Approach for the redefinition of the damping factor of railway bridges

In dynamic assessments of railway bridges, the damping properties of the bridge – usually summarised in Lehr's damping factor – have a decisive influence on the generated calculation results. However, the conservative definition of the EN 1991-2 regarding the damping factor to be applied in calculations often yields uneconomical dynamic assessments of railway bridges. The specification of EN 1991-2 usually reveals significant discrepancies between measurements and standard specifications when in-situ measurements are carried out on structures, and the actual damping properties are determined based on measurement data. Building on this background, the research activities at the Institute of Structural Engineering/Research Unit Steel Structures at TU Wien focus on developing novel approaches for determining the damping factor of railway bridges. The overarching aim is to formulate a realistic approach as an alternative to the specifications in EN 1991-2, thereby avoiding overly conservative dynamic assessments of railway bridges. This contribution presents the latest developments regarding a possible redefinition of the damping factor for railway bridges, specifically steel bridges. Here, a measurement campaign conducted on 15 railway bridges within the existing Austrian network serves as the basis for the analysis. The results highlight two essential relations: By including several system parameters ( in contrast to the exclusively span-dependent standard specifications), clear dependencies of the damping properties can be identified. Furthermore, when using damping factors identified from measurement data as the basis for data, the measurement and evaluation method used to determine the damping factor is of key importance. For example, measurements taken as a result of train crossing are to be regarded as critical, whereas harmonic force-excited vibration excitations provide reliable measurement results.