Abstract Summary
A realistic and economical computational assessment of the dynamic behaviour of railway bridges requires, first and foremost, input parameters that correspond to reality. In this context, the computationally applied damping properties of the structure have a decisive influence on the results in the prediction of resonance effects. Concerning the damping factors used in dynamic calculations, the EN 1991-2 standard prescribes damping factors depending on the type of structure and the span. However, these factors can be regarded as very conservative values. As a result, in-situ measurements on the structure are often necessary to classify a bridge categorized as critical in prior dynamic calculations as non-critical. Regarding in-situ tests, a measurement-based determination of the damping factor is inevitably accompanied by a scattering of the generated results. This scattering is due to the measurement method used. Also, it results from the individual scope of action of the person evaluating the test and this person's interpretation of the measurement data. With this background, this contribution presents methods and analysis tools for determining the damping factor, intending to reduce the scatter of the results, and limiting the scope of action of the person evaluating the test. Methods and analysis tools are discussed for methods in the time domain and the frequency domain. The standard method for damping determination in the frequency domain is the bandwidth method, whereby the damping factor is determined based on an amplitude-frequency response generated from measurement data. Concerning the time domain, the damping factor is determined based on the decay process after dynamic excitation via the logarithmic decrement. With regard to the dynamic excitation before decay processes, excitation mechanisms with high energy input are to be selected so that the measured decay process has several clearly identifiable oscillation cycles. For both measurement methods (time and frequency domain), the derivation of determination equations for the damping factor is performed assuming a single degree of freedom (SDOF) vibrating system with linear system properties (constant mass, stiffness, and damping). This contribution presents more advanced procedures and methods as extensions of the standard methods. In both cases, the basic idea is to adjust the amplitude-frequency response generated by measurement (frequency domain) or the recorded decay process (time domain) using the least square method by a mathematically defined curve in such a way that the greatest possible agreement between measurement data and approximation is achieved. Based on several in-situ tests on existing bridges, the procedure for determining the damping factor is explained, and the methods are compared in the time and frequency domain. It is shown that a clearly defined evaluation algorithm can significantly reduce the scattering of results. Furthermore, it is shown that the amount of dissipated energy substantially affects the generated damping factors. Higher energy dissipation results in higher damping factors, which means that using excitation methods with high energy input (e.g. excitation by train crossing) leads to higher and, thus, favourable damping factors concerning dynamic calculations.
