Caracterização experimental e numérica do comportamento frágil de ligações com cavilhas em estruturas de madeira
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2011
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A resistência e durabilidade das estruturas de madeira depende essencialmente do desempenho das suas ligações que são consideradas pontos críticos das estruturas, sendo responsáveis por 80% das falhas verificadas neste tipo de estruturas. A fissuração frágil é uma das possíveis causas de rotura das ligações não sendo, no entanto, devidamente tratada pelos procedimentos de projecto actuais, que se baseiam no Modelo Europeu de Cedência, especialmente desenvolvido para previsão de modos de rotura dúcteis. O problema da fissuração frágil coloca-se de forma ainda mais premente nas ligações do tipo cavilha com transferência de momentos, pois a transferência de momentos implica a solicitação local da madeira segundo direcções que apresentam componentes normais ao fio da madeira.
Assim, propõe-se o estudo de uma ligação aço-madeira-aço, com duas cavilhas de modo a garantir transferência de momentos, sendo investigado o efeito da distância entre cavilhas e da distância da cavilha ao topo da peça de madeira. Este estudo é conduzido recorrendo quer a um programa experimental de ensaios de flexão das ligações, quer a modelos de elementos finitos tridimensionais das ligações. Tendo o programa experimental ilustrado modos de rotura essencialmente frágeis, procurou-se modelar as ligações recorrendo à Teoria de Weibull para materiais frágeis, assim como recorrendo a modelos de dano coesivo, com elementos finitos de interface.
A distância entre cavilhas demonstrou ter um efeito mais significativo na rigidez e resistência da ligação do que a distância da cavilha ao topo da peça de madeira. Recorrendo a um modelo de elementos finitos com comportamento elasto-plástico da madeira, de acordo com o modelo de Hill generalizado, foi possível descrever a rigidez dos provetes testados. A Teoria de Weibull permitiu melhorar as previsões das cargas de rotura em relação à abordagem baseada nas tensões de pico observadas na análise de elementos finitos. A abordagem baseada no modelo de dano coesivo mostrou ser promissora, uma vez que, recorrendo aos parâmetros de fractura da madeira, permitiu descrever a iniciação e a propagação do dano numa das séries testadas. Para além disso, o modelo de dano revelou aptidão no sentido de prever tanto a rigidez inicial como a carga máxima.
The strength and durability of timber structures depends on the mechanical performance of the joints which are generally assumed the critical points of these structures, being responsible for about 80% of the observed failures. Brittle cracking is one possible failure mode which is not conveniently handled by the current design procedures, available in codes of practice for timber structures, such as the European Yield Model, which has been specifically proposed to assess the ductile failure modes of timber joints. The brittle cracking is a major concern for dowel type moment resisting timber joints, since the moment transfer leads to local loading of the wood, around dowel holes, along various directions with some tensile stress component normal to the grain. This study is concerned with the investigation of the mechanical behaviour of steel-wood-steel moment resisting joints with two dowels. The distance between dowels and the distance between the dowel and the end of the wood member are investigated by means of an experimental programme of three-point bending monotonic quasi-static tests. Furthermore, three-dimensional finite element models are proposed to investigate the behaviour of the joints considered in the experimental programme. Since the experimental programme has shown brittle failure models of the joints, they were alternatively modelled using the Weibull failure theory as well as using damage cohesive models with interface finite elements. The distance between dowels has a strong influence on stiffness and strength of the joints. The end distance did not contribute to the increase of the mechanical performance of the joint. The proposed finite element model with elastoplastic behaviour, according to a generalized Hill yield theory, predicted satisfactorily the stiffness of the tested joints. The Weibull failure theory allowed the prediction of the ultimate loads with increased accuracy with respect to the maximum stress criterion. Cohesive zone modeling has revealed that damage onset and propagation in wood is well captured for a tested series, in agreement with the observations carry out during the experimental tests. The model also permits to predict both the initial stiffness and ultimate load.
The strength and durability of timber structures depends on the mechanical performance of the joints which are generally assumed the critical points of these structures, being responsible for about 80% of the observed failures. Brittle cracking is one possible failure mode which is not conveniently handled by the current design procedures, available in codes of practice for timber structures, such as the European Yield Model, which has been specifically proposed to assess the ductile failure modes of timber joints. The brittle cracking is a major concern for dowel type moment resisting timber joints, since the moment transfer leads to local loading of the wood, around dowel holes, along various directions with some tensile stress component normal to the grain. This study is concerned with the investigation of the mechanical behaviour of steel-wood-steel moment resisting joints with two dowels. The distance between dowels and the distance between the dowel and the end of the wood member are investigated by means of an experimental programme of three-point bending monotonic quasi-static tests. Furthermore, three-dimensional finite element models are proposed to investigate the behaviour of the joints considered in the experimental programme. Since the experimental programme has shown brittle failure models of the joints, they were alternatively modelled using the Weibull failure theory as well as using damage cohesive models with interface finite elements. The distance between dowels has a strong influence on stiffness and strength of the joints. The end distance did not contribute to the increase of the mechanical performance of the joint. The proposed finite element model with elastoplastic behaviour, according to a generalized Hill yield theory, predicted satisfactorily the stiffness of the tested joints. The Weibull failure theory allowed the prediction of the ultimate loads with increased accuracy with respect to the maximum stress criterion. Cohesive zone modeling has revealed that damage onset and propagation in wood is well captured for a tested series, in agreement with the observations carry out during the experimental tests. The model also permits to predict both the initial stiffness and ultimate load.
Descrição
Dissertação de Mestrado em Engenharia Mecânica
Palavras-chave
Estruturas de madeira , Resistência dos materiais , Cavilhas