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Laminated composite plates and shells have been widely used in aeronautical, mechanical, and naval structures. Stability is a major concern in the design of these structures due to their high slenderness. Therefore, it is necessary to properly analyze their post-critical behavior, assessing their sensitivity to initial imperfections and load-carrying capacity. This paper presents a methodology based on the Isogeometric Analysis to study the stability of laminated plates and shallow shells. In this formulation, the geometry and displacement field are described using NURBS basis functions. Appropriate integration schemes are used to avoid the locking problem for thin-walled plates and shells. The proposed formulation was applied in the stability analysis of various examples and excellent results were obtained. The influence of the composite layup and geometric imperfections on the buckling load and post-critical behavior is studied. New bifurcation solutions to a well-known laminated shell buckling benchmark problem are presented.

Os perfis de aço formados a frio estão sujeitos a fenômenos de instabilidade, tanto local, quanto global, que influenciam na capacidade de carga do elemento comprimido. A utilização de enrijecedores pode aumentar a capacidade de carga e a eficiência desse elemento. Apresenta- se, nesse artigo, um estudo numérico de colunas de seções do tipo U enrijecido com enrijecedores intermediários na alma, submetidas à compressão axial. Os perfis adotados são seções comumente utilizadas como colunas (montantes) em sistemas construtivos industrializados constituídos de elementos metálicos pré-fabricados, conhecidos como Light Steel Frame. O estudo baseia- se em análises numéricas através do Método dos Elementos Finitos, onde são obtidos as cargas críticas e os respectivos modos de flambagem elástica das seções, através de análise linear, e são feitas avaliações do comportamento pós-flambagem das seções através de análise não-linear. Os comprimentos das colunas foram variados ao longo das séries analisadas, buscando-se identificar os fenômenos de instabilidade predominantes. Cold-formed steel sections are subject to instability phenomena, either local or global, which can influence the load capacity of compressed elements. The use of longitudinal stiffeners can provide increasing load capacity and element efficiency. This paper presents a numerical investigation of cold formed steel sections with web intermediate stiffeners, subjected to axial compression. The sections considered in this study are the channel ones commonly used as columns in residential Steel Framing Systems. The study is based on finite element method analysis, and the critical loads and respective elastic buckling modes are obtained through linear analysis. Non-linear inelastic analyses are conducted to obtain column ultimate-load capacity and to study its post-buckling behavior. Different column lengths were adopted throughout the analyses, to evaluate their behavior under the different types of local instability.

O presente trabalho apresenta proposta de análise numérica de perfis de aço formados a frio submetidos à compressão centrada. Trata-se de investigação, realizada por meio de elementos finitos, com vistas à análise da influência de imperfeições geométricas iniciais presentes em perfis U simples e U enrijecido, sob os aspectos de sua forma, magnitude e sentido. A imperfeição geométrica é adotada na forma de modos de falha local, global e distorcional, observando a suscetibilidade a tais modos para a seção de interesse. Cada tipo de imperfeição é avaliado de forma isolada e, em seguida, de forma associada (conjunta). São apresentados aspectos referentes à estratégia de modelagem adotada na construção dos modelos numéricos, bem como aspectos de interesse com relação às análises não-lineares aqui consideradas. Para fins de obtenção da força de colapso, fica examinada a possibilidade de empregar modos de falha isolados para representar as imperfeições geométricas, porém, com a amplitude previamente ajustada. Para os casos de imperfeições associadas, para representar a imperfeição geométrica, se destaca o papel do sentido da imperfeição do tipo global com relação a sua associação às imperfeições dos tipos local e distorcional para perfis U enrijecidos, bem como à imperfeição do tipo local para perfis U simples. The work presents a numerical analysis on cold-formed steel members compressed between pinned ends. One is about an inquiry carried through in finite elements in which it was studied the influence of initial geometric imperfections, existing in plain channels and lipped channels, under the aspects of its form, magnitude and signal. The geometric imperfection is adopted on the forms of the local, global and distortional buckling modes, observing the susceptibility to such modes for each type of section. Each imperfection is evaluated on the isolated way and after that of associated way. Referring aspects to the strategy of modeling adopted in the construction of the numerical models are presented, as well as aspects of interest with regard to the developed nonlinear analyses. For ends of attainment of the ultimate load, it is examined the possibility to use isolated buckling modes to represent the geometric imperfections, as long as the amplitude it is adjusted. The cases that had applied coupled imperfection to represent the geometric imperfection detach the paper of the signal of the imperfection of the global type with regard to its association to the imperfections of the types local and distortional for lipped channels and, to the one of the local type for plain channels.

The optimized plate structure consists of a simply supported square base plate stiffened with an orthogonal grid of flat stiffeners welded to the base plate by fillet welds. The uniformly distributed compressive load acts biaxially in the plane determined by the centre of gravity of T-sections, which consist of a part of the base plate and of a stiffener. In the optimization process the number of stiffeners as well as the thicknesses of the base plate and flat stiffeners, which minimize the cost function and fulfil the design constraints, as sought. The cost function includes the cost of material, assembly, welding and painting. Constraints relate to the global buckling, local buckling of base plate parts and stiffeners as well as to the deflection due to shrinkage of welds. To illustrate the effectiveness of the mathematical methods, the problem is solved by the Rosenbrock’s hill-climb algorithm as well as by entropy-based unconstrained minimization.

OBJETIVO: Comparar os resultados obtidos pelas técnicas de "laser in situ keratomileusis" (LASIK) e "photorefractive keratectomy" (PRK) na correção de miopia e astigmatismo em olhos previamente submetidos à cirurgia de descolamento de retina (DR) com "buckle" escleral. MÉTODOS: Vinte e cinco olhos de 22 pacientes com alterações refracionais significativas após a cirurgia de DR foram submetidos à cirurgia refrativa. Em 14 olhos de 13 pacientes foi realizado LASIK e em 11 olhos de 9 pacientes, PRK. O intervalo mínimo entre a cirurgia de DR e a cirurgia refrativa foi de 12 meses. O tempo de seguimento foi de, pelo menos, 12 meses. RESULTADOS: Doze meses após a cirurgia, a média do equivalente esférico (EE) no grupo submetido ao LASIK diminuiu de -6,49 D antes da cirurgia para -0,17 D e a média do cilindro de -1,10 D para -0,23 D. A média do EE no grupo submetido ao PRK foi reduzida de -5,35 D para +0,02 D e a média do cilindro, de -1,38 D para -0,54D. Em ambos os grupos, 11 olhos apresentaram melhora da acuidade visual sem correção de pelo menos 4 linhas. CONCLUSÃO: Tanto o LASIK quanto o PRK foram seguros e eficazes para a correção do erro refracional induzido após a cirurgia de DR. Nossos resultados não apresentaram diferenças significativas entre os procedimentos. Estudos posteriores envolvendo maior amostragem e seguimento mais prolongado contribuirão para melhor avaliação da cirurgia refrativa em pacientes submetidos à cirurgia com "buckle" escleral. PURPOSE: To compare the results of "laser in situ keratomileusis" (LASIK) and "photorefractive keratectomy" (PRK) to correct myopia and astigmatism in eyes previously submitted to retinal detachment (RD) surgery with scleral "buckle". METHODS: Twenty-five eyes of 22 patients submitted to scleral buckling procedure with significant refraction changes after surgery were evaluated. Fourteen eyes of 13 patients underwent LASIK and 11 eyes of 9 patients underwent PRK. Minimum time between RD surgery and refractive surgery was 12 months. All patients were followed-up for at least 12 months. RESULTS: Twelve months after surgery, mean spherical equivalent in the LASIK group changed from -6.49D preoperatively to -0.17D and mean astigmatism decreased from -1.10D to -0.23D. Spherical equivalent in the PRK group decreased from -5.35D before surgery to +0,02D and average astigmatism changed from -1.38D to -0.54D. In both groups, uncorrected visual acuity (UCVA) improved at least 4 lines in 11 eyes. CONCLUSION: Both LASIK and PRK were safe and effective to correct refractive error after RD surgery. Our results showed no significant difference between procedures. Further studies with more patients and long-term follow-up are required to better evaluate the effects of refractive surgery in patients submitted to scleral buckling procedure.