Ladder Sleeper and New Track Structures Development

Hajime WAKUI
Manager,Structural Engineering Group,Structure Technology Development Division

Senior Research Engineer,Structural Engineering Group,Structure Technology Development Division

Hiromi INOUE,
Senior Research Engineer,Structural Engineering Group, Structure Technology Development Division

Newly invented Ladder Sleeper, which is comprised of longitudinal PRC beams mounted on the underside of each pair of rails as well as multiple steel connectors which connect these longitudinal beams at designated spacings, is introduced.

key words: Track Structure, Longitudinal Sleeper, Ladder Sleeper, Maintenance

1. Ladder Sleeper development

The conventional concrete sleeper (crosstie) came about as a substitute for the wooden crosstie. Although it fulfills a role beyond that of a wooden Crosstie substitute on modern high-speed railways, due to its comparatively heavy weight and large rail-fastening capacity, it does have limitations in coping with issues that must be solved from the view point of the need to reduce track maintenance.
On the other hand, the longitudinal sleeper method comprised of a beam structure lengthwise along the rail superbly fulfills the function of the sleeper (ballast pressure reduction effect, etc.) and, if practical application of this were possible, the outlook would be an overall lengthening of the ballast maintenance cycle. It has, therefore, been a focus of attention among railway researchers for many years.
Based on the valuable history of challenges that have been made in developing the longitudinal sleeper method, there are two prerequisites in terms of structural design required in its actualization, as indicated in the following:

  1. Assuring that the weight per unit track length does not become too great compared to that of the crosstie.
  2. Assuring that it functions sufficiently as a tie (gauge maintenance function)
The Ladder Sleeper illustrated in Fig.1 was developed in order to fulfill these prerequisites. The Ladder Sleeper has a rigidly connected mixed structure comprised of longitudinal PRC beams and steel pipe connectors and is a unified precast plant-manufactured product in ladder form using cast concrete. The minimum unit length of the Ladder Sleeper is 5m and can be selected in 2.5m increments. The maximum length was set at 12.5m as the longest length that could be reasonably transported.

2. New track structures using Ladder Sleeper

As shown in Fig.2, when a comparison is made between the Ladder Sleeper and the crosstie (mono-block type PC, two-block type RC), it could be said that the Ladder Sleeper represents an effort to use the mono-block type in the rail direction in order to strongly enhance the sleeper function, while maintaining the tie function by incorporating the rational concept of the steel connector for the two-block type. On the other hand, in comparison to slabs (plant-type and frame-type), it could be said that the Ladder Sleeper has extremely advanced toward moving away from the plate type, which was the first type of slab, toward the utilization of the frame-type slab, for economic and environmental reasons. However, the frame-type slab is strictly a plane member and has a basically different structure compared to the Ladder Sleeper, which is comprised of mixed beam members.
Thus, the Ladder Sleeper is a next-generation rail support component that can be used universally as an alternative to both crossties and slabs. Possible applications including, five types of tracks are described in Fig.3.

3. Conclusions

  1. The Ladder Sleeper was developed as a longitudinal sleeper method. The most important structural features include the pre-tensioning PRC structure using only deformed PC steel wires as the main reinforcements, a longitudinal beam with the minimum cross-section and the rigidly connected structure combining longitudinal beams and steel pipe connectors.
  2. The Ladder Sleeper represents a next-generation rail support component incorporating the functions and properties of conventional crossties and slabs and the usage of five types of Ladder Track from ballast to non-ballast were pointed out.
The Ladder Track can be thought of as a compound vibration-proof rail that is formed through the lamination of steel and concrete rails, as shown in Fig.4. The realization of a mild load environment, approaching that of rubber tire systems, by combining this compound vibration proof rail with resilient wheels, will be an important issue for the steel-wheel and rail system. There are clear possibilities of the Ladder Sleeper playing an important role in proceeding with the optimization of the steel-wheel and rail system, from both maintenance and environment stand point. In preparation for its actualization in the near future, we look forward to receiving continued guidance.