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  • 1.
    Abbasiverki, Roghayeh
    et al.
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Concrete Structures.
    Ansell, Anders
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Concrete Structures.
    Malm, Richard
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Concrete Structures.
    Analysis of shallowly buried reinforced concrete pipelines subjected to earthquake loads2014In: Nordic Concrete Research, ISSN 0800-6377, no 51, p. 111-130Article in journal (Refereed)
    Abstract [en]

    Buried reinforced concrete pipelines are widelyused in e.g. water and wastewater systems. Failure of these infrastructures mayresult in drastic effects and recently they have been brought into focus asvital components in safety systems for nuclear power installations. The highlevel of safety has here lead to a demand for reliable earthquake risk analyses.In this paper, methods are compared and the use of seismic design loadsdemonstrated. FE analysis in 2D of soil-pipe interaction under seismic wavepropagation is performed. The performance of concrete pipes subjected toseismic waves with different frequency content is evaluated with respect todifferent soil condition but also water mass effect.

  • 2.
    Ahmed, Lamis
    et al.
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Concrete Structures.
    Ansell, Anders
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Concrete Structures.
    Malm, Richard
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Concrete Structures.
    Numerical modelling and evaluation of laboratory tests with impact loaded young concrete prisms2016In: Materials and Structures, ISSN 1359-5997, E-ISSN 1871-6873, p. 1-14Article in journal (Refereed)
    Abstract [en]

    Numerical modelling in combination with in situ measurements, observations and laboratory testing will be important to future establishment of reliable guidelines for efficient civil and engineering work involving concrete casting close to e.g. blasting operations. Results from laboratory tests with impact loaded young concrete prisms are here evaluated using a 3D finite element model. Solid elements are used implementing a non-linear material model, capable of describing cracking during stress wave propagation. The position of cracks and measured particle vibration velocities are calculated and compared with laboratory test results. The damaging effect of impact vibrations is evaluated using crack width and fracture energy as damage criteria. Alternative geometry for the test prisms, with a notched section, is analysed. This will give one wide crack at the centre of the prism instead of two or three cracks distributed over its length which will make future laboratory test more efficient and reliable. Recommended damage limits at concrete ages of 4, 6, 8 and 12 h are given, based on numerical calculations for concrete strength class C25 and C50.

  • 3.
    Ahmed, Lamis
    et al.
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Concrete Structures.
    Ansell, Anders
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Concrete Structures.
    Malm, Richard
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Concrete Structures.
    Numerical modelling and evaluation of laboratory tests with impact loaded young concrete prisms2016In: Materials and Structures, ISSN 1359-5997Article in journal (Refereed)
    Abstract [en]

    Numerical modelling in combination with in situ measurements, observations and laboratory testing will be important to future establishment of reliable guidelines for efficient civil and engineering work involving concrete casting close to e.g. blasting operations. Results from laboratory tests with impact loaded young concrete prisms are here evaluated using a 3D finite element model. Solid elements are used and a non-linear material model implemented, capable of describing cracking during stress wave propagation. The position of cracks and measured particle vibration velocities are calculated and compared with laboratory test results. The damaging effect of impact vibrations is evaluated using crack width and fracture energy as damage criteria. Alternative geometry for the test prisms, with a notched section, is analysed. This will give one wide crack at the centre of the prism instead of two or three cracks distributed over its length which will make future laboratory test more efficient and reliable. Recommended damage limits at concrete ages of 4, 6, 8 and 12 h are given, based on numerical calculations for concrete strength class C25 and C50.

  • 4.
    Ahmed, Lamis
    et al.
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Concrete Structures.
    Malm, Richard
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Concrete Structures.
    Ansell, Anders
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Concrete Structures.
    Finite element simulation of shotcrete exposed to underground explosions2012In: Nordic Concrete Research, ISSN 0800-6377, no 45, p. 59-74Article in journal (Refereed)
    Abstract [en]

    An elastic finite element model is used tosimulate theinducedstress waves from blasting, propagating in rock towards shotcrete on a tunnel wall. Due to the inhomogeneous nature of the rock, the stress wavesattenuate onitsway from the point of explosiontowardsthe shotcrete on the rock surface. Material damping for the rock-mass is estimated from in-situ measurements. The vibration resistance of the shotcrete-rock support system depends on the material properties of the shotcrete. Age-dependent material properties are varied to investigate the behaviour of young shotcrete subjected to blast loading. Finally, finite element analysis results are presented and verified through comparison with other numerical models, measurements and observations.

  • 5.
    Andersson, Andreas
    et al.
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Structural Design and Bridges (name changed 20110630).
    Carlsson, Fredrik
    Lunds Tekniska Höskola.
    Enckell, Merit
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Structural Design and Bridges (name changed 20110630).
    Enochsson, Ola
    Luleå Tekniska Universitet.
    Karoumi (Redaktör), Raid
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Structural Design and Bridges (name changed 20110630).
    Malm, Richard
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Concrete Structures.
    Plos, Mario
    Chalmers Tekniska Högskola.
    Sundquist, Håkan
    KTH, School of Architecture and the Built Environment (ABE).
    Täljsten, Björn
    LuleåTekniska Universitet.
    Wiberg, Johan
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Structural Design and Bridges (name changed 20110630).
    Ülker, Mahir
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Structural Design and Bridges (name changed 20110630).
    Modern mät- och övervakningsmetodik för bedömning av befintliga broar2007Report (Other academic)
  • 6.
    Andersson, Andreas
    et al.
    KTH, Superseded Departments, Mechanics.
    Malm, Richard
    KTH, Superseded Departments, Mechanics.
    Measurement evaluation and FEM simulation of bridge dynamics2004Independent thesis Basic level (professional degree), 20 credits / 30 HE creditsStudent thesis
    Abstract [en]

    The aim of this thesis is to analyse the effects of train induced vibrations in a steel Langer beam bridge. A case study of a bridge over the river Ljungan in Ånge has been made by analysing measurements and comparing the results with a finite element model in ABAQUS. The critical details of the bridge are the hangers that are connected to the arches and the main beams. A stabilising system has been made in order to reduce the vibrations which would lead to increased life length of the bridge.

    Initially, the background to this thesis and a description of the studied bridge are presented. An introduction of the theories that has been applied is given and a description of the modelling procedure in ABAQUS is presented.

    The performed measurements investigated the induced strain and accelerations in the hangers. The natural frequency, the corresponding damping coefficients and the displacement these vibrations leads to has been evaluated. The vibration-induced stresses, which could lead to fatigue, have been evaluated. The measurement was made after the existing stabilising system has been dismantled and this results in that the risk of fatigue is excessive. The results were separated into two parts: train passage and free vibrations. This shows that the free vibrations contribute more and longer life expectancy could be achieved by introducing dampers, to reduce the amplitude of the amplitude of free vibrations.

    The finite element modelling is divided into four categories: general static analysis, eigenvalue analysis, dynamic analysis and detailed analysis of the turn buckle in the hangers. The deflection of the bridge and the initial stresses due to gravity load were evaluated in the static analysis. The eigenfrequencies were extracted in an eigenvalue analysis, both concerning eigenfrequencies in the hangers as well as global modes of the bridge. The main part of the finite element modelling involves the dynamic simulation of the train passing the bridge. The model shows that the longer hangers vibrate excessively during the train passage because of resonance. An analysis of a model with a stabilising system shows that the vibrations are damped in the direction along the bridge but are instead increased in the perpendicular direction. The results from the model agree with the measured data when dealing with stresses. When comparing the results concerning the displacement of the hangers, accurate filtering must be applied to obtain similar results.

  • 7.
    Ansell, Anders
    et al.
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Concrete Structures.
    Ekström, Tomas
    Energoretea AB.
    Hassanzadeh, Manouchehr
    Lund University, Building Materials.
    Malm, Richard
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Concrete Structures.
    Crack propagation in buttress dams: Application of non-linear models - Part II2010Report (Other academic)
    Abstract [en]

    The largest and most important concrete dams in Sweden are buttress dams. These consist of a large number of concrete monoliths formed by a front-plate with a supporting buttress. Cracks have been observed in some of these dams which in a long-term perspective may affect their safety.

    Concrete dams located in cool areas are often subjected to severe environmental impacts. Recent assessments and investigations of a buttress dam built 1954 in northern Sweden points out several types of cracks. The front-plate of the dam was not heat insulated on the downstream side when constructed, which has led to freeze-thawing damages in the plate. However, in 1994 a heat insulation wall that prevents ice-formation and protects the front-plate against frost damage was installed. It is located between two buttresses in the dam, from the rock up to the dam crest. The wall has most probably led to increased mechanical stresses in the pillars as a result of contraction and expansion due to seasonal temperature changes.

    A finite element model based on non-linear fracture mechanic, plasticity theory and damage mechanics has been utilized to study crack development in a buttress dam. The combined effects of restrained thermal displacements and loads caused by water were studied. The development of cracks due to seasonal temperature variations was simulated, especially with respect to the effect of the insulating wall installed some 40 years after the completion of the dam. The results show that the seasonal temperature variation causes high tensile stresses at different locations on the dam, and that the cracks can be initiated from at least four locations. Thermal stresses in combination with the load caused by water were shown to be the reason for cracking. The results point out that the addition of the insulating wall greatly contributed to the development of cracks in the buttress. A more suitable placement of the insulation wall could have prevented the cracking of the pillars.

  • 8.
    Ansell, Anders
    et al.
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Concrete Structures.
    Ekström, Tomas
    Energoretea AB.
    Hassanzadeh, Manouchehr
    Lund University, Building Materials.
    Malm, Richard
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Concrete Structures.
    Verification of the Cause of the Cracks in a Buttress Dam2009In: ICOLD 23rd CONGRESS PROCEEDINGS, 2009Conference paper (Refereed)
    Abstract [en]

    Buttress dams located in cool areas are often subjected to severe environmental impacts. Recent assessments and investigations of a buttress dam in northern Sweden points out several types of cracks. The dam was built 1954. The front plate was not heat insulated on the downstream side when constructed, which has led to freeze-thawing damages in the plate. However, in 1994 a heat insulation wall protecting the front plate was installed. It is located between each pillar in the dam, from the rock up to the crest and located approximately in the middle of the pillars. The insulation wall has most probably led to increased mechanical stresses in the pillars as a result of contraction and expansion due to seasonal temperature changes. A finite-element model based on non-linear fracture mechanic has been utilized to study crack development in the buttress dam. The results show that the seasonal temperature variation causes high tensile stresses at different locations on the dam, and that the cracks can be initiated from at least four locations on the dam. All types of cracks can propagate simultaneously. The analysis indicates that the addition of an insulation wall, which did not include the whole body of the pillars, has caused diagonal cracks in the pillars. The insulation wall prevents the ice-formation on the front plate and protects it against frost damage. However, a more suitable placement of the insulation wall could have prevented the cracking of the pillars.

  • 9.
    Ansell, Anders
    et al.
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Concrete Structures.
    Malm, Richard
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Concrete Structures.
    Modelling of Thermally Induced Cracking of a Concrete Buttress Dam2008In: Nordic Concrete Research: Research projects 2008 / [ed] J. Silfwerbrand, Oslo: The Nordic Concrete Federation , 2008, p. 30-31Conference paper (Refereed)
  • 10.
    Ansell, Anders
    et al.
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Concrete Structures.
    Malm, Richard
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Concrete Structures.
    Modelling of thermally induced cracking of a concrete buttress dam2008In: Nordic Concrete Research, ISSN 0800-6377, Vol. 38, p. 69-88Article in journal (Refereed)
    Abstract [en]

    Some of the larger hydropower dams in Sweden are buttress dams, consisting of up to 100 concrete monoliths formed by a front plate with a supporting buttress. Cracks have been observed in some dams, through the buttresses and at the base close to the rock foundation. The combined effects of restrained thermal displacements and loads caused by water are studied through finite element analysis. The results demonstrate the use of a non-linear material model and show that it is possible to follow the formation and propagation of the cracks. The analyses indicate that thermal stresses in combination with the loads caused by external water pressure are the reason for the cracking.

  • 11.
    Ansell, Anders
    et al.
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Concrete Structures.
    Malm, Richard
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Concrete Structures.
    Strukturanalys för bärande konstruktioner2010Report (Other academic)
  • 12.
    Eriksson, Daniel
    et al.
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Concrete Structures.
    Gasch, Tobias
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Concrete Structures.
    Malm, Richard
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Concrete Structures.
    Ansell, Anders
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Concrete Structures.
    Freezing of partially saturated air-entrained concrete: A multiphase description of the hygro-thermo-mechanical behaviourManuscript (preprint) (Other academic)
    Abstract [en]

    Even though air-entrained concrete is usually used for concrete structures located in cold climates that are exposed to wet environments, frost damages are frequently detected during inspections. However, it is often hard to assess the extent and severity of these damages and, thus, there is a need for better tools and aids that can complement already established assessment methods. Several studies have successfully shown that models based on poromechanics and a multiphase approach can be used to describe the freezing behaviour of air-entrained concrete. However, these models are often limited to the scale of the air pore system and, hence, hard to use in applications involving real structures. This study proposes a hygro-thermo-mechanical multiphase model which describes the freezing behaviour of partially saturated air-entrained concrete on the structural scale. The model is implemented in a general FE-code and two numerical examples are presented to validate and show the capabilities of the model. The first concerns a series of experimental tests of air-entrained cement pastes, whereas the second aims to show the capability of the model to account for an initial non-uniform distribution of moisture. The results show that the model can reproduce the freezing behaviour observed in the experimental tests on a structural scale as well as being capable of describing freezing induced deformations caused by non-uniform moisture distributions.

  • 13.
    Gasch, Tobias
    et al.
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Concrete Structures. Vattenfall R&D.
    Facciolo, Luca
    Vattenfall R&D.
    Eriksson, Daniel
    Vattenfall R&D.
    Rydell, Cecilia
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Concrete Structures. Vattenfall R&D.
    Malm, Richard
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Concrete Structures. Vattenfall R&D.
    Seismic analyses of nuclear facilities with interaction between structure and water: Comparison between methods to account for Fluid-Structure-Interaction (FSI)2013Report (Other academic)
    Abstract [en]

    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.

  • 14.
    Gasch, Tobias
    et al.
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Concrete Structures.
    Hansson, Håkan
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Concrete Structures.
    Malm, Richard
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Concrete Structures.
    Hassanzadeh, Manouchehr
    Vattenfall AB / Lund University.
    Concrete Support Structure for Hydroelectric Generators Subjected to Rotor Dynamic Loads2014In: International Symposium on Dams in a Global Enviromental Challange, Bali, 2014Conference paper (Other academic)
    Abstract [en]

    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.

  • 15.
    Gasch, Tobias
    et al.
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Concrete Structures.
    Malm, Richard
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Concrete Structures.
    Effects of aging concrete in support structures for hydroelectric machinery2014In: XXII Nordic Concrete Research Symposium, Reykjavik, 2014, Vol. 50, p. 237-240Conference paper (Other academic)
    Abstract [en]

    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.

  • 16.
    Gasch, Tobias
    et al.
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Concrete Structures.
    Malm, Richard
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Concrete Structures.
    Ansell, Anders
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Concrete Structures.
    A coupled hygro-thermo-mechanical model for concrete subjected to variable environmental conditions2016In: International Journal of Solids and Structures, ISSN 0020-7683, E-ISSN 1879-2146, Vol. 91, p. 143-156Article in journal (Refereed)
    Abstract [en]

    It is necessary to consider coupled analysis methods for a simulation to accurately predict the long-term deformations of concrete structures. Among other physical fields that can be considered, both temperature and moisture have a significant influence on the deformations. Variations of these fields must therefore be included implicitly in an analysis. This paper presents a coupled hygro-thermo-mechanical model for hardened concrete based on the framework of the Microprestress-Solidification theory. The model accounts for important features of concrete such as ageing, creep, shrinkage, thermal dilation and cracking; all of these under variable temperatures and moisture conditions. It is discussed how to implement the proposed model in a flexible numerical framework that is especially suitable for multi-physics analyses. The capabilities of the model are shown through the analysis of three experimental data sets from the literature, with focus on creep and shrinkage. Overall, the agreement between the analysis and experimental results is good. Finally, a numerical example of a concrete gravity dam with dimensions and loads typical to northern Sweden is analysed to show the capabilities of the model on a structural scale.

  • 17.
    Gasch, Tobias
    et al.
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Concrete Structures.
    Malm, Richard
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Concrete Structures.
    Eriksson, Daniel
    Vattenfall Engineering .
    Hassanzadeh, Manouchehr
    Vattenfall Engineering .
    Probabilistic analyses of crack propagation in concrete dams: Part II2013Report (Other academic)
    Abstract [en]

    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.

  • 18.
    Gasch, Tobias
    et al.
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Concrete Structures.
    Malm, Richard
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Concrete Structures.
    Nordström, Erik
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Concrete Structures.
    Hassanzadeh, Manouchehr
    Lund University, Division of Building Materials..
    Non-linear analyses of cracks in aging concrete hydro power structures2016In: Dam Engineering, ISSN 0958-9341, Vol. 26, no 3, p. 1-26Article in journal (Refereed)
    Abstract [en]

    The concrete structures at Swedish hydro power facilities were built during the early to mid-20th century and many of them are starting to exhibit age related wear and deterioration. It isimportant to ensure the integrity of these concrete structures from a dam safety perspectiveand also to secure a safe operation of the power facility in the future. With the latter in mind,this paper aims to study the concrete structures that house the power generating machinery ofthe facility, especially the parts close to the generator where the loads from the power unit aresupported. Cracks observed in these structures will reduce its stiffness, which affects theoperation of the rotating machinery. This paper presents and discusses some generalconsiderations and loads that are of importance for this type of structures and highlights sometypical cracks that have been observed in Swedish hydro power facilities. To complement thisdiscussion, a case study is presented of a hydro power facility where cracks have been foundin the concrete support structure of the power unit, especially at the interconnections betweenthe unit and the concrete. The most likely cause of these cracks are investigated through nonlinearfinite element analysis considering mechanical loads as well as physical loads such asdrying shrinkage and temperature variations. It is concluded that the long-term physicalloading is the most probable cause of the observed cracks. However, the operation of thepower unit and changes in its operational pattern can cause further propagation of thesecracks. Finally, suggestions on possible enhancement of the analysis methods used in the casestudy are proposed and discussed for further studies of this type of concrete structures.

  • 19.
    Gasch, Tobias
    et al.
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Concrete Structures.
    Malm, Richard
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Concrete Structures.
    Nordström, Erik
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Concrete Structures.
    Hassanzadeh, Manouchehr
    Lund University, Division of Building Materials.
    Non-linear analyses of cracks in aging concrete hydro power structures2016In: Dam Engineering, ISSN 0958-9341Manuscript (preprint) (Other (popular science, discussion, etc.))
    Abstract [en]

    The concrete structures at Swedish hydro power facilities were built during the early to mid-20th century and many of them are starting to exhibit age related wear and deterioration. It isimportant to ensure the integrity of these concrete structures from a dam safety perspectiveand also to secure a safe operation of the power facility in the future. With the latter in mind,this paper aims to study the concrete structures that house the power generating machinery ofthe facility, especially the parts close to the generator where the loads from the power unit aresupported. Cracks observed in these structures will reduce its stiffness, which affects theoperation of the rotating machinery. This paper presents and discusses some generalconsiderations and loads that are of importance for this type of structures and highlights sometypical cracks that have been observed in Swedish hydro power facilities. To complement thisdiscussion, a case study is presented of a hydro power facility where cracks have been foundin the concrete support structure of the power unit, especially at the interconnections betweenthe unit and the concrete. The most likely cause of these cracks are investigated through nonlinearfinite element analysis considering mechanical loads as well as physical loads such asdrying shrinkage and temperature variations. It is concluded that the long-term physicalloading is the most probable cause of the observed cracks. However, the operation of thepower unit and changes in its operational pattern can cause further propagation of thesecracks. Finally, suggestions on possible enhancement of the analysis methods used in the casestudy are proposed and discussed for further studies of this type of concrete structures.

  • 20.
    Gasch, Tobias
    et al.
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Concrete Structures.
    Nässelqvist, Mattias
    ÅF.
    Hansson, Håkan
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Concrete Structures.
    Malm, Richard
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Concrete Structures.
    Gustavsson, Rolf
    Vattenfall Engineering.
    Hassanzadeh, Manouchehr
    Vattenfall Engineering.
    Cracking in the concrete foundation for hydropower generators: Part II2013Report (Other academic)
    Abstract [en]

    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.

  • 21.
    Gasch, Tobias
    et al.
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Concrete Structures.
    Sjölander, Andreas
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Concrete Structures.
    Malm, Richard
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Concrete Structures.
    Ansell, Anders
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Concrete Structures.
    A coupled multi-physics model for creep, shrinkage and fracture of early-age concrete2016In: 9th International Conference on Fracture Mechanics of Concrete and Concrete Structures / [ed] John E. Bolander, Eric N. Landis, Victor E. Saouma, 2016Conference paper (Refereed)
    Abstract [en]

    The behaviour of concrete at early-age is complex and involves several physical fieldssuch as temperature, moisture and deformations. In this paper a hygro-thermo-chemo-mechanicalmodel for the analysis of early-age concrete based on a combination of models from the literature ispresented. The chemical model is based on the reaction degree concept, also used to define internal actions such as self-desiccation and ageing of mechanical properties. A mechanical model based on the Microprestress-Solidification theory for concrete creep is used, that in a simplified manner alsoconsiders concrete fracture. The model has been implemented in a numerical framework suitable for coupled multi-physics problems. It is here applied to a case study of an un-reinforced concrete tunnel plug made of a low-pH self-compacting concrete. Good agreement is generally obtained with measurements and hypotheses previously made on the behaviour of the plug are verified.

  • 22.
    Goldgruber, Markus
    et al.
    Institut für Wasserbau und Wasserwirtschaft, Technische Universität Graz.
    Malm, Richard
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Concrete Structures.
    Nonlinear Seismic Simulation of an Arch Dam using XFEM2014Conference paper (Refereed)
    Abstract [en]

    The “Committee on Computational Aspects of Analysis and Design of Dams” in International Commission on Large Dams (ICOLD) is responsible for organizing benchmark workshops every second year, where the 12th benchmark workshop was held in the city of Graz back in October 2013. One of the three topics in this benchmark workshop was titled “Fluid Structure Interaction, Arch Dam - Reservoir at Seismic Loading”, formulated by the Institute of Hydraulic Engineering and Water Resources Management from Graz University of Technology. The focus of this topic was on the interaction between the reservoir and the dam, and thereby studies the influences of different approaches to simulate the reservoir. To keep the results comparable between the 13 participants the simulations had to be performed just linear. However, structures like arch dams undergo nonlinear behavior. Due to the massive amount of concrete, such structures are divided into almost independent vertical blocks acting like cantilevers. Furthermore, the hydrostatic water pressure can lead to openings in the contact plane between dam and the foundation. A cooperation between Graz University of Technology and KTH Royal Institute of Technology led to the idea to investigate the structures behavior by taking into account the contacts ( block joints, abutment) and cracking (tensile failure), due to the seismic acceleration by using XFEM. The results are pointing out the possibilities and borders of such complex nonlinear simulations.

  • 23.
    Hansson, Håkan
    et al.
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Concrete Structures.
    Malm, Richard
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Concrete Structures.
    Initial study of oblique hard target projectile impact of normal and high strength concrete targets2011In: Nordic concrete research: Research projects 2011:  Proceedings of XXI Nordic Concrete Research Symposium / [ed] D.H. Bager, 2011, p. 63-66Conference paper (Refereed)
    Abstract [en]

    The ability to predict penetration resistance in concrete is necessary to evaluate the vulnerability of protective designs for impacts by penetrating weapons, or deformable projectiles. The paper presents experimental work regarding oblique projectile impact of both normal strength and high performance concrete targets with modern type of hard target penetrators. Furthermore, finite element (FE) analyses of non-normal projectile impacts of the normal strength concrete targets are presented, and its limitations discussed.

  • 24.
    Hansson, Håkan
    et al.
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Concrete Structures.
    Malm, Richard
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Concrete Structures.
    Non-linear Finite Element Analysis of Deep Penetration in Unreinforced and Reinforced Concrete2011In: Nordic Concrete Research, ISSN 0800-6377, Vol. 44, p. 87-107Article in journal (Refereed)
    Abstract [en]

    Penetration and perforation of concrete targets are studied by the use of numerical simulations to enhance the understanding of the penetration phenomenon. Comparisons were made with test results obtained for both reinforced and unreinforced 48.0 MPa normal strength concrete. The studied projectiles were made as generic models of penetrators for buried hardened target defeat. Varying impact velocities and angles for the penetrators were investigated. The simulations gave reasonable results for the different simulation cases, with the best results were obtained for reinforced concrete targets.

  • 25.
    Hassanzadeh, Manouchehr
    et al.
    Lund University, Building Materials.
    Malm, Richard
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Concrete Structures.
    Nordström, Erik
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Concrete Structures.
    Analysis of Displacements and Crack Formations in Foundations for Hydropower Generators2012Conference paper (Refereed)
    Abstract [en]

    A rather extensive program for improvement of the Swedish hydropower plants is ongoing. The aims are to secure future production and to maintain and further develop an already high dam safety. In connection with earlier work, which dealt with assessment of an existing buttress dam where a non-linear finite element model was applied to determine the cause of the observed cracks. The results showed that the non-linear finite element method is a powerful tool to determine the structural behaviour of large concrete structures. The study in this paper is a continuation of the previous project, aiming at applying the method to other parts of dam structure such as foundation supporting the generator (stator and rotor), rotor spider, turbine shaft, spiral casing, turbine and draft tube.

    The hydropower plant, which is studied, was constructed in the early forties. During the inspection, structural damages (cracks) were discovered around some of the stator and rotor spider supports. 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, some times even several times during one day. The purpose of this study is to illuminate the complex stress conditions in the generator foundations of a hydropower plant and to reveal the causes of the stresses and to verify their role in formation of the cracks.

    The structural behaviour of a foundation has been analysed taking into account the transient thermal gradients in combination with dead loads and some of the operational loads imposed to the foundation. A three dimensional non-linear finite element model has been applied in order to analyse formation and propagation of the cracks. The analyses showed that based on the assumption made, the concrete foundation cracks mainly on the outside but also near some of the stator supports due to the combination of mechanical and thermal loads. However, the studied loads cannot explain all of the types of damages that can be found in-situ. It is likely that especially the drying shrinkage may be the one of the reasons for the cracks that has been found near the stator supports and especially the rotor spider supports.

    It is important from a dam safety perspective to determine the causes of the structural cracks that have been found in-situ and also to evaluate the effect of the reduced stiffness due to cracking, since a reduced structural stiffness can 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.

  • 26.
    Hellgren, Rikard
    et al.
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Concrete Structures.
    Malm, Richard
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Concrete Structures.
    Johansson, Fredrik
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Soil and Rock Mechanics.
    Ríos Bayona, Francisco
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Soil and Rock Mechanics.
    Pull-out tests of 50-year old rock bolts2016In: Proceedings of the International Symposium on Appropriate technology to ensure proper Development, Operation and Maintenance of Dams in Developing Countries, Johannesburg, 2016, p. 263-272, article id 258Conference paper (Refereed)
    Abstract [en]

    In this paper, detailed numerical and analytical calculations are presented with the aim to study the effect of rock bolts and the influence of degradation on the dam safety. In addition, results from inspections and pull-out tests are presented from a case study with rock bolts that has been installed for 50 years. All tested bolts have with margin withstood higher stresses than the maximum allowed stresses according to Swedish standards. At visual inspection, no bolts have shown any major signs of degradation and all investigate bolts are in very good condition.

  • 27.
    Hellgren, Rikard
    et al.
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Concrete Structures.
    Malm, Richard
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Concrete Structures.
    Persson, Anton
    Klasson Svensson, Emil
    Estimating the effect of ice load on a concrete dams displacement with regression models2017In: Proceedings ICOLD 2017 International Symposium: Knowledgebased dam engineering, Prag, 2017Conference paper (Refereed)
    Abstract [en]

    In colder climates, the ice load is an important factor to consider when assessing the dam safety. The size of the ice load is associated with uncertainties, regarding both its maximum size and seasonal variation. There is today no reliable method to determine the ice load acting on a dam, since available measurements and theories results in a large scatter regarding the maximum ice loads.

    In this project, the possibility to determine the ice load based on traditional measurements of a dam behavior is investigated. It’s evaluated if the ice thickness is a significant predictor for the displacement of a Swedish arch dam. A case study on inverted pendulums was performed where dynamic regression models were used to calculate the displacement of the dam.

    The model showed significant results which could explain the displacements. The ice thickness shows an effect on a 5 % significance level. The results indicate that the seasonal ice load is large enough to be traceable, and could be determined from conventional pendulum measurements. Since pendulums are installed on a large number of dams, a method that estimates the ice load from those measurements could vastly reduce uncertainties regarding ice loads on dams.

  • 28.
    Isander, Anders
    et al.
    E.ON Vattenkraft.
    Nilsson, Carl-Oscar
    E.ON Vattenkraft.
    Malm, Richard
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Concrete Structures. SWECO Infrastructure.
    L'évaluation et la réhabilitation structurelle du barrage béton a contrefort de Storfinnforsen en Suède2013Conference paper (Refereed)
    Abstract [en]

    The Storfinnforsen hydro power dam was completed in 1954 and is the largest concrete buttress dam in Sweden. A few years after completion, horizontal cracks were found in the lower parts of the front-plates and freeze-thaw damage was detected on the upstream side of the front-plates. This resulted in a structural rehabilitation program where cracks were grouted and an insulating wall was installed to reduce the thermal gradient over the thickness of the front-plate. Despite these measures, additional cracks were found on the dam several years after where especially diagonal cracks from the inspection-gangway were found in the buttress walls. In this paper, numerical analyses are presented which shows that most cracks found in-situ have developed or propagated as a result of the seasonal temperature variations. One important finding is that the location of the insulating wall had contributed to increased stresses in the buttress wall and is a likely cause of the new cracks. Therefore, an alternative placement for the insulating wall is presented which reduces the stresses in the buttress wall.

    As a result of these studies, comprehensive rehabilitation and life-extension program is now being performed to maintain and further develop an already high dam safety of Storfinnforsen and in Ramsele which is a similar dam downstream. The program consisted of moving the insulating walls, improving the stability with ground anchorage tendons, strengthening the front-plates and in addition to widen the road on the dam crest to allow for heavy traffic.

  • 29.
    Johansson, Fredrik
    et al.
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Soil and Rock Mechanics.
    Ekström, I.
    Rito Pi, C.
    Malm, Richard
    Carlsson, V.
    FEM-analysis of a concrete dam in northern Sweden2015In: Association of State Dam Safety Officials, Dam Safety 2015, Association of State Dam Safety Officials , 2015Conference paper (Refereed)
    Abstract [en]

    Krångfors is a concrete dam located in northern Sweden that was constructed in three stages from 1928 to 1973. This expansion in different stages resulted in a complicated structural design. Extensive cracks have been noticed in parts of the structure. Two main hypotheses for the cause of these cracks were initially established; 1) alkali-silica reaction (ASR) and 2) large temperature variations in the structure. Petrographic analyses of concrete cores showed that the concrete ballast consisted of greywacke with potential for ASR. However, accelerated testing of concrete cores showed that the potential for expansion due to ASR was limited. To analyze the effect from temperature variations, three dimensional FEManalyses were performed. The results from these FEM-analyses are the main focus of this paper. These analyses showed that adding the annual temperature variation and possible shrinkage to the FEM-model gave results that could explain the observed cracks.

  • 30.
    Larsson, Stefan
    et al.
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Soil and Rock Mechanics.
    Charbit, Benjamin
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Soil and Rock Mechanics.
    Malm, Richard
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Concrete Structures.
    Ansell, Anders
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Concrete Structures.
    Pilotstudie för utveckling av förenklad beräkningsmetod för jordstabilisering med skivor av kalkcementpelare2011Report (Other academic)
  • 31.
    Larsson, Stefan
    et al.
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Soil and Rock Mechanics.
    Malm, Richard
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Concrete Structures.
    Ansell, Anders
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Concrete Structures.
    Numerisk simulering av lateralt belastade kalkcementpelare2012In: Bygg & teknik, ISSN 0281-658X, no 1, p. 25-28Article in journal (Other (popular science, discussion, etc.))
  • 32.
    Larsson, Stefan
    et al.
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Soil and Rock Mechanics.
    Malm, Richard
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Concrete Structures.
    Charbit, Benjamin
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Soil and Rock Mechanics.
    Ansell, Anders
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Concrete Structures.
    Finite element modelling of laterally loaded lime-cement columns using a damage plasticity model2012In: Computers and geotechnics, ISSN 0266-352X, E-ISSN 1873-7633, Vol. 44, p. 48-57Article in journal (Refereed)
    Abstract [en]

    The behaviour of laterally loaded lime-cement columns in a shear box was studied. Laboratory tests are presented together with numerical analyses where the columns are simulated by a concrete damage plasticity model that considers stiffness degradation. Seven model tests were investigated where the columns were installed in a single column pattern and in rows with different column overlap in order to investigate the influence of the degree of overlapping of the columns in the rows. The results of the numerical evaluations showed good agreement with the experimental shear stress-displacement relation and a good accuracy with respect to the fractures developed.

  • 33.
    Malm, Richard
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Concrete Structures.
    Analys av sprickbildning och sprickbredd vid plant spänningstillstånd i balkliv av armerad betong2005Report (Other academic)
    Abstract [sv]

    I denna förstudie har analys av spänningar, sprickinitiering och sprickpropagering studerats för stora betongkonstruktioner i brukgränstillstånd. Speciellt har sneda sprickor studeras som vanligtvis uppstår broars balkliv, som ett resultat av kombinerad belastning av normal- och tvärkrafter. Det har studerats hur dessa förhållanden avspeglas i dagens dimensioneringsnormer och standarder. För att tydligare belysa skillnaden mellan de metoder som finns i olika normer har ett beräkningsexempel på Gröndalsbron och Alviksbron använts. Beräkningsmetoder som används vid dimensionering i brukgränstillstånd antar att strukturen är sprucken och befinner sig i stabiliserat sprickmönster. Beräkningsmetoder som utvecklats i vetenskapliga artiklar, så som t.ex. Modifierade tryckfältsteorin, baseras generellt på laboratorieförsök där nytillverkade, ospruckna balkar belastas för att registrera de sprickbredder, töjningar och spänningar som uppstår. Eftersom betongkonstruktioner i princip alltid har sprickor kommer dessa metoder att underskatta sprickbredder och armeringsspänningar i brukgränstillstånd. Vid jämförelse med verkliga uppmätta sprickbredder på Gröndalsbron gav följande två metoder mycket god överensstämmelse; generella metoden i NS 3473 samt BBK’s fackverksmodell med spricklutningen som ger lika armeringsspänningar i de två riktningarna. Beräkningarna visar att metoden i Eurocode 2 och BBK’s fackverksmodell där spricklutningen antas motsvara huvudspänningsriktningen, tenderar att överskatta sprickbredden.

    Det finns ett flertal olika metoder för att beskriva uppsprickning och sprickpropagering i finita element (FE) formuleringar. I denna rapport har fördelar och nackdelar med dessa olika metoder beskrivits och dessutom har några vanliga konventionella finita element program studerats. Vid FE modellering av uppsprickning hos spröda material finns generellt två inriktningar smeared crack eller discrete crack. Skillnaden mellan dessa är att vid modellering med smeared crack-metoder krävs ingen förkunskap om var sprickorna kommer att uppträda, medan vid discrete crack-metoder kan strukturen endast spricka vid fördefinierade områden. Vid dimensionering av betongkonstruktioner är smeared crack-metoder mest lämpliga, eftersom kunskapen om var uppsprickningen kommer att ske oftast saknas.

  • 34.
    Malm, Richard
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Structural Design and Bridges (name changed 20110630).
    FE Analysis of Cracking in Concrete due to Shear Loading2008In: Nordic Concrete Research: Research projects 2008 / [ed] J. Silfwerbrand, 2008, p. 202-203Conference paper (Refereed)
    Abstract [en]

    Shear type failure in concrete is often complicated to calculate due to its brittle nature. The strongly non-linear behaviour results in a significant drop in load carrying capacity which usually leads to numerical convergence difficulties. There are several different constitutive models developed to describe the behaviour of concrete. A realistic model of the concrete behaviour enables accurate simulation of actual structural behaviour in service and in ultimate limit state. In this paper non-linear finite element (FE) analyses of concrete beams subjected to in-plane forces resulting in a shear type failure are analysed with different continuum based concrete models.

  • 35.
    Malm, Richard
    Vattenfall Power Consultant AB.
    Low-pH Concrete Plug for Sealing the KBS-3V Deposition Tunnels2012Report (Other academic)
    Abstract [en]

    In SKB’s main alternative for final repository of radioactive material, KBS-3V, the backfilled deposition tunnels will be separated from the remaining tunnel system with concrete plugs. These concrete plugs will be designed for a life span of 100 years and their function shall maintain until the transport tunnels outside the plug are backfilled and the natural geohydrological conditions have been restored.

    The purpose of this report is to document the results and the evaluation from this project and motivate the choice of the most appropriate design for closing the deposition tunnels in the spent fuel repository. The purpose has also been to investigate and present the loads acting on the plug system and determine the load capacity of the concrete plug. This report is the result of a project conducted between 2009-01-01 – 2010-12-31 and the project group has made its assessment based on the conditions and requirements that are present today.

    The entire design of the plug system is part of this project, where the plug system consists of a filter, a bentonite seal and a cast-in-place concrete plug. Two different conceptual design alternatives for the concrete plug have been studied in this report, one long tapered plug and one dome shaped plug. The results in this report focus on the choice of the conceptual design for the concrete plug and its possibility to assist the entire plug system to satisfy its requirements.

    It is a complicated task to dispose the radioactive waste and it sets high technical requirements on the design and the production of the backfill and the closing of the deposition tunnels. The aim of this project is to design and develop a plug system suitable for production. This is done by the means of numerical calculations and analyses. The primary function of the concrete plug is to act as a resistance to the external loads originated from the axial expansion of the backfill and the water pressure. However, the entire plug system has a requirement on being watertight, which also affects the design of the concrete plug. In the spent fuel repository, low-pH concrete should be used instead of conventional concrete. The reason for this is to the largest extent to reduce the negative effect that basic materials could have on the function of the bentonite clay. For this purpose, a new low-pH concrete recipe has been developed and this changes the conditions for using reinforcement, cooling and grouting compared to the use of conventional concrete.

    The report shows the possibilities to use an unreinforced plug made of low-pH concrete as a resistance in the deposition tunnels. Today, some parameters are unknown and some data may be classified as uncertain, primarily regarding the long-term properties of the low-pH concrete material and the bentonite clay. It will take several years until all questions can be answered and a full-scale test is vital to validate the assumptions and the performed numerical simulations. The report should therefore be considered based on that data and conclusions will be studied further and be experimentally verified under realistic and controlled conditions.

  • 36.
    Malm, Richard
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Structural Design and Bridges.
    Predicting shear type crack initiation and growth in concrete with non-linear finite element method2009Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

     

    In this thesis, the possibility to numerically describing the behaviour that signifies shear type cracking in concrete is studied. Different means for describing cracking are evaluated where both methods proposed in design codes based on experiments and advanced finite element analyses with a non-linear material description are evaluated. It is shown that there is a large difference in the estimation of the crack width based on the calculation methods in design codes. The large difference occurs due to several of these methods do not account for shear friction in the crack face.

    The finite element method is an important tool for analysing the non-linear behaviour caused by cracking. It is especially of importance when combined with experimental investigations for evaluating load bearing capacity or establishing the structural health. It is shown that non-linear continuum material models can successfully be used to accurately describe the shear type cracking in concrete. A method based on plasticity and damage theory was shown to provide accurate estimations of the behaviour. The methods based on fracture mechanics with or without inclusion of damage theory, overestimated the stiffness after crack initiation considerably. The rotated crack approach of these methods gave less accurate descriptions of the crack pattern and underestimated the crack widths. After verification of the material model, realistic finite element models based on plasticity and damage theory are developed to analyse the cause for cracking in two large concrete structures. The Storfinnforsen hydropower buttress dam is evaluated where the seasonal temperature variation in combination with the water pressure have resulted in cracking. With the numerical model the cause for cracking can be explained and the crack pattern found in-situ is accurately simulated. The model is verified against measurements of variation in crest displacement and crack width with close agreement. The construction process of a balanced cantilever bridge, Gröndal Bridge, is numerically simulated and a rational explanation of the cause for cracking is presented. It is shown that large stresses and micro-cracks develop in the webs during construction, especially after tensioning the continuing tendons in the bottom flange. Further loads from temperature variation cause cracking in the webs that is in close agreement with the cracking found in-situ. The effect of strengthening performed on this bridge is also evaluated where the vertical Dywidag tendons so far seem to have been successful in stopping further crack propagation.

     

  • 37.
    Malm, Richard
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Structural Design and Bridges (name changed 20110630).
    Shear cracks in concrete structures subjected to in-plane stresses2006Licentiate thesis, monograph (Other scientific)
    Abstract [en]

    After only two years of service, extensive cracking was found in the webs of two light-rail commuter line bridges in Stockholm, the Gröndal and Alvik bridges. Due to this incident it was found necessary to study the means available for analysing shear cracking in concrete structures subjected to in-plane stresses. The aim of this PhD project is to study shear cracking with these two bridges as reference. In this thesis, the first part aims to study the possibility of using finite element analysis as a tool for predicting shear cracking for plane state stresses. The second part is concerning how the shear cracks are treated in the concrete design standards.

    Shear cracking in reinforced beams has been studied with non-linear finite element analyses. In these analyses the shear cracking behaviour was compared to experiments conducted to analyse the shear failure behaviour. Finite element analyses were performed with two different FE programs Abaqus and Atena. The material model used in Atena is a smeared crack model based on damage and fracture theory with either fixed or rotated crack direction. The material model used in Abaqus is based on plasticity and damage theory. The fixed crack model in Atena and the model in Abaqus gave good results for all studied beams. For the two studied deep beams with flanges the results from the rotated crack model were almost the same as obtained with the fixed crack model. The rotated crack model in Atena gave though for some beams a rather poor estimation of the behaviour.

    The calculation of crack widths of shear cracks has been studied for the long-term load case in the serviceability state for the Gröndal and Alvik bridges, with the means available in the design standards. The methods based on the crack direction corresponding to the principal stress and do not include the effect of aggregate interlocking seems to be too conservative. Two of the studied methods included the effect of aggregate interlocking, it was made either by introducing stresses in the crack plane or implicitly by changing the direction of the crack so that it no longer coincide with the direction of principal stress. For calculations based on probable load conditions, these methods gave estimations of the crack widths that were close to the ones observed at the bridges. Continuous measurements of cracks at the Gröndal and the Alvik bridges have also been included. Monitoring revealed that the strengthening work with post-tensioned tendons has, so far, been successful. It also revealed that the crack width variations after strengthening are mainly temperature dependent where the daily temperature variation creates movements ten times greater than those from a passing light-rail vehicle. Monitoring a crack between the top flange and the webs on the Gröndal Bridge showed that the top flange was moving in a longitudinal direction relative to the web until the strengthening was completed. The crack widths in the sections strengthened solely by carbon fibre laminates seem to increase due to long-term effects.

  • 38.
    Malm, Richard
    et al.
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Structural Design and Bridges.
    Andersson, Andreas
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Structural Design and Bridges.
    Field testing and simulation of dynamic properties of a tied arch railway bridge2006In: Engineering structures, ISSN 0141-0296, E-ISSN 1873-7323, Vol. 28, no 1, p. 143-152Article in journal (Refereed)
    Abstract [en]

    This paper deals with the dynamic effects on a tied arch railway bridge during train passages. The bridge is located in Ange municipality in central Sweden. Large vibrations of the hangers were observed during train passages and field measurements have been performed to study the train induced vibrations of the hangers. According to the Palmgren-Miners linear damage rule, there is a great risk of fatigue in the threaded parts of the hangers. The low damping in the hangers has a large influence of the risk of fatigue failure. A 3D finite element model of the bridge has been developed where the dynamic vehicle-bridge interaction was modelled using contact surface formulation with a sprung mass train system. The measured data results are compared with the results from the FE model to give a better understanding of the dynamic behaviour of the bridge.

  • 39.
    Malm, Richard
    et al.
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Concrete Structures.
    Ansell, Anders
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Concrete Structures.
    Cracking of Concrete Buttress Dam Due to Seasonal Temperature Variation2011In: ACI Structural Journal, ISSN 0889-3241, E-ISSN 1944-7361, Vol. 108, no 1, p. 13-22Article in journal (Refereed)
    Abstract [en]

    The largest and most important hydropower dams in Sweden are concrete buttress dams. These consist of a large number of concrete monoliths formed by a frontplate with a supporting buttress. Cracks have been observed in some of these dams, and these may affect their safety in the long term. As an important tool for the condition assessment of such dams, a finite element model based on nonlinear fracture mechanics and plasticity theory has been used to study crack development in a buttress dam. The combined effects of restrained thermal displacements and loads due to water were studied. The development of cracks due to seasonal temperature variations was simulated, especially the effect of an insulating wall installed some years after the completion of the dam. Thermal stresses, in combination with the load caused by water, were shown to be the reason for cracking. The addition of an insulating wall greatly contributed to the development of cracks in the buttress.

  • 40.
    Malm, Richard
    et al.
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Concrete Structures.
    Ansell, Anders
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Concrete Structures.
    Non-linear analyses of cracking in segmentally constructed concrete box-girder bridgesIn: Engineering Structures, ISSN 0141-0296Article in journal (Refereed)
  • 41.
    Malm, Richard
    et al.
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Concrete Structures.
    Ansell, Anders
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Concrete Structures.
    Nonlinear analysis of thermally induced cracking of a concrete damIn: ACI Structural Journal, ISSN 0889-3241, E-ISSN 1944-7361Article in journal (Refereed)
  • 42.
    Malm, Richard
    et al.
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Concrete Structures.
    Ansell, Anders
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Concrete Structures.
    Verifying the Behaviour of a Concrete Buttress Dam Subjected to Temperature Variations2011In: Nordic concrete research: Research projects 2011: Proceedings of XXI Nordic Concrete Research Symposium / [ed] D.H. Bager, 2011, p. 191-194Conference paper (Refereed)
    Abstract [en]

    Numerical simulations and experimental results are presented for a 60 year old buttress dam, subjected to temperature variations. The dam has several cracks, that likely have originated from temperature variations. The first step of the analyses was to recreate the current crack-pattern of the dam, by simulating cyclic seasonal temperature variations. After this, detailed simulations of the temperature variation were performed to compare the numerical results with the variations in crack widths and displacements measured for one year. The finite element model shows good agreement with the observed crack-pattern and the measured variations in crack widths and displacements.

  • 43.
    Malm, Richard
    et al.
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Concrete Structures.
    Eriksson, Daniel
    Vattenfall Power Consultant.
    Gasch, Tobias
    Vattenfall Power Consultant.
    Hassanzadeh, Manouchehr
    Lund University, Building Materials.
    Probabilistic Analyses of Thermal Induced Cracking in a Concrete Buttress Dam2011In: Risk Analysis, Dam Safety, Dam Security and Critical Infrastructure Management / [ed] Ignacio Escuder-bueno, Enrique Matheu, Luis Altarejos-garcfa, 2011Conference paper (Refereed)
    Abstract [en]

    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.

  • 44.
    Malm, Richard
    et al.
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Concrete Structures.
    Gasch, Tobias
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Concrete Structures.
    Finite element analyses of an arch dam subjected to seismic loads and hydrodynamic forces2014In: XXII Nordic Concrete Research Symposium, 2014, Vol. 50, p. 465-468Conference paper (Other academic)
    Abstract [en]

    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.

  • 45.
    Malm, Richard
    et al.
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Concrete Structures.
    Gasch, Tobias
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Concrete Structures.
    Eriksson, Daniel
    Vattenfall Engineering.
    Hassanzadeh, Manouchehr
    Vattenfall Engineering.
    Evaluating Stability Failure Modes due to Cracks in a Concrete Buttress Dam2013In: Changing Times: Infrastructure Development to Infrastructure Management, United States of America: U.S. Society on Dams , 2013, p. 415-424Conference paper (Refereed)
    Abstract [en]

    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.

  • 46.
    Malm, Richard
    et al.
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Concrete Structures.
    Gasch, Tobias
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Concrete Structures.
    Eriksson, Daniel
    Vattenfall R&D.
    Hassanzadeh, Manouchehr
    Vattenfall R&D.
    Probabilistic analyses of crack propagation in concrete dams: Part 12013Report (Other academic)
    Abstract [en]

    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.

  • 47.
    Malm, Richard
    et al.
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Concrete Structures.
    Hassanzadeh, Manouchehr
    Lund Universiity, Building Materials.
    Gasch, Tobias
    Vattenfall Power Consultant.
    Eriksson, Daniel
    Vattenfall Power Consultant.
    The Influence of Cracks on the Structural Behaviour of a Buttress Dam2011In: Studies on Modern Technologies and Long-term Behavior of Dams / [ed] Jia Jinsheng, Zhang Shugang, Xu Zeping, Xu Yao, China WaterPower Press , 2011, p. 677-685Conference paper (Refereed)
    Abstract [en]

    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.

  • 48.
    Malm, Richard
    et al.
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Concrete Structures.
    Hassanzadeh, Manouchehr
    Lund University, Building Materials.
    Gasch, Tobias
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Concrete Structures.
    Eriksson, Daniel
    Vattenfall R&D.
    Nordström, Erik
    Vattenfall Hydro.
    Cracking in the concrete foundation for hydropower generators: Analyses of non-linear drying diffusion, thermal effects and mechanical loads2013Report (Other academic)
    Abstract [en]

    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.

  • 49.
    Malm, Richard
    et al.
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Concrete Structures.
    Hassanzadeh, ManouchehrSWECO.Hellgren, RikardKTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Concrete Structures.
    Proceedings of the 14th ICOLD International Benchmark Workshop on Numerical Analysis of Dams2017Conference proceedings (editor) (Refereed)
    Abstract [en]

    Every second year, the Technical Committee A (Computational Aspects of Dam Analysis and Design) of ICOLD organizes an international benchmark workshop on numerical analysis of dams. The purpose is to share knowledge and experience regarding numerical modelling within the fields of dam safety, planning, design, construction as well as operation and maintenance of dams. The Terms of Reference for Committee A state; ”BenchmarkWorkshops are organised to compare numerical models between one another and/or with reference solutions, including the dissemination and publication of results”.

     The 14th ICOLD International Benchmark Workshop on Numerical Analysis of Dams was held from the 6th to the 8th September 2017 in Stockholm, the capital of Sweden. It was organized by the Local Organizing Committee, on behalf of the Swedish National Committee of ICOLD (SwedCold) and the ICOLD Technical Committee A. It was hosted by KTH Royal Institute of Technology, Sweden’s largest technical research and learning institution and one of Europe’s leading technical universities. With respect to hydropower and dam engineering, KTH has for many years been active within of the Swedish hydropower Center (SVC), a key centre for research and educational activities in Sweden. SwedCOLD established in 1931, and is one of the oldest members of the International Commission On Large Dams, ICOLD. The development of hydro power for electricity production was initiated in the late 19th century and made the large scale industrialisation in Sweden possible. About 2000 hydropower plants are in operation in Sweden today, producing almost half of the total electricity supply. Considering the many and relatively old dams in Sweden, organizing the benchmark workshop was important to increase the knowledge regarding managing aging hydropower structures.

    The 14th ICOLD International Benchmark Workshop has addressed current challenges regarding design and maintenance of existing dams and improved the understanding of these by exchange of experience on the use of numerical modelling for design, performance evaluation and safety assessment of dams. On behalf of the organizing committee, I would like to express my sincere appreciation to the formulators/moderators of each theme for their outstanding work to prepare the themes, reviewing the papers and evaluating the results. I also want to thank all participants to the benchmark workshop for their contributions and their presence to this benchmark workshop. Finally, the support from the Technical Committee A and the support from the companies and organizations that sponsored this benchmark workshop is gratefully acknowledged.

  • 50.
    Malm, Richard
    et al.
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Structural Design and Bridges (name changed 20110630).
    Holmgren, Jonas
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Concrete Structures.
    Cracking in deep beams owing to shear loading. Part 1: Experimental study and assessment.2008In: Magazine of Concrete Research, ISSN 0024-9831, E-ISSN 1751-763X, Vol. 60, no 5, p. 371-379Article in journal (Refereed)
    Abstract [en]

    In this paper, laboratory tests to failure of ten large deep beams with I-shaped cross-sections are presented. All beams had the same geometry with a shear span-to-depth ratio of 1.25 but differed in the amount of the vertical and horizontal web reinforcement. The presented results from the measurements consist of load-deformation curves, crack widths and crack patterns and strain distribution near the supports. The ultimate loads for these beams have been calculated with two strut-and-tie models and one truss model. The first strut-and-tie model calculates the tensile contribution of both reinforcement and concrete and takes into account their influence on the principal tensile stress. The second strut-and-tie model is a modification of the first one where the stress distribution along the strut is redefined. The third method is the truss model that is incorporated in a Design Code. The truss model gave the best result for the beams with a higher reinforcement ratio that exhibited in a shear compressive failure. The diagonal tensile failure that occurred in the beams with a small amount of web reinforcement was best captured with the modified strut-and-tie model.

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