Time-frequency analysis of suspension bridge response for identification of vortex induced vibrations
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It is increasingly more frequent to install continuous monitoring systems on large bridges around the world. These systems are then used to monitor the loading and structural behavior, which generally translates into continuous estimates of the bridges’ modal properties. Because of practical reasons, in general only the lower frequency range of the signals is stored and analyzed, and only a limited number of sensors can be installed. This means that for the particular case of suspension bridges it is not feasible to instrument each hanger. This paper shows that it is possible to detect vortex induced vibrations on hangers indirectly by monitoring the accelerations at the deck. The study is based on the acceleration signals collected by a permanent monitoring system installed on the deck and pylons of the Hardanger suspension bridge (1310 m span) in Norway. Additionally, the temporary measurements of some of the hangers are used to assess the validity of the predictions. The signals are analyzed in time-frequency domain by means of Short-Term Fourier Transform and Continuous Wavelet transform. The paper includes some discussion on the advantages and disadvantages of both methods for the task at hand. The results clearly show the presence of frequencies associated to vortex shedding on the hangers and how these frequencies change with wind speed. The ideas reported in this study should allow owners of large bridges to obtain more information from their existing monitoring systems.