Breaking the glass roof: Building with ETFE architecture

by Katie Daniel | June 28, 2016 2:58 pm

by David Capezzuto
Tensile structures have been used for millennia. When indigenous peoples required shelter that was lightweight and structurally sound, fabrics made from animal hides and easily transportable elements were the most viable solution. At the Roman Colosseum, a retractable Velarium provided shading for a more comfortable spectator experience. Now, ethylene tetrafluoroethylene (ETFE) is offering new opportunities.

A relatively new product within the industry, ETFE is a fluorine-based plastic that remains strong across a wide range of temperatures and is highly resistive to corrosion. It was developed from polytetrafluoro-ethylene (PTFE)—also known as Teflon—a strong, lightweight, fire-retardant fabric membrane that was originally formulated for space exploration apparel, but later used for architectural applications and transparent envelopes.

What is ETFE?
While glass structures provide comparable sunlight transmission and insulation, ETFE is highly durable, more transparent, and significantly lighter—it is approximately one percent the weight of glass. Initially used in agricultural applications, ETFE has since been used on high-profile projects such as the Eden Project botanical attraction (Cornwall, England), the Allianz Arena (home to soccer’s FC Bayern Munich), and the Beijing National Aquatics Center—the famous Water Cube featured at the 2008 Summer Games. ETFE film is now considered a premium material for transparent cladding applications ranging from roofing to façade construction to traditional skylight applications to long-span structures.

Few building materials can match ETFE for its design flexibility and performance value. For China’s Nantong Park Bon-Garden Greenhouse, the material’s thermal performance and light transmission properties support thriving horticulture while still achieving a unique illuminated dome design. At Empire City Casino at Yonkers Raceway in New York, a porte-cochere with a 1020-m² (11,000-sf) pneumatic ETFE film roofing system showcases the material’s ability to meet the unique aesthetic and practical needs—providing not only an eye-catching design element, but also shelter and shade for occupants with high expectations for comfort.

ETFE is also one of the most lightweight and transparent cladding materials available. Due to its low coefficient of friction, neither dust nor dirt sticks to it. As it is ultraviolet (UV) transparent, it neither discolors nor structurally weakens over time. A highly sustainable product, the manufacturing byproducts of ETFE can be remolded into new ETFE products such as tubing components, wires, or castings.

Performance characteristics
ETFE film brings in numerous benefits for occupants and the building owner. The film can be between 90 and 95 percent transparent, allowing for UV transmission and photosynthesis for agricultural applications. The solar performance ranges of ETFE film systems are also flexible, as they can incorporate multiple frit patterns on one or multiple layers.

Standard or custom printed patterns and a range of colors can be applied during the extrusion process to provide design continuity with the rest of the structure and contribute toward solar control properties. ETFE films are also extremely elastic. Up to 600 percent at breaking point, they are still structurally resistant. The tensile strength at the limit of elasticity/plasticity is 21 to 23 N/mm² (3045 to 3335 lbf/si), but tensile strength to breaking point is 52 N/mm² (7542 lbf/si). For the structural calculation, a limit of 15 N/mm² (2175 lbf/si) is considered a conservatively realistic estimate.

ETFE does not degrade under exposure to environmental pollution, UV light, harsh chemicals, or extreme temperatures, making it an exceptionally long-lasting material. ETFE film also has about 70 percent acoustic transmission, making it ideal for projects expecting loud noises. During design development, sound transmission should be considered, as it will indeed transmit sound beyond the ETFE system and to nearby adjacent properties.

From the extruding of the film to transportation to the site, ETFE is sustainable and energy-efficient. Compared to other cladding materials, the design-build process leaves a small carbon footprint. ETFE systems comprise materials with low embodied energy that are demountable and recyclable. The low softening temperature of ETFE film makes the process of recycling the film efficient and economical. ETFE also enhances insulation and daylighting, contributing to the building’s global energy efficiency. It is also exceptionally lightweight compared to competing materials, allowing substructure support systems and concrete foundations to be designed more efficiently and cost-effectively, contributing to a reduced carbon footprint.

For applications in which glass would typically be used, ETFE delivers superior thermal performance. For double- or triple-layer pneumatic systems, multiple layers of film are welded into panels inflated with low-pressurized air to stabilize the film and provide the system’s thermal property. In a single-layered application, an R-value of approximately ‘1’ can be achieved. In a two-layer system, an R-value of about 1.6 can be reached. A three-layer ETFE assembly has an R-value around 0.51 K·m²/W (2.9 sf·F·h/Btu) or a U-value around 1.99 W/m²·K (0.35 Btu/(h·sf·F). Integrating internal blankets of aerogel into the system further increases the level of thermal performance.

Another key characteristic of ETFE is the air inflation system. A pneumatic ETFE cushion system is generally fed by one or more inflation units. Each unit consists of two redundant blowers forming a backup system for guaranteed structural stability. A series of pressure sensors continuously monitors the internal pressure of the ETFE cushions, maintaining them between 240 and 290 Pa (5 and 6 psf). One unit can feed a roof ranging from 1400 to 2325 m² (15,000 to 25,000 sf). They are UL-certified and run on an 110V power, with a power consumption of less than 1kW.

Due to the non-adhesive surface of ETFE, deposits are washed away by rain, resulting in a ‘self-cleaning’ effect. However, it is necessary to perform yearly inspections, including all necessary checks on the air blower system and filter replacements. The ETFE film and its attachments must also be inspected to prevent any further deterioration.

Overall, ETFE film structures are low-maintenance systems. Should there ever be a puncture or tear—which typically results from flying debris from weather-related instances—repairs in the field can be done. In these instances, if it is a small hole or tear, the membrane can simply be repaired with a patch. If it is a larger tear in the film, replacing the affected ETFE panel can be done without needing to replace the entire ETFE roofing system.

ETFE films have been rated under various national and international standards for fire resistance as self-extinguishing with no burning drops. ETFE is classified under several different standards, including:

• ASTM E84, Standard Test Method for Surface Burning Characteristics of Building Materials, Class A;
• UL 94VTM, Tests for Flammability of Plastic Materials for Parts in Devices and Appliances−Thin Material Burning Test, Class 0;
• EN 13501-1, Fire Classification of Construction Products and Building Elements−Part 1: Classification Using Data From Reaction to Fire Tests, Class B-s1-d0; and
• National Fire Protection Association (NFPA) 701, Standard Methods of Fire Tests for Flame Propagation of Textiles and Films.

Due to the material’s high resistance and elasticity, ETFE is ideal where sudden extreme loads, such as earthquakes or blasts, may occur. Unlike glass, which can shatter and cause major concerns under such situations, ETFE deflects the heavy load.

Conventional construction versus ETFE
There are several basic differences found between conventional construction and building with an ETFE assembly. With conventional construction, heavy weight and rigidity are the standard requirements—therefore, the design must follow limited geometry, often leading to high construction and development costs and limited aesthetic options.

With ETFE, structural form and integrity are achieved through tension. This allows architects to break away from traditional geometric shapes and create freeform designs cost-effectively. To create a structure that harmonized with its surrounding landscape, the design-build team for Empire City Casino at Yonkers Raceway designed an exotic porte-cochere that appeared to emerge out of a nearby hillside. As the structure was a gateway to nighttime entertainment, the design also incorporated custom-colored light-emitting diodes (LEDs). Achieving this dynamic aesthetic cost-effectively through conventional construction would be impractical, if not unfeasible.

ETFE also features numerous engineering benefits. In conventional post-and-beam construction, structure is created through compression. With ETFE, structural loads are carried by internal air pressure compensating for external wind and snow pressures, resulting in a lightweight structural element. It can be difficult to span great distances without providing support columns with conventional construction. With tensile architecture, weight is comparatively negligible and larger spans are significantly easier to achieve.

British architect Sir Michael Hopkins has summarized the unique benefits of tensile architecture.

“Increasingly, we are exploring highly-efficient multi-functional elements, where the structural performance, enclosure, light, and thermal transmittance are combined in a single element,” he said. “These are the reasons we use membrane.”

Design process steps
The ability to achieve dynamic designs is a key benefit of tensile architecture. Its two basic forms are single layer, where tension is achieved through mechanically tensioning the ETFE; and double or triple layer cushion constructions, where tension is achieved through pressurization or inflation.

Throughout the design-build process, contractors specializing in tensile structures work with architects to provide project condition details to be used in construction documents or, more frequently, in the design-build specifications included for the specialty contractor.

Making the most of ETFE’s performance characteristics requires interaction between many geometric forms, materials, and tensioning options. Design can be aided by sophisticated software to help architects and engineers create nearly any imaginable design.

Consultation with a contractor that specializes in ETFE architecture is an extremely valuable step. Architects who work with ETFE film structures recognize this cooperative effort as a best practice. Working with a specialty contracting firm, with in-house design, software, data, and fabrication resources, can minimize risk for the client, designers, architects, and engineers.

ETFE structures are generally specified as design-build projects. In designing the structure, the geometry need to be addressed first. Next, testing how the membrane will interact with the support structure helps determine how to meet the load requirements. Throughout the entire design-build process, coordination is critical to the structure’s overall aesthetics and performance.

Tensile contractors
Special software packages used in-house by specialty contractors incorporate finite element analyses to provide detailed output for proposed designs. Full design responsibility typically rests with the tensile contractor and is inclusive of both the membrane and structure, which is usually composed of steel and/or cables.

Membrane products are purchased in stock lengths and widths. The engineering and detailing required to pattern the material should be included in the scope of the tensile contractor’s work. During membrane fabrication, stock lengths are cut to the designed patterns. Then, the patterns are assembled to the desired panel sizes required for installation.

Since most tensile contractors provide design-build services, they are also responsible for detailing all of the tensile structure’s various elements. Detailed drawings of each element are required to effectively produce fabricated elements. Typically, performance specifications are prepared by the architect and engineer. It is the responsibility of the pre-qualified specialty contractor to provide a proposal that meets the outlined criteria.

During the construction phase, forces applied to the structure during installation must be analyzed to prevent unbalanced loads. Since the integrity of a structure relies on elements erected and set in tension, detailed engineering and planning is necessary for the membrane, steel, cables, and the methods developed to install the system.

Testing of the membrane
Tests must be completed to evaluate the film’s behavior under load and determine its stress limits. With a sufficient safety factor, the design stress limit will be around 15 N/mm². Some projects require custom tests to demonstrate adequate fire performance.

A material-testing regimen is also required in the design development process, which is critical to the structure’s life span and quality assurance (QA) processes. These tests should be provided by the specialty contractor through in-house resources or by third-party facilities approved by internationally recognized tensile organizations.

Extensive testing ensures ETFE film strength and flexibility, and that the membrane and selected connections can withstand seasonal weather extremes. A weather machine that produces accelerated weathering effects is used to test the membrane’s ability to withstand UV rays in wet and dry conditions. Light transmission and reflectance are also measured.

Construction and installation
After a project is approved through design development, and manufacturing and testing are underway, the contracting firm’s project managers and engineers collaborate with the general contractor on the construction methodology and coordinate erection procedures and scheduling. Throughout construction, site superintendents manage the installation of the structural supporting steel, extrusions, cables (if required), and the ETFE film. Due to the lightweight nature of membrane, installing an ETFE roof or façade system is a cost-effective solution that requires less structural steel to support the roof compared to conventional building materials, enabling long spans of column-free space.

Since ETFE structures require some maintenance, building owners often work with the contractor to ensure adequate aesthetic and structural longevity of the steel, cables, or tensile membrane, as well as the air inflation system for a pneumatic structure. These services can range from simple cleanings to comprehensive structural reviews and modifications.

Covered for the future
ETFE film systems are durable, highly transparent, and lightweight in comparison to glass structures and feature exceptional light transmission, solar control, shading, and elasticity. The design-build process is highly energy-efficient and cost-effective compared to conventional construction.

The membrane’s physical properties allow for freeform designs that can serve practical occupant needs and facilitate ambitious aesthetic features. Working with a specialty tensile contractor is an industry best practice, allowing the architect and client to achieve their mutual visions on time and on budget.

The evolution of ETFE architecture to-date indicates further growth for this versatile material and the category in general. A byproduct of PTFE, it is a historically significant building material that should pave the way for even more dynamic freeform designs with uncompromised performance value.

David Capezzuto is the director of business development at Birdair Inc. With the company for more than 25 years, he began his career as a business development manager. Capezzuto has managed many key projects and initiatives through the Americas, including development of the company’s business plans and strategies. He can be reached via e-mail at dcapezzuto@birdair.com[1].

Source URL: https://www.constructionspecifier.com/breaking-the-glass-roof-building-with-etfe-architecture/

---