In this paper a direct system identification method is presented for identification of general linear system physical parameters (structural mass, damping and stiffness matrices) using direct inverse solution of the equations of motion in the frequency domain. When the input and output (responses) measured data are noise free, the proposed method can identify physical parameters exactly. When the input and outputs are mixed with measurement noise, an optimization process has been added to the method in order to improve the accuracy of the identified physical parameters of the system. The validity and efficiency of the proposed method is tested on a multistory-non-proportional-damping-structure subjected to sweep harmonic forces at the first story. It is shown that the proposed method can estimate the system parameters with a high level of accuracy. In the case of Gaussian white noise root mean square (RMS) is equal to 3% of the RMS of the structure response, stiffness matrix will be identified with around 2% error. Therefore identified matrices could be used for the damage detection process. The sensitivity of the proposed method to the noise level and damping of structure has also been investigated.
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