IDENTIFICATION OF MODAL QUANTITIES FROM 2 EARTHQUAKE RESPONSES

The work deals with the identification of modal parameters of a struct ure from earthquake records when the input ground motion is unknown. T his may occur, for example, owing to instrumental malfunctions. The pr ocedure is based on the assumption that at least two responses are ava ilable and consists of two main steps. In the first one, modal frequen cies are estimated by searching relative minima of a function that inv olves the ratio of the Fourier amplitudes of the two records, while th e second phase is devoted to the identification of other modal quantit ies (i.e. effective participation factors and modal dampings). Once th e identification process has been completed, an estimate of the unknow n base input may be performed by means of the IFFT algorithm. The prop osed approach has been checked against both finite element simulations of simple structures and field measurements on real buildings.