The footbridge in Tours
In the city of Tours, in France, a new footbridge has been constructed. As part of commissioning, vibration measurements are carried out. By means of modal analysis, dynamic parameters up to 6 Hz of the footbridge are identified:
• Natural frequencies for all detected eigenmodes,
• Damping ratios for all detected eigenmodes,
• Mode shapes for all detected eigenmodes.
To perform this, the company Wölfel Engineering has used the RECOVIB Tiny to measure the bridge vibrations.
Thanks to RECOVIB Tiny, you no longer need to unwind metres of cables or install sophisticated wireless networks. (to long for Wi-Fi)
A shaker system is used for excitation of the bridge structure with two different
orientations: in horizontal lateral direction and in vertical direction.
9 RECOVIB Tiny are positioned on the bridge deck.
Nine RECOVIB Tiny are fixed to the bridge thanks to the magnets
Several measurements were recorded for both shaker configurations on both bridge sides. Based on broad band excitation measurements vibration data is evaluated: mode shapes analysis are calculated and afterwards natural frequencies and damping ratios are identified using curve fitting procedures from experimental modal analysis (EMA).
|Time||25 January 2017|
|Measurement System|| 9 units of 3- axis shock & vibration data logger Recovib Tiny
Manufacturer: Micromega Dynamics
|Excitation|| Type: Electro dynamic shaker
Manufacturer: Wolfel Monitoring Systems Gmbh+ Co. KG
Frequency range: 0.5 Hz to 200 Hz
Maximum force: 500 N
6 (on bridge)
2 (on opposite bridge side)
1 (on shaker)
| Manufacturer: Micromega Dynamics
Model: 3-axis vibration micro logger Recovib Tiny
Measurement range: ± 2g
Frequency range: 0 Hz to 640 Hz
|Calculation|| Transfer function (Shaker force to vibration velocity):
– Window function: Hanning; width: 10 s; overlapping: 50 %
Integration (vibration acceleration to vibration velocity):
– High pass filter with 0.1 Hz cut-off frequency
|Analysis|| Mode shapes are visualized based on measurement results of RECOVIB Tiny
sensors providing more degrees of freedom for visualization
An engineer from the Wölfel team, ready to launch the shaker and the vibration analysis
Mode shapes Analysis
The instrumentation campaign was able to identify the first four main resonances frequencies:
- Rocking mode: 1,4 Hz
- Torsional mode: 1,9 Hz
- Bending mode (Horiz.): 3 Hz
- Bending mode (vert.): 4 Hz
Eigenmode no. 1: Rocking mode, mainly horizontal transversal direction
Eigenmode no. 2: Torsional mode
Eigenmode no. 3: Bending mode, in horizontal transversal direction
Eigenmode no. 4: Bending mode, in vertical direction
For the purpose of carrying out this vibratory analysis on the footbridge, Woelfel Engineering chose to use the new vibration recorder, the RECOVIB Tiny, an extremely practical device for a fast and efficient vibration diagnosis.
Its wireless technology and intuitive operation provides substantial time savings in the performance of the measurements.
The RECOVIB Tiny is the only range of vibration and shock recorder that allows several sensors to be used simultaneously in a synchronized manner enabling the performance of modal analyses and operational deflection shape analyses on large structures such as this footbridge.