This paper presents a technique for identification of non-linear hysteretic
systems subjected to non-stationary loading. In the numerical simulations, a
Bouc-Wen model was chosen for its ability to represent the properties of a wide
class of real hysteretic systems. The parameters of the model are computed
instantaneously by approximating the internal restoring force surface through
an "ad hoc" polynomial basis. Instantaneous estimates result from time-varying
spectra of the response signals. A numerical application of interest to
earthquake engineering is finally reported
