Synopsis

When looking a catapult that is ready to throw a stone, one can observe that there is very little displacement of the beam when the stone is put on the catapult (figure 1). This is due the fact that the rope act as a column and takes the load of the stone. The rope has been prestressed by the bending energy stored in the wooden beam. By adding weight on the catapult the rope will lose tension. The same concept can be used to shorter the span of a beam or a bridge. For instance, a cable can be set up at mid span of a beam. In order to have an approximately flat resulting unloaded with external forces configuration, the initial configuration must be curved. The second step is to load the beam with a cable that will act as a column

Acknowledgement

Facts & Figures

  • 350m2
  • 2kms of GFRP tubes
  • 500k$
  • 1 day for erection

Client & Design

  • ACD
  • TESS
  • NAVIER

Partners

  • Ferrari
  • Esmery Caron

From shape to shell

Overall design process

The goal of the design process is to identify a gridshell structure that works and which respect as faithfully as possible the architectural project – a shape and a program. It represents “the path from shape to structure”. Its progress, sequential and iterative, revolves around three major stages: shape, mesh and structure (Fig. 7). It is not trivial to go through this complex process. Indeed, for each step, the method, the tool, the criteria, that offer both a sufficient explorative richness to find out enough candidate solution, and the means to evaluate and compare the suitability of those solution, have to be found.

From sketch to shape

The first step of the process consists in building a precise geometric model from the sketch of the architect and to evaluate its mechanical potential (Fig. 8). At this stage, the goal is to estimate the probability a given shape would lead to the generation of a structurally feasible gridshell. The figure shows a selection of 3 slightly different 3D shapes, derived from the targeted shape, based on an analysis of their principal curvatures. Stresses in the grid are mainly due to the bending of the profiles. They derive directly from their geometric curvature. Thus, the principal curvatures of the surface – because they give a qualitative measurement of the local curvature of any curve drawn on a surface – are relevant indicators to evaluate the stress rate of a grid laying on it. Particularly, one should ensure the following condition is satisfied everywhere, where r is the pipe’s outer radius, Rmin is the minimum principal radius, E is the flexural modulus, σk,flex the characteristic flexural strength (§5.3) and γlt the long-term partial coefficient of material resistance. Ideally, the shape is controlled by few key parameters. Thus, it can be adapted and optimized through an iterative process, towards this criterion (1).

From shape to mesh

During the second step, the candidate surface is meshed and the mechanical potential of the resulting grid is evaluated. At this stage, we try to estimate the probability a given mesh could lead to the generation of a viable gridshell structure (Fig. 9). Simultaneously, meshes are compared according to their architectural relevance. This time, the geometric curvature of the polylines drawn on the surface is the criterion to characterize the mechanical potential of the grid. In particular, one should ensure the following condition is satisfied everywhere, where Rspline is the spline’s local curvature radius: The mesh is obtained by the compass method, described in [11], which develops a regularly spaced grid on a surface from two secant directrix. For a given shape there are an infinite number of meshes. The aim is to identify at least one grid, suitable towards architectural and structural criteria (Fig. 9). The figure shows resulting meshes and flat grids, depending the directrix. The laboratory tried various numerical methods to generate such grids [12]. Here, a specific software [13], developed for rhino & grasshopper, allows generating this kind of mesh on any nurbs surface. It performs the following elementary operations: surface meshing with the compass method, trimming, control of geometry’s integrity and flattening of the grid. The tool also generates automatically a text file, which can be imported in structural analysis software, containing all the required information to perform the formfinding of the structure. An add-on facilitates loads application of various complexities (snow, wind, etc.), which is tricky for free forms.

Construction details

In this project, one can identify 4 major structural details : the swivel coupler for connecting com- posite tubes to assemble the grid (Figure 8a); the steel sleeve for connecting several composite tubes to make long members from initially short piece of tubes (Figure 8b); ground anchorages for fixing the structure to the concrete slab (Figure 8c) and the lacing edge beam of the fabric (Figure 8d). Note that the tricky issue of connecting steel and composite parts is solved in a similar way through sleeve and anchorage details.

Sleeves are major components in the structural system. The presented component is a great innovation compared to the composite gridshells built previously, where members were simply interrupted or overlapped. By establishing mechanical and architectural continuities between pipes, this sleeve brings closer the real behavior and the theoretical behavior of the shell.

The sleeve is a steel system that sets up mechanical continuity between two adjacent composite pipes for both tension and bending. It is made of three parts: two connectors linked by a threaded rod (Fig. 12b). Each connector is a 48.3×2.9mm steel pipe, slightly larger than the composite pipes on which it is put on, with a welded M20 nut at one of his end. The connector is pinned to the composite pipe with three 10mm bolts. Some structural adhesive is also employed to fill gaps and to guaranty a good rigidity of the assembly. However, the sleeve is designed ignoring the adhesive contribution to the mechanical strength of the system. A M20 threaded rod links the two connectors. It allows tension forces and bending moments to pass form one pipe to the other. It cannot transfer any twisting moment.

Tension forces are transferred from the composite pipe to the connector through shear in the pins. Thanks to a lower bearing resistance in the composite than in the steel, each of the three pins can be gradually loaded. When loading the system, at first, only one of the three pins is really in contact with both the steel pipe and the composite pipe, because of inevitable small manufacturing gaps. When increasing the axial load, this pin starts to “eat” into the composite pipe until the second pin comes also in contact (Fig. 13). This scheme is reproduced until another failure mode happened. For this mode of composite failure, which prevails in this case, the total bearing capacity of the assembly is thus three times the capacity of a single pin.

Bending moments are transferred through the threaded rod of the sleeve. This part is designed to reach simultaneously the two following qualitative criterions. Firstly, the rod bending stiffness should be roughly equivalent to the composite bending stiffness itself to preserve curvature’s continuity along the system. Note that this continuity is of prime importance from an architectural point of view. Secondly, the steel quality of the rod should be adjusted so it starts its plastification when the composite pipe tends to approach its maximum design stress (a third of the yield stress). Thus, the rod acts as a “fuse” which concentrates rotational deformations avoiding failures in the critical elements of the structure (the composite pipes).

Codes for composite materials

Beyond the technical difficulties related to both design and structural analysis of the shell, the regula- tory framework was a vital issue for the project’s success. Because it was the first time a structure of this kind would host regularly a large number of people in a long-term period, the question of its re- liability over time was a major issue. To be built the gridshell had to comply with existing standards, which do not take into account such an innovative edifice, all in composite material. The strategy adopted to bypass this obstacle consisted in making the most of the existing regulatory framework to justify the compliance of a structure that would not, at first sight, be taken into account by stan- dards that does not include composite materials. As far as possible, the design was led in compliance with the Eurocodes, where the structural design is made according to limit states under normalized loadings (self-weight, snow, wind, etc.). Despite the Eurocodes do not directly take into account com- posite materials, they propose some probabilistic methods to introduce new materials (Annexe D). As far as possible, the mechanical properties of the GFRP pipe were determined by tests in conformance with these methods. Otherwise, values where taken according to the Eurocomp [16]. In some cases, as the sleeve, the design of the construction details has also benefited from this approach.

References

  • All
  • Active Bending
  • Gridshell

 The ephemeral cathedral of Créteil : a 350m2 lightweight structure made of a GFRP composite gridshell – L. du Peloux et al. (2015)

Authors

L. du Peloux, Jean-François Caron, Frédéric Tayeb, Olivier Baverel

Abstract

The Ephemeral Cathedral of Créteil (Paris, France) is a gridshell structure made of composite materials. Built in 2013, this religious edifice of 350m2 is a temporary church meant to gather the parishioners during the two-years renovation of their permanent cathedral. This large-scale prototype represents a first in the building industry, which still shows excessive apprehension for the use of non-traditional materials such as composites, especially when it comes to structural applications. Based on a previous successful experience the gridshell was prefabricated and erected by the parishioners themselves.

Date

Jul. 2015

Conference

JNC 2015 (Lyon, France)

Links

> permalien
> pdf

 The ephemeral cathedral of Créteil : a 350m2 lightweight gridshell structure made of 2 kilometers of GFRP tubes – L. du Peloux et al. (2015)

Authors

L. du Peloux, Frédéric Tayeb, Jean-François Caron, Olivier Baverel

Abstract

The Ephemeral Cathedral of Créteil (Paris, France) is a gridshell structure made of composite materials. Built in 2013, this religious edifice of 350m2 is a temporary church meant to gather the parishioners during the two-years renovation of their permanent cathedral. This large-scale prototype represents a first in the building industry, which still shows excessive apprehension for the use of non-traditional materials such as composites, especially when it comes to structural applications. Based on a previous successful experience the gridshell was prefabricated and erected by the parishioners themselves.

Date

May. 2015

Conference

CIGOS 2015 (Paris, France)

Links

> permalien
> pdf

 Le design paramétrique au service de la foi – O. Namias (2013)

Authors

O. Namias

Abstract

Pendant la reconstruction de leur cathédrale, les fidèles du diocèse de Créteil continueront à suivre la messe sous une toile légère soutenue par des tiges de fibre de verre et de métal, un ouvrage expérimen-tal développé par le laboratoire Navier (Ecole des Ponts) en association avec le bureau d’études T/E/S/S.

Date

Oct. 2013

Journal

D'Architectures

Links

> permalien
> pdf

 Faith can also move composite gridshells – L. du Peloux et al. (2013)

Authors

L. du Peloux, F. Tayeb, O. Baverel, J-F. Caron

Abstract

This paper presents the overall design and construction process of a gridshell in composite materials at Créteil, in Paris suburb. This religious edifice of 350m² is a temporary cathedral meant to gather the parishioners during the two-years renovation of their permanent cathedral. It can accommodate up to 500 people and complies with all the required performances for such a building: structural stiffness, fire safety, waterproofness, lightning, thermal comfort, etc. This project arises thanks to a long-term collaboration between T/E/S/S and NAVIER and marks the accomplishment of a ten-years research project in the area.

Date

Sept. 2013

Conference

IASS 2013 (Wroclaw, Poland)

Links

> permalien
> pdf
> bibtex

 Cathédrale éphémère à Créteil – C. Simon (2013)

Authors

C. Simon

Abstract

Alors que les travaux de transformation de la cathédrale de Créteil (Val-de-Marne) débutent sous la houlette d’Architecture-Studio, les fidèles ont migré vers la construction légère qui doit les abriter jusqu’en septembre 2014. Cette cathédrale éphémère, capable d’accueillir jusqu’à 360 personnes assises, est constituée d’une structure courbe de type « gridshell », une maille composée de tubes fins en matériau composite recouverte d’une toile en PVC précontraint. Conçue par les chercheurs du laboratoire Navier de l’Ecole Nationale des Ponts et Chaussées et le bureau d’études T/E/S/S, elle a été montée avec des bénévoles en une quinzaine de jours, en janvier.

Date

Feb. 2013

Journal

Le Moniteur

Links

> permalien
> pdf

 Reportage ecclesia magasine – E. Chapelle (2013)

Authors

E. Chapelle

Abstract

A Créteil, installation d'une "cathédrale éphémère" en attendant la fin du déploiement de la cathédrale actuelle : reportage sur place. Émission du Mardi 22 janvier 2013.

Date

Jan. 2013

Journal

Radio Notre Dame

Links

> permalien
> mp3

 Reportage CNRS images – H. Colombani (2013)

Authors

H. Colombani

Abstract

Que ce soit les performances thermiques, l'esthétique ou le confort de vie, les nouvelles solutions recherchées pour le bâtiment tendent à revisiter de façon innovante l'utilisation des matériaux connus depuis longtemps comme la brique, la céramique, le bois..., mais aussi à employer de nouveaux matériaux aux propriétés intéressantes. Jean-François Caron, chercheur en mécanique des matériaux au laboratoire Navier, présente un pultrudé, matériau innovant composé de fibres de verre et de résine, qui offre une grande liberté architecturale de par sa capacité de déformation et permet de créer n'importe quelle forme ou dimension (cathédrale éphémère de Créteil).

https://player.vimeo.com/video/131307380

Date

May. 2013

Journal

CNRS

Links

> permalien
> video