Description of the research
The question of which materials for elastic gridshells is at the roots of the development of these structures at the Navier laboratory, but things actually came out the other way round. Historically, the expertise of the laboratory was material science and we were looking for “taylor made” applications for composites materials, for applications where composites might definitively replace other traditional like in the sport and leisure industry. The structural typology that came out after a few months is elastic gridshells.
A first study on the selection of the most adapted materials for grid shell and also on the design criteria of such materials in applications with permanent bending stresses was conducted by Douthe in his PhD thesis and in (Douthe 2010). A detailed analysis of the failure mode and of creep behaviour was further carried out by Kotelnikova-Weiler in her PhD thesis and in (Barboura 2010) (Barboura 2011) (Kotelnikova-Weiler 2013). A paper summarising how to chose the most adapted material for a gridshell was done in (Kotelnikova-Weiler 2013 IJSS) in collaboration with Gengnagel team in Berlin.
- O. Baverel
- J.-F. Caron
- C. Douthe
- N. Kotelnikova-Weiler
- Active Bending
- Composite Material
- Building Physics
Materials for Actively-Bent Structures – N. Kotelnikova Weiler et al. (2013)
N. Kotelnikova Weiler, C. Douthe, E.Lafuente Hernandez, O. Baverel, C. Gengnagel
Active bending structures need materials with specific mechanical properties such as large admissible strain and sufficiently high stiffness to prevent buckling. This paper proposes to investigate the materials that could be used following Ashby's selection method. Then it focuses on the most affordable materials which are glass fibre reinforced polymers (GFRP) and natural fibre reinforced polymers (NFRP). As the initial selection is based on short term characteristics, the long term behaviour of fibre reinforced polymers is then addressed based on recent durability studies which are needed to ensure the performance, reliability and safety of a structure. It is shown that, depending on the loading type (tension, bending, torsion, alone or in combination...) and on the nature of the components (fibres, matrix and interfaces), the material undergoes several phenomena reducing its mechanical performances and potentially leading to its failure. Finally, this knowledge of the materials, which allows for a better understanding of the specific relation between the material and the active bending structures, is used to give a framework for stress limitations and recommendations for further optimisation of reliable structures.
Kinetic of fibre ruptures in a UD composite material with a viscoelastic matrix subject to traction [Cinétique de ruptures de fibres dans un matériau composite UD soumis à la traction avec une matrice viscoélastique] – N. Kotelnikova-Weiler et al. (2013)
N. Kotelnikova-weiler, O. Baverel, J.-F. Caron
Structural applications of fiber reinforced polymers are strongly dependent on long-term durability studies. Subject to sustained loading, the material undergoes several phenomena reducing its mechanical performances and potentially leading to its failure. To understand the creep rupture mechanisms and identify relevant mechanical parameters that will need to be adjusted during the composition of the material, modeling at the fibers scale seems adequate. In this article a shear-lag model will be presented. This 2D model integrates a viscoelastic behavior of the matrix and a stochastic distribution of fibers strength. Results showing the main phenomena occurring in the stress redistribution in the composite material will also be presented.
Optimisation mécanique et énergétique d’enveloppes en matériaux composites pour les bâtiments – N. Kotelnikova-Weiler (2012)
The majority of existing buildings does not follow present energy efficiency regulations. In order to fulfill environmental requirements it seems impossible, in this context, to consider a global demolition-reconstruction policy. Renovation programmes need to be implemented. An innovative energy efficiency improvement solution is proposed, enabling to explore urbain energy ressources presently underexploited. The concept is to add, around the building, an external envelope whose main function would be to collect energy. An optimization tool aiming at finding the optimal geometry and collectors' spatial distribution on the envelope's surface, is developed. Its validation is carried out on simple cases, it is then applied in situations showing strong contrasts: optimization during summer and winter, in Oslo and in Tunis, with close obstacles partially masking the Sun. In order to build the emerging complex geometries, the use of gridshell structures is proposed. These structures are obtained through elastic deformation of an initially plane grid made of composite material slender beams. This raises the problem of composite materials durability under sustained loading. In order to study the long-term behavior (creep and creep rupture) of these materials composed of reinforcing fibres and a polymeric viscoelastic matrix, a micromechanical model is developed. This shear-lag type model allows studying the influence of the constituents' mechanical properties on the lifespan of the composite under permanent pure traction or combined shear and traction loadings.
Experimental study of the influence of matrix system on pultruded GFRP composites’ creep beahviour under flexural loading – S. Barboura et al. (2011)
Long-term behaviour of glass-resin pultruded composites for structural application in construction – S. Barboura et al. (2010)
S. Barboura, N. Kotelnikova-weiler, J.-F. Caron, O. Baverel
Gridshell structures in glass fibre reinforced polymers – C. Douthe et al. (2010)
C., Douthe, O. Baverel, J.-F. Caron
Applications in which composite materials are used as bearing structural elements are rare and generally copy structural solutions that have been optimized for steel for years. Unfortunately these materials have very different properties, therefore specific construction methods and specific structural forms have to be invented for composite materials. Gridshell structures may be one of those. So, after a brief history of gridshells, a demonstration with the Ashby's method that glass fibre reinforced polymers are an alternative for this kind of structure is shown. An original experimental test for the identification of the parameters of the short and long time behaviours of bent pultruded tubes is then detailed. Afterwards a scale one prototype of composite gridshell is presented and loaded. Results of the experimental tests are compared to numerical results of a non-linear analysis done with the dynamic relaxation method. The authors then concluded on the feasibility of such composite gridshells and on their potential for future development.
Etude de structures élancées précontraintes en matériaux composites, application à la conception des gridshells – C. Douthe (2007)
Les matériaux composites sont des matériaux nouveaux qui possèdent une grande déformabilité et une grande raideur. Les structures de bâtiment qui requièrent ces deux propriétés sont peu nombreuses et les gridshells en sont un bon exemple. En effet, ces structures à double courbure sont obtenues par déformation élastique puis rigidification d'une grille plane sans raideur en cisaillement. Les grands déplacements et les grandes rotations qui surviennent durant la phase de montage de ces structures élancées précontraintes nécessitent la prise en compte de non-linéarités géométriques importantes. Un outil numérique spécifique reposant sur la méthode de la relaxation dynamique est donc développé et validé. Il permet une nouvelle approche de la forme des gridshells et la mise au point d'une méthode de recherche de forme originale. Il est également utilisé pour l'étude géométrique et l'analyse structurelle de prototypes de gridshells en matériaux composites construits sur le site de l'École Nationale des Ponts et Chaussées.
Formfinding of a grid shell in composite materials – C. Douthe et al. (2006)
C. Douthe, O. Baverel and J.-F. Caron
The advantages of the use of glass fiber composites for grid shell are presented. The shape of grid shells results from a post-buckling state of tubes. To bypass the difficulty to predict the geometry of the final equilibrium state, the large rotations which occur during the erection process are modelled using the dynamic relaxation algorithm. This paper proposes an adaptation of this method for structures prestressed by bending through the development of a computer program. It includes the validation of this numerical tool through comparisons with a finite elements software. Then an application to the form-finding of a grid shell and the study of its stability under standard loading conditions will be presented. Finally the authors conclude on the technical and economic feasibility of this composite grid shell.