Structural health monitoring of the KW51 bridge based on detailed strain mode shapes: Environmental influences versus simulated damage

Strain mode shapes are attractive features for vibration-based monitoring as their sensitivity to local damage is known to be high. However, the assessment of their performance in situ has remained an open problem. This work presents the continuous dynamic strain monitoring of a steel railway bridge with FBGs for one year. The natural frequencies and strain mode shapes of ten modes are automatically identified from operational strain time histories, on an hourly basis. The influence of temperature on these modal characteristics is quantified and then investigated with a FE model of the bridge. A clear influence of temperature on the natural frequency of most modes is identified, especially during frost periods, contrary to the strain mode shapes, which are mostly insensitive to temperature changes, as confirmed also by the FE model. The FE model is employed to investigate also the influence of simulated local damage on the modal characteristics.