Today a dynamic analysis is often performed with the use of advanced finite element software. An advanced model normally produces precise results. Analyzing these large models of bridges is however time consuming and a detailed description of the geometry, supports and material properties must be known. In an early stage of a project a reduced model could significantly reduce the time needed to perform the final dynamic analysis. The aim of this thesis was to investigate the dynamic response of railway bridges for high-speed trains and to develop a reduced model that can be used early in a project, to provide a quick dynamic analysis of bridges with several spans. The reduced model was based on the fact that the fundamental dynamic behavior of certain type of bridges may be described by the dynamic behavior of 2-dimensional Bernoulli beam elements. The bridge model was developed as a MATLAB function and consists of a selectable number of Bernoulli beam elements. Three different approaches of modeling the train load were compared. The first method of modelling the train was made as moving point-loads, the second as a distributed load and the third as a separate mass and spring model. In order to make the developed MATLAB function easy to use, to perform quick analysis and with good accuracy, the numbers of input parameters were minimized. This was made by setting some of the properties of the reduced model as fixed standard values. The values were based on result of extensive modelling studies. Finally, two bridges with realistic dimensions and materials were analyzed using both the reduced model and a more advanced model. The reduced model was proved to provide basically the same results as the more advanced model. The time needed to model the bridge and to perform the analysis was approximately 10 minutes when using a standard home pc, which was a significant reduction of the time needed to make the advanced model and to perform the analysis
