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UBC Theses and Dissertations

Studies on seismic behavior of concrete gravity dams Yoneda, William James

Abstract

This Master's thesis presents three independent studies related to the safety of concrete gravity dams. 1. Dam-Gate Hydrodynamic Interaction Study 2. Damping Study 3. Shear Key Study The three studies are self-contained, presented independently in Chapters 2 - 4 of this thesis. Work on this thesis started in winter 2002. The analytical work was completed in summer 2004. This thesis was written as part of the Professional Partnership Program with the University of British Columbia and British Columbia Hydro. A modal analysis of a 2DOF simplified model representing a portion of an existing gravity dam structure was performed. The main purposes of this study were to evaluate the effect of dam-gate interaction on the hydrodynamic loads and to evaluate the effect of varying the natural frequency of the gate. It was found that the modal interaction between the dam and gate structures caused variation in the hydrodynamic loads acting on the gate and upstream face of the dam. One of the recommendations for retrofitting the gates is to decrease the natural frequency of the gate to below the natural frequency of the dam. This essentially increases the flexibility of the gate system and reduces the amount of hydrodynamic loads generated on the gate and upstream surface of the dam. An analytical study to test the Half-Power Bandwidth method of estimating damping in concrete gravity dams was performed successfully. The objectives of this study were to test this method of evaluating structural damping and to recommend a reasonable estimate of structural damping in concrete gravity dams. The damping values computed by this method were found to be much lower than expected (less than 2%) and therefore, unrepresentative of the damping that is likely believed to be present in the concrete gravity dams analyzed in this study (approximately 5 - 10%). An exploratory study involving the finite-element modeling of a shear key system was performed. The stress patterns that developed in the finite element model under applied horizontal load were similar to those exhibited in the early stages of the cracking sequence presented by Bakhoum (1991). In order to model the later stages of the cracking sequence, it would be necessary to implement nonlinear material models that are capable of modeling loading beyond the linear range to failure.

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