Ethylene-Tetrafluoroethylene copolymer (ETFE) is one of the trending innovative cladding materials in the market which is known primarily through iconic projects such as Eden in Cornwall UK, Allianz Arena Stadium in Munich and National Aquatic Centre a.k.a.Watercube in Beijing. Being very thin, lightweight and when used in air-filled cushion system its enormous strength and a range of adaptive environmental attributes render ETFE as a very creative solution to many projects.
ETFE has been known since the 1940s, which was an accidental spin-off man-made fluoropolymer during DuPont’s research program to develop an insulation material for industrial machinery that was resistant to friction and abrasion, immune to radiation and effective at both extremely high and low temperatures. Until 1970 it was not commercialized after which it was used in a wide range of applications in the petroleum, automotive, aerospace and nuclear industries.
First architectural use of ETFE was done in Burgers Zoo in Arnheim, Holland in 1982 for a rectification of roof cladding works.
ETFE is used as an extruded film to be used as a light weight cladding material. It can be both used on façade or roof applications where the detailing is similar. Thickness of the ETFE skin can be as thin as 100microns ranging up to 250 microns which is designed by the ETFE specialist as per design requirements. The film can either be used in the form of an inflated multilayer pillow (named as Pneumatic system or Air filled cushion system) or as a single skin when tensioned against a supporting curved structure in the manner of a tent. (fig. 1)
Salient Features of ETFE as a Facade Material
- ETFE is fairly resistant to tears and punctures and has a good tensile stiffness. The relatively high tensile strength and modulus enable the polymer to maintain its form when stressed.
- ETFE membrane is immune to UV light, atmospheric pollution and other forms of environmental degradation. ETFE has shown no visible signs of deterioration in Burgers Zoo application which was built 32 years ago.Expected lifetime of the material is anticipated greater than 40 years.
- Light penetration in ETFE can be as high as 95% however this would be in a diffused form and may pose a design concern where the clarity of the cladding is vital for the design.
- ETFE can be supplied in colored or printed format to reduce solar glare/transmission. However, the colors are limited to blue, white and red.
- ETFE is a self-cleaning material and does not require a frequent cleaning. As it is formed with extrusion process ETFE has a very smooth and slippery surface.
- Since ETFE has an extremely smooth and non-adhesive surface it is difficult for algae to grow on the surface. If the detailing is done properly for the interfaces, dirt cannot accumulate.
Fire Performance of ETFE
Fire performance of the ETFE is studied on a design case basis. Although ETFE is a combustible material, it is self- extinguishing and low inflammable due to presence of fluorine in its chemical structure and a low oxygen index. ETFE can act as a self-venting during fire. Material shrinks away when the temperature rise above 200ºC and creates a natural smoke vent. ETFE does not melt and drip when shrinking away as other plastic materials and produces toxic fumes only when burned at temperatures above 800 degrees Celsius.
When designing ETFE roofs& canopies, water drainage is studied in particular in case of an air pump system failure or a deflation of an ETFE pillow. In this respect, appropriate fall or slope should be provided in the design to facilitate natural drainage and minimize the risk of pooling water in case of a failure and to mitigate excessive stretching of material when filled with water. There are mechanical means of dealing with water drainage of the roof patented to each ETFE specialist. However these are not recommended as these valve systems can cause air infiltration through the fixtures.
Maintaining an ETFE Facade
For regular maintenance, approximately at every six months visual inspections should be conducted to detect any damages to the pillows and to check the air pump systems. Small holes that may appear in the cushions are normally acceptable as the inflation system works at quite low pressures and it would just have to work a little harder to compensate for the hole. The hole can be temporarily patched while are placement cushion is being maintained. ETFE suppliers have specialist methods for patching as ETFE can be difficult to bond to due to its smooth surface.
Punctures or tears less than 100mm long can be repaired on site by using a specialist ETFE tape welded onto the foil. If a cushion needs to be replaced, access can be achieved using a net system the way ETFE cushions are installed.
TYPES OF ETFE APPLICATIONS:
Single Skin ETFE Applications:
- Single skin application is an integrated system with aluminum perimeter clamping and water sealant system with stainless steel cables. ETFE when used as single skin requires stainless steel cables at the welded ETFE pockets at about every one meter spacing to increase the strength of the material. Spacing of the cables depends on the design requirements and structural conditions.
- Single skin ETFE membrane are used where the U-values are not a consideration. Single layer ETFE enclosure systems cannot take the advantage of pneumatic cushion technology with respect to the elongation of the material that initiates a load reduction cycle.
Pneumatic ETFE Applications:
- Drumming effect of rain on ETFE may become a design concern depending on the use and function of ETFE cladding applications. In this respect in projects such as hospitals, museums, schools acoustic performance shall be studied if sound levels are a concern. In order to mitigate, preventive design methods can be adopted to reduce the impact of the noise generated by rain on the internal environment. One method to improve the acoustic performance is to install an anti-drumming ETFE mesh over the ETFE skin. By doing that water can pool on the perforated mesh and dampen the noise from the rain.However an anti-drumming mesh contradicts the benefits of the smooth surface of ETFE with respect to self-cleaning aspect. Hence, unless it is not absolutely necessary it is recommended to have a provision on the rain noise mitigation while designing the ETFE and make the decision whether to install the anti-drumming mesh after the roof has been in operation for 3 or 4 years.
- Arup has been involved in many ETFE buildings in the world some of which are listed below.
- Two of the most recent projects with ETFE cladding in Singapore are used as case studies while discussing the ETFE properties and design considerations.
ITE College in Ang Mo Kio
ITE (Institute of Technology and Education) College in Ang Mo Kio is completed in 2012 which comprises total of six ETFE canopies. Arup helped Architect (RSP Architects Singapore) during the design and execution of the canopies.
Four of the trapezoidal shaped canopies at the fly roof area a.k.a. Pod Garden Canopies house green walls and gardens underneath. Two of the canopies with circular and an elliptical roof form cover an amphitheater and a forum where public events take place.
For Pod Garden canopies it was crucial to use a material that allows Ultraviolet (UV) light penetration for allowing plant growth and photosynthesis for the green walls and gardens along the alley which is not possible with safety laminated glass. Architectural was looking for a transparent material allowing UV light transmission and adequate shading and thermal comfort to the pedestrians.
ETFE can penetrate 99% of the complete spectrum which includes visible, infrared and ultra violet making it the best possible material to be considered for botanical garden, natural turf applications for stadiums or canopies above the gardens such as this project.
Architect wanted to create shadows with leaf patterns on the walls and floors to harmonize with the green walls surrounding the arrival gardens. This was possible through the frit pattern application to ETFE pillows with almost any pattern to provide some form of shading. In ITE project the leaf pattern was customized as per Architectural intent with 60% frit.
Substantial savings from the steel structure and aluminum framing were achieved due to ultra-light weight of the ETFE cladding. ETFE has a density of 1.75gr/m³ which equals to 780g /m² for a single layer of ETFE skin. This is 3% of the weight compared to a 6+6 laminated glass panel. Reduction of the self-weight of the cladding material allowed using slimmer steel sections at the roof. Pod garden canopies spanning 35mtare formed with approximately 12 pillows with 4m width and in a single length.
Birds are unable to stand on the cushions due to their slippery surface.However, they can perch on the cover caps between ETFEcushions and pecking holes in the ETFE cushions which is one of the issuesthat may damage the ETFE cushions. To prevent the possible bird pecking and damaging bird wires are installed on the top of the aluminum extrusions. Having springs on the both ends of the stainless steel wires birds are unable to keep their balance when landing on the wires and this helps to avoid damage risks.
At forum and amphitheater canopies with curvilinear forms reaching to 30 m span 70% print is selected to provide thermal comfort to the occupants. AS the canopies house public events, acoustical performance of the material was important. ETFE is transparent to sound; meaning that sound travels through and very little noise is reverberated back. In the case of noise generated within the area below the canopy, this is beneficial as the ambient noise will go straight through the roof and will be therefore less difficult to control. This was beneficial in the case of the forum and amphitheater canopies. But this also means that noise generated outside the covered area i.e. plant and equipment noise will be heard within the space which may become a design concern for other applications and particular acoustics study shall be done in such cases.
ITE was a very fast track project where ETFE helped with reducing the installation duration to one third of the time that would be required for glass canopy installation. Being able to pass large spans and installation methodology installation time of ETFE is much shorter than compared to any other cladding material. ETFE can be produced as long as required from handling perspective 50m length is a recommended span; in this project maximum length utilized was about 35mt long.
With the innovative use of ETFE many advantages are gained throughout the project with savings on costs, shorter installation duration and meeting the project performance requirements such as UV penetration, shading with customized patterns, acoustical transparency and large spans with minimal framing.
Sports Hub a.k.a. National Stadium
Reaching to its final days of completion, Sports Hub is one of the biggest and innovative iconic projects of Singapore with an ultra-thin moving roof spanning 310m over the stadium housing 55,000 people at the public and sports events.
Roof is mainly made of 2 layers; ETFE moving roof supported on runway trusses and operating above an aluminum standing seam fixed roof. Running over the runway trusses, moving roof can be closed and opened in about 25 minutes.
20,000m2 moving roof is cladded in a multi-layer ETFE pillow and incorporate a matrix of LED lights, making it one of the largest addressable LED screens in the world.
By using an ultra-light weight cladding material on the moving roof, the result is a super-efficient shell dome structure, with a total steel weight of 8,057 metric tons. At a structural weight per square meter of just over 100kg/m2 , this would be considered efficient even for a roof that was half this span.
ETFE pillows with 3m width by 50m length for an individual pillow covering the enormous roof with a minimal use of aluminum extrusion and steel sub frame. LED lights are incorporated within the ETFE design and when lit at night any shapes can be created on the giant screen as well as the national flag of Singapore.
Natural turf is used in the design with the UV light penetration when the roof is in closed conditions, other times roof can be opened to allow for direct sun light penetration in to the space.
Solar shading to the roof is provided by applying frit pattern on the upper ETFE skin and diffused light is provided to avoid increased solar heat gain and glare issues to ensure spectator comfort levels. High insulation properties of ETFE pillows provide a shelter from the high solar heat gain while keeping the cooled air within the dome.
Sports Hub was awarded Green Mark Gold Plus status with incorporation of innovative green technologies and will be one of the signature projects of Singapore.