There are 3 common geometric forms for tensile membrane structures and they fall into the following categories:

• Arch supported
• Conical
• Hyperbolic paraboloid

 It is also possible to create a 'hybrid' form by combining these 2 or 3 geometric shapes creating a simplistic and dynamic design.

In simple terms, these forms have internal membrane support structures (rigid, soft, curved or straight) and perimeter boundaries consisting of curved edge cables or straight beams, masts/columns with guy cables. In the example of a conical, the central mast provides the internal rigid support (balering) and the catenary edge cables with masts/cables to the perimeter.

When designing tensile membrane structures it is important to have a 3 dimensional shape with highs and lows. These surfaces have double curvature so as to ensure that under all loading conditions the membrane will be able to adequately resist the loads and remain in tension.

The form is generated by breaking up the surface into individual panels that will be cut out of a roll of fabric (typically 1.5 - 4m wide). These panels will have curved edges that when matched to an adjacent panel edge will produce the curvature of the surface.When joined together and stressed in a structure,these panels need to distort within the body of the panel to form the final shape. If it distorts too much it may result in "wrinkling" or unintended stresses within the membrane surface. This can have a negative impact on the structure both esthetically and structurally.Using more seams to achieve the shape can be used to alleviate this situation however this can increase fabrication time and fabric "wastage" (the amount of fabric discarded after plotting the individual cutting panels). Efficient "nesting" (alignment of the panels on the roll) of the panels can minimise the wastage, but fabrication time is difficult to minimise if there is a high density of seams within the surface.

Important Points when Carrying out a Project in Tensile Membrane

Correct Selection of the Membrane Whether it is PVC, PTFE or ETFE.

PVC coated polyester fabrics come with either acrylic or PVDF (Polyvinyl dene fluoride) top coating.The PVDF coating is the preferred coating for the harsh India environment. PVC/PVDF tends to be used more often due to cost and general performance.

PTFE (polytetrafluoroethylene) coated glass fabrics - is commonly known as teflon coated fibreglass. PTFE unique combination of inertness, thermal stability and surface self cleaning properties make PTFE ideal for projects requiring superior weather and fire resistance.

ETFE (ethyl tetra fluoro ethylene) foil - is lightweight (it weighs 1% of the equivalent-sized glass panel) and highly transparent to UV light. ETFE has better insulation properties than glass when considered as a 2 or 3 layer cushion.

Prestress of the Membrane.
Prestress is an important aspect of tensile membrane structures and must be carried out correctly to the grade and type of membrane used. Minimum prestress for type II PVC membrane is approximately 2kN/lm and for a similar Grade of PTFE membrane, approximately 3.5kN/lm.

Fall(Slope) of Membrane
During design analysis check the slope of the membrane at all locations to avoid pooling of water. If the prestress is low pooling of water will be exacerbated.Also, the greater the fall, the greater the velocity of water runoff resulting in better self cleaning characteristics of the membrane in dusty environments.

Steelwork

Steelwork should be correctly detailed. Bolted connections are preferred to ensure correct tolerances at site ensuring the membrane fits perfectly with no wrinkles to the membrane. Site fabrication and welding should be avoided.

Durable Fixing Hardware
All fixing hardware should be aluminium, hot dipped galvanized steel or stainless steel to ensure long term performance and durability of the tensile membrane structure.

Waterproofness
Correct detailing of membrane flashing will ensure a waterproof structure.

Richard MacDonald President - India operations Taiyo Membrane India (MakMax)