Remote vibration measurement system for spatial measurement of civil structures

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In order to detect local damage in structures using vibration measurement, measurement and identification of vibration modes with higher natural frequencies is needed. Also, measurement of these modes is increasingly becoming important as evaluation of environmental vibration/noise effect to surrounding residence is required in urban areas to ensure living comfort. However, in civil structures, the difficulties of measuring spatial vibration and disturbance decrease the identification accuracy of higher modes by conventional methods, such as accelerometers, because of its scale and high level environmental noise. Besides, densely distributed measurement tends to be costly since setting of accelerometers requires works at high positions with construction of temporary support for workers. In this project, applicability of Laser Doppler Vibrometer is studied to reliably measure vibration characteristics of spatially large civil structures under microscopic vibration such as ambient vibrat … More ion. By this technology, remote measurement can be achieved by scanning of surface by Laser beam, which potentially eliminates dangerous labor works at high position. In this study, non-stationary, and random components in measured ambient vibration are eliminated by sweeping the area of interested and stacking the time histories recorded repeatedly. In this way natural frequencies and mode shapes of higher modes can be identified with high accuracy by assuming the ambient vibration as a stationary process even without the knowledge of input excitation. Then, the measured modal frequencies and mode shapes after damage and measured data before, damage are compared to inversely solve the location and extent of stiffness change which will indicate the degree of damage in the structure. The method is demonstrated by experiments of plate vibration with additional mass which simulates local stiffness degradation. Finally, field measurement was conducted at actual elevated highways, and the applicability of the measurement is demonstrated. Less