Tore Dahlberg

Load impact on railway track due to unsupported sleepers

Abstract Ballasted railway tracks will settle as a result of permanent deformations in the ballast and in the underlying material layers. The settlement is caused by the repeated traffic loading and the severity of the settlement depends on the quality and the behaviour of the ballast, the sub-ballast, and the subgrade. As the behaviour of the material is not exactly the same under all sleepers, and since the loading of the track is irregular, the amount of settlement will differ from one sleeper to another. A result of this is that the sleepers are not always fully supported, and some sleepers may even become completely unsupported (voided). A gap appears between the sleeper and the ballast bed. As soon as the track geometry starts to deteriorate, the variations of the train/track interaction force increase, and this speeds up the track deterioration rate. This paper presents a computer model by which the dynamic train/track interaction can be simulated. The influence of one or several voided sleepers on the train/track interaction force and on the track dynamics is investigated. Track settlement due to hanging sleeper(s) is discussed.

Dynamic Train/Track/Ballast Interaction - Computer Models and Full-Scale Experiments

Abstract A numerical method has been developed by which the vertical dynamic behaviour of a railway track subjected to the loading of a moving train may be investigated. The interacting train and track are both modelled as dynamic systems and the compound train/track system is treated as a whole. In the track model, the rail is treated as a Rayleigh-Timoshenko beam discretely supported, via railpads, by rigid sleepers. Below each sleeper, a rigid mass accounts for the mass of the ballast and those parts of the subgrade that participate in the vibration. The rigid mass is connected to the adjacent masses, to the foundation and to the sleeper by linear springs and viscous dampers. The mass, stiffness and damping of these track components and also the sleeper spacings can be arbitrarily varied. The contact between the wheelsets and the rail is modelled by non-linear Hertzian spring elements. The model permits calculation of deflections, accelerations and forces in various track components, and also enables e...

Vertical Dynamic Train/Track Interaction - Verifying a Theoretical Model by Full-Scale Experiments

SUMMARY A numerical method has been developed by which track deformations and the dynamic forces acting between train wheels and rails may be simulated. The method allows for calculation of deflections, accelerations and sectional forces in various track components. The work has resulted in increased possibilities to investigate how parameters such as speed, axle load, wheel base of a bogie, rail corrugations, wheelflats etc influence the dynamic behaviour of track and vehicle components. The computational method may serve as a tool for optimizing the track with reference to life of track components. In order to verify the computational method, a full-scale measurement program was accomplished on the West Coast line in Sweden. Measurements of strains and accelerations in the track were made simultaneously with measurements on the train of rail/wheel contact forces and accelerations. Test results will be presented and compared to calculations. In order to investigate the influence of pad stiffness, the sof...

OPTIMIZATION CRITERIA FOR VEHICLES TRAVELLING ON A RANDOMLY PROFILED ROAD - A SURVEY

SUMMARY A 5-DOF plane vehicle model is studied. A randomly profiled road is assumed to impart normally distributed stationary vertical random displacements to the front and rear wheels. Several vehicle performance criteria based on response mean square spectral densities are discussed. It is emphasized that these performance criteria contain more information than do the simple response standard deviations. A computer program has been developed for optimization of two or more of the system parameters to make a vehicle response (or a weighted sum of responses) a minimum. Constraints on parameters and responses can be introduced. Ride comfort, road holding, energy absorption, fatigue failure and first-passage failure are studied. Numerical examples are given.

Vehicle-Bridge Interaction

SUMMARY With the emergence of high-speed trains, dynamic loads on bridges have changed. A method for estimation of the time-dependent vehicle-bridge interaction forces has been developed in the present paper. The increase (or decrease) of the bridge response due to dynamic effects is determined. The moving constant-force problem is reviewed in some detail. Results obtained by the present method for the moving-mass problem are compared with existing experimental and theoretical results as reported in the literature. A parametric study of bridge responses is made. The parameters varied are the vehicle speed, the ratio of vehicle mass to bridge mass, the ratio of vehicle eigenfrequency to bridge eigenfrequency, and the relative damping of the vehicle. Finally, the influence of an initial bridge deflection is discussed.

Comparison of ride comfort criteria for computer optimization of vehicles travelling on randomly profiled roads.

SUMMARY This paper discusses and compares some ride comfort criteria which may be suitable for randomly vibrating vehicles. The criteria consider single-figure measures based on response mean square spectral densities for plane linear combined vehicle-passenger models. Eight different measures divided into three groups are studied. The two vehicle suspension damper stiffnesses are numerically optimized with respect to the eight measures of ride comfort. The results are compared and discussed. Two optimizations with respect to five vehicle parameters are also reported.

Reduction of Wind-Induced Random Vibrations by Use of an Optimal Dynamic Absorber

The efficiency and stability of a dynamic vibration absorber in reducing wind-induced random vibrations of a structure will be investigated. The mean of the peak values of a critical stress resultant (design force) will be minimized by attaching an optimally tuned and damped linear absorber to the structure. The primary structure is modelled as a lightly-damped one-degree-of-freedom linear system. The influence of the inherent damping (not very well known in practice) of the primary structure on the choice of optimal absorber parameters and on the final response is found to be small. Alternative objective functions are studied. In a typical example, an absorber mass equal to one percent of the vibrating mass of the primary structure is found to reduce the mean of the peak values of the stress resultant by one third.