190 E. Mola et al. Fig. 23.3 (a) Force-displacement diagram of a friction pendulum device under seismic conditions; (b) Friction pendulum bearing modelled by nine springs Table 23.1 Linear modal analysis results: modal frequencies MI–TO way Mode f (Hz) Vibration mode of the whole MI-TO way Vibration mode of the investigated spans Mode f (Hz) Vibration mode of the whole MI-TO way Vibration mode of the investigated spans 1 1.27 2 1.29 3 1.32 4 1.37 5 1.45 6 1.53 7 1.58 8 1.59 9 1.61 10 1.63 11 1.65 12 1.70 13 1.73 14 1.75 23.4 Operational Modal Analysis Operational modal analysis was applied to process the experimental data: wind, vibrations induced by the surrounding working area, nearby traffic etc. could provide suitable environmental excitation for modal parameters identification. In order to ensure the robustness of the results, the time duration of test was eight hours: this has allowed stabilizing the spectra with a signal/noise ratio good enough to get reliable results in the next modal analysis identification. In addition, it was necessary to check temperature changes to eventually correct any deviation in the modal parameters depending on its value: this dependence was proved to be negligible in this case. The availability of a large amount of data coming from low noise transducers allowed averaging over long time windows, in order to obtain frequency spectra characterized by an acceptable signal to noise ratio.
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