MS9.6 - Dynamics of Railway infrastructures

Using axle box accelerations (ABA) measured from rail vehicles is a cost-efficient way of monitoring railway track conditions. Existing techniques are focused on identifying a specific type of degradation, usually at a single track layer, such as at the rail top, fastenings, insulated joints, ballast, etc. The basic principle is to identify unique ABA features that correspond to a particular degradation. For instance, increased magnitudes of wavelet power spectrum between certain frequencies were used for the detection of singular rail surface defects, such as squats. A common approach for identifying such ABA features is through sensitivity analysis, which employs a vehicle-track interaction (VTI) model and simulates track degradations via parameter changes in the VTI model. However, different types of degradation may cause similar changes in specific ABA features. In this case, a wider range of ABA features should be investigated to determine if they can distinguish between different degradations. In addition, conventional sensitivity analysis only varies one parameter at a time while fixing the other parameters, making it difficult to consider the combined effects of multiple degradations. As a result, it is still challenging to distinguish between various types of track degradations, particularly when they occur at different track layers at the same time. Instead of focusing on a specific type of track degradation, this paper aims to identify interpretable and generic ABA features measured on a baseline track without degradations. Furthermore, the most contributing track parameters or operational variables to different ABA features are determined through global sensitivity analysis, where multiple parameters of a VTI model are varied simultaneously to simulate the effects of combined degradations under various operational conditions. The outcome of this paper can be used to establish a quantitative relationship between ABA features and track parameters, which can be applied to track condition monitoring. First, ABA features are identified through field measurements at a well-maintained plain track. We apply the synchrosqueezed wavelet transform to the ABA signals and construct a feature space with peak frequencies and magnitudes of the power spectrum. Furthermore, based on hammer tests and pass-by measurements, different peak frequencies are associated with different mode shapes of the VTI system and thus can be used to monitor the condition of their corresponding components. Identified ABA features serve as a baseline for calibrating and validating the VTI model. Subsequently, a high-dimensional parameter space is defined to represent track conditions (such as the track geometry and railpad/ballast stiffness) and operational variables (such as the axle load and vehicle speed). The parameter space covers both the nominal and degraded values, as well as the uncertainties associated with them. We sample from the parameter space to obtain multiple sets of space-filling input parameters for the VTI model. In such a way, different track parameters are varied simultaneously across multiple samples. Through simulations with sampled parameters, a variance-based global sensitivity index is calculated to determine the most sensitive ABA features associated with each track parameter.

In this work, the dynamic response of six portal frame bridges are experimentally evaluated through full-scale controlled dynamic testing. The study aims to (i) experimentally identify the modal characteristics of portal frame railway bridges with different span lengths and subsoil conditions, (ii) provide high-quality experimental data that can be used for system identification and experimental validation of numerical models; and (iii) study the influence of Soil-Structure Interaction (SSI) on the fundamental modal parameters and resonant response of bridges. In the case of short span bridges, the measured modal properties identify the substantial contribution of the surrounding soil on the global damping of the system and highlight the importance of the SSI on the dynamic response. On the contrary, the identified fundamental damping ratios of the long span bridges are in the range of the recommended design values and seems not to be influenced by the SSI effects. The results show that there is a need to review the recommended modal damping ratios for this type of bridges in the code provisions and design manuals.

For railway bridges on high-speed lines, dynamic analyses are usually required to ensure that vibrations from passing trains do not exceed certain limits, e.g. regarding the vertical deck acceleration. Using over-simplified models may lead to inaccurate results that sometimes show theoretical exceedance for bridges that are not susceptible to dynamic loading. This may cause unnecessary changes in the design of new bridges or speed restrictions on existing bridges. This paper presents an efficient 2D beam model of a continuous single-track concrete slab bridge with span lengths of 13 + 17 + 13 m. At the end supports, the deck has a 1.5 m overhang with an integral structure in interaction with the adjacent embankment. The columns are rigidly connected to the bridge deck. All supports are built on slab foundations, and one of the supports sits on a stratum of sandy silt that will affect the dynamic behaviour. Experimental testing has been performed using a hydraulic exciter with a controlled frequency and a known input. The resulting frequency response functions (FRF) clearly show the first three vertical bending modes. Testing with increased load amplitude shows a trend of decreased natural frequency and increased damping, especially for the first mode. The response from passing trains was also recorded. The geometry of the 2D model is based on design drawings and geotechnical documents. The boundary conditions are updated based on the experimental FRFs. The model is then used to simulate the response from passing trains and compared with the experimental data. Good agreement is generally found between the 2D model and the experimental data, both regarding the FRFs and passing trains. The model further shows the importance of accounting for soil-structure interaction in dynamic analysis, otherwise excessive vibrations may occur.

This paper investigates energy harvesting on railway bridges. The electromechanical behaviour of a cantilever-based energy harvester is represented by an analytical model for the estimation of the energy harvested from train-induced bridge vibrations. A genetic algorithm constrained to geometry and structural integrity is studied to solve the design optimisation problem of 3D printed energy harvesters tuned to the fundamental frequency of the bridge. Additive manufacturing by 3D printing of the substructure of the harvester is implemented to maximise the design flexibility and energy performance. The design and manufacture procedure of an optimal device prototype with PLA substructure is presented for a real bridge in an is service High-Speed line. The optimal design prototype is experimentally validated under laboratory conditions. Finally, the performance of energy harvesting is evaluated from experimental data measured by the authors.