Stiffness contributions of ballast in the context of dynamic analysis of short span railway bridges

Railway bridges are subjected to intensive dynamic loading. In structural calculations, the resulting response is
estimated by dynamic calculations. As the basis of these calculations, the system’s natural frequencies are to be
numerically determined. Various studies have shown that a discrepancy between these measured natural fre-
quencies and the numerical determination often occurs for short-frame bridges. In common practice, the ballast
is assumed to be non-contributing to the global structural stiffness. In some cases, rheological models are
developed to consider possible ballast stiffnesses based on small-scale model tests. The resulting spring-damper
models are based on empirical values and are not directly related to the material behaviour of ballast.
This paper discusses the ballast stiffness at the material level using relationships known from soil dynamics.
The research shows that the material properties of the ballast can be determined in terms of stress and shear
strain dependence. An equivalent ballast stiffness can be determined and considered in dynamic calculations for
a known stress state and corresponding superstructure accelerations. ... mehrThe paper presents an approach for
determining vertical and horizontal ballast stiffness according to shear strains and superstructure accelerations.
This developed approach is validated by experimental tests using a small-scale bending beam and accompa-
nying numerical calculations.
Furthermore, a parameter study shows that the stiffness of the ballast does not contribute significantly to the
global bending stiffness. Thus, the first bending natural frequency is hardly influenced by the ballast track.