A fabric structure’s material selection, proper design, engineering, fabrication and installation all work together to ensure a sound structure. The material’s role in the structure’s performance makes the selection process especially important. This is particularly true with tensile and air-supported structures because their membranes, as well as their frames, carry the loads.
Most fabric structures use fabrics rather than meshes or films. The fabrics typically are coated and laminated with synthetic materials for greater strength and/or environmental resistance. Among the most widely used materials are polyester laminated or coated with polyvinyl chloride (PVC), woven fiberglass coated with polytetrafluoroethylene (PTFE) or silicone. Meshes, films and other materials also have appropriate applications.
Polyester is the most frequently used base material because of its strength, durability, cost and stretch. Polyesters laminated or coated with PVC films generally are the least expensive for longer-term fabrications.
Laminates usually consist of vinyl films over woven or knitted polyester meshes (called scrims or substrates). Coated fabrics typically use a high-count, high-tensile base fabric coated with a bondable substance for extra strength. One fabric manufacturing method places polyester fabric under tension before and during the coating processes. The result is that yarns in both directions of the weave have identical characteristics, giving the fabric increased dimensional stability.
Lighter fabrics (200 to 270g/m2) commonly are used as acoustic and insulated liners suspended beneath a structure’s envelope. For long-term exterior use, heavier materials are needed: 20- to 26-oz. (680 – 880gm) fabrics with topcoatings of polyvinyl fluoride (PVF, of which Tedlar is an example) or polyvinylidene fluoride (PVDF, of which Vidar, Fluorex® and Kynar® are examples). These topcoatings provide a protective finish to withstand environmental degradation.
Vinyl-coated polyester is the most common fabric for producing flexible structures, such as custom-designed awnings, canopies, walkways, tent halls, smaller air-supported structures and light member-framed structures.
Vinyl-coated polyester is composed of a polyester scrim, a bonding or adhesive agent, and exterior PVC coatings. The polyester scrim supports the coating (applied initially in liquid form) and provides the tensile strength, elongation, tear strength and dimensional stability of the finished fabric. The scrim is made of high-tenacity, continuous-filament yarns, which have high dimensional stability, and can be bent thousands of times without losing any tensile properties. The base fabric’s tensile strength is determined by the size (denier) and strength (tenacity) of the yarns and the number of yarns per linear inch or meter. The bigger the yarn and the more yarns per inch, the greater the finished product’s tensile strength. For architectural applications, base fabrics typically weigh between 2.5 and 10 oz/yd2, with a tensile strength between 300 (2.662 N/5cm) and 650 lbs/in (5.60 N/5cm), although fabrics intended only for tent use may have lower measurements.
The adhesive agent provides a chemical bond between the polyester fibers and the exterior coatings and prevents wicking of moisture into the fibers. Wicking is the capillary like action of fiber to absorb water, which could result in freeze-thaw damage.
The PVC coating liquid (vinyl Organisol or Plastisol) contains chemicals to achieve desired properties regarding color, water resistance, mildew resistance and flame retardancy. The fabrics also can be made with high levels of light transmission or complete opaqueness. After the coating is applied to the scrim, the fabric goes through a heating chamber to dry the liquid coating.
Vinyl-laminated polyesters are used for awnings, tents and low-tension frame structures. Technically, a laminated fabric consists of a reinforcing polyester scrim that is calendared between two layers of unsupported PVC film. In general use, it refers to two or more layers of fabric or film joined by heat, pressure and an adhesive to form a single ply.
With an open-weave or mesh polyester scrim, the exterior vinyl films bond to themselves through the openings in the fabric. Heavier base fabrics, though, are too tightly constructed to permit this lamination process, so an adhesive must bond the exterior films to the base fabric.
A good chemical bond is important to prevent delamination and is critical in developing the proper seam strengths. The adhesive enables the seam, created by welding vinyl-coated fabric to another piece of the same material, to meet a structure’s shear forces and load requirements at all temperatures. By preventing wicking of moisture into the scrim’s fibers, the adhesive prevents fungal growth or freezing that can affect the exterior coating’s adhesion to the scrim. In response to EPA regulations, the adhesives are water-based.
Using an open-weave scrim such as mesh might make these fabrics more economical, depending on the number and type of features required in the vinyl. What weight is necessary to withstand abrasion and wear? Is flame resistance needed? Is a particular color required? What width? Virtually any color, plus UV resistance, abrasion resistance, and colorfastness can be formulated into the vinyl, but the more of these features incorporated, the higher the cost.
Another widely used base material is woven fiberglass coated with PTFE (also known as Teflon®) or silicone. The glass fibers are drawn into continuous filaments, which are bundled into yarns. The yarns are woven to form a substrate. The fiberglass has a high ultimate tensile strength, behaves elastically and does not undergo significant stress relaxation or creep. The PTFE coating is chemically inert, withstands temperatures from minus 100F to 450F (minus 73C to 232C), is immune to UV radiation and can be cleaned with water.
PTFE-coated fiberglass is available with as much as 25% translucency, providing diffused interior light. Its ability to provide natural daytime lighting and its highly reflective surface for efficient nighttime interior lighting can reduce energy consumption.
For these and other reasons, fiberglass-based fabrics have been the material of choice for stadium domes (both air- and cable-supported) and many other permanent structures, particularly in the United States. Another reason some industry experts cite for this is a perception among code officials that its high melting temperature and lack of creep, or long-term elongation, make it superior to polyester. Other industry insiders note that polyester, like fiberglass, melts rather than burns at high temperatures, and that properly constructed, polyester structures may be equally durable.
Because of the differences in how polyester and fiberglass perform in fire-resistance tests, PTFE-coated fiberglass is the only membrane material that currently meets the U. S. model building codes definitions of a noncombustible material. (The three U.S. model codes are currently being reviewed and soon will be consolidated into one code.) This is a more accurate reason for the PTFE-coated fiberglass preference, but it raises questions about whether standards applied to other building materials should be applied to membranes.
This material is constructed of PTFE fibers woven into a fabric. As of now, only one such material is available. Woven PTFE combines the environmentally-resistant advantages of the material with its ability to withstand repeated flexing and folding, an advantage it has over coated-fiberglass fabrics. Such flexibility makes it an especially good option for convertible structures; however, it is a rather expensive material and is not as strong as either polyester or glass.
Perhaps the newest development in the fabric structures arena is the introduction of ETFE (ethylene tetrafluoroethethylene), a transparent membrane with fabric like qualities and the advantages of PTFE, such as a self-cleaning capability. Resistant to atmospheric pollution and UV light, ETFE has a very long expected lifespan of more than 20 years. Effective thermal performance (average U value is 2.6W/m2K for a two-layer system) and high light transmission (95% visible light, 85% UV light) enable a range of applications where traditional materials, such as glass, would not be practical. It is more than 20 times lighter than glass (0.35kg/m2 for ETFE vs. 15kg/m2 for glass) and is ecologically friendly and energy efficient as its constituent materials are fluorspar, hydrogen sulphate and trichloromethane, all non petrochemical derivatives. It is 100% recyclable.
Meshes, netting and film
These are the least-used materials for fabric structures. Mesh is a broad term for any porous fabric with open spaces between its yarns. It can be made from almost any fiber by a variety of methods, including knitting, weaving and extrusion. In some cases it acts as a substrate to beef up other fabrics or is coated to produce specific characteristics.
For architectural use, meshes typically are available as polyester weaves lightly coated with vinyl or as knitted fabrics using high-density polyethylene (HDPE), polypropylene or acrylic yarns. Polyester mesh dyes well, is strong, has a low water absorption rate and can be economical. Nylon often is used in industrial applications because of its strength and resistance to chemicals, although it does have a high water absorption rate and may cost more than polyester. Often used in agriculture, recreation and containment, polypropylene and HDPE are inert, so they can’t be stained or dyed, and are less expensive than polyester or nylon. Polypropylene, however, does have a comparatively low melting rate, a factor in some industrial applications.
Meshes can provide shade as well as shelter from wind. Since they’re porous, meshes are little good against rain. Still, they are inexpensive and have been used for some low-cost membrane structure applications. For obvious reasons, meshes are not used in traditional air-supported structure design.
Netting is considered a type of mesh, usually tight with small holes. Netting finds use in stadium interiors behind goals, golf ranges and courses, playground equipment and structures, horticulture, zoos, construction sites and other areas where protection or containment is needed.
Netting consists of a nylon, polyester or polypropylene with extruded or spun yarns that is knotted or raschel knitted to form the material. Each material has its advantages and appropriate applications. Polyester holds dye better than nylon but is more expensive; nylon is easier to coat, but has a higher water absorption rate and doesn’t hold dye as well. Polypropylene floats on water, is durable and chemically resistant, but can’t be dyed. Raschel knitting is a newer, faster manufacturing method than knotting. One drawback is that the knitted material can unravel, which can be thwarted by heat-setting the netting to shrink and stabilize the fibers.
Films are transparent polymers extruded in sheet form without a supporting substrate. They are not laminated or coated. Examples include clear vinyl, polyester or polyethylene. These films are cheaper than textiles, but they are neither as strong nor as durable.
Films are much weaker in tension, though more elastic, than scrim-based fabrics. Films sometimes have application in air-inflated structures. Air-inflated structures are composed of fabric tubes in which the air is pressurized, but the structure’s interior itself is not. Some air-inflated roofs or building envelopes have been made using two or three layers of films to form air pillows. The film layers are thermally welded and sealed, and the resulting pillows are inflated by small fans. The inflation increases the internal pressure to prestress the surface, creating load resistance. Such film pillows are framed by an aluminum extrusion perimeter, which must accommodate some structural movement.
Films range in thickness from 30 to 200 microns and can be produced with levels of translucency varying from 25% to 95% light transmission. Films are low weight, have a life expectancy of 20 to 25 years and highly resist dirt. The inflated pillows exhibit good thermal insulation values. More research needs to be done to develop a range of standard reliable, economical details, for instance, to improve the water seals and reduce wicking.
Blackout material, sometimes called blockout material, is an opaque or nontranslucent fabric. Primarily used for tents, the fabric is a laminate that sandwiches a dark opaque layer between two white exterior layers. Because no light transmits through the tent top or walls made with blackout material, lighting and heat can be controlled. In addition, any stains, dirt, repairs or slightly mismatched panels on the tent’s exterior will not be noticeable from the inside.
Blackout fabric also has its disadvantages. Heating may be necessary, as the tent’s interior may be colder than using nonopaque fabric. Heating and lighting, of course, will increase the operating costs. The multiple layers make the fabric heavy and harder to handle, and increase the possibility of delamination over time. The fabric is more expensive that other tent materials, pinholes or snags in it are apparent to occupants, and often, the interior color does not exactly match the exterior color.
Shadecloth, often a knitted fabric such as monofilament polyethylene, originally found use as agricultural crop protection. It has been adapted for tension structures whose purpose is solar shading. Shadecloth can be manufactured in a variety of colors, offers stretch and resiliency and remains flexible without tensile-strength loss under a range of conditions. Light transmission can vary from 20% to 90% shade factor; its UV filter construction can range from 30% to 70%.
Nylon, which may be laminated with vinyl films, is stronger and more durable than polyester, but has a higher cost and more stretch. It may be a good choice for some small jobs, but it stretches too much for use in large buildings.
Spandex is difficult to use in long-term exterior applications because of the wind’s effect on its seams and construction, but its stretch and splash of colors can contribute to interior spaces or temporary exterior use, for instance, at festivals and special events.
Solution-dyed acrylic and modacrylic have gained use, particularly for small shading structures. Their wide range of colors and modacrylic’s flame retardancy make the materials attractive to designers (see the Awnings & Canopies section for more information).
Kevlar® is an excellent lightweight fabric for construction, but it is very expensive, it is rarely used for large structures, such as dome roofs.
Product test data is almost the only way to establish a measure of relative quality. Many variables enter into the process of making fabrics, which may make one manufacturer’s product significantly different from its competition’s in one or more aspects. Test results provide the best indicator of such differences. Many suppliers have invested considerable money and time to test and characterize their products, and routinely provide information about the properties of their fabrics, including:
- strip tensile strength
- grab tensile strength
- trapezoidal tear strength
- tongue tear strength
- adhesion strength
- flame resistance
- finished weight
- base fabric weight
- available topcoatings
- resistance to cold cracking
- dead load
- structural properties
- life expectancy
Tensile strength data is a basic indicator of relative strength. It’s fundamental for architectural fabrics that function primarily in tension.
Tear strength is important because if a fabric ruptures in place, it generally does so by tearing. This occurs when a local stress concentration or local damage results in the failure of one yarn, which increases the stress on remaining yarns.
Adhesion strength is a measure of the strength of the bond between the base material and coating or film laminate that protects it. The measure is useful for evaluating the strength of welded joints for connecting strips of fabric into fabricated assembly.
Flame retardancy is not the same as flame proofing. Fabric with a flame-retardant coating can withstand a point source even if it is very hot, but a flame-retardant material still will burn if a large ignition source is present. The larger the ignition source, the more total heat energy is available to the fabric fibers behind the protective coating, The more heat energy gets in, the faster and more successfully the fabric reaches a temperature at which it catches fire and burns from the inside out. Typical tent fires, for example, begin with small ignition sources, but ultimately the flammability of the tent’s contents contributes to the fabric’s response.
Flame-retardancy tests measure the self-extinguishing feature of fabric when subjected to a flame. The industry has developed AF-1 and AF-2 classifications for architectural fabrics. Both types must have a flame spread rating of 25 or less and provide at least a Class C roof covering. In addition, AF-1 fabrics must pass tests related to resistance to external fire exposure and interior flame spread. In certain temporary or nonbuilding structures, fabrics that meet NFPA 701 (flame resistance), or NFPA 701 in conjunction with a Class C classification, may suffice. Manufacturers should provide confirming information on which of the NFPA or ASTM tests their products pass.
Most architectural fabrics have some form of topcoating applied to their exterior coating to improve cleanability. The topcoats are acrylic solutions, polyurethane-acrylic solutions, PVDF solution coats or a PVF film lamination. The topcoat provides a hard surface on the outside of the material and minimizes plasticizer migration. The barrier helps prevent dirt from sticking to the material and allows the fabric to be cleaned with water. As the material ages, the solution-coated top finishes will erode and the material will collect more dirt and be harder to clean. Thicker-solution topcoats last longer than thin coats, but coatings that are too thick will embrittle and crack when folded.
For permanent air structures and tensile structures, use of a 1-mil (25.4 microns) PVF film, particularly if long-term cleanability and appearance is an issue. The 1-mil PVF film is 10 times as thick as the solution topcoats and will eliminate plasticizer migration.
The fabric’s top finish should relate to the structure’s long-term aesthetic requirements. Structures used for warehousing and industrial applications generally don’t require high levels of cleanability. Air-supported structures for sports events, tennis courts or golf ranges require a moderate level of cleanability. Custom tensile structure for amusement parks and music pavilions generally require the highest level of cleanability.
A fabric’s most fundamental properties are related to stress versus strain (unit load versus unit elongation), expected service life, the mechanisms of joining the material together (welding, gluing, etc.) and the behavior of the material in or around a fire. With this information, you are reasonably assured of being able to design a safe project.
For stress versus strain, data should be in the form of both uniaxial and biaxial information that characterizes the fabric in terms of its stiffness, elasticity and plasticity. The information is essential to effective modeling of the material’s response under load in a load-carrying application. Shear strength, shear strain and Poisson’s ratios are more difficult to obtain, but are fundamental for analyzing fabric as a structural material.
Fabric manufacturers should be able to provide evidence of the fabric’s long-term performance in a representative environment based on testing aged samples.
Other properties come into play in evaluating a fabric’s viability in a project. Finding information about these properties may be more difficult to obtain, but worth asking about to gain a full picture of the fabric’s performance in a project. Some properties include:
- shading coefficients
- general solar, optical, thermal performance data
- acoustical data
- dimensional stability
- seam strength and stability
- construction method
- general handling ability, including abrasion resistance, foldability, etc.
Shading coefficients; solar, optical, thermal performance data
Building occupants’ thermal comfort depends on the air temperature surrounding them and the radiant temperature of the surfaces enclosing them. The qualities that make fabric structures attractive — their low mass and translucency — also can contribute to rapid temperature changes in response to external conditions. As a result, it can feel quite different at various locations in the space, depending on proximity to surfaces in contrasting thermal states caused by cloud cover, wind speed or the sun’s intensity. Ignoring this effect could result in uncomfortable and inefficiently maintained environments.
To understand a fabric membrane’s thermal behavior, look to the properties information that its manufacturer supplies. Specifically, it should offer summer and winter U-values and shading coefficients, and optical information about the fabric’s transmittance, absorptance and reflectance, ideally at all wavelengths of thermal radiation and all angles of incidence.
We generally think of fabric as absorbing sound but unfortunately, coated fabrics used for roofs and other structures are not efficient sound-absorbing materials. Although it is true that fabrics will exhibit reasonable sound-absorbing properties at lower (bass) frequencies, at middle and high frequencies the fabric’s sound absorption is low. (Some coated fabrics can be designed to provide good sound absorption, but they are not impervious so they cannot be used in external roof or structure construction.)
Thus, other materials or installations must provide sound absorption when a coated fabric is used for an arena or stadium roof, retail store, airport terminal or similar application. Coated sound-absorbing fabrics often are installed beneath the impervious roof fabrics. The distance between the exterior roof fabric and the interior sound-absorbing fabric affects the sound absorption and its relation to frequency. Avoid small spacing between the two fabrics since doing so limits sound absorption.
For large spaces, it often is not possible using coated fabrics alone to provide the required absorption for reverberation control.
To take advantage of a coated fabrics’ ability to reflect sound, the structure’s shape must play as great a role as the fabric itself. Double-curved surfaces can reflect sound in many directions. Since the fabric does not provide a 100% acoustical barrier, the shape must be carefully designed.
Their sound reflectivity makes tensile structures especially suitable for acoustic music performances, in which it’s important for sound to reflect back to the artists so they may hear themselves. Properly designed saddle-shaped surfaces both reflect and diffuse sound.
For amplified performances, the interior of the tensile structure may need fabric liners or other materials to absorb sound. Because tensile structure fabrics reflect the middle- and high-range sounds, lower frequencies may go through the membrane, making the sound too bright for amplified music. A variance may be needed for certain performances in which the sound beyond the structure exceeds municipal decibel-level requirements.
With PVC-coated and PVC-laminated polyesters, color selection will affect the colorfastness and UV resistance of the finished material. Certain bright colors and pastel shades will tend to fade with time. Highly translucent material also will not have the UV light resistance as compared to materials with high levels of titanium dioxide (white pigment) in the exterior coatings.
More so than with traditional construction, in the design of membrane structure, the material’s properties must be taken into consideration early in the design process. Only some fabrics, for instance, can accommodate a sharp edge on a four-point cover. In the case of a retractable structure, a designer must know whether the fabric can be folded, and the folding volume. Fiberglass yarns, for example, lose tensile strength when folded. Speak directly with fabricators and installers to learn the material’s maintenance needs, whether it can be walked on during installation, and whether special tools and equipment are needed for installation.