Keywords: eigen frequency, damping, dynamic train load, resonance, bridges, filler beams, short and medium span, fatigue.

A realistic computation of the reaction of railway bridges to train crossings can be challenging. Depending on span and construction type there can be huge differences between the predicted and measured response. The behaviour of filler beam bridges subjected to dynamic train loads is described in this paper. Differences between calculated and measured fundamental frequencies, damping ratios, deflections and accelerations are shown, giving special attention to resonance phenomena.

Differences between simplified calculation methods and more explicit simulations of train crossings with numerical models according to current standards are pointed out, as well as the restricted applicability of the simplified approaches.

Many researchers have already identified that not taking the influence of structural boundary conditions into account is one of many reasons for the differences between calculated and measured values. A monitoring system installed on the German filler beam bridge Erfttalstraße is used to investigate the dynamic behaviour of the bridge. Crossings of different train types with different velocities and the resulting accelerations und deformations are compared to the respective calculated ones. The characteristics of the individual boundary elements are described and possibilities to include their influence into dynamic calculations are discussed.

Furthermore fatigue phenomena are investigated, though for this particular filler beam bridge a fatigue check is not required as there are no girder joints. As expected the bridge shows an excellent resistance, not only in theory, when subjected to unfavourable fatigue load models but also assessed by real monitoring results.

Strain time-histories, measured during two representative days have been used to calculate the relevant steel tensile stresses and concrete compressive stresses. The results from these two days have been extrapolated to an expected service life of one hundred years.

All steel stresses are below the threshold value for the fatigue strength and therefore do not have to be considered. The cumulative concrete damage is very low, again proving that a fatigue check of filler beam bridges is justifiably not required.

The influence of the boundary elements is currently being investigated in more detail in the German research project DYNABRIDGE, funded by the Research Association Steel Application FOSTA. The project aims at the separation of permanent and only temporarily occurring effects. This will allow for the consideration of the permanent effects within the dimensioning process. This means that for example seasonal effects on the stiffness such as ballast layer freezing have to be ruled out as well as effects changing the stiffness during the day.

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