Novel evaluation methods for data-based determination of damping factors in the frequency and time domains with application on railway bridges

Abstract

A realistic and economical dynamic assessment of railway bridges requires input parameters that correspond to reality. In this context, the applied damping properties of the structure have a decisive influence on the results in the prediction of resonance effects and further in the assessment of the compatibility between rolling stock and railway bridges. The standard prescribes damping factors depending on the type of structure and the span to be used in dynamic calculations. However, these factors can be regarded as very conservative values which do not represent reality. Thus, in situ measurements on the structure are often necessary to classify a bridge categorised 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 due to the measurement method used and as a result of the individual scope of action of the test-evaluating person and this person’s interpretation of the measurement data. This paper presents novel evaluation methods and analysis tools for determining the damping factor based on measurements in the frequency and time domains, intending to reduce the scatter of the results and limit the scope of action of the person evaluating the test. The main aim is to provide simple and easy-to-use evaluation algorithms for practical applications without additional data transformations and to define clear principles of action for the data-based evaluation of realistic and high damping factors. Based on in situ tests on 15 existing railway bridges, the data-based procedure for determining the damping factor is explained, and the methods are compared in the time and frequency domains. It is shown that a clearly defined evaluation algorithm can significantly reduce the scattering of results. Furthermore, it is demonstrated that forced vibration excitation and evaluation in the frequency domain provide the best results in reliable, reproducible, and high damping factors.