Our new vibration recorder the RECOVIB Tiny: mode shape analyses of large structures such as bridges

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).

 

Measurement settings

 

 

  Time  25 January 2017
  Place  Tours, France
  Object  Footbridge Sanitas
  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

Model: BD.5

Frequency range: 0.5 Hz to 200 Hz

Maximum force: 500 N

  Sensors

   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.

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