Comparison of ballasted and ballastless bridges for high speed trains

Abstract: The purpose of the project is to investigate the difference in performance between ballasted and ballastless railway bridges dedicated to high speed trains by taking into account both static and dynamic requirements. The main questions are: a) whether choosing a ballastless bridge results in a more slender section due to the lower self-weight b) if the design of bridges for high speed trains is governed by the static or by the dynamic requirements. The method followed was to first make a complete static design of a ballasted and a ballastless bridge, and then subject them to a 2D dynamic analyses in order to see if the cross section dimensions must be changed. Some of the bridges required a more thorough dynamic analyses, and for these, a 3D model was developed. The analysed bridge is a simply supported beam with a T section carrying one track. Some variations were also considered, namely a simply supported bridge with a double T section carrying two tracks, as well as a single track bridge in two spans. It was found that all of the analysed bridges are somewhat more slender for the ballastless alternative, and require a 10 -30% less reinforcement. Simply supported bridges carrying one track are governed by the dynamic requirements; the bridges in two spans are for shorter spans governed by the statics and for longer spans by the dynamics. Bridges in double T fulfilled all the requirements according to the 2D analyses, but were found to be greatly affected by the 3 dimensional effects and failed to satisfy the criteria when these were taken into account. Finally, the optimal design according to these analyses is a ballastless bridge in a simple T section. If the bridge constructed should carry two tracks, then it should be constructed as two T beams that are not connected to one another in order to avoid the unfavourable 3D effects.