The tensile structure business has grown considerably in the last 10 years and is predicted to grow exponentially in the coming years. Such structures are becoming bigger and more sophisticated, as contractors, engineers and architects develop more confidence in their designs and reinforce them with their execution. Although the field may have evolved and more clients are interested in using them, they are still considered to be special – a new technology. Tensile surface structures do not figure widely in the design vocabulary of architects, engineers,urban planners, building owners and national authorities, and till that happens, their application will continue to be constrained.

Therefore, there is a need for people to be better informed about the general behaviour and the advantages and disadvantages of using tensile surface structures in relation to more conventional buildings. For instance, the internal environment is seen as a key issue - how do such enclosures behave and to what functions are they suited? How do these structures differ from ‘normal’ buildings in sense of maintenance, engineering and most importantly cost?

Achieving increasingly more with less and less this requirement of our time is met in an exemplary manner by textile structures. They draw on the origins – the ancient form of the tent is ever present and yet the best achievements appear to come to us from the future. No other type of architecture benefits from its primary conditions in as direct a manner in none other does the shape follow the function as rigidly none other achieves as much span with as little material and energy expenditure while conserving the resources in the same manner. Short building times, cost effectiveness, urban development qualities and long life make textile structures appear to be the chance for a new type of architecture for mankind and its environments of the future.

History & Background:

The history of tensile structures – and tent-type housing is as old as mankind itself. Leaves, barks leather and felt all came much before fabrics. Woven architectural fabrics have been around longer than most other construction materials. The Romans used woven leather tents in their campaigns across Europe and nomadic cultures have used fabrics as cover for thousands of years, be it the North American Teepee, the black goat hair tent of the Bedouins, or the Mongolian yurt. With the advent of industrialization,fabrics did not translate into an acceptable building material because of ephemeral qualities.

Fabrics and foils as building materials were revisited in the nineteen sixties due to a cultural interest in inflatables, instant cities and alternative lifestyles. The early sixties being a time for general revival in architecture, experimentation reigned supreme on the international scene. Be it in the pioneering lightweight structures of Frei Otto in Germany, or the inspirational drawings of Archigram in the UK, to the influences of Buckminster Fuller in geodesic domes, or the inflatable structures of Water Bird in the States, and thus, 20th century fabric structures were born.

The advantages and the characteristics of the structures remained unchanged- ease of erection, simplicity of means, low mass, high mobility, aesthetics of shape. But all these traditional tent structures relied on materials that lasted only a few years.Cotton, polyester, nylon all of them when un-coated, burn themselves or are susceptible to flames. These fabrics deteriorate substantially in the exposure to pollution and UV rays. They lose their form because of their extremely plastic behaviour, of deflecting under sustained loads (creep action). Thus these structures still continued to be viewed in the realm of temporary structures, and enlarged versions of vernacular architecture.

Fabric architecture moved towards PVC coated polyester fabric, which is the ancestor of today’s polyester fabrics, and today the most widely used membrane structure material. The influence of space exploration in the 70s also saw the introduction of Teflon coated glass fibers, which can withstand tremendous temperature differences and harsh outtry,door climates. The material adapted to be used in baking oven conveyor belts and during the past twenty years has become a permanent structural membrane for many larger architectural projects. From the yachting industry, Kevlar was introduced which has a greater strength-to-weight ratio than steel. From the airplane industry, Spectra fibers have been introduced into architectural parlance. Vectran, a high performance thermoplastic yarn spun from liquid crystal polymers is slowly beginning to be used in architectural applications.

In addition, many materials that we are familiar with such as stainless steel can now be woven into textiles that embody tremendous structural and have an immense architectural potential. Although these materials have been circulating in the international market for well over 30 years, in India, predominantly structures are made in PVC coated polyester fabrics which allow a greater degree of flexibility be it in the case of cost, or handling or design.

Membrane Materials

Most tensile structures utilize fabrics rather than meshes or films. The fabrics are typically coated and or laminated with synthetic materials for greater strength and environmental resistance. The most widely used materials are woven polyester cloths coated with polyvinyl chloride (PVC) and woven fiberglass coated with either polytetrafluoroethylene (PTFE) or silicone.

PVC Coated Polyester Materials

This material has been widely used for fabric structures for over twenty years, and is usually the most frequently specified material. The material is easily handled and welded using fabric structures high frequency welders. International convention has defined four grades of fabric based on mechanical properties, weights and strengths from grade I to grade IV. There are several types of PVC fabrics classified according to surface coatings see below. The life span of a PVC coated polyester architectural fabric should exceed fifteen years.

Most architectural PVC polyesters have some sort of top-coating applied to their exterior or weathering surface. The top-coating improves the appearance of the material, extends its life and allows the material to be readily cleaned or washed by rainwater (self cleaning). The topcoats are applied in different ways depending on the nature of the topcoat and the required thickness. Lacquers are sprayed on whereas thicker coatings are “knife applied” or laminated to the PVC. The thickness of top coatings has a direct relationship with the longevity of the PVC membrane. Topcoatings will degrade over time leaving the PVC surface exposed to airborne pollutants, UV degradation, wind and weather. The presence of a topcoat also tends to inhibit the migration of the plasticizers which give PVC its elastic and flexible properties. Migration and degradation of these plasticizers cause the PVC to become brittle, to blister and delaminate. Different types of top-coating s include acrylic solutions, PVDF solutions and PVF film laminations.

Acrylic Topcoat

This commonly used finish is also the most economical and most widely available. It is a thin,sprayapplied solution which gives a transparent glossy finish to the PVC. The acrylic coatings have a good resistance to UV degradation. The thinness of the coating application means that this material is easy to fabricate and repair by high frequency or hot air welding. Acrylic topcoats are ideal for fabrics that are used for temporary structures and demountable structures such as marquees, circus tents, track side curtains,rock concert venues and warehouses.

100% PVDF Topcoat

Polyvinylidene Fluoride (PVDF) is made up of fluoride, carbon and hydrogen. The compatibility of the carbon and fluoride is such that it offers a resistance to UV degradation and atmospheric chemical attack, which is far superior to the acrylic topcoat.Controlled exposure tests show that colour differences and reduction in brilliance are significantly less with PVDF than with its acrylic counterpart over time.

PVDF topcoats also offer resistance to algae and fungal attack. They have good self-cleaning properties and therefore need less maintenance during their lives.These properties combine to give a membrane a life span of 15 to 20 or more years depending on site quality of the membrane itself. Like acrylic topcoats 100% PVDF topcoats are highly flexible and resistant to cracking, making them easy to handle during installation. The production procedures where the PVDF is chemically grafted to the PVC, as well as the polymers used, limits the choice of colours available. White is the only standard available colour.

The chemical-resistant properties of the 100% PVDF are such that the finished topcoated material cannot be welded to itself in its raw state. To effect a weld on 100% PVDF material the top-coating must either be abraded off or the material must be butt welded. This extra operation increases the price of the fabrication and carries risks associated with the grinding depth calibration of the abrasion machine, and the complete covering of the abraded seam strip by the overlap. Inaccuracies in both these areas can significantly weaken the welded seam. Site repairs are also difficult to administer accurately as they usually require manual abrading of the membrane using sandpaper. This coating is marketed under the trade names Fluotop and Kynar.

PVDF/PVC Top-Coating

This top-coating is effectively a dilution of the PVDF topcoat. This gives the advantages of being both more economical to produce and to fabricate.The saving in fabrication costs is derived from the finished fabric being weldable without need for abrasion. The diluted effect of the PVDF however means that environmental resistance is reduced along with longevity.

PTFE Coated Glass Fabric

PTFE glass fabric is a frequently specified material due to its longevity. It has a life expectancy of approximately 30 years depending on conditions. Its base fabric is made up of glass fibres which are drawn into continuous filaments then bundled together in yarns. These yarns are then woven to form a substrate. The woven fibreglass has a high tensile strength, behaves elastically and does not undergo significant stress relaxation or creep. The glass fibre is also completely incombustible.

The flexing behaviour of the glass is however inherently poor. This leads to cracking, poor handling ability and self-abrasion within the coating. The PTFE or Teflon coating is also non-combustible. These coatings, being very inert, have a low coefficient of adhesion. This quality means that the coating itself has good self-cleaning ability. In the finished fabric however the self-cleaning ability is slightly diminished by the grainy surface of the membrane under tension, providing small indentations in which airborne solids can accumulate.

Most PTFE membrane is an off-white/brown colour when it leaves the mill or fabrication plant. This discoloration bleaches to white in the presence of UV light. For this reason PTFE fabric should not be used indoors or in permanent shade without being pre-bleached. Prebleaching can be undertaken but it is expensive because it involves the material being cooked in an industrial oven for long periods at temperatures exceeding 250 degrees C.

The glass scrim combined with the bleached coating results in a fabric with good light transmittance. Fabrication of PTFE membrane requires specialized welding techniques under controlled environmental conditions. It also requires extra care in handling and packaging due to susceptibility to cracking and self abrasion. These properties contribute to its high cost and to the need for additional tensioning hardware for the finished fabric structure. The tensioning of PTFE glass fabric is a slow process,as it requires incremental adjustment over long periods on site. This factor also contributes to its cost. PTFE glass cloth is often specified for high profile projects, for example, stadium roofs where longevity is required and where budget is not an issue. It is also suitable for desert and marine conditions.

Silicone Coated Glass Cloth

This material has a similar base cloth to PTFE glass membrane. However, the silicone coating gives the finished fabric a more flexible rubbery form. It therefore does not suffer from stress cracking in the same way. The silicone coating is more economical and 100% fire proof. This finished fabric is available in a range of colours, widths and thicknesses.The fabric is seamed by sewing or via an adhesive bond. Its high translucency and fire rating make it suitable for indoor tensioned ceilings and atrium shades. Only the most advanced silicone coated fabrics have any self-cleaning properties, so it is not often specified for outdoor use.

Cottons

These materials are environmentally friendly and are easily dyed to any colour. They can be treated to make them non-combustible and therefore suitable for indoor ceilings and exhibition structures. These materials quickly discolour in outdoor conditions, as they are vulnerable to biological attack. Unlike most other fabrics however, they can be laundered.

Coated Nylons

Usually used for yacht sails and hot-air balloons,nylon has a high strength/low weight ratio making it suitable for very lightweight structures, drapes and exhibition work. It can be packed and folded without creasing and is available in a wide range of colours and coatings. Nylon can be fire treated, UV treated and waterproofed.

Behaviour of Fabric Structures

Fabric structures give natural diffuse light but with reduced heat load. The high reflectivity of the white membrane fabric is very efficient, because of the amount of heat it reflects. When compared to polycarbonate or glass as a roof glazing system which are extremely translucent and emit a lot of heat, and increase the internal cooling costs, or have to be shaded by internal shading devices like blinds etc. Fabrics can prove to be a very good alternative especially when you are dealing with tropical climates, where heat radiation and transmission is the ruling criterion for roofing structures. They can be built to emit a lot of light (up to 13%) or no light (0%). All this is without any substantial change in the overall budget of the fabric roof.

Light penetrates into tensile fabric membrane with natural light and at night the artificial light provides an ambience of. In day time the light transmission is typically 5% - 20%, it is just sufficient to eliminate or greatly reduce the need for artificial lighting in day time. With little or no artificial lighting a 1 of 4 heat load is reduced. Absorption of solar energy in the fabric structures is typically 4% to 17%. Thus, these roofs heat / cool down very quickly due to their low masses. They also absorb very less amount of energy (as compared to conventional solid structures, which trap heat due to their mass).

With coloured fabrics typically PVC architecture fabric, the heat absorption is less, but particularly dark colored fabric membranes, the absorption of heat is very high and the reradiated effect can be strong and therefore unpleasant. For this reason white should be chosen for enclosed or shade structures in tensile fabric structures application.

This also dictates the form of the structure. When the form of the tensile is not shallow, and has a mass of volume of air that can absorb the radiation and which offers constant ventilation through a higher point, the structure stays cooler. As a result, climatologically, tensile structures offer brilliant and remarkably efficient solutions.

Structural Efficiency:

Fabric structures offer both the roof and the cladding in one structural element. In this the system, the form and the elements are a complete package and act as a whole. Thus, deflections and the finish and overall appearance of the structure becomes very important, as there is not much “surface finish” that one can do once the structure is executed. A greater understanding of the form and other aspects could result in stunning results, sadly to say, an equal proportion of ignorance may result in equally disastrous results. For increasing the efficiency of these structures, be it in the respect of costing or the material consumption, one has to try to design such structures with large spans with minimum number of corners and edges which min membrane plates corners, as the more the number of terminations, more the number of end plates and accessories – which are by far the most expensive components in the whole structural system and have to be engineered and CNC cut to form. Secondly, by increasing the symmetry or modular aspect of the structure, the structure can be organised more easily, the details mass produced, and the exclusivity of the structure curtailed. However, this may result in seriously limiting the creative capability of the designer. Thirdly, by changing the edge detailing, structural support and finishes one can help in substantially changing the budget of the structure. Simply changing the end plates from MS with galvanised finish to Stainless steel 316, one could simply triple the budgets allotted for the end plates. Same applies for the cables. There are different ways and criterions along which the clients and the designers need to probe to decide on where and how to allow for particular types of end terminations and finishes. Finally, Complex design and heavy engineering work on very difficult design, can throw the entire budget in a controversy so large – that it may end up in the cancelling of the project all together. As the American saying goes, “if it isn’t broke, don’t fix it” more often than not, tensile structure designers are faced with a very apologetic approach to their designs. Continually being compared to conventional structures and their possibilities and strengths, and pushed to the limit for the structures to do what they are intrinsically not designed to do, the project eventually results in a very expensive and sometimes an abuse of material, labour and sensitivity. The simpler, truer to form the structure is developed, the lighter, better,more efficient and cheaper the solution shall evolve.

Practical Advantages of Tension Fabric Structures:

•  Seam and Curve in the tensile fabric structures reflect tension forces hence create eye catching character.Also they have the ability to create a sense of order and establish a datum in the space which can be a very powerful order in the overall structure.
• Structure, lighting, fire sprinkle and other exposed elements of services in the space complement each other and if designed in a proper fashion may result in increasing the beauty of the overall structure.

• Tensile structures offer a very low maintenance when compared to glass. This is especially when compared to PVC fabrics, which can be easily patched in the case of fire, vandalism or accidental damage. Glass on the other hand would have to be replaced resulting in high cost, and difficulty especially after the site is finished.
• The fabric is factory manufactured off the construction site. This minimizes site interruptions, delays and interferences between the roofing and the site labour, as both works can operate independently to each other. Also, as a result of this,stringent quality control and pre-fabrication norms can be imposed on to the material. Also, it results in smaller site overheads.
• The membrane is factory welded into single weatherproof skin eliminating expansion joints, or the effects of thermal contraction and elongation, which can be easier accommodated by compensation and re-tensioning.
• By using a fabric, a larger span with more coverage can be achieved with a greater efficiency of material. Also, the fabric can be erected at a single go, thereby reducing the presence on site of different agencies, and the speed of construction of the whole roof, would far outstrip the time taken for other pre-fabricated roofs.

M. Eng. Bhavini Mistry M. Archineer M.St.Partner FREITAGMANN Visiting Faculty at the CEPT University
M.Eng. Shehzad Irani M. Archineer M.St.Partner FREITAGMANN CEPT University in the Architecture and ID Departments