Characterization of railroad crosstie movements by numerical modeling and field investigation

Crossties are critical components of the railroad track to facilitate load distribution to track substructure. Recent instrumentation at Kingston, Rhode Island indicated that a crosstie could have translations as well as rotations under train load repetitions. However, previous laboratory experiments on crossties did not fully take the crosstie rotations into consideration. In this paper, actual movements of railroad crossties under moving train loads were first characterized by numerical modeling and field investigation, and then the effect of tie rotations on track ballast performance was evaluated by discrete element modeling (DEM). A vehicle dynamics model coupled with a three-dimensional finite element track model were utilized to simulate the crosstie movements. To obtain the crosstie movements under train passage, field measurements were conducted on an Amtrak high-speed passenger line and a freight railroad short line. The measuring units were mounted on top of crossties to record the accelerations in vertical, lateral, and longitudinal directions and the Euler angles in roll, pitch and yaw. The translations of crossties were obtained by the double integration of accelerations and the rotations were represented in terms of the Euler angles. Both modeling results and field measurements indicate that crossties have not only translations but also rotations under moving trains. In addition, angular velocity and angular acceleration measured in the field tests indicate that the rotations could cause extra moments up to 5000 N-m on top of the ballast layer. The extra moments may not cause significant track failure, but may accelerate the deterioration of track components. Further, a DEM program was used to evaluate the effect of the crosstie rotations on track ballast. The results of DEM show that the crosstie rotations would increase the acceleration of individual ballast particles and contact forces between ballast particles.