A new method called direct eigensystem realization algorithm (DERA) is developed for structural modal parameter identification. A direct procedure is presented for obtaining the minimal realization of a linear system from observations of the input and output in time domain, then modal parameters are obtained from the eigenvalues and eigenvectors of the system transition matrix. Because of the use of direct input-output observations some weaknesses of a few of schemes of modal identification which use impulse response function as initial data have been avoided. An improved algorithm of the basic procedure is also presented for noisy observations. Effectiveness of the algorithm is proved by good numerical results and the high accuracy is remained for close-spaced frequency modes.
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