Operational Modal Analysis of Large Bridges
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As a part of the new coastal highway E39 which is being planned and built along the westcoast of Norway by the Norwegian Public Roads Administration, state of the art methodsof structural dynamics needs to be developed and used to cross the deep and wide fjords.Operational modal analysis aims to find the modal properties; natural frequencies, dampingratios and mode shapes of a structure while it is under operating conditions using its vibration data. Several methods of operational modal analysis are developed and they can broadly be divided into time-domain and frequency-domain. Theory of operational modal analysis and the methods chosen are presented in this thesis. The main objective of this thesis was to develop MATLAB functions which implements severalof these methods. The methods are performed on two case studies. A shear frame with lowand high damping where the exact modal parameters are known, this is done to check thatthe methods work properly and to see how damping influences the accuracy of the methods.Then the methods are used on the Hardanger Bridge, a bridge in operating conditions. Themodal properties are extracted for each of the methods and then compared on account ofaccuracy. The methods are also compared regarding ease of use and computational efficiency. Most of the methods found the natural frequencies for the low damping shear frame, but forhigh damping some of the methods proved to be inadequate. For damping ratio and modeshapes only a few of the methods managed to give satisfactory results. For the HardangerBridge most of the methods managed to find the natural frequencies, but only a few managedto estimate the damping ratios and mode shapes. Overall, Cov-SSI proved to be the bestmethod.