Tension membrane structures

Tension membrane structures offer an aesthetic,  practical, and cost-effective way to provide roofing to either new or existing areas. These shade  supporting structures have been put into use for most  part of the twentieth century and have found wide  spread applications including, sports facilities,  convention centres, concert halls, shade roofs of large  commercial buildings, airport roofs, gas stations and terminals, to mention only a few.  The membrane part in the structure is pre-stressed and thus enables the structure to maintain its form.

 Two types of shapes are found in tension membrane  structures. The first type is the 'anticlastic structures' with two double curvatures in the  opposite direction while the second is a   'synclastic structures' with the double curvatures  in the same direction. Anticlastic structures can  take a variety of shapes and forms including,  the arched vault, hyper and cone. Technically speaking a tension membrane  structure is a combination of elements, which carry only tension and no compression or bending. This  is the reason why the use of compression rings or  beams, that form the bending or compression elements,  is used in most tensile structures.

Why Tension Membrane Structures?

The semi-translucent nature of fabric structures is  what makes them a favourite with engineers and architects looking for roofing systems to cover large  areas, such as sports stadia or terminals. Fabric  structures help in increasing the sustainability quotient of  a building in more ways than one. The fabric allows for  entry of natural light, while cutting down the  transmission of heat. The high reflectivity of the  membrane makes it an ideal alternative to glass as a  roof glazing system.  Tension membrane structures are usually reinforced  using either PVC /Polyester or PTFE based coatings. This makes the fabric structure perform well from the fire  performance perspective too. For example, a tension membrane structure with PTFE coating is rated non  combustible as per ASTM 136, making them completely safe. Additionally the inert nature of the fabric aids in  self-cleaning, a characteristic which makes them  perfectly suited for application over large areas.

The  dependency on artificial lighting is vastly reduced. The unique properties of light reflectance and  transmission also offer exciting possibilities for lighting  after dark. Directing lights under the canopy to reflect  off the underside is a great way to use uplighters, but  more even lighting can be achieved under the fabric by  shining lights down on the fabric from above.

The thermal insulation achieved with a single layer  of either PVC/Polyester or PTFE membrane with a typical weight of around 1200gm per sq metre and a  U value of approximately 4.5 W/m2K, is more or less  similar to that of glass. White is mostly the preferred  colour when it comes to tension membrane structures.  This is because with dark coloured membranes, the  absorption of heat is very high. Dark coloured  membranes can also re-radiate heat. White is  therefore the preferred choice in the case of tension  membrane fabrics.

Roofing and Cladding

With tension membrane structures, it is possible to  have both the roofing and cladding in one single structural element. Typically the seam and curve of the  fabric structure that reflects the tension is aesthetically  pleasing, while also being important for the structural  integrity. Due to the integration of the roofing and  cladding, the structures are also easy to clean and  maintain, when compared to glaze glass roofing. The  roof, in the case of tensile membrane structures is  factory welded and therefore easy to install, apart from  acting as a weatherproof skin that does not contain  expansion joints. All these factors enable rapid  construction and coverage of large areas.

Span Capabilities

An excellent span capability is another factor that  puts fabric ahead of other materials. While every other  possible roofing material requires rigid intermediate  support, it is not the case with fabric structures. The  fabric can span from one boundary to another in one  unbroken (sweep). This ensures that there are no sealing  related issues that need to be addressed. The amazing tensile capacity of fabric helps to  reduce the number of components that make up the  supporting framework to a minimum, thus enabling a  structure that is much more light-weighted when  compared to other types of construction. On the flip side  though, the structures incorporating the concept need  large foundations in order to prevent wind currents  lifting the canopy. This factor is offset by the fact that in  terms of cost foundations are cheaper to prepare, than  the visible above ground construction components that  are exposed to the vagaries of weather and therefore,  more prone to damage.