Apparent Periods of a Building. II: Time-Frequency Analysis

In this, the second of a two-part paper, the analysis of the apparent frequency of a seven-story reinforced-concrete hotel building in Van Nuys, Calif., is extended to consider its time-dependent changes, both short and long term. The instantaneous apparent frequency is measured by two methods: windowed Fourier analysis and zero-crossings analysis. The results show that it changes from earthquake to earthquake and during a particular earthquake. The results also suggest ''self healing'' believed to result from settlement of the soil with time and dynamic compaction from aftershock shaking. Implications of such high variability of the system frequency on structural health monitoring, control of response, as well as on the design codes are discussed. Nonlinear response of the foundation soil acts as a sink of the incident seismic wave energy. It is suggested that it could be exploited in future designs to serve as a powerful and inexpensive energy-dissipation mechanism. reinforced-concrete hotel in Van Nuys, Calif. (VN7SH) by Fourier analysis. We also described an approximate model in which the soil-structure interaction phenomena are viewed in the simplest possible form, via a rigid foundation model. Be- cause the Fourier transform of the response is evaluated from the entire time history of response, the results in Part I gave an insight only into the overall changes of the apparent system frequency from one earthquake to another. The trend that emerged from the results of the Fourier analysis is a reduction of the apparent system frequency with increasing levels of shaking at the site. In this part, we study the time and ampli- tude-dependent changes of by time-frequency analysis. The ˜ f description of the building, recorded strong motion data, earth- quake damage, and full-scale ambient vibration tests con- ducted following the Northridge earthquake of 1994 were pre- sented in Part I and will not be repeated here.

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