Identifying structural properties of a steel railway bridge for structural health monitoring using laser Doppler vibrometry

Steel railway bridges are indispensable to the safety, serviceability, and sustainability of the U.S. freight and rail network. Reduction in the stiffness of steel railway bridges is responsible for their strength-related and serviceability-related failure modes. Recent advances in the development of remote sensing techniques such as laser Doppler vibrometry (LDV) have enabled bridge engineers to assess the structural performance of bridges with the information that conventional visual inspection cannot provide. In this paper, experimental work and data analysis on the identification of structural parameters (generalized stiffness, generalized mass, and generalized damping) of a steel railway bridge under the application of a moving commuter train, using a portable LDV system, are presented. Background and train-induced vibration measurements (velocity and displacement) at the midspan of the bridge girder were measured. Time-domain displacement measurements in free vibration and forced vibration were used to identify the frequencies representing background noise (55.67 Hz), train loading (0.996 Hz), and the fundamental mode (8.698 Hz) of the bridge, using a single-degree-of-freedom bridge model. From the analysis, the damping ratio of the bridge was found to be 1.322% and the train speed to be 57.1706 mph (25.5575 m/s). A bounding approach was proposed to address the uncertainties in our calculations by providing the upper bound and the lower bound values of generalized stiffness, generalized mass, and generalized damping of the bridge. Monitoring the change in these structural properties can better assist decision makers in routine maintenance and asset management of steel railway bridges and other critical civil infrastructures.