| 1. |
- 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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| 2. |
- Andersson, Andreas, 1980-
(författare)
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Capacity assessment of arch bridges with backfill : Case of the old Årsta railway bridge
- 2011
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The work presented in this thesis comprises the assessment of existing arch bridges with overlying backfill. The main objective is to estimate the load carrying capacity in ultimate limit state analysis. A case study of the old Årsta railway bridge is presented, serving as both the initiation and a direct application of the present research. The demand from the bridge owner is to extend the service life of the bridge by 50 years and increase the allowable axle load from 22.5 to 25 metric tonnes. The performed analyses show a great scatter in estimated load carrying capacity, depending on a large number of parameters. One of the factors of main impact is the backfill material, which may result a significant increase in load carrying capacity due to the interaction with the arch barrel. Based on theoretical analyses, extensive conditional assessments and the demand from the bridge owner, it was decided that the bridge needed to be strengthened. The author, in close collaboration with both the bridge owner and the persons performing the conditional assessment, performed the development of a suitable strengthening. The analyses showed a pronounced three-dimensional behaviour, calling for a design using non-linear finite element methods. Due to demands on full operability during strengthening, a scheme was developed to attenuate any decrease in load carrying capacity. The strengthening was accepted by the bridge owner and is currently under construction. It is planned to be finalised in 2012. The application of field measurements to determine the structural manner of action under serviceability loads are presented and have shown to be successful. Measured strain of the arch barrel due to passing train has been performed, both before, during and after strengthening. The results serve as input for model calibration and verification of the developed strengthening methods. The interaction of the backfill was not readily verified on the studied bridge and the strengthening was based on the assumption that both the backfill and the spandrel walls contributed as dead weight only. The finite element models are benchmarked using available experimental results in the literature, comprising masonry arch bridges with backfill loaded until failure. Good agreement is generally found if accounting for full interaction with the backfill. Similarly, accounting for the backfill as dead weight only, often results in a decrease in load carrying capacity by a factor 2 to 3. Still, several factors show a high impact on the estimated load carrying capacity, of which many are difficult to accurately assess. This suggests a conservative approach, although partial interaction of the backfill may still increase the load carrying capacity significantly.
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| 3. |
- Arvidsson, Therese
(författare)
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Train–Bridge Interaction : Literature Review and Parameter Screening
- 2014
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- New railway lines are continuously being constructed and existing lines are upgraded. Hence, there is a need for research directed towards efficient design of the supporting structures. Increasingly advanced calculation methods can be motivated, especially in projects where huge savings can be obtained from verifying that existing structures can safely support increased axle loads and higher speeds.This thesis treats the dynamic response of bridges under freight and passenger train loads. The main focus is the idealisation of the train load and its implications for the evaluation of the vertical bridge deck acceleration. To ensure the running safety of train traffic at high speeds the European design codes set a limit on the vertical bridge deck acceleration. By considering the train–bridge interaction, that is, to model the train as rigid bodies on suspension units instead of constant moving forces, a reduction in bridge response can be obtained. The amount of reduction in bridge deck acceleration is typically between 5 and 20% for bridges with a span up to 30 m. The reduction can be higher for certain train–bridge systems and can be important also for bridge spans over 30 m. This thesis aims at clarifying for which system parameter combinations the effect of train–bridge interaction is important.To this end, a thorough literature survey has been performed on studies in train–track–bridge dynamics. The governing parameters in 2D train–bridge systems have been further studied through a parameter screening procedure. The two-level factorial methodology was applied to study the effect of parameter variations as well as the joint effect from simultaneous changes in several parameters. The effect of the choice of load model was thus set in relation to the effect of other parameter variations.The results show that resonance can arise from freight train traffic within realistic speed ranges (< 150 km/h). At these resonance peaks, the reduction in bridge response from a train–bridge interaction model can be considerable.From the screening of key parameters it can furthermore be concluded that the amount of reduction obtained with a train–bridge interaction model depends on several system parameters, both for freight and passenger train loads. In line with the European design code’s guidelines for dynamic assessment of bridges under passenger trains an additional amount of damping can be introduced as a simplified way of taking into account the reduction from train–bridge interaction. The amount of additional damping is today given as function of solely the bridge span length, which is a rough simplification. The work presented in this thesis supports the need for a refined definition of the additional damping.
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| 4. |
- 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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| 5. |
- Du, Guangli, 1985-
(författare)
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Towards Sustainable Construction: Life Cycle Assessment of Railway Bridges
- 2012
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Since last few decades, the increased pressure from the environmental issues of natural resource depletion, global warming and air pollution have posed a great challenge worldwide. Among all the industrial fields, bridge infrastructures and their belonged construction sector contribute to a wide range of energy and raw materials consumptions, which is responsible for the most significant pollutions. However, current bridges are mainly designed by the criterion of economic, technique, and safety standards, while their correlated environmental burdens have unfortunately rarely been considered. The life cycle assessment (LCA) method has been verified as a systematic tool, which enables the fully assessment and complete comparison for the environmental impact among different bridge options through a life cycle manner. The study presented in this thesis is focused on railway bridges, as the LCA implementation is under great expectations to set a new design criterion, to optimize the structural design towards the environmental sustainability, and to assist the decision-making among design proposals. This thesis consists of two parts: an extended summary and three appended papers. Part one gives an overview introduction that serves as a supplementary description for this research work. It outlines the background theory, current development status, the LCA implementation into the railway bridges, as well as the developed excel-based LCA tool. Part two, includes three appended papers which provides a more detailed theoretical review of the current literatures and knowledge associated with bridge LCA, by highlighting the great challenging issues. A systematic flowchart is presented both in Paper I and Paper II for how to model and assess the bridge life cycle, together by coping with the structural components and associated emissions. This flowchart is further illustrated on a case study of the Banafjäl Bridge in Sweden, which has been extensively analyzed by two LCA methods: CML 2001 method and streamlined quantitative approach. The obtained results can be contributed as an analytical reference for other similar bridges. Based on the theoretical review and analytical results from case studies, it has been found that the environmental profile of a bridge is dominated by the selected structural type, which affects the life cycle scenarios holistically and thus further influences the environmental performance. However, the environmental profile of the structure is though very case specific; one cannot draw a general conclusion for a certain type of bridge without performing the LCA study. The case study has found that the impact of material manufacture phase is mostly identified significant among the whole life cycle. The availability of the inventory data and project information are appeared as the major problem in the bridge LCA study. Moreover, lack of standardized guideline, criteria and input information is another key issue. A criterion is needed to illustrate what are the qualified limits of a bridge to fulfill the environmental requirements. Therefore, the development of LCA for railway bridges still needs further collaborative efforts from government, industry and research institutes.
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| 6. |
- Gonzalez, Ignacio
(författare)
-
Application of monitoring to dynamic characterization and damage detection in bridges
- 2014
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The field of bridge monitoring is one of rapid development. Advances in sensor technologies, in data communication and processing algorithms all affect the possibilities of Structural Monitoring in Bridges. Bridges are a very critical part of a country’s infrastructure, they are expensive to build and maintain, and many uncertainties surround important factors determining their serviceability and deterioration state. As such, bridges are good candidates for monitoring. Monitoring can extend the service life and avoid or postpone replacement, repair or strengthening works. The amount of resources saved, both to the owner and the users, by reducing the amount of non-operational time can easily justify the extra investment in monitoring.This thesis consists of an extended summary and five appended papers. The thesis presents advances in sensor technology, damage identification algorithms, Bridge Weigh-In-Motion systems, and other techniques used in bridge monitoring. Four case studies are presented. In the first paper, a fully operational Bridge Weigh-In-Motion system is developed and deployed in a steel railway bridge. The gathered data was studied to obtain a characterization of the site specific traffic. In the second paper, the seasonal variability of a ballasted railway bridge is studied and characterized in its natural variability. In the third, the non-linear characteristic of a ballasted railway bridge is studied and described stochastically. In the fourth, a novel damage detection algorithm based in Bridge Weigh-In-Motion data and machine learning algorithms is presented and tested on a numerical experiment. In the fifth, a bridge and traffic monitoring system is implemented in a suspension bridge to study the cause of unexpected wear in the bridge bearings.Some of the major scientific contributions of this work are: 1) the development of a B-WIM for railway traffic capable of estimating the load on individual axles; 2) the characterization of in-situ measured railway traffic in Stockholm, with axle weights and train configuration; 3) the quantification of a hitherto unreported environmental behaviour in ballasted bridges and possible mechanisms for its explanation (this behaviour was shown to be of great importance for monitoring of bridges located in colder climate) 4) the statistical quantification of the nonlinearities of a railway bridge and its yearly variations and 5) the integration of B-WIM data into damage detection techniques.
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| 7. |
- González, Ignacio, 1985-
(författare)
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Study and Application of Modern Bridge Monitoring Techniques
- 2011
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The field of monitoring is one of rapid development. Advances in sensor technologies, in data communication paradigms and data processing algorithms all influence the possibilities of Structural Health Monitoring, damage detection, traffic monitoring and other implementations of monitoring systems. Bridges are a very critical part of a country’s infrastructure, they are expensive to build and maintain, and many uncertainties surround important factors determining the serviceability and deterioration of bridges. As such, bridges are good candidates for monitoring. Monitoring can extend the service life and avoid or postpone replacement, repair or strengthening work. Many bridges constitute a bottleneck in the transport network they serve with few or no alternative routes. The amount of resources saved, both to the owner and the users, by reducing the amount of non-operational time can easily justify the extra investment in monitoring. This thesis consists of an extended summary and three appended papers. The thesis presents advances in sensor technology, damage identification algorithms and Bridge Weigh-In-Motion techniques. Two case studies are carried out. In the first a bridge and traffic monitoring system is implemented in a highway suspension bridge to study the cause of unexpected wear in the bridge bearings. In the second a fully operational Bridge Weigh-In-Motion system is developed and deployed in a steel railway bridge. The gathered data was studied to obtain a characterization of the site specific traffic.
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| 8. |
- Hansson, Håkan, 1969-
(författare)
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Warhead penetration in concrete protective structures
- 2011
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The analysis of penetration of warheads in concrete protective structures is an important part of the study of weapon effects on protective structures. This type of analysis requires that the design load in the form of a warhead is determined, and its characteristic and performance within a protective structure is known. Constitutive equations for concrete subjected to weapon effects have been a major area of interest for a long time, and several material models for concrete behaviour are developed. However, it is not until recent years that it has been possible to use finite element (FE) analyses to simulate the behaviour of concrete targets during projectile penetration with acceptable results. The reason for this is a combination of several factors, e.g. development of suitable material models for concrete, enhancement of numerical methodology and affordable high capacity computer systems. Furthermore, warhead penetration has primary been of interest for the armed forces and military industry, with a large part of the conducted research being classified during considerable time. The theoretical bases for concrete material behaviour and modelling with respect to FE analyses of projectile penetration are treated in the thesis. The development of weapons and fortifications are briefly discussed in the thesis. Warheads may be delivered onto a protective structure by several means, e.g. artillery, missiles or aerial bombing, and two typical warhead types were used within the study. These warhead types were artillery shells and unitary penetration bombs for the use against hardened targets, with penetration data for the later warhead type almost non-existing in the literature. The penetration of warheads in concrete protective structures was therefore studied through a combination of experimental work, empirical penetration modelling and FE analyses to enhance the understanding of the penetration phenomenon. The experimental data was used for evaluation of empirical equations for concrete penetration and FE analyses of concrete penetration, and the use of these methods to predict warhead penetration in protective structures are discussed within the thesis. The use of high performance concrete increased the penetration resistance of concrete targets, and the formation of front and back face craters were prevented with the use of heavily reinforced normal strength concrete (NSC) for the targets. In addition, the penetration depths were reduced in the heavily reinforced NSC. The evaluated existing empirical penetration models did not predict the behaviour of the model scaled hardened buried target penetrators in concrete structures with acceptable accuracy. One of the empirical penetration models was modified to better describe the performance of these penetrators in concrete protective structures. The FE analyses of NSC gave reasonable results for all simulation cases, with the best results obtained for normal impact conditions of the penetrators.
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| 9. |
- Hedebratt, Jerry
(författare)
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Industrial Fibre Concrete Floors : Experiences and Tests on Pile-Supported Slab
- 2012
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Pile supported floor slabs have often been designed solely in ultimate limit state ULS and then foremost with uniformly distributed loadings UDL. The investigation of serviceability limit state SLS has been of simpler nature, even according to the governing codes of practice.Often it has been minimum‑reinforced with the presumption that full friction to the supporting ground is present, whit‑out any inspection, which by the Swedish code of practice even more reduced the addition of crack reinforcement. The cracks have not been controlled, before they in fact have occurred. For pile supported floor slabs the ground support will be there still, at least for a time, after the casting. As the ground settles, as dehydration always will occur, and drainage and the covering roof the precipitation to reach the ground, the slab will often be completely free bearing between the piles. The minimum reinforcement is based on the assumption that only the upper layer is needed to reinforce due to dehydration shrinkage – despite that the whole floor section in time will obtain the same moisture profile and also shrinkage magnitude. One often excludes the influence of creep and temperature and the affect from external loading and local variance of restraints in calculations in the SLS. Research on behaviour in SLS has been modest; in spite of that the contractors and the client and finally the end‑user of the floors often suffer from these problems.It has by this thesis been established that the shrinkage of the concrete used for industrial floors is large 0.9‑1.1 ‰, and that the problem foremost arise from cracking and problems with joints and unevenness in the floor. The integrated method for design and production of industrial floors is a way to the solution, but requires that all involved assign to co‑operate to 100 %. Furthermore it is required that one selects the proper materials to the proper design and the proper production method. If one will save cost this will often be on materials; which will lead to reduced reinforcement content and reduced concrete thickness. This way is wrong and will in end make the client suffer economically. A way to solve this has been to cast the floors with steel fibre concrete SFC; from the beginning often a little bit thicker and with moderate steel fibre content and complementary reinforcement, compared to present execution. The competition from abroad has nevertheless shaped solutions that with thinner slabs and less traditional reinforcement and invalid design calculations compete on faulty grounds. This work demonstrates how this make the floor suffer in ULS and SLS.Trough full‑scale testing (half of a normally loaded industrial floor in matter of geometry) where a pile supported floor slab has been simulated by a flat‑slab floor cast in steel fibre concrete, it has been shown that the solution with steel fibre concrete performs well in slabs for industrial floors. On one hand it gives the opportunity to production wise superior methods for placing concrete which potentially could gain the environment with reduced reinforcement content, and on the other hand SFC brings a ductile failure behaviour for loadings with much larger magnitudes than in normal ULS design, and further SFC provides with a stiffer response and with possibility to construct slabs with small creep deformation.Finally it has been established that, when it comes to short‑term point loadings (ULS) and with long-term point loadings (SLS) one can rely on the bearing capacity and the tough behaviour of SFC. And that one may exert an influence on both limit states, through variation of the SFC and the reinforcement content. This is shown for a real bearing structure, the pile supported industrial floor, and that in a safe way.
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| 10. |
- Ilina, Elena
(författare)
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Understanding the application of knowledge management to the safety critical facilities
- 2010
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Challenges to the operating nuclear power plants and transport infrastructures are outlined. It is concluded that most aggravating factors are related to knowledge. Thus, of necessity, effective knowledge management is required. Knowledge management theories are reviewed in their historical perspective as a natural extension and unification of information theories and theories about learning. The first line is identified with names as Wiener, Ashby, Shannon, Jaynes, Dretske, Harkevich. The second line – with Vygotsky, Engeström, Carayannis. The recent developments of knowledge management theorists as Davenport, Prusak, Drew, Wiig, Zack are considered stressing learning, retaining of knowledge, approaching the state awareness of awareness, and alignment of knowledge management with the strategy of the concerned organizations. Further, some of the details and results are presented of what is achieved so far. More specifically, knowledge management tools are applied to the practical work activities as event reporting, data collection, condition assessment, verification of safety functions and incident investigation. Obstacles are identified and improvements are proposed. Finally, it is advised to continue to implement and further develop knowledge management tools in the organizations involved in various aspects of safety critical facilities.
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| 11. |
- Jansson, Robert
(författare)
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Fire Spalling of Concrete : Theoretical and Experimental Studies
- 2013
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Fire spalling of concrete is not a new phenomenon. To some degree there has always been a risk during rapid heating of concrete. Therefore, to a certain degree the effect of fire spalling is included in the bank of data from fire tests and fires on which our understanding of the fire resistance of concrete is based. However, the development and modern use of more dense concrete mixes have produced cases of very severe fire spalling which have increased the urgency to understand this phenomenon. In this context, the use of an addition of polypropylene (PP) fibres to the mix to limit the amount of spalling has been one topic of interest for this thesis.During fire tests on a post-tensioned concrete structure made of spalling sensitive concrete, it has been shown that substantially lower amounts of PP fibres than 2 kg/m3, which is recommended in the Eurocode (1992-1-2:2004), can be used with successful results.As part of this study, another important aspect has emerged, i.e. the impact the test method used can have on the fire spalling depths observed in concrete specimens. This has been known for many years but is seldom discussed in the scientific literature. In this thesis it has been shown that results from tests on unloaded cubes do not necessarily correspond to results seen on larger loaded slabs. In the results presented, none of the tested cubes spalled whereas some of the large slabs spalled to the degree that the reinforcement became fire exposed. Further, the difference in spalling depths between small and large post-tensioned slabs was shown to be substantial; although in general the ranking in severity from least to greatest spalling correlated between these two specimen sizes. The correlation to larger specimens was much vaguer in the case when the small slabs were not loaded in compression as there sometimes was no spalling in the small slabs.From time to time the randomness of the fire spalling of concrete has been mentioned. To investigate this further, an analysis of 110 fire tests performed on small slab type specimens was performed. This analyse showed that the spalling behaviour had a good repeatability between two identical tests, which proved that the so called “random factor” relating to spalling depth was low for the chosen data set. It was also possible to make a multiple least squares fit of test parameters that could be used to predict the spalling behaviour which also underlines that a substantial stochastic factor was not present.Regarding the influence of different factors, the results compiled on the influence of ageing show that for three of the tested Self-Compacting Concrete (SCC) mixes, the amount of spalling was reduced with age whereas for the fourth mix (which included the highest amount of limestone filler, 140 kg/m3) the spalling was not reduced with age. In this test series no systematic influence of the intensity of the fire, between standard fire exposure and the more severe hydrocarbon fire, on the spalling depth was detected for this type of specimen. The only major difference was that spalling started earlier during the more severe fire exposure.Pressure measurements conducted as part of the work within this thesis, supported by results from the literature, indicate that there is no relationship between pressure rise due to moisture and fire spalling. Based on this and the fact that the spalling event in many cases happens at relatively low temperatures where the saturation vapour pressure is low two alternative factors to explain the function of PP fibres have been presented: (i) PP fibres reduce the moisture content in the critical zone close to the heated surface which affects the mechanical properties advantageously, and (ii) PP fibres amplify moisture movement leading to larger drying creep and shrinkage which locally relax the thermal stresses.To investigate the influence of the presence of moisture on the compressive strength, specimens were tested after being boiled for varying periods of time in a water bath. The study showed a remarkable reduction of strength due to boiling of the mortar specimens. After boiling mortar in a water bath for 3, 10 or 20 minutes, i.e. approximately the same time span as the initiation of fire spalling during fully developed fires, the strength was only 64% of the corresponding value for a dry specimen. As no strength change was detected between the specimens boiled 3, 10 or 20 minutes, and that the corresponding saturation pressure for steam at 100ºC is negligible compared with the tensile strength of concrete, it was concluded that pore pressure is not a significant contributor to the measured reduction in strength. It appears that the presence of moisture itself rather than an increased pressure is the most important factor reducing strength. This is a clear indication that moisture plays a key role in the fire spalling of concrete but in a different way from previously assumed.
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| 12. |
- Johansson, Christoffer, 1985-
(författare)
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Simplified dynamic analysis of railway bridges under high-speed trains
- 2013
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The world-wide development of new high-speed rail lines has led to more stringent design requirements for railway bridges. This is mainly due to the fact that a train at high speeds can cause resonance in the bridge superstructure. In order to avoid problems of this kind, it has become essential to perform dynamic simulations, which are usually carried out in a time consuming finite element program. The main reason for not using an analytical solution is that this type of solution only exists for simple bridges, e.g., simply supported bridges.The aim of this thesis, is therefore the development of a simplified analytical model for preliminary dynamic analyses of railway bridges. The model is then used in several studies, both parametric and probabilistic ones, to determine the dynamic response of various railway bridges under moving loads. Special attention is paid to acceleration levels in the bridge superstructure, because previous studies have shown that these are often decisive. In the design, both the model and the numerical simulations can be used with considerable effectiveness to help engineers to define their structural systems.All bridges, throughout this thesis, have been modelled with elastically supported multi-span Bernoulli-Euler beams. The amount of dissipated energy is modelled using modal damping. The train load is modelled by concentrated loads, which implies that the vehicle-bridge interaction is not considered. A model with these characteristics can be used to analyse simply supported concrete bridges as well as multi-span steel bridges. Another advantage of the proposed method is that the solution is exact, because the equation of motion has been solved using a Laplace transform. A lot of effort has been made to have a model both as simple as possible and flexible enough to be able to study a wide variety of structures.The results of the case studies have shown that concrete bridges are more suitable than steel and composite bridges for use in the new high-speed lines. The simulations have also shown that short-span railway bridges have problems in meeting the design requirements. This confirms the high acceleration levels that were recorded in France, just after the inauguration of the new high-speed line between Lyon and Paris, on several short-span bridges. Furthermore, it was found that a multi-span bridge has a reduction in its dynamic response of up to 60 % compared to a similar simply supported bridge.
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| 13. |
- Le, Thanh Nam, 1986-
(författare)
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Corotational formulation for nonlinear analysis of flexible beam structures
- 2012
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Flexible beam structures are popular in civil and mechanical engineering. Many of these structures undergo large displacements and finite rotations, but with small deformations. Their dynamic behaviors are usually investigated using finite beam elements. A well known method to derive such beam elements is the corotational approach. This method has been extensively used in nonlinear static analysis. However, its application in nonlinear dynamics is rather limited. The purpose of this thesis is to investigate the nonlinear dynamic behavior of flexible beam structures using the corotational method. For the 2D case, a new dynamic corotational beam formulation is presented. The idea is to adopt the same corotational kinetic description in static and dynamic parts. The main novelty is to use cubic interpolations to derive both inertia terms and internal terms in order to capture correctly all inertia effects. This new formulation is compared with two classic formulations using constant Timoshenko and constant lumped mass matrices. This work is presented in the first appended journal paper. For the 3D case, update procedures of finite rotations, which are central issues in development of nonlinear beam elements in dynamic analysis, are discussed. Three classic and one new formulations of beam elements based on the three different parameterizations of the finite rotations are presented. In these formulations, the corotational method is used to develop expressions of the internal forces and the tangent stiffness matrices, while the dynamic terms are formulated into a total Lagrangian context. Many aspects of the four formulations are investigated. First, theoretical derivations as well as practical implementations are given in details. The similarities and differences between the formulations are pointed out. Second, numerical accuracy and computational efficiency of these four formulations are compared. Regarding efficiency, the choice of the predictor at each time step and the possibility to simplify the tangent inertia matrix are carefully investigated. This work is presented in the second appended journal paper. To make this thesis self-contained, two chapters concerning the parametrization of the finite rotations and the derivation of the 3D corotational beam element in statics are added.
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| 14. |
- Le, Thanh-Nam
(författare)
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Nonlinear dynamics of flexible structures using corotational beam elements
- 2013
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The purpose of this thesis is to develop corotational beam elements for the nonlinear dynamic analyse of flexible beam structures. Whereas corotational beam elements in statics are well documented, the derivation of a corotational dynamic formulation is still an issue.In the first journal paper, an efficient dynamic corotational beam formulation is proposed for 2D analysis. The idea is to adopt the same corotational kinematic description in static and dynamic parts. The main novelty is to use cubic interpolations to derive both inertia terms and internal terms in order to capture correctly all inertia effects. This new formulation is compared with two classic formulations using constant Timoshenko and constant lumped mass matrices.In the second journal paper, several choices of parametrization and several time stepping methods are compared. To do so, four dynamic formulations are investigated. The corotational method is used to develop expressions of the internal terms, while the dynamic terms are formulated into a total Lagrangian context. Theoretical derivations as well as practical implementations are given in detail. Their numerical accuracy and computational efficiency are then compared. Moreover, four predictors and various possibilities to simplify the tangent inertia matrix are tested.In the third journal paper, a new consistent beam formulation is developed for 3D analysis. The novelty of the formulation lies in the use of the corotational framework to derive not only the internal force vector and the tangent stiffness matrix but also the inertia force vector and the tangent dynamic matrix. Cubic interpolations are adopted to formulate both inertia and internal local terms. In the derivation of the dynamic terms, an approximation for the local rotations is introduced and a concise expression for the global inertia force vector is obtained. Four numerical examples are considered to assess the performance of the new formulation against two other ones based on linear interpolations.Finally, in the fourth journal paper, the previous 3D corotational beam element is extended for the nonlinear dynamics of structures with thin-walled cross-section by introducing the warping deformations and the eccentricity of the shear center. This leads to additional terms in the expressions of the inertia force vector and the tangent dynamic matrix. The element has seven degrees of freedom at each node and cubic shape functions are used to interpolate local transversal displacements and axial rotations. The performance of the formulation is assessed through five examples and comparisons with Abaqus 3D-solid analyses.
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| 15. |
- Leander, John, 1976-
(författare)
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Improving a bridge fatigue life prediction by monitoring
- 2010
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- For steel railway bridges, fatigue is the main reason for a limited service life. An initial prediction of the fatigue life is usually performed using loads and conservative assumptions stated in the governing standards. The result is used for decisions of further actions. Even if the result is an insufficient residual life, some bridges are not easy to strengthen or replace without causing large traffic disturbance. In this licentiate thesis, the real load effect is studied by monitoring a bridge during service. The aim is to improve the fatigue life prediction by reducing the uncertainties in the traffic load and finally attain a longer theoretical service life of the bridge. A case study of the Söderström Bridge in central Stockholm, one of Sweden’s most important railway bridges, is incorporated in the studies. Previously performed theoretical assessments have shown alarming results regarding the remaining fatigue life for some of the structural members in the bridge. To verify the result and hopefully receive more favorable stress ranges, the bridge was instrumented with a monitoring system in 2008. The bridge and the monitoring campaign are thoroughly presented. Owing to the large data volume produced by the monitoring system, robust routines and procedures for quality assurance have been required. A deterministic fatigue assessment is presented based on 43 days of continuous measurements comprising more than 17 thousand train passages. Unfortunately, also this fatigue assessment indicates high fatigue damages. A large monitoring system with many gauges enables a statistical evaluation of the quality of the measurements. By the principle of Analysis of Variance, a comparison of the stress range spectra can be performed with the aim of finding deviant spectra and corrupt gauges. An additional aspect is the length of the monitoring period, meaning the required duration for obtaining a stable result. A model for the convergence progress is proposed which could be incorporated in a monitoring system for use during service. The extensive monitoring program of the Söderström Bridge has resulted in reliable stress range spectra. By curve fitting and goodness-of-fit tests measured spectra have been compared with known statistical distributions. These distributions can be used for future fatigue reliability assessments.
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| 16. |
- Leander, John
(författare)
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Refining the fatigue assessment procedure of existing steel bridges
- 2013
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This thesis treats the fatigue assessment process of steel bridges. The purpose is to identify areas with potential of enhancement with an overall aim of attaining a longer service life. The aging bridge stock in Sweden and in many other developed countries is an impending economical burden. Many bridges have reached their expected service life and increased axle loads, speeds, and traffic intensity further accelerate the deterioration. An immediate replacement of all our bridges approaching their expected service life will not be possible. For economical and environmental reasons, effort should be put on extending the theoretically safe service life as far as possible. Fatigue is one of the major reasons limiting the service life of steel bridges .A specific example is the Söderström Bridge in Stockholm, Sweden. Fatigue cracks have been found in the webs of the main beams and theoretical assessments have shown an exhausted service life. As a mean to reduce the uncertainties in the theoretical assessments a monitoring campaign was started in 2008 and continued in periods until 2011. The first continuous period of 43 days of measurements in 2008 forms the experimental foundation for this thesis.A fatigue life prediction involves (i) an estimation of the load effect, (ii) an estimation of the resistance, and (iii) the selection of a prediction model. This thesis treats in some sense all three of them. Considering part (i), the load effect, a theoretical study on the influence of dynamics has been performed. The quasi static approach suggested in the standards does not necessarily reflect the true dynamic behavior of the structure. Performing a dynamic moving load analysis gives for all cases studied a more favorable fatigue life. A further enhancement of the assessment is to perform in situ measurements. The uncertainties related to dynamics, loads, and structural behavior are thereby captured in the response. Routines for processing the measured response and performing life predictions are treated, moreover the quality of the measured response.The resistance, part (ii), in form of the fatigue endurance has a funda- mental influence on the fatigue life. As an attempt to reach a more adequate fatigue endurance a refined assessment of a typical joint is performed using linear elastic fracture mechanics (LEFM). A descriptive detail category is suggested which renders a somewhat more favorable resistance.Part (iii), the prediction model, is treated considering the safety format. A reliability-based model is suggested which enables a consideration of the uncertainties in each stochastic variable. Thereby, all aforementioned parts and uncertainties can be combined within the same prediction. The reliability-based model is used for a code calibration of partial safety factors to be used in semi-probabilistic assessments according to the standards.The implementation of the suggested procedures is shown with a numerical example. The outcome should not be generalized but the example clearly shows an increase in fatigue life with the proposed enhancements of the assessment procedure.
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| 17. |
- 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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| 18. |
- Safi, Mohammed, 1983-
(författare)
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Life-Cycle Costing : Applications and Implementations in Bridge Investment and Management
- 2013
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- A well-maintained bridge infrastructure is a fundamental necessity for a modern society that provides great value, but ensuring that it meets all the requirements sustainably and cost-effectively is challenging. Bridge investment and management decisions generally involve selection from multiple alternatives. All of the options may meet the functional demands, but their life-cycle cost (LCC), service life-span, user-cost, aesthetic merit and environmental impact may differ substantially. Thus, life-cycle analysis (LCCA, a widely used decision-support technique that enables comparison of the LCC of possible options), is essential. However, although LCCA has recognized potential for rationalizing bridge procurement and management decisions its use in this context is far from systematic and the integration of LCCA findings in decisions is often far from robust. Thus, the overall objective of the work underlying this thesis has been to contribute to the development of sustainable bridge infrastructures while optimizing use of taxpayers’ money, by robustly incorporating life-cycle considerations into bridge investment and management decision-making processes.The work has introduced a full scheme for applying LCCA throughout bridges’ entire life-cycle. Several practical case studies have been presented to illustrate how an agency could benefit from use of a bridge management system (BMS) to support decisions related to the management of existing bridges and procure new bridges. Further developments include a comprehensive approach incorporating a novel LCCA technique, “LCC Added-Value Analysis”, which enables procurement of the most cost-efficient bridge design through a fair design-build (D-B) tendering process. A further contribution is a novel, holistic approach designed to enable procurement of bridges with the maximal possible sustainability (life-cycle advantages) under D-B contracts. The approach combines LCC Added-Value analysis with other techniques that make bridges’ aesthetic merit and environmental impact commensurable using an adapted concept named the willingness-to-pay-extra (WTPE).The systematic analytical procedures and potential of LCCA to deliver major savings highlighted in this thesis clearly demonstrate both the feasibility and need to integrate LCCA into bridge procurement and management decisions. This need has been recognized by Trafikverket (the Swedish Transport Administration), which has implemented a software tool developed in the research (BaTMan-LCC) in its bridge and tunnel management system (BaTMan). This thesis introduces readers to the field, considers BaTMan and the bridge stock in Sweden, discusses the developments outlined above and obstacles hindering further implementation of LCCA, then presents proposals for further advances.
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| 19. |
- Selander, Anders, 1978-
(författare)
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Hydrophobic Impregnation of Concrete Structures : Effects on Concrete Properties
- 2010
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Hydrophobic impregnations often referred to as water repellent agents, today mainly consisting of alkylalkoxysilanes, are often used on concrete to prolong the service life of the structure. This is accomplished by protecting the reinforcement bars from chlorides or by changing the moisture content inside. When the concrete is treated with a water repellent agent the properties of the surface layer becomes hydrophobic and thereby water droplets are stopped from entering, still allowing water vapour to pass through. This change can reduce chloride ingress and stop heavy rain from penetrating through the surface layer. This thesis presents results concerning how the properties of concrete are affected by a hydrophobic impregnation. Moisture transport and fixation in the surface layer of the concrete are studied as well as the secondary effects of more practical use such as the effect on chloride ingress, water absorption and humidity level. It also presents results on how the penetration depth and concentration of the water repellent agent (i) depend on a number of parameters, and (ii) affect the outcome of the treatment. Water repellent treatments on a number of different concrete structures in Stockholm, ranging from tunnel to high-rice building, are evaluated as well. The three most important factors for the penetration of any water repellent agent into concrete are time, porosity and degree of saturation. A semi-empirical equation is derived that gives an idea on how much these factors affect the efficient penetration depth of the water repellent agent. The depth and concentration have a major effect on the performance of the treatment. The moisture diffusion coefficient for a water repellent treated concrete is close to constant and not nearly as dependent on the relative humidity (RH) as for untreated concrete. Unlike untreated concrete, where capillary suction plays an important role for the moisture transport at high RH, the vapour transport is the dominant transport mechanism even at high RH for water repellent treated concrete. The moisture fixation is affected by a water repellent treatment and the effect is clearest at high moisture levels. The main reason for this is that the capillary porosity is affected by the treatment to a relatively high degree while the gel porosity to a large extent remains unaffected. A hypothesis is presented which suggests that the RH inside the concrete at the time of the treatment affects not only the depth and concentration but also in which range of pore radii the water repellent agent is present and active. The durability of hydrophobic impregnations can be divided into surface effects and in depth effects. The first is sensitive to the environmental and mechanical loadings and normally disappears within a year while the later can be long lasting if a sufficient depth is reached. Hydrophobic impregnations are not the answer to all problems in concrete related to moisture, but if correctly used it can prolong the service life of the structure which will lead to savings of natural resources and thus both economical and environmental savings for the community.
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| 20. |
- Ülker-Kaustell, Mahir, 1975-
(författare)
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Essential modelling details in dynamic FE-analyses of railway bridges
- 2013
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The increased need to reduce the use of fossil fuels imposes higher demands on the efficiency of rail transportation. Therefore, an improved knowledge regarding the dynamic properties of railway bridges and infrastructure for railway traffic in general is required. Typically, increased train speed, longer trains and increased axle loads increase the dynamic response in railway bridges. Modelling details for bridge structures such as the flexibility of the foundations, radiation damping in the subsoil and the embankments as well as hysteretic effects in bridge bearings and the track superstructure are typically neglected. The reason for this is that suitable models which consider the influence of such effects in engineering calculations have not yet been implemented in the effectual design codes. This thesis is mainly based on a case study of a ballasted, simply supported steel-concrete composite bridge, which shows a considerable variation in the natural frequencies and damping ratios depending on the amplitude of vibration. Furthermore, the natural frequencies were found to increase significantly during the winter. It is well known that the dynamic properties of typical civil engineering structures are dependent on the amplitude of vibration. However, the fact that certain railway bridges exhibit such non-linear behaviour also for very small amplitudes of vibration has been shown only during later years. This has been verified by means of measurements of the free vibrations after train passages on three typical Swedish beam bridges for railway traffic. Possible sources to this amplitude dependency have been identified primarily in the bridge bearings and the track superstructure. Models of these structural components, based on the so called Bouc-Wen model, have been implemented in a commercial finite element program and was used in a preliminary study. The results indicate that roller bearings and pot bearings can give rise to a non-linear mode of vibration, characterised by two different states. At very small amplitudes of vibration (. 0:1m=s2), no movement over such bearings occur (state 1) since their initial resistance to motion is not overcome. Depending on parameters such as the longitudinal stiffness of the foundations and substructures, the beam height over the supports as well as the bearing type, there is an amplitude of vibration at which the initial resistance to motion is completely overcome (state 2). The bearings are then free to move, with a resistance characterised by the kinematic friction (pot bearings) or the rolling resistance (roller bearings). During the transition from state 1 to state 2, the frequency decreases continuously towards an asymptotic value and the damping initially grows considerably, from a value which corresponds quite well to the recommendations of the Eurocodes and then returns to a value similar to that in state 1. The preliminary study indicates that it is possible to design certain bridges so that this increase in damping is optimal over the relevant range of amplitudes of vibration.
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| 21. |
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| 22. |
- Wennström, Jonas
(författare)
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Life Cycle Costing in Road Planning and Management : A Case Study on Collision-free Roads
- 2014
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Construction of infrastructure does not only mean large capital investments but also future costs to operate and maintain these assets. Decision making in planning and design of roads will impact the need of future operation and maintenance activities. Additionally, infrastructure management is often under increasing pressure of aging structures, limited budgets and increased demands from public which require transparency in the decision making. Life cycle costing is a methodology that takes into account costs throughout an asset’s life cycle including investment, operation, maintenance and disposal. Despite the methodology’s existence for more than 40 years, the practical application is often reported to be scarce in both private and public sectors. Implementation in road planning and management means a high complexity where the life cycle costing can to be applied from early planning, design, construction and management in which all influence life cycle cost. Life cycle costing can also be applied in many different ways, level of detail and for different type of studies.For effective implementation of life cycle costing in road planning, design and management, different considerations need to be understood. In this thesis the application of life cycle costing has been studied through case study research. The main case selected was an investment to convert a single carriageway road to a, so called, sparse collision-free road. Through widening and separation between driving directions the traffic safety is significantly improved. However, in recent years increased operation and maintenance costs have been associated with the road type. Especially concerns regarding increased road user cost during road works have been expressed. This case was examined in two case studies from different perspectives. The first one was to study the implications on project appraisal and the second one examined the possibility to optimise pavement design.Results from cost benefit analyses based on established road appraisal techniques indicated that operation and maintenance related costs had limited impact on profitability. The second study also indicated that future cost can be influenced differently depending on criteria for optimal alternative. Based on economic analyses using established techniques, increased operation and maintenance liabilities appear to be of limited concern, in contrary to the perception. In future research this need to be set in context of road management with refined analysis in order to study implications for future management.
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