Technical experiment assessment in France
for membranes: Marseille and Nice Stadia
Adelyne Albrecht1, Adrien Escoffier1, François Consigny1
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Elioth, EGIS Concept
Abstract
In France, building control may require a technical experiment assessment (ATEx)
for a fabric tensile structure. The design and fabrication rules are taken from a
recommendation document based on experience and not up-to-date with technology
and calculation software. This document has a curvature criterion, out of which an
ATEx is usually required. Foreign contractors have to adapt to this more
conservative French regulation and learn the ATEx procedure.
For both stadia projects in Nice and Marseille, flat ETFE on cables, flat fabric and
curved surface fabrics were considered. Non-traditional flat fabric and ETFE on
cables are less efficient structural systems than a curved tensile surface, as they
cannot develop equilibrium without deforming; water-ponding is also an issue for
flat roofs. In-depth analysis of the different options was performed in the early stage
of the projects. Eventually, fabric on arches was adopted for the horizontal parts and
again refined strategies of calculation were developed to assess the impact of
phenomenon which cannot be modelled in the calculation software. Both stadiums
adopted a flat membrane for the façade.
On the Nice stadium, ETFE was used for the transparent inner ring. Testing
procedures to assess the properties regarding pretension, creep and temperature
effects were agreed on, along with the design rules, to be submitted to the ATEx.
Keywords: Stadium roof, Flat membrane, Membrane on arches, Appréciation
technique d’expérimentation (ATEx), Innovation, ETFE.
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Introduction
Elioth, part of French engineering group EGIS, has been working on the design of
the roof structures and membrane covers for both Nice Stadium (Allianz Riviera)
and Vélodrome de Marseille stadium since 2010. In this paper we present the French
validation procedure for those covers and offer some feedback which we believe can
prove useful to foreign design practices and contractors.
In summer of 2010, the design team formed by Scau architects and the engineering
company Egis, together with the building company Bouygues won the architectural
competition for the refurbishment, the design and construction of a new cover, for
the 75-year-old football stadium in Marseille.
This stadium will be covered with PTFE on arches for the roof, and flat PTFE for
the vertical façade.
Figure 1 : Marseille stadium
In September 2010, the design team formed by Wilmotte & Associés architects and
the engineering company Egis, together with the building company Vinci won the
architectural competition for the design and build of a new football stadium in Nice.
This project is further developed in [2].
This stadium is covered with PVC on arches for part of the roof, and flat single layer
ETFE on cables for the façade and part of the roof.
Those projects are part of the several newly built football stadiums in France,
motivated by the hosting of the Euro cup in 2016.
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Figure 2 : Nice stadium
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French regulation and ATEx procedure
In France, a structural membrane does not have a proper design standard. A design
guide called “Recommandations pour la conception des ouvrages permanents de
couverture textile” [1] exists, which gives design criteria for curved membranes
consistent with the French codes approach. However it does not cover the latest
technologies, and is not consistent with the Eurocode design approach, which is now
used for the supporting structure. It does not describe either how the calculations
have to be performed.
This guide is used as the basis of the design, and the rules and criteria are required to
be verified. Those criteria happen to be more conservative than in other European
countries’ regulations, which can come as a surprise for foreign contractors. When
the guide lacks some specifications (material properties, safety factors etc), an
agreement has to be found with the building control organisation and validated with
the ATEx when there is one.
When a project involves technologies not covered by a standard, an ATEx procedure
is usually required. Therefore in a project involving a structural membrane an ATEx
is likely to be undertaken. The procedure can be light and easy for traditional
material and installation processes, but it can also go deep in its requirements for
tests, calculations, analyses, process and installation description etc. That is why this
procedure has to be anticipated well in advance and taken into consideration in the
project planning. The cost of it can also sometimes be a factor for decision, although
usually not for large-scale projects.
The ATEx procedure is presided by the CSTB (Scientific and Technical Centre for
Building):
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“The Technical Assessment procedure requires that reliable justifications that often
take a long time to collect should be provided, to ensure that the Recommendation is
constructive and sufficiently credible for the customer, the prescriber, the foreign
importer, the insurance company, etc.
The procedure is available to anyone who asks for it: either the innovation promoter,
or users of the said innovation (clients, designers, contractors, technical inspectors
and insurance companies).
The assessment relates to safety, feasibility, probable operation of the innovation in
service, probability and severity of any foreseeable disorders, the possibility of
making repairs if necessary.”
The comittee is composed of experts from the main French building institutions. It
makes a decision based on the technical file: approved, refused, or pending for
further justifications to provide within a delay.
When an ATEx concerns a project (as opposed to a product), it is delivered for this
project only. If the technology and installation process has already been successfully
used on another building that can serve as a reference and simplify the procedure.
For a membrane, the following considerations can be a motivation for the need for
an ATEx:

The curvature: if the curvature is not within the criteria for the guide
(membrane too flat)

The fixing system: aluminium rails and stainless steel locks are not covered
by the guide.

The scale of the pieces to be laid

The material, especially with ETFE film which is not a fabric

The support structure if it has a part in the design assumptions or
installation strategy.
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Stadia projects with PVC and PTFE membranes
Both Nice stadium and the Vélodrome de Marseille are covered with fabric on
arches.
Figure 3 : Prototype of Marseille stadium
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Figure 4 : Omega profile and tensioning bolt system for the flat PTFE
façade
For Marseille, the following issues had to be studied.
Water-ponding
The end bays of a radial strip have a smaller curvature as they span between an arch
and a straight steel member of the primary structure. Calculation under SLS snow
load and with the deformed geometry of the roof was performed to prove that the
water could always be evacuated. Iterated calculation with accumulated water was
performed under ULS snow-load to check the resistance of the membrane. An
additional calculation was required as the membrane was then resting on a primary
structure bracing, with ensuing load take-down and member check.
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Figure 5 : Illustration of water-ponding.
Modelling and design of arch supports
The arches have a mix of fixed, pinned and sliding connections with the main
structure. Their restraint conditions are not straight-forward. Second order
calculations were performed to show that the sliding had little impact on the
effective out-of-plane buckling length.
Leaving aside the non-linear contact between the fabric and the arch, which is
usually dealt with by removing the arches or the supports from the model for upward
wind loading, the determination of the actual out-of-plane forces in the arch is
complex:
In section, the membrane is never symmetrical across the arch plane. The tension
being almost constant on either side of the arch (like a pulley), the resultant applies
an out-of-plane load onto the arch, the most loaded side pushing the arch out.
On the other hand, the membrane applies a friction force onto the arch, which is
difficult to assess, and has the opposite effect, pulling the arch towards the most
loaded side.
In the usual structural calculation software, these phenomena cannot be accurately
represented by replacing the arches with supports, even sliding ones, nor by tying
the 2D fabric elements to the 1D arches elements, nor with compression / tension
links or other tricks. Different models were tried, amongst them one where the 2D
elements tied to the arch was replaced with sliding cables with equivalent property.
It was verified that the Mz moment in the arch had a different sign depending on the
linkage with the fabric. However, the forces obtained with the simple model where
the fabric is linked to the arch proved generally to be enveloping the ones obtained
with the other models.
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The buckling check of the arches proved also an issue, as French National Annex to
EC3 uses method A for interaction factors, and the actual bending diagrams were
not as simple. Again, some non-linear calculations were performed to check that the
analytical method using first-order results did not lead to under-dimension the
arches.
Figure 6: Study of connection model between the fabric and the
arches. The forces in the arches are very sensitive.
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Inversion of curvature
A flat membrane, when stretched, will never get slack, although it may become
deformed in both directions. The inversion of curvature should not lead to fatigue
failure through repeated slackness.
In an anticlastic fabric, each load direction is supposed to solicit one of the two
fabric yarn directions. Under high loading, the non-working direction can see its
curvature inverted; in such an event both directions take the load. During the
inversion, the surface line gets shorter, and a theoretical calculation shows the total
loss of tension along the line. The actual tension in the yarn, folded by the tension of
its orthogonal thread, cannot be calculated. An easy and convincing method for
testing the fatigue resistance under repeated inversions of curvature is still being
discussed.
Figure 7 : Loss of tension in the transverse direction for a hypar in a
standard structural calculation software
The other main issues on Marseille stadium included the fabrication and installation
process (the PTFE pieces being 80m in length), testing and validation of the
stainless steel straps for fixing, structural resistance of secondary steel members on
the façade designed to allow control of the pretension.
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Flat ETFE single-layer on cables [2]
During the ATEx, the main focus was for clarifying the allowable stress in ETFE to
take into account the creep.
The study was motivated by the fact that ETFE behaviour under stress had mostly
been studied for inflated cushions and that there was a risk that creep would occur
and lead to a loss of pretension.
Experimental tests were carried out under varying loading and climatic conditions to
determine the stress limit that would cause remaining strain in the ETFE film. We
found that this creep limit checked against ELS loads was the governing criterion for
the film.
References
[1] BIGER, BARITEAU, et al. Recommandations pour la conception des
ouvrages permanents de couverture textile (Partie I), Recommandations
pour la confection et la mise en œuvre des ouvrages permanents de
couverture textile (Partie II), Annales de l’ITBTP, 1997 (mis-à-jour
2007).
[2] ESCOFFIER, A., et al. Nice Stadium: Design of a flat single layer ETFE
roof, Proceedings of the 2013 TensiNet symposium, TensiNet, Istanbul,
2013.
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