ResourcesContinuing Education

Designing with structural fabrics

Architects may be passionate about exterior tensioned membranes, but designing them properly requires 3-D thinking and carefully tailored details.

By Wendy Talarico

Continuing
Education

Use the following learning objectives to focus your study while reading this month’s ARCHITECTURAL RECORD / AIA Continuing Education article.

Learning Objective:
After reading this article, you will be able to:

1. Explain the advantages of using fabrics in a building's design.

2. Describe which elements of a building may be of fabric

3. Identify the fibers and coating used for structural fabrics.

Fabric in architecture captures the imagination. It conjures romantic images: tent canvas staked out in a forest or luxuriously draped curtains making shade for desert nomads. The sculptural shapes an architect can achieve with fabric-from taut, firm roof peaks to pillowy curves-cannot be accomplished with any other material. And the atmosphere inside a fabric structure is unlike that of a conventional building: The diffuse daylight, the acoustics, the shape of the walls or ceilings, even the way the air moves through the space are all different.

Todd Dalland, FAIA, and Nicholas Goldsmith, FAIA, of FTL Happold Design and Engineering Studio in New York City, are as entranced with tensile-fabric structures today as they were when they were first introduced to them in college; in 1971, Dalland made a kind of pilgrimage, hitchhiking from Cornell University to New York City to see Frei Otto's tensile structure in the courtyard of the Museum of Modern Art. Goldsmith later worked with Otto in Germany.

While some firms incorporate fabric in their design work, FTL Happold specializes in it. They design and engineer all types of fabric structures, from air-supported domes to tensile-membrane roofs. FTL Happold has even developed its own software for structural designs (as have a few other engineering firms and some of the fabricators, including Birdair in Amherst, N.Y.).

In addition to designing the structures, FTL Happold also gets about half its business from consulting with other architects who want to use fabric in their designs. Together with the British-based engineering firm Buro Happold, which is in partnership with FTL, the integrated staff provides the engineering, specifies the cutting patterns, or helps the architect develop the design. "We're trying to show other architects that working with fabric is not that hard, once you try it," Dalland says. But engineering a tensile-membrane structure is challenging, even though computer modeling helps in understanding the stresses and loads and in visualizing the finished product.

Fabric is more widely used overseas than in the United States, especially in Europe, Japan, Mexico, and South America, says Bruce Wright, editor of Fabric Architecture, a publication of the Industrial Fabrics Association International in St. Paul, Minn. That's because there is a greater use of canopies and awnings in these countries to limit heat gain. Since air conditioning is often considered a luxury outside the United States, tensile-membrane roofs are popular because they allow daylight while providing shade.

Cut from different cloths

Structural fabric may be used as the entire shell of a building, or as the roof or the cladding. In a smaller role, it may serve as a shading system, skylight, or entrance canopy. There are mesh fabrics (used for shading structures), clear fabrics, liners that improve the aesthetics and performance of outer fabrics, and a range of exterior fabrics for permanent structures. All of these fall into two main types of cloth: polyester and fiberglass. The coatings applied affect the fabric's performance. Considered most durable among these is DuPont Teflon polytetrafluoroethylene (PTFE) and a newer variation of this called TEDLAR. All of these help prevent ultraviolet degradation of the base fabric while adding strength and water- and dirt-resistance.

 

Case Study

Project: The Columbus Center, Baltimore Architect: Zeidler Roberts Partnership, Toronto
Engineer: Peter Sheffield and Associates
Fabricator: Birdair, Amherst, N.Y.

Standing next to the iconic aquarium in Baltimore's Inner Harbor, the Columbus Center, a biotechnology laboratory and exhibition space, is a landmark in its own right, thanks to the graceful fabric roof and entry canopy that swoops across the facade, shading the vast expanse of glass there from the sun. The PTFE-coated fiberglass membrane attracts people "like a circus tent," says Eb Zeidler, faia, of Zeidler Roberts. "It just looks like an attraction, like something interesting must be going on inside." At night, the fabric is lit from within, making the structure glow like a lantern beside the harbor.

The architects, who have done many fabric structures, consider the material a good, inexpensive way to create organic forms. "Using metal and glass to accomplish the same shapes would be too expensive and look too stiff," Zeidler says. "Fabric is a wonderful material in that it is affordable and unique."

A two-layer membrane was selected for the Columbus Center because of its insulation value-primarily to keep the air conditioning inside the space during Baltimore's hot, humid summers. The reflective properties of the fabric and the lack of artificial lighting also help keep it cool inside.

The structural membrane is supported by 10-inch-wide steel posts. Four small skylights are inserted into the fabric so that the light in the spaces below is variable. The steel frames of the skylights also serve as clamps for the fabric. All of this was preassembled on the ground and hydraulically hoisted into place.

One of the goals of the center is to attract children and young people who might not otherwise learn about biotechnology. But private research is also conducted there, giving the building a kind of dual personality. The fabric, installed on the exhibition side, clearly signals that something good is waiting inside for the young students. W.T.

Photo by Michael Dersin, Alain Jaramillo
Steel post supports and skylights are visible over an interior walkway.

The type of fabric that's used depends on the building type. Permanent structures typically use Teflon-coated fiberglass because of its longevity. Manufacturers say the material will last 50 years, but independent design professionals who have worked with it predict a more conservative 30 years. Owing to the material's relative newness (the first Teflon-coated fiberglass roof was installed 25 years ago and is still going strong), actual records of its longevity are not available. Its installed cost is about $60 per square foot.

Deployable structures are typically PVC-coated polyester, though the material is also commonly used on permanent buildings. Teflon-coated fiberglass is not meant to be packed and unpacked, Dalland says. "You set it up once and it stays." But polyester is more flexible. The fabric has an installed cost of about $40 per square foot. Its fabric replacement cycle is about 15 years by most estimates but, again, actual figures vary. When used on deployable structures its life span may be shortened due to the rigors of setting it up and taking it down.

Polyethylene fabric, developed in 1994, consists of a woven polyethylene scrim that is also coated on both sides with polyethylene films. The material has a high weight-to-strength ratio, says Jay Alberts, business development manager for Intertape Polymer Group, a manufacturer in Bradenton, Fla. This material has a great deal of potential, Dalland says. "Its biggest advantage is its low cost and its greenness," he continues. "It's the only fabric that is 100 percent recyclable, and it costs about half as much as PVC-coated polyester. Manufacturers already recycle the scraps and can recycle entire roofs."

PVC-coated polyester is also being recycled, though not to the extent possible with polyethylene (which is a mono-material). An experimental but promising new process called Vinyloop, developed by two European textile manufacturers, separates the polyester fiber from the PVC compound found in postconsumer vinyl products. Teflon-coated fiberglass typically comes only in white, but pvc-coated polyester fabrics come in many colors. Fabric weights, measured in pounds per square yard, range from 4 ounces to 48 ounces, depending on wind- and snow-loading criteria.

How it performs

Tensile fabric offers many performance advantages, but the biggest is probably its strength-to-weight ratio. "It enables you to cover large areas with little material. That makes it easy to install and lessens the amount of energy used in the building process," Dalland says. "It can also mean a lighter building structure, because fabric as a cladding material is much lighter than glass or stone." The lightweight fabric also interacts more readily with natural forces than a material that's more rigid, which is one reason that the interior of a tensile-membrane structure feels so different from that of a conventional building. "You don't feel so isolated from the outdoors," Dalland adds.

Structural fabric must perform up to the same codes, standards, and loading criteria as other structural materials. It must meet fire-performance standards; the base fabrics must be noncombustible or flame-retardant.

 

Case study

Project: Palm Springs Regional Airport
Architect: Gensler, Santa Monica, Calif.
Fabricator: Birdair, Amherst, N.Y.


Palm Springs, synonymous with the great outdoors, felt its airport should signal this association. Like Fentress Bradburn and Associates' Denver International Airport, the peaked structure at the Palm Springs terminal is meant to symbolize the surrounding mountains, beckoning visitors. "We wanted the airport to feel like a resort," says Andrew Cohen, aia, project director for Gensler. "The gateway to every city should be memorable in some way. The fabric makes this a landmark."

The PTFE-coated fiberglass membrane protects and shades the airport's open-air concessions and seating areas. There is even a putting green on site, since Palm Springs is a golfers' mecca.

"We realized that, with the tensile material for shade, we didn't need to close off and air condition these spaces. The fabric provides a luminous, natural light while keeping temperatures 25 to 30 degrees cooler during the day," Cohen adds. The waiting rooms are sealed and air conditioned, but there is no fabric membrane over these areas.

The architects had done some fabric structures, but this was their largest to date. The 39,000-square-foot structure was installed over a 10-week period. The fabric is supported by tension rings and cables; the cables, in turn, are supported by steel masts. Turnbuckles are used to tension the cable. The system provides large spans but is extremely lightweight.

The energy savings as a result of use of the fabric were "incredible," Cohen says. That's because of the lack of air conditioning and artificial lighting. Single layers of fabric were used throughout the project. W.T.

Photo by RMA Photography

Canopies at the Palm Springs Regional Airport are made of PTFE-coated fiberglass fabric. They are hung from the cables attached to a tension ring.

"Sprinkler installation for fabric roofs, when necessary, requires special attention in order to integrate the sprinkler drops and head locations formally and functionally with the fabric shapes and openings," says Goldsmith. "Variances for performance-based fire-resistance analysis can be obtained in appropriate situations," says Douglas Radcliffe, vice president of sales and marketing for Birdair.

For stability, the fabric in tensile structures must curve equally in opposing directions-known mathematically as a hyperbolic paraboloid. The fabric is usually oriented so that the fibers run parallel to these curves. As a result, stresses are balanced. Fabric is somewhat elastic, but, properly designed, it won't sag or creep.

Translucency is another principal advantage of fabric. Daylight, filtered through the woven yarns, is soft and even, eliminating the need for supplemental lighting. That can mean significant cost savings for energy, particularly for shopping malls, convention centers, and places where lighting needs are high. After dark, the fabric lets light out, creating a structure that glows in the night. Light transmittance varies according to the type of fabric and coatings and the number of layers, colors, and, of course, the position of the building in relation to the path of the sun and other structures.

The material's performance is not as impressive when it comes to R-values. A single layer has an R-value of about 1. This is improved with double or even triple layers, which add air spaces that help insulate. And low E films can be laminated onto the fabric layers. But when the total energy package is examined, it may be that fabric will save money.

That's because natural lighting, which virtually eliminates the need for electric lights, lowers energy costs and avoids heat gain from the lamps. The highly reflective surface of fabric also lowers the amount of heat gain from solar radiation, especially compared to conventional roofs.

This dovetails with the relatively new approach toward building services design at FTL Happold, where only the portions of rooms that are occupied by people are heated or cooled, and only during the times they are occupied, but that's another story.

 

Case study

Project: Burj Al Arab Hotel, Dubai
Architect: W.S. Atkins & Partners, London
Fabric: Dyneon, Oakdale, Minn.
Fabricator: Skyspan Europe, Germany

This is the kind of project architects dream of: a luxurious, spare-no-expense hotel and conference center on a man-made island in Dubai. Opened less than a year ago, the hotel features a brilliant white fiberglass and PTFE-coated vertical cloth membrane that's meant to resemble the billowing sail of a traditional Arabic sailing ship.

The 161,000-square-foot, two-layer screen covers the open side of the V-shaped steel-and-glass building, creating an atrium lobby. A more standard choice for this area would have been glass, but in the extreme desert conditions, this would have introduced heat and glare. The fabric provides a soft, milky light and lessens heat gain. At night, it is used as a gigantic projection screen onto which the faces of high-ranking guests are shown [May 2000, page 321].

The vertical installation may mark the first time that tensile fabric was used as part of the facade, says Michael Wolf, project manager for Skyspan. The application was more difficult to engineer than a typical horizontally applied tensile membrane. The fabricators, who also did the engineering, created a fabric model and used it for structural analysis and for cutting patterns.

Photo Courtesy of Skyspan
The coated Teflon fabric skin of the Burj Al Arab Hotel in Dubai is divided into 12 parts.

The 650-foot-tall screen is divided into 12 fields averaging 7800 square feet each. Installing a single membrane for the whole area would have placed too much stress on the fabric.

The fields are each supported by steel arches and pairs of steel rods that are also tied to the structure. The fabric was clamped to the arches at ground level, then the entire assembly was lifted into place and unfurled. The screen stops about 45 feet above the ground to allow for the steel-and-glass entry facade.

The desert heat and light will take a toll on the fabric's longevity, though the fabricators estimate that it will hold up for up to 50 years. The smaller individual fields, however, are more easily replaced than the entire assembly. W.T.

 

Structural components and installation

Tensile-membrane structures are supported by masts, arches, or external trusses. The masts give fabric roofs their iconic peaks, which are typically steel, but may be aluminum if the structure is relocatable. Mast-supported structures are less expensive than the other two, but the spans are shorter and there are more interior columns. Arches allow larger spans but require greater structural gymnastics. External trusses allow a wide-open space but, once again, require more costly engineering and materials.

Working with fabric requires some of a dressmaker's skills and sensibilities. At FTL Happold, models are handsewn using white panty hose and are supported with brass tubes, polyester threads, and Crazyglue. On the building itself, darts, seams, and other details are common. These are calculated and drawn by FTL Happold with special software and executed by the fabricators.

The bolts of cloth for actual construction come in varying widths, from about 6 to 14 feet. During design, much thought is given to creating a pattern that minimizes waste during the cutting process. The fabric may be cut with lasers, using CAD drawing files for guidance. Or it may be cut by hand, using templates and cutting equipment borrowed from the fashion industry. Fabric seams achieve full design loading, and vary in width from 1 to 3 inches wide, depending on fabric type and on wind and snow loading; they are made by RF (radio-frequency welding), or heat welding.

As well as being predesigned and pre-engineered, the entire tensile structure and its supporting structures are prefabricated off-site in factories. The fabric is assembled at the plant, put on a roll, and transported to the site, where it is unfurled into place.

Contractors, usually ironworkers, work with sheets that are as large as possible but can still be handled. Wind is a problem, according to Birdair's Radcliffe. "If winds are above 20 miles per hour, we won't install. It's like wrestling with a 10,000-square-foot kite." During installation, the fabric is tethered with a guideline.

The only work done on-site is the foundations. Masts are connected to these via pins; some play is necessary to allow the mast to move slightly to accommodate loads. The assembly is stayed in several directions with cables. Replacing the fabric on a permanent structure, when it is worn, is a significant undertaking that should be considered during design. Generally the portion of the building adjacent to the tensile structure will have to be closed during the replacement time.

Try it, you'll like it

Fabric is not for everyone; nor is it for every building type. But as Dalland says, "A space with a fabric roof or wall invariably becomes the premium space within a building. Yet, despite these advantages, most architects have never touched a fabric structure." He adds, "I bet most secretly want to."

 

 

Proper design, engineering, material selection, and fabrication all work together to ensure a sound tensile-fabric structure. Since the industry is constantly changing with many new players, architects should check out a variety of fabric suppliers and structure fabricators to determine which company works best for them. The list below is not definitive; often suppliers will double as fabricators and vice versa. For more information, visit the Industrial Fabrics Association International's Web site at www.ifai.com.
Rita F. Catinella

Fabric suppliers

Anchor Industries Inc.
Evansville, Ind. 800/255-5552. www.anchorinc.com

Chemfab Corp.
Salt Lake City. 801/262-3654.
www.chemfabcorp.com

Cover-All Building Systems
Saskatoon, Canada.
800/615-4777.
www.coverall.net

Ferrari S.A.
Cedex, France.
33/04-7497-6649.

Hansen Weatherport Corp.
Olathe, Colo. 970/323-5932.
www.weatherport.com

Intertape Polymer Group
Montreal. 800/565-2000.
www.intertapepolymer.com

Losberger US LLC
Mooresville, Ind. 800/964-8368.
www.losberger.com

Mehler Coating Fabrics Inc.
Charlotte, N.C. 704/527-7188.

Seaman Corporation
Wooster, Ohio. 800/927-8578.
www.seamancorp.com

Snyder Manufacturing Inc.
Dover, Ohio. 330/262-1111.
www.SnyderMan.com

Taconic
Petersburg, N.Y. 518/658-3202.
www.4taconic.com

Taiyo Kogyo Corporation
Tokyo. 81/03-3719-37.

Universal Fabric Structures
Quakertown, Pa. 800/634-8368.
www.ufsinc.com

Weatherhaven
Burnaby B.C. Canada. 604/451-8900.
www.weatherhaven.com

 

Structural fabricators

Advanced Structures Inc.
Marina Del Rey, Calif. 310/301-1984.
www.asidesign.com

Birdair Inc.
Amherst, N.Y. 800/622-2246
www.birdair.com

Canvas Specialty
Los Angeles. 800/423-4082.
www.can-spec.com

J&J Carter Limited
Bagingstoke Hampshire, U.K. 44/01256 8114.
www.jjcarter.com

Landrell Fabric Engineering
Monmouthshire, U.K.
44/01291 6277
www.landrell.com

Permafab Pty Ltd.
Sydney, Australia. 61/02
9858 36.

Rainier Industries, Ltd.
Seattle. 800/869-7162.
www.rainierindustries.com

Rubb Buildings Inc.
Sanford, Maine. 800/289-7822.
www.rubb.com

Skyspan (UK) Ltd.
Wiltshire, U. K. 44/ 01722 331599.
www.skyspan.com

Sullivan & Brampton
San Leandro, Calif. 800/257-5900.
www.sullivanandbrampton.com

Tensile Designs International
Kansas City, Mo. 800/58-SHADE.
www.tensiledesigns.com

Tentnology
Surrey, B.C. Canada.
800/663-8858.
www.tentnology.com

 

Questions:

  1. What advantages do fabrics offer to a building's design?

  2. Why are fabrics used more in other countries than in the United States?

  3. Fabric is used for which elements in a building?

  4. Structural fabrics are made of what fibers and coatings?

  5. How are fabric structures supported?

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