Le benzo-a-pyrène (BaP) est un hydrocarbure aromatique polycyclique (HAP) cancérogène pour l’homme, qui contamine toutes les sphères de notre environnement. Son métabolite, le BaP-7,8-diol-9,10-époxyde (BPDE) est considéré comme son cancérogène ultime. Le BPDE se lie à l’ADN, formant des adduits qui doivent être réparés et qui seraient responsables des dommages à l’ADN et de la cancérogenèse induite par le BaP. Les adduits BPDE-ADN et les dommages à l’ADN (bris simple-brin [BSB] à l’ADN, aberrations chromosomiques [AC], échanges entre chromatides-sœurs [ÉCS] et micronoyaux [MN]) ont été mesurés dans les lymphocytes humains exposés à de faibles concentrations de BaP, provenant de jeunes volontaires non-fumeurs et en santé. Suite à l’exposition au BaP, le niveau d’adduits BPDE-ADN et la fréquence des AC et des MN augmentent significativement, puis diminuent aux concentrations les plus élevées de BaP testées, suggérant une induction du métabolisme de phase II du BaP. Lors de la mesure des ÉCS, nous obtenons une courbe dose-réponse linéaire, indiquant la production d’un autre type de lésions devant être réparées par le système de réparation par recombinaison homologue. Ces lésions pourraient être des bris à l’ADN ou des bases oxydées (8-OH-dG), ce qui est suggéré par l’analyse des corrélations existant entre nos biomarqueurs. Par ailleurs, la comparaison de la courbe dose-réponse des hommes et des femmes montre que des différences existent entre les sexes. Ainsi, les ÉCS, les AC et les MN sont significativement augmentés chez les hommes à la plus faible concentration de BaP, alors que chez les femmes cette augmentation, quoique présente, est non significative. Des différences interindividuelles sont également observées et sont plus importantes pour les adduits BPDE-ADN, les MN et les AC, alors que pour les ÉCS elles sont minimes. Les analyses statistiques effectuées ont permis d’établir que quatre facteurs (niveau d’exposition au BaP, adduits BPDE-ADN, fréquence des AC et nombre de MN par cellule micronucléée) expliquent jusqu’à 59 % de la variabilité observée dans le test des ÉCS, alors qu’aucun facteur significatif n’a pu être identifié dans le test des AC et des MN. L’analyse du mécanisme de formation de nos biomarqueurs précoces permet de suggérer que les bris à l’ADN et les bases oxydées devraient être classées comme biomarqueurs de dose biologique efficace, au sein des biomarqueurs d’exposition, dans le continuum exposition-maladie du BaP, étant donné qu’ils causent la formation des biomarqueurs de génotoxicité (ÉCS, AC et MN). Par ailleurs, le test des AC et des MN ont permis de confirmer l’action clastogénique du BaP en plus de mettre en évidence des effets aneugènes affectant surtout la ségrégation des chromosomes lors de la division cellulaire. Ces effets aneugènes, reliés à l’étape de progression dans la cancérogenèse, pourraient être particulièrement importants puisque l’exposition au BaP et aux HAP est chronique et dure plusieurs années, voire des décennies. La compréhension des mécanismes régissant la formation des biomarqueurs étudiés dans cette étude, ainsi que des relations existant entre eux, peut être appliquée à de nombreux contaminants connus et émergents de notre environnement et contribuer à en évaluer le mode d’action. / Benzo-a-pyrene (BaP) is a polycyclic aromatic hydrocarbon (PAH) classified as carcinogenic to human, and is present throughout our environment. Metabolic activation of BaP leads to production of BaP-7,8-diol-9,10-epoxide (BPDE), considered as its ultimate carcinogenic metabolite. BPDE can bind to DNA, forming BPDE-DNA adducts at the origin of BaP-induced DNA damage and carcinogenesis. BPDE-DNA adducts and DNA damages (DNA single-strand breaks [SSBs], chromosomal aberrations [CAs], sister chromatid exchanges [SCEs] and micronuclei [MNs]) are measured in human lymphocytes exposed to low BaP concentrations, taken from non-smoking healthy young subjects. Following BaP exposure, BPDE-DNA adduct levels, as well as CA and MN frequencies raise significantly, and then decrease to the higher BaP concentrations tested, suggesting metabolic enzyme saturation or induction of BaP phase II metabolism. As for SCEs test, a linear dose response curve is obtained, suggesting that production of additional DNA lesions requiring homologous recombination repair may occur. These lesions could be DNA breaks or oxidized DNA bases (8-OH-dG), as indicated by correlation analysis performed between our biomarkers. Additionally, when comparing the dose-response curves for men and women separately, some differences show up. Indeed, SCEs, CAs, and MNs are significantly increased in men at the lowest BaP concentration tested, while in women, this increase is present but not significant. Interindividual differences are also present and are more considerable for BPDE-DNA adducts, MNs and CAs, whereas they are very low for SCEs. Statistical analysis showed that four factors (BaP exposure level, BPDE-DNA adducts, CA frequency and number of MN per micronucleated cell) significantly explained up to 59 % of observed variability in SCE test, while no such factors could explain the observed variability in CA and MN test. Following analysis of mechanisms underlying the formation of early biomarkers, we suggest a modification of the Exposure-Disease Continuum of BaP. We propose that DNA breaks and oxidized DNA bases should be classified as biomarkers of biologically effective dose (part of the exposure biomarkers), as their presence are at origin of early biomarkers of genotoxicity (SCEs, CAs and MNs). On the other hand, CA and MN tests confirmed clastogenic properties of BaP, and highlighted aneugenic effects influencing mostly chromosome segregation during cell division. These aneugenic effects, linked to the progression step of carcinogenesis, could be of particular importance given that exposure to BaP and other PAHs (smoking, occupational exposure) are chronic and may last for decades. Understanding the mechanisms playing a role in early biomarkers formation, as well as the relations existing between them, can be largely applied in our environment to many known and emerging contaminants, thus contributing to characterize their mode of action.

Imager des objets non-périodiques à une échelle nanométrique et à une échelle femto seconde est un vrai challenge à notre époque. Les techniques d'imagerie " sans lentille " sont des moyens puissants pour répondre à ce besoin. En utilisant des sources ultrarapide (~fs) et cohérente (ex. laser à électron libre ou harmoniques d'ordres élevés), ces techniques nous permettent de reconstruire des objets à partir de leur figure de diffraction, remplaçant les optiques conventionnelles du système d'imagerie par un algorithme informatique. Dans ce travail de thèse, je présent des expériences d'imageries en utilisant un rayonnement extrême-UV (15~40 nm) produit par la génération d'harmoniques d'ordre élevé d'un laser infrarouge puissant. Ce manuscrit est constitué d'une introduction, un chapitre de background théorique, trois chapitres de travail de thèse et une conclusion générale avec perspectives. La première partie du travail de thèse porte sur les développements et caractérisations de la ligne de lumière avec l'objectif de générer maximum de photons harmoniques cohérents avec un front d'onde plat. La deuxième partie est consacrée aux expériences et analyses de trois techniques d'imageries " sans lentille " : Imagerie par diffraction cohérente (CDI), Holographie par la transformée de Fourier (FTH) et Holographie avec références étendues (HERALDO). Ces derniers nous permettent de reconstruire des objets avec une résolution spatiale de 78 nm dans le cas de CDI et de 112 nm dans le cas de HERALDO, tous les deux avec une résolution temporaire de 20 fs. La troisième partie est une première application physique de l'imagerie sur la ligne harmonique. Il s'agit des études statiques et dynamiques de nano-domaines magnétique avec une résolution spatiale sub-100 nm à l'échelle femto seconde. Perspective des techniques d'imagerie 3D et développement potentiel de la ligne d'harmoniques sont présentés à la fin.

The thesis studies three distinct issues in monetary economics using a common dynamic general equilibrium approach under the assumptions of rational expectations and nominal price rigidity. The first chapter deals with the so-called "liquidity trap" - an issue which was raised originally by Keynes in the aftermath of the Great Depression. Since the nominal interest rate cannot fall below zero, this limits the scope for expansionary monetary policy when the interest rate is near its lower bound. The chapter studies the conduct of monetary policy in such an environment in isolation from other possible stabilization tools (such as fiscal or exchange rate policy). In particular, a standard New Keynesian model economy with Calvo staggered price setting is simulated under various alternative monetary policy regimes, including optimal policy. The challenge lies in solving the (otherwise linear) stochastic sticky price model with an explicit occasionally binding non-negativity constraint on the nominal interest rate. This is achieved by parametrizing expectations and applying a global solution method known as "collocation". The results indicate that the dynamics and sometimes the unconditional means of the nominal rate, inflation and the output gap are strongly affected by uncertainty in the presence of the zero lower bound. Commitment to the optimal rule reduces unconditional welfare losses to around one-tenth of those achievable under discretionary policy, while constant price level targeting delivers losses which are only 60% larger than under the optimal rule. On the other hand, conditional on a strong deflationary shock, simple instrument rules perform substantially worse than the optimal policy even if the unconditional welfare loss from following such rules is not much affected by the zero lower bound per se. The second thesis chapter (co-authored with Andrea Pescatori) studies the implications of imperfect competition in the oil market, and in particular the existence of a welfare-relevant trade-off between inflation and output gap volatility. In the standard New Keynesian model exogenous oil shocks do not generate any such tradeoff: under a strict inflation targeting policy, the output decline is exactly equal to the efficient output contraction in response to the shock. I propose an extension of the standard model in which the existence of a dominant oil supplier (such as OPEC) leads to inefficient fluctuations in the oil price markup, reflecting a dynamic distortion of the economy's production process. As a result, in the face of oil sector shocks, stabilizing inflation does not automatically stabilize the distance of output from first-best, and monetary policymakers face a tradeoff between the two goals. The model is also a step away from discussing the effects of exogenous oil price changes and towards analyzing the implications of the underlying shocks that cause the oil price to change in the first place. This is an advantage over the existing literature, which treats the macroeconomic effects and policy implications of oil price movements as if they were independent of the underlying source of disturbance. In contrast, the analysis in this chapter shows that conditional on the source of the shock, a central bank confronted with the same oil price change may find it desirable to either raise or lower the interest rate in order to improve welfare. The third thesis chapter (co-authored with Andrea Pescatori) studies the extent to which the rise in US macroeconomic stability since the mid-1980s can be accounted for by changes in oil shocks and the oil share in GDP. This is done by estimating with Bayesian methods the model developed in the second chapter over two samples - before and after 1984 - and conducting counterfactual simulations. In doing so we nest two other popular explanations for the so-called "Great Moderation": (1) smaller (non-oil) shocks; and (2) better monetary policy. We find that the reduced oil share can account for around one third of the inflation moderation, and about 13% of the GDP growth moderation. At the same time smaller oil shocks can explain approximately 7% of GDP growth moderation and 11% of the inflation moderation. Thus, the oil share and oil shocks have played a non-trivial role in the moderation, especially of inflation, even if the bulk of the volatility reduction of output growth and inflation is attributed to smaller non-oil shocks and better monetary policy, respectively. / La tesis estudia tres problemas distintos de macroeconomía monetaria utilizando como marco común el equilibrio general dinámico bajo expectativas racionales y con rigidez nominal de los precios. El primer capítulo trata el problema de la "trampa de liquidez" - un tema planteado primero por Keynes después de la Gran Depresión de 1929. El hecho de que el tipo de interés nominal no pueda ser negativo limita la posibilidad de llevar una política monetaria expansiva cuando el tipo de interés se acerca a cero. El capítulo estudia la conducta de la política monetaria en este entorno en aislamiento de otros posibles instrumentos de estabilización (como la política fiscal o la política de tipo de cambio). En concreto, se simula un modelo estándar Neo-Keynesiano con rigidez de precios a la Calvo bajo diferentes regimenes de política monetaria, incluida la política monetaria óptima. El reto consiste en resolver el modelo estocástico bajo la restricción explícita ocasionalmente vinculante de no negatividad de los tipos de interés. La solución supone parametrizar las expectativas y utilizar el método de solución global conocido como "colocación". Los resultados indican que la dinámica y en ocasiones los valores medios del tipo de interés, la inflación y el output gap están muy influidos por la presencia de la restricción de no negatividad. El compromiso con la regla monetaria óptima reduce las pérdidas de bienestar esperadas hasta una décima parte de las pérdidas obtenidas bajo la mejor política discrecional, mientras una política de meta constante del nivel de precios resulta en pérdidas que son sólo 60% mayores de las obtenidas bajo la regla óptima. Por otro lado, condicionado a a un choque fuerte deflacionario, las reglas instrumentarias simples funcionan mucho peor que la política óptima, aun si las pérdidas no condicionales de bienestar asociadas a dichas reglas no están muy afectadas por la presencia de la restricción de no negatividad en si. El segundo capítulo de la tesis estudia las implicaciones de la competencia imperfecta en el mercado del petróleo, y en concreto la existencia de un conflicto relevante entre la volatilidad de la inflación y la del output gap de un país importador de petróleo. En el modelo estándar Neo Keynesiano, los choques petroleros exógenos no generan ningún conflicto de objetivos: bajo una política de metas de inflación estricta, la caída del output es exactamente igual a la contracción eficiente del output en respuesta al choque. Este capitulo propone una extensión del modelo básico en la cual la presencia de un proveedor de petróleo dominante (OPEP) lleva a fluctuaciones ineficientes en el margen del precio del petróleo que reflejan una distorsión dinámica en el proceso de producción de la economía. Como consecuencia, ante choques provinientes del sector de petróleo, una política de estabilidad de los precios no conlleva automáticamente a una estabilización de la distancia del output de su nivel eficiente y existe un conflicto entre los dos objetivos. El modelo se aleja de la discución los efectos de cambios exógenos en el precio del petróleo y se acerca al análisis de las implicaciones de los factores fundamentales que provocan los cambios en el precio del petróleo en primer lugar. Esto último representa una ventaja clara frente a la literatura existente, la cual trata tanto los efectos macroeconómicos como las implicaciones para la política monetaria de cambios en el precio del petróleo como si éstos fueran independientes de los factores fundamentales provocando dicho cambio. A diferencia de esta literatura, el análisis del capitulo II demuestra cómo frente al mismo cambio en el precio del petróleo, un banco central puede encontrar deseable bien subir o bajar el tipo de interés en función del origen del choque. El tercer capitulo estudia el grado en que el ascenso de la estabilidad macroeconómica en EE.UU. a partir de mediados de los 80 se puede atribuir a cambios en la naturaleza de los choques petroleros y/o el peso del petróleo en el PIB. Con este propósito se estima el modelo desarrollado en el capitulo II con métodos Bayesianos utilizando datos macroeconómicos de dos periodos - antes y después de 1984 - y se conducen simulaciones contrafactuales. Las simulaciones permiten dos explicaciones alternativas de la "Gran Moderación": (1) menores choques no petroleros; y (2) mejor política monetaria. Los resultados apuntan a que el petróleo ha jugado un papel no-trivial en la moderación. En particular, el menor peso del petroleo en el PIB a partir de 1984 ha contribuido a una tercera parte de la moderación de la inflación y un 13% de la moderación del output. Al mismo tiempo, un 7% de la moderación del PIB y 11% de la moderación de la inflación se pueden atribuir a menores choques petroleros.

Nightclubs and restaurant bars have become major sources of noise pollution particularly in areas close to residential dwellings. The purpose of this study was to investigate the impact of noise emanating from nightclubs and restaurant bars on the community of Melville, Johannesburg. This study followed both qualitative and quantitative research methods. A total of 100 respondents were randomly sampled within the study area. Qualitative data was collected using a structured questionnaire. A calibrated sound level meter was used to measure environmental noise levels at 10 different measuring points. The research finding revealed that about:  87% of noise levels measured with the sound level meter did not comply with officially acceptable levels of 40dB at night.  69% of respondents indicated that the main source of noise is pollution is nightclubs.  78% of respondents described noise as annoying, disturbing and unwanted.  57% of respondents indicated that members of their household have suffered from sleeping disorders due to noise activities at night disrupting their sleep patterns and resulting in irritability and fatigue. The noise measurements were taken on weekends and public holidays during the day from 10h00 to 14h30 and at night from 22h00 to 02h30. The research findings revealed that the residents of Melville experienced high level of noise at night with nightclub as major source of noise and as a result the majority of the sampled population complained about irritability, fatigue and sleeping disorders due to exposure to noise. The outcome of this research indicated the need of health education on the adverse effects of noise pollution and the need of sound insulation at places of entertainment. Implementation of a noise management policy is needed in order to effectively control and manage the noise pollution in its area of jurisdiction and regular noise level monitoring by constantly taking noise measurements by law enforcements officers. / Department of Environmental Sciences / M. Sc. (Environmental Management)

Les méthodes perturbatives, telle que l’approximation de Born, sont nécessaires pour résoudre les problèmes inhérents à l’ionisation d’atomes et de molécules par impact d’électrons ou de positrons. Afin d’optimiser les calculs nécessaires pour le second terme de l’approximation de Born, nous avons commencé par étudier l’ionisation de l’atome le plus simple : celui d’hydrogène. Nous avons utilisé une base contenant un grand nombre d’états (294) nous permettant d’éviter la relation de fermeture qui nécessite l’introduction d’un paramètre qui représente la valeur moyenne d’excitation. Nos résultats ont montré un bon accord avec l’expérience surtout pour les faibles énergies des électrons éjectés. Nous avons ainsi pu montrer l’importance de la contribution des états du continuum (représentés par des pseudo-états), en particulier pour les transitions de type dipolaire. Pour la double ionisation de l’atome d’hélium, nous avons appliqué la même méthodologie de calcul numérique complet tout en incluant 20 états et pseudo-états intermédiaires et en utilisant une fonction d’onde corrélée d’interaction de configuration, on a trouvé pour les grandes énergies d’incidence (5 keV) que l’effet du terme Born 2 est presque nul. Par contre l’application de « SBA » avec la relation de fermeture pour l’état fondamental et les premiers états excités montre une petite différence avec « FBA » en particulier en dehors de la région du transfert. Dans le cas des faibles énergies d’incidence (601 eV) il était attendu d’obtenir un effet important de « SBA » surtout que des études sur les (e,3-1e) de l’hélium montrent un déplacement du pic principal par rapport à « FBA ». Donc on peut dire que les 20 états intermédiaires contribuent de façon insuffisante et qu’il faudra considérer beaucoup plus d’états / The perturbative methods, such as Born approximation, are necessary to solve the problems concerning the ionization of atoms and molecules by electrons or positrons impacts. In order to use Born approximation in an optimized way, we tested it on the simplest atom « Hydrogen » by using a basis of large amount of intermediate states (294) and complete numerical calculation without using the closure approximation whose application needs the introduction of a parameter which is the excitation mean value. Our results proved a significant agreement with the experiment particularly for small energies of the ejected electrons. We also proved an important contribution of the continuum (represented by the pseudo-states), and particularly the dipolar transition. For the double ionization of Helium atom, we applied the same methodology of complete calculation by including 20 intermediate states and pseudo-states and by using a configuration interaction wave function, we found that for high incident energy (5 keV) the effect of the second term of Born is almost zero. However, the application of the « SBA » with the closure approximation by using the fundamental state and the first excited states show a slight difference relative to the « FBA » particularly outside the transfer region. In case of low incident energy (601 eV), it was expected a crucial effect of the « SBA » especially that previous studies of (e, 3-1e) of Helium show a significant shift of the main peak relative to the « FBA ». So we can say that 20 intermediate states are not enough and the application of the « SBA » needs more states

The Russian wheat aphid (RWA) (Diuraphis noxia Kurdjumov) is one of the important insect pests of wheat (Triticum aestivum L.), barley (Hordeum vulgare L.) and other grasses. To date, there are four RWA biotypes identified in South Africa. The virulent biotypes emerged, partly due to climate change and new genetic variations within populations of RWA; hence there is a need to improve host-plant resistance, as an effective control measure. Bread wheat (Triticum aestivum L.) accession Cereal Introduction (CItr) 2401 is known to be resistant to all RWA biotypes worldwide. The goal of this study was to use a backcrossed near-isogenic line (NIL) BC5F5 mapping population, developed from a cross between CItr 2401 and susceptible Kavkaz, to identify and validate single nucleotide polymorphism (SNP) markers linked to the resistance phenotype in CItr 2401. This was achieved by (i) conducting a preliminary study that evaluated the suitability of simple sequence repeat (SSR) markers previously reported in literature for discriminating stacked RWA resistance genes and, (ii) employing SNP markers for the first time in a RWA resistance study as a future alternative to the widely used SSR markers. None of the tested SSR markers showed potential use in marker-assisted selection (MAS). The mapping population was phenotypically evaluated for RWA resistance using the four South African biotypes, viz. RWASA1, RWASA2, RWASA3 and RWASA4. Analysis of variance (ANOVA) showed significant (P<0.001) differences of genotypes after confirming the normality of residuals and homogeneity of variance. The Illumina iSelect 9,000 wheat SNP platform was used to genotype the two crossing parents and a selection of 24 NIL genotypes from the mapping population. Eight SNP markers found to be linked to the phenotype were converted to breeder-friendly and high-throughput Kompetitive allele-specific polymerase chain reaction (KASP) markers. The designed KASP markers were validated on the two crossing parents, the 24 NIL sent for SNP genotyping, on the mapping population and on the preliminary study genotypes for their effectiveness. The KASP assays developed in this study will be useful for stacking the RWA resistance from CItr 2401 with other Dn genes effective against the RWA. / Life and Consumer Sciences / M. Sc. (Life Sciences)

Practical work forms an integral part of physical science. Most schools in Venda have no laboratories for practical work. Teachers use the telling method. Students learn by rote. They learn without understanding science concepts. They cannot apply their knowledge to real life situations. This research investigates the use of equipment from inexpensive material in teaching physical science. Teachers trained to improvise equipment use them in classroom teaching. These help teachers to understand many science concepts and use the process approach in their teaching. The research found that when science is taught through experiments with improvised apparatus, student learning occurs at various domains of science education. The bad effects of lack of facilities for practical work are mostly eliminated. Meaningful learning leads to the understanding of science concepts. Students construct their own knowledge, apply it to any situation and enjoy learning the subject. / Curriculum and Instructional Studies / M. Ed. (Didactics)

No description available.

In this work the Point Charge Electrostatic Model (PCEM), the Simple Overlap Model (SOM) and the Method of Equivalents Nearest Neighbors (MENN) were applied to a well known series of oxides, namely, Gd2O3, Y2O3, Lu2O3, In2O3 and Sc2O3, all doped ion Eu3+ with the purpose of discussing the charge of interaction and some aspects of the crystal field effect. To this end, calculations were made of the crystal field and crystal field strength parameters and splitting of the 7F1 level of the luminescent ion. By using the luminescent site local structure, the PCEM, as expected, led to satisfactory results only from the qualitative point of view. With the SOM and the MENN it was possible to reproduce the experimental splitting of the 7F1 energy level and its sublevels, with physically acceptable charge factors. Only in some cases the NN charge has been greater than its valence. A discussion on the position of the charge of interaction also has been made. / Neste trabalho o Modelo Eletrostático de Cargas Pontuais, o Modelo de Recobrimento Simples e o Método dos Vizinhos Equivalentes foram aplicados a uma série bem conhecida de óxidos, a saber, Gd2O3, Y2O3, Lu2O3, In2O3 e Sc2O3, todos dopados com o íon Eu3+, com o objetivo de discutir a carga de interação e aspectos do efeito do campo cristalino. Para isso, foram feitos cálculos de parâmetros do campo cristalino, de parâmetro de força do campo cristalino, dos subníveis e desdobramento do nível 7F1 do íon luminescente. Usando a estrutura local do sitio luminescente, o modelo eletrostático, como esperado, levou a resultados satisfatórios apenas do ponto de vista qualitativo. Já com o modelo de recobrimento simples e com o método dos vizinhos equivalentes foi possível reproduzir o desdobramento experimental do nível de energia 7F1 e os seus subníveis, com fatores de carga fisicamente plausíveis. Em apenas alguns poucos casos a carga dos primeiros vizinhos foi maior que a valência respectiva. Também está feita uma discussão sobre a possível posição desta carga de interação.

Equações diferenciais de quarta ordem aparecem naturalmente na modelagem de oscilações de estruturas elásticas, como aquelas observadas em pontes pênseis. São considerados dois modelos que descrevem as oscilações no tabuleiro de uma ponte. No modelo unidimensional estudamos blow up em espaço finito de soluções de uma classe de equações diferenciais de quarta ordem. Os resultados apresentados solucionam uma conjectura apresentada em [F. Gazzola and R. Pavani. Wide oscillation finite time blow up for solutions to nonlinear fourth order differential equations. Arch. Ration. Mech. Anal., 207(2):717752, 2013] e implicam a não existência de ondas viajantes com baixa velocidade de propagação em uma viga. No modelo bidimensional analisamos uma equação não local para uma placa longa e fina, suportada nas extremidades menores, livre nas demais e sujeita a protensão. Provamos existência e unicidade de solução fraca e estudamos o seu comportamento assintótico sob amortecimento viscoso. Estudamos ainda a estabilidade de modos simples de oscilação, os quais são classificados como longitudinais ou torcionais. / Fourth order differential equations appear naturally when modeling oscillations in elastic structures such as those observed in suspension bridges. Two models describing oscillations in the roadway of a bridge are considered. In the one-dimensional model we study finite space blow up of solutions for a class of fourth order differential equations. The results answer a conjecture presented in [F. Gazzola and R. Pavani. Wide oscillation finite time blow up for solutions to nonlinear fourth order differential equations. Arch. Ration. Mech. Anal., 207(2):717752, 2013] and imply the nonexistence of beam oscillation given by traveling wave profile with low speed propagation. In the two-dimensional model we analyze a nonlocal equation for a thin narrow prestressed rectangular plate where the two short edges are hinged and the two long edges are free. We prove existence and uniqueness of weak solution and we study its asymptotic behavior under viscous damping. We also study the stability of simple modes of oscillations which are classified as longitudinal or torsional.