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  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.
11

Dinâmica não linear de um pórtico plano sob carregamento não ideal: análise numérica e experimental. / Nonlinear dynamics of a portal frame excited by nonlinear load: experimental and numerical analysis.

Flavio José Garzeri 04 October 2001 (has links)
Nesta tese, apresenta-se a análise numérica e experimental do comportamento dinâmico não linear de um pórtico excitado por uma fonte não ideal - um motor elétrico de corrente contínua desbalanceado - como exemplo dessa classe de problemas. Elabora-se um modelo matemático com quatro graus de liberdade: dois relacionados com o deslocamento horizontal e vertical do ponto de apoio da máquina e dois com os parâmetros de funcionamento do motor elétrico. Adota-se a formulação Lagrangeana para gerar as equações de movimento contendo termos não lineares até ordem cúbica. A solução numérica é obtida através do método de Runge-Kutta com passo adaptativo. Ensaios dinâmicos e estáticos foram realizados com o motor e com o sistema completo, com vistas a validar o modelo matemático. Um sistema de medição digital armazenou todos os dados obtidos em arquivos de fácil leitura pelos programas atualmente disponíveis. Filmagens de alta velocidade e fotografia estroboscópica registraram movimentos típicos relacionados com comportamentos não lineares. Os resultados numéricos e experimentais mostram boa correlação entre si, além de apresentarem alguns dos fenômenos associados ao se forçar uma estrutura a passar por uma de suas ressonâncias excitando-a com um dispositivo com potência limitada, como o efeito Sommerfeld. Outros fenômenos, devido ao comportamento geometricamente não linear da estrutura, são também detectados, tais como saturação modal e transferência de energia. / Numerical and experimental analysis of the nonlinear dynamics of a portal frame excited by a non-ideal source - an unbalanced direct current motor - is presented in this thesis as an example of this class of problems. A four degree of freedom model is elaborated: two of them related to the horizontal and vertical structural displacements and two others to the functioning parameters of the motor. A Lagrangian approach for deducing the equations of motion, up to cubic non-linear terms is followed. The numerical solution is obtained through Runge-Kutta algorithm with adaptive step. Static and dynamic tests were performed with the motor and with the complete system, in order to validate the mathematical model. A digital acquisition system recorded all data in computer files, ready to be read by available commercial programs. High speed filming and stroboscopic photography were used to register typical movements related to nonlinear behavior. Numerical and mathematical results show good correlation, as well as present some phenomena related to passage through resonance with limited power excitation such as the Sommerfeld effect. Some other phenomena, related to the nonlinear behavior of the structure are also detected, as modal saturation and energy transference.
12

2D-model of a portal frame railway bridge for dynamic analysis

Kylén, Joakim January 2010 (has links)
No description available.
13

Soil-Structure Interaction Analysis of Portal Frame Railway Bridges : Numerical Analysis of Two Case Study Bridges

Sandqvist, Nils, Milicevic, Marko January 2020 (has links)
This thesis concerns dynamic Soil-Structure Interaction (SSI) analysis of portal framerailway bridges. Dynamic problems are common for bridges used for high speedrailway traffic. The passing trains induce harmonic loads on the bridges causingvibration amplitudes that may cause damage to the bridge structures and userdiscomfort.Previous studies have shown that the effects of SSI are substantial for short spanportal frame bridges. The damping ratio of the system is greatly increased due to theenergy dissipation properties of the surrounding soil causing significant changes in thedynamic response of the structure. Therefore, it is of interest to investigate the effectsof SSI for portal frame bridges with longer spans.Two case study bridges with span lengths of approximately 16m have been investigatedin detail in this study. Dynamic analyses of the bridges and train passage simulationshave been performed. The results show that SSI significantly increases the dampingratio which leads to lower vibration amplitudes. It is also possible to draw theconclusion that more accurate results are achieved when modeling fixed foundationsrather than using static spring foundations to replicate the stiffness of the subsoil.Moreover, a simplified modeling approach accounting for the effects of SSI is proposed.The proposed method provides satisfactory results, but more future work may increasethe quality of the results further. To validate the conclusions from this study, a proposalfor experimental validation is presented. Performing full-scale dynamic tests on thestudied bridges would enable further comparison and validation of the results.
14

Dynamic Soil-Structure Interaction of a Portal Frame Railway Bridge - Numerical Analysis on a Case Study Bridge

Ikzer, Rita January 2018 (has links)
In the field of structural dynamics, a broader knowledge about relevant phenomena that affect the dynamic behavior of railway bridges is vital for structural engineers and design code administrators. The knowledge might benefit in an increased understanding of e.g. the resonance phenomena, and in improvements of the existing design codes. A phenomenon that has received more attention in recent times is the so called soil-structure interaction (SSI), as it may significantly contribute to the stiffness and damping of a structural system. Previous investigations have suggested that the influence of SSI might be crucial for short and relatively stiff structures such as portal frame bridges. Yet, this effect is usually neglected due to the lack of simple models and guidelines. Dynamic analyses have been performed on a short-span closed portal frame railway bridge, situated on the Bothnia Line, where the effect of the surrounding and underlying soil and the ballasted track, has been investigated. This has been accomplished through the adoption of multiple boundary conditions to consider different forms of soil-structure interactions. The vertical bridge response has been studied by numerical three-dimensional models, both with full FE-models and simplified models appropriate for practical design purposes. More specifically the natural frequencies and damping ratios have been scrutinized. Theoretically, it has been identified that the contribution of the soil on the global damping is largely influential, as it has been indicated that the damping ratio of the fundamental bending mode is seven times greater than the, in this case, significantly conservative recommended design value. Furthermore, SSI has shown to increase the natural frequencies which consequently shifts the critical resonant speed, allowing for higher speeds. The bridge response is predominantly affected by the backfill soil, yet the modal damping contribution is equally substantial from the backfill and the subsoil. Moreover, it has been established that the proposed simplified model is promising and in good agreement with the full model. It has also been resolved that train passages on the surrounding soil play an important role on the dynamic bridge response. Unfortunately, the simplified model has proven to be incapable of considering these train loads, implying that further development is needed to attain an adequate model that may be implemented for portal frame bridges of short span. Applying only elastic constraints on the vertical degree of freedom at the foundation is a simplified modeling approach that fails to capture the soil behavior in an accurate manner, and is therefore not recommended for future research projects. While on the subject of future investigations, the effect of SSI should be studied on other bridges to externally validate the obtained results. / Inom strukturdynamik är det essentiellt att erhålla en bredare kunskap om relevanta fenomen som kan påverka det dynamiska beteendet av järnvägsbroar. Detta gäller för både yrkesverksamma ingenjörer och administratörer av normer och standarder för att få en ökad förståelse av exempelvis resonansfenomen samt för revidering och förbättring av befintliga normer. Ett fenomen som på senare tid har fått mer uppmärksamhet är den så kallade jord-struktur interaktionen eftersom den kan ha en signifikant inverkan på styvheten och dämpningen av ett system. Tidigare undersökningar har tytt på att effekten av jord-struktur interaktionen kan vara avgörande för korta och relativt styva broar som exempelvis plattrambroar. På grund av bristen på enkla modeller och riktlinjer är denna effekt ofta försummad. Dynamiska analyser har utförts på en kort sluten plattrambro belägen på Botniabanan, där påverkan av motfyllningen, underliggande jorden och det ballasterade spåret har utretts. Detta har åstadkommits genom att beakta olika randvillkor för att ta hänsyn till diverse former av jord-struktur interaktioner. Den vertikala responsen i bron har studerats genom tredimensionella numeriska modeller både med detaljerade FE-modeller och med praktiskt lämpade förenklade modeller, där i synnerhet egenfrekvensen och dämpningskvoten har analyserats. Bidraget från jorden har påvisat sig ha en avsevärd inverkan på den globala dämpningen då det framgick att dämpningskvoten för den fundamentala böjmoden är sju gånger större än det, i denna fallstudie, betydligt konservativa rekommenderade dimensioneringsvärdet. Dessutom har jord-struktur interaktionen lett till ökade egenfrekvenser som följaktligen skiftat den kritiska resonanshastigheten vilket tillåter högre hastigheter. Motfyllningen har haft en avsevärd effekt på responsen av bron, medan bidraget till ökningen i modala dämpningen har fördelats lika mellan motfyllningen och underliggande jorden. Vidare är den föreslagna förenklade modellen lovande och i god överenstämmelse med den detaljerade modellen. Det har även konstaterats att tågpassager på motfyllningen spelar en viktig roll för den dynamiska responsen. Dessvärre har den förenklade modellen misslyckats med att ta hänsyn till dessa tåglaster, vilket indikerar att en vidareutveckling krävs för en implementerbar adekvat modell för plattrambroar av korta spännvidder. Ett förenklat modelleringsalternativ är applicering av enbart elastiska randvillkor i den vertikala frihetsgraden av bottenplattan. Detta alternativ har visat sig vara otillräckligt för att efterlikna den underliggande jordens beteende och undanbedes för framtida studier. På tal om framtida projekt bör jord-struktur interaktionen utredas på andra broar för att externt validera resultaten.
15

Finite Element Analysis of the Dynamic Effect of Soil-Structure Interaction of Portal Frame Bridges - A Parametric Study

Dagdelen, Turgay, Ruhani, Shaho January 2018 (has links)
In Sweden, the railway sector currently faces the challenge of developing its first high-speed railway line, in response to the need to provide faster domestic and international transport alternatives. High-speed train passages on railway bridges can cause resonance in the bridge superstructure, which induce high accelerations that should not exceed the limits stipulated in the current design code. The most common bridge type adopted in Sweden is the portal frame bridge, an integral abutment bridge confined by surrounding soil. The soil possesses inherent material damping and radiation damping that allows energy dissipation of train-induced vibrations. Both the damping and the natural frequency of the soil-structure system influence the acceleration response of the bridge superstructure. Therefore, it is necessary to investigate the effect of soil-structure interaction on portal frame bridges. Within this thesis, a numerical parametric study was performed to gain knowledge of the dynamic effect of the relative deck-abutment stiffness on the soil-structure interaction of portal frame bridges. For four span lengths, three different boundary conditions were analyzed in the form of i) no soil, ii) backfill, and iii) half-space. The analysis was performed on two- and three-dimensional finite element models. The backfill and subsoil were modeled with both direct finite element approach, and with a simplified approach using Kelvin-Voigt models and frequency-dependent impedance functions. Furthermore, time was devoted to investigating the nonlinear compression-only behavior of the interaction between the backfill and the abutments to allow separation. The results presented in the thesis illuminate the essence of including soil-structure interaction in the dynamic analysis as both the modal damping ratio and the natural frequency increased drastically. The effect of backfill on short span bridges has shown to be more prominent on the reduction of the train-induced vibrations. For longer spans, the subsoil proved to be more significant. For the simplified models the modal damping ratios of the different span lengths have been quantified as a logarithmic trend of the first vertical bending mode. Two-dimensional models have been problematic when using plane stress elements due to the sensitivity of the element thickness on the response. Thus, such models are only recommended if validation with corresponding three-dimensional models and/or field measurements are possible. By allowing separation of the soil-structure interface, the effect of contact nonlinearity on the acceleration response has been more suitable with direct finite element approach - in which static effects of the soil are accounted for - contrary to the simplified nonlinear models with compression springs. / Järnvägssektorn i Sverige står inför utmaningen att utveckla den första höghastighetsbanan med syftet att erbjuda snabbare inhemska och internationella transportalternativ. Passager av höghastighetståg på järnvägsbroar kan orsaka resonans i brons överbyggnad vilket resulterar i höga accelerationer som inte får överskrida begränsningarna i dimensioneringsnormen. I plattrambroar, vilka är främst förekommande i Sverige, utförs broplattan inspänt i rambenen omslutna av jord. Jorden bidrar utöver styvhet, även med material- och strålningsdämpning där vibrationer i jorden inducerade av tågpassager tillåts dissipera. Accelerationerna i brons överbyggnad påverkas av dämpningen och egenfrekvensen av jord-struktur systemet. Med anledning av detta är det väsentligt att undersöka effeken av jord-struktur interaktionen på plattrambroar. I detta examensarbete har en numerisk parametrisk studie utförts för att erhålla kunskap om effekten av den relativa styvheten av broplattan och rambenen på jord-struktur interaktionen av plattrambroar. Fyra spännvidder har undersökts för tre olika randvillkor där i) ingen jord, ii) motfyllning samt iii) halvrymd har beaktats. Analysen utfördes på två- och tredimensionella finita element modeller. Motfyllningen respektive underliggande jord modellerades med finita element på ett direkt- samt förenklat tillvägagångssätt där Kelvin-Voigt modeller och frekvensberoende impedansfunktioner användes. Mellan motfyllningen och rambenen har separation tillåtits där det icke-linjära förhållandet av interaktionen undersöktes med tryckbeteenden för fjädrarna. Resultaten belyser vikten av att inkludera jord-struktur interaktionen i dynamiska analyser p.g.a. ökningen den medför för den modala dämpningen och egenfrekvensen. För korta spännvidder, påvisades det att effekten av motfyllningen var mer framstående för reduktionen av vibrationerna orsakade av tåg. För längre spännvidder framgick det däremot att underjorden hade en större påverkan. Effekten av jord-struktur interaktionen på spännvidderna kvantifierades som ett logaritmiskt samband för den modala dämpningen av första vertikala böjmoden. Tvådimensionella modeller har varit problematiska när plana spänningselement användes p.g.a. känsligheten i responsen orsakad av variationer i elementtjockleken. Därav rekommenderas tvådimensionella modeller endast om validering mot tredimensionella eller fältmätningar är möjliga. När separation tilläts i gränsytan av jord-struktur interaktionen, visade det sig att direkt tillvägagångssätt med finita element var mer lämplig med hänsyn till det icke-linjära kontaktbeteendet. Detta eftersom de statiska effekterna av jorden påverkade accelerationsresponsen markant. De statiska effekterna har inte varit möjliga att simulera i dem förenklade icke-linjära modeller med tryckfjädrar.

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