AMBIENT MODAL TESTING OF THE VESTVEJ BRIDGE USING RANDOM DECREMENT

摘要

This paper presents an ambient vibration study of the Vestvej bridge. The bridge is a typical Danish two-span concrete bridge which crosses a highway. The purpose of the study is to perform a pre-investigation of the dynamic behaviour to obtain information for the design of a demonstration project concerning application of vibration based inspection of bridges. The data analysis process of ambient vibration testing of bridges has traditionally been based on auto and cross spectral densities estimated using an FFT algorithm. In the preanalysis state the spectral densities are all averaged to obtain the averaged spectral densities (ASD). From the ASD the eigenfrequencies of the structure can be identified. This information can be used in the main analysis, where all modal parameters are extracted from the spectral densities. Due to long cabling and low response levels (small ambient loads) the response measurements might have a low signal to noise ratio. Thus, it might be difficult clearly to identify physical modes from the spectral densities. The Random Decrement (RD) technique is another method to perform the data analysis process in the time domain only. It is basically a very simple and easily implemented technique. In this paper it is demonstrated how the RD technique can be used in the preanalysis state in combination with the FFT algorithm, and how the technique can be used in a full analysis.