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USES AND EFFECTS: Tensile Fabric Structures have intriguing potential for practical and innovative use in structures. They have properties distinct from other building materials and systems, and require a clear understanding of the guiding principles if they are to be handled successfully. To date, the main usage has been for roof canopies, but opportunities exist for more diverse use. Tensile structures provide a range of dramatic forms that not only serve to reduce heat gain and provide shelter, but also to designate special importance for the area that it covers. Fabric mesh can be used when visibility has to be maintained through the membrane. Most often, tensile structures are used for a dramatic effect over performance and outdoor exhibition areas. Designed as permanent or demountable canopies, they act as a foil for lighting and the projection of images, and can also be used for protecting audiences from the elements. Fabric possesses a unique translucent quality, allowing diffused light to permeate through to the covered area. This can allow softened natural illumination to provide a background light source during the daytime, or enable theatrical effects to be exploited using artificial light at nighttime. Fabric mesh can be used when visibility has to be maintained through the membrane. Fabric is the most lightweight building material that can be used for architectural applications. Its tensile strength affords it incredible spanning potential, with a minimum of structure to support its weight. The fabric planes can be manufactured in huge panels, limited only by the size of the manufacturing facility where it is made. As a result of having no rigidity, the panels may be rolled up, enabling ease of transportation, and minimal on-site fabrication. Tensile structure design is a relatively new structural approach, with most development occurring in the last four decades. Their usefulness has been accelerated by the development of two key factors. Firstly, synthetic fabrics have been created which combine sufficient strength to span large distances with the durability to stand as fairly permanent structures. Secondly, there has been the rapid advancement of computers, which enable accurate 3D structural analysis and pattern cutting to be carried out in an economically viable manner. Additionally, the rising cost of building materials make the light weight tensile structure an option for covering areas that might otherwise require excessive and expensive amounts of more traditional materials. The fabrics used in modern architectural applications are of two different types, each of which have individual characteristics, PVC coated polyester and polytetra-fluorothylene (PTFE) coated glass fibre. Types: Many tensile and fabric structures can be placed in one of four categories: · “Classic” Tensile Fabric Structures: using masts and
shaped fabric forms with anchors attaching it either to the ground or to other structures Fabric structures may be divided into two categories: tension and
air-supported. Air-supported structures use a membrane, supported by air pressure, to act
as the walls and roof of the structure; whereas tension structures use compression,
tension, and fabric elements for structural stability. Some examples of this type of
structure are at Rutgers University at Newark’s gymnasium and Essex County Community
College’s tennis court covering. Tension structures use a membrane, supported by a
cable net, to act as the roof of a structure. An example would be any “dome”
sports arena. TENSION STRUCTURES: The basic types of tension structures are cable domes, mast supported, arch supported, radial tent and saddle roofs. The distinction between roof and wall is indistinct. Tension structures can economically span large distances without internal obstructions without the need for any mechanical systems. Architecturally, these structures can be interesting since many different and dynamic shapes can be produced. Tensile structures can be incredibly complex, and most often require computer analysis of the different joints, cables, and compression/tension members. In a tension structure, all of the members are either in tension or compression. Removal of even one member would cause the structure to collapse, due to the non-redundant nature of the system. Structural Principles: Tension structures consists of cables and fabric in tension. The cables carry the gravity loads while stability and resistance to wind uplift is provided by the weight of the roof deck system. The most common type, the cable dome, consists of a center tension ring surrounded by a number of hoop rings that are also in tension; these are surrounded at the perimeter by a compression ring. Shapes: Saddle Roof AIR SUPPORTED STRUCTURES Air supported structures are held up by air pressure. The inside of the structure is pressurized like a balloon. While this could seem at first to be uncomfortable to the occupants of the structure, the pressure differential is no greater than that of ordinary barometric fluctuations. Common uses of air supported structures include sports stadiums, the “bubbles” used to cover tennis courts and pools, and many other temporary shelters. Types: The two basic types of air supported structures are high profile and low profile. Profile refers to the height to the structure relative to its span. High profile structures are typically used for temporary or storage facilities and are often free standing, which means they have no foundation upon which they rest. Low profile structures are used to span long distances such as sports stadiums, also low profile structures tend to be placed upon a building rather then the ground itself, thus being used as roofs. This is due to the forces involved in supported the structure. High profile air supported structures are less common today because the cost of comparable tension fabric structures has been reduced considerably. Advantages and Disadvantages: First costs for an air-supported roof always have compared favorably with those of conventional roof structures. On a cost-per-seat basis, the advantage is even more evident. The savings come from lower construction and supporting structure costs plus overall economy of design. Architecturally, the designs are very elegant and dramatic. Unintentional deflation and the cost associated with it is the major problem with these structures. The most common cause of deflation is accumulating snow and resulting ponding. The introduction of design refinements, such as computer patterning and greater knowledge and planning on the part of operators has helped reduce the occurrences in the recent past. Structural Principle: Air pressure is used to support and force in all directions. This force is used to support the fabric. The cables do not support the fabric, but hold it down. The fabric is attached to the cables in panels resulting in a hybrid membrane. The hybrid membrane transfers the stresses from the fabric to the cables. The cables are attached to a compression ring, which resists the uplifting forces. Configurations: The most basic shape is a low profile oval with a diagonal cable pattern and a funicular compression ring. Funicular implies that there are no bending moments in the compression ring. A rectangular shape with modified corners and two way cable systems will keep a compression ring funicular. One way cable systems in a modified rectangular structure produce moments in the compression ring. High profile air supported structures may use one or two way cable systems or just fabric alone. (From Tensile Structures, edited by Frei Otto) RESOURCES: This and more information can be found at these sites: http://www.landrell.com/landrell.htmhttp://www.benevento.peoples.it/~baku/main.htmlhttp://koenig.njit.edu/civil/gen.html |