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Fabric Roofing

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Fabric tent of yore represents an archetypal form of building that is getting rediscovered in the modern architecture. The tensile fabric membrane enables large areas to be spanned, is light and easy to convert and dismantle. As a building medium, fabric offers limitless possibilities of three-dimensional forms.

With advancements in cable supported structures and mechanically induced pretension (air pressure systems), the tensile fabric roofing is increasingly being incorporated in existing and new structures across the world. The selection of tensile fabric material depends on the structure design, aesthetic appeal and location requirements for the application. The three external fabric types commonly used in tensile fabric structures are PVC (polyvinyl chloride) coated fabric, PolyesterSilicon coated fabric and Teflon coated glass P.T.F.E. (polytetrafluroethylene). The polyester and glass make up the woven element forming the sub-strait, to which PVC, Silicone or Teflon coatings are applied.

PVC coated polyester available in many colours is the least expensive with design life of almost 20 plus years, robust, easy to install and de-mountable. PVC fabric is translucent and the addition of white pigments increases its resistance to UV rays while additives like anti fungal treatment, UV reflector and absorber agents are used to enhance its PVC properties. The Silicone coated and Teflon glass coated fabric has a higher tensile strength and design life of more than 25 years. But being glass based they are more brittle and therefore cannot be used for repeated flexing. However, Silicone and Teflon are chemically inert, resistant to moisture, UV rays and microorganisms and have self-cleaning properties.

The formwork for the fabric roofing consists of open or closed systems. An open system structure looks best when used as an independent stand-alone statement. It comprises a fabric perimeter supported by an integral cable and requires larger foundations than a closed system. A closed system structure consists of rigid members around the edge to support the fabric and requires smaller foundations. Also, controlling water is much easier in a closed system as consists of framework that lends itself well to integral drainage. The supporting structure can be used as down-pipes to allow water to be led away without visible attachments. Anil Shah, Technical Director, Aakruti Tenso Structures and Facades Pvt Ltd. briefs, “The tensile structures and roofing solutions space is a rapidly evolving area of structural and architectural engineering. There are two shapes evident in tension fabric structures. Anticlastic structures like cone, arched vault and hypar 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. 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.”

Also, an important consideration in the attachment of the fabric to the support system is the flexibility of connection for displacement and rotation. The uneven loads imposed on the structure during assembly, affects its working life span. The same is considered during analysis and has a direct effect on the connection system. Connections from the fabric to the support system should always be adjustable. Teflon coated fabrics require re-tensioning once the fabric has settled over a period of a few weeks. Moreover, conventional structures have an internal rigidity in order to be stable but as fabric structures are mainly fabric and cables, they rely on their form and internal pre-stress to remain stable. Thus, fabric structures are more difficult to design and their behaviour more complicated. As a building medium, fabric requires a different approach to that of conventional roofing materials and the following design factors considered:

• The loads that will be exerted on connecting buildings and/or ground

• Foundation pads

• Any services adjacent to foundations to be re-routed if required

• Management of rainwater/ rainwater run-off

• Need for add-ons such as lighting, protection or bird netting

Fabric structures are more capable than equal spans of concrete or steel due to their very high strength weight ratio. Much greater spans can be achieved by reinforcing the fabric with webbing or cables with support structure of edge tripods, central masts or ‘push-ups’. Anil Shah adds, “Tensile fabric membrane can withstand immense amounts of load conditions, is suited to endure severe climatic conditions and finds a variety of application with structural requirements like domes, skylights and canopies. It provides higher light transmission during daytime and is an excellent alternative to polycarbonate or glass as roof glazing system. It absorbs solar energy (4%-17%), reduces the heat load and is low maintenance as compared to glass.”

The latest innovation in the fabric roofing segment is the development of fabric with three layer construction and aerogel as insulation material. It offers the same architectural beauty as PTFE fibreglass tensile fabric membrane, but with the added benefit of a feather-light insulation layer that traps air to prevent heat loss and solar heat gain. The aerogel filler has an air content of 95 percent and is sandwiched between PTFE (Teflon®) fabrics. The outer PTFE layer comes standard with PTFE coating that helps to keep the roof clean; the next layer in is a aerogel blanket, an extremely lightweight thermal insulating material; and the innermost layer is a PTFE glass fibre interior liner that provides continuous vapour and effective acoustical barriers. The novel material offers many advantages in tensile fabric applications. It helps maintain daylight harvesting by transmitting and diffusing natural light, enhanced temperature control and innovative sustainability. For instance, with a notable insulation value of R-12, the material enhances thermal efficiency by retaining cool air in warm conditions and warm air in cold and reduces HVAC energy consumption and costs. In addition, the fabric is malleable, durable and fade resistant as also water, mold and mildew repellent.

Case Study – Windsor Realty, Mumbai

The building’s existing two-level Cafeteria with a terrace used for open-air sit out required to be covered to protect against excessive sun & rains. The challenge was to transfer all the loads to existing columns – beams without disturbing existing interiors- exteriors. Aakruti Tenso Structures and Facades Pvt Ltd engaged in the designing, manufacturing and installation of tensile membrane structures collaborated with architect Cherag Bardoliwala of Talati & Panthaki Associates to offer unique textile roofing.

Anil Shah
Technical Director,
Aakruti Tenso Structures and Facades Pvt Ltd.

The total plan area of the terrace is 300sqm or 3000sqft. Fabric – Tensile membrane structure was the only option feasible due to limitations in data available for the existing architectural structure size & location. Also, the architectural roofing was to be extended further then outer periphery of the terrace by at least a meter, complicating the structural issue further.

Aakruti Tenso designers and engineers decided to provide a structural design form free of any bending moments, where in all the reaction forces would be transferred directly to the major columns located from the levels below. Members were mounted on columns through pin joints and fairly long members were used to ensure the coverage as desired. At locations where the columns were not in the vicinity, designers creatively took supports from adjacent beams with slanted members meeting at the desired anchor points again having Pin joints in the base.

This reduced on site work time as all the members were now fabricated to final form in the factory and only the base plates with flanges for pin joints were inserted at the column positions & re-claded again. The normal functioning of the Cafeteria continued right until the fabric was installed.

Design & Structure

Detailed analysis of the structure was carried out and in the absence of local codes for construction of tensile membrane structures, British standards were followed to generate wind loads depending on geometry, height of structure, height from sea level, distance from sea, surface roughness and topology. The dimensional detailing of each individual structural member was carried out depending on the maximum load generated in load combinations.

Almost all the members were fabricated form hollow round tubular ERW pipes of Diameter, 138mm X 6 mm. The base plates of 25mm. thickness with 20-25mm. flanges for pin joints were machined – fabricated. 12mm galvanized wire strands were used to reinforce fabric, as well as to support struts in angular positions.

The fabric used to span 20 meters X 15 meters was Type 2 Ferrari 902 S. The weld joint of 38mm Lap was used. The single piece fabric was lifted to the upper level. The fabric was fixed in aluminum kedar profile, which in turn was clamped on the steel structure through self threading screws. The rest of the fabric was anchored through assembly of SS316 end plates, turn buckles, M20 studs, ball washers, nuts etc. The whole installation was completed in three days.

 

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