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Development of non-contact scanning vibration measurement system for real-scale structures

In order to rationalize vibration monitoring in structures, this paper addresses a non-contact scanning vibration measurement system employing laser doppler vibrometers. Generally, in the case of measurement for concrete members or dirt-adhering steel members, monitoring by laser doppler vibrometers is extremely difficult due to low laser reflectance. In this study, it is experimentally verified that the return or reflected laser beam quantity depends on subtle differences in the surface condition of a measurement object. Therefore, by searching the vicinity of the insufficient measurement point for the optimum point which provides a maximum reflected laser beam quantity, a remarkable improvement of measuring accuracy for real-scale structures can be achieved. As an example, vibrations of steel girders and reinforced concrete deck of actual bridges are measured, so as to verify the measuring accuracy of the developed system, and eigen local mode shapes of the members are able to be identified.