Decoupled Fourier Amplitude spectrum to identify the inelastic dynamic characteristics of structures under earthquake loading

Earthquake damage in structures is related to the co-seismic elongation of the structural period that is used as a global damage proxy. The study introduces a novel and straightforward technique to estimate the predominant elastic and inelastic period of buildings during earthquake shaking. The proposed technique introduces the Decoupled Fourier Amplitude Spectrum (DFAS), which aims to decouple the spectrum of the response from the seismic excitation and allows the identification of the co-seismic vibration period during damaging earthquakes. The DFAS technique is applied to numerically-calculated linear and nonlinear responses of earthquake-excited SDOF systems in order to highlight the usefulness of the DFAS-based period identification. Further corroboration of the DFAS technique is provided by calculating the predominant inelastic frequency using structural responses recorded from three existing buildings during their excitation by severe earthquake events that led to structural damages. The DFAS-identified predominant inelastic frequency of the systems was found in good agreement with results based on well-established time-frequency transformations. The proposed technique is expected to facilitate the damage assessment and the associated quantification of limit states on the basis of either monitored responses of existing structures or numerical predictions.