Tensile Fabric Structures

A tension fabric structure is a membrane (fabric) maintained under pre-stress (tension) state by structural elements and supporting systems.

Advantages of Tensile Fabric Structures

• » Can disperse natural light with condensed heat load (Excellent alternative to polycarbonate or glass as roof glazing system).
• » Higher light transmission during daytime (Sufficient to reduce artificial lighting requirement by 5% - 20%).
• » Absorbs solar energy (4%-17%) and reduces the heat load.
• » Low maintenance as compared to glass.
• » Minimal site interruptions since the membranes structures are manufactured in the factories.
• » No extension joints since the membranes structures are welded into a single weatherproof skin.
• » Designed for rapid construction.
• » Larger span for more coverage.

There are two shapes evident in tension fabric structures.

Anticlastic Structures

Anticlastic structures are pure tensile fabric surfaces having a curvature at a given point and in a particular direction, which is of the opposite sign to the curvature at that point in a perpendicular direction.

These are available in many free tension fabric forms and shapes such as

Cones : Single cone, multiples cones, fixed edges, centenary edges, cables edges, height variations, inverted cones etc.

Arched Vault : Parallel arches and crossed arches.

Hypar : Two opposing high points and two opposing low points.

Synclastic Structures

Synclastic structures are air-supported surfaces having a curvature at a given point and in a particular direction that is of the same sign as the curvature at that point in the perpendicular direction.

For such air-supported structures, the fabric envelope is supported by pressurized air but most of the fabrics derive their strength from their double curved shape.

Two types of Tensile Fabrics are used in Tension Fabric Structures

Polyvinyl Chloride (PVC)

On the basis of mechanical properties, weight and strength the International Convection has differentiated four distinct grades of fabrics. After the shape analysis, a project engineer decides the grade of fabric to used.

• » PVC has been widely used for fabric structures for over twenty years.
• » The fabric is translucent and the addition of white pigments increases its resistance to UV rays.
• » Additives like anti fungal treatment, UV reflector and absorber agents are used to enhance PVC properties.
• » For improving its durability, an extra coat of pigmented coating can be added.
• » Flexibility can be improved by adding plasticizers (these might eventually drift to the surface attracting dirt and turning the PVC brittle, discolored and hard).
• » PVC is welded by using high frequency welders.

The coating on the PVC is of utmost importance. A smooth and shiny coating facilitates to keep the fabric clean, it therefore essential to select the most appropriate fabric type during fabrication and installation.

PVC Fabrics are classified into Several types Depending upon their Surface Coating

Acrylic lacquer coating is suited for marquee type colored structures that are cleaned regularly, reducing the chances of the structure getting dirty or damaged. This type of coating does not provide very high resistance to UV rays. An acrylic lacquer coating has 3-5 years warranty.

Polyvinyl DeneFlouride (PVDF) coating is widely used due to its durable nature. It sheds dirt easily and has a higher resistance to UV rays. One of its major drawback is that is not weldable in its pure form. To overcome this, many manufacturers dilute the PVDF with PVC to create an excellent quality, durable finish that needs no special preparation for welding. A PVDF coating has 7-12 years warranty depending upon the structure type and fabric.

Most of the fabric structures are faced with the wicking problem, i.e. water seeping the length of the thread lines of the fabric. This leads to the discoloration of the fabric and probable de lamination. To prevent this anti-wick polyester fiber is used.

Poly Tetra Fluoro Ethylene Coated Fibreglass (PTFE)

Another type of Tensile Fabric Structure is Poly Tetra Fluoro Ethylene Coated Fibreglass.

• » PTFE is chemically inert but bleaches white when exposed to sunlight
• » It is unaffected by UV rays
• » Has self cleaning properties making it virtually maintenance free
• » Is fire resistant with a life span of over 25 years
• » Limited colors, but specific shades can be custom made
• » PTFE properties allow it to be fused and defused by the same instrument. This is very useful as damaged panels can be repaired on the site itself
• » It has the second lowest co-efficient of friction amongst all the materials making it difficult to handle during manufacturing and installation phases

Attribute	PVC	PTFE
Durability	10-15 years	Over 25 years
Chemical reactions	The fabric weakens on exposure to UV rays	Inert to almost all chemicals and solvents, but bleaches white when exposed to sunlight
Lighting requirements	The fabric is clear thus decreasing artificial lighting	As much as 25% translucency providing interior diffused light
Fire rating*	Moderate	Higher
Cost**	Moderate	At least five times more than PVC

*How fire resistant a material is depends on the base fabric and the kind of coating on it. All the membrane materials will melt under high temperatures but the degree will vary.

**The cost of the fabric is a very small component of the overall project cost. The choice of fabric might not significantly alter the overall project cost.