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Remote modal study of reinforced concrete buildings using a multipath lidar vibrometer

Abstract:

Over recent years there has been growing interest in building frequency analysis using ambient vibrations in the fields of structural engineering with application to earthquakes, health monitoring, or operative testing. The direct applications are (1) to define the modal characteristics of existing buildings for modeling their seismic response, (2) to monitor the long-term variations in their structural health, and (3) to detect and localize changes in the structure such as those produced by earthquake damage. Simultaneously, velocity measurements with laser remote sensing techniques have gained interest for several applications. For example, coherent lidar systems enable accurate measurement of the vibration velocity of remote targets. This allows operative modal analysis (OMA) of potentially damaged buildings, for their diagnosis from a safe distance after a seismic event. This paper compares the frequency analyses obtained using sensitive velocimeter sensors and coherent lidar sensors, applied to a number of existing reinforced concrete (RC) buildings. Ambient vibrations recorded by velocimeters are processed using the frequency domain decomposition method for defining building frequency and mode shape, while the laser remote sensing approach uses the coherent lidar method to measure velocity, frequency, and mode shape. The results of a real-scale trial on five buildings in Grenoble, France, are presented. The reliability of this technique for remote structural diagnosis is discussed and the modal parameters, as measured by lidar at a range of 200 m and by in situ velocimeters, are compared. The results from the two instruments were in good agreement, which leads us to conclude on the ability of the coherent lidar approach to assess the modal frequencies and mode shapes of existing buildings at long range and without any retroreflectors on the structure for structural and earthquake engineering purposes.