Keywords: bridge dynamics, natural frequencies, ballast stiffness, material approach.

Railway bridges are subjected to intensive dynamic stresses due to train crossings. The resulting system response (accelerations) is estimated in advance through dynamic calculations. The basis of the calculations is the knowledge of the natural frequencies. Various studies have shown that a discrepancy between the measured and the numerical determination of these natural frequencies often occurs for short frame bridges. In common practice, the ballast is assumed to be non-supporting regarding the global structural stiffness. In some cases, rheological models of vertical springs and dashpots are used to consider the load distribution in vertical direction.

This publication discusses ballast stiffness on the material level. This leads to a stiffness contribution of the ballast to the horizontal stiffness (fraction of bending stiffness) and thus to a stiffening of the structure. It is shown that ballast can be defined in terms of stress and shear strain dependence. If ballast stresses and expected superstructure accelerations are known, an equivalent ballast stiffness can be determined and taken into account in a dynamic calculation using a linear approach. The material approach is verified by a scaled component test and a parallel numerical reference calculation.

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