Optimisation of slab track design considering dynamic train–track interaction and environmental impact

A recently developed Flexible Track System Model integrated with a multibody dynamics software tool was used to simulate the dynamic interaction between a vehicle and two innovative slab track designs, comparing their performance with respect to conventional ballasted track. The design concepts are presented and the modelling assumptions are given. Simulations are then carried out to quantify, for example, the impact of ballasted track degraded state using the case of a hanging sleeper. In comparison, the benefit of the two innovative track designs is highlighted as they prevent this type of localised defect from occurring. The alternative track designs were also shown to be capable of carrying a vehicle safely and with less impact on the system over a localised weakened formation support, by simulating a washout event.

One of the problems with high-speed tracks in service is the damage of their cement asphalt mortar layer. The development of cement asphalt mortar damage can lead to gradually failure of the slab supporting. In fact, the cement asphalt mortar damage is uniform. The effect of cement asphalt mortar damage location seriously affects the dynamic behavior of the track and even operational safety. It is important to clearly understand the effect of cement asphalt mortar damage location on dynamics, which is helpful to direct track maintenance. In this study, a three-dimensional coupling dynamic model of a vehicle and a slab track is developed. Using the proposed model, the wheel–rail contact forces and the displacements and accelerations of the track are calculated to study the influences of the size and location of the damaged cement asphalt mortar zone on the dynamic characteristics of the track system.

Numerical simulation platform for slab track systems subjected to a moving vehicle

Slab tracks are used worldwide in high-speed railways. The slab may be cast in situ, resulting in a continuous length of concrete, or it may be constructed in discrete precast sections laid end to end. According to structural characteristics of the China Railway Track System (CRTSII) slab track system, a model for dynamic analysis of vehicle-track-subgrade coupling system has been developed. Based on the model, a new type of slab track element is presented, and the associated stiffness matrix, mass matrix and damping matrix for the element are deduced. This element includes rail, rail fastening and pad, prefab slab, cement-asphalt mortar, hydraulically bonded layer, and subgrade. By means of the Lagrange equation, a numerical method for coupling the moving wheel and the rail with explicit formula is presented and the associated finite-element equation is formulated. As application examples, parameter studies on the track vibration of the slab track structure, such as stiffnesses and dampings resulting from the rail pad, CA mortar (cement-asphalt mortar) and subgrade, are investigated. To understand dynamic behavior of track transition from conventional ballast track to slab track, the effects of train speed and track stiffness on track vibration in the transition are evaluated. The obtained results show (1) reasonably chosen parameter values of the slab track structure and suitably specified stiffness of the transition will significantly improve the behavior of the track performance; (2) changing track stiffness in transition has an influence which increases with an increase of railspeed, on the vertical rail acceleration and the wheel/rail contact force; (3) smoothing of track stiffness in transition can reduce track vibration and improve the operational quality of the train. Countermeasures include long ties, additional rails, hot mix asphalt (HMA) underlayment, slab track approach, stone columns and piles to strengthen weak subgrade, rail seat pads, rubber tie mats, and other countermeasures.

Rail cant parameter, applied in order to attain suitable movement of wheel on rail, directly influences in dy- namic interaction of track-vehicle. In this paper, the effect of geometrical parameters on the behaviour of dynamic interaction of wheel-rail is being investigated through a parametric study. A complex dynamic analyses using ADAMS/RAIL are being applied for one section of actual executed line and the records of geometrical parameters have been registered for this purpose. The necessary measurements have been recorded by KRAB measuring machine, safety levels and serviceability have been controlled based on these data. The results of the analysis show that non-conformity of the rail seat slope relative to its desired value, strictly influences on the coefficient of derailment (about 18 percent), horizontal dynamic forces (about 17 percent), and results in severe rail wear (about 18 percent). However, vertical dynamic forces and safety index are not of any concern form the derailment point of view, since it is within the acceptable range of the standards. The comparison of analyses' results in various operational speeds shows a remarkable impact on the vertical dynamic forces (up to 13 percent) and derailment index (up to 8 percent).

A finite element post-processor for fatigue assessment of welded structures based on the Master S-N curve method

In reinforced concrete structures, the degree of deterioration due to an alkali aggregate reaction is influenced by the amount of reinforcement in the concrete, because the reinforcement restrains the expansion due to the alkali aggregate reaction. To investigate the effect of reinforcement steel on the deterioration of concrete, experimental works were made using concrete prism specimens with various amounts of steel reinforcement. Expansion and crack patterns were measured for 2 years. And the compression tests of the concrete prism specimens were caried out after 5 year's storage. Compressive strength, stress-strain relationship and Young's modulus were measured for each specimen. At the same time, acoustic emission characteristics of each specimen during compressive loading were investigated.The test results show that the deterioration of concrete due to the alkali aggregate reaction decreased extremely when the steel reinforcement ratio was more than 0.71%. One of the reasons is considered that the growth of cracks is restricted by the chemical prestress induced in the concrete with the expansion of concrete due to the alkali aggregate reaction, resulting the decrease of water supply to the inner part of concrete.

The dynamic compliance of different railway tracks on different layered soils have been calculated using a combined 6nite-element and boundary element method: conventional track on homogeneous soil with different stiffness, extreme, normal and inverse layering of the soil, plates in and on top of the soil (slab track on various soils), conventional track with elastic rail pads. The compliance functions of the track can easily be introduced into a coupled analysis of the vehicle-track-interaction to study the dynamic axle-loads and the dynamic diplacement amplitudes, which have also been measured at three different locations of a high speed line in Germany.

A railway track dynamics model based on modal substructuring and a cyclic boundary condition

To effectively reduce the railway vibration and its environmental impact, vibration mitigation measures are increasingly used. The vibration reduction effect of railway tracks is described quantitatively by insertion loss (IL). ILs obtained from in situ measurements under moving train loads and laboratory tests under artificial excitation differ significantly due to the different track loading state between these two methods. The differences of track loading state are induced by the moving effect of train passages and the preloads effect of vehicle masses, the latter of which is a significant factor to discuss in this paper. In order to study the static preload by vehicle masses influence on the vibration reduction effect in isolated tracks, the steel spring floating slab track and regular slab track, as a reference case, were compared. First, a theoretical simplified model was constructed, following which a finite–infinite element coupled model was built, which was calibrated by experimental test results. Impact loads were applied to both tracks with preloads using unsprung wheelsets or sprung vehicle-body masses, with the total mass varying from 0 t to 30 t. The results demonstrate that the increase in preload of unsprung mass makes the natural frequencies further reduced, and the peak IL value increased from 39 dB to 48 dB. The increase in preload has a significant effect on vibration responses below 5 Hz, and the application of the preload has different effects on the reduction effect in different frequency ranges.

The numerical solution of the dynamics of a railway vehicle interacting with supporting structures is facilitated by mathematical models and evaluation techniques. In fact, the vehicle multibody modelling, the supporting structure models, and the Hertz and Kalker theories concerning the wheel-rail contact forces are indispensable guides to the dynamic analysis of railway systems. The present work is concerned with an evaluation method for the normal direction contact force component at the point of contact between the rail and wheel profiles. It proposes an alternative approach, which eliminates the typical problems of the Hertz formulation. The evaluation method developed ignores the contact stiffness and is based on the elastic deformation of the supporting structure, which, as the simulations demonstrate, leads to physically more acceptable results and shorter calculation times.

Dynamic vehicle–track interaction at high vehicle speeds is investigated in a frequency range from about 20 Hz to 3 kHz. The inclusion of wheel rotation in the vehicle model is investigated by implementing a structural dynamics model of a rotating wheelset in an existing simulation environment. Calculated wheel–rail contact forces using this comprehensive flexible wheelset model are compared with contact forces based on less detailed, non-rotating wheelset models. Load cases including broad-band wheel–rail excitation due to vertical rail irregularities (rail corrugation) and impact excitation caused by a local deviation from the nominal wheel radius (wheel flat) are considered. Viewed from the excitation point, each wheelset resonance peak of multiplicity two splits into two peaks; the separation of the two peaks increases with increasing rotational speed. If the wheelset model is excited at a frequency where two different mode shapes, due to the wheel rotation, have coinciding resonance frequencies, the contact force calculated for the rotating wheelset model differs significantly from those of the non-rotating models. Further, the use of a flexible rotating wheelset model is recommended for load cases leading to large magnitude contact force components in the high-frequency range (above 1.5 kHz). In particular, the influence of the radial wheel eigenmodes with two or three nodal diameters is significant.

Investigation of rail irregularity effects on wheel/rail dynamic force in slab track: Comparison of two and three dimensional models

The vertical dynamic interaction between a high-speed railway vehicle and slab track is simulated in the time domain using a two-dimensional track model. The transient interaction problem is solved using an extended state-space vector approach in combination with a complex-valued modal superposition technique for the track model. The presented slab track model includes two layers of concrete beams, where the panels of the upper layer are described by possibly coupled beams of finite length, while the roadbed in the lower layer is a continuous beam. Both rail and concrete layers are modelled using Rayleigh-Timoshenko beam theory. The presented model is applied to calculate the influences of foundation stiffness gradient and track design parameters on various track responses. In particular, the influences of the thickness of the roadbed on the load distribution on the foundation, and of the stiffness of the rail pad on the bending moment in the concrete panels, are investigated in two demonstration examples.

열차 진동 저감을 위한 플로팅 슬래브궤도의 설계에 있어서 주행 안전, 승차감 및 사용성을 확보하는 것이 매우 중요하다. 본 연구에서는 플로팅 슬래브궤도에서 열차 주행안전과 승차감, 사용성 확보를 위한 요구조건을 분석하여 제시하였고, 열차-궤도 상호작용을 고려한 동적 해석기법을 적용하여 시스템 고유진동수, 스프링 지지간격 및 배치방법, 감쇠비 등 주요 시스템 설계변수에 따라 일반 콘크리트궤도와의 접속구간을 포함한 플로팅 슬래브궤도 구간에서의 열차 및 궤도의 동적 거동을 분석하였다. 연구결과에 따르면 일반 궤도와 플로팅슬래브궤도 간의 접속구간에서의 지지강성의 차이에 의해 윤중 변동율, 레일 응력, 레일 인상력 등의 동적 응답이 크게 증가하는 것으로 나타났으며, 따라서 접속구간에서 스프링 지지간격을 좁히거나 스프링 강성의 차이를 완화시키는 방안이 주행안전과 궤도 사용성 확보를 위해 효과적인 것으로 나타났다. 한편 차체 가속도로 평가하는 승차감은 접속구간에서의 지지강성의 차이에 의해서는 거의 영향을 받지 않고, 시스템 튜닝 주파수에 의해 가장 큰 영향을 받는 것으로 나타났으며, 승차감 확보를 위해서는 적절한 시스템 튜닝 주파수를 선정하는 것이 매우 중요한 것으로 나타났다. 이 밖에 감쇠비, 스프링 간격, 열차속도에 따른 영향을 분석하였다. It is of great importance to assure the running safety, ride comfort and serviceability in designing the floating slab track for mitigation of train-induced vibration. In this paper, for this, analyzed are the system requirements for the running safety, ride comfort and serviceability, and then, the behavior of train and track at the floating slab track including the transition zone to the conventional concrete slab track according to several main design variables such as system natural frequency, arrangement of spring at transition, spacing of spring isolators, damping ratio and train speed, using the dynamic analysis technique considering the train-track interaction. The results of this study demonstrate that the discontinuity of the support stiffness at the transition results in a drastic increase of the dynamic response such as wheel-rail interaction force, rail bending stress and rail uplift force. Hence, it is efficient to decrease the spacing of springs or to increase the spring constants at the transition to obtain the running safety and serviceability. On the other hand, the vehicle body acceleration as a measure of ride comfort is little affected by the discontinuity of the stiffness at the transition, but by the system tuning frequency; thus, to obtain the ride comfort, it is of great significance to select the appropriate system tuning frequency. In addition, the effects of damping ratio, spacing of springs and train speed on the dynamic behavior of the system have been discussed.