Hanging tie study using the “moving deflection spectrum”

A hanging tie is a form of railroad track distress that occurs when voids have developed beneath the ties due to uneven ballast settlement and improper maintenance practices. It will lead to an increase in dynamic impact loading on the top of the ballast thereby further deteriorating the track structure. This paper proposes a fast, nondestructive screening method to identify the hanging tie problem. The method utilizes a dynamic track model to characterize the track’s “moving deflection spectrum” under different tie-supporting scenarios. The model includes a moving dynamic load and ties with discrete supports. The moving deflection spectrum shows dynamic responses of the “track moving deflection” in the frequency domain. The modeling results indicate that a significant discrepancy exists in the moving deflection spectrum depending on whether or not there is a hanging tie condition. To validate this method, preliminary field tests were carried out on both Boston Metro and St. Louis Metro lines. Then the moving deflection spectrum generated by the model is compared with the moving deflection spectrum in the field tests as measured by the accelerometers installed on a high-rail vehicle. Results show that the method is effective in identifying the hanging tie problem and has great potential to be employed by the rail industry in the future.