<p>After only two years of service, extensive cracking was found in the webs of two light-rail commuter line bridges in Stockholm, the Gröndal and Alvik bridges. Due to this incident it was found necessary to study the means available for analysing shear cracking in concrete structures subjected to in-plane stresses. The aim of this PhD project is to study shear cracking with these two bridges as reference. In this thesis, the first part aims to study the possibility of using finite element analysis as a tool for predicting shear cracking for plane state stresses. The second part is concerning how the shear cracks are treated in the concrete design standards.</p><p>Shear cracking in reinforced beams has been studied with non-linear finite element analyses. In these analyses the shear cracking behaviour was compared to experiments conducted to analyse the shear failure behaviour. Finite element analyses were performed with two different FE programs Abaqus and Atena. The material model used in Atena is a smeared crack model based on damage and fracture theory with either fixed or rotated crack direction. The material model used in Abaqus is based on plasticity and damage theory. The fixed crack model in Atena and the model in Abaqus gave good results for all studied beams. For the two studied deep beams with flanges the results from the rotated crack model were almost the same as obtained with the fixed crack model. The rotated crack model in Atena gave though for some beams a rather poor estimation of the behaviour.</p><p>The calculation of crack widths of shear cracks has been studied for the long-term load case in the serviceability state for the Gröndal and Alvik bridges, with the means available in the design standards. The methods based on the crack direction corresponding to the principal stress and do not include the effect of aggregate interlocking seems to be too conservative. Two of the studied methods included the effect of aggregate interlocking, it was made either by introducing stresses in the crack plane or implicitly by changing the direction of the crack so that it no longer coincide with the direction of principal stress. For calculations based on probable load conditions, these methods gave estimations of the crack widths that were close to the ones observed at the bridges. Continuous measurements of cracks at the Gröndal and the Alvik bridges have also been included. Monitoring revealed that the strengthening work with post-tensioned tendons has, so far, been successful. It also revealed that the crack width variations after strengthening are mainly temperature dependent where the daily temperature variation creates movements ten times greater than those from a passing light-rail vehicle. Monitoring a crack between the top flange and the webs on the Gröndal Bridge showed that the top flange was moving in a longitudinal direction relative to the web until the strengthening was completed. The crack widths in the sections strengthened solely by carbon fibre laminates seem to increase due to long-term effects.</p>

After only two years of service, extensive cracking was found in the webs of two light-rail commuter line bridges in Stockholm, the Gröndal and Alvik bridges. Due to this incident it was found necessary to study the means available for analysing shear cracking in concrete structures subjected to in-plane stresses. The aim of this PhD project is to study shear cracking with these two bridges as reference. In this thesis, the first part aims to study the possibility of using finite element analysis as a tool for predicting shear cracking for plane state stresses. The second part is concerning how the shear cracks are treated in the concrete design standards. Shear cracking in reinforced beams has been studied with non-linear finite element analyses. In these analyses the shear cracking behaviour was compared to experiments conducted to analyse the shear failure behaviour. Finite element analyses were performed with two different FE programs Abaqus and Atena. The material model used in Atena is a smeared crack model based on damage and fracture theory with either fixed or rotated crack direction. The material model used in Abaqus is based on plasticity and damage theory. The fixed crack model in Atena and the model in Abaqus gave good results for all studied beams. For the two studied deep beams with flanges the results from the rotated crack model were almost the same as obtained with the fixed crack model. The rotated crack model in Atena gave though for some beams a rather poor estimation of the behaviour. The calculation of crack widths of shear cracks has been studied for the long-term load case in the serviceability state for the Gröndal and Alvik bridges, with the means available in the design standards. The methods based on the crack direction corresponding to the principal stress and do not include the effect of aggregate interlocking seems to be too conservative. Two of the studied methods included the effect of aggregate interlocking, it was made either by introducing stresses in the crack plane or implicitly by changing the direction of the crack so that it no longer coincide with the direction of principal stress. For calculations based on probable load conditions, these methods gave estimations of the crack widths that were close to the ones observed at the bridges. Continuous measurements of cracks at the Gröndal and the Alvik bridges have also been included. Monitoring revealed that the strengthening work with post-tensioned tendons has, so far, been successful. It also revealed that the crack width variations after strengthening are mainly temperature dependent where the daily temperature variation creates movements ten times greater than those from a passing light-rail vehicle. Monitoring a crack between the top flange and the webs on the Gröndal Bridge showed that the top flange was moving in a longitudinal direction relative to the web until the strengthening was completed. The crack widths in the sections strengthened solely by carbon fibre laminates seem to increase due to long-term effects. / QC 20101119

Esta pesquisa apresenta uma contribuição ao entendimento do efeito da variação do estado de tensões e variação de umidade na compactação, na suscetibilidade à deformação permanente dos solos de subleito. Assim, um programa experimental foi desenvolvido considerando as variáveis o estado de tensões e a umidade, que são importantes na análise da evolução da deformação permanente dos solos de subleito. Para esta avaliação, foi utilizado um solo arenoso laterítico proveniente da região de Dois Córregos, estado de São Paulo, Brasil. Foram realizados ensaios de caracterização e ensaios triaxiais de cargas repetidas (módulo de resiliência e deformação permanente). Adicionalmente, foi analisado o efeito do acúmulo de deformação permanente nos valores de módulo de resiliência, que foi efetuado a partir de ensaios realizados antes e após o ensaio de deformação permanente. Além disso, de posse dos resultados de deformação permanente foi verificada a ocorrência do Shakedown. Concluiu-se que um aumento de tensões causa um aumento na deformação permanente, não sendo possível avaliar o efeito da variação de umidade na deformação permanente. Observou-se a ocorrência do Shakedown, porém não foi possível definir seu limite. Não foi possível o enquadramento da deformação permanente no modelo de Monismith, sendo proposto, por meio de regressão múltipla, um modelo para avaliação da deformação permanente, considerando as variáveis, umidade, número de aplicações de carga e estado de tensões. / This research presents a contribution to understanding the effect of the variation of the state of stress and water content in the evolution of the permanent deformation of subgrade soils. A lateritic sandy soil from the region of Dois Córregos, State of São Paulo, Brazil, was used throughout the characterization and triaxial repeated loads tests (resilient module and permanent deformation tests). Additionally, it was analyzed the effect of the accumulation of permanent deformation in the values of resilient module from tests performed before and after the permanent deformation tests, as well as it was verified the occurrence of Shakedown. It was concluded that an increase of stress causes an increase in permanent deformation, but it was not possible to evaluate the effect of the variation of moisture content in permanent deformation. It was also observed the occurrence of Shakedown, but it was not possible to set its limit. Furthermore, it was not possible to fit the permanent deformation testing data in the model of Monismith, being proposed, by means of multiple regression, a model for the assessment of permanent deformation considering the variables moisture content, number of load applications and state of stress.

Esta pesquisa apresenta uma contribuição ao entendimento do efeito da variação do estado de tensões e variação de umidade na compactação, na suscetibilidade à deformação permanente dos solos de subleito. Assim, um programa experimental foi desenvolvido considerando as variáveis o estado de tensões e a umidade, que são importantes na análise da evolução da deformação permanente dos solos de subleito. Para esta avaliação, foi utilizado um solo arenoso laterítico proveniente da região de Dois Córregos, estado de São Paulo, Brasil. Foram realizados ensaios de caracterização e ensaios triaxiais de cargas repetidas (módulo de resiliência e deformação permanente). Adicionalmente, foi analisado o efeito do acúmulo de deformação permanente nos valores de módulo de resiliência, que foi efetuado a partir de ensaios realizados antes e após o ensaio de deformação permanente. Além disso, de posse dos resultados de deformação permanente foi verificada a ocorrência do Shakedown. Concluiu-se que um aumento de tensões causa um aumento na deformação permanente, não sendo possível avaliar o efeito da variação de umidade na deformação permanente. Observou-se a ocorrência do Shakedown, porém não foi possível definir seu limite. Não foi possível o enquadramento da deformação permanente no modelo de Monismith, sendo proposto, por meio de regressão múltipla, um modelo para avaliação da deformação permanente, considerando as variáveis, umidade, número de aplicações de carga e estado de tensões. / This research presents a contribution to understanding the effect of the variation of the state of stress and water content in the evolution of the permanent deformation of subgrade soils. A lateritic sandy soil from the region of Dois Córregos, State of São Paulo, Brazil, was used throughout the characterization and triaxial repeated loads tests (resilient module and permanent deformation tests). Additionally, it was analyzed the effect of the accumulation of permanent deformation in the values of resilient module from tests performed before and after the permanent deformation tests, as well as it was verified the occurrence of Shakedown. It was concluded that an increase of stress causes an increase in permanent deformation, but it was not possible to evaluate the effect of the variation of moisture content in permanent deformation. It was also observed the occurrence of Shakedown, but it was not possible to set its limit. Furthermore, it was not possible to fit the permanent deformation testing data in the model of Monismith, being proposed, by means of multiple regression, a model for the assessment of permanent deformation considering the variables moisture content, number of load applications and state of stress.

Diploma thesis deals with reconstruction of old beam bridge in town Ledeč nad Sázavou. Three variants of reconsttruction were done. Detailed structural design report was done to one of them, streithening with post-tensioning. Calculation of load effect was solved with finite elemet method via software on grillage model. Construction was checked on ultimate limit state and serviceability limit state. Necessary drawings were elaborated to chosen method.

High electron mobility transistors (HEMTs) realized using AlxGa1-xN/GaN are relatively new technology which is prominent for high-speed and high-power applications. Some of the main problems with this technology were identified as dynamic RDSon, current collapse and threshold voltage instabilities due to the off-state stress. What was less investigated is the effect of the off-state stress on the leakage current at lower voltages. In this work, multiple devices with various initial leakage currents are stressed at different stress conditions (drain voltage, temperature, duration) and the development of drain-source on-state resistance (RDSon), threshold voltage (Vth) and drain-source leakage current (IDSS) after stress are tracked. It was found out that devices with initially higher leakage exhibit higher RDSon and Vth before stress, which simulations attributed to the higher Al mole fraction in the back-barrier or less unintentional doping in the channel layer. During the off-state stress (VDS = 900 V), the leakage current shortly rises and then sharply drops, presumably because of the charge redistribution in the back-barrier. After the stress, no larger changes were observed for RDSon and Vth , but they were for the leakage current i.e., initially low leakage devices had post-stress leakage increase, while initially high leakage devices had post-stress leakage decrease. This is assumed to be caused by the charge redistribution. Parasitic capacitance measurements showed the rise of the pre-stress input, output and reverse transfer capacitances with the pre-stress leakage, which could presumably be explained by higher Al mole fraction inducing more charges in the channel layer, deeming higher Al mole fraction in the back-barrier as a main assumed cause for all the observed effects. After the stress, capacitance changes were tentatively explained by the charge redistribution in the back-barrier. Finally, high temperature was shown to significantly reduce the observed long time to recovery. However, more measurements are needed to further observe this influence. Additionally, more experiments (e.g., on wafer, G-ω measurement, etc.) are needed in general to further investigate the mechanisms behind these memory effects. / Transistorer med extra hög elektronmobilitet, sk HEMT-ar (high electron mobility transistor) kan fabriceras med hjälp av materialen galliumnitrid (GaN) och aluminium-galliumnitrid (AlxGa1-xN). GaN HEMT-ar lämpar sig mycket väl för kraftkomponenter för höga frekvenser. Några av problemen med denna nya teknologi är hög resistans mellan ”drain” och ”source”, RDSon, reduktion av mättnadsströmmen (”current collapse”) och instabiliteter hos styrets tröskelspänning (Vth) när komponenterna stressas i avslaget tillstånd. Mindre känt är effekterna av stress för en avslagen komponent på läckströmmen (IDSS) vid låga påkänningar. I detta arbete har komponenter med varierande läckströmmar utsatts för olika typer av stress, t ex drain-spänning och höjd temperatur under olika långa tider samtidigt som parametrarna RDSon, Vth och IDSS har uppmätts. Det kunde noteras att komponenter med initialt högre läckströmmar hade högre RDSon och Vth innan de utsattes för stress, vilket med hjälp av simuleringar kunde visas bero på högre Al-molfraktion i skiktet under AlxGa1-xN-lagret, alternativt lägre dopning i kanal-lagret. Vid stress (VDS = 900 V) med komponenterna i avslaget tillstånd ökade läckströmmen kortvarigt för att sedan minska, troligtvis beroende på att laddningar omfördelas i lagret under kanal-skiktet. Efter stress sågs inte några större förändringar hos RDSon och Vth , men förändringar syntes på läckströmmen: komponenter med initialt låga läckströmmar visade ökning av läckströmmen efter stress, medan komponenter med initialt höga läckströmmar visade en post-stress minskning av läckström. Detta antas bero på den nämnda omfördelningen av laddningar djupare ner i komponenterna. Mätningar av parasitisk kapacitans visade på en ökning av pre-stress in- och ut-kapacitanserna (Ciss, Coss) samt ”gate-drain”-kapacitansen som funktion av pre-stress läckströmmar. Detta kan möjligtvis förklaras av att en högre andel Al inducerar mer laddningar i kanalen, vilket indikerar att högre Al-molfraktion i skiktet under kanal-lagret är orsaken till många av de effekter som setts. Kapacitansförändringar efter stress förklaras troligen av att laddningarna i detta lager återgår. Till sist noteras också att hög temperatur tydligt reducerade den långa tiden som komponenterna behövde för att återhämta sig. Det var också tydligt att mer mätningar är nödvändiga för att ytterligare säkerställa de uppmätta förändringarna, t ex mätningar av G-ω på icke kapslade chips, skulle kunna öka förståelsen för dessa minneseffekter.