The longitudinal track-bridge interaction of ballasted track in railway bridges: Experimental determination of dynamic stiffness and damping characteristics

Considering the longitudinal track-bridge interaction (TBI) of ballasted track significantly influences the corre-sponding calculation results in dynamic analyses of railway bridges. However, there is still a need for realistic and test-based characteristic parameters (dynamic stiffness and damping values). This paper focuses on an experimental determination of the dynamic properties of the longitudinal track-structure interaction. Based on extensive experiments using a large-scale test facility, the ballasted track's dynamic stiffness and damping values are determined for practical applications. The test spectrum covers a wide frequency and amplitude range, whereby influences due to vertical load of the track (loaded/unloaded) and climatic conditions (summer/winter) are also examined. The tests show that the dynamic properties change significantly with the vertical load and climatic influence. Different damping mechanisms occur depending on the excitation frequency regarding the damping properties. Both friction-based and viscous damping mechanisms appear in the lower frequency range, while only viscous damping occurs in the upper frequency range. The resulting damping characteristics act frequency-dependent and also slightly amplitude-dependent. The stiffness characteristics of the longitudinal track-structure interaction show for each configuration (loaded/unloaded, summer/winter) a non-linear behaviour deviating from the normative specifications, which acts primarily displacement-dependent and almost frequency-independent. Concerning the frozen ballast bed, EN 1991-2 equates the state of the frozen ballast bed with a fixed (ballastless) track. The tests show that this assumption ('frozen = ballastless') does not correspond to reality, whereby the temperature level fundamentally influences the longitudinal stiffness in this respect. While at temperatures just below zero (-2 degrees C), no significant differences to the non-frozen ballast bed are discernible; the dynamic properties change considerably at temperatures far below zero (-10 degrees C). Furthermore, the tests also allow observing the ballast bed liquefaction phenomenon, which occurs at resonance when the excitation frequency matches the natural frequency.