Tensile Structure - Tensile Fabric Structure and Outdoor Tensile Structure

What does "tensile fabric" mean?
"Tensile roofing" is a term is used simply to describe fabric that is under tension, i.e. being pulled in opposing directions. It can take many forms, though is generally based around the three shapes; the cone , Double Curve Barrel Vault and the hyperbolic shape.. This means that any fabric can be a piece of tensile fabric - it is not restricted to a particular fabric type.

Can any shape be created?
In broad terms the answer to this is yes, although there are a few rules and a number of exceptions.

The rules pivot around the fact that all canopies are created from 3 inherent shapes, the Barrel vault , the Cone and the Hypar . Every canopy uses the curves and principles of these three forms and the three may be combined on a single canopy.

Some forms, such as the 'inflated look' structure can only be created by inflating, however it is possible, with the use of double curvature (Hypar being the easiest example of this) and spars to create the appearance of inflation. This can be done by pushing the fabric into shape, in the same way that a wind surfing sail is pushed into shape by its battens. This is shown on the installation for Amadeus at Heathrow.

Is tensile fabric stretchy?
Contrary to popular belief the fabric used for tensile fabric structures is generally not stretchy. In fact a non stretchy fabric is preferred, as it offers the best predictability for the design and installation of the structure.

Stretchy fabrics such as 'Lycra' and other 'Lycra' based fabrics can be used for certain applications but due to their low fire retardency, can generally not be used for interior or exhibition use in the UK.

If the fabric is not stretchy then how are the curved forms created?
The fabric form is computer generated using a specific Software/Programmes for the industry. This 'Shape' is then converted into flat panel which then can be but out to create paper patterns or the information can be used to computer cut the panels on a plotter/cutter.

Although the flat fabric is not specifically stretchy with will stretch a very small amount particularly across the bias of the fabric. If this stretch is more than a specific percentage, then the stretch amount has to be compensated for and thus the additional length that it will provide has to be deducted from the panel size.

Why and how is the fabric tensioned?
Because of the nature of fabrics used, the presence of the double curve and the process of manufacturing, tensioning is always required in order to induce the correct shape and take out any minor creases in the fabric.

This tensioning can take many forms from simply hand pulling a canopy into place and Velcro fixing, to tensioning a corner of a canopy with a rigging screw or turn buckle [photo of rigging screw tensioner] depending on the size of the structure and the way in which it is to be used.

Can my tensile fabric have straight edges?
In general terms the answer to this question is yes, however it can be affected by the size and design of the canopy and the visual detail that is required. For this reason we recommend that you talk to us at the early stages of a design, allowing us to advice on the appropriate detail.

What fabrics are best used for interiors and exhibitions?
As far as 'form' and 'appearance' are concerned any type of fabric can be used to create a tensile fabric form, but we find the best to be:

Coated glass fabric such as Natte and Sunscreen
Cotton fabrics such as Proban Cotton

But there are restrictions due to other factors:

Certain applications require higher fire ratings (see fabric chart).
Brittle fabrics can fracture during manufacture and result in lines appearing in the cloth (uncoated glass fabrics) that, although do not effect the cloths strength, can be unsightly.
Some uncoated glass fabrics are hazardous to handle and therefore are slow to work with or inappropriate for some applications.

What fabrics are best used for exterior applications?
As per the interior and exhibition fabric recommendation, any type of fabric can be used to create a tensile fabric form for exterior use, but considerations to fire rating, water repelling qualities, wind loading, life span, UV breakdown are all factors that can effect the decision of what fabric type.

Typical exterior fabrics include:

PVC Coated Polyester
PVDF Coated Polyster
Cotton Canvas
PVC on Polyester Mesh
PTFE

What fire ratings do the fabrics have to achieve for interior use?
Because of fire regulations only glass based fabrics and high quality cottons can be used for most interior and exhibition applications.

The glass fabric can be used for all environments as it simply does not burn (rated to BS476 pt 6 Class O (UK), M1 (France),B1 (Germany) allowing it to be used for screens, canopies and ceilings. The cotton fabrics however are rated lower (BS 5867), which depending on the application can restrict its use. Further advice on the fire rating of fabrics can be provided on request.