HISTORIC MASONRY MONITORING BY MOTION MAGNIFICATION ANALYSIS

Vibration monitoring of historic structures in urban environment is a relevant issue for health survey and early damaging detection in sustainable and enhanced resilient cities. This study explores the potentialities of vibration monitoring by Motion Magnification Analysis. Motion magnification acts like a microscope for motion in video sequences, but affecting only some groups of pixels. The motion magnification uses the spatial resolution of the video-camera to extract physical properties from images to make inferences about the dynamical behaviour of the object, e.g. to visualize at least the first mode shape, no matter its dimensions, since any point on the surface of the object can be considered a virtual sensor. Recently, a number of experiments conducted on simple geometries like rods and other small objects, as well as on bridges, showed the reliability of this methodology compared to accelerometers and lasers. Researchers have been also very interested in assessing the method's feasibility, since conventional devices are surely more precise, but more expensive and much less practical. In this paper, we give an introduction to MMA and describe its application to the analysis of two full-scale historic masonry walls tested on shaking tables. This is an interesting point, because the size of tested walls is larger than usual small experimental set-ups implemented in MMA testbed until now. Results showed that MMA allowed a visual identification of fractures in advance. Moreover, we performed some conventional calculation for modal analysis of the walls, such as FRF and PSD, on MMA output data. Though the used equipment (camera, tripod and lighting) was of low quality, in order to test the methodology in an unfavourable environment with very high data noise, the estimate of the first modal frequency showed a good agreement with modal analysis by a more conventional optical system used as reference.