A Multi-Step Approach for Identification of Structural Modifications Based on Operational Modal Analysis

In this paper, an approach for detecting damage and structural modifications is implemented, based on a combination of several indicators derived from the recorded ambient dynamic structural response. In a multi-step procedure, the normalized cumulative power spectral density (NCPSD) evaluated at several structural nodes and a thereof derived "NCPSD damage index" is proposed to detect a modification of the structure. A subsequent sophisticated superposition of a series of mode shape quantities yields a "mode shape damage index" as a function of the structural coordinates that reveals the specific location of this structural modification. Furthermore, it is discussed how environmental effects from varying temperatures on natural frequencies can be substantially reduced. Therefore, long-term monitoring of the structure is required to define relations that allow translating the proposed damage parameters contaminated by temperature effects into the domain of the structure in a reference state. The efficiency of the proposed method is tested on an old iron railway bridge, subjected to a series of structural modifications, such as stepwise reducing the stiffness of local elements, adding mass and an additional flexible support. From the results of this comprehensive study it can be concluded that this nondestructive approach is a promising tool for reliable structural health monitoring (SHM).