ABSTRACT
ABSTRACT: The paper gives an overview of the necessary conditions for a successful structural condition assessment by vibration measurements. First a critical overview is given about the advantages and disadvantages of the different excitation sources: forced, ambient and impact excitation. For damage level 1, the advantage of ARX-models, simulating the relation between the histories of eigenfrequencies and the histories of temperatures, over classical regression techniques is enlightened Other new methods like the one based on features extracted from spatial filtering will be presented as well. Many methods have been proposed for damage identification, levels 2 and 3. The paper concentrates on the most versatile of them: finite element updating. At the expense of building a FE-model of the structure, all acquired data can be used: natural frequencies, scaled or unscaled mode shapes and modal strains. Potential damage is simulated in a FE-model of the structure by adopting a parametric representation of this damage. Mathematically, a constrained optimization problem is solved. The objective function is defined as a sum of squared differences. The residual vector contains the differences in the identified modal data (and possibly some derived quantities), such as the natural frequencies, the mode shapes, the modal curvatures, etc. If the objective function contains multiple local minima, a global optimization algorithm should be used, like the Coupled Local Minimisers (CLM) method. Two examples illustrate the use of FE-updating for damage assessment. The first application concerns the well known Z24 bridge in Switzerland. The second example is the Tilff bridge. New in this project was the use of very accurate optical fibers for strain measurements. Finally, a series of conclusions are highlighted which can be considered as guidelines for good practice of vibration monitoring.
