ETFE panels line the inner ring of the new canopy at Hard Rock Stadium in Miami Gardens, Florida. The material also surrounds four structural masts, which rise 150 feet above the canopy and 350 feet above the ground to anchor the cables that support the canopy (Wayne Stocks/Thornton Tomasetti).
The summer of 2016 saw the opening of two sports venues that use ETFE in their cladding. At Hard Rock Stadium in Miami, the transparent polymer foil lines the inner ring of a newly built canopy as well as openings for its structural mast supports. In Minneapolis, 246,000 square feet of the lightweight material cover the south-sloping roof of U.S. Bank Stadium, making it the largest ETFE roof in North America. Located at opposite ends of the climate scale – sun, heat and hurricane winds in Florida, bitter cold and heavy snow in Minnesota – these two venues demonstrate ETFE’s versatility.
Detail of the south-facing, sloped ETFE roof. The north-facing portion of the roof is conventional metal deck (courtesy Minnesota Vikings).
ETFE has been part of the architectural lexicon in other countries for nearly 30 years, but has not been widely used in the U.S. We believe the high-visibility use of ETFE at the Hard Rock and U.S. Bank stadiums (as well as in other buildings, like ARTIC and The Shed) marks a turning point in its adoption as a viable option for transparent roofs, skylights and building envelopes in the U.S., especially since there are now nearly a half dozen qualified specialty fabricators available in the U.S. market that can produce and install ETFE in complex applications.
Our façade engineering team has expertise in a wide range of materials, so we can fully evaluate the options specific to every project and develop technical responses that capitalize on the material’s advantages and address potentially problematic characteristics. While ETFE isn’t right for every job, it can be a great solution when used in suitable applications. Here are some reasons why ETFE can be a game changer for sports venues.
ETFE transforms enclosed facilities into light-filled, dynamic spaces. Enclosed facilities have four-season functionality, which expands programmability and increases revenue for owners and operators. But the dark, confined atmospheres of conventionally roofed venues are no longer in step with patron expectations. At the 66,200-seat U.S. Bank Stadium, which hosts NFL football, MLS soccer, NCAA basketball and baseball, high school sports, motocross races, concerts, conventions and other events, the ETFE roof admits abundant natural light, creating an outdoor feel while protecting occupants from the elements.
Interior of U.S. Bank Stadium (courtesy Minnesota Vikings).
It keeps the grass green. ETFE admits the full spectrum of light, including ultraviolet. Its solar control options provide flexibility in the amount of sunlight that can enter, balancing spectator comfort with the light needed to keep natural turf fields healthy. At Hard Rock Stadium, the ETFE ringing the edge of the central opening protects occupants from the elements while exposing the field to more natural light than an opaque canopy would allow.
The transparent ETFE lets in light to keep the grass field healthy (Thornton Tomasetti).
It enables lighter, more cost-effective supporting structures. ETFE systems are extremely light and flexible. At U.S. Bank Stadium, the roof’s low weight enabled our structural engineers to provide primary support for it with a single 970-foot steel ridge truss. The result? Cost savings and a striking visual lightness.
The light roof allowed structural engineers to support it with this elegant, minimalist truss (Thornton Tomasetti).
It is extremely durable and requires minimal maintenance. ETFE and PTFE are fluorocarbon-based polymers developed by DuPont for integrity over a wide temperature range. Both are corrosion- and chemical-resistant, and do not break down, discolor or weaken from UV exposure. While some components (gaskets, for example) will require replacement over the system’s lifetime, accelerated aging tests in laboratories suggest that these polymers can last more than 50 years before needing replacement. A very low coefficient of friction prevents dust and debris from sticking, so that it is carried away by rain – or by simply spraying it with water in dry climates.
In Minneapolis, preventing snow buildup on the roof was critical. A climate analyst helped us understand potential snow behavior; then we worked with architect HKS to shape the roof slope so snow would slide off. We also used continuous ETFE panels (at 340 feet, the longest ever fabricated) that act like slides, sending the snow into a specially designed snow gutter that melts it and carries the water away (courtesy MSFA).
It provides an outdoor feel at a fraction of the cost of a retractable roof. ETFE combines a much lower initial cost with intrinsic durability and low-maintenance performance. And in cold climates, a retractable roof would remain closed for many months per year. ETFE lets in natural light every day, in all weather conditions. In Minneapolis, the outdoor feel can be augmented by opening five 55-foot-wide movable glass-and-steel panels set into an end wall.
While our team found ETFE to be the best material for the roof of U.S Bank Stadium, glass was a better choice for this large end wall. The wall has five enormous operable panels that open to admit fresh air (courtesy Darb02 via Wikimedia Commons).
Its solar performance can be adjusted to the project’s needs. ETFE foil can be produced with tints or frit patterns customized for the specific environment and program. Multiple layers printed with frits can be used to “tune” the shading for different seasons. At U.S. Bank Stadium, we performed detailed solar studies to determine the density of frit required to reduce solar heat gain.
It has significant protective-design and fire-performance benefits. As an elastic material, ETFE has tremendous ability to absorb impact loading without risk of fracture or breakage. It has passed large and small missile tests, rendering it acceptable for use on secure U.S. and British government buildings. This impact resistance was a critical factor at Hard Rock Stadium, enabling the canopy to meet local standards for hurricane resistance. When exposed to fire, ETFE’s chemical composition does not allow combustion or burning. It can melt, creating holes that vent smoke from the fire.
Right: Revit model of the canopy at Hard Rock Stadium; left: detail of ETFE panels during installation (Thornton Tomasetti).
It is sustainable. Relative to the covered area and amount of material used, the embodied energy of ETFE cushions is less than that of glass, when used in the same application. The transparency contributes to natural daylighting, reducing the need for electric illumination, which typically constitutes the largest portion of a building’s energy demand. With multilayer cushions, the thermal resistance of ETFE can match or better that of a typical IGU (insulated glazing unit, often used for façades). Useful Wintertime heat buildup can be countered by intelligent ventilation strategies (via the stack effect) during warmer seasons. Thus, ETFE can help create an offset from the typical mechanical requirements for conditioning the space in both warm and cool seasons.
Solar studies carried out by our sustainability practitioners early in the design phase helped validate the effects of proposed geometry and material use in Minneapolis (left) and Miami (right) (Thornton Tomasetti).
Its acoustic properties can reduce excessive interior noise. No one notices acoustics done right, but when it’s wrong, it’s a problem for players, spectators and broadcasters. ETFE film has 70 percent acoustic absorption/transmittance, which dramatically reduces reverberations and creates more comfortable spaces. But it can also let exterior sounds – especially noise from rain falling on a roof – into the facility, so it’s important to carefully study the potential for both positive and negative acoustic effects when considering an ETFE system. Once the effects are understood, designers can take a number of different measures to “tune” the acoustics to mitigate unwanted noise.
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