Abstract
Today, one of the most important variables for the design of high-speed railway bridges is the vertical acceleration of the deck, whose maximum value is limited by the Standards and it constitutes one of the Serviceability Limit States (SLE) associated to traffic safety. In particular, simply supported isostatic bridges of short-to-medium span lengths belonging to conventional railway lines are quite critical in this regard, since they may experience unadmissible vertical acceleration levels due to resonance situations.
The problem of excessive vibrations has led to the decision of replacing some old bridge decks to improve their dynamic behavior when subjected to new traffic requirements. This decision must be based on rigorous dynamic calculations and according to the Standards. Traditionally, planar numerical simply supported beam models have been very common in literature, which may be advantageous in terms of computational costs although it may not be accurate enough for double track bridges, due to the contribution of three-dimensional modes. Another common practice of modelling is to neglect the effect of the elastic supports (neoprene) located on the bridge abutments, a trend that has been extended to orthotropic plate models and grillage analisis. In the framework of this Thesis the influence of these practices of modeling in the verification of the SLE of vertical acceleration is evaluated, through the study of the dynamic behavior of a set of isostatic bridges of different types and relations width /span length. The behaviour of the bridges of study has been simulated using spatial finite element models, based on the approximation of the thin orthotropic plate and including the vertical stiffness of the elastic bearings. The convenience of using three-dimensional models over the traditional beam-type and the influence of the elastic bearings in the prediction of their dynamic behaviour has been quantified with the implemented numerical models. 
As an alternative to the classical strengthening or deck replacement process in existing railway bridges that exhibit an unadmissible dynamic behaviour when subjected to higher specifications, in this work the possibility of increasing their level of damping using viscoelastic dampers is studied. The proposed solution transforms the vertical oscillations of the slab into shear deformation of the viscoelastic material, achieving an energy dissipation and therefore, the attenuation of the resonant behavior of the structure. The particular mechanical properties of viscoelastic materials, dependent on aspects such as the excitation frequency, the cycles of oscillation experienced by the damping device or the ambient temperature, have encouraged the use of two different numerical models in a view to simulate their dynamic behavior in the retrofitting system. In a  first approach, and through the use of a simplified law of the dynamic behavior of the damping material, a dimensioning procedure of the retrofitting system has been proposed, based on an analytical approach. Finally, using a more realistic law of the viscoelastic behavior (based on the fractional derivatives), the effectiveness of the proposed damping system and also of the dimensioning methodology has been proven, through its application to the example of a bridge with excessive vertical oscillations. Numerical results show that the dynamic response of the structure can be significantly reduced with the proposed retrofitting system.