Profile laser scanners for the monitoring of structures

Scientists at the TU Darmstadt have developed a new method for detecting changes to bridge profiles. It works without touching it, just with laser scanners.

Profile scanners as part of a load test on a historic masonry bridge.Photo: Florian Schill

Profile scanners as part of a load test on a historic masonry bridge.

Photo: Florian Schill

Engineering structures such as bridges are exposed to high loads. In addition to the aging processes of the material, environmental influences and an increased traffic volume are added. Regular checks of the bridge profiles are therefore indispensable. However, conventional methods are associated with a great deal of effort. Sensors are mounted at critical points of the structure and removed after the measurement. In some cases, blockages of the respective roads or railroad tracks take place. In addition, deformations are only detected selectively. Namely, where the sensors are mounted. The scientists of the TU Darmstadt have now found a way to scan complete structures from a distance using a laser. Blocking is therefore not necessary, and the process takes place over a larger area.

More than 100,000 bridges – Efficient measuring system required

Safety must be the top priority for buildings. This has once again been proven by the collapse of a four-lane motorway bridge in the Italian city of Genoa. However, there are more than 100,000 bridges in Germany. Of these, over 40,000 bridges on freeways and highways are subject to growing traffic. Another 25,000 bridges bear the weight of trains on railway lines. Their monitoring is clearly regulated in Germany (DIN 1076). The main tests with extensive measurements must be carried out at least every six years, in addition to special situations such as after floods or possible damage from a serious accident. The logistical challenge for a careful monitoring of the structures is therefore very high. “In order to handle the high and increasing number of monitoring tasks efficiently, a modern and practicable system is required,” says Andreas Eichhorn, who is responsible for the Geodesy of Measurement Systems and Sensors (GMSS) at TU Darmstadt.

Team has been able to perform bridge measurements with a standard profile laser scanner. Static and dynamic deformations, such as deflections of a bridge, have been determined with an accuracy of tenths of a millimeter and not only for certain points, as is the case with sensor technology, but also for complete bridge profiles. The basis of the measuring system is a Z + F Profiler laser scanner. Such devices work according to the phase measurement principle. For bridge monitoring, the laser beam is guided over the test object. The repetition rate of the measurement intervals is up to 200 hertz, with a maximum data acquisition rate of one million points per second.

Spatial representation of a train crossing with two wagons. The six figures (lines) correspond to the spatial representation of a defined point in time during the crossing. Presentation: Anna Sviridova / Florian Schill

Space-time representation of a train crossing with two wagons. The six figures (lines) correspond to the spatial representation of a defined point in time during the crossing. Presentation: Anna Sviridova / Florian Schill

Successful test runs on wind turbines and noise barriers

The procedure can be performed from a distance of no more than 120 meters. Since it is non-tactile, that is, without touching, it can also detect locations of structures that are difficult to access. The bridges are accordingly recorded in their entirety. The results show how the structure behaves at rest, how much it deforms under load and whether these deformations are still within tolerable limits. Measurement and evaluation of the data are largely automated by means of a special software. The researchers acknowledge that their measurements are associated with slightly higher uncertainty than the data obtained using conventional methods. But they were sufficient to reliably detect typical structural deformations and assess the condition of the bridges. In addition, the laser scanner procedure is faster and less complicated.

Although the scientists initially focused on bridges, in principle it was also possible to capture other structures using this principle. As a test, the researchers have already scanned wind turbines, noise barriers and factory halls. Thanks to the high scanning rate of the scanner, they were able to capture dynamic building parameters such as attenuation measurements.

Scan buildings with microwaves

TU Darmstadt has already developed new methods for bridge monitoring. A team headed by Matthias Becker from the Department of Physical Geodesy and Satellite Geodesy has used microwaves to measure positional deviations of structures. This technique is typically used to monitor unstable slopes, for example. Also in this case, non-contact measurements from a long distance, which made fatigue-related material damage such as cracks or changes in the elastic parameters visible. However, the microwaves were sensitive to humidity and changes in air pressure, which affected the results.

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