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- Numerical models for seismic analysis of arch damsPublication . Alegre, André; Oliveira, Sérgio; MENDES, PAULO; Proença, Jorge; Carvalho, Ezequiel; Matsinhe, BrunoThis paper presents a study on the seismic behaviour of large arch dams under strong earthquakes. The numerical simulations are carried out using DamDySSA, a 3D finite element program developed for dynamic analysis of concrete dams, which includes calculation modules for linear seismic analysis and for non-linear seismic analysis, considering the effects of joint movements and tensile and compressive damage in concrete. The case studies are the 132 m-high Cabril dam, in Portugal, and the 170 m high Cahora Bassa dam, in Mozambique. The seismic response results computed using linear and non-linear models are compared, in order to investigate the influence of the joint movements on the structural response, and to analyse the resulting concrete damage under a strong seismic load. Overall, there was a release of arch stresses in the upper part of both dams, due to the opening of the vertical joints, and consequently an increase of vertical stresses along the main cantilevers, causing concrete tensile failure. Furthermore, this study emphasized the potential of the developed numerical methods and thus of DamDySSA for predicting the non-linear seismic behaviour of arch dams.
- Use of SSHM data for dynamic behavior analysis and damage detection on large dams. The cases of Cabril dam and Cahora Bassa damPublication . Alegre, André; MENDES, PAULO; Carvalho, Ezequiel; Matsinhe, Bruno; Oliveira, Sérgio; Proença, JorgeThis paper presents recent results from studies on the dynamic behavior of large concrete dams in normal operational conditions, based on the comparison between experimental results, extracted from continuous dynamic monitoring data, and numerical results from finite element analysis. The case studies are Cabril dam (132 m high), in Portugal, and Cahora Bassa dam (170 m high), in Mozambique, two large arch dams that have been under Seismic and Structural Health Monitoring since 2008 and 2010, respectively. The monitoring data analysis is performed using DamModalID, a program specifically prepared for automatic frequency domain modal identification, while the numerical simulations are carried out utilizing DamDySSA4.0, a 3D finite element program developed for dynamic analysis of concrete dams. The focus in this work is on studying the evolution of the main natural frequencies over time, namely (i) to analyze the influence of the reservoir water level variations on the dynamic properties of the dam-reservoir-foundation system and hence on the frequencies’ values, and (ii) to evaluate the correlation with the deterioration and ageing of dams, ultimately aiming at the detection of eventual structural changes due to concrete damage. Based on the presented results, the relation between the frequencies’ values and the water level variations made it possible to verify that the dynamic response of both dams is clearly influenced by the reservoir level. Furthermore, the comparison between the identified frequencies and the values computed with reference models, considering linear behavior, enabled to conclude that the dynamic behavior of both dams has not changed over the last decade, despite the existing deterioration phenomena, and thus they can continue operating normally.
- Seismic and structural health monitoring systems for large dams: theoretical, computational and practical innovationsPublication . Oliveira, Sérgio; Alegre, André; Carvalho, Ezequiel; MENDES, PAULO; Proença, JorgeThis paper is focused on the study of the dynamic behaviour of two large arch dams, and it presents some innovations for the improvement of Seismic and Structural Health Monitoring (SSHM) systems for dams. The work describes a methodology based on the integrated use of software for automatic monitoring data analysis and of computational 3D fnite element (3DFE) models for dam dynamic behaviour simulation. The monitoring data analysis software was developed for automatic modal identifcation, in order to obtain natural frequencies and mode shapes, for automatic detection of vibrations induced by seismic events, to be distinguished from those caused by other operational sources, and for comparison between results retrieved from measured vibrations and numerical results from 3DFE modelling. The numerical simulations are carried out using a 3DFE program developed for dynamic analysis of dam-reservoir-foundation systems, based on a solid–fuid coupled formulation and considering the dam-water dynamic interaction, including calculation modules for complex modal analysis and for linear and non-linear seismic analysis. The case studies are two large arch dams that have been under continuous dynamic monitoring over the last ten years: Cabril dam (132 m high), the highest dam in Portugal, and Cahora Bassa dam (170 m high), in Mozambique, one of the highest dams in Africa. The SSHM systems installed in both dams have similar schemes and were designed to continuously record accelerations in several locations at the upper part of the dam body and near the dam-foundation interface, using uniaxial and triaxial accelerometers. The most signifcant experimental results from continuous dynamic monitoring are presented and compared with numerical results for both dams, with emphasis on the evolution of natural frequencies over time, including the vibration mode shapes for various water levels, and on the measured accelerations during low-intensity seismic events. Furthermore, the main results of non-linear seismic response simulations are provided, considering the efects due to joint movements and tensile and compressive concrete damage, aiming to assess the seismic performance of both dams based on the Endurance Time Analysis method.
- Modal identification methods in time domain numerical tests and applications in civil engineeringPublication . Prior, Ana; Oliveira, Sérgio; Freitas, MatildeThis work intends to contribute to the development and use of time domain modal identification methods, namely the Maximum Likelihood Method (MLM) and the Covariance-Driven Stochastic Subspace Identification Method (SSI-Cov). Both methods are studied using numerically generated data and the SSI-Cov method is applied to the analysis of dam vibration records, due to environmental excitation.
- Seismic safety assessment of arch dams using an ETA-based method with control of tensile and compressive damagePublication . Alegre, André; Oliveira, Sérgio; MENDES, PAULO; Proença, Jorge; Ramos, Rafael; Carvalho, EzequielThe seismic safety assessment of large concrete dams remains a significant challenge in dam engineering, as it requires appropriate analysis methods, modern performance criteria, and advanced numerical models to simulate the dam seismic behavior. This paper presents a method for seismic safety assessment of arch dams based on Endurance Time Analysis (ETA), using tensile and compressive damage results from a robust formulation for seismic analysis considering joint opening/sliding and concrete non-linear behavior (finite element program DamDySSA, under development in LNEC). The seismic performance is evaluated by controlling the evolution of the damage state of the dam, according to predefined performance criteria, to estimate acceleration endurance limits for tensile and compressive damage. These acceleration limits are compared, respectively, with the peak ground accelerations prescribed for the Operating Basis Earthquake (OBE) and Safety Evaluation Earthquake (SEE), aiming to evaluate the dam seismic performance relative to both earthquake levels efficiently, using a single intensifying acceleration time history. The ETA-based method is applied to the cases of Cabril Dam (132 m-high) and Cahora Bassa Dam (170 m-high), confirming its usefulness for future seismic safety studies, while the potential of DamDySSA for non-linear seismic analysis of arch dams is highlighted.