Cyril Douthe

Materials for Actively-Bent Structures

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 Ashbys 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.

Analytical Investigations on Elementary Nexorades

Nexorades or reciprocal frames can be seen as a practical way to reduce the complexity of connections in spatial structures by connecting reciprocally the members by pairs. This reduction of the technological complexity of the connections is however replaced by a geometrical complexity due to numerous compatibility constraints. The purpose of this article is to make explicit these constraints for elementary structures and to solve analytically the resulting system of equations. Applications to regular polyhedrons are presented and a practical realization (a 3 m high dodeca-icosahedron) is shown. In the brief conclusion, perspectives for complementary analytical developments for spatial structures are drawn forth.

Generalised cyclidic nets for shape modelling in architecture

The aim of this article is to introduce a bottom-up methodology for the modelling of free-form shapes in architecture that meet fabrication constraints. To this day, two frameworks are commonly used for surface modelling in architecture: non-uniform rational basis spline modelling and mesh-based approaches. The authors propose an alternative framework called generalised cyclidic nets that automatically yield optimal geometrical properties for the envelope and the structural layout, like the covering with planar quadrilaterals or hexagons. This framework uses a base circular mesh and Dupin cyclides, which are natural objects of the geometry of circles in space, also known as Möbius geometry. This article illustrates how complex curved shapes can be generated from generalised cyclidic nets. It addresses the extension of cyclidic nets to arbitrary topologies with the implementation of a ‘hole-filling’ strategy and also demonstrates that this framework gives a simple method to generate corrugated shells.

Stability of Pseudo-Funicular Elastic Grid Shells

The paper presents some results on the influence of the pre-stress induced by the erection method of elastic grid shells on their buckling capacity. It starts with the numerical methods and their validation with the study of a prebuckled arch. Then, a form-finding scheme using low-speed dynamics is used to generate automatically a family of elastic grid shells, and their buckling capacity is compared to the one of grid shells with the exact same geometry, but without any pre-stress. The paper demonstrates finally that the pre-stress decreases by a few percent the buckling capacity of elastic grid shells.

Marionette Meshes: Modelling free-form architecture with planar facets:

We introduce an intuitive method, called Marionette, for the modelling of free-form architecture with planar facets. The method takes inspiration from descriptive geometry and allows to design complex shapes with one projection and the control of elevation curves. The proposed framework achieves exact facet planarity in real time and considerably enriches previous geometrically constrained methods for free-form architecture. A discussion on the design of quadrilateral meshes with a fixed horizontal projection is first proposed, and the method is then extended to various projections and patterns. The method used is a discrete solution of a continuous problem. This relation between smooth and continuous problem is discussed and shows how to combine the marionette method with modelling tools for smooth surfaces, like non-uniform rational basis spline or T-splines. The result is a versatile tool for shape modelling, suited to engineering problems related to free-form architecture.

Morphological and Mechanical Investigation of Interconnected Elastic Gridshells

This paper describes the structural reinforcement of elastic gridshells by interconnecting several gridshells together. Reference for the present investigation is taken from the composite (GFRP) gridshell structure built for the Forum of the Solidays music festival in Paris in 2011. The principle of interconnection is illustrated with various structures which are generated using numerical toolbox developed by the laboratory for the automatic meshing of free-forms structures on Rhino. Then the effect of the interconnection on the structural behaviour is briefly discussed based on elementary loading cases. Finally, the feasibility of the concept as well as its architectural and morphological potential is show by the construction of a double gridshell prototype. The authors conclude on the possible future developments of such morphologies which could give interesting and unexpected architectures.

Gridshell in composite materials: Towards large span shelters

ABSTRACT In 1975, the gridshell of Mannheims Bundesgartenschau revealed the extraordinary aesthetic potential of a new type of lightweight structures with very interesting mechanical properties; it however did not become widespread. This article shows how composite materials can be an original and profitable solution for such constructions. In the first section, the authors recall the principal characteristics of gridshells. They explain then why glass fibres reinforced polymers provide a very attractive solution for this kind of application. The second section is dedicated to the numerical modelling of the structure. The basic principles of the dynamic relaxation method are presented. The third section shows the results of the measures taken on the prototype built at the Navier Institute and compares them with the results of the numerical simulations. A fourth section deals with the roofing solutions of such structures, and shows scale: 1 realisation. Finally the authors conclude on the potential of such structures for larger span constructions.