Source: Fabric Architecture | Sourcebook 2009
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.
Topcoatings
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.
Structural properties
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
- colorfastness
- cleanability
- 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.
Acoustical properties
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.
Colorfastness
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.
Handling considerations
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.
