A tensile structure is a construction of elements, carrying only tension and no compression or bending. Tensile structures are the most common type of thin-shell structures. Being economically feasible, a tensile membrane structure is often used as a roof, because they can stretch over large distances looking attractive. Most tensile structures are supported by some form of compression or bending elements, such as masts (as in the Millennium Dome), compression rings or beams. Tensile structures have been used since long in tents allowing it to withstand loads. Steady technological progress has increased the popularity of fabric-roofed structures. The low weight of the materials makes construction easier and cheaper than standard designs, especially, when vast open spaces are to be covered. Its lightweight nature, translucent and reflective properties, and environmental adaptability could also be taken advantage of, in building construction.
History of Tension Fabric Structures
Russian engineer Vladimir Shukhov was one of the firsts, to develop practical calculations of stresses and deformations of tensile structures, shells and membranes. Covering the area of 27,000 sq m, Shukhov designed eight tensile structures and thin-shell structures’ exhibition pavilions for the Nizhny Novgorod Fair, in 1896. In the 1950s, architects and engineers began to take a renewed interest in using tension, as the primary method of transferring loads in structures. Two main figures, namely, Frei Otto and Horst Berger of Germany were responsible for the advancement in this investigation of tensile structures. The concept was later championed by German architect-engineer Frei Otto, who first used the idea in the construction of West German Pavilion at Expo 67, in Montreal. Later, Otto used the idea for the roof of the Olympic Stadium of the 1972 Summer Olympics, in Munich . Some of the current well-known structures, utilizing tension fabric include the largest cable-supported roof in the world are the London Millennium Dome and the Haj Terminal in Jeddah, designed in parts by Berger. Currently, it is one of the largest tensile structures in the world .
Concept Design and Criteria for Shape Finding
The concept design is the most important stage of the design process. A bad concept will reverberate throughout the design, manufacture and installation process to impair the appearance and performance of the final product. A number of factors need to be taken into consideration:
a) Geometric constraints of the site and adjacent buildings
b) Sun-shading levels required and Sun angles
c) Air-flow and ventilation of the space
d) Light transmission requirements for the space below
e) Availability and positions of anchorage points
f) Need for continuous-sealed perimeter anchorages
g) Aesthetic considerations and compatibility with adjacent elements
h) Achievement of adequate curvature to minimize fabric stresses and movements
i) Drainage of rainwater and avoid ponding
j) Suitable fabric slopes, to ensure adequate self-cleansing
k) Nature of supporting structure and tensile elements
Different Types of Tensile Structures
A two-dimensional tension fabric membrane can take planar tensile forces, but it cannot take significant forces perpendicular to this plane. Therefore, in addition to being pre-stressed, tension fabric must take a certain three-dimensional shape, in order to remain stable. These shapes were discovered by Otto and Berger during their investigation of natural forms, such as soap bubbles. There are two types of general shapes: Anticlastic and Synclastic Shapes.
1) Anticlastic Shapes are created by having the radii of the principal curvatures on opposite sides of the tension fabric surface. As a result, when loaded at a particular point, tension will increase on one curve of the membrane and leave the opposite curve. Thereby, preserving equilibrium and keeping the structure stable. In order to keep anticlastic shapes, some kind of structural frame or support is necessary in the form of cables or steel beams. Some examples of anticlastic shapes are saddle, cone and wave forms.
2) Synclastic Shapes are characterized by having the radii of the principal curvatures on the same side of the fabric (Fig. 2). In order to counteract external forces, pressure from the within is necessary. This is why synclastic shapes are associated with air-inflated structures. The difference of pressure created by air pumped into the building is able to counteract the external forces, in the form of wind or snow .
Methods used for Concept Design
a) Wireframe Computer Models – The simple computer models are the starting point, for investigating various fabric forms and developing the concept design into a practical form, to ensure the design criteria are satisfied.
b) Fully-rendered Computer Models – Models of the final design of the fabric structure can be added to the model of the whole building. They are considered very useful for the final presentations to clients.
Anchors, Foundations and Supporting Structures
The provision of adequate anchorages and supporting structures must be addressed at the earliest stages of the design process. Failure, to address these issues early on, could result in the tensile structures being impossible to include, once the design and construction has progressed beyond a certain stage. Even small canopy structures impose significant forces on the sub-structures. Large structures will require special design features, to be able to resist the tensile forces. Tensile fabric structures impose forces, which have significant horizontal and vertically upward components. These are not the types of forces, which buildings are normally designed for. If special provision has not been made for these forces, it is crucial that the tensile structure be designed, taking these into account.
Fabric Options – PVC and PTFE
There are three main types of tension fabric used in architectural applications today: PVC-coated Polyester, Silicon-coated Fiberglass, and Teflon-coated Fiberglass. PVC-coated polyester is not only the cheapest and easiest to manage of the three, but also it has the shortest life span. Teflon and silicon-coated fiberglass are more durable and are more expensive . However, the basic structure of the material is similar, for all the three. Usually made out of polyester or fiberglass, the bottom layer of the tension fabric is a base fabric. This fabric is created with fibers weaved in and out of each other, which run perpendicular or in the warp and welt directions. The base fabric is extremely important, because it dictates a number of the final fabric properties, including stress and strain properties . They are relatively translucent and reflective, in addition to the incredible strength of the base fabrics. For example, Teflon-coated fiberglass has a reflectivity of 70% .
Factors Influencing Fabric Performance
Observation of fabric structures in different situations in various parts of the world, show a particular environment in which the fabric is situated. It has a very significant effect on its performance. The main factors influencing the fabric performance are as follows:
a) Geographic latitude and the temperature of the fabric
b) UV radiation level reaching the fabric
c) Humidity level
d) Pollution level, as well as, the type of pollutants
e) Dust level
f) Frequency and nature of cleaning operations
g) Deposition of vegetable matters (leaves, etc.) onto the fabric
h) Staining resulting from rainwater run-off, over other building materials
i) Exposure to direct rainfall, to assist in dirt and dust removal
PVC / Polyester Fabric: Properties and Characteristics
PVC / Polyester fabric consists of a woven polyester base cloth, which is coated with PVC and another top coating. They can be classified into 2 basic categories, depending on the type of protective top-coating:
a) Acrylic Lacquer
b) PVDF / Acrylic Lacquer Alloy Coatings in varying proportions
These top coatings have a large influence on the performance and appearance of the fabric. They, not only provide the fabric with some of its UV resistance, but also, vastly improve its self-cleaning characteristics. In general, fabrics with acrylic coatings have not performed well over long term, in tropical countries. Most of the examples appear to attract and retain significant amounts of dirt and dust after relatively short times, in service. On sites with high UV levels, the acrylic coatings break down fairly quick and the deterioration in appearance can occur within a few years, after installation. The fabrics with PVDF / acrylic alloy coatings are the most commonly used. They have been in service for about 25 years. The PVDF / acrylic coating is heat-fused onto the base fabric as part of the manufacturing process. The top surface of the fabric has a smooth slippery feel, so it is very effective in repelling dirt and resisting mould growth. With the anticipated life being directly proportional to the amount of PVDF in the top coating, useful lives of 15-25 years are achieved with these fabrics.
PTFE / Fibreglass Fabric: Properties and Characteristics
The first outdoor PTFE / fibreglass structure was erected in California, almost 40 years ago. It is still performing well in service. Therefore, it is likely that structures, which are made with today’s PTFE / fibreglass fabrics, will achieve useful lives in excess of 50 years. PTFE / fibreglass fabric is very effective in repelling dirt. Structures, which have been inspected after years in service, have been found with perfectly clean surfaces. Even though, they have never had cleaning maintenance. Three of the new stadiums, built for the Soccer World Cup in South Africa in 2010 had PTFE / fibreglass roofs. The main disadvantage of PTFE fabric is its high cost – the overall cost of structures, using this fabric is almost twice of the PVC structures.
Light and Heat Transmission
One of the main advantages of fabric is its translucent properties – on an average, architectural fabrics transmit about 13% of the light falling on the top surface. This results in a very pleasant light and airy feel to the space below. It can also result in significant cost savings on lighting. Fabric is also very effective in reducing the transmission of radiant heat from the Sun. It is a material, which has been significantly underutilised in the climatic conditions, prevailing in South Africa. However, global warming is likely to result in increased usage of tensile fabric structures in future.
Lighting of Fabric
Lighting is very effective in emphasising the aesthetic appearance of fabric structures and should always be included, whenever possible. Both back-lighting and front-lighting can be used, depending on the effect that is desired.
Fabric and Fire
PVC has fire retardant properties and achieves a class 2 fire rating. It means, the fabric is self-extinguishing and does not produce drips of molten fabric. The fire rating is accepted by most approval authorities for use, generally as a roof enclosure. PVC has an added advantage in a fire situation. The fabric seams will separate at about 100ºC, thereby, allowing a very early venting of toxic fumes and smoke. This is a major advantage in saving lives of people who, may get trapped in the building.
Fabric is easy to clean. It can be done using soft brushes, light duty, non-acidic detergents and copious rinsing water. Personnel can access the fabric by means of ropes and use soft-soled shoes, to walk on the fabric .
Case Study 1: Denver International Airport, USA
Denver International Airport was completed in 1994. It is the World’s third largest airport. The Teflon-coated fibreglass roof of the airport is designed to resemble the peeks of the Rocky Mountains in winter, capped with snow. The tensile structure has stood the test of time. The structure has not failed under the extreme weather conditions, it experiences .
Case Study 2: Haj Terminal, Jeddah International Airport, Saudi Arabia
In 1977, SOM – Skidmore Owens & Merrill Architects and Engineers was commissioned by the Saudi Arabia Government to design a terminal, to serve the pilgrims to Mecca during Haj at the International Airport, in Jeddah. 80,000 people a day had to be accommodated in transition between buses and airplanes. To protect people against the heat of the desert Sun, 440,000 m² of space had to be covered. An enclosed building was too expensive. During the conceptual design, concrete and metal roof schemes were abandoned because they absorb too much heat. Fabric structure alternatives were studied, in consultation with Horst Berger of Geiger Berger Associates, whose fabric roof for the Bicentennial in Philadelphia had proven effective in improving comfort, on hot days. PTFE-coated glass fiber fabric reflects 70% of the Sun’s heat, radiating out during night eliminating electric light because of its translucency. SOM chose a fabric structure concept, which divides the building into 10 modules. Each module covers 320 x 137m consisting of 21 tent units, with a plan dimension of 45.75 m x 45.75 m. Two groups, of five modules each, were arranged along the two sides of a central access road. 20 gates for the planes are located at the opposite ends of the modules.
To keep the space below open, the structural concept by SOM’s Fazlur Khan suspends the tent units from high masts, which were located at the four corners of each tent. Interior supports consist of single columns. Along the edges and the corners, two or four columns were combined into frames, to resist the lateral forces. This arrangement of the support system gives the architecture of the world’s largest roof, its powerful image .
Tension fabric is still a relatively new building material, despite its more extensive use over the past decade or so, in architectural applications. Discovering necessary forms and amount of pre-stressing can be extremely complex. Sometimes vaguely understood, much care is taken in designing structures involving this material. However, the advantages of tension fabric cannot be disputed. Especially over large areas, it is an incredibly lightweight, material saving and energy conserving solution for roofing systems. In short, it is an excellent option for architects and engineers for designing sustainable structures.
Dr. Mohammad Arif Kamal
Associate Professor, Architecture Section
Aligarh Muslim University, Aligarh
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