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UBC Theses and Dissertations

Ambient vibration assessment of Ruskin Dam dynamic properties Kemp, Bradley Gary


Concrete gravity dams are significant civil engineering structures whose failure can produce catastrophic results. Field appraisal of their dynamic properties would be useful to validate numerical models used for structural analysis to determine, for instance, seismic resistance. The suitability of ambient vibration testing and analysis to provide meaningful dynamic properties of a concrete gravity dam has been studied. Tests were conducted at B.C. Hydro's Ruskin Dam, located near Mission, B.C. Tests were completed at two reservoir levels to permit calibration of a numerical model. Two distinct methods of frequency domain based analysis were used to determine dynamic properties. The first method used proven ambient vibration techniques, based on a "relative" single inputsingle output system, with no quantification of excitation. Relative transfer function relationships were constructed between a reference location on the dam and all other points on the dam at which measurements were taken. The selection of potential natural frequencies was completed by consideration of the power in the signals recorded. Final selection of natural frequencies and associated mode shapes was based on the resulting gain, phase and coherence values of the relative transfer functions. In the second method, the field measurements were considered to represent a single inputsingle output system with output noise, as the bedrock signal was shown to represent the majority of input. For this approach, natural frequencies were indicated as peaks in crossspectral density functions with high coherence and corresponding transfer functions having phase factors equal to 90 degrees. Two natural frequencies and corresponding mode shapes were identified. Confidence of prediction was found to be highest for the first of these. Although this was concluded to be a meaningful result, there is clearly a need for developing additional ambient vibration theory which can be implemented to identify dynamic properties. To demonstrate the usefulness of the dynamic properties obtained through the ambient vibration testing and analysis, a numerical model of the dam utilizing the finite element method was subjected to modal analysis with which to complete calibration/parametric studies. Calibration was found successful only for the first natural frequency, probably because the numerical model failed to include the effects of water compressibility, which were shown through rudimentary calculations to be significant. Calibration to the first natural frequency is, however, considered meaningful as concrete gravity dam response is attributed mainly to the fundamental mode.

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