p/Cover Story/Sep - Roof & Facade

Roof&Facade Asia Feature: Translucent & Transparent Materials
ETFE in Membrane Constructions
– Important aspects of ETFE
cushion structures
Mr Dieter Linke, covertex GmbH / Engineering + Design GbR, Germany, highlighted the
potential for the use of ETFE (ethyl tetra fluoro ethylene) cladding in the building industry, in
his presentation at FLUOROPLAST 2004 World Congress which addressed the theme
‘Fluoropolymers in Architecture - Construction – Design – Engineering’. The event, held on 28
and 29 April 2004 in Zurich, Switzerland, was organised by Maack Business Services of Zurich.
(The next FLUOROPLAST World Congress will be held in Zurich from 14 to 15 March 2006).
Introduction
Traditional architectural membranes are coated fabrics.
Wind and snow loads are carried by the fabric.
The coating protects the fabric against environmental
impacts and it allows for the weld assembly of
the panels.
These membranes with PE-fabric and PVC
coating or glass fibres with PVC coating are light
and very strong – but never transparent.
Typical characteristics of ETFE are its
transparency (also combined with printing), natural
light transmittance for visible light and UV-rays, its
low weight and the possibility to build extended and
variable curved surfaces in one piece. These
characteristics make it very interesting for
architectural purposes.
But as ETFE membranes in roofing and cladding
are not reinforced, they have to carry the loads
without fabric. Thus, ETFE membranes have only a
fractional amount of tensile strength, compared to
coated fabric materials.
It is obvious that the supporting structure and
the detailing for the ETFE attachment must take this
fact into account. A good way to do so, is to use ETFE
in claddings as air supported cushions.
It is a great challenge to merge the architect’s
intentions with the special needs of ETFE
structures, in order to achieve a good, durable and
economic building.
But to do so, knowledge about the less obvious,
4 Roof & Facade Asia • September 2004
though very important technical aspects relating to
the use of ETFE as a structural element in buildings,
is required, too. As building with ETFE has very
little history, principals, architects, and engineers
usually are not aware of the sometimes very rigid,
boundary conditions.
Depending on the type of supporting structure
and the physical needs of the cladding, like thermal
insulation, shading, and internal humidity, two
main design groups can be identified - ETFE
cushions and mechanically prestressed ETFE in a
single layer.
First, important aspects about ETFE cushions will
be highlighted. Against this background, the special
design features of the world’s largest ETFE cushion
project, Allianz Arena Soccer Stadium, Munich, will
be explained, and towards the end, the prospects
with single layer ETFE constructions, will be
discussed.
Typical configuration of ETFE cushions
ETFE cushions basically consist of one inner and one
outer foil. Both are stabilised by automatically
controlled, moderate, overpressure of the air
trapped between them. This pressure is retained by
fans. The layers are usually welded together along
the edges and attached to the support structure with
special aluminium extrusions.
Static mode of operation
Inward-aimed external loads (wind pressure, snow),
are led to the inner foil by the internal overpressure.
It carries the load, hammock-like.
Outward-aimed external loads (wind suction) are
carried by the outer layer in similar manner. Internal
pressure and external wind suction add up to a
Allianz Arena Soccer Stadium, Munich, Germany: The world’s
largest ETFE cushion project.
certain extent and need to be calculated carefully.
The overpressure usually amounts to 250 Pa and
can be changed up to the equivalent snow load, if
necessary.
Owing to the instability in windy conditions, a
lack of air pressure should be avoided. If water
ponding is excluded by well designed slopes on the
roof, a lack of air pressure is usually not a static
danger. The foils are designed for the expected loads
and it is not important, whether, for example, snow
load effects are applied to the inner layer via air
cushion or applied directly.
EXTERNAL LOAD E.G. SNOW
HORIZONTAL
SUPPORT
REACTION
INTERNAL OVER PRESSURE
VERTICAL
SUPPORT
REACTION
RADIUS OF OUTER FOIL
OUTER
INNER
RISE
OF
ARCH
Limited strength of ETFE foil
The performance of a cushion-carrying load is
dependent on the deployed thickness of the foil
which is usually between 150 and 250 microns and
on its radius of curvature. With typical building
code design loads, the free span of a long,
rectangular ETFE cushion, can reach up to
approximately 4 m. Its length is limited only by
practical considerations.
High tension forces at the attachment structure
and/or the space for the dip, must be taken into
account. A radius of 3.5 m to 4 m for the curvature
of a cushion layer between the attachment lines, is
a good value to start with, for the design. A small
radius reduces stress but increases the rise of the
arch of the cushion foils.
Supported with cable nets, to reduce the static
free span of the foil, ETFE cushions have been built
with up to 50 m in diameter.
Safe air supply
The normal excess pressure of 250 Pa equals 0.0025
bar or 0.25 kN/sqm. The surplus amount of air
required to provide this pressure in the cushion, is
only a few litres. The air to fill the cushions and to
replace losses, is led into the cushions via pipes and
hoses, and is usually taken from outside of the
building.
Except under extremely windy conditions or
when there is a danger of water ponding, the air
overpressure in the cushion is not a vital element in
the static performance of the cushions. But lack of
air pressure could have negative effects on inner
foils, internals, thermal insulation and durability of
the entire construction.
Usually, two electric engines with two fans (not
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