MS9.9 - Dynamics of Railway infrastructures

Vehicle-bridge systems are continuously subjected to environmental and operational variations that complicate the process of identifying structural damage. In general, damage is defined both as a substantial alteration of the physical characteristics of the material, as well as the modification of the geometric characteristics of any part of the structure, including changes in the boundary conditions and connectivity of the system itself. The loads that mostly affect the structural integrity of bridges are associated to vehicular traffic. This paper proposes a method for damage detection in railway bridges which combines two approaches of dynamic analysis: Model-Based and Data-Based. Two models of a bridge need to be created for this purpose. The first model is a finite element numerical model (FEM), which is calibrated and updated through the data collection campaign of a continuous or discontinuous monitoring system of the structure in terms of accelerations and deformations. The purpose of this model is to have a reference benchmark against which damage can be measured. The simulation is carried out in a supervised learning fashion, through which damage indexes can be determined through the comparison of the experimental data with the benchmark FEM. The second model is based on an autoregressive (AR) model, which uses both experimental and numerical data to conduct damage identification. The effectiveness of this approach arises as a result of the AR parameters being directly proportional to the stiffness of the structure. Therefore, the time series of continuously identified AR parameters can be used as damage-sensitive features. The effectiveness of the proposed approach is firstly validated through a simplified bridge model that takes into account the vertical and lateral dynamic interactions of the vehicle-structure system. Afterwards, the proposed methodology is applied to a real in-operation bridge, the Mascarat Viaduct in Alicante (Spain). The viaduct, built in the beginning of the 20th century, is part of the railway network of the Valencian community FGV (Ferrocarriles de la Generalitat Valenciana) belonging to line 9 of the TRAM of the province of Alicante. The presented results and discussion evidence that the fusion of the two approaches facilitates the damage identification problem in railway bridges. The usefulness of the proposed methodology lies in the fact that bridge data acquired in damaged conditions are generally scarce or even non-existent.

A realistic and economical computational assessment of the dynamic behaviour of railway bridges requires, first and foremost, input parameters that correspond to reality. In this context, the computationally applied damping properties of the structure have a decisive influence on the results in the prediction of resonance effects. Concerning the damping factors used in dynamic calculations, the EN 1991-2 standard prescribes damping factors depending on the type of structure and the span. However, these factors can be regarded as very conservative values. As a result, in-situ measurements on the structure are often necessary to classify a bridge categorized as critical in prior dynamic calculations as non-critical. Regarding in-situ tests, a measurement-based determination of the damping factor is inevitably accompanied by a scattering of the generated results. This scattering is due to the measurement method used. Also, it results from the individual scope of action of the person evaluating the test and this person's interpretation of the measurement data. With this background, this contribution presents methods and analysis tools for determining the damping factor, intending to reduce the scatter of the results, and limiting the scope of action of the person evaluating the test. Methods and analysis tools are discussed for methods in the time domain and the frequency domain. The standard method for damping determination in the frequency domain is the bandwidth method, whereby the damping factor is determined based on an amplitude-frequency response generated from measurement data. Concerning the time domain, the damping factor is determined based on the decay process after dynamic excitation via the logarithmic decrement. With regard to the dynamic excitation before decay processes, excitation mechanisms with high energy input are to be selected so that the measured decay process has several clearly identifiable oscillation cycles. For both measurement methods (time and frequency domain), the derivation of determination equations for the damping factor is performed assuming a single degree of freedom (SDOF) vibrating system with linear system properties (constant mass, stiffness, and damping). This contribution presents more advanced procedures and methods as extensions of the standard methods. In both cases, the basic idea is to adjust the amplitude-frequency response generated by measurement (frequency domain) or the recorded decay process (time domain) using the least square method by a mathematically defined curve in such a way that the greatest possible agreement between measurement data and approximation is achieved. Based on several in-situ tests on existing bridges, the procedure for determining the damping factor is explained, and the methods are compared in the time and frequency domain. It is shown that a clearly defined evaluation algorithm can significantly reduce the scattering of results. Furthermore, it is shown that the amount of dissipated energy substantially affects the generated damping factors. Higher energy dissipation results in higher damping factors, which means that using excitation methods with high energy input (e.g. excitation by train crossing) leads to higher and, thus, favourable damping factors concerning dynamic calculations.

This work is devoted to the analysis of the vibratory response of High-Speed (HS) multi-track railway bridges composed by short-to-medium simply-supported spans. The vibrational response of such struc-tures may be influenced by three-dimensional modes (first torsion and first transverse bending modes of the deck) and, therefore, the use of planar model may not be adequate. In particular, it aims to investigate the influence of three geometrical aspects usually disregarded in numerical models used to evaluate the Serviceability Limit State of traffic safety in such structures: (i) the deck obliquity, (ii) the presence and correct execution of transverse diaphragms at the supports, and (iii) the number of successive simply-supported spans weakly coupled through the ballast track layer. The influence of these aspects is ana-lysed from the correlation of a detailed numerical model and experimental measurements on an in-service High Speed (HS) multi-track railway bridge. From the reference model, a set of variants accounting for different levels of deck obliquity and dia-phragm configurations are envisaged and the maximum transverse acceleration over the platform is determined under railway excitation. The analysis is extended to bridges with an increasing number of successive spans covering the lengths of interest. Special attention is paid to the particular location of the maximum response and to the participation of modes different from the longitudinal bending one. Finally, a numerical-experimental comparison of a real bridge response under two train passages is presented for the straight and oblique models, and the response adjustment along with the actual bridge performance are assessed.