Railway transition zones present a major challenge in railway track design mainly due to abrupt jumps in stiffness and differential settlements that result from crossing stiffer structures such as bridges or culverts. Despite numerous efforts to mitigate these transition effects at both the superstructure and substructure levels, a comprehensive solution remains elusive. Substructure-level interventions have demonstrated some effectiveness but are often cost-prohibitive and challenging to implement in existing operational railway transition zones. In contrast, mitigation measures at the superstructure (rail, sleepers, rail-pads, under-sleeper pads) level can be easily installed but have shown limited improvement in site measurements. This study evaluates the influence of different sleeper configurations in transition zones and reduced sleeper spacings on the operation-driven dynamic amplifications in railway transition zones, employing a recently proposed criterion based on the total strain energy in the track-bed layers (ballast, embankment, and subgrade). In addition to this, the influence of the loss of contact between sleepers and ballast (i.e., hanging sleepers), which typically results from the differential settlement, is studied. The first part of the paper provides useful insights regarding the interventions (and/or initial design) in the sleeper configuration and spacing, whereas the second part of the work highlights the need for interventions to deal with the loss of contact between sleeper and ballast. A 2-dimensional finite element model of an embankment-bridge transition was used for the analysis. The results show that it is not possible to mitigate the transition effects completely using the interventions involving sleeper spacing and configuration.@en

Railway transition zones (RTZs) experience higher rates of degradation compared to open tracks, which leads to increased maintenance costs and reduced availability. Despite existing literature on railway track assessment and maintenance, effective design solutions for RTZs are still limited. Therefore, a robust design criterion is required to develop effective solutions. This paper presents a two-step approach for the formulation of a preliminary-design criterion to delay the onset of processes leading to uneven track geometry in RTZs. Firstly, a systematic analysis of each track component in a RTZ is performed by examining spatial and temporal variations in kinematic responses, stresses and energies using a finite element model of an embankment-bridge transition. Secondly, the study proposes an energy-based criterion to be assessed using a model with linear elastic material behavior and states that an amplification in the total train energy in the proximity of the transition interface is an indicator of increased (and thus non-uniform) degradation in RTZs compared to the open tracks. The correlation between the total strain energy (assessed in the model with linear material behaviour) and the permanent irreversible deformations is demonstrated using a model with non-linear elastoplastic material behavior of the ballast layer. In the end, it is claimed that minimising the magnitude of total strain energy will lead to reduced degradation and a uniform distribution of total strain energy in each trackbed layer along the longitudinal direction of the track will ensure uniformity in the track geometry.@en

The railway transition zone where the track transitions from a ballasted track to a slab track, is a crucial area that can experience amplified dynamic responses. This work aims to develop a deeper insight into the mechanisms leading to the amplified dynamic response in railway transition zones. The study employs a finite element model to investigate the amplification of total strain energy due to the phenomena of reflection and redistribution of energy close to the transition interface. The results of the study are obtained for three case studies involving non-reflecting boundary (representing an energy sink) and homogeneous material along the vertical direction of the track, and the responses are studied for individual and combined effects in comparison to a benchmark case. The findings of the study show that eliminating the phenomena results in no dynamic amplification in total strain energy in railway transition zones. The conclusion highlights the importance of understanding these phenomena in order to design an efficient railway transition structure.@en

Railway transition zones (RTZs) are regions where abrupt track stiffness changes occur that may lead to dynamic amplifications and subsequent track deterioration. The design challenges for these zones arise due to variations in material properties in both the depth (trackbed layers composed of different materials) and longitudinal directions of the track, as well as temporal variations in mechanical properties of materials due to several external factors over the operational period. This research aims to investigate the effects of these variations in material properties (i.e., of the resulting stiffness distributions in vertical and longitudinal directions) on the behaviour of RTZs, assess from this perspective the performance of a novel transition structure called the SHIELD, and establish a methodology for designing a robust solution to mitigate the dynamic amplifications in these zones. Results indicate that stiffness variations in both vertical and longitudinal directions significantly influence the dynamic behaviour of the RTZs. The study also suggests a permissible range of stiffness ratios to control the amplification of strain energy in the most critical components of RTZs, both in the initial state as well as during the operational phase (where material properties may vary over time). Moreover, the proposed methodology offers a valuable tool for the design and evaluation of RTZs and is applicable to various transition types and a broad spectrum of material properties.@en

Railway transition zones are the most critical part of the railway infrastructures that experience 4-8 times more degradation compared to open tracks. Despite several attempts to reduce the maintenance and operation costs in these critical zones, a robust and comprehensive solution remains unknown. In recent studies, a robust safe hull inspired energy limiting design of a transition structure was proposed for an embankment-bridge transition (without ballast layer over the bridge) to deal with operation-induced degradation. However, this solution was investigated in detail for only this particular type of transition. In this work, the scope of this mitigation measure is extended for an embankment-bridge transition with ballast running over the bridge and its performance is evaluated using a strain-energy criterion. It was concluded in the end that the safe hull inspired energy limiting design can effectively mitigate the operation-induced dynamic amplification for more than one type of railway transition zones. @en

The maintenance and repairs of transition zones pose significant challenges in the railways industry due to their accelerated degradation rates in comparison to free tracks. These zones, characterized by track property variations, amplify their response when a source travels along the tracks. The resulting amplifications in stress and strain fields lead to differential settlements, affecting both track's stability and passenger comfort. This study investigates the influence of incorporating active control forces and moments at the interface of transition zones to mitigate the excessive degradation of soft domains.

Three 1-dimensional models are developed, which include active control forces and moments as a novel method to minimize the amplification of the response in the soft domain of transition zones, as well as, the derivation of these forces. The purpose of these three models is to give the reader an inside view of the method to derive these active control forces with models that increase in complexity.

The first model involves an Euler-Bernoulli beam resting on a piece-wise inhomogeneous Winkler foundation. The second model extends this by including a shear beam and a second layer of foundation, with the first layer being homogeneous and the second layer piece-wise inhomogeneous (Kerr foundation). The third model is a hybrid model between the first two, on which the soft domain is represented by the description of the second model, and the rigid domain by the description of the first model. Numerical solutions in the time-domain were applied to these models, and the study focused on a transition zone subjected to a constant amplitude moving load with constant velocity, traveling from a soft to stiff domain. The purpose of the first model is to give the reader a general idea on the derivation of a very simplistic model. The second model's purpose is to better represent the different elements of a railway structure, on which the shear beam and the lower layer of springs represent the mobilised soil under the tracks, while the top layer of springs and the Euler-Bernoulli beam represent the ballast, the sleepers and the rail. Finally, the third model was made to represent a transition zones, on which the soil is discontinued due to a man-made structure, e.g. a concrete bridge, which leads the structure under the ballast, in the rigid domain, to be consider infinitely stiff and to be represented just by an Euler-Bernoulli beam on a Winkler foundation.

Findings reveal that the active control forces and moments are capable of fully mitigating the dynamic amplification in soft domains of transition zones, however, they increase the dynamic amplifications in the stiff domain. Moreover, the shape that these forces take over time is dictated by the interaction between both domains of the transition zone, when the interface of the transition zone has continuous elements, the forces take a similar shape to the eigenfield of the system, while when there is discontinuous elements over the interface, the forces take a 'flipping' shape. Furthermore, of the two parameters studied (velocity of the moving load and vertical stiffness), the dominating parameter in the response of the system depends upon the regime that the system is being subjected to. For relatively low and extremely high velocities of the moving load, the system is dominated by the vertical stiffness ratio, while for medium velocities of the moving load, the system is dominated by the velocity of the moving load. Finally, transition zones with discontinuous elements require significantly more energy to be absorbed and added into the system by the active control forces to mitigate the dynamic amplifications in the soft domain of the system.

These thesis offer valuable insights for preliminary designs of active control forces aimed at diminishing the dynamic amplification of soft domains of transition zones railway.

This comprehensive study addresses the persistent issue of railway transition zone degradation, evaluating the efficacy of the most commonly used mitigation measures and proposing a novel Safe Hull-Inspired Energy Limiting Design (SHIELD) of a transition structure. Firstly, this work assesses the traditional transition structures, including horizontal and inclined approach slabs and transition wedges, using commonly studied responses (kinematic response and stress) and a recently proposed criterion based on total strain energy minimization. The second part of the paper evaluates the newly introduced transition structuress (SHIELD) using the same criterion as used for the evaluation of the traditional transition structures. A detailed investigation of existing and a new design using a 2-dimensional finite element model shows SHIELD’s effectiveness in managing energy flow at transition zones and provides reasoning behind the ineffectiveness of the other commonly used transition structures. The study demonstrates the robustness and comprehensiveness of the recently developed energy-based criterion and its applicability to different types of transition zones. Moreover, it highlights the potential of SHIELD as a solution to address the complexities associated with the design of railway transition zones.@en

ransition zones in railway tracks experience strong amplification of stress and strain fields due to the passage of train over inhomogeneity. The inhomogeneity in these zones can be attributed to changes in mechanical properties of material along the longitudinal direction of the track, and to displacement/traction discontinuities at interfaces leading to an amplified response in railway transition zones (TZ) with respect to the open tracks. In this paper, different kinds of inhomogeneities are considered in isolation and in combination to study the effects on railway track components in transition zone. The first type of inhomogeneity considered is non-uniformity of materials at various levels of the track along the longitudinal direction. The second type of inhomogeneity that will be considered arises from displacement and traction discontinuities at the interface of soil and structure and at the interface of sleepers and ballast (hanging sleepers). The results provide necessary insight for the design of effective mitigation measures to prevent the amplified response in railway TZ.@en

This paper studies the effectiveness of adding auxiliary rails as a mitigation measure for degradation in transition zones of railway tracks. More specifically, it investigates the settlement mechanisms counteracted by the additional rails. Results show that when the system’s response is in the quasi-static regime, adding auxiliary rails over the soft part of the transition zone is beneficial while adding them over the whole transition zone is not. Furthermore, the auxiliary rails have a beneficial impact also when the system’s response is in the dynamic regime; the beneficial effect is caused by the improved load distribution to the supporting structure and not from counteracting the dynamic response amplification that occurs at transition zones. While this mitigation measure has been previously investigated, the contribution of this study lies in a more in-depth analysis of the mechanism through which auxiliary rails can mitigate the degradation at transition zones.@en

Human-induced rhythmic loading is an increasingly critical aspect in the design process of civil engineering structures such as sports stadiums and floors accommodating gym and aerobic classes. There are two main reasons for this trend. First, structures are becoming more slender with improvements in materials and construction techniques and modern trends in architectural designs. And second, crowds are getting livelier than previously was the case, their activities can become better synchronized due to the presence of various auditory and visual stimuli at above mentioned events.

In recent years there is also a growing discussion on the strength and stability of event structures. A very common type of structure is an event deck structure. Visitors gather on top of these decks during festivals and in some cases festival organizers place bars underneath them to efficiently use the structure. Personnel working in these bars experience the movement of the structure due to a dancing crowd and tend to feel uncomfortable. Engineers at Tentech are aware of this phenomenon through contacts with clients.

Nowadays, in the Netherlands, event deck structures are designed to withstand a vertical static load of 5 $kN/m^2$, prescribed by Dutch design codes. The amplitude of this static load is based on a dense static crowd. But according to existing literature, a synchronically jumping crowd can cause a vertical load which far exceeds the design load prescribed by design codes. This provides a reason to further investigate the extreme load case of a synchronically jumping crowd on an event deck structure.

A missing element in the standard design of structures subjected to a synchronically jumping crowd is the consideration of dynamic interaction between human and structure. In this research the focus is on how human-structure interaction (HSI) can influence the human-induced loading and the internal stresses caused by that loading in the structure. This is done by building a 3D finite element model in Abaqus.

A 3D finite element model is used to easily vary in the position of a jumper on the structure. Modelling group effects such as the coordination factor is also easier when using a 3D model. And in the case of an event deck structure a 3D model will result in more detailed results compared to 2D models because mechanical properties of an event deck can vary over the third dimension.

A mass-spring system is suggested to represent a jumping person. To simulate a synchronically jumping crowd, multiple mass-spring systems are used. By assigning an initial velocity to the mass in the mass-spring system, it is possible to simulate a similar mechanism as a jumping person colliding with a structure. By analysing the force in the spring over time it is possible to draw conclusions on the influence of human-structure interaction on the impact peak force. And by comparing the impact forces with the reaction forces which are measured at the base of the structure, the effects of structural vibrations on the internal stresses are determined.

It is found that taking HSI into account does influence the impact peak force and the internal stresses of the structure. But for an event deck structure with a natural frequency of 20 Hz or higher, this only accounts for an individually jumping person. It is found that the more people jump on a structure, the less significant the effect of HSI will be, even when a crowd tries to jump synchronically. This is caused by the natural time lag between each jumping person.

An important subject in the research on the behaviour of railway structures is the validation of numerical models by means of in-track measurements. Many forms of track measurements have taken place all over the world, mainly in the sense of strain and acceleration measurements, but a more unexplored area is that of the direct measurement of stresses in the ballast.
The aim of this research is to find the best way to equip a railway sleeper with measuring instruments in order to use it for validation of computational models. The sleeper must be able to measure the the vertical velocity of the sleeper as well as the normal stresses on the ballast-sleeper interface over time.

In order to predict the quantities to be measured, a finite element model was build for this research with the Kratos open source software package. The model comprises one single sleeper on top of a volume of ballast, both in solid elements. A dynamic load was applied vertically on top of the sleeper to simulate the passing of a double axle bogie. Linear elastic springs were fixed on top of the sleeper to represent force of the rail pulling the sleeper back to its initial place.
In order to analyse the effect of hanging sleepers, interface elements were used in between the ballast and the sleeper to simulate hanging gaps. The idea is that these elements transfer no stresses when the surfaces are physically separated, and adopt Mohr-Coulomb criteria as soon as the surfaces approach each other. This method turned out to be very difficult to implement, despite numerous discussion with the software engineer it was not possible to script this in the FE software in such a way that all calculations converged without any errors.
During the modelling of the sleeper, it was found that the forces on the sleeper are highly dependent on the resistance of the rail. The rails have such a high bending stiffness that they hardly show any curvature over the length of only a few meters, so the sag of the rail is mainly dependent on the behavior of the surrounding sleepers. It was concluded that, to get a good picture of the behaviour, it is not sufficient to model only one individual sleeper, but it is necessary to look at a larger part of the railway track in order to involve the coupling between the sleepers.
The results from the computational model were used to determine the magnitude range and the frequency domain of the values to be measured, in order to be able to select the right measuring instruments. The results were also used to select the right positions for the sensors on the sleeper. Due to the constraints described above, a certain margin of uncertainty has been adopted.

The vertical speed of the sleeper is best measured with accelerometers, the large amount of previous applications of this type of measurement has shown that this measurement works well and will not lead to problems.
It was considered to use pressure cells for the measurement of interface stresses, though this device turned out to be difficult to fix at the sleeper and it gives very little information on the distribution of stresses. It was concluded that the best way to measure the ballast-sleeper interface stresses is by using the matrix based tensile surface sensor (MBTSS). This instrument is able to capture the stress distribution on a surface over time, by measuring the electrical current flow through a matrix of conductive lines, the current is resisted when certain forces are applied on the surface. Tekscan is a manufacturer that offers a wide range of MBTSS types, including sensors previously used in railway measurements. A certain protection will be needed to protect the sensor from being damaged by the ballast particles.
Previous applications of MBTSS in railway structures showed the calibration of the sensor to be very difficult. This presumably has to do with the way the stresses are distributed over the sensor surface. Since the stresses of the ballast are often compressed to very small contact points, it should be made sure the stresses will not get lost between the sensels, that are the points on the surface where the stress magnitudes are measured. The advice is to cover the MBTSS with an under sleeper pad, this is a relatively stiff mat that, in addition to offering protection to the sensors, also spreads the ballast forces. Laboratory tests will have to be performed in advance of the field measurement to test whether a correct calibration is possible. It can not be made sure that a fully successful calibration method will be found and therefor additional measurements are recommended for a validation for the stress magnitudes. A first validation can be found by equating the sum of the vertical forces with the mass times the acceleration of the sleeper. To find the sum of the vertical forces, the load on top of the sleeper at the rail-sleeper connection, has to be measured as well. This can be realized by placing MBTSS between the baseplate of the rail and the sleeper. Similar measurements described in literature proved this type of measurement to be feasible. Furthermore, the result of the acceleration measurement will have to be used here as well. A second validation can be performed by determining the moment distribution over the length of the sleeper, this can be conformed to the magnitude and distribution of the vertical stresses on the sleeper. The most reliable way to perform this validation is by using fiber measurements to find the moment distribution over the whole of the length. The most important positions over the length of the sleeper are under the rails and in the middle of the sleeper, because the peaks in the moment distribution are expected here based on the computational model calculations.

This work presents a numerical simulation of the church of San Francesco, Amatrice, and subsequent interpretation of the evolution of damage and collapse mechanisms that were caused by the main events of the seismic sequence that hit the Apennine area of Central Italy in 2016. The study primarily focuses on the response of the church with reference to the two main events: seismic shocks of 24th August and 30th October 2016. The dynamic non-linear analysis was performed on the structure using Abaqus CAE 2017, which was intended at simulating the damage and collapse that were observed on the real structure subsequent to the main seismic events. The study also contributes to suggest preventive interventions/material enhancements that could have either limited or mitigated the damage, thereby avoiding the collapse of the Church. In an effort to reciprocate the seismic events, the time-history of the accelerograms recorded at the AMT fixed station and of the amplified local accelerogram were considered as seismic input. Furthermore, this study presents simulations of the response of the structure subjected to the main seismic events simulated in a continuous chronological sequence and the effects of local and global interventions on the seismic response of the entire structure. This study indicates the application of numerical simulation as an efficient tool for seismic analysis of masonry structures, with the obtained results ranging within the acceptable margin of errors. In addition, the simulations can be used to analyse the proposed interventions, while taking into account the limitations of the software and computational techniques. This paper highlights the interesting conclusions related to the effects of local and global interventions in case of the church of San Francesco and the role of amplification of the accelerations for the site of Amatrice.@en

This work is aimed at the numerical interpretation of the evolution of damage and collapses observed on the Civic Tower of Amatrice, caused by the main events of the seismic sequence of 2016 in the Apennine area of Central Italy. In particular, the study considers the response of the tower with reference to the two main events, occurred on August 24th and October 30th, 2016 respectively. Non-linear dynamic analyses were carried out by developing a finite element model and the behaviour of the tower was investigated with reference to the damage and partial collapses produced by the main events. In dynamic numerical analyses, the accelerograms corresponding to the main seismic events obtained with the study of the site effects were used as seismic input. Moreover, studies have been carried out to understand if and which of the interventions of reinforcement/seismic improvement, realized at the beginning of the years ‘80, have been determinant to limit the damage, avoiding the complete collapse of the tower. The results of the study, on the one hand, allow to highlight a good correspondence between the evolution of the actual damage observed on the tower and the damage assessed by numerical analyses thus demonstrating the validity of the numerical models set up for the analyses. On the other hand, however, they have made it possible to underline how the presence of the improved material and structural reinforcement interventions carried out at the beginning of the years ‘80 have contributed to avoid the complete collapse of the tower.@en