The influence of ballast on the vibrations of railway bridges

Sammanfattning: This thesis investigates two aspects concerning the influence of the ballast on vibrations of railway bridges. The first one is the additional stiffness due to the ballast. For that, a special finite beam element with a non-linear longitudinal stiffness associated to the slip at the interface between the ballast and the bridge is implemented and used to calculate vertical accelerations during and after the passage of a train. The second one is the load distribution of the axle loads through the ballast. To investigate this, a parametric study is carried out on a two dimensional model with plane elements. Based on this load distribution, different load shapes are created, which are used in dynamic analyses in Matlab on three different bridge models. The conclusions are that a linear and a bi-linear longitudinal stiffness at the interface give similar maximum accelerations. Moreover, the critical train velocities are increased with the additional stiffness of the ballast. The study also shows that distributed loads, in general, lowers the maximum accelerations, especially on short bridges.