Reference measurement of the structure in April 2004
To get a target orientated estimation of the monitoring system, the first step was a detailed reference measurement. Hereby, the sensors were placed in a very fine grid on the structure, and were loaded by ambient excitation. The results of this test were the natural frequencies, mode shapes and damping ratios. In additional a Finite Element Model (FEM) was built and fitted to the results of the experimental test (Model Updating). This reference model as like as the reference measurement forms the base for future monitoring processes - read more.
Vibration monitoring during the Viennese City Marathon on May, 16th 2004
In course of the 21st Viennese City Marathon run on Sunday May, 16th 2004, Reichsbrücke has been crossed by over 20,000 runners. A couple of cases in the near past have shown that walking or running by a big number of people induces vibrations which can be a problem for the structure. To provide the safety of Reichsbrücke during a big event like this, a very sensitive measuring system for recording the vibrations was installed. The data has shown, that this bridge is strong enough to withstand future marathons crossing it. The data has indeed a very clear effect by the runners, but these didn't pose a threat to the structural safety at any time - read more.
Analyzing of the ship collision at June, 10th 2004
During a turn over manoeuvre of a passenger ferry on the Danube, at June, 10th the ship collided into a main column of the bridge. Experts of MA29 have inspected the structure after the collision and gave following resume: light damage of cover layer of column, but no significant damages and no danger due to safety. The data before and after the collision of the health monitoring system were used to give additional statements - read more.
Start up of the system in December 2004
After Installation of the monitoring system, as well as the connection of the server to broadband internet, the regular operation could start. In December 2004, the system was still in a testing phase, but since January 2005, the autonomous measuring of the system began and the date are transferring in real time to the monitoring headquarter where it will be analyzed - read more at Monitoring System.
Recent results / reference measurement
To get a database for the continuous monitoring, the structure was pre detailed measured which forms the reference measurement. Main target here, was to evaluate the dynamic properties like natural frequencies, mode shapes and damping ratio. By using a mobile test system with - Wilcoxon Sensors - which are very sensitive, it was possible to even get with the slight ambient excitation, representative results.
Analyzing of the data was done by support of the software MACEC of the university Leuven, which uses the so called "Stochastic Subspace Identification" method, to extract the modal parameters. This technology is nowadays state of the art in structural monitoring, because the results, natural frequencies, mode shapes and damping ratios, is very accurate here.
time domain signal (upper left), frequency domain signal (bottom left) and stability program of MACEC (right)
Parallel to the analysing of test results, a Finite Element model was built to numerically analyse it. This model was created with the software MATHFEM to evaluate the modal parameters on theoretical way. The model were fitted to the test results (model updating), where the parameters for variation were modal masses and stiffness of the structure.
Procedure for updating of FE-models
First vertical bending mode (FE-Model)
In course of the 21st Viennese city marathon on Sunday, May, 16th 2005, which started at the Austria Center / UNO - City, 20,000 participants crossed the bridge. In past times, it figured out, that walking or running of a big group of people, can heavily excitate the bridge, which can bring problems due to serviceability or even in some cases of safety.
To provide the safety of Reichsbrücke due to collapse, arsenal research in cooperation with MA 29, has installed a very sensitive testing system to determine the vibration characteristics. Therefore, sensors were placed on the bridge, which can continuous measure the existing accelerations in three directions (vertical, transversal and longitudinal). These results can be compared to the excitaions of normal traffic (tramway and underground/metro).
One of the main dynamical loading on bridges are next to the traffic loading, human induced vibrations by walking running or jumping. Normally, human induced vibrations will cause serviceability problems, where the comfort is restricted or even to uselessness of the bridge. It is very rare, that human induced vibrations will cause serious stability problems (inroad or fatigue). Important on this combination is the avoiding of subjective danger impressions (panic reaction) of humans.
Mainly vertical and horizontal dynamic forces acting bridges, were induced by rhythmic movements of humans. Here the following aspects are of main interest:
- Step frequency from walking , running or jumping
- Number of persons
- "Lock-in" effect (synchronisation between people and bridge)
Normally, the frequency of walking or running ranges from 2 Hz to 2.5 Hz. Next to vertical excitations, mainly side (horizontal) excitations are of interest. The horizontal vibration induced by human walking are the reasons of spectacular vibration problems in last few years (millenium bridge). It was possible to compare the huge number of runners with the excitation of regular traffic with the monitoring of Reichsbrücke.
To identify the maxima values of vibration, the sensors were positioned in the largest span with about 170 m length (approximately 40% of the main span). By using triaxial accelerometers, it was possible to monitor the reactions of the bridge in all directions in spaces, where the transversal direction of runners, was of main interest. Comparisons of the excitation frequencies with those of the reference measurement evaluated natural frequencies, could give an assessment over the unwanted resonance effects. Bigger part of vibration problems in the last years is due to resonance appearances.
Analyzer and accelerometers in Reichsbrücke
Analysing of measured data provides draw conclusions from the vibration behaviour as well as the condition of the whole structure. The sensors are measuring an acceleration signal, which represent the actuel excitation of the structure. Target was it, to catch the vibrations of the rhythmic runners and to compare it with "normal" vibrations, coming from traffic and tramways, underground / metro respectively. With this tool of vibration measurement, it is possible to assess the bridge on a very efficient way. Because of the real time recording of data, it is possible to continuous examine the excitations and to face it with those from regular traffic.
Vibration signal during an underground/metro crossing
The test on the day of marathon has been already performed on Sunday May, 16th 2004, 8:00 am, to examine the behaviour of the bridge in "dormancy" (without street traffic). Before starting of the marathon, there were just few vehicles as well as the underground / metro on the bridge. Following table presents the occurred acceleration for the cases "regular traffic" and "marathon".
Comparison of the measured accelerations
Acceleration during marathon run
Acceleration during regular traffic
As expected, the underground / metro gave the biggest values on accelerations for the structure, where the stimulation energy is decisive in the higher frequency range. Crossing of passenger cars has only shown fewer consequences due to the dynamic behaviour of the bridge. Crossing of trucks could be clearly identified in the signal, but were far below the signals of underground /metro.
Significant vibrations of the bridge came from runners, who induced by sidewise stepping next to vibrations in the vertical direction also vibrations in the transversal direction. The excitation frequency of the runners was about 2.5 Hz.
Analysing of test results has shown, that the fourth vertical bending mode of Reichsbrücke is at 2.75 Hz. So the excitation energy of the runners were concentrated in this mode, which can be clearly seen in the frequency domain spectrum. The amplitude level of the natural frequency in the spectrum is a value to define the energy content. In comparison, for the regular traffic by "ambient" excitation, the energy content over the frequency bandwidth is almost constant distributed. Dangers in according to resonance vibrations and extraordinary high vibration amplitudes were not given.
Frequency domain spectra marathon (left) and regular traffic (right)
Vibrations in lower frequency range are more noticeable for humans, then higher frequency ranges. By the concentration of vibration energy during the marathon in the fourth mode of the bridge, the vibration is subjective for humans more noticeable than the higher frequent excitation by the underground /metro.
In general, the test results have shown that the loading by the runners are clearly under those of the underground /metro traffic. Acceleration values due to running are much higher than those from passenger cars and trucks. In particular the transversal component of force, which are induced by human running, is compared to truck traffic, 3 times higher and compared to passenger car traffic, 6 times higher.
The measured data but represented neither for serviceability (indisposition for the user) nor for load bearing safety a major problem.
Comparison of the measured accelerations (vertical)
Comparison of the measured accelerations (transversall)
Recent results / ship collision
In course of a turn over manoeuvre on June, 10th 2004 of the German passenger ferry "European Viking" on the Danube river near Reichsbrücke, the ship collided against a main column of the bridge.
To support the safety inspections of MA29, arsenal research provided dynamic tests in the box girder of the structure, which is a representative position at about 40% of the main span. The already existing data of the reference measurement were compared with these of the collision from June, 10th 2004, were especially the dynamic behaviour in horizontal direction (direction of collision) were taken into account.
Position of the sensor in Reichsbrücke for assessment of collision
Because lower natural frequencies of a bridge are strongly depended on actual supporting conditions, - and the frequencies are very good represented at the chosen position of the sensor - using of dynamic tests in case of extraordinary events as an additional tool for assessment, is very good applicable for this kind of purposes.
The analysing in this form is an additional method to the safety inspections of MA 29. For a more detailed assessment, a more complex measurement with more sensors would be necessary.
Measured time domain acceleration signal before (left) and after (right) collision
Frequency spectral before (dark blue) and after (light blue) collision