The most important quality in choosing a material for a fabric structure is its fire resistance. National Fire Protection Association (NFPA) 701 is the most common fire test for textiles and films. The American Society for Testing and Materials (ASTM) is another recognized standard for a wide range of materials, and ASTM E-84, 108, and 136 are common tests related to fabrics for membrane structures.
The latest architectural fabrics used for a building envelope respond to heat and light much differently than previous generations of fabric; they also offer features and benefits different than conventional construction materials. Architectural fabrics can be manufactured to vary in translucency from 1 to 95 percent and, in thermal resistance, from a single pane of glass to that of a conventionally insulated structure, while still maintaining adequate daylighting. A fabric roof can be a source of interior light at night if artificial light is directed onto its highly reflective surface.
Selecting the right fabric
The performance of today’s architectural fabrics depends upon the weaving pattern, choice of substrate, and coating. Each composite has unique properties and characteristics that suit it to different applications. Most materials presented have a minimum of stretch and shrinkage in a wide range of temperature and humidity conditions, and coatings that prevent mildew, staining, and streaking. Choice of a material calls for understanding of its light reflectivity and light transmission. Reflectivity is the amount of light the surface of the material reflects; transmission is the amount of light that penetrates the material. Most fabrics allow some amount of light transmission, but some materials come with a blackout scrim between layers and allow no light to penetrate, so light and heat from the sun can be controlled.
All the materials come in some shade of white; some are also available in a limited range of colors, depending on supply and demand. The proper selection of membrane material will be based on the proposed size, form, function, and desired longevity of the structure and the economics of the project.
How are these membranes fabricated?
The covering of a tensioned fabric structure is referred to as the membrane. It can be fabricated a number of ways based on the material chosen and the orientation of the seams. All aspects of a fabric structure should be derived from the same computer model or full-scale mockup. Computer-generated patterns are the most widely accepted template for fabrication; smaller structures, such as awnings, are patterned directly off a full-scale mockup.
Seams determine the appearance of joined panels. The seams can be sewn, glued, electronically welded, or heat-sealed. Seam styles can be parallel or radial to a mast. Butt seams are joints produced by placing two adjacent pieces directly beside one other and covering the join with a strip of material; lap seams are joints made by overlapping the edges of the material. Reinforcements—multiple layers of material applied to specific areas of a membrane to strengthen it where concentrated tension loads exist—are also a part of the fabrication process and differ from project to project.
What’s available Today?
Architectural fabrics in common use today include:
PTFE (polytetrafluorethylene)-coated fiberglass
Woven PTFE by Gore called Tenara
PVC (polyvinylchloride)-coated polyester
Stretch fabrics (spandex)
High-density polyethylene (HDPE)
ETFE (ethylenetetrafluorethylene) foil
PTFE-coated fiberglass is the worldwide preferred material for large-scale permanent structures or structures requiring long life and specific construction code compliance (IN-9). PTFE has excellent weather, temperature, and chemical resistance, as well as durability and strength. Its life span is over thirty years, and it is manufactured in accordance with such standards as ASTM E-108 and E-84, meaning that it is noncombustible. PTFE varies in translucence from 7 to 15 percent, and reflects between 68 and 75 percent of incident sunlight. The transmitted is evenly dispersed and free of shadows and glare.
Before installation, PTFE has an irregular off white or slightly brown color, which is the result of the manufacturing and fabrication process. Once exposed to direct sunlight, its external surface bleaches to a milky white within a matter of days. PTFE comes in colors, but manufacturers require a minimum order. The material requires heat-sealing of FEP (fluorinated ethylene propylene) between layers at the seams to join fabric pattern sections. PTFE comes 10 to 12 feet wide depending on the manufacturer. It is considerably more expensive than PVC and is not very flexible. Manufacturers include Saint Gobain, Verseidag, and Fibertech.
Silicone-coated fiberglass is an inexpensive alternative to PTFE fiberglass with many of its attributes. It has very high tensile and tear strength and is more flexible than most other materials. Silicone has had a reputation for years of getting dirty rather quickly and being problematic at the seams; however, the topcoat has been improved and fabricators are willing to use the material. The seaming process requires an adhesive that takes less time to cure completely than PTFE, which reduces labor cost. The seaming process is more efficient and the quality of the seam strength more consistent. Silicone coated fiberglass does not generate any toxic fumes while burning, which makes it safer than PTFE or PVC. It is long lasting, flame resistant, dimensionally stable, and available in a range of colors and translucence. The material comes 6 to 10 feet wide, depending on the manufacturer. Manufacturers include Fabrimax and P-D Interglas.
Woven PTFE is a 100-percent fluoropolymer fabric made with high-strength PTFE. It offers durability, strength, and flexibility. It transmits up to 40 percent of light. It combines good light- and water resistance with the ability to withstand repeated flexing and folding, an advantage coated fiberglass fabrics. The material is pliable enough for retractable and deployable structures. It is rather expensive and is not as strong as either PTFE or polyester. This material comes 6 to 8 feet wide and has a 25-year life span. For colors, a minimum order is required. The manufacturer is W. L. Gore.
PVC-coated polyester is the most cost-effective membrane material and, therefore, an ideal choice for both temporary and permanent tension structures. The material is soft, pliable, and less expensive than PTFE. It is available in a variety of weights to meet a wide range of structural requirements. This material is sealed with a radio-frequency (RF) welder or hot air sealer. A number of different topcoats allow panels to be RF welded easily; however, PVC with topcoats of polyvinyl fluoride (PVF) and polyvinylidene fluoride (PVDF), which provide the base material with a much cleaner and maintenance free surface, require additional work in the shop. Both top-of-the-line PVF and PVDF require that the topcoat or film where two panels are to meet be ground off in order for them to be RF welded. This is time consuming and requires great care in order to keep the seams clear of dirt, mold, and mildew. PVC material has a life span range between 15 to 20 years depending on the topcoat chosen. It comes in a variety of colors and translucence. The material be found as a perforated mesh. PVC is subject to creep—stretching under load—an can also d thus requires periodic retensioning. The material comes in widths of 54 to 98 inches. Manufacturers include Ferrari, Mehler, Naizil, Seaman, and Verseidag.
PVC-laminated polyester is used primarily for temporary structures and stationary and retractable awnings and canopies. Vinyl laminates are two or more layers of fabric or film joined together by heat, pressure, and a water-based adhesive to form a single ply. These materials are lower in cost and have a shorter life span than coated materials. They come in a variety of colors and in stripes and patterns. Fabric comes 54 to 98 inches wide; the life span is 8 to 10 years. Manufacturers include Herculite and Snyder.
Theatrical draperies are used for interior applications only. These are fabrics used primarily in theaters and places of public assembly where fire resistance is required. The materials available vary in quality, texture, width, and cost. Some have very short life spans, while others are manufactured to last a lifetime. Theatrical drapery project do not necessarily need to be in tension. Recommended distributors of theatrical draperies include Rose Brand and Dazian.
Stretch fabrics such as spandex are materials that stretch rather easily in multiple directions. They are used for both temporary and interior projects. The material can be dyed or silk screened, and are used often at trade shows and special events. The life span varies depending on the application.
HDPE is manufactured and used in a variety of ways. The material can be made for shading only or engineered and woven for complete water protection. Shade mesh comes in a variety of styles, colors, and shade factors from 50 to 95 percent. A high-density polyethylene fabric provides high tensile strength, ultraviolet (UV) stability, and high UV absorption. Coated polyethylene produces higher strength-to-weight properties than many traditional membrane fabrics. HDPE is 100-percent recyclable since it is a combination of high- and low-density polyethylene. Expected life of the fabric is 10 to 12 years. The shade cloth comes 8 to 12 feet wide. It is especially well suited to dry and hot climates and where protection from sun and hail is desired. Manufacturers include Sunports, Interwrap, and ECP.
ETFE foil is a polymer resin from the same family as PTFE. It is produced in very thin sheets and is manufactured to be installed as inflated pillows, also referred to as cushions or foils. It is an alternative to structural glass for long-span structures and, because of its light weight, is a way of reducing the size of the primary structural system. ETFE foils are supported by a constant air flow supplied by an inflation system consisting of a centrifugal fan unit and emergency backup, with humidity controls and filters to prevent moisture and dirt from getting inside the pillows. The material has low tear propagation, is UV resistant, inert to chemicals, and 100-percent recyclable. Multiple layers of ETFE can provide an effective thermal enclosure. ETFE can be designed with unique patterns on the film, providing a range of light transmission. It can be used in a single layer for smaller structures such as awnings and canopies. Because ETFE requires both fabricating experience and specialized equipment for joining panels, it is best purchased directly through a specialty contractor.
The best way to determine the most appropriate material for your application is by contacting the manufacturers and requesting a fabric sample.
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