| 1. |
- Eriksson, Daniel, 1987-
(författare)
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Multiphase models for freeze-thaw actions and mass transport in concrete hydraulic structures
- 2021
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- A crucial task for civil engineers is to make appropriate designs of new concrete structures and assessments of existing structures to ensure a long service life and sustainable use of the infrastructure. This doctoral thesis aims to increase the understanding of how advanced mathematical models can be used to describe phenomena and processes governing concrete degradation and thereby ultimately contribute to improving tools for design and assessments. The focus is on degradation processes that cause commonly observed concrete damage types in hydraulic structures exposed to cold climates and soft water. During a structure's service life, it is subjected to various deteriorating actions, but for the typical exposure conditions considered in this work, degradation due to freeze-thaw exposure and calcium leaching is of particular concern for the durability. Hence, the work related to improved modelling has been focused on phenomena related to these two degradation processes of concrete and how they may interact to produce damaging synergy effects.All developed models in this doctoral project treat concrete as a multiphase porous medium and use poromechanics to describe the coupled hygro-thermo-mechanical behaviour of the material. Moreover, since the overall aim concerns degradation in hydraulic structures, the model development has focused on obtaining formulations applicable for structural-scale simulations. The models presented in this thesis describe long-term water absorption into air-entrained concrete and the response of partially saturated air-entrained concrete exposed to freeze-thaw conditions. In the latter models, the phase changes and the freeze-thaw hysteresis are explicitly considered in the formulations. The presented simulation examples are performed using the Finite Element Method (FEM), and the capabilities of the models are verified with experimental data from the literature. Additionally, accelerated leaching experiments on air-entrained concrete are presented, where the influence of leaching on the formation and melting of ice inside the pore space due to pore structure alternations are investigated.The main research contribution of this work is the development and evaluation of advanced models applicable for structural-scale simulations that describe essential processes and phenomena related to freeze-thaw exposure of air-entrained concrete. The experimental work shows the significant influence of calcium leaching on the freeze-thaw processes, and the results can also facilitate future development of models considering some of the interactions causing damaging synergy effects. Adopting a multiphase modelling approach has been found suitable for describing the coupled processes and including interactions between different deterioration mechanisms. The theoretical models can also help gain further insights and improve the understanding of the phenomena, and thus, e.g. aid in developing more simplified models suited for daily engineering applications.
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| 2. |
- Gasch, Tobias
(författare)
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Multiphysical analysis methods to predict the ageing and durability of concrete
- 2019
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- With the societal demand for sustainability and the increasing age of infrastructure, a crucial task for the civil engineering community is to improve the durability of concrete structures. This thesis aims to contribute to such development through theoretical studies using mathematical modelling and numerical simulations. During its service life, a concrete structure is subjected to many different actions, ranging from mechanical loads to chemical and physical processes. Hence, a sound modelling strategy requires multiphysics and the inclusion of coupled chemical and physical fields (e.g. temperature, moisture and cement hydration) in addition to methods that describe mechanical integrity of the material. Conditions and phenomena critical for concrete structures at hydropower facilities have been of particular interest to study.The thesis presents several mathematical models of various complexity to describe the multiphysical behaviour of concrete using a material point description. A significant focus is on models that describe the mechanical behaviour of concrete where aspects such as ageing, cracking, creep and shrinkage are investigated. For the creep behaviour, a state-of-the-art model based on the Microprestress–Solidification (MPS) theory is reviewed and further developed. The appended papers (III to IV) presents a mathematical framework for the modelling of durability aspects of concrete based on multiphase porous media theory. The governing equations are derived with the Thermodynamically Constrained Averaging Theory (TCAT) as a starting point. It is demonstrated how this framework can be applied to a broad variety of phenomena related to durability; from the casting and hardening of concrete to the long-term absorption of water into air-entrained concrete. The Finite Element Methods (FEM) is used to solve the proposed mathematical models, and their capabilities are verified using experimental data from the literature.The main research contribution is the development and evaluation of theoretical models that advance the understanding and improve knowledge of the ageing and durability of concrete and concrete structures. More precisely, it is shown how multiphysical models and the developed multiphase framework can be used to gain insights on the material behaviour of concrete at smaller scales while they are also applicable to structural-scale simulations. During all model development, the efficient solution of structural problems has been fundamental. Through case studies and several examples from the literature, it is exemplified how these models can be used to enhance the performance and thereby increase the durability of concrete structures.
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| 3. |
- Hellgren, Rikard, 1988-
(författare)
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A systems approach to ice loads on concrete dams
- 2022
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Dams are mainly used for the storage of water to electricity production and irrigation, or for river regulation. Continuous work to assure a high dam safety level is a prerequisite to minimize the risk for the uncontrolled release of water. An essential part of the safety evaluation of concrete dams is to understand the loads they are exposed to and the expected response of the dam. Under normal conditions, the behaviour of concrete dams is, to a great extent, governed by the ambient variation in temperature and water level. For concrete dams in cold climates, the large variation in ambient temperatures between summer and winter is particularly significant. In addition, these dams may be subjected to a pressure load from the expansion or movement of an ice sheet on the reservoir. The current guidelines for these ice loads are based on the dam's location and state that concrete dams must be designed for a line load of 50-250 kN/m. Thus, the ice load constitutes a significant part of the total load, especially for small dams. Despite its relatively significant impact, the knowledge about ice loads is insufficient, and the magnitude and return period of ice loads constitute one of the greatest uncertainties during stability evaluations of concrete dams. Furthermore, an apparent contradiction is that measurements and models indicate that ice loads are higher than the recommended values. Simultaneously, there are no reported dam failures where the ice-load has been addressed as the reason for the breach.To increase the knowledge about ice loads and the structural behaviour of concrete dams, this thesis applies an approach where the ice and the dam are parts of a structural system. The thesis contains six studies investigating the dam's, the ice's or the system's response to external loads. Studies of the dam are aimed at increasing the understanding of the normal behaviour of concrete dams. Studies of ice loads include measurements, and a major contribution from this project is the development of a 1$\times$3 m$^2$ ice load panel, the installation of the panel on a concrete dam, and subsequent measurements during six winters. In addition, a systematic review and meta-analysis of previous measurements have been performed. The studies of the different parts have been connected through two studies of the entire system. The first is a parameter study where the static interaction between ice and dam is simulated to quantify how geometric variations of the reservoir and ice affect the mechanical ice loads. The second study searches for detectable influence from the ice load in the measured behaviour of concrete dams.The results show that the structural behaviour of concrete dams under normal conditions is primarily governed by the variation in water level and temperature. For the studied dams, these effects are significantly larger than the influence from damage and degradation. Ice load measurements and simulations show that ice loads varies significantly along the dam. This variation makes it difficult to quantify the impact of external factors on the magnitude of ice loads. Despite this difficulty, three independent analyses show that ice thickness, water level change, and the dam's properties have an evident effect on the magnitude of ice loads. If current guidelines are to be updated to consider local conditions at the dam, these three parameters should be included. Ice loads of the magnitudes measured and specified in the current guidelines should have a notable impact on the behaviour of a dam during normal operation. However, such an impact has not been found in the eight dams studied within this project. This result indicates that the ice loads measured locally do not necessarily represent the global ice load that acts on the entire structure.
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| 4. |
- Hellgren, Rikard, 1988-
(författare)
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Condition assessment of concrete dams in cold climate
- 2019
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Dams in many countries are approaching their expected service life. Proper assessment of the aging dams structural health increase the knowledge of the current safety, and allow for better planning of renovation and rebuilding investments. The behavior of concrete dams is, to a great extent, governed by the ambient variation in temperature and water level. In cold regions, the ice sheet formed in the reservoir may subject a pressure load on the dams. Theoretically, this load has a significant impact on the structural behavior of dams. Despite this, the maximum magnitude, as well as the seasonal variation of the ice load, constitute the most considerable uncertainty in the safety assessment of dams.This thesis presents research that examines how to model the expected behavior of dams in cold climate. The underlying problem is to predict the response of dams due to variation in the external conditions. Since the ice load is such a vital part of the external conditions in cold climate, the understanding and modeling of ice loads have been given extra attention. Models suitable to predict the long-term behavior of dams can be divided between theoretical, data-based, and hybrid. Prediction accuracy is essential to set alert thresholds, and in that regard, the data-based models are generally superior.The major contribution of this thesis is the design and installation of a prototype ice load panel with direct measurement of the ice pressure acting on a dam. The panel is attached on the upstream face of the dam and is large enough so that the whole thickness of the ice sheet is in contact with the panel. The predicted ice load from the best available model that includes loads from both thermal events and water level changes did not correspond to the measured ice loads. As there are no validated models or measurement methods for ice load on the dam, continued research is necessary, both through further measurements to increase knowledge and development of models.
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| 5. |
- Schönbeck, Pia
(författare)
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Data based configuration decisions in hospital construction projects
- 2022
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The purpose of hospital buildings is to enable the diagnosis, treatment and caring of patients. Hospital configurations are therefore complex and deficient functionality at the final delivery of construction projects are a known issue. Configuration management is essential to ensure that all required functions are fulfilled. Digital building model data can provide the necessary information to support management of hospital configurations. Data based decisions are dependent on a baseline that contains all required functions. Thereafter, connections between data regarding functions, solutions and production methods should be established. This knowledge provides opportunities for automated data analyses giving real time accurate configuration information, enabling the prevention of deviations from the client’s intent and streamlining processes. The studies in this compilation thesis investigate this subject from different perspectives. Literature reviews, case studies and quantitative analyses of digital building data were the methods used. The results show that the systematic management of data and information can ensure hospital configurations that fulfil required functions at handover. These studies provide a conceptual model showing how configuration information applies to the construction project processes. The proposed detailed decision model shows how data from building models can support hospital configuration decisions in construction projects. Studies show that hospital construction projects management of configuration changes information is deficient. Manually managed logs contained insufficient information about the changes, while the building model data unused by the investigated projects was significantly more reliable. A risk analysis shows that data based configuration decisions in construction projects can prevent hospital building deficiencies, or facilitate recovery if they have already occurred. Digital technologies can improve the accuracy of configuration information, on which automated and industrial production equipment are dependent. However, construction project research seldom addresses the potential synergy and sustainability effects of different technologies. All case studies performed show that the current configuration information management in hospital projects is incomplete. Consequences found were cost increase, delays, reduced client value and corrective rework. The overall conclusion is that data based decisions can improve the control over hospital configurations, ensuring healthcare functions, construction project outcomes and sustainability. This thesis identifies a new research field by showing the potential of data based configuration decisions.
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| 6. |
- Sjölander, Andreas, 1983-
(författare)
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Analyses of shotcrete stress states due to varying lining thickness and irregular rock surfaces
- 2017
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Shotcrete is sprayed concrete applied pneumatically under high pressure and was invented in the beginning of the 1900's. This new technique decreased the construction time and since steel fibres were introduced in the shotcrete during the 1970's, shotcrete has been the primary support method for tunnels.Tunnels excavated with the drill and blast method creates a highly irregular rock surface which results in a shotcrete lining with varying thickness. The structural behaviour as well as the loads acting on the shotcrete lining depends on the interaction between the shotcrete, rock and rock bolts. There are several parameters influencing this interaction, e.g. bond strength, the stiffness of the rock and thickness of the shotcrete. All of these parameters are difficult to predict accurately which makes the structural design of the lining to a complex problem.This thesis present the first part of a research project with the long-term goal to improve the understanding of the structural behaviour of the shotcrete lining. To achieve this, numerical modelling have been used to study the build up of stresses and cracking of shotcrete when subjected to restrained loading caused by e.g. temperature differences and drying shrinkage. The response in the lining when subjected to a gravity load from a block has also been studied. The model is capable of describing the non-linear deformation behaviour of both plain and fibre reinforced shotcrete and uses presented in situ variations in thickness to more accurately account for the effects of expected variations in thickness. The thesis discuss and demonstrate the effect of important loads that acts on the shotcrete lining and how the irregular geometry of the rock surface in combination with the varying thickness of the shotcrete affect the development of stresses in the lining. It is also discussed how a full or partial bond failure affect the structural capacity of a shotcrete lining.
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| 7. |
- Abbasiverki, Roghayeh
(författare)
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Numerical modelling considerations for analysis of concrete hydraulic structures subjected to high-frequency seismic loads
- 2023
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Concrete hydraulic structures are of great importance in today's society. When situated in areas with hard bedrock, these structures may become extra vulnerable to seismic excitations as these here are dominated by high-frequency vibrations which can have disastrous consequences for slender structures. The aim of this thesis was to investigate special considerations that must be made when conducting analyses of such hydraulic structures during high-frequency excitations. Underground and on the ground structures were investigated separately. Underground concrete pipelines and concrete buttress dams were selected for the study because their behaviour when exposed to seismic excitations is dominated by their stiffness. The most effective models and modelling methods for the seismic analyses of such structures were implemented and evaluated. Two-dimensional finite element (FE) models were developed for the dynamic analysis of underground concrete pipelines loaded by seismic waves propagating from bedrock through soil. The interaction between the bedrock and the surrounding soil was investigated with respect to rock geometry and soil properties. The surface of dam foundations is commonly irregular, resulting in nonuniform motions at the dam-foundation interface. The free-field modelling methods for concrete dam foundations were adapted in order to accurately describe the propagation of earthquake vibrations from the source to the ground surface. The implementation of a threedimensional FE model for concrete buttress dams was investigated. Two different methods for free-field modelling are presented, which can be implemented independently of the software used. The seismic loads are applied as effective earthquake forces at non-reflecting boundaries. In the first method, the free-field motions at the non-reflecting boundaries are determined by the so-called domain reduction method using the direct FE calculation. In the second method, the free-field motions are analytically determined based on the onedimensional wave propagation theory. The results are also compared with the massless foundation modelling approach, in which the topographical amplifications are neglected. It was demonstrated that a two-dimensional model can effectively account for pipeline behaviour. The most important aspect of the models is the ability to capture bending deformations, as segmented structures such as pipelines are vulnerable in this respect. Nonuniform bedrock reduces the safety of concrete pipeline, especially because of bending deformations in the pipe and joints. The massless method gave unreliable results for analyses of dams, especially for high-frequency excitations. The analytical method was also unreliable in estimating the non-linear behaviour of the dams. But, a new time domain deconvolution method was developed to transform the earthquake motion from the foundation surface to the corresponding input motion at depth. It wasfound that free-field modelling of foundations using the direct FE method can accurately capture the topographic amplifications of the seismic excitations. It was shown that a three-dimensional model is required for seismic evaluation of concrete buttress dams. The topographic amplification of high-frequency waves at the surface of canyons had a significant effect on the response of this type of dam.
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| 8. |
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| 9. |
- Bryne, Lars Elof, 1969-
(författare)
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Time dependent material properties of shotcrete for hard rock tunnelling
- 2014
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- In this thesis different mechanical properties for shotcrete (sprayed concrete) such as compression strength, bond strength, bending tensile strength, elastic modulus, free and restrained shrinkage as a function of its age was investigated. One of the main issues was to investigate the difference between ordinary cast concrete and shotcrete. Reliable material data for young and hardening shotcrete is scarce which in the past have made such comparisons difficult. Also, less accurate data representative for cast concrete has often been used in numerical modelling and design analyses. The focus of the project has particularly been on the properties bond strength and restrained shrinkage for which two new testing methods has been developed and evaluated. Microstructural studies have also been performed as a complement to the bond strength testing.The bond to rock is one of the most important properties for shotcrete used as rock reinforcement. During the very first time after spraying the physical properties and the bond to the rock depend on the set accelerator and the micro structure that is formed. The investigation of early age bond strength of shotcrete is of great importance both from a production perspective and a safety perspective. The newly developed method was tested and evaluated and proved that it can be used for bond strength testing already from a couple of hours after shotcreting. The bond, or adhesion, depends on several factors such as texture of the rock, the type of accelerator, application technique, etc. In this work the development of the microstructure in the interfacial transition zone (ITZ) and strength of the bond was investigated. The results show that the bond strength is related to the hydration process, i.e. the strength gain of the shotcrete. The early development of the ITZ was here studied using a scanning electron microscope (SEM) making it possible to observe changes over time, before and after proper cement hydration.Restrained shrinkage cracking of shotcrete, especially in the case of shotcrete sprayed on soft drains that are parts of a tunnel lining not continuously bonded to the rock, can be detrimental for the sustainability of an infrastructure tunnel system. Maintenance and repair costs can be high over time. It is shown that the developed test method realistically captures the behaviour of shotcrete drains on hard rock in situ. The method can be used in the evaluation of different technical solutions for avoiding or minimizing shrinkage cracks in shotcreted soft drains. It can also be used to assess the performance of shotcrete fully bonded to a rock surface, with respect to the ability to prevent cracking or to distribute possible shrinkage damage into several fine cracks instead of one wide.
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| 10. |
- Enzell, Jonas
(författare)
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Toward Realistic Failure Evaluations for Concrete Buttress Dams
- 2023
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Concrete dams, complex structures supporting massive loads, have traditionally been assessed using simplified 2D analytical stability analyses based on the rigid body assumption. Previous studies have shown that 3D behavior, such as the interaction between the monoliths and the valley's geology, can greatly impact the load-bearing capacity of gravity dams but remains largely unexplored in buttress dams. Internal failure modes have also been shown to impact the load-bearing capacity and failure modes of concrete dams. The dam breach geometry and breach development time are important factors for flooding simulations used for emergency plans. There are no available methods for estimating the breach parameters for concrete dams. Instead, they are usually assumed based on simplified national recommendations, which introduces large uncertainties in the analysis. Thus, developing methods to estimate failure behavior in concrete gravity and buttress dams could significantly enhance flood simulation accuracy.This licentiate thesis aims to develop more realistic analysis methods for determining the load-bearing capacity and failure behavior of concrete buttress dams. To achieve this aim, studies using physical model tests were conducted to determine the 3D effects of the boundary conditions and the interaction between the monoliths and verify the results from finite element simulations. Numerical studies were performed to examine the failure behavior of concrete buttress dams and to determine suitable methods for such simulations. The results from the physical model tests suggest that 3D effects significantly impact the load-bearing capacity and the failure behavior of concrete buttress dams. Therefore, the entire dam should be considered in stability analyses rather than just single monoliths. The numerical studies showed that finite element models could successfully simulate dam failures, including the 3D behavior of concrete buttress dams and internal failure modes. However, there remain questions about the best methods for representing phenomena such as first-order roughness, valley shape, and fracture planes in these models.The model tests showed that while dam failures can occur abruptly with little to no initial signs of displacement, the presence of rough foundations, cohesion, and rock-strengthening measures in real-world dams suggests actual dam failures may not be as sudden. The results helped establish knowledge in the field to potentially create better alarm limits for automatic monitoring systems.
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| 11. |
- Eriksson, Daniel, 1987-
(författare)
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Numerical models for degradation of concrete in hydraulic structures due to long-term contact with water
- 2018
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The durability of concrete is of major concern in all types of concrete structures where the combined effect of exposure conditions and the type and quality of the concrete material usually determines the rate of degradation. Furthermore, there are synergy effects between different deterioration mechanisms, which means that the combined rate of degradation is higher than the sum of the individual rates of each mechanism. Therefore, to accurately predict the residual service life of existing structures or when designing new structures, it is essential to consider all these aspects. This means that various chemical and physical processes, as well as how these interact, must be taken into account in models aiming to be used for service life predictions.This thesis presents the first part of a research project with the aim to investigate common deterioration mechanisms of concrete in hydraulic structures, and to improve the knowledge how these and other related phenomena can be described using mathematical models. The objective is also to study how different mechanisms interact and to find suitable approaches to account for these interactions in the models. To this end, a literature survey on commonly detected damage in hydraulic structures is presented. In addition, it also addresses in what types of and where in hydraulic structures the various damage types are usually observed. The mathematical models presented in this part of the project are focused on long-term water absorption in air-entrained concrete as well as on freezing of partially saturated air-entrained concrete. Both models are based on a multiphase description of concrete and poromechanics to describe the coupled hygro-thermo-mechanical behaviour. The thesis also presents some of the basic concepts of multiphase modelling of porous media, including discretization of the models using the finite element method (FEM). Furthermore, it covers the simplifications that are usually introduced in the general macroscopic balance equations for mass, energy and linear momentum when modelling cement-based materials.To verify the developed models and to show their capabilities, simulation results are compared with experimental data, in situ measurements and other simulations from the literature. The results indicate that both models perform well and can be used to predict long-term moisture conditions in hydraulic structures as well as freezing-induced strains in partially saturated air-entrained concrete, respectively. Even though no interactions with other deterioration mechanisms are included in the models, the development and use of these have given insights to which parameters that are important to consider in such extensions. Furthermore, based on the insights gained, the complexity of describing the full interactions between several mechanisms in mathematical models is also discussed. It is concluded that models aiming to be used for service life predictions of hydraulic structures in day-to-day engineering work need to be simplified. However, the type of advanced models presented in this thesis can serve as a basis to study which aspects and parameters that are essential to consider in simplified prediction models.
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| 12. |
- Li, Shicheng
(författare)
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CFD and data-driven modeling for safe spillway discharge
- 2023
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- A spillway is a crucial dam structure that regulates water levels and prevents the dam from overtopping. The majority of the dams in the world were constructed before the 1980s. Compared to the current design flood standards, many spillways are undersized, e.g., in Sweden. The higher floods also change the air-water flow properties, giving rise to risks in spillway operations. High-velocity flow is a major concern in spillway design. If the flow rate exceeds 20 m/s, there is a risk of cavitation. Another issue connected with the increased floods is energy dissipation. Many existing energy-dissipating arrangements are insufficient or constructed only for a design flood standard at the time of dam construction. To this end, it is essential to assess the discharge behaviors of spillways, including discharge capacity, two-phase flow features, energy dissipation, cavitation, etc. Engineering solutions for improved hydraulic performance are also critical to ensure safe operation.This thesis focuses on CFD and data-driven modeling for safe spillway discharge, presenting new insights into spillway hydraulics, improved methods for accurate flow predictions, and structural modifications for better hydraulic performance. The research consists of five parts. (1) For discharge estimation, ensemble learning models are established to forecast the river flows, aiming to provide accurate inflow information, even multiple days ahead. To directly assess spillway discharge capacity, three data-driven models are constructed, which yield accurate flow predictions and outperform empirical correlations. (2) For modified designs, several unconventional step layouts are conceived, with their hydraulic behaviors investigated. The modified configurations include steps with chamfers and cavity blockages and inclined steps. The goal is to devise engineering solutions to enhance energy dissipation and reduce cavitation risks. (3) For improved two-phase flow modeling, a population balance model is first established to account for the evolutionary process of air bubbles. Then machine learning techniques are integrated for error correction. The resulting model generates accurate results, with 99.6% and 89.6% of the velocity and air concentration predictions within a 20% relative error range. (4) For cavitation reduction, an aerator is often used to artificially entrain air into the flow. This part develops an embedded multi-gene genetic programming model for accurate predictions of aerator air demand. Compared with the best empirical formula, the proposed approach improves the fitness indexes by 23% and reduces the errors by 48%. (5) This part reviews the theoretical basis and deals with several aspects of physical scale modeling, supported exclusively by field measurements. For proper upscaling, model-prototype comparisons indicate that the approach flow velocity at the aerator should exceed the 7.00–7.50 m/s threshold, or the Reynolds number should be above 1.58 ×106. If this condition was not met, attempts to scale up the air flow would lead to errors.
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| 13. |
- Li, Shicheng
(författare)
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Numerical simulations of flow discharge and behaviours in spillways
- 2021
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- A spillway is an important component of a dam and serves as a flood release structure. It achieves controlled discharge of water and protects the dam from overtopping. The majority of the hydropower dams were built before the 1980s, and many spillways are undersized in light of the present design flood guidelines. Another issue that arises in connection with the high design floods is the energy dissipation capacity. Many existing energy-dissipating arrangements are insufficient or construed only for a design flood standard at the time of dam construction. The increment in the flood discharges requires that the energy dissipation should be improved to obtain sufficient capacity or higher efficiency. In addition, the high-velocity flow is a major concern in the design of spillways. If the flow velocity exceeds approximately 20 m/s, the risk of cavitation may arise. In Sweden, many dams belong to this category. To address these issues, an assessment of their discharge behaviours is required. Innovative engineering solutions for better energy dissipation and cavitation mitigation are also necessary for safe operation. This thesis presents machine learning based methods for discharge estimation. Three data-driven models are developed to study the discharge behaviours of the overflow weirs. Their reliability is validated through the comparison with the experimental and empirical results. These models are capable of giving accurate predictions and show superiority over the conventional approaches. With high accuracy and adaptability, data-driven models are an effective and fast alternative for spillway discharge prediction. This research also focuses on the hydraulic design of stepped spillways, aiming to devise innovative engineering solutions to enhance energy dissipation and reduce cavitation risks. Consequently, several unconventional step layouts are conceived and their hydraulic behaviours are investigated. The modified configurations include steps with chamfers and cavity blockages, non-linear steps and inclined steps. This part attempts to gain insight into the effects of the step geometries on the spillway hydraulics via computational fluid dynamics, which provides references for engineering applications.
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| 14. |
- Magnusson, Johan, 1967-
(författare)
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Shear in Concrete Structural Elements Subjected to Dynamic Loads
- 2019
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Concrete structural elements subjected to severe dynamic loads such as explosions at close range may cause shear failures. In the Oklahoma City bombing in 1995 two concrete columns on the ground level were reported to have failed in shear. Such shear failures have also been reported to occur in several experimental investigations when concrete beams and slabs were subjected to blast or impact loads. The dynamic shear mechanisms are not yet fully understood and it is therefore of research significance to further investigate these mechanisms. The main objective of the research presented in this thesis is to experimentally and theoretically analyse shear failures of reinforced concrete elements subjected to uniformly distributed dynamic loads.The experimental work consisted of concrete beams of varying concrete grades and reinforcement configurations subjected to blast loads. One series involved testing of steel fibre reinforced concrete (SFRC) beams and the other series involved tests with concrete beams reinforced with steel bars. The former investigation showed that SFRC beams can resist certain blast loads. In the latter investigation, certain beams subjected to blast loads were observed to fail in flexural shear while the same beams exhibited flexural failures in the static tests. Such shear failures specifically occurred in beams with relatively high reinforcement contents. With these experiments as reference, numerical simulations with Ansys Autodyn were performed that demonstrated the ability to predict flexural shear failures.A direct shear failure mode has also been observed in experiments involving concrete roofs subjected to intense distributed blast loads. In several cases, the roof slabs were completely severed from their supporting walls along vertical or near-vertical failure planes soon after the load had been applied. Theoretical analyses of the initial structural response of beams subjected to distributed loads were conducted with the use of Euler-Bernoulli beam theory and numerical simulations in Abaqus/Explicit. These analyses show that the initial structural response consists of shear stresses and bending moments developing at the supports. The remaining parts of the beam will be subjected to a rigid body motion. Further simulations with Abaqus shows that that dynamic direct shear failure appears to be due to a deep beam response with crushing of the compressive struts at the supports, and therefore differs from a static direct shear mode. The results also showed that parameters such as element depth, amount of reinforcement, load level and load duration played a role in developing a dynamic direct shear failure.
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| 15. |
- Malm, Richard, 1980-
(författare)
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Predicting shear type crack initiation and growth in concrete with non-linear finite element method
- 2009
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- 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.
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| 16. |
- Schönbeck, Pia
(författare)
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Use of configuration information in construction projects to ensure high performance of healthcare facilities
- 2020
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The transfer of information between the different phases of construction projects is limited, which complicates the quality assurance of configurations. Functional requirements on the building configuration are often ambiguous and lack measurable acceptance criteria. Vaguely described criteria provide an insufficient basis for the design and verification acceptance values. The contractor is often the first to consider the feasibility just before or during the production phase. However, feasibility including production methods should be considered with the establishment of each design solutions. Costs or time consequences are often the basis for change decisions, while the investigations of the impact on the building configuration and functionality are insufficient. A continuous flow of information with functional requirements as a controlling factor is required to control configurations. Functionality that ensure high quality patient care are essential in health care facilities, therefore the control of the configurations are particularly important.This licentiate thesis shows how configuration information in healthcare construction projects can ensure necessary building functionality. The following information areas are identified as necessary for control of configurations: function, verification, design solution, production and change control. All information is related and changes in one area affect the others, directly or indirectly during construction projects. The studies that are the basis of this licentiate thesis show that the management of configuration information was deficient in healthcare construction projects, especially regarding availability and interconnectivity. In addition, the information was not available at the right time during the construction process, which impaired configuration control. For example, verification methods were not available until the end of production. This precluded verification of intended functionality through large parts of the construction project. Detailed change information from digital models of buildings can significantly improve control of configurations, but this requires standardisation of input data.In the manufacturing industry, configuration management is practised to ensure that products fulfil the required functions throughout their entire lifecycle. Development of new technologies, such as digital processes and industrialised construction, require that construction projects develop working processes similar to those of the manufacturing industry. The risk of unnecessary rework with subsequent cost increases, delays and environmental impact decrease with increased control of the configuration. In construction projects, systematic management of configuration information can ensure delivery of healthcare facilities with intended functionality.
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| 17. |
- Sjölander, Andreas, 1983-
(författare)
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Structural behaviour of shotcrete in hard rock tunnels
- 2020
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Tunnels in hard and jointed rock are normally excavated in an arch shape to enable the rock mass to support its weight. Since the beginning of the 1980's, fibre reinforced shotcrete (FRS) in combination with rock bolts have been the dominating support method for hard rock tunnels. This type of rock support is a complex composite structure in which the structural behaviour depends on interaction between shotcrete, rock and bolts. The design is commonly based on a rock mass classification system in combination with analytical solutions or finite element (FE) modelling. However, the in-situ variations of important properties of the shotcrete are normally neglected.The aim of this thesis is to describe and explain how the variations in shotcrete thickness and bond strength affect the structural behaviour and capacity for a shotcrete lining. Especially, the influence of local variations in shotcrete thickness and bond strength has been studied in detail. For this purpose, a numerical framework capable of simulating bond failure, cracking of FRS and pull-out failure of grouted rock bolts have been developed. Moreover, in-situ data for shotcrete thickness and bond strength have been collected and analysed to characterize the variations in important shotcrete parameters.The results in this thesis show that when shotcrete is subjected to shrinkage, local variations in shotcrete thickness affects the crack pattern. However, the number and width of the cracks are similar to the case with uniform thickness. Most importantly, a pattern of fine and narrow cracks develops in unreinforced shotcrete subjected to shrinkage when a continuous bond to the rock exists. When shotcrete is subjected to the load from a loose block, the force is transferred to the surrounding rock through bond stresses distributed over a narrow band. Simulations have shown that the structural capacity, with respect to bond failure, depends on the shotcrete thickness. Moreover, a strong linear correlation was found between the mean value of the bond strength and shotcrete thickness around the perimeter of the block and the structural capacity. Local weak areas, i.e. with low bond strength or thickness, may exist around the perimeter without having a significant effect on the structural capacity. Design of bolt-anchored shotcrete linings is based on failure modes previously derived from experimental testing. This thesis has contributed to an increased understanding of the failure mechanisms of the lining and has confirmed that the design can be based on individual failure mechanisms.
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| 18. |
- Abbasiverki, Roghayeh
(författare)
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Analysis of underground concrete pipelines subjected to seismic high-frequency loads
- 2016
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Buried pipelines are tubular structures that are used for transportation of important liquid materials and gas in order to provide safety for human life. During an earthquake, imposed loads from soil deformations on concrete pipelines may cause severe damages, possibly causing disturbance in vital systems, such as cooling of nuclear power facilities. The high level of safety has caused a demand for reliable seismic analyses, also for structures built in the regions that have not traditionally been considered as highly seismically active. The focus in this study is on areas with seismic and geological conditions corresponding to those in Sweden and Northern Europe. Earthquakes in Sweden for regions with hard rock dominated by high-frequency ground vibrations, Propagation of such high-frequency waves through the rock mass and soil medium affect underground structures such as pipelines.The aim of this project is investigating parameters that affect response of buried pipelines due to high-frequency seismic excitations. The main focus of the study is on reinforced concrete pipelines. Steel pipelines are also studied for comparison purposes. The effects of water mass, burial depth, soil layer thickness and non-uniform ground thickness caused by inclined bedrock are studied. The results are compared to those obtained for low-frequency earthquakes and the relationship between strong ground motion parameters and pipelines response is investigated. It is shown that, especially for high frequency earthquake excitations, non-uniform ground thickness due to inclined bedrock significantly increase stresses in the pipelines. For the conditions studied, it is clear that high-frequency seismic excitation is less likely to cause damage to buried concrete pipelines. However, the main conclusion is that seismic analysis is motivated also for pipelines in high-frequency earthquake areas since local variation in the ground conditions can have a significant effect on the safety.
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| 19. |
- Ahmed, Lamis
(författare)
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Models for analysis of shotcrete on rock exposed to blasting
- 2012
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- In underground construction and tunnelling, the strive for a more time-efficient construction process naturally focuses on the possibilities of reducing the times of waiting between stages of construction. The ability to project shotcrete (sprayed concrete) on a rock surface at an early stage after blasting is vital to the safety during construction and function of e.g. a tunnel. A complication arises when the need for further blasting affects the hardening of newly applied shotcrete. If concrete, cast or sprayed, is exposed to vibrations at an early age while still in the process of hardening, damage that threatens the function of the hard concrete may occur. There is little, or no, established knowledge on the subject and there are no guidelines for practical use. It is concluded from previous investigations that shotcrete can withstand high particle velocity vibrations without being seriously damaged. Shotcrete without reinforcement can survive vibration levels as high as 0.5−1 m/s while sections with loss of bond and ejected rock will occur for vibration velocities higher than 1 m/s. The performance of young and hardened shotcrete exposed to high magnitudes of vibration is here investigated to identify safe distances and shotcrete ages for underground and tunnelling construction, using numerical analyses and comparison with measurements and observations. The work focuses on finding correlations between numerical results, measurement results and observations obtained during tunnelling. The outcome will be guidelines for practical use. The project involves development of sophisticated dynamic finite element models for which the collected information and data will be used as input, accomplished by using the finite element program Abaqus. The models were evaluated and refined through comparisons between calculated and measured data. First, existing simple engineering models were compared and evaluated through calculations and comparisons with existing data. The first model tested is a structural dynamic model that consists of masses and spring elements. The second is a model built up with finite beam elements interconnected with springs. The third is a one-dimensional elastic stress wave model. The stress response in the shotcrete closest to the rock when exposed to P-waves striking perpendicularly to the shotcrete-rock interface was simulated. Results from a non-destructive laboratory experiment were also used to provide test data for the models. The experiment studied P-wave propagation along a concrete bar, with properties similar to rock. Cement based mortar with properties that resembles shotcrete was applied on one end of the bar with a hammer impacting the other. The shape of the stress waves travelling towards the shotcrete was registered using accelerometers positioned along the bar. Due to the inhomogeneous nature of the rock, the stress waves from the blasting attenuate on the way from the point of explosion towards the shotcrete on the rock surface. Material damping for the rock mass is therefore accounted for, estimated from previous in-situ measurements. The vibration resistance of the shotcrete-rock support system depends on the material properties of the shotcrete and here were age-dependent properties varied to investigate the behaviour of young shotcrete subjected to blast loading. The numerical simulations require insertion of realistic material data for shotcrete and rock, such as density and modulus of elasticity. The calculated results were in good correspondence with observations and measurements in-situ, and with the previous numerical modelling results. Compared to the engineering models, using a sophisticated finite element program facilitate modelling of more complex geometries and also provide more detailed results. It was demonstrated that wave propagation through rock towards shotcrete can be modelled using two dimensional elastic finite elements in a dynamic analysis. The models must include the properties of the rock and the accuracy of the material parameters used will greatly affect the results. It will be possible to describe the propagation of the waves through the rock mass, from the centre of the explosion to the reflection at the shotcrete-rock interface. It is acceptable to use elastic material formulations until the material strengths are exceeded, i.e. until the strains are outside the elastic range, which thus indicates material failure. The higher complexity of this type of model, compared to the engineering models, will make it possible to model more sophisticated geometries. Examples of preliminary recommendations for practical use are given and it is demonstrated how the developed models and suggested analytical technique can be used to obtain further detailed limit values.
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| 20. |
- Ahmed, Lamis
(författare)
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Models for analysis of young cast and sprayed concrete subjected to impact-type loads
- 2015
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The strive for a time-efficient construction process naturally put focus on the possibility of reducing the time of waiting between stages of construction, thereby minimizing the construction cost. If recently placed concrete, cast or sprayed, is exposed to impact vibrations at an early age while still in the process of hardening, damage that threatens the function of the hard concrete may occur. A waiting time when the concrete remains undisturbed, or a safe distance to the vibration source, is therefore needed. However, there is little, or no, fully proven knowledge of the length of this distance or time and there are no established guidelines for practical use. Therefore, conservative vibration limits are used for young and hardening concrete exposed to vibrations from e.g. blasting.As a first step in the dynamic analysis of a structure, the dynamic loads should always be identified and characterized. Here it is concluded that impact-type loads are the most dangerous of possible dynamic loads on young and hardening concrete. Shotcrete (sprayed concrete) on hard rock exposed to blasting and cast laboratory specimens subjected to direct mechanical impact loads have been investigated using finite element models based on the same analysis principles. Stress wave propagation is described in the same way whether it is through hard rock towards a shotcrete lining or through an element of young concrete. However, the failure modes differ for the two cases where shotcrete usually is damaged through loss of bond, partly or over larger sections that may result in shotcrete downfall. Cracking in shotcrete due to vibrations only is unusual and has not been observed during previous in situ tests. The study of shotcrete is included to demonstrate the need of specialized guidelines for cases other than for mass concrete, i.e. structural elements or concrete volumes with large dimensions in all directions.Within this project, work on evaluating and proposing analytical models are made in several steps, first with a focus on describing the behaviour of shotcrete on hard rock. It is demonstrated that wave propagation through rock towards shotcrete can be described using two-dimensional elastic finite element models in a dynamic analysis. The models must include the material properties of the rock and the accuracy of these parameters will greatly affect the results. It is possible to follow the propagation of stress waves through the rock mass, from the centre of blasting to the reflection at the shotcrete-rock interface. It is acceptable to use elastic material formulations until the strains are outside the elastic range, which thus indicates imminent material failure. The higher complexity of this type of model, compared with mechanical models using mass and spring elements, makes it possible to analyse more sophisticated geometries. Comparisons are made between numerical results and measurements from experiments in mining tunnels with ejected rock mass and shotcrete bond failure, and with measurements made during blasting for tunnel construction where rock and shotcrete remained intact. The calculated results are in good correspondence with the in situ observations and measurements, and with previous numerical modelling results. Examples of preliminary recommendations for practical use are given and it is demonstrated how the developed models and suggested analytical technique can be used for further detailed investigations.The modelling concept has also been used for analysis of impact loaded beams and concrete prisms modelled with 3D solid elements. As a first analysis step, an elastic material model was used to validate laboratory experiments with hammer-loaded concrete beams. The laboratory beam remained un-cracked during the experiments, and thus it was possible to achieve a good agreement using a linear elastic material model for fully hardened concrete. The model was further developed to enable modelling of cracked specimens. For verification of the numerical results, earlier laboratory experiments with hammer impacted smaller prisms of young concrete were chosen. A comparison between results showed that the laboratory tests can be reproduced numerically and those free vibration modes and natural frequencies of the test prisms contributed to the strain concentrations that gave cracking at high loads. Furthermore, it was investigated how a test prism modified with notches at the middle section would behave during laboratory testing. Calculated results showed that all cracking would be concentrated to one crack with a width equal to the sum of the multiple cracks that develop in un-notched prisms. In laboratory testing, the modified prism will provide a more reliable indication of when the critical load level is reached.This project has been interdisciplinary, combining structural dynamics, finite element modelling, concrete material technology, construction technology and rock support technology. It is a continuation from previous investigations of the effect on young shotcrete from blasting vibrations but this perspective has been widened to also include young, cast concrete. The outcome is a recommendation for how dynamic analysis of young concrete, cast and sprayed, can be carried out with an accurate description of the effect from impact-type loads. The type of numerical models presented and evaluated will provide an important tool for the work towards guidelines for practical use in civil engineering and concrete construction work. Some recommendations on safe distances and concrete ages are given, for newly cast concrete elements or mass concrete and for newly sprayed shotcrete on hard rock.
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| 21. |
- Ansell, Anders, et al.
(författare)
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Concrete Structures
- 2012
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Rapport (övrigt vetenskapligt/konstnärligt)
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| 22. |
- Gasch, Tobias
(författare)
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Concrete as a multi-physical material with applications to hydro power facilities
- 2016
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- During its lifetime, a concrete structure is subjected to many different actions, ranging from mechanical loads to environmental actions. To accurately predict its integrity from casting and throughout its service life, a modelling strategy is required that considers mechanical loading but also implicitly accounts for physical effects such as temperature and moisture variations. This is especially true for large concrete structures found in many infrastructure applications such as bridges, nuclear power plants and dams. Modelling concrete as a multi-physical material is becoming an increasingly used approach for which large research efforts are being made, including the development of more refined mathematical and numerical methods as well as considering more physical and chemical variables in the coupled model.The research project, of which this licentiate thesis is the first phase, aims at investigating aging concrete structures at hydro power facilities, with focus on the internal structures of the power plants. This thesis presents a review of advanced mathematical methods and concepts for modelling aging concrete found in the literature which can later be applied to study such structures. The focus is on models that describe the deformational behaviour of concrete where aspects such as aging, cracking, creep and shrinkage are investigated. However, in order to accurately describe such phenomena, a multi-physical approach is adopted where moisture and temperature variations in the concrete are studied. Also, models that describe the chemical behaviour related to hydration and thus in extension aging, are also reviewed and introduced in the multi-physical framework. The use of such models are discussed in the context of the finite element method (FEM), in which coupled models are implemented, verified and applied in the appended papers using two different FE codes.Several verification examples are presented covering different aspects of the implemented models, both in isolation and coupled in a multi-physical setting. By comparing the numerical results with experimental data from the literature it can be shown that it is possible to predict most aspects of aging concrete that have been of interest here. While these examples are all on a laboratory scale, numerical examples and case studies are also provided that exemplify how the models can be applied on a structural scale. By using the developed analysis tools, valuable information and insights can be gained on aging concrete structures and these tools will in the next phase of the research project be applied to large concrete structures at hydro power facilities.
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| 23. |
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| 24. |
- Johansson, Morgan, et al.
(författare)
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Explosionslast och dynamisk strukturrespons i armerade betongkonstruktioner
- 2023
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Ingår i: Bygg och Teknik. - 0281-658X .- 2002-8350. ; 115:6
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Tidskriftsartikel (populärvet., debatt m.m.)abstract
- Med syfte att frigöra attraktiv markyta för ny bebyggelse finns det imånga svenska städer idag ett önskemål att förtäta den befintliga stadsmiljön. En sådan åtgärd kan dock resultera i minskade avstånd mellan byggnader och transportled, vilket kan medföra ökade krav med hänsyn till bland annat explosioner. För att skapa en optimal helhetslösning för sådana olycksscenarion behöver de tre delområdena riskhantering, explosionslast och strukturrespons samordnas på ett korrekt sätt. I dagens explosionshantering finns dock betydande brister bland både riskanalytiker och konstruktörer, något som ofta beror på otillräcklig kommunikation och kunskap.
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| 25. |
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| 26. |
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| 27. |
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| 28. |
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| 29. |
- Magnusson, Johan
(författare)
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Structural concrete elements subjected to air blast loading
- 2007
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- In the design of structures to resist the effects of air blast loading or other severe dynamic loads it is vital to have large energy absorbing capabilities, and structural elements with large plastic deformation capacities are therefore desirable. Structures need to be designed for ductile response in order to prevent partial or total collapse due to locally failed elements. The research in this thesis considers experimental and theoretical studies on concrete beams of varying concrete strength. The nominal concrete compressive strength varied between 30 MPa and 200 MPa. A total of 89 beams were tested of which 49 beams were reinforced with varying amounts of tensile reinforcement. These beams were also reinforced with stirrups and steel fibres were added to a few beams. The remaining 40 beams were only reinforced with steel fibres with a fibre content of 1.0 percent by volume. Two different fibre lengths having constant length-to-diameter ratio were employed. The tests consisted of both static and air blast tests on simply supported beams. The blast tests were performed within a shock tube with a detonating explosive charge. All experimental research focused on deflection events, failure modes and loads transferred to the supports. The dynamic analyses involve single-degree-of-freedom (SDOF) modelling of the beam response and the use of iso-damage curves. Also, the dynamic support reactions were calculated and compared with test results. For beams with tensile reinforcement, the failure mode of some beam types was observed to change from a flexural failure in the static tests to a flexural shear failure in the dynamic tests. Beams with a high ratio of reinforcement and not containing steel fibres failed in shear, whereas beams with a lower ratio of reinforcement failed in flexure. The introduction of steel fibres prevented shear cracks to develop, thus increasing the shear strength of the beams. The presence of steel fibres also increased the ductility and the residual load capacity of the beams. Beams subjected to air blast loading obtained an increased load capacity when compared to the corresponding beams subjected to static loading. The SDOF analyses showed good agreement with the experimental results regardless of concrete strength and reinforcement amount. The results of using iso-damage curves indicate conservative results with larger load capacities of the beams than expected. The theoretical evaluations of the dynamic reactions were in agreement with the measured average reactions, both in amplitudes and in general shape. The experimental results with steel fibre reinforced concrete beams indicate that the dynamic strength was higher than the corresponding static strength and that the toughness was reduced when increasing the compressive strength. Beams of normal strength concrete failed by fibre pull-out while a few beams of high strength concrete partly failed by fibre ruptures. It may be favourable to use shorter fibres with smaller aspect ratios in structural elements of high strength concrete and subjected to large dynamic loads. Further research should involve studies on the size effect, on different boundary conditions, on different types of structural elements and on the combination of blast and fragment loads. The theoretical work should involve analyses both with the use of SDOF modelling and finite element analysis.
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| 30. |
- Peterson, Viktor
(författare)
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Assessment of shear and energy‐absorption capacity of reinforced concrete elements under impulsive loads
- 2023
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Impulsive loads have been observed to cause brittle shear failure in reinforced concrete elements designed for ductile failure modes under static loads. Brittle failure modes exhibit poorer energy absorption capabilities compared to ductile flexural failure modes due to their limited deformation capacity, leading to premature failure. The discrepancy between the responses under static and extreme dynamic loads arises from inertia and wave propagation effects, which tend to increase as the load duration decreases relative to the fundamental period of the element. This thesis investigated the occurrence of shear failures in reinforced concrete elements subjected to impulsive loads, both experimentally and numerically, and evaluated to what extent current analysis methods for impulse-loaded structures can predict shear failure. Furthermore, the study examined the influence of crucial parameters on the energy absorption capacity during flexural failure modes when shear failure was inhibited. The results demonstrated that shear-plug damage, prevalent during impact loads, may lead to premature shear failure during sequential impact testing. This occurred for a statically flexure-critical beam with a significantly larger static flexural-shear capacity relative to its flexural capacity. Similar conclusions applied to the residual static capacity after an initial impact introduced shear-plug damage. These findings indicate potentially severe consequences of shear-plug damage, which should be considered when assessing structures damaged by impact loads. The energy absorption capacity of reinforced concrete elements is closely related to the plastic work capacity of the reinforcement. The experimental study showed how the plastic work capacity varied with reinforcement properties, concrete properties, and impact velocity using static and dynamic four-point flexural tests. The results revealed that the reinforcement type, specifically whether the steel is mild or stiff, governs the strain distribution during static and low-velocity impact testing. Generally, stiff steels result in strain localization before rupturing, indicating a lower plastic work capacity. Factors such as stress and strain capacity also proved significant. However, as the impact velocity increased, wave propagation effects governed strain distribution rather than reinforcement type. Numerical studies comparing results with outcomes using proposed design methods indicated agreement for support reactions used to verify the shear capacity in the later stages of the response. However, this agreement decreased in the initial stages of the response. This may be because the dynamic equilibrium method only considers a global response, while the local response due to wave propagation is influential in the initial stages of the response. Today, resources such as Biggs [8] and the Swedish Fortifications Agency [86] recommend using two stages of the response to determine the internal forces; an elastic global response and a later elastoplastic global response. From the observations in the papers, it is suggested to add a third initial stage of the response considering wave propagation effects. However, it is deemed that this response stage only has a significant effect for high-intensity blast loads with short rise times relative to the shear wave velocity.
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| 31. |
- Peterson, Viktor, et al.
(författare)
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Numerical simulations of shear reinforced RC beams subjected to blast loads
- 2022
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Ingår i: <em>fib Symposium</em>. - : fib. The International Federation for Structural Concrete.
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Konferensbidrag (refereegranskat)abstract
- The numerical simulations presented in the present paper were on beam models geometrically identical to the beams previously tested in a shock tunnel but now provided with increased shear reinforcement consisting of vertical stirrups. The tested beam failed in brittle flexural shear during the blast loading in the tunnel. The results from the numerical investigation showed that brittle shear failures of beams subjected to blast loads may be prevented by dense shear reinforcement. The guideline for impulse loaded structure showed to underestimate the dynamic shear capacity as it is not dependent on the actual transverse reinforcement amount. This is however considered in the model used for determining the static shear capacity, meaning that further work developing a similar model fit to impulse loading is advised. To realize this development a new test series is proposed.
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| 32. |
- Peterson, Viktor, et al.
(författare)
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On the Residual Static and Impact Capacity of Shear-Reinforced Concrete Beams Subjected to an Initial Impact
- 2022
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Ingår i: Applied Sciences. - : MDPI. - 2076-3417. ; 12:22, s. 11377-
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Tidskriftsartikel (refereegranskat)abstract
- Impact loads in previous research showed to induce brittle responses of statically flexure-critical reinforced concrete (RC) beams designed for ductility. The impact load may produce flexural shear damage modes similar to that observed during quasi-static loads and local shear damage under the impact zone. The occurrence of shear damage modes during impact tests has been investigated extensively, but their effect on the residual quasi-static and dynamic capacity is not fully understood. For this aim, an initial high-velocity impact test initiated severe shear damage to RC beams. The beams were then tested quasi-statically and by sequential impact testing using the same setup as the initial tests. The results indicate a flexure-dominated response during sequential impact tests for beams containing extreme shear reinforcement amounts, favouring the energy-absorption capacity. Significant shear and flexural damage occurred for beams with less shear reinforcement, indicating a hybrid response that varied throughout the tests. The tests for the residual quasi-static capacity indicated severe consequences from initial local shear damage on the capacity, as shown by the brittle response of the beam with the most shear reinforcement. However, wide initial flexural cracks instead showed a favourable effect, as there was an indication of transfer from brittle to ductile failure. For beams showing both global and local shear damage, it was concluded that global shear damage modes were critical for the residual static and dynamic shear capacity.
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| 33. |
- Peterson, Viktor, et al.
(författare)
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The effect of explosions in road tunnels on critical structural elements
- 2023
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Ingår i: Expanding Underground - Knowledge and Passion to Make a Positive Impact on the World. - : CRC Press/Balkema. ; , s. 3263-3271
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Konferensbidrag (refereegranskat)abstract
- In the present paper, the walls of a concrete frame used for over-decking in a road tunnel were first designed using the relevant design provisions. The response of the wall was then analysed using non-linear finite element analysis, and the results of the finite element analysis were compared to what was predicted using the design guidelines. Additionally, simulations of the blast load resulting from gas leakage during transportation were conducted as a separate study. The results from two vapour clouds containing gases of hydrogen and propane were compared.
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| 34. |
- Rydell, Cecilia, 1978-
(författare)
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Seismic high-frequency content loads on structures and components within nuclear facilities
- 2014
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Sweden is generally considered to be a low seismicity area, but for structures within nuclear power facilities, the safety level demand with respect to seismic events are high and thus, these structures are required to be earthquake-resistant. The seismic hazard is here primarily considered to be associated with near-field earthquakes. The nuclear power plants are further founded on hard rock and the expected ground motions are dominated by high frequencies. The design earthquake considered for the nuclear facilities has an annual probability of 10-5 events, that is, the probability of occurrence is once per 100 000 years. The focus of the study is the seismic response of large concrete structures for the nuclear power industry, with regard not only to the structure itself but also to non-structural components attached to the primary structure, and with emphasis on Swedish conditions. The aim of this licentiate thesis is to summarize and demonstrate some important aspects when the seismic load is dominated by high frequencies. Additionally, an overview of laws, regulations, codes, standards, and guidelines important for seismic analysis and design of nuclear power structures is provided.The thesis includes two case studies investigating the effect of seismic high-frequency content loads. The first study investigates the influence of gaps in the piping supports on the response of a steel piping system subjected to a seismic load dominated by high amplitudes at high frequencies. The gaps are found in the joints of the strut supports or are gaps between the rigid box supports and the pipe. The piping system is assessed to be susceptible to high-frequency loads and is located within the reactor containment building of a nuclear power plant. The stress response of the pipe and the acceleration response of the valves are evaluated. The second study investigates the effect of fluid-structure interaction (FSI) on the response of an elevated rectangular water-containing concrete pool subjected to a seismic load with dominating low and high frequencies, respectively. The pool is located within the reactor containment building of a boiling water reactor at a nuclear power plant. The hydrodynamic pressure distribution is evaluated together with the stress distribution in the walls of the tank.From the two case studies, it is evident that the response due to a seismic load dominated by high frequencies and low frequencies, respectively, is different. Although the seismic high-frequency load may be considered non-damaging for the structure, the effect may not be negligible for non-structural components attached to the primary structure. Including geometrical non-linear effects such as gaps may however reduce the response. It was shown that the stress response for most of the pipe elements in the first case study was reduced due to the gaps. It may also be that the inclusion of fluid-structure interaction effects changes the dynamic properties of a structural system so that it responds significantly in the high frequency range, thus making it more vulnerable to seismic loads dominated by high frequencies. In the second case study, it was shown that even for a seismic load with small amplitudes and short duration, but with dominating high-frequency content, as the Swedish 10-5 design earthquake, the increase of the dynamic response as fluid-structure interaction is accounted for is significant.
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