Application Note: Operational Modal Analysis on a bridge structure

For analysing big structures the Operational Modal Analysis is used. This application note shows how to use SIRIUS® instruments for data acquisition and DS-MODAL-OMA software for the in-depth modal parameter extraction. Furthermore it gives some hints how to plan the test (reference sensors, virtual points) and what has to be taken care of (sensors, amplifiers).

INTRODUCTION

Together with the Italian consultant company ESSEBI Srl (http://www.essebiweb.it/index.php) we did a measurement to get the mode shapes of a high speed train bridge segment, approx. 16m long and 10m wide. Because there were only 15 sensors available, the task was split into two measurements, changing sensor positions between.

MEASUREMENT SETUP

EQUIPMENT USED

• DAQ system: 2 x SIRIUSi-6xACC,2xACC+, synchronised with sync cable
• Acceleration sensor PCB 393A03 (1 V/g)
• Acceleration sensor PCB 393B12 (10 V/g)

SOFTWARE USED

• Dewesoft X2 SP6 (for data acquisition)
• DS-MODAL-OMA package (for post processing)

PHOTO SETUP

GEOMETRY AND SETUPS

Because there were only 15 acceleration sensors available, the measurement had to be split into two setups. Two triaxial reference sensors were kept at the same positions as references (red arrows). The green arrows show the orientation of the sensors.

In the picture below you already see the finished model.

It also includes virtual points (without arrows), which can be linked manually to existing sensors by the use of equations. If you assume some parts of the structure as rigid, these points can enhance your geometry animation and give a more realistic behaviour even if it was not possible to mount a sensor in each point.

DATA ACQUISITION

For modal analysis of big structures we are primarily interested in the lower frequencies, therefore the sampling rate was set to 500 Hz. The whole measurement chain must be able to measure low frequencies, so we used acceleration sensors covering the low end with 0.5 Hz resp. 0.05 Hz. The new SIRIUSi-ACC amplifiers are able to measure down to 0.1 Hz by default.

The measurement duration was around 45 minutes for each setup, in the picture below you can see the raw data with the short segments of the train passing by (around 7 seconds each). During the test measurement it was noticed that when mounting the very sensitive 10V/g sensors in upright position (Z axis), they went to overload when the train was passing by, because they have a smaller range. So the mounting orientation had to be changed.

DATA EXPORT

For both setups the data was exported to UFF file format (Universal File Format) for easy import to DS-MODAL-OMA for post processing. You can define the channel attributes (response node, response direction) e.g. 14Z+ already in Dewesoft X2 or later when importing in DS-MODAL.

DS-MODAL-OMA ANALYSIS

MODAL IDENTIFICATION

For data import of two different setups, the files must have the same length. In DEWESoft you can easily mark and export an exact time interval with the “time selector”.

The data (500 Hz sampling rate) was decimated to 60 Hz first, which results in a spectrum up to approx. 30 Hz.

After calculation of the Half PSD, the modal identification technique Narrowband OMA was applied. You can refine the result by checking the curve-fitting in the “Enhanced view”.

MODE SHAPES

In the picture below you see two example mode shapes, the first bending (9.77 Hz) and the first torsional (10.81 Hz) mode.

If you like further information on the DEWESoft Data Acquisition Systems or Software and reside in Australia or New Zealand, please contact Metromatics on +61 7 3868 4255 or sales@metromatics.com.au  

For outside this area, contact Dewesoft directly on +43 31 32 2252 or sales@dewesoft.com