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Rail turnouts are built to enable flexibility in the rail network as they allow for vehicles to switch between various tracks, therefore maximizing the utilisation of existing rail infrastructure. In general, railway turnouts are a safety-critical and expensive feature to a rail system as they suffer aggressive operational loads, in comparison to a plain rail track, and thus require frequent monitoring and maintenance. In practice, great consideration is given to the dynamic interaction between the turnouts components as a failed component may have adverse effects on the performance of neighbouring components. This paper presents a nonlinear 3D finite element (FE) model, taking into account the nonlinearities of materials, in order to evaluate the interaction and behaviour of turnout components. Using ABAQUS, the finite element model was developed to simulate standard concrete bearers with 60 kg/m rail and with a tangential turnout radius of 250 m. The turnout structure is supported by a ballast layer, which is represented by a nonlinearly deformable tensionless solid. The numerical studies firstly demonstrate the importance of load transfer mechanisms in the failure modes of the turnout components. The outcome will lead to a better design and maintenance of railway turnouts, improving public safety and operational reliability.

Rail operators are considerably demanded by the public and other stakeholders to be more efficient than ever. As a result, the maximisation of utilisation and flexibility of rail network is one of the key strategies in rail asset management. A railway turnout is a critical part of the railway where tracks cross over one another at an angle to divert a train from the original track. It allows for train vehicles to cross over or switch between various tracks and in turn maximising the utility of tracks and assets. Its main components include rails, switches, crossings, steel plates, bearers, ballast, and subgrade (as shown in Figure

Typical components of a railway turnout.

Railway track structures experience static, dynamic, and often impact loading conditions due to wheel/rail interactions associated with the abnormalities in either a wheel or a rail over their life cycle [

extra length of turnout bearers in comparison with standard sleepers,

centrifugal forces through curved pairs of rails,

forces and bending moments induced from points motors and other signaling equipment,

impact forces induced by wheel-rail interaction,

mechanical rail joints.

On this ground, this numerical study was initiated by a recent number of reportedly broken concrete bearers on a mixed-traffic line in New South Wales (NSW), Australia. Due to the complexity of the loadings and damage modes in railway turnouts, this study aims to establish a three-dimensional (3D) finite-element (FE) model. The 3D FE model adopts an elastoplastic region of bending and shear deformation of materials. The 3D FE model was developed based upon a common tangential turnout used in Australia. This study indicates that the crossing panel is where turnout bearers experience the greatest bending moment and shear force and it provides the critical force envelopes for design improvement of turnout bearers.

Railway turnout systems have generally been analysed using a grillage beam method (Manalo et al., 2010) [

This paper presents a nonlinear 3D FE analysis using ABAQUS considering the whole turnout, which comprises bearers, rail, guard rails, crossing nose, rail pads, baseplates, and guardrail support plates. The benefits of modelling in 3D space are to incorporate the effects of the neighbouring bearers and to take into consideration the longitudinal forces of the continual rail. The boundary conditions of the central 3D model can be simulated enabling vibrations to radiate beyond the model [

In general, the surface conditions of the wheel and rail play a critical role in the W-R contact force, in addition to geometric irregularities, train speeds, and type of track structure. The large contact force will accelerate the deterioration rate of the turnout crossing. Sun et al. [^{3} N/m as the effective vertical rail damping rate per wheel, and ^{6} N/m as the effective vertical rail stiffness per wheel.

In addition, lateral resistance is usually designed to reinforce the structural integrity of the rail and turnout. Considerations are only given to lateral force values sustained for distances of 2 metres or more. Unless supported by appropriate technical justification, vehicles attempting to negotiate a lateral ramp discontinuity in track alignment, when travelling on a curve at maximum normal operating speed and at maximum cant deficiency, without exceeding a total lateral force level per axles of 71 kN, should introduce lateral action that can be calculated using the following formula [^{6} N/m as the effective lateral rail stiffness per wheel. Based on these formulas, the dynamic forces can be estimated for the design of turnout components.

The FE model comprises entirely 3D deformable solids, straight and curved rail, bearers of varying length, and a ballast layer as the track support. This study focuses on the behaviour of the bearer and ballast; therefore, a suitably accurate rail seat load within a tangential configuration is required for the analysis. Steel rails were modelled in 3D space to account for their cross-sectional properties, the width of the contact patch between the wheel and rail, the width of the rail web, and the width of the rail footing. The rail and switch rail profiles were validated against rail authority’s specifications [

Design properties of materials.

Materials | Elastic modulus (MPa) | Compressive strength (MPa) | Tensile strength (MPa) |
---|---|---|---|

Concrete | 38,000 | 36–55 | 4.0–6.30 |

Prestressing tendon | 200,000 | — | 1,700 |

Steel rails | 205,000 | — | — |

The elastic modulus of steel rails and crossing is defined by the initial slope of the stress-strain relationship to the extent of the upper yield threshold, as illustrated in Figure

Stress-strain relationship of structural steel [

Stress-strain relationship of concrete [

The ballast layer is simulated as a hardening-soil (HS) model [

A sensitivity analysis has been undertaken for mesh sizes for each rail component. As the mesh sizes and the material densities are different between the two tied objects, a tie constraint is generated to allow for ABAQUS to automatically optimise and refine the interface mesh. Tie constraints are applied to the rail and the concrete bearers to represent the rail fasteners. Instead of frictional interaction and the effect of submersed bearers in a ballast layer, the bearers are tied onto the underlying ballast layer to greatly reduce computational effort. As all members are tied, translational and rotational degrees of freedom will be distributed throughout. All tie constraints were taken as surface to surface, as opposed to a simplified node to surface, as this allowed for uniform distribution between the tied components. The interface between bearer or sleeper and ballast has been treated with contact surface elements where the bearers can lift freely as ballast is modelled using tensionless solid elements [

A fixed boundary condition is applied to the bottom most surface of the subballast to idealise the substructure and a symmetrical constraint is applied to the ends of the rail to idealise a continuous rail within the relevant plane, in this case the

Loading configuration is in accordance with Standards Australia [

Railway traffic loads-axle loads.

300LA load case

Load steps: 300LA coupled locomotive design loading on turnout; (top) load step 2, (middle) load step 36, and (bottom) load step 48

The above load set is applied to the model at 600 mm increment initially to coincide with turnout bearer spacing, or referred to hereafter as load sets. A total of 48 load steps (including model initiation) are modelled to generate the overall movement of the locomotive negotiating the turnout. When approaching the crossings, the load step is later set at 50 mm increment. Figure

The numerical model was previously calibrated using the field measurement data [

Resultant deflection of sleeper 47 and computational time with varying ballast mesh size.

Mesh size (mm) | Deflection (mm) | Computational time (s) |
---|---|---|

60 × 60 | 2.54 | 24,784 |

70 × 70 | 2.32 | 12,638 |

80 × 80 | 2.28 | 10,824 |

90 × 90 | 2.59 | 5,655 |

100 × 100 | 2.54 | 5,547 |

Midpoint deflections of each bearer along the turnout.

Tie constraint between rail and bearer

Maximum recorded deflection at each bearer (midpoint)

Displacement envelope of the bearer right underneath the crossing (number 47).

Bearer 47 (red) experiences the greatest deflection

Deflection response of bearer 47 at each load step

In practice, frequent maintenance of supporting bearers and fastening systems can often be observed in the field even though those components have been designed in accordance with engineering standards. This study has therefore investigated such an important issue. It is found that the bearers, which undergo the greatest deflection of a coupled locomotive pass, are the bearers underneath the crossing nose (maximum at bearer number 47). The sensitivity analysis illustrates the maximum deflection in all bearers with the passing of a moving couple train load, 300LA [

Effect of mesh sizes on the deflection of the bearer underneath the crossing nose (number 47).

The shear stress response of crossing bearer (number 47) at the most critical loading is illustrated Figure

Shear force envelope of the bearers upon wheel impact.

Shear force envelope of bearer 47 upon wheel impact

Shear stress distribution of bearer 47 (under load step 36)

Bending stresses and moment envelopes of turnout bearers.

Bearer 21 (red) in relation to load step 18 (yellow)

Bending stress of bearer 21 under load step 18

Bending moment envelope of bearer 21 with computed values (blue) against specifications (black)

Bearer 47 (red) in relation to load step 36 (yellow)

Bending stress of bearer 47 under load step 36

Bending moment envelope of bearer 47 with computed values (blue) against specifications (black)

Due to the unchanging response of the modelled result and the fact that the bearers under the crossings experience critical loading, it is believed that the moment envelope overlooks the adverse loading configurations of a turnout system and, instead, idealises the moment response to that of straight rail. Figures

Bending moment envelope of FE model, bearer 47 (blue), and specifications (black).

Bending moment envelope of bearer 21. Bending moment of FE model, bearer 21 (blue), and specifications (black).

It is concerning the fact that the maximum bending moment simulated, being 322 kNm, is greater than that specified by about 800%. Also that no change in loading condition or material property has been changed is noteworthy; yet, by involving a greater number of points in which to create an accurate stress diagram, the loading may be allowed to deviate so greatly. As the increase in bending moment is concentrated within the midspan, it can be deduced that the midspan could become more susceptible to permanent deformations and cracking that was not designed to not be designed to accommodate for the adverse loading.

The resultant bending diagram depicted in Figure

This paper firstly presents a development of three-dimensional finite element model of a tangential turnout system for an investigation into the failure modes that were arisen from the field observations and measurements on a mixed traffic rail line whereas broken concrete bearers and loose fasteners were reported routinely. A 3D FE model has been established and validated for the analysis of a complete turnout system. The primary objective of this study is to determine the critical location, be able to realise the critical deflection, and validate shear force and bending moment envelopes of a turnout system. To address this, ABAQUS has been employed to carry out all modelling and postprocessing of a complete 3D turnout. From the detailed analysis, turnout bearers right underneath crossing panel experience the highest load actions, resulting in the largest deformations. It is also found that the turnout sleeper or bearer underneath the closure rails or where there is a change in rail curvature is subjected to a high level of vertical loading, sometimes exceeding its design load limits. These results are of significant importance to rail engineers and track designers, in order to establish a safer and more reliable turnout system. Future work will evaluate the effects of ballast voids and pockets on the dynamic behaviour and lateral sliding of turnout systems.

The authors declare that there is no conflict of interests regarding the publication of this paper.

The authors are grateful to University of Western Sydney and RailCorp for the support throughout this study. Also, the last author wishes to thank Australian Government for his Endeavour Executive Fellowships at Massachusetts Institute of Technology, Harvard University, and Chalmers University of Technology.