On development of base shear versus roof drift curves using earthquake-response data.

摘要

The nonlinear behavior of a building during a strong earthquake limits the use of classical signal processing tools to analyze its response data. There are methods based on Fourier transform, such as the Short-Time Fourier transform and the Spectrogram, which are tailored to process nonstationary data but with the assumption that the response could be represented using piece-wise linear behavior. An approach based on the Empirical Mode Decomposition (EMD) method that takes into account the nonlinear behavior of buildings subjected to strong earthquake motion and the nonstationary characteristics of its response has been developed.;The performance of EMD to decompose combinations of sinusoidal signals and response of single degree-of-freedom (SDOF) systems to base excitation is presented. A correction to the instantaneous frequency of sinusoidal signals with varying frequencies is also discussed.;In the basic EMD approach, acceleration data recorded in a building are processed using a sifting procedure to obtain a set of Intrinsic Mode Functions (IMFs). In theory, each IMF corresponds to a distinct process involved in the total response, i.e., each IMF is assumed to be associated with a single component of the total response out of many involved. A procedure to identify the meaningful IMFs associated with the fundamental structural response, the equivalent of fundamental mode response in linear systems, is developed. The fundamental response from the IMF is used to generate an estimate of the dominant global lateral-load versus drift curve, also known as capacity curve, for the structure. The accuracy of the estimate depends on the spatial distribution of the available response data and the information about the structure.;To demonstrate the method proposed in this study, the capacity curve of a single degree-of-freedom (SDOF) system subjected to earthquake base excitation was estimated. The Bouc-Wen model was used to describe the restoring force on the SDOF system. The estimated capacity curve was compared with the force-displacement curve generated from the cycles of the response of the SDOF system. The curves are in good agreement with each other. The method was applied to estimate the actual capacity curves of the 7-story reinforced concrete hotel building in Van Nuys, California using recorded acceleration data at the roof of the building during the 1971 San Fernando and 1994 Northridge earthquakes. Results are in accordance with the findings of other researchers.;In general, the EMD based method which takes into account the nonlinear and nonstationary behavior of buildings provides an efficient way of estimating capacity curves based on actual recorded data during earthquakes. The results allow insightful review of load-displacement curves generated through the pushover analysis used in practice.