Transverse vibrations in existing railway bridges under resonant conditions: Single-track versus double-track configurations

In this paper, the dynamic performance of a certain kind of existing bridge under the action of high-speed railway traffic is addressed. Short-to-medium-span simply supported structures are susceptible to experiencing harmful transverse vibrations under resonant conditions, leading to excessive levels of vertical acceleration, the subsequent premature deconsolidation and degradation of the ballast layer, passenger discomfort and high maintenance costs of the corresponding lines. This dynamic problem, which can require the reduction of the design velocity or the temporary interruption of the railway service to undertake corrective actions, has been reported in Europe, China and Japan during the last decades (European Rail Research Institute (2000) [13], Duan (1995) [30], Ishibashi (2004) [15]). In particular, single-track simply supported short bridges appear to be quite critical structures in this regard. In what follows, the authors present an evaluation of the dynamic performance of three real bridges on the Spanish railway network, located in double-track branches but composed of structurally independent decks. The objective of the investigation is to compare their performance with that of hypothetical modified double-track versions of the original decks. An orthotropic plate finite element model, whose properties are updated from experimental tests performed on the bridges in the past, is used to predict the structural dynamic responses in the time domain. Finally, the predominant effect of the increase in the mass of the bridges over the increase in the eccentricity of the tracks, the reduction in the natural frequency experienced by the torsion mode, and the relevance of three-dimensional modal contributions is proven, leading to remarkable reductions of the maximum acceleration levels in the multiple-track modified structures.