Identification of changes in the stiffness and damping matrices of linear structures through ambient vibrations

The practical difficulties presented by forced vibration testing of large structures, such as tall buildings, dams or bridges, led to a vast interest in structural monitoring through ambient vibrations, which usually allows the proper identification of modal properties, natural frequencies, damping and modes of vibration. Changes in these modal properties constitute an indication of structural damage, which may then be assessed on the basis of experimental evidence. The authors describe an approach to determine the so-called damage damping and stiffness matrices, which are essential to identify the location and intensity of damage. No restrictions are introduced on the damping matrix of the system. The approach requires ambient vibration data records of all relevant co-ordinates used in the structural model, which are processed employing the SSI-Cov method in conjunction with a modal analysis procedure. The identification method is illustrated with examples using experimental measurements on a reinforced concrete beam and a reduced scale mufti-storey plane frame structure, also laboratory tested, as well as the theoretical simulated vibration response of a mufti-storey concrete frame subjected to seismic excitation. Additional difficulties presented by field monitoring of large structures are briefly examined at the end of the paper. Copyright (C) 2007 John Wiley & Sons, Ltd.