Tensile fabric structures offer uniquely designed spaces that can create uncluttered spans of space. With their design flexibility and light-transmitting qualities, tensile membrane systems are considered a cost-effective option that can lead to a reduction in required building materials. Found everywhere from stadium roofs to retractable canopies and static structures, tensile architecture can also be integrated with glass or other glazing materials, such as in glass curtainwalls and glass skylights.
Tensile architecture is the use of cables and rods in tension to provide support to structural members, according to Peter J. Katcha, MBA, director of North American sales at SEFAR Architecture, St. Petersburg, Fla. Tensile architecture can consist of glass, canopies, tension fabric structures or anywhere cables and rods are used in tensile forms to provide structural support for an overhead apparatus.
Richard Appleby, business development manager at Kansas City, Mo.-based Structurflex LLC, adds that tensile architecture is a very lightweight, structurally stable alternative to conventional construction. “It clearly adds a strong architectural element to what’s normally considered a very generic proponent of a building,” he says.
Tensile Architecture and Steel
Michele Roth, business development manager at Birdair Inc., Amherst, N.Y., notes that the structural steel is an integral component in supporting the membrane structure and defining the boundaries of its shape. This includes enclosed facilities such as the Denver airport, where its free-flowing tensile structure springs from a more orthogonal base to the top of steel masts, and also includes amphitheaters or entrance canopies, where combinations of masts and perimeter steel create peaks and edges to which the membrane spans.
“Depending on the desired form the architect or client desires, the supporting structure can be placed to create boundaries and changes in direction for the membrane surface,” Roth says. “For example, you can create a linear peak in the structure by placing a steel beam as a ridge and clamping the membrane to the beam. Between such supports as these, the membrane will want to behave naturally to balance the forces applied to it, creating a unique shape. This shape will be characterized by similar but opposite curves in two directions, also known as an anticlastic form.”
“Tensile architecture allows you to employ the most efficient transfer of loads to support a structure,” Katcha notes. “So cables and rods are the most efficient means of transferring loads to a building for example, or across long spans. Instead of using concrete pilings or typical truss work, tensile cables are very efficient in terms of their weight and ability to manage high loads over long distances.”
Appleby notes that Structurflex uses ornamental steel support systems rather than post and beams, as it is more sensitive to the required finishes, shapes, contours and details.
“Cables are used to provide the tension to pre-stress the fabric to create the shape,” explains Katcha. “Fabric leverages the concept of the tensile architecture and creates a very efficient form, one that can span long distances and the interaction of that cable is what pre-stresses the fabric into shape. The fabric is patterned to make sure there is shape in the structure, because you can’t use flat fabric. If you have flat fabric then water, snow, etc., will pond on that fabric. So you have to have some pitch and conical shape.”
Fabric Materials
There are a variety of materials available to create tensile fabric structures. The most common is a PVC membrane. With a life span of more than 20 years, PVC membranes often have additional protective PVDF fluoropolymer coatings on both sides to protect the surface and making it easy to clean.
PTFE is the most durable membrane used in tensile membrane systems. It comprises of a woven glass fibre base coated with polytetrafluoroethylene (PTFE), a highly inert coating generally unaffected by environmental contaminants and ultraviolet light. PTFE has a life span of more than 30 years and has fire resistant properties, in addition to being Energy Star and Cool Roof Rating Council certified.
SEFAR weaves and coats PTFE fabrics, and is the manufacturer of TENARA Fabric, a 100 percent fluoropolymer with a backbone of high-tenancity PTFE yarn. “The PTFE fabrics are incredibly durable, but they are also highly translucent and self-cleaning,” Katcha explains. “The transmissions of light and durability are really the value propositions to people in the fabric architecture market.”
Another option is Ethylene Tetrafluoroethylene (ETFE), which offers a lightweight alternative to glass. It is a transparent extruded film or foil with similar light transmission to glass, but at 1 percent of the weight. ETFE has a life span of more than 20 years and excellent weathering abilities. Additionally, it can be applied in single-, double- or multiple-layered applications.
Energy Efficiency
According to Structurflex, tensioned membrane structures typically reflect approximately 75 percent of external heat and light, which removes the inherent heat load of alternative clear and transparent materials, such as glass or polycarbonates. The fabrics typically allow 9 to 18 percent transmission of daylight, providing softly diffused light within the structure, reducing or eliminating the need for artificial lighting.
Tensile membrane structures have high sun reflectivity and low absorption of sunlight, which greatly reduces the solar energy and heat gain that enters the structure, resulting in less energy use, explains Roth. The membrane also allows for natural daylight to enter into the interior, making it a comfortable space while reducing the costs of electrical energy and reducing the need for artificial lighting during the day.
Tensioned membrane structures comply with the same design standards and loading regulations as conventional building structures. They are also designed to withstand wind, snow, heavy rains, seismic activities and uplift.
Nexen Stage at MacDonald Island Park, Fort McMurray, Alberta, Canada
Completed in 2014, the Nexen Energy (a CNOOC Ltd. Company) Stage at SMS Equipment Stadium at Shell Place in MacDonald Island Park, Fort McMurray, Alberta, Canada, is part of a major expansion project while will allow Shell Place to host a variety of large-scale events at the outdoor performance center.
MacDonald Island Park is operated by the Regional Recreation Corp. of Wood Buffalo, Fort McMurray, and is Canada’s largest community recreation, leisure and social center. In addition to the Nexen Energy Stage and SMS Equipment Stadium, Shell Place will include a baseball/softball tournament center, field house, conference center and community park.
For the project, Birdair Inc., Amherst, N.Y., supplied an approximately 30,000-square-foot PTFE canopy structure that overlooks the Nexen Energy Stage and Molson Outdoor Rink. Designed by 360 Architecture, Kansas City, Mo., and general contractors Clark Builders, Edmonton, Alberta, the canopy is designed to mimic the undulating northern lights and has become widely identified with Shell Place. The canopy is made of PTFE fiberglass membrane with a steel supporting structure, creating an open, inviting space that will come to life during events. Read Jones Christoffersen Consulting Engineers, Vancouver, was the project engineer.
St. George Staten Island Ferry Terminal, Staten Island, N.Y.
Originally designed and constructed in 1996, the light canopy frames at the St. George Terminal esplanade in Staten Island, N.Y., were outdated in both their light source and their surrounding materials. The original design was comprised of an A-series light fixture with spherical acrylic housing that became difficult to maintain over the years, requiring a new type of light and more durable material.
Designed by FTL Design Engineering Studio, New York City, the project utilizes multiple layers of St. Petersburg, Fla.-based SEFAR Architecture’s TENARA Fabric 4T40HF in a star pattern installed around the center cone of each of the 12 light canopies. The pattern accommodates the shape of the canopy frame and its additional layers closer to the center post reinforce the fabric at its most tensioned spot.
The retrofitted canopies also feature an induction lighting system that uses a polycarbonate lens and is ballasted for cooling. The TENARA Fabric diffuses the light and expands in tension, cleanly billowing out above the light to fill in the canopy’s frame. Completed in September 2014, the project team also included tensile fabric membrane, custom hardware and installation by Eventscape Inc., Toronto; lighting designer Leni Schwendinger, New York City; and the New York City Department of Transportation.
**St. George Photos: Photos: ©FTL Design Engineering Studio
McGrath Amphitheatre, Cedar Rapids, Iowa
Cedar Rapids, Iowa, was severely flooded in 2008 and creating an amphitheatre for outdoor entertainment was part of the city’s healing process. With a tight budget, the project was originally speced for structural framing. During the bid process, Structurflex, Kansas City, Mo., bid with the initial specifications and also offered a less expensive alternative, winning the project.
Working with Watertown, Mass.-based Sasaki Associates’ original concept, Structurflex design team developed an affordable structure that met the original design intent. As engineer of record for the project, Structurflex took on responsibility for the steel framing, membrane and cable systems. The project utilized 6,500 square feet of PTFE Coated Fibreglass (Sheerfill II).
Clean, smart connection details allowed a seamless erection process in the field, while adjusting tensioning and clamping systems simplified the challenges of installing PTFE fiberglass during cold weather. Considered the “jewel in the crown” of the Cedar Rapids riverfront redevelopment plan, the McGrath Amphitheatre opened in August 2013.
