To meet the economic, social and infrastructure needs of the community for safe and efficient transportation systems, long span bridges have been built throughout the world. Long span bridges are one of the most challenging kinds of structures in civil engineering. The cable-stayed bridges are of great interest mainly as an alternative and a more economic solution than the one of suspension bridges. In addition, the fiber reinforced polymer (FRP) composites are, nowadays, successfully used for constructing modern bridges, where the significant weight saving provides additional benefits. Because of the great flexibility, modern long-span cable-stayed bridges are usually very susceptible to dynamic loads especially to the earthquake and strong winds. Therefore, the earthquake-resistant and wind-resistant designs become one of key issues for successful construction of bridges. The objective of the present research is to develop a very efficient spline finite strip technique, for modelling and analysis of both conventional and hybrid FRP cable-stayed bridges. The study falls into the categories of bending, free vibration, seismic, and aerodynamic flutter analysis. The spline finite strip method (SFSM) is one of the most efficient numerical methods for structural analysis of bridges, reducing the time required for estimating the structural response without affecting the degree of accuracy. In the finite strip method, the degrees of freedom could be significantly reduced due to the semi-analytical nature of this method. However, the previous versions of SFSM are not able to model the entire bridge system. For that reason, the structural interactions between different structural components of the bridge could not be handled. In addition, the vibrations and displacements of the towers and cables could not be investigated. In the present formulation, all these obstacles have been eliminated. Moreover, the proposed finite strip technique is very efficient and accurate due to the drastic reduction in the formulation time, simplicity of data preparation, rapid rate convergence of the results, and the semi-analytical nature. Last but not least, and for the first time, a fully finite strip solution is extended to the area of wind engineering. Using the spline finite strip discretization, the aerodynamic stiffness and mass properties of the long-span cable-stayed bridge are derived. The aerodynamic properties along with the structural properties of long-span plates and bridges are formulated in the aerodynamic equation of motion and are used to analyze the flutter problem. The accuracy and efficiency of the proposed advanced finite strip method is verified against the finite element and field measurement results. The results demonstrate that this methodology and the associated computer code can accurately predict the dynamic and aerodynamic responses of the conventional and FRP long-span cable-stayed bridge systems. The outcome of the present research will lead to a comprehensive structural analysis of bridges in the framework of the proposed discretization which is more efficient and straightforward than the finite element analysis.

Apresenta-se neste trabalho um estudo teórico-experimental sobre a instabilidade de perfis formados a frio submetidos à flexão. A instabilidade distorcional se faz comum na presença de tensões de compressão atuando sobre perfis enrijecidos e fabricados com aços de elevada resistência mecânica. A parte teórica abrange os métodos de cálculo analíticos e numéricos para a análise de instabilidade distorcional de perfis de seção aberta formados a frio. Na parte experimental inclui-se o estudo de perfis formados a frio com seções do tipo U enrijecidos submetidos aos ensaios à flexão. Nestes ensaios variou-se a altura de alma e espessura de chapa procurando-se abranger maior número de condições geométricas para análise da estabilidade distorcional. Inclui-se também a análise de instabilidade numérica dos perfis do programa experimental através do método de resistência direta via método das faixas finitas. Com base nos resultados experimentais, numéricos e na análise teórica do problema, verificou-se o procedimento adotado pela NBR14762/2001 e efetuou-se comparação entre curvas de resistência propostas para o dimensionamento de perfis formados a frio à flexão. Foi verificado que o fenômeno de instabilidade distorcional pode ser o estado limite último crítico para o dimensionamento dos perfis formados a frio. / This work present a study theoretical-experimental about the distortional buckling of cold-formed steel members under bending. The distortional buckling is common for members with edge-stiffened sections and manufactured with steel of high strength. The theorical part present the analitic and numerical methods for stability analysis by distortional buckling. The experimental part included the study of cold-formed with sections channel altering flange width and thickness by proof in flexural members. Included too the numerical analysis by Direct Strength Method in Finite Strip Method on members with the same configurations of experimental investigation. Based on the experimental measurements, numerical analysis and in the theorical analysis of work, verified the procedures adopted in brazilian code NBR14762/2001 and compared with others distortional curve proposed for to design of cold-formed members under bending. Verified that the distortional buckling may control the design being the critical mode for the ultimate limit state.

This thesis aims to reduce the reinforcement areas in lattice girder elements after large amounts have been observed in several projects. Existing handbooks and materials for designing lattice girder elements have been analyzed. A revised calculation model for design that is adapted to Eurocode and the Swedish national annex EKS 11 has then been developed. The calculation model should be seen as a proposal to how lattice girder elements can be designed. A reference project has laid the basis for testing the calculation model. Simultaneously two finite element models have been established in FEM-Design and smaller calculations have been performed in WIN-Statik: Concrete Beam.  The results show that the reinforcement areas have been reduced. An effective method of designing lattice girder elements is to extract forces and moments from a finite element program. These are then inserted into the calculation model together with other required data and by making the desired adjustments. The calculation model then designs the lattice girder element. Alternatively, the lattice girder element is designed using the FEM-software whilst the lattice girder is designed using the calculation model. Numerous methods of finding the design-moments have been studied. The results show that it is not satisfactory to calculate the moments with respect to a lattice girder element or a single strip. Thus, the whole slab must be taken into consideration.

Para o projeto de estruturas com perfis de aço formados a frio, é fundamental a compreensão dos fenômenos da instabilidade local e global, uma vez que estes apresentam alta esbeltez e baixa rigidez à torção. A determinação do carregamento crítico e a identificação do modo de instabilidade contribuem para o entendimento do comportamento dessas estruturas. Este trabalho avalia três metodologias para a análise linear de estabilidade de perfis de aço formados a frio isolados, com o objetivo de determinar os carregamentos críticos elásticos de bifurcação e os modos de instabilidade associados. Estritamente, analisa-se perfis de seção U enrijecido e Z enrijecido isolados, de diversos comprimentos e diferentes condições de vinculação e carregamento. Determinam-se os carregamentos críticos elásticos de bifurcação e os modos de instabilidade globais e locais por meio de: (i) análise com o Método das Faixas Finitas (MFF), através do uso do programa computacional CUFSM; (ii) análise com elementos finitos de barra baseados na Teoria Generalizada de Vigas (MEF-GBT), via uso do programa GBTUL; e (iii) análise com elementos finitos de casca (MEF-cascas) por meio do uso do programa ABAQUS. Algumas restrições e ressalvas com relação ao uso do MFF são apresentadas, assim como limitações da Teoria Generalizada de Viga e precauções a serem tomadas nos modelos de cascas. Analisa-se também a influência do grau de discretização da seção transversal. No entanto, não é feita avaliação em relação aos procedimentos normativos e tampouco análises não lineares, considerando as imperfeições geométricas iniciais, tensões residuais e o comportamento elastoplástico do material. / For the design of cold formed steel members, it is essential to understand the effects of local and global instability, since these members typically have a high slenderness and low torsion stiffness. The determination of critical loads and the associated buckling modes contribute to understand the behavior of these members. This work performs a evaluation of three methods for linear stability analysis of isolated cold-formed steel members in order to determine the elastic critical loads and the corresponding buckling modes. Specifically, Ue and Ze shape members were studied with various length, different boundary conditions and loads. The elastic critical loads and buckling modes are determined by means of: (i) analysis with the Finite Strip Method (FSM), by the computer program CUFSM, (ii) beam finite element analysis based on the Generalized Beam Theory (FEM-GBT), by GBTUL program, and (iii) Finite Element Method with shell analysis using ABAQUS program. Some restrictions and warnings regarding the use of the FSM are presented, as well as limitations of the Generalized Beam Theory and precautions to be taken in the shell models. It is also analyzed the influence of the degree of discretization of the cross section. In the present study, no evaluation was made with respect to normative procedures neither nonlinear analyses considering the initial geometric imperfections, residual stresses and elastoplastic behavior of the material.

Para o projeto de estruturas com perfis de aço formados a frio, é fundamental a compreensão dos fenômenos da instabilidade local e global, uma vez que estes apresentam alta esbeltez e baixa rigidez à torção. A determinação do carregamento crítico e a identificação do modo de instabilidade contribuem para o entendimento do comportamento dessas estruturas. Este trabalho avalia três metodologias para a análise linear de estabilidade de perfis de aço formados a frio isolados, com o objetivo de determinar os carregamentos críticos elásticos de bifurcação e os modos de instabilidade associados. Estritamente, analisa-se perfis de seção U enrijecido e Z enrijecido isolados, de diversos comprimentos e diferentes condições de vinculação e carregamento. Determinam-se os carregamentos críticos elásticos de bifurcação e os modos de instabilidade globais e locais por meio de: (i) análise com o Método das Faixas Finitas (MFF), através do uso do programa computacional CUFSM; (ii) análise com elementos finitos de barra baseados na Teoria Generalizada de Vigas (MEF-GBT), via uso do programa GBTUL; e (iii) análise com elementos finitos de casca (MEF-cascas) por meio do uso do programa ABAQUS. Algumas restrições e ressalvas com relação ao uso do MFF são apresentadas, assim como limitações da Teoria Generalizada de Viga e precauções a serem tomadas nos modelos de cascas. Analisa-se também a influência do grau de discretização da seção transversal. No entanto, não é feita avaliação em relação aos procedimentos normativos e tampouco análises não lineares, considerando as imperfeições geométricas iniciais, tensões residuais e o comportamento elastoplástico do material. / For the design of cold formed steel members, it is essential to understand the effects of local and global instability, since these members typically have a high slenderness and low torsion stiffness. The determination of critical loads and the associated buckling modes contribute to understand the behavior of these members. This work performs a evaluation of three methods for linear stability analysis of isolated cold-formed steel members in order to determine the elastic critical loads and the corresponding buckling modes. Specifically, Ue and Ze shape members were studied with various length, different boundary conditions and loads. The elastic critical loads and buckling modes are determined by means of: (i) analysis with the Finite Strip Method (FSM), by the computer program CUFSM, (ii) beam finite element analysis based on the Generalized Beam Theory (FEM-GBT), by GBTUL program, and (iii) Finite Element Method with shell analysis using ABAQUS program. Some restrictions and warnings regarding the use of the FSM are presented, as well as limitations of the Generalized Beam Theory and precautions to be taken in the shell models. It is also analyzed the influence of the degree of discretization of the cross section. In the present study, no evaluation was made with respect to normative procedures neither nonlinear analyses considering the initial geometric imperfections, residual stresses and elastoplastic behavior of the material.

The finite strip method (FSM) is a very efficient numerical method employed for performing the structural analysis of slender structures, such as cable-stayed bridges; the strip discretization of the model allows for the usage of a lower number of degrees of freedom, in comparison with the finite element method (FEM), while, as it will be discussed in the current research, the results obtained from both methods are in relatively good agreement. Moreover, to address the latest developments in the area of smart construction materials used for long-span bridges, the fiber reinforced polymer (FRP) composites were implemented for the bridge deck modeling, as part of a hybrid composite FRP cable-stayed bridge, and an extend laminate integrated finite strip method (LFSM) was applied for estimating the static structural performance of the hybrid composite FRP long-span cable-stayed bridge under several concentrated and uniformly distributed loadings. The free vibrations analysis was conducted for the Kap Shui Mun Cable-stayed Bridge model, and the natural frequencies were compared with the ones obtained from an FE model of the same bridge. One of the advantages of using the integrated finite strip method is that number of vibration modes, which can be included in the dynamic analysis when the effect of a sweeping sinus and a seismic loading are investigated when a conventional FE analysis would fail to converge. The outcomes of this research will set the stage for the hybrid long-span cable-stayed bridges modeling by the laminate integrated finite strip method (LFSM) which is more efficient and straightforward than the finite element analysis, for performing the static, free vibration, time domain, and frequency domain analyses.

Thin-walled cold-formed steel (CFS) has exhibited inherent structural and architectural advantages over other constructional materials, for example, high strength-to-weight ratio, ease of fabrication, economy in transportation and the flexibility of sectional profiles, which make CFS ideal for modern residential and industrial buildings. They have been increasingly used as purlins as the intermediate members in a roof system, or load-bearing components in low- and mid-rise buildings. However, using CFS members in building structures has been facing challenges due to the lack of knowledge to the fire performance of CFS at elevated temperatures and the lack of fire design guidelines. Among all available design specifications of CFS, EN1993-1-2 is the only one which provided design guidelines for CFS at elevated temperatures, which, however, is based on the same theory and material properties of hot-rolled steel. Since the material properties of CFS are found to be considerably different from those of hot-rolled steel, the applicability of hot-rolled steel design guidelines into CFS needs to be verified. Besides, the effect of non-uniform temperature distribution on the failure of CFS members is not properly addressed in literature and has not been specified in the existing design guidelines. Therefore, a better understanding of fire performance of CFS members is of great significance to further explore the potential application of CFS. Since CFS members are always with thin thickness (normally from 0.9 to 8 mm), open cross-section, and great flexural rigidity about one axis at the expense of low flexural rigidity about a perpendicular axis, the members are usually susceptible to various buckling modes which often govern the ultimate failure of CFS members. When CFS members are exposed to a fire, not only the reduced mechanical properties will influence the buckling capacity of CFS members, but also the thermal strains which can lead additional stresses in loaded members. The buckling behaviour of the member can be analysed based on uniformly reduced material properties when the member is unprotected or uniformly protected surrounded by a fire that the temperature distribution within the member is uniform. However if the temperature distribution in a member is not uniform, which usually happens in walls and/or roof panels when CFS members are protected by plaster boards and exposed to fire on one side, the analysis of the member becomes very complicated since the mechanical properties such as Young’s modulus and yield strength and thermal strains vary within the member. This project has the aim of providing better understanding of the buckling performance of CFS channel members under non-uniform temperatures. The primary objective is to investigate the fire performance of plasterboard protected CFS members exposed to fire on one side, in the aspects of pre-buckling stress distribution, elastic buckling behaviour and nonlinear failure models. Heat transfer analyses of one-side protected CFS members have been conducted firstly to investigate the temperature distributions within the cross-section, which have been applied to the analytical study for the prediction of flexural buckling loads of CFS columns at elevated temperatures. A simplified numerical method based on the second order elastic – plastic analysis has also been proposed for the calculation of the flexural buckling load of CFS columns under non-uniform temperature distributions. The effects of temperature distributions and stress-strain relationships on the flexure buckling of CFS columns are discussed. Afterwards a modified finite strip method combined with the classical Fourier series solutions have been presented to investigate the elastic buckling behaviour of CFS members at elevated temperatures, in which the effects of temperatures on both strain and mechanical properties have been considered. The variations of the elastic buckling loads/moments, buckling modes and slenderness of CFS columns/beams with increasing temperatures have been examined. The finite element method is also used to carry out the failure analysis of one-side protected beams at elevated temperatures. The effects of geometric imperfection, stress-strain relationships and temperature distributions on the ultimate moment capacities of CFS beams under uniform and non-uniform temperature distributions are examined. At the end the direct strength method based design methods have been discussed and corresponding recommendations for the designing of CFS beams at elevated temperatures are presented. This thesis has contributed to improve the knowledge of the buckling and failure behaviour of CFS members at elevated temperatures, and the essential data provided in the numerical studies has laid the foundation for further design-oriented studies.

Le but final de cette thèse est la proposition dune technique de résolution du problème du sixième degré de liberté des méthodes de bandes finies et déléments finis de plaque et de coque surbaissée déformable en cisaillement (le problème de rotation dans le plan) et de lappliquer dans la formulation des bandes finies et des éléments finis afin danalyser le comportement des structures à parois minces. Tout dabord, le présent travail introduit les caractéristiques des profils à parois minces tels que formes de la section, nuances dacier et imperfections initiales de type géométrique (défaut de rectitude, de planéité), structural (contraintes résiduelles) ou matériel (écrouissage). Ceux-ci sont nécessaires aux analyses ultérieures. Ensuite, une étude bibliographique aborde les méthodes de calcul des barres à parois minces tant analytiques que semi-empiriques ou numériques. Les méthodes analytiques ont été basées sur les théories de VLASOV et de BENSCOTER et une méthode intitulée Generalized Beam Theory développée par SCHARDT avec ses collègues depuis le début des années 1970 en Allemagne. Les méthodes semi-empiriques tiennent compte de linfluence du voilement sur le comportement global par le concept de largeur et de section effectives. Les méthodes numériques sont la méthode des éléments finis de type plaque, de type coque et la méthode des bandes finies. Puis, une technique originale est proposée avec succès pour introduire dans les relations standard déformation-déplacement des théories des plaques et des coques surbaissées, la rotation dans le plan pour assurer la conformité des variables nodales rotatives aux jonctions spatiales. Lapplication de cette technique sert à mettre au point les bandes finies et les éléments finis de type plaque et coque surbaissée. Il est proposé un programme déléments finis, nommé FENALYSE, qui est capable danalyser la linéarité, la non-linéarité et le flambement des structures à parois minces qui sont composées ou peuvent être modélisées par plaques planes et coques surbaissées. Tandis que les bandes finies ne sont développées que pour calculer le flambement des profils à parois minces qui sont simplement appuyés et un programme intitulé FLAMBANDE est proposé. Les verrouillages de cisaillement et de membrane sont éliminés par la technique dintégration réduite. La description lagrangienne actualisée est utilisée dans lanalyse non-linéaire. Les éléments finis de type plaque et de type coque surbaissée permettent de considérer, entre autre, les phénomènes inhérents aux profils à parois minces tels que la torsion non uniforme avec gauchissement, la distorsion de la section, les phénomènes dinstabilité couplées, la plasticité, les contraintes résiduelles, les imperfections locales et globales, le changement de la limite délasticité sur la section. Plusieurs exemples numériques tant académiques que pratiques sont réalisés afin de montrer la fiabilité de ces éléments finis.