Mode tracking for a steel railway bridge in presence of changing support conditions

Abstract 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 data, typically accelerations. This process is repeated on a regular, e.g. hourly, basis. In a second step, which is referred to as the mode tracking, the modes obtained from multiple OMA are linked across time windows, resulting in a subset of modes which can be tracked in time. The mode tracking typically relies on criteria that measure the similarity of the natural frequencies and mode shapes, assuming these do not change significantly over two time windows. In this paper, we consider the case of a bowstring railway bridge in Halle, Belgium, where the modes of the bridge can be assigned to two different states. This is observed from the identified natural frequencies, which are characterized by sudden shifts between consecutive time windows, which complicate the mode tracking. Displacement measurements at the bearings show that the transition is due to a change in support conditions under thermal action. It is explained how these transitions are explicitly accounted for in the mode tracking.