Condition monitoring of building structures during earthquakes using a wavelet-based analysis technique.

The vibrational characteristics of building frames can change during an earthquake due to yielding or damage to structural members or connections. In concentrically braced steel frames, brace members yield in tension and buckle in compression. This results in temporal variations in the stiffness of the structure and frequency content of its response. This paper presents condition monitoring of the seismic response of a set of concentrically braced frame systems. In particular, a time-frequency tool based on wavelet analysis is employed to investigate the dynamic responses of concentrically-braced frames displayed in shake table tests. Damage in the form of brace yielding and buckling is detected by comparing table and frame response accelerations using wavelet analysis. This comparison is carried out by examining frequency-banded wavelet coefficients of the ground motion and elastic inelastic frame response, and by evaluating the corresponding energy contributions of the different frequency bands. Furthermore, a wavelet-based equivalent linearisation technique is employed to determine the temporal equivalent natural frequencies of the frames throughout the response. It is observed that the frame stiffness reduces significantly in the later stages of the tests due to the extensive yielding and elongation experienced by the brace members.