Summary

Rails carrying trains along a fixed right of way are highly flexible under heavy stresses and must be tightly fastened to a strong base, the railway ties, at frequent intervals to maintain their true alignment. Today’s concrete crossties were developed to replace wooden crossties, and naturally took the same shape. Since the crosstie system provides poor load dispersal, however, the crossties gradually sink under the repetitive loads applied onto the track. Lateral movement resistance in this system is also weak. From the 1940s to the 1960s, experiments were carried out in France, the Soviet Union and Japan with longitudinal sleepers laid in parallel pairs under the rails. The goal was to create a railway track requiring a minimum of maintenance. However, none of the experiments were successful. Based on this experience, the Structural Engineering Group of Japan’s Railway Technical Research Institute has developed the “Ladder Sleeper,” in which parallel longitudinal concrete beams are held together by transverse steel pipes. It is a unified precast plant-manufactured product in ladder format. The lateral steel pipes are embedded into the longitudinal concrete beams. Ladder Sleepers provide continuous support to the rails and act with them as composite rails. When laid in stone ballast, the resulting ground pressure is reduced and more even for the same overall weight as for concrete crossties. On viaducts and in tunnels, where it is desirable to lay a non-ballasted track, Ladder Sleepers still offer advantages. They can either be laid on a resilient mat on the concrete trackbed, or they can be installed as a floating track insulated from the concrete trackbed by discrete resilient mountings. The ballasted and non-ballasted Ladder Tracks result in ensuring train safety, assuring maintenance reduction, and environmental improvements, promising major progress in achieving railway efficiency and improving railway management.

Keywords: Track structure, Longitudinal sleeper, Ladder Sleeper, Ladder Track, Viaduct

Introduction

How to support both rails, and in which part to adjust the level and line to compensate for the trackbed settlement, are dominant points from the viewpoints of ensuring train safety, easing maintenance ,mitigating both noise and ground-borne vibration, and also as a means for reducing construction costs for railways. Rail-support components, such as sleepers and slabs, therefore play the most important role in railway structure system, since they form the interface between rails and trackbed.

The Ladder Sleeper, as shown in Fig.1, is a next generation rail-support component, incorporating the functions and properties of both conventional crossties and slabs. Ladder Sleepers are manufactured as two longitudinal concrete beams joined by transverse steel pipes, which act as gauge ties. Transverse steel pipes are embedded into longitudinal concrete beams. The finished product looks like a ladder laid on the ground - hence the name. This configuration results in an ultra-light-weight sleeper that is roughly the same weight per track length as concrete crosstie systems.

In a new design, as shown in Fig.2, which is scaled up for use on heavy haul lines with up to a 40 ton axle load, pairs of shoulders for Pandrol clips are cast into the top of the sleeper at 750 mm intervals. Also, the distance between any neighboring two of the transverse pipes is set at 3.0 meters, corresponding to four rail fastening intervals. The minimum unit length is 6 meters, since two pipes are obviously necessary for structural and gauge stability. The maximum is 12 meters (four pipes) weighing some 4 tons, which is the most that can be conveniently handled at the work site. The purpose in having different length units is to enable curved track to be laid. By using an adjustable insulated spacer between the Pandrol shoulder and the rail, as shown in Fig.3, for example, or by using Pandrol shoulders on a movable base plate, a 200 meter radius curve can be laid using the 6 meter long units, whereas the 12 meter units are used on more than 2000 meter radius curves.

The ballasted Ladder Track may not offer the same level of resistance to longitudinal creep as concrete crosstie systems do. Transverse steel plates, named anti-creep panels, as shown in Fig.1, which react against the ballast, can therefore be installed between the longitudinal beams, to offer the required longitudinal creep resistance.

In the Ladder Track system, a continuous rubber buffer strip can be inserted between the rail and longitudinal beam. Another rubber buffer can be attached to the underside of the longitudinal beam. This type of construction makes for a strong composite rail, as shown in Fig.4, that consists of a conventional steel rail sitting on top of a new “concrete rail” (the longitudinal beam).

When laid in stone ballast, Ladder Sleepers contribute to further improved train safety by bridging across weak spots in the subgrade, where partial subsidence or collapse has occurred, for example after a washout or an earthquake. As regards ballast pressure, a much more even distribution is achieved by employing Ladder Sleepers. The maximum pressure is approximately halved, compared to using conventional crossties. As a consequence, the settlement rate is far lower than that experienced with concrete crossties. This all spells a big reduction in maintenance costs.

Not only does this new rail reduce the need for maintenance, it also greatly reduces both noise and ground-borne vibration, especially when combined with resilient wheels.

Furthermore, because of their configuration, Ladder Sleepers are highly resistant to track buckling in hot weather and/or during earthquakes. It could eliminate the track buckling problem in continuous welded rail (CWR) track, even in sharp curves.

On viaducts and in tunnels, where it is desirable to lay a non-ballasted track, Ladder Sleepers still offer advantages. Several designs for different types of track structure have been developed.

Patents for Ladder Track system have been filed in a number of countries.

Structural design keypoints on Ladder Sleeper

Because of its excellent crack resistance, pretensioned partially-prestressed concrete (PPC) was adopted for longitudinal beams. Newly developed indented prestressing wires of three or five strands, which have very good adhesion to the concrete, are arranged close to the top and bottom surfaces, as shown in Fig.2. Stress transfer between the indented wires and the concrete relies solely on this bond.

The beam width is set at 450 mm at the upper surface. The beam depth under the rail seat is chosen between 165 and 185 mm, depending on both the applied axle load and anticipated beam support conditions. The transverse steel pipe is 80 mm in diameter and 9 mm thick.

The effect of unsupported gaps 2.0 and 4.0 meters long, appearing symmetrically under both beams, as well as appearing non-symmetrically under one side beam, was analyzed under quasi-static wheel loads. These calculations were used to determine the crack-moment resistance and the ultimate-moment capacity for the longitudinal beam, as well as to determine the optimum diameter and thickness for the transverse steel pipe, by means of a limit state design method. These unsupported gaps, especially 4.0 meter ones, are extreme cases in the ballasted track and serve to emphasize that Ladder Sleepers can enhance safety under conditions where the subgrade has partially collapsed, for example after a washout or an earthquake.

Drop-weight impact tests and FEM analyses were performed to simulate the effect of wheel flats. Longitudinal beam bending moments per unit impact wheel load, as shown in Fig.5, have a dynamic response characteristic, compared to a static burden. Comparing the results between the Ladder Sleeper and the crosstie, there is a big difference in loading durations, during which the bending moments reach the maximum values. However, there is not such a big difference in the maximum moment values between the Ladder Sleeper and the crosstie. These factors confirmed that Ladder Sleepers had been correctly designed to withstand wheel flat impacts.

Maintenance reduction effect in ballasted Ladder Track

Ballast pressures, generated under the Ladder Sleeper, are controlled by the width and depth of the longitudinal beam. However, the maximum beam width may be restricted to a certain value, such as 40 to 45 cm, from the viewpoint of easing tamping work requirements. The relation between the maximum ballast pressure and the ballast-subgrade modulus is shown in Fig.6 with the beam depth as a parameter, in which the beam width was fixed at 45 cm and the wheel loads were set at 80 kN. The beam depth does not affect the maximum ballast pressure very much. However, the beam bending stiffness decreases the number of ballast pressure pulses. The beam depth is, therefore, a very important factor, as well as the beam width, from the viewpoint of minimizing track deterioration.

The maximum pressure immediately under the wheels is approximately halved, compared to conventional crossties, and the pressure gradient is very much smoother. It is therefore no surprise that repetitive loading tests showed settlement rates to be about eight times less under the middle of Ladder Sleepers, compared with conventional track, as shown in Fig.7.

However, settlement becomes greater at the Ladder Sleeper ends, emphasizing the need for developing a joint such as shown in Fig.8. The aim is to connect the adjacent longitudinal beams within a steel channel using poured mortar and through bolts. However, at intervals, it will be necessary to allow longitudinal expansion and contraction of the sleepers to take place at the joints.

Ladder Sleepers are designed to be tamped or stone-blown by machine to maintain the required rail top level. Obviously, the tamping heads have to be turned through 90°, but the sleepers are flexible enough to be lifted by an on-track machine and the principle is almost the same as for crosstie tracks, as shown in Fig.9.

The 40 ton standard gauge version has been installed in the High Tonnage Loop at the AAR’s Pueblo test center, where comprehensive studies on the ballasted Ladder Track are to be carried out.

Floating or semi-floating Ladder Track

Recently, non-ballasted tracks have become preferred on viaducts and in tunnels, where a concrete trackbed is laid, from the viewpoint of their almost maintenance-free performance. However, non-ballasted tracks usually have environmental problems, such as entailing rolling noise and ground-borne vibration.

Floating Ladder Tracks, as shown in Fig.10, for example, could be a breakthrough for non-ballasted tracks. The track panel is insulated from the concrete trackbed by discrete resilient mountings. Resilient mountings, as shown in Fig.11, which support vertical loads with shear deformation in the rubber sheath, are placed at 1.5 meter intervals. Resilient mountings can also support longitudinal and transverse loads. Another floating Ladder Track, in which both side faces of the longitudinal beam are supported, is now under development.

These floating Ladder Tracks can disperse vibrational energy along the track and reduce the ground-borne vibration, combined with the vibration dissipation effect embodied in resilient mountings. When rails were continuously supported, rolling noise could be greatly reduced, as well as achieving a less noise radiation area for the composite rail construction. It is also very easy to install noise absorber panels near the track.

A semi-floating Ladder Track is very promising, too, in which discrete resilient mats are used in place of the above mentioned resilient mountings, as shown in Fig.12. In this type, both ends of transverse steel pipes only need to be cramped longitudinally and transversely, not vertically, at appropriate intervals, in order to withstand longitudinal and transverse loads.

These floating or semi-floating Ladder Tracks are also applicable on a concrete trackbed above earthworks. The level and line accuracy can be maintained by adjusting with both rail-fastenings and discrete mountings (or mats) to compensate for the settlement of earthworks.

Innovation in viaduct and bridge structure

Conventional viaducts and bridges are usually designed and constructed based on the condition that stone ballast is fully loaded. As a result, a wide concrete trackbed, some 10 meters wide for double tracks, including sidewalks along the tracks, for example, is necessary, and the overall structure seems to become massive and less economical, compared to guideway structures for magnetically levitated systems, in which mono-box girders for supporting each track are eventually used.

Floating or semi-floating Ladder Tracks have much less width (some 2.0 meter width for standard gauge) and ultra light weight (some 600 kg/each track meter), compared to conventional ballasted track. By adopting floating or semi-floating Ladder Tracks, economical and slender mono-box girders for supporting each track, similar to those of magnetically levitated systems, can be easily introduced into viaducts and bridges, as well as due to their less noise and vibration performance. The mono-box girders are to be manufactured at factories and can be conveyed by means of ordinary trailer-truck transportation facilities. A new viaduct, based on this concept, is proposed as shown in Fig.13, whose weight above piers can be halved, compared to that for short-spanned frame viaduct, popularly used in Japan as an alternative to embankments. The transverse natural frequency for the new viaduct can easily be maintained at more than 2 Hz during strong earthquakes, which is the key factor in order to ensure train running safety during strong earthquakes.

Concluding remarks

The Ladder Sleeper, in which longitudinal precast concrete beams, which are held to the desired gauge by embedded steel pipes, provide continuous support to the rails and act with them as composite rails, represents a next-generation rail-support component, incorporating the functions and properties of conventional crossties and slabs.

When laid in stone ballast, the resulting ground pressure is reduced and more even for the same overall weight as for concrete crossties. When laid on a concrete trackbed, floating and semi-floating Ladder Tracks are recommended, from the viewpoint of superior environmental performance, as well as resulting in economical viaducts and bridges.

The realization of a mild load environment, including less rail-corrugations and rail-shellings, is the most important theme for the steel-wheel and rail system to be optimized, with adopting resilient wheels if needed. The Ladder Sleeper could play a very important role for its optimization and a computer mechanics simulation is to be carried out by using the model as shown in Fig.14.

Assuring train safety, reducing maintenance costs, improving environmental problems, and reducing construction costs for infrastructure, such as viaducts and bridges, that will accompany the switch to Ladder Track system, promise major progress in railway efficiency and railway management.

BEAM DEPTH

BEAM WIDTH : 45 CM

ULTRA-LIGHT-WEIGHT

SOUND BARRIER WALL

SOUND BARRIER PANEL

FLOATING LADDER TRACK

WALL TYPE PIER

BOGGY (RIGID BODY)

CABIN (RIGID BODY)

WHEELSET (FEM)

CONNECTOR (FEM)

LONGITUDINAL BEAM (FEM)

RAIL (FEM)

GAP

ULTRA-LIGHT-WEIGHT

SOUND BARRIER WALL

SOUND BARRIER PANEL

FLOATING LADDER TRACK

WALL TYPE PIER

BOGGY (RIGID BODY)

CABIN (RIGID BODY)

WHEELSET (FEM)

CONNECTOR (FEM)

LONGITUDINAL BEAM (FEM)

RAIL (FEM)

GAP

MONO-BOX GIRDER