| 1. |
- Gasch, Tobias, et al.
(författare)
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Concrete Support Structure for Hydroelectric Generators Subjected to Rotor Dynamic Loads
- 2014
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Ingår i: International Symposium on Dams in a Global Enviromental Challange. - Bali.
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- In earlier times, the generators of the hydropower plants ran more or less continuously, while nowadays there are many planned starts and stops. The hydropower stations are thereby, due to the new pattern of operation, subjected to loads that they were not originally designed for. The aim of this study is to understand the complex interaction between the power generating system and the supporting concrete structure, during this new operational pattern.During inspections, cracks were discovered in the concrete structure of the power house, near the stator and rotor spider supports, at several hydropower stations in Sweden. In a previous phase of this project it was shown that these cracks initiated due to the combined effect of drying shrinkage, mechanical loads and variations in temperature due to starts and stops. Cracking of the concrete structure reduces its stiffness, which may result in larger loads acting on the structure and vibrations exceeding the unit’s strict tolerance limits.In this part of the study, the behaviour of a concrete support structure subjected to rotor dynamic loads during normal operation has been studied. A detailed 3D numerical model has been developed which include hydropower unit. The results of this study show that a reduced structural stiffness of the concrete support structure, due to cracking, influences the behaviour of the rotating system.
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| 2. |
- Gasch, Tobias, et al.
(författare)
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Cracking in the concrete foundation for hydropower generators : Part II
- 2013
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Rapport (övrigt vetenskapligt/konstnärligt)abstract
- An extensive program for improvement of the hydropower plants in Sweden is currently on-going. The aims are to secure future production and to maintain and further develop an already high dam safety.During inspection, cracks were discovered in the concrete foundation, near the stator and rotor spider supports, at some hydropower stations in Sweden. The cracks were believed to be related to new patterns for generator operation, thereby changing the dynamic loading of the stator and rotor spider supports. Previously the generators ran continuously, while nowadays there are an increased number of stops and starts, sometimes even several times during one day. Increased dynamic forces due to runaways, and also other dynamic events such as emergency stops, may also contribute to increased stress levels and cracking of the foundation. Furthermore, although extreme loads such as short circuits of the generator seldom occurs, the influence on the dynamic forces acting on the supporting structure and concrete foundation may be strongly influenced during such events.The objective of this study is to understand the complex interaction between the power generating system (stator, rotor, turbine, etc.) and its supporting concrete structure. It is important from a dam safety perspective to determine the causes of existing structural cracks in the foundation. Furthermore, to be able to predict further crack propagation of the concrete foundation will help to determine future maintenance requirements.A three dimensional non-linear finite element model developed earlier was used to evaluate a methodology for analyses of the interaction between the generator and the concrete foundation. The influence of cracks in the concrete foundation was investigated by including the fracture pattern obtained in earlier FE analyses of time-dependent thermal and moisture gradients. These analyses showed that the drying shrinkage induced cracking inside the concrete foundation and especially close to the supports of the stator and the rotor spider. The obtained fracture pattern for the previous analysis was used as input for this study, with the concrete foundation’s changed structural properties and their influence on the interaction with the generator considered in the analyses. Furthermore, deadweight and operational load were also included in the analyses.The study show that FE models with a cracked concrete foundation can be used to analyse structural interaction betwee foundation and generator components during operation of a hydro power generator. The crack pattern can be determined by FE analyses, or by in-situ measurements of existing concrete cracks for a specific concrete foundation. The analyses show that further studies are needed regarding the combined effects from thermo-mechanical loads, drying shrinkage, creep and dynamical loads caused by the generator. The combined effects may further increase the stress levels for the concrete foundation, especially locally near perforations, and stator and rotor spider supports. These analyses should be performed with an increased numerical resolution for both the concrete foundation and the supporting structure for the generator, with an increased accuracy for the local stress variations near perforations of the foundation and also at the supports for the generator. This research area will be further investigated within a recently started research project at KTH, financed by the Swedish Hydropower Centre.
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| 3. |
- Gasch, Tobias, et al.
(författare)
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Effects of aging concrete in support structures for hydroelectric machinery
- 2014
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Ingår i: XXII Nordic Concrete Research Symposium. - Reykjavik. ; , s. 237-240
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- At many of the Swedish hydropower plants, cracks have been observed in the concrete power station. Although the presence of cracks in these massive concrete structures does not pose an immediate threat to the structural safety, it of course affects its durability. Besides this, and perhaps as important in this application, the presence of cracks reduces the structural stiffness which affects the operation of the machinery.A case study is presented, where cracks have been found in the concrete support. Furthermore, analysis methods to evaluate the status of the concrete support; mainly through the use of finite element analysis are proposed.
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| 4. |
- Gasch, Tobias, et al.
(författare)
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Probabilistic analyses of crack propagation in concrete dams : Part II
- 2013
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Rapport (övrigt vetenskapligt/konstnärligt)abstract
- Several concrete buttress dams in northern Sweden have been found to be subjected to, more or less severe, cracks according to recent assessments and investigations. Theoretical analyses and field measurements have shown that most of these cracks have developed or propagated as a result of the seasonal temperature variations. Most dams in Sweden were built for more than 50 years ago and it is therefore important to also consider the influence of long-term effects and degradation to assess the dam. The ordinary sliding and overturning stability analyses may not be sufficient when the supporting structure is cracked, since the cracks may comprise the integrity and the homogeneity of the structure.The work presented in this report is a continuation of the work on advanced numerical methods for studying crack propagation in concrete dams presented in Björnström et al. (2006), Ansell et al. (2008), Ansell et al. (2010) and Malm et al. (2013). In the latter parts of the project the main focus has been on the development of probabilistic analysis methods for studying crack propagation, mainly with respect to the stochastic variation of material properties of concrete but also with regard to loading conditions. The concepts of the used probabilistic analysis methods were introduced in a previous part of the project, (Malm et al. 2013). In the previous part, stochastic spatial distribution of material properties was only studied within local areas where a crack was expected to be developed. In the work presented in this report, this concept has been expanded to cover stochastic spatial distribution of material properties within an entire buttress wall of one monolith. A sensitivity study is also presented regarding variations in the assumed temperatures of the seasonal temperature loading.To increase the accuracy of the numerical model a new FE-model was developed with a refined mesh compared to previously used meshes. The new mesh size was chosen with respect to the smallest characteristic crack length of a concrete sample in the stochastic population. All other aspects of the model were defined equally as in previous models.The results of the probabilistic analyses with respect to variation in material properties showed a significant increase in developed cracks, compared to a deterministic analysis. The main crack patterns were, however, similar, but additional cracks were developed adjacent to previous cracks and the inclination of some cracks was changed. These findings should be included when assessing different dam stability failure modes of buttress dams. The sensitivity study of the assumed temperatures showed that it was mainly the low temperatures in combination with temperature differences that initiate cracking in the monolith.
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| 5. |
- Gasch, Tobias, et al.
(författare)
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Seismic analyses of nuclear facilities with interaction between structure and water : Comparison between methods to account for Fluid-Structure-Interaction (FSI)
- 2013
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Rapport (övrigt vetenskapligt/konstnärligt)abstract
- Methods to describe the interaction between fluids and solids has been one of the biggest focus points for the research within the field of computationalengineering for the recent years. This area is of interest to a variety ofengineering problems, ranging from flow in blood vessels, aerodynamics andof course the interaction between water and civil engineering structures. Thetypical civil engineering application of fluid-structure interaction (FSI)encountered in a nuclear facilities is obtained at seismic loading, where the nuclear facilities consists of water filled pools of various sizes, for example the spent fuel and condensation pools. These water filled pools contribute with added mass to the structure, which lowers the natural frequency of thestructure as well as hydrostatic and hydrodynamic pressure that acts on thewalls of the pool due to wave propagation in the fluid. In addition, as the pools also have a free water surface towards the environment of thestructure, free surface wave propagation also has to be accounted for; i.e.sloshing. This introduces extra non-linearity to the problem, since a freesurface constitutes a boundary condition with an unknown location.The main part of this report constitutes as a state-of-the-art summary whereconcepts important for FSI analyses are presented and important differencesare discussed. Due to the different interests of the numerous disciplinesengaged in this research area, a large number of methods have been developed, where each has different strengths and weaknesses suited for the problem in mind when developing the method. The focus of this report havebeen to describe the most important numerical techniques and the categories of methods that or of most interest for civil engineering problems, such as simplified analytical or mass-spring models, Acoustic Elements, ArbitraryLagrangian-Eulerian (ALE) and coupled Eulerian-Lagrangian (CEL).Thereafter two benchmark examples are presented, intended to highlightdifferences between the different methods. In the first study, sloshing of aliquid tank is studied where the numerical methods are compared toexperimental results, regarding the movement of the free water surface. In addition, the hydrodynamic (fluid) pressures on the walls of the tanks arecompared between the different numerical methods. It was shown that mostanalysis methods give accurate results for the sloshing wave height whencompared with the experimental data. It should however be mentioned that the tank was only excited by a simple harmonic motion with a frequency thatdo not give rise to any resonance waves in the water body.Also when it comes to fluid pressure good agreement between the differentanalysis methods was found, although no experimental data was available forthis parameter. It was also noticed that for the sloshing tank, most of the change in pressure occurred close to the free surface of the water, which indicates that it mainly consists of a convective pressure, i.e. from the sloshing. Thereby, finite element programs that account the impulsive mass incivil engineering FSI problems should not be used for this type of analysis. In the second study, the numerical methods are compared based on differenttypes of seismic input, such as a large earthquake with mainly low frequencycontent typically like an earthquake on the US west coast and one smallerearthquake with relatively higher degree of high frequency content typicallylike a Swedish type of earthquake. One important observation was that the relative increase in induced stresses in the structure, with and withoutconsideration of the water was significantly larger for the Swedish earthquakethan for the US earthquake. One possible reason for this may be that the Swedish earthquake is not large enough to excite the relatively stiff structurewithout any water, but it will induce significant dynamic effects in the waterwhich give rise to higher stresses in the concrete as well. However, this shows that it is very important to include water in seismic analyses.
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| 6. |
- Malm, Richard, et al.
(författare)
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Concrete arch dam at seismic loading with fluid structure interaction
- 2013
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Konferensbidrag (refereegranskat)abstract
- A concrete arch dam have been analyzed during seismic loading with a model based on acoustic elements to describe the water and infinite elements as quiet boundaries to prevent wave reflection. The results have also been compared with a simplified model based on Westergaards added mass approach. The simplified model is only used, in this study, for comparison with the more advanced model with acoustic elements. Therefore the results from this simplified model are just used as a rough estimate of the induced stresses and displacements. Despite this, the simplified Westergaard model gives similar results compared to the more advanced model with acoustic elements for the water and infinite elements for the boundaries. The largest difference between the models often occurs at the nodes in the base of the arch dam, which may be due to poor discretization. Generally, the Westergaard added mass gives higher maximum principal stresses at the base on the upstream side than the acoustic model, while often underestimating the min principal stresses at the base on the downstream side. Both models show high tensile stresses near the base of the arch dam that would result in cracks.
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| 7. |
- Malm, Richard, et al.
(författare)
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Cracking in the concrete foundation for hydropower generators : Analyses of non-linear drying diffusion, thermal effects and mechanical loads
- 2013
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Rapport (övrigt vetenskapligt/konstnärligt)abstract
- An extensive program for improvement of the hydropower plants in Sweden is currently on-going. The aims are to secure future production and to maintain and further develop an already high dam safety.During inspection, cracks were discovered in the concrete foundation, near the stator and rotor spider supports, at some hydropower stations in Sweden. The cracks were believed to be related to the function of the stator supports and to new patterns of generator operation. In earlier times, the generators ran continuously, while nowadays there are many stops and starts, sometimes even several times during one day. The objective of this study is to understand the complex interaction between the power generating system (stator, rotor, turbine, etc.) and the supporting concrete structure. It is important from a dam safety perspective to determine the causes of the structural cracks that have been found in-situ.A three dimensional non-linear finite element model has been developed in order to analyse formation and propagation of the cracks. Several different load effects have been studied in this project in addition to the mechanical loads during operation. The new pattern of generator operation with several starts and stops lead for instance to variations in temperature which have been studied. Besides this, the uneven drying shrinkage of concrete has also been studied in this project. Thereby, the structural behaviour of a concrete foundation for the power generating system has been analysed taking into account the transient thermal and moisture gradients in combination with dead loads and some of the operational loads imposed to the foundation.The analyses shows that reinforced concrete structure that constitute a support to the generator is subjected to cracking due to the loads considered in this study, where the cracks near the supports are caused by a combination of mechanical loads, long-term drying shrinkage and temperature variations. The analyses showed that even after 20 years, the moisture content in the centre of the thicker part in the concrete foundation still had a high relative humidity. At the same time the concrete close to the free surfaces and the slender parts of the concrete foundation had reached the same relative humidity as the environment. Thereby, a large difference in drying shrinkage is obtained between different parts of the concrete foundation and thereby large forces due to restrain. The analyses showed that the drying shrinkage induced cracking inside the concrete foundation and especially close to the supports of the stator and the rotor spider which coincides with location of the cracks found in-situ.The results show that the cracks found in-situ can be simulated and explained with advanced numerical methods. The results also indicate that the dynamic effect from the loads caused by the power generating system have to be studied further, since a reduced structural stiffness due to cracking may result in larger loads imposed on the structure from the magnetic eccentricity and turbine imperfections or alternatively lead to a fatigue failure of for instance the reinforcement.
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| 8. |
- Malm, Richard, et al.
(författare)
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Evaluating Stability Failure Modes due to Cracks in a Concrete Buttress Dam
- 2013
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Ingår i: Changing Times. - United States of America : U.S. Society on Dams. ; , s. 415-424
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Konferensbidrag (refereegranskat)abstract
- Several concrete buttress dams in northern Sweden have been found to be subjected to, more or less severe, cracks according to recent assessments and investigations. Theoretical analyses and field measurements have shown that most of these cracks have developed or propagated as a result of the seasonal temperature variations. Most dams in Sweden were built for more than 50 years ago and it is therefore important to also consider the influence of long-term effects and degradation to assess the condition of the dam. In this paper, simulations have been performed with detailed 3D nonlinear numerical analyses in order to study crack initiation and crack propagation due to stochastic variation in material properties, which represent concrete degradation. The structural response due to loads from gravity, hydrostatic water pressure and thermal seasonal effects have been considered in the studies. It was shown that weak material properties near the crack-tip will govern the trajectory of the crack. According to the analyses, significant amount of cracking can occur in the front-plate and buttress if the strength of the concrete is reduced, which lead to new potential failure modes. In addition, the extent of cracking in the buttress dam is largely governed by the induced cracking in the front-plate since these cracks have a tendency to propagate into the concrete buttress. Based on the calculated extent of cracking in the dam body, different failure modes can be assessed in order to determine the dam stability failure.
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| 9. |
- Malm, Richard, et al.
(författare)
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Finite element analyses of an arch dam subjected to seismic loads and hydrodynamic forces
- 2014
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Ingår i: XXII Nordic Concrete Research Symposium. ; , s. 465-468
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- A concrete arch dam subjected to seismic ground accelerations has been analysed using the finite element method. The response of the concrete structure is calculated through dynamic implicit analyses using two different modelling approaches, the Westergaard added mass approach with hydrodynamic inertia forces from a finite water volume and a model based on acoustic elements. The models show high tensile stresses near the base of the dam which indicate a risk for cracking. The study demonstrate that the choice of damping, the type of seismic excitation and use of quiet boundaries have a significant influence on the result.
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| 10. |
- Malm, Richard, et al.
(författare)
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Probabilistic analyses of crack propagation in concrete dams : Part 1
- 2013
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Rapport (övrigt vetenskapligt/konstnärligt)abstract
- Several concrete buttress dams in northern Sweden have been found to be subjected to, more or less severe, cracks according to recent assessments and investigations. Theoretical analyses and field measurements have shown that most of these cracks have developed or propagated as a result of the seasonal temperature variations. Most dams in Sweden were built for more than 50 years ago and it is therefore important to also consider the influence of long-term effects and degradation to assess the dam. The ordinary sliding and overturning stability analyses may not be sufficient when the supporting structure is cracked, since the cracks may comprise the integrity and the homogeneity of the structure.This project is a continuation from previous projects presented by Björnström et al. (2006), Ansell et al. (2008) and Ansell et al. (2010). In these previous projects, Storfinnforsen hydropower dam located in northern part of Sweden was studied and the purpose of the projects was to explain the cause for cracking found in situ. In the present project, the previously developed numerical model was verified against measured variations in crest displacement and crack width due to temperature variations during one year. The results showed that the numerical model, which was used, could predict both variations in displacements and crack width with good accuracy compared to the measurements on the actual dam.The studies of this project which are presented in this report are focused on the crack propagation due to stochastic variations in material properties. This was performed with probabilistic analyses based on a local model of an inclined crack in the supporting buttress. Monte-Carlo simulations were performed where each element was randomly assigned a concrete strength according to an assumed material distribution. It was shown that weak material properties near the crack-tip will govern the propagation and the trajectory of the crack. The results also showed that the average crack propagation from the probabilistic analyses differed both regarding inclination and length compared to the deterministic analysis which was based on mean values. In addition to the local analyses, global analyses were also performed, where stochastic variations in material properties were assigned to the whole monolith. The analyses showed that using design values or characteristic values of the material strength may give a different failure mode compared to the case where mean values of the material properties are used. When considering nonlinear properties for verification analyses, it is important to base the material properties on as accurate material properties as possible. In addition, the extent of cracking in the buttress dam is largely governed by the induced cracking in the front-plate since these cracks have a tendency to propagate into the concrete buttress. The results also showed that if the dam have been subjected to degradation and thereby has reduced concrete strength, the thermal stresses could induce significant cracking in the monolith which could lead to new failure modes. Based on the calculated extent of cracking in the dam body, different failure modes should be assessed in order to determine the dam stability failure.
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| 11. |
- Malm, Richard, 1980-, et al.
(författare)
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Probabilistic Analyses of Thermal Induced Cracking in a Concrete Buttress Dam
- 2011
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Ingår i: Risk Analysis, Dam Safety, Dam Security and Critical Infrastructure Management.
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Konferensbidrag (refereegranskat)abstract
- Recent assessments and investigations of buttress dams in northern Sweden reveal several types of cracks. The theoretical analysis and field measurements have showed that the most of the cracks are either developed or propagated as a result of the seasonal temperature variations. Cracks influence the behaviour of the dams in different ways, such as reducing the tightness of the dam and increasing the hydraulic pressure within the material/structure. Furthermore, cracks may have an impact on the stiffness and stability of the dam. The ordinary sliding and overturning stability analyses are not sufficient when the supporting structure is cracked. The cracks may comprise the integrity and the homogeneity of the structure. A cracked, and for that matter even repaired structure, can’t be regarded as a homogenous structure and should be treated accordingly. Consequently, other types of models instead of the conventional design models should be utilized for the stability analyses of the cracked and repaired dams.The mode of the failure is one of the decisive elements considering determination of the probability of the failure. The conditions for crack initiation and the trajectory of the crack propagation are the decisive factors which govern the failure mode. Ordinary design methods and advanced numerical models which are based on the elastic behaviour of the structure can’t be utilized, since these models are not able to describe the non-linear behaviour and to predict the failure mode of the structure.A finite-element model based on non-linear fracture mechanics is being utilized to study crack development in a buttress dam. The aim of the study was to reveal crack trajectories and different probable failure modes, and moreover to determine the influences of the cracks on the overall behaviour of the structure. In a real structure the loading (mechanical and environmental) and boundary conditions are decisive factors regarding initiation, propagation and trajectory of the cracks. Furthermore, the material properties and their statistical distribution may influence the formation of cracks and the mode of failure.
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| 12. |
- Malm, Richard, 1980-, et al.
(författare)
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The Influence of Cracks on the Structural Behaviour of a Buttress Dam
- 2011
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Ingår i: Studies on Modern Technologies and Long-term Behavior of Dams. - : China WaterPower Press. - 9787508490182 ; , s. 677-685
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Konferensbidrag (refereegranskat)abstract
- Buttress dams located in cold areas are often subjected to severe environmental conditions. Recent assessments and investigations of buttress dams in northern Sweden reveal several types of cracks. Theoretical analysis and field measurements have showed that the most of the cracks are either developed or propagated as a result of the seasonal temperature variations.Cracks influence the behaviour of the dams in different ways, such as reducing the tightness of the dam and increasing the hydraulic pressure within the cracks. Furthermore, cracks may have impact on the stiffness and stability of the dam. The ordinary sliding and overturning stability analyses are not sufficient when the supporting structure is cracked. The cracks may comprise the integrity and the homogeneity of the structure. A cracked, and for that matter even repaired structure, can’t be regarded as a homogenous structure and should be treated accordingly. Consequently other types of models instead of the conventional design models should be utilized for the stability analyses of the cracked and repaired dams.There are at least two major aspects which must be considered when a cracked or repaired structure is being analysed. The first aspect is the principle of superposition and the second aspect is probability of the failure. It is well known that a cracked structure does not behave linearly, consequently the principle of superposition can’t be applied to determine the overall effects of the several simultaneously acting events.The mode of failure is one of the decisive elements considering determination of the probability of the failure. The condition for crack initiation and the trajectory of the crack propagation are the decisive factors which govern the failure mode. Ordinary design methods and advanced numerical models which are based on the elastic behaviour of the structure can’t be utilized, since these models are not able to describe the non-linear behaviour and to predict the failure mode of the structure.A finite-element model based on the non-linear fracture mechanics has been utilized to study crack development in a buttress dam. The aim of the project was to study crack trajectories and to determine the influences of cracks on the overall behaviour of the structure, for instance in the global stiffness of the structure and possible failure modes. The paper will present the structure, the numerical model and the results.
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| 13. |
- Rydell, Cecilia, et al.
(författare)
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Interaction between structure and water in seismic analyses of nuclear facilities
- 2013
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Konferensbidrag (refereegranskat)abstract
- The objective of this paper is to evaluate different approaches to account for fluid-structure interaction (FSI) in seismic analyses of nuclear facilities. Different methods to account for FSI, from simplified to highly advanced numerical methods, are briefly reviewed and some important concepts are discussed. A benchmark example of a simple tank sloshing problem is included to evaluate the use of different FSI methods.The main conclusion from the study is that it is of great significance to first of all include the effect of FSI. When considering the response of a tank subjected to a load of periodic nature, as in the benchmark example, the hydrodynamic effects are very important, since they increase the load effect on the structure. It is also observed that the simplified methods, in which the hydrodynamic effects are included as a mass-spring system, results in much higher stresses in the structure than if the fluid is included as continuum elements. However, the more advanced methods lead to extra computational time and also require more from the analyst. With the focus of this project being the global response of the structure, most methods describe the fluid unnecessarily complicated and phenomena such as splashing and turbulence are of little interest. The main aspects that influence the structure are the mass and inertia of the fluid along with the surface waves, the sloshing. Considering this, simplified methods such as elements with acoustic equations, and even mass-spring systems, to represent the fluid, often give results that are accurate enough.
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| 14. |
- Rydell, Cecilia, et al.
(författare)
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Stresses in water filled concrete pools within nuclear facilities subjected to seismic loads
- 2014
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Ingår i: Nordic Concrete Research. - 0800-6377. ; :51, s. 43-62
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Tidskriftsartikel (refereegranskat)abstract
- This paper presents a study on water filled pools within nuclear facilities subjected to seismic loads. The type of structure studied is an elevated rectangular concrete tank, supported by the reactor containment, which is a high cylindrical concrete structure. Seismic analysis is performed using finite element models, accounting for fluid-structure interaction (FSI) between the water and the concrete structure. The stresses in a concrete pool are calculated, also investigating the changes in stresses as additional cross-walls are added. The effects from earthquakes dominated by low and high frequencies are evaluated, representative for conditions at the West coast of North America and Northern Europe, respectively. It is shown that the coupled fluid-structure systems have more significant modes in the high frequency range compared to the models without water, that is, for frequencies at which the Northern European type earthquake has significant energy compared to the Western North American earthquake. The seismic analyses show that the relative increase of hydrodynamic pressure is higher when the outer walls of the pool are stiffened due to the inclusion of additional cross-walls. With the inclusion of additional cross-walls, modes with lower natural frequencies, although still relatively high, become more important for the hydrodynamic pressure response. Leading to a higher stress response in the outer walls of the pool for models including the additional cross-walls compared to models without cross-walls. The study indicates that the effect from fluid-structure interaction is of great importance also for seismic loads with relatively high-frequency content.
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