The learning objectives of this presentation are:
How a tensile structure works.
Design options.
Appropriate and inappropriate applications.
Cost implications against life cycle costs.
This schematic process diagram summarises the 5 main stages of a project, these are:
Concept, Design, Analysis, Fabrication, Erection and Maintenance.

The areas I will concentrate on are Concept and Design, I will touch on analysis, fabrication and erection only to the extent that they have a major influence on the concept and design, the same with fabric types.

Tensile fabric structures are an environmentally sensitive medium and an inexpensive way to create an organic form. The biggest performance advantage is its strength to weight ratio, which saves on materials (most fabrics can be recycled). Being lightweight and flexible; fabric interacts better with natural forces than a rigid material, this combined with its daytime translucency and night-time luminosity gives a magical feeling of being outdoors, combined with the security and comfort of indoors. It is an Architects dream material allowing experimentation with form to create exciting new solutions to conventional design problems.

Codes and Standards.
There are no codes and standards in the UK relating specifically to tensile fabric structures. The lack of widespread knowledge of this medium and the lack of recognition of this type of construction in building codes requires that the manufacturer provide a high degree of technical validation in order to fulfil their obligation to assure public safety and adherence to building codes. The same loading criteria and standards of other building methods apply. Good manufacturers will have documented their own working methods, which will as far as possible incorporate existing codes and practices, a copy of our own ‘codes & standards’ is available on request. We can also offer help with pulling tender packages together.

Formalisation of codes and standards for tension fabric structures is presently being considered by a committee under the EU, and should be available within two years.



The Nature of Tensile Fabric Structures.

Introduction.
Fabric is unique as an architectural tool, the sculptural forms that can be achieved are offered by no other medium, however, certain simple rules must be obeyed.

What is tension?
Tension is the force used to pull the molecular structure of a material apart. It is the most efficient way of using any material because it utilises the whole cross section at maximum efficiency rather than just the material at the extremes of the cross sectional form, as in bending and compression loads. Take the example of a stick; it will break under compression or bending loads, long before it would be pulled apart by tension. Tension loads maximise the load capacity of materials, or to put it another way, requires the least material.

What is a tensioned fabric structure?
True tensile fabric structures are those in which every part of the fabric is in tension. The fundamental rule for stability is that a tensioned fabric structure must curve equally in opposite directions, this gives the canopy stability. This is known as an anticlastic form and mathematically as a hyperbolic paraboloid. I will come to this later.

Common Misconceptions.
It is commonly believed that fabric structures cannot cope in heavy weather conditions. This is untrue. A fabric structure can be designed for almost any condition, heavier fabrics and more 3 dimensional forms will cope with, for example, extreme wind and snow loads. We ourselves have built structures in typhoon and tornado zones.

It is commonly believed that the fabric is stretchy or elastic in nature; again this is untrue. If the fabric were elastic it would balloon under wind loads and settle under snow. A typical structural external fabric has a tensile strength of 10 tonnes per linear metre and will creep no more than a few percent after 20 years of extreme conditions. The fabric is ‘alive’ and does ‘creep’, which we take into consideration during the engineering, but basically fabric needs to be thought about as being totally inert in the initial stages. The complex 3dimensional form of a canopy is achieved not by elasticity but by a cutting pattern where strips of material, between 1m and 2m wide, with non-parallel sides are sewn or welded together.

What is tension in the fabric of a fabric tensile structure?
We put the fabric of a tensile structure under tension. We do not stretch the fabric into position. It is cut and bonded together to make its final shape. We will load the fabric during erection. This loading or tension which we have pre-engineered is called pre-tension or pre stress.

Pre-tension is the most efficient way of resisting live loads snow, wind etc.

A person can happily walk over a tensile fabric structure once tensioned, the fabric is extraordinary tight. If you throw a brick onto the fabric it will simply bounce off. These imposed loads or live loads are therefore appropriate when the designer wishes to use the minimum amount of material for either functional or aesthetic reasons.

Open and Closed Systems.
The pre-tension is transmitted to a support system, which can be either closed or open or a mixture of the two. An open system is best defined by having a cable around the perimeter of the fabric, this transmits the load to the support system. These loads can be massive and require large concrete tie down blocks. A closed system is best defined by having rigid members around the edge. These are designed to counter the pre-tension put upon the fabric more like a conventional building. Closed system structures require smaller foundations. Both systems open and closed can be combined which is particularly useful if one is trying to abut to a building and gain weather protect.

Concept.

Testing Initial Concepts for Viability.
True tensile fabric structures must have double curvature designed into the fabric. These curves work in opposite directions to each other to resist imposed loads (outside forces), giving 3 dimensional stability. This mathematically is called a hyperbolic paraboloid and is the anticlastic form. The low points resist uplift and the high points resist downloads. The easiest way to understand this is by using a soap bubble model. I do not suppose that there is any one in this room who has not occasionally blown a common soap-bubble, and while admiring the perfection of its form, and the marvelous brilliancy of its colours, wondered how such a object can be so easily produced.

I hope that none of you are yet tired of playing with bubbles, because, as I hope we shall see, there is more in a common bubble than is first apparent.

Soap-bubble Demonstration.
Taking a basic wire frame in the shape of a hyperbolic paraboloid we dip this into soapy water, pulling the frame out you will see a soapy film suspended within the frame. This saddle-like elastic skin of liquid represents our fabric with its anticlastic form. This skin is the minimum surface area of that frame, due to the surface tension of the liquid It has the least surface area that can "web" within the frame. The more irregular and the flatter the fabric is, the more we need to load the material to stabilise the shape. The fabric should have sufficient curvature in both directions preferably roughly similar but at the same time not too extreme. A proportion of 4:1 between horizontal span and vertical articulation is desirable.

A contractor’s experience regarding the most efficient form should be sought at an early stage, particularly if cost is an issue. However, by doing this exercise with a soap bubble model you can see the stresses in the skin by its colour change.

Making a Model Demonstration.
At this early stage making a stocking model from a pair of stockings or lycra material is very helpful to visualise what can now be quite a complex three dimensional form. I find that using a cardboard box with one side cut out and pinning the fabric out is very helpful. By inserting objects, such as a pencil beneath the fabric and deforming it upwards you can start to appreciate what the fabric could look like (without the use of Superglue).

With our box being our required coverage and our ‘pencils’ being our internal support structure we can consider spans. Large dynamic sweeps of fabric have to be supported and have to resist the worst case of uplift and down loads from snow, wind etc.



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