Rui Calçada

Integer Programming to Optimize Tamping in Railway Tracks as Preventive Maintenance

In the area with railway engineering, scheduling preventive maintenance actions is an important issue for engineers and railway administrations because the optimization of maintenance actions at a preventive level allows maintenance costs to be reduced during the life cycle of the track, with the guarantee of its geometrical quality. This paper describes a model designed to optimize the tamping operations in ballasted tracks as preventive maintenance. The model, formulated as mixed 0–1 linear program, considers real technical aspects as constraints, which is a novel approach in the optimization of track maintenance over time. Global optimization is used to predict and to schedule tamping, taking into account four aspects: the evolution of the track degradation over time; the track layout; the dependency of the track quality recovery on the track quality at the moment of the maintenance operation; the track quality limits that depend on the maximum permissible train speed. Computational experience with two track stretches of the Portuguese Northern Railway Line is included to highlight the efficacy of the proposed methodology.

Influence of train dynamic modelling strategy on the prediction of track–ground vibrations induced by railway traffic

This paper presents a study developed in order to identify the influence and relevance of the mechanical properties of the train in the prediction of vibrations induced by passenger railway traffic. Based on experimental data and numerical simulation, a discussion about the influence of the modelling strategy adopted for the train simulation is performed. Special emphasis is placed on the study of the influence of the unsprung and semi-sprung masses of the train on the train–track interaction mechanism. From the developed study it was found that the modelling strategy adopted for the train simulation is a key factor in the prediction of the free-field vibrations. Moreover, the accuracy of the prediction is considerably increased if the train simulation considers the unsprung and semi-sprung (bogies) masses of the train.

Design and construction of backfills for railway track transition zones

The initial geometry of a railway track continually degrades over its life-cycle. Changes in the track alignment give rise to variations in the dynamic axle load which accelerate track degradation, with consequences for maintenance and availability of the line. This behaviour is particularly evident at some critical locations that are associated with abrupt changes in the track’s vertical stiffness, such as transitions to bridges or other structures. In order to mitigate this problem, careful design and construction is required, for which several recommendations have been suggested in the literature. However, studies based on the maintenance records of existing high-speed lines have shown that the problem of track degradation associated with stiffness variations is far from being solved. This paper presents a short review on the design of transition zones. A case study on the design and construction of a transition zone on a new Portuguese railway line is analysed. Results of conventional laboratory and cyclic load triaxial testing on granular materials and in situ mechanical characterization of the layers are presented. Relevant aspects regarding the construction are addressed and discussed. The results obtained at the substructure level seem to indicate that the design of the transition zone was successful in minimizing settlement and achieving a gradual stiffness increase as a bridge is approached.

A direct method for analyzing the vertical vehicle-structure interaction

Article history: Received 24 June 2011 Revised 24 September 2011 Accepted 3 October 2011 Available online 10 November 2011

Finite-element model calibration of a railway vehicle based on experimental modal parameters

This article describes the experimental calibration of a three-dimensional numerical model of an Alfa Pendular train vehicle based on modal parameters. The dynamic tests of the carbody and bogie of the vehicle allowed the determination of the frequencies and modal configurations of 13 vibration modes, by applying the data-driven stochastic subspace identification method. In addition, a dynamic characterisation test of the passenger-seat system was also conducted. The calibration of the model was performed using a submodelling/multistep approach involving two phases, the first one focused on the calibration of the model of the bogie under test conditions and the second one focused on the calibration of the complete model of the vehicle. The calibration was performed through an iterative method based on a genetic algorithm and allowed to obtain optimal values of 17 parameters of the numerical model. For the pairing of the vibration modes, real and complex, a recent technique was used based on the calculation of the modal strain energy. The stability of a significant number of parameters considering different initial populations demonstrated the robustness of the algorithm. The comparison of experimental and numerical responses before and after calibration revealed significant improvements in the numerical model and a very good correlation between the responses obtained with the calibrated model and the experimental responses.

Under sleeper pads in transition zones at railway underpasses: numerical modelling and experimental validation

In railway lines, transition zones between different track support conditions normally evidence higher degradation rates, thus requiring additional maintenance to ensure safety and service quality. Studies based on numerical simulations indicate that under sleeper pads (USP) can minimise those degradation rates. The study presented herein focuses on the influence of USP on the dynamic behaviour of transitions to underpasses, in an attempt to fill the gap between numerical and field studies. To that aim, the authors used finite element method models, calibrated and validated with field measurements. These models take into account the train–track interaction and include all relevant track components and backfill geomaterials. This study shows that soft USP have a significant influence over the tracks dynamic behaviour: amplifying rail displacements and sleeper accelerations, and inducing abrupt variations in the track vertical stiffness and oscillations in train–track forces. To benefit from the use of USP, the authors highlight the need to carefully design stiffness properties of USP and determine their arrangement at transitions. An improved design for the transition zone is proposed.

Hybrid Fiber-Optic/Electrical Measurement System for Characterization of Railway Traffic and Its Effects on a Short Span Bridge

The characterization of traffic effects on a short span railway bridge in Northern Portugal with a new hybrid platform that allows the simultaneous assessment of signals generated by a sensing network composed of both electrical and fiber Bragg grating-based sensors was demonstrated. A commercial fiber-optic-based train characterization system using a bridge weight-in-motion algorithm was also developed and tested. By using only three fiber Bragg grating sensors the system allows on-motion determination of train speed and weight distribution.

On the use of under sleeper pads in transition zones at railway underpasses: experimental field testing

The railway track at transition zones to bridges or to other structures usually requires considerably more interventions to maintain the same open track service quality and safety levels. Under sleeper pads (USP) have been reported to contribute to reduce degradation and frequency of maintenance operations in open ballasted tracks, and recent numerical studies have suggested its use in transition zones. In this study, the influence of using USP at transition zones to underpasses was investigated. The structural behaviour of two transitions to similar box culverts was compared. USP were added to the sleepers in one of the transitions. Track characterisation using non-destructive tests is presented and analysed. Measurements of track response from passing trains are also presented and discussed. Results indicate that USP influence the dynamic behaviour of the track, increasing its vertical flexibility and amplifying both rail displacements and sleeper accelerations.

Dynamic monitoring of railway track displacement using an optical system

With the increases in traffic, axle loads and travelling speed, the dynamic monitoring of railway tracks and structures is becoming more and more important to ensure a high level of safety and comfort. This situation is particularly critical at transition zones where rapid changes of track stiffness occur. This paper presents a contactless system to measure track displacements and its application in an embankment/underpass transition zone, located on the Northern line of the Portuguese railway network where the Alfa Pendular tilting train travels at a maximum speed of 220 km/h. The system is based on a diode laser module and a position sensitive detector (PSD). The PSD receives the laser beam emission and the detection of the centre of gravity of the beam spotlight on the PSD area enables the calculation of the displacement. Before field application, static and dynamic laboratory validation tests were performed in order to evaluate the system performance for different laser to PSD distances, and an accuracy of 0.01 mm was achieved using data acquisition rates of up to 15 kHz. The optical measuring system proved to be an efficient and flexible way to measure absolute and relative rail displacements in the field, enabling the detection of track deformability differences along the transition zone, even for the passage of trains at high speed (220 km/h).

Updating and validation of the dynamic model of a railway viaduct with precast deck

This study describes the calibration and experimental validation of the dynamic model of a railway viaduct with precast deck. Global modal parameters of the structure and local modal parameters of the upper slab of the deck are identified based on a dynamic test. The calibration of the numerical model is done using a genetic algorithm that allows obtaining optimal values of 11 parameters of the numerical model. The inclusion of local modal parameters proved to be crucial, as various parameters of the numerical model do not have significant influence on global modal parameters. Mode pairing between numerical and experimental vibration modes is performed using a recent technique based on modal strain energy. The experimental validation of the calibrated numerical model is done by the comparison between numerical responses and experimental responses obtained in a dynamic test under railway traffic. This dynamic test shows the existence of a nonlinear behaviour of the viaducts supports. There is an excellent correlation between numerical and experimental responses for different train speeds with the adjustment of the longitudinal supports stiffness of the calibrated model.