On the efficiency of mitigation measures in reducing the amplified response at transition zones in railway tracks: tuned mass dampers, auxiliary rails, and under-sleeper pads

Abstract

In transition zones, railway tracks experience significant inhomogeneity in their mechanical properties—more specifically in vertical stiffness. In such areas, conventional tracks (soft tracks) are typically encountered with other engineering structures with noticeably larger stiffness, such as bridges and culverts (stiff tracks). This inhomogeneity together with the passage of high-speed trains leads to amplification in dynamic response, which in turn results in faster degradation and higher cost of maintenance at transition zones. In practice, various mitigation measures have been adopted which have led to improvement in track performance to a certain degree.

This thesis is mainly focused on the feasibility of using the tuned mass damper (TMD), as a novel mitigation measure, for improving the aforementioned undesired behavior. Additionally, the efficiency of two already existing corrective measures, namely auxiliary rail and under sleeper pad (USP), is investigated at transition zone.

The track is modeled as an infinite one-dimensional Euler-Bernoulli beam resting on a piecewise-homogeneous and continuously distributed Kelvin foundation. For each mitigation measure, semi-analytical solutions are derived through the Fourier transform method. Regarding TMD analysis, mechanical parameters are optimized by an evolutionary algorithm (NSGA-II), in which the discrepancy between the soft and the stiff tracks’ wavenumbers is minimized. In regard to auxiliary rail, two configurations with multiple number of extra rails (ERs) are evaluated; ERs over soft track only, and ERs over all domains. Additionally, USPs with different stiffness are considered for their arrangement along the track. The efficiency corresponding to each mitigation measure is mainly evaluated through dynamic amplification factor (DAF) and power input.

The system with TMD demonstrates a significant reduction in DAF amplitude corresponding to the load velocity for which the optimization is performed. This improvement is also evident for velocities close to the aforementioned load speed. In fact, the addition of TMD results in presence of a free propagating wave behind the load and decreasing the critical velocity in the corresponding system. The outcomes corresponding to power input suggest a significant reduction in potential damage to the foundation due to the employment of TMD.

Furthermore, the application of ER leads to improvement in dynamic performance of the track by increasing the critical velocity to a larger value, at which the corresponding DAF indicates no reduction. In addition, considering more than one ER along the track does not lead to a noticeably better result compared to when only one ER is added. Moreover, applying ER over soft track leads to inhomogeneity in bending stiffness and mass corresponding to the beam element at transition point. Therefore, the system with ER over all domains indicates a better dynamic behavior. Potentially, less damage to the foundation can be signified in the system with ER according to the power input response.

Finally, USP can significantly affect the equivalent stiffness of the track. It is concluded that the efficiency of USPs in mitigating the amplified response is strongly dependent on their stiffness and arrangement along the track, as well as the stiffness variation in the supporting structure; improper design of USPs alignment can adversely result in even more amplified responses.