B. Steck, E. Robbe & N. Humbert


Figure 12. Display of the detection of normal mode using the previous post treatment and modal identification with mode shape for the combination of two detections calculated with different reference sensors (758 and 1004)


4.3. Verification of the installation’s ability to represent mode shapes In the case study of the arch dam, the presence of four boxes at the crest influenced the number and placement of sensors. To validate this setup, a punctual vibration measurement was conducted using five additional sensors (orange circles in Figure 2, resulting in a total of nine sensors on the crest of the structure and one on the rock. One hour of recording was performed in October 2024. An operational modal analysis (OMA) was conducted on this recording, identifying the first seven modes and their corresponding mode shapes. These mode shapes are compared in the following figure with those obtained from the four main stations. The comparison reveals that for the first four modes, the mode shapes obtained using four stations are similar to those obtained with nine sensors, although the mode shapes appear less smooth. For higher frequency modes, the mode shapes are significantly degraded when considering only four stations. If these modes are of specific interest, additional stations should be added to the setup.


Finally, when significant changes in the stiffness or mass of the studied structure are anticipated, the number of stations must be increased because the mode shapes could be altered and make the chosen station locations suboptimal.


150 | Dam Engineering | Vol XXXIII Issue 3


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