Modal Analysis of Structures by Means of Optical Methods

The experimental assessment of modal frequencies (shapes and damping) of structures is generally performed in terms of acceleration, velocity or displacement by using respectively accelerometers, velocimeters (also known as seismometers of geophones) or interferometers (laser scanner and similar). These are not, however, the only possible approaches.

In this work some results than can be achieved by using normal video-cameras currently on the market (passive optical methods) are presented. Two types of approach are possible in this case: Lagrangian or Eulerian. Digital Image Correlation is a quite well known Lagrangian approach, but its application is more frequently restricted to laboratory conditions, due also to the impact of variable illumination in outdoor environments. Optical Flow methods are less known Eulerian approaches, with a number of limitations when their underlying assumptions do not hold.

In this paper, some original variations within these main categories are studied and applied to a real structure. In all cases it is necessary to achieve sub-pixel resolution, because the movement of civil structures is often smaller or much smaller than the real size covered by a pixel. Limits and possible future developments of the proposed techniques are then discussed.