Investigation of the seismic response of braced frames using wavelet analysis

A time-frequency tool based on wavelet analysis is employed to investigate the dynamic responses of concentrically braced frames that were subjected to earthquake excitations in a series of shake table tests. The frames investigated each possess a pair of diagonal braces and were subjected to a record of the El Centro ground motion, scaled to cause elastic and inelastic responses. Measurements of the ground (table) and response (test mass) accelerations were obtained using accelerometers. Wavelet coefficients quantifying these motions in each of eight frequency bands are determined, and the degree of correlation between these coefficients is evaluated for the elastic and inelastic response cases. These data are attributed to physical observations of the response of the test frames, and it is shown that the application of wavelet analysis allows yielding and buckling events to be detected from a limited set of measurements, namely ground and building floor accelerations. A wavelet-based equivalent linearization technique is used to determine the temporal variations in frame stiffness that occur due to brace yielding and buckling. It is shown that throughout most of the duration of its response, the frame displays a stiffness that is much lower than expected.