Reliability analysis of base sliding of concrete gravity dams subjected to earthquakes

Concrete gravity dams are typically constructed in blocks separated by vertical contraction joints. The design of straight concrete gravity dams is traditionally performed by assuming each block to be independent, except for gravity dams in valleys with relatively small width to height ratios. Un...

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Main Author: Horyna, Tomáš
Format: Others
Language:English
Published: 2009
Online Access:http://hdl.handle.net/2429/9991
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spelling ndltd-UBC-oai-circle.library.ubc.ca-2429-99912018-01-05T17:35:02Z Reliability analysis of base sliding of concrete gravity dams subjected to earthquakes Horyna, Tomáš Concrete gravity dams are typically constructed in blocks separated by vertical contraction joints. The design of straight concrete gravity dams is traditionally performed by assuming each block to be independent, except for gravity dams in valleys with relatively small width to height ratios. Understanding the 2-D behaviour of individual monoliths is thus considered relevant and 2-D models are usually employed in safety evaluations of existing dams. During a strong seismic event, low to medium height concrete gravity dams tend to crack at the base as opposed to tall dams, which attract high stresses and cracking at the level of a slope change on the downstream side of a dam. The state-of-the-practice in the seismic evaluation of concrete gravity dams requires that the failure mode of the dam monolith sliding at its base be considered. This study focused on the post-crack dynamic response of existing concrete gravity dams in order to investigate their safety against sliding considering non-linear effects in the damfoundation interface. Sliding response of a single monolith of a low to medium height concrete gravity dam at the failure state was studied and, therefore, the monolith separated or unbonded from its foundation was considered. The work included experimental, analytical and reliability studies. During the experimental study, a model of an unbonded concrete gravity dam monolith was developed and tested using a shake table. The model, preloaded by a simulated hydrostatic force, was subjected to a selected variety of base excitations. Other effects, such as hydrodynamic and uplift pressures were not considered in the experiments. A strong influence of amplitude and frequency of the base motions on the sliding response of the model was observed during the tests. Simple and more detailed numerical models to simulate the experiments were developed during the analytical study. It was observed that a simple rigid model could simulate acceptably the tests only in a limited range of excitation frequencies. A finite element (FE) model simulated the experiments satisfactorily over a wider range of dominant frequencies of the base accelerations. The numerical models were used to simulate the seismic response of a 45 m high monolith of a concrete gravity dam subjected to three different earthquake excitations for varying reservoir's water level. The agreement between the results using the simple rigid and the FE models was found acceptable. The results of the numerical simulations were used in a reliability analysis to calculate probabilities of failure of the 45 m high monolith. Probability of failure was defined here as an annual chance of exceeding an allowable amount of the monolith's base sliding during an earthquake. The peak ground acceleration (PGA), the characteristics of the time history, and the reservoir's water level were considered as random parameters during this study. Using the FE model, the annual probabilities of failure ranged from 1. 1E-8 for the mean PGA of 0.2g and 20 cm of allowable sliding to 1.3E-3 for the mean PGA of 0.6g and 1 cm of allowable sliding. The probabilities of failure using the simple rigid model were found close to those using the FE model. It was concluded that the computationally less demanding simple rigid model may be adequately used in reliability calculations of low to medium height concrete gravity dam safety against base sliding. Applied Science, Faculty of Civil Engineering, Department of Graduate 2009-07-02T22:25:39Z 2009-07-02T22:25:39Z 1999 1999-11 Text Thesis/Dissertation http://hdl.handle.net/2429/9991 eng For non-commercial purposes only, such as research, private study and education. Additional conditions apply, see Terms of Use https://open.library.ubc.ca/terms_of_use. 21018793 bytes application/pdf
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description Concrete gravity dams are typically constructed in blocks separated by vertical contraction joints. The design of straight concrete gravity dams is traditionally performed by assuming each block to be independent, except for gravity dams in valleys with relatively small width to height ratios. Understanding the 2-D behaviour of individual monoliths is thus considered relevant and 2-D models are usually employed in safety evaluations of existing dams. During a strong seismic event, low to medium height concrete gravity dams tend to crack at the base as opposed to tall dams, which attract high stresses and cracking at the level of a slope change on the downstream side of a dam. The state-of-the-practice in the seismic evaluation of concrete gravity dams requires that the failure mode of the dam monolith sliding at its base be considered. This study focused on the post-crack dynamic response of existing concrete gravity dams in order to investigate their safety against sliding considering non-linear effects in the damfoundation interface. Sliding response of a single monolith of a low to medium height concrete gravity dam at the failure state was studied and, therefore, the monolith separated or unbonded from its foundation was considered. The work included experimental, analytical and reliability studies. During the experimental study, a model of an unbonded concrete gravity dam monolith was developed and tested using a shake table. The model, preloaded by a simulated hydrostatic force, was subjected to a selected variety of base excitations. Other effects, such as hydrodynamic and uplift pressures were not considered in the experiments. A strong influence of amplitude and frequency of the base motions on the sliding response of the model was observed during the tests. Simple and more detailed numerical models to simulate the experiments were developed during the analytical study. It was observed that a simple rigid model could simulate acceptably the tests only in a limited range of excitation frequencies. A finite element (FE) model simulated the experiments satisfactorily over a wider range of dominant frequencies of the base accelerations. The numerical models were used to simulate the seismic response of a 45 m high monolith of a concrete gravity dam subjected to three different earthquake excitations for varying reservoir's water level. The agreement between the results using the simple rigid and the FE models was found acceptable. The results of the numerical simulations were used in a reliability analysis to calculate probabilities of failure of the 45 m high monolith. Probability of failure was defined here as an annual chance of exceeding an allowable amount of the monolith's base sliding during an earthquake. The peak ground acceleration (PGA), the characteristics of the time history, and the reservoir's water level were considered as random parameters during this study. Using the FE model, the annual probabilities of failure ranged from 1. 1E-8 for the mean PGA of 0.2g and 20 cm of allowable sliding to 1.3E-3 for the mean PGA of 0.6g and 1 cm of allowable sliding. The probabilities of failure using the simple rigid model were found close to those using the FE model. It was concluded that the computationally less demanding simple rigid model may be adequately used in reliability calculations of low to medium height concrete gravity dam safety against base sliding. === Applied Science, Faculty of === Civil Engineering, Department of === Graduate
author Horyna, Tomáš
spellingShingle Horyna, Tomáš
Reliability analysis of base sliding of concrete gravity dams subjected to earthquakes
author_facet Horyna, Tomáš
author_sort Horyna, Tomáš
title Reliability analysis of base sliding of concrete gravity dams subjected to earthquakes
title_short Reliability analysis of base sliding of concrete gravity dams subjected to earthquakes
title_full Reliability analysis of base sliding of concrete gravity dams subjected to earthquakes
title_fullStr Reliability analysis of base sliding of concrete gravity dams subjected to earthquakes
title_full_unstemmed Reliability analysis of base sliding of concrete gravity dams subjected to earthquakes
title_sort reliability analysis of base sliding of concrete gravity dams subjected to earthquakes
publishDate 2009
url http://hdl.handle.net/2429/9991
work_keys_str_mv AT horynatomas reliabilityanalysisofbaseslidingofconcretegravitydamssubjectedtoearthquakes
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