Synopsis

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.

Acknowledgement

First of all, the authors would like to thank the Catholic Church of Créteil for their trust and their courage, which permitted to get into this ambitious and successful project. Secondly, the authors would greet the engineers from T/E/S/S, which have developed this challenging project during 18th months. They made a valuable work and permitted research works to become a reality throughout this amazing edifice. They also thank the firm Viry for the supervision of construction works, including the delicate erection stage. Thirdly, the authors would like to thank warmly all the people involved in the construction process: the numerous parish’s volunteers, the technicians and researchers from the Laboratory Navier and the engineers from T/E/S/S and Viry firms. Beyond the technic, their enthusiasm made this project a powerful human experience. Fourthly, the authors would like to thank the local firms for their work: BATEM (concrete), Eloi (steel), Esmery Caron (fabric), Solutions Composites (composite material), Axmann (connections), ENSG.

Facts & Figures

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

Client & Design

Partners

  • Viry

Project Overview

Context and challenges

Creteil is a city of 90.000 inhabitants in the southeast suburb of Paris. It started to urbanize itself in the late 50’s, impelled by the French architect Charles-Gustave Stoskopf. In 1976 he designed Notre Dame of Créteil, a modest catholic church made of concrete, which became a cathedral 10 years later. Recently, the diocese of Creteil has undertaken a major architectural redevelopment project of its cathedral, including a timber shell covering the religious area and the creation of a new cultural area. Once transformed, the edifice shall be more visible, more hospitable and livelier for citizens. Inevitably, such a molt takes time and a temporary place of worship was required to ensure liturgical services during the two-years work. In November 2011, T/E/S/S, the structural design office in charge of the cathedral renovation project, made an ambitious proposal to the diocese. Based on a previous successful experience – the construction of a composite gridshell for the festival Solidays (see video [1]) – T/E/S/S suggested that rather installing a basic tent, the parishioners should construct themselves a temporary cathedral (see video [2]). The overall cost of the project is 400k€, about 30% more than a standard tent of the same size that would have been rented for 2 years.

Architectural form of the building

The generation of the form was driven by two objectives that of providing a variety of appropriate internal spaces within which the community could assemble, and externally to be a welcoming and visually interesting form. Today, the internal organization of a Roman Catholic Church is in large part driven by the post Vatican II vision of a religious celebration being a collective gathering of the community around the Eucharist, center of spiritual life. A circular seating arrangement is often considered the most convivial form to create a sense of belonging while minimizing a sense of hierarchy (Fig. 2).

However the community is not only using the building for religious celebration but also for encounters on a more informal manner, for example spontaneous gatherings after religious ceremonies. In the early Roman church, such gathering of the community was facilitated by the presence of an anti-space to the main space called a narthex, through which one passed on entering the church. It was therefore felt appropriate that the formal freedom which the gridshell system offered would be used to explore forms composed of an agglomeration of major and a minor volumes which contain the two functions: formal and informal gatherings (Fig. 3).

Formal explorations were undertaken using modeling clay. The final form is based loosely on two adjacent semi spherical volumes of different size, which are merged into one complex form. Externally the fear of the design team was that the totally convex blob form could look intimidating. It was therefore decided that the two spherical virtual forms, which would be joined to make the final form, would be arranged not in a symmetrical axial manner, but in an asymmetrical curved composition. The resulting form seen in plan is convex on one side and concave on the other. The concave form in plan allows for double curvature to be introduced into what would be otherwise a simpler blob and gives sensuality and visual interest to the building.

Placing of the building on site

The temporary cathedral is located on a land owned by the municipality, which is used for sporting and other communal gatherings. The curve in the building defines an external area where the church community could meet in the open air and this is where the entrance to the church is situated. The building was positioned on the site so that the entrance addresses a grass planted area forming a garden forecourt or “parvis”. A service building housing plant, toilets and vestry are housed in a portacabin positioned to the rear of the building.

It is formally quite difficult to integrate doors, which must be verticals, into a complex geometry. Either the gridshell could be deformed to accommodate the geometrical requirements of doors, or the doors could be integrated into an independent form. The latter approach was chosen. In looking for forms to house the doors, reference was made to the conical monumental doorways with rings of concentric decoration, which welcome the faithful to Romanesque and Gothic churches in France. The conical forms were found to by coherent to the overall geometry of the building. The entrance doors were therefore inserted into a conical hooded form made of rolled steel plates and stiffened by concentric steel tubes, which not only make reference to historic precedence but also refer to the gridshell to be discovered inside.

The cone of the entrance doors was positioned in the concave side of the building giving access directly to the narthex part of the internal volume (Fig. 4). To the rear of the church is situated a service door. The steel hood, which houses this door, is curved tightly around the door and takes up an ovoid form.

Technical description

The gridshell structure is made of long glass-fiber pipes (∅ext 42mm) pinned together with scaffold swivel couplers. The structural members of the grid, all of different length, are built from one, two or three composite pipes connected with steel sleeves (§6.2). The length of the pipes is restrained to 12m so standard trucks can deliver them. The pipes are organized in three layers. The first two layers are placed perpendicularly on the ground. They form the “quadrangular primary grid”. The distance between the pipes of these two layers is constant so the grid is regular. This primary grid is elastically deformed to obtain the final shape. The third layer of pipes acts as bracing. It gives the structure a shell behavior. Those pipes are fixed to the primary grid once it has been shaped. The structure is anchored to a concrete strip footing with a special steel system, which ensures the transit of loads from the composite structure to the ground. A similar system enables to fix the structure to the doors.

A PVC coated fabric, tailor patterned, covers the structure. It includes transparent strips that bring daylight inside. The fabric is stretched on a devoted peripheral edge beam with a double lacing system (halyard and strap). At the ground, the lacing edge beam is made of a bent composite rod nailed regularly to the concrete slab. At the doors, it is made of a steel arch welded to the doorframes. The membrane is waterproof and as a continuous membrane has no joints except at the perimeter. At the perimeter a continuous strip of membrane is prefixed to the internal face of the membrane and fixed to the ground slab being clamped between the concrete and an aluminum flat bar. At the doors, the flexible strip is riveted to the doorframes.

Daylight

The gridshell is covered in a PVC membrane, which is opaque. How to introduce daylight into the interior was a major subject of reflection. The simplest way found was to use transparent membrane placed occasionally on the membrane. A small amount of light was required in the interior to create a contemplative atmosphere. The lights would in consequence glow and would be seen as luminous insertions in the vault, like stars in the celestial vault or the apse of some Romanesque churches. The stars were patterned on the joints of the PVC membrane. The almond shape came from simplification of the cutting into the panels either side of the joints and to avoid stress concentrations around cuts in the membrane. This shape, known as Mandela, is frequently used in Marian religious imagery. The distribution of the transparent insertions is quite uniform but gets denser above the pinnacle.

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.

Research and Testing

Flexural strength

Naturally, the characteristic flexural strength (σk,flex) of the GFRP tube is needed to verify if the structure complies with Eurocodes. This parameter has a critical impact on the structure’s reliabil- ity because in this particular application stresses in the tubes are mainly due to bending. Thus, it was important to con- firm the manufacturer’s value by assays.

Three-point flexural tests were led with and without connexions (tightening torque set to 20Nm) to determine the characteris- tic strength according to the protocol of the Eurocode (Annex D) :

For five tests, the factor kn,5% is 1.80 , assuming a normal distribution. One can note that the connections scatter the re- sults more. Finally, the manufacturer value of 400MPa (ASTM D790) was confirmed and retained for further calcu- lations.

Sliding of connections

Delivery, cutting, gluing, connexions

Sleeves testing

The graph below shows the tensile test of a 3 pins connection between a connector and the corresponding composite tube (Fig. 16). Till 35kN the graph reflects the elastic behavior of the composite tube, with slight deviations corresponding to pins rearrangement. Then the compressive stress applied by each pin to the compo- site tube exceeds its compressive strength. Progressively, the pins are pulled of through the tube under a residual force that tends to stabilize at around 20kN.

Construction Works

Ground works

Placing & Concrete slab and footing.

Prefabrication of the gridshell

Delivery, cutting, gluing, connexions

Grid erection

Delivery, cutting, gluing, connexions

Bracing

Delivery, cutting, gluing, connexions

Covering with a PVC membrane

Delivery, cutting, gluing, connexions

The People

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