Tension membrane structures offer an aesthetic, practical, and cost-effective way to provideroofing to either new or existing areas. These shade supporting structures have been put into usefor most part of the twentieth centuryand have found wide spread applicationsincluding,
sports facilities, convention centers, concert halls, shade roofs of large commercial buildings, airport roofs, gas stations and terminals, tomention only a few. The membrane part inthe structure is pre-stressed and thus enables the structure to maintain itsform.
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 doublecurvatures in the same direction.Anticlastic structures can take avariety of shapes and forms including, the arched vault, hyper and cone. Technically speaking a tensionmembrane structure is a combination ofelements, which carry only tension and no compression or bending. This is the reason why the use of compressionrings or beams, that form the bending or compression elements, is used in mosttensile structures.
Why Tension Membrane Structures?
The semi-translucent nature of fabric structures is what makes them a favourite with engineersand architects looking for roofing systems to cover large areas, such as sports stadia or terminals.Fabric structures help in increasing thesustainability quotient of a building inmore ways than one. The fabric allows for entry of natural light, while cutting down the transmission of heat. The high reflectivityof the membrane makes it an idealalternative to glass as a roof glazingsystem. Tension membrane structures areusually reinforced using either PVC/Polyester or PTFE based coatings. This makes the fabric structure perform wellfrom the fire performance perspectivetoo. For example, a tension membrane structure with PTFE coating is ratednon combustible as per ASTM 136, makingthem completely safe. Additionally the inert nature of the fabric aids in self-cleaning, a characteristic which makesthem perfectly suited for applicationover large areas.
The dependency onartificial lighting is vastly reduced. The unique properties of lightreflectance and transmission also offerexciting possibilities for lighting after dark. Directing lights under the canopy to reflect off the underside is a great way to useuplighters, but more even lighting canbe achieved under the fabric by shininglights down on the fabric from above.
The thermal insulation achieved with a single layer of either PVC/Polyester or PTFE membrane witha typical weight of around 1200gm per sq metre and a U value of approximately 4.5 W/m2K, is moreor less similar to that of glass. Whiteis mostly the preferred colour when itcomes to tension membrane structures. This is because with dark coloured membranes, the absorption of heat is very high. Darkcoloured membranes can also re-radiateheat. White is therefore the preferredchoice in the case of tension membranefabrics.
Roofing and Cladding
With tension membrane structures, it is possible to have both the roofing and cladding in onesingle structural element. Typically the seam and curve of the fabric structure that reflects the tension isaesthetically pleasing, while also beingimportant for the structural integrity.Due to the integration of the roofing and cladding, the structures are also easy toclean and maintain, when compared toglaze glass roofing. The roof, in thecase of tensile membrane structures is factorywelded and therefore easy to install, apart from acting as a weatherproof skin that does notcontain expansion joints. All thesefactors enable rapid construction andcoverage of large areas.
An excellent span capability is another factor that puts fabric ahead of other materials. Whileevery other possible roofing materialrequires rigid intermediate support, itis not the case with fabric structures. The fabric can span from one boundary to anotherin one unbroken (sweep). This ensuresthat there are no sealing related issuesthat need to be addressed. The amazing tensile capacity of fabric helps to reduce the number of components that make upthe supporting framework to a minimum,thus enabling a structure that is muchmore light-weighted when compared toother types of construction. On the flip side though, the structures incorporating theconcept need large foundations in orderto prevent wind currents lifting thecanopy. This factor is offset by the fact that in terms of cost foundations are cheaper toprepare, than the visible above groundconstruction components that are exposedto the vagaries of weather and therefore, more prone to damage.