Truss Railway Bridge Indirect Frequency Estimation by Time-Shifted Accelerations Difference

In the field of Structural Health Monitoring the modification of the natural frequencies of an infrastructure can be used for monitoring the occurrence of a damage and to follow its evolution. This paper deals with the identification of the first flexural frequency of a railway bridge from the accelerations measured on board of a transiting train. In a real scenario there are several external disturbances that can affect the effectiveness of any drive-by method, among these there is the track geometrical irregularity. To deal with this issue, the authors propose an approach based on the technique of time-shifted signals subtraction. A mathematical formulation is derived, where the role played by the ratio between shifting time and signal period in the performances of the time-shifted subtraction operation is clarified. To check the method, train-bridge dynamical simulations including track irregularity were performed. As a result, sensing points onboard positioning as well as a sufficient target bridge frequency excitation during train transit emerged as key factors to be met simultaneously for the success of the methodology illustrated in this work.