Abstract Summary
Vibration-based damage detection commonly relies on a monitoring of the modal characteristics of a structure over time. It is assumed that damage will result in a change of the structural stiffness, which will in turn lead to a change of the natural frequencies and the displacement/strain mode shapes. The monitoring of the modal characteristics generally consists of two consecutive steps. First, the modal characteristics are identified by application of operational modal analysis (OMA) on a time series of response measurements, typically accelerations. This is for example repeated on an hourly basis. In a second crucial step, which is referred to as the mode tracking, the modes obtained from the OMA are grouped, resulting in a limited set of physical modes that occur in multiple time frames. The mode tracking typically relies on the assumption that the natural frequency and mode shape of a given mode do not alter significantly between two consecutive time frames. In this paper, we consider the case of a bowstring railway bridge in Halle, Belgium, where the structural dynamic behavior is characterized by two clearly separate states. This is observed from the identified natural frequencies which are characterized by sudden shifts between consecutive time frames. It is explained how these transitions are explicitly accounted for in the mode tracking, leading to a significant improvement of the mode tracking accuracy. Displacement measurements at the bearings show that the transition is due to a change in support conditions under thermal action.
