Identification of Vibration Mode Shapes for a Bridge Deck under Ambient Vibration for the Health Monitoring

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Vibration measurement is a powerful technique for structural health monitoring. Identification of high order vibration characteristics, especially vibration mode shapes, is needed to detect local damage in structures. However, in civil infrastructures, the difficulties of measuring spatial vibration and input excitation decrease the identification accuracy of these higher modes. Laser Doppler Vibrometer can be applied to reliable measurement of vibration of spatially large structures under microscopic vibrations such as ambient vibration. In this study, a spatial vibration measurement system is established, and non-stationary component in measured ambient vibrations are eliminated by sweeping the area of interest and repeatedly stacking the time histories recorded. In this way, mode shapes can be identified from these vibration measurement results without knowledge of input excitation.Vibration mode shapes of a bridge deck are identified using this laser vibration measurement system. However, in this case, lots of optical noise is contained in the time histories recorded for that made of concrete, because of the reduction of laser reflection quantity. To overcome this, for measurement points which are impossible to measure vibration, many sub-measurement points around that are set, and laser reflection quantity of each sub-measurement point is measured, respectively. By calculating the mean values, optimum measurement points that give maximum laser reflection are selected. Based on this function, the vibration mode shapes of the bridge deck are identified under ambient vibration, and it is verified experimentally that non-contact vibration measurement using Laser Doppler Vibrometer is possible to apply for bridge health monitoring.