structural Health Monitoring SHM have to be provided already in the construction phase, the use of MoDiTa does not even require structural retrofitting.
In general, natural frequencies and damping constants are used for the current condition assessment of a bridge structure. To collect the necessary initial data, the system is positioned as close as possible to the bridge, Fig. 2. This procedure leads to an increase in measurement accuracy by reducing meteorological influences. The results from the monitoring are adjusted, mathematically modelled and statistically processed. Local damage to the bridge structure is reflected in significant changes in the natural frequencies and damping constants. Both parameters are derived from the ambient window, which spans from the loading of the train to the transition to the resting state, fig. 3. The non-uniform deformations during the train crossing result from the punctual loading by the train axles. In this context, the state of health of the south bridge in Mainz-Weisenau (truss bridge) and the railway bridge in Lahnstein (bar arch bridge) were examined.
The analysis of the parameters leads to consistent results in Lahnstein on two days with different stand and target points. Consistent parameters indicate that there is no evidence of damage at the discretely observed bridge areas. At the south bridge in Mainz, moderate deformations are observed during train crossings, but no significant swinging out after the train crossing. Apparently this bridge is very stiff, the methods of the SHM do not provide any information on the condition of the bridge. In general, the final and expert interpretation of the derived parameters is done in an interdisciplinary exchange.
The optical system offers itself as an economical alternative for monitoring aged bridge structures. So far, its applicability has only been proven for steel bridges.