Fabric Architecture - Technical

Continual Professional Development (CPD).

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What is a tensioned fabric structure?
True tensile fabric structures are those in which every part of the fabric is in tension. A tensioned fabric structure must curve equally in opposite (vertical) directions, this gives the canopy 3 dimensional stability. This is an anticlastic form and seen most simply as a hyperbolic paraboloid. A proportion of 4:1 between horizontal span and vertical articulation is desirable. The more irregular and the flatter the form is, the more we need to load the material to stabilise the shape. The fabric is loaded during erection, called pre-tension or pre stress.

Fabric is inelastic in nature. If the fabric were elastic it would distort under wind and snow loads. A typical external fabric has a tensile strength of 10 tonnes per linear metre and will creep a few percent after 20 years. Fabric needs to be thought of as being totally inert. The complex 3-dimensional form of a canopy is achieved not by elastic fabric but by a cutting pattern where strips of material with non-parallel sides are sewn or welded together.

Advantages.

Unique building medium.

Lightweight and flexible, fabric interacts with and expresses natural forces.

Tensile fabric structures are an environmentally sensitive medium. Tension is the most efficient way of using any material, it utilises the material at maximum efficiency rather than just the material at the extremes of the cross sectional form, as in bending and compression loads.

Fabric structures have higher strength/weight ratio than concrete or steel.

Most fabrics can be recycled.

A fabric structure can be designed for almost any condition, heavier fabrics and more 3 dimensional forms will cope with extreme wind and snow loads.

Disadvantages.

Fabric structures being mainly fabric and cables have little or no rigidity and therefore must rely on their form and internal pre-stress to perform the this function.

As a rule of thumb spans greater than 15 metres should be avoided however, much greater spans can be achieved by reinforcing the fabric with webbing or cables.

Loss of tension is dangerous for the stability of the structure and if not regularly maintained will lead to failure of the structure.

If an "open" system (see below) structure is to abut a building then care needs to be taken with loadings.

Thermal values limit use. This can be overcome with thermal lining and double skins at the cost of translucency.

Trying to successfully control water from an open system structure is difficult and requires guttering.

Process:

Concept

Design

Analysis

Fabrication

Erection.

Design factors:

Location (Wind and snow loads)

Foundations

Drainage

Acoustic Performance

Fire resistance

Thermal values

UV attack

Condensation

Translucency

Reflectivity

Lighting

Maintenance

Access

Vandalism

Fabrics.

PVC (polyvinyl chloride) coated polyester,

Silicon coated glass and

Teflon coated glass P.T.F.E. (polytetrafluroethylene). The polyester and glass are the

woven element, the sub-strait, the PVC, Silicon and Teflon coatings are applied to this.

PVC coated polyester is the least expensive, design life of 15 to 20 years due to ultra violet attack. Besides cost it has the advantage of being very robust, easier to ship and erect. Comes in many colours. Most economic material nearly always PVC coated polyester if maintenance/renewal schedules in place. Additional initial cost of the glass fabrics approx. 50% more expensive for extended life of 10 years.

Silicon glass has higher tensile strength than polyester, but being glass it is brittle, subject to damage from repeated flexing. Not subject to ultra violet attack, 30+ year design life. Silicon and Teflon are almost completely chemically inert, resistant to moisture and micro-organisms and have self cleaning properties.

All types of fabric can be used internally if fire retarded, most commonly used is PVC coated glass cloth.

Support Systems.
An open system has a cable around the perimeter of the fabric-loads can be massive and require large foundations-most sculptural form. A closed system has rigid members around the edge. Closed system structures require smaller foundations. Open and closed systems can be combined. A support structure can be edge tripods, central masts or push-ups. In a closed system the edge will be supported continuously by a "kader" in an extruded section.

Codes and Standards.
There are no codes and standards in the UK relating specifically to tensile fabric structures, manufacturer must provide a high degree of technical validation in order to assure public safety and adherence to building codes. The same loading criteria and standards of other building methods apply.

Fabric Architecture and Signature Structures.
Founded in 1984 and has completed hundreds of projects all over the world. Fabric Architecture deals mostly with one off design and build projects while Signature Structures deals with pre-designed structures, where the design, patterning and engineering have already been undertaken.

The trinity of material, form and process.
As we face a future of doubling the world population in the next 40 years it is all our responsibility to maximise the materials we use from worlds store our cupboard. Using Lightweight materials and putting them in tension is one of the fundamental ways that this can be achieved.

Board (1)
As Todd Dallard, one of the industries founders said `A space with a fabric roof becomes a premium space within a building. Yet despite these advantages, most Architects have never touched a fabric structure!

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