Effectiveness of the Random Decrement Technique in Modal Identification of Structures using Ambient Vibration Response

Abstract

The random decrement Technique (RDT) is a time domain procedure, where the structural responses to operational loads are transformed into random decrement functions, which are proportional to the correlation functions of a system’s operational responses, which can be considered equivalent to free vibration responses. Ambient vibration test of a structure usually produces noisy response and the existing modal identification techniques often fail to produce accurate results in such case. Due to contamination of ambient vibration with white Gaussian noise, preprocessing may be required to clean up the vibration signal in order to detect the modal properties accurately. In this paper, the Random Decrement Technique is used as a preprocessing technique to remove the redundancy form the data by use of cross correlation in Ambient Vibration test. The method has been tested for a steel cantilever beam having a length of 1000 mm rectangular cross section with 10 mm height and 65 mm width. The beam was instrumented with 5 wireless sensors placed at every 200 mm to measure the acceleration response. Both ambient and forced vibration tests were conducted for the beam. As expected, the modal identification method failed when the ambient vibration response was used without preprocessing. When preprocessed with RDT, the ambient vibration response produced modal properties similar to that obtained using forced vibration response.