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Determination of structural dynamic properties of three buildings in Vancouver, B.C., from ambient vibration surveys To, Ngok-Ming

Abstract

The dynamic properties, namely the natural frequencies, mode shapes and damping values of three downtown Vancouver buildings have been obtained by means of ambient vibration surveys. The buildings surveyed include an asymmetrical hi-rise - the Harbour Centre, and two frame structures -Toronto Dominion Bank and IBM Towers. Three hi-rise buildings of identical design - the Gage Residences - were also surveyed to examine their respective dynamic properties. Two independent measurement systems were employed so that important data could be duplicated and statistically stable spectra obtained to determine damping. Two damping estimation methods were used - the autocorrelation and partial moment methods. A man excited test was also performed on the T/D Tower. The measured natural frequencies of the Harbour Center were much higher than published theoretical values (64% higher in one case). This suggests that the analytical model used was too flexible and may not have accounted for the interactions of non-structural elements. In the case of the T/D and IBM Towers the frequencies were found to agree closely with the results obtained from theoretical analyses. The natural frequencies of the asymmetrical building - the Harbour Centre - were expected to be very close together (.01 Hz apart, in some cases, as indicated by an existing analytical study). A special smoothing process was employed to search for the close frequencies. Although this search did not reveal frequencies spaced at 0.01 Hz, it is still possible that frequencies exist at closer than .01 Hz or that they have very unequal Fourier amplitudes at a .01 Hz spread. In either case, the smoothing process was not able to separate such frequencies. The results obtained for the Harbour Center E-W direction suggest that the frequencies of the fundamental and second modes in this direction may be equal, or nearly equal, and that the measured 'fundamental' mode shape is not uniquely defined. The partial moment method for damping estimates seems to provide stable results for a given length of record, and seems to be little affected by the smoothing process. The low damping values obtained by this method for the Harbour Centre, T/D and IBM Towers are typical of damping determinations by ambient vibration tests. Stable estimates of power spectral density or Fourier spectra were achieved easily in this experimental programme. The autocorrelation method therefore could be confidently used to secure good damping estimates. The damping values obtained by this method are comparable to the results found from the partial moment technique. The man-excited test results for the T/D Tower also provided damping estimates which were within the range of values determined by the partial moment and autocorrelation methods. The natural frequencies and damping values (using only the partial moment method) obtained for the three Gage Towers on the UBC Campus were almost the same; structurally the Towers therefore may be regarded as being identical.

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