Effects of moving loads on railway bridges considering the structure of railway track

Resumen

A significant increase of the high-speed railway network has been built in Spain and other European countries for the last decades. Due to the complex Spanish orography, several tunnels and bridges are necessary to accommodate the railway layout to the specific site conditions. Already from the first installation of high-speed viaducts, concerns regarding dynamic effects caused by train loads and eventual resonance called the attention of engineers and scientists. As a result, dynamic analyses are typically required at the design stage of railway bridges in order to verify the structural safety, serviceability and passenger comfort. Despite the experience gained for the last years, new challenges have arisen due to the introduction of technological innovations in railway construction. Among them, track types different from the traditional ballasted track are being introduced, including ballastless track systems or slab track.

Ballastless track has been identified as a promising alternative against ballasted track due to the reduced maintenance costs and excellent quality control. Ballastless track has been recently installed in several railway bridges of the new high-speed lines in northern Spain (e.g. new access to Galicia). Due to its practical interest, the main goal of the present TFM is to analyze whether the track structure (i.e. ballasted or ballastless) can play a role in the dynamic performance of railway bridges and the passenger comfort.

As the term ballastless track can include several track types which can affect differently to the whole structural response, the present TFM has covered the analysis of so-called monolithic and independent ballastless track systems, depending on the interaction degree of the track with the bridge girder. The former can be a design consisting of an in-site continuous reinforced concrete slab in which twin-block sleepers are embedded. The later can be implemented as floating precast concrete slabs separated from the bridge girder with an intermediate elastomeric layer or mat. In addition, a reference ballasted track has been also considered in the study.

In the present TFM, the purpose has been to study the influence of the above-mentioned track types on the dynamic behavior of railway bridges under moving loads. The methodology has consisted of numerical simulations including the different elastic components between the track and the bridge. The loads have consisted of a set of moving loads due to the passage of high-speed commercial trains at different velocities. Two types of bridges have been studied, according to the common bridge typologies of the Spanish network, namely a short single-span bridge and a long statically indeterminate bridge.

Comparisons have been carried out in terms of displacements, accelerations and dynamic amplifications at the bridge and the rail. The results have indicated that the short bridge is rather sensitive to dynamic effects caused by moving train loads, especially for speeds over 300 km/h. Moreover, it has been obtained that ballastless track systems can help in reducing accelerations and dynamic amplifications at the bridge with respect to ballasted tracks. In turn, the long multi-span bridge has shown to suffer much less dynamic effects than the short bridge.