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 its 3-dimensional stability.

This is often referred to as 'double curvature' or an 'anticlastic form' and mathematically it is known as a hyperbolic paraboloid.

A proportion of 4:1 between the horizontal span and the 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 complex 3-dimensional form of a canopy is achieved not by elastic fabric, which is stretched into position but by a cutting pattern where strips of material with non-parallel sides are sewn or welded together. The fabric is cut and bonded together to make its final 3-dimensional shape.

We will load the fabric during the installation process. 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 such as snow, wind etc. A tensile fabric structure, once it is tensioned, can take a large amount of applied weight and the fabric is extraordinarily tight. If something is thrown onto the fabric, it will 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.

Common Misconceptions

It is commonly believed that fabric structures cannot cope in heavy weather conditions - this is not true. 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. Fabric Architecture has built structures in typhoon and tornado zones.

It also is commonly believed that 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.

Fabric, being flexible and normally woven, is a ‘live’ medium that stretches across the diagonal plain to a greater extent than in the direction of the weave, this can be most easily seen with a piece of netting.

Certain fabrics can also have more stretch in one direction than in the other. This is due to the threads of the weft being woven in and out of the warp threads, which are already tightened.

If an uneven load is applied only on the weft threads, they will straighten and crimp the warp threads, subtly changing the shape of a canopy or part of a canopy. This must be allowed for in the patterning of the fabric and the erection of the canopy. Distortion is more evident in material where the threads are first coated and then woven.

Where the base fabric is woven and then coated, the coating applied to both sides of the base fabric helps to keep the threads at their original spacing.

Fabric Types

There are many different fabrics with various features and benefits to each one, however there are three basic external fabric types most commonly used in tensile fabric structures:

External fabrics

• PVC (polyvinyl chloride) coated polyester
• Silicon coated glass and
• Teflon coated glass P.T.F.E. (polytetrafluroethylene).

The polyester and glass make up the woven element, which forms the sub-strait. The PVC, Silicone and Teflon coatings are then applied to this.

PVC coated polyester is the least expensive and has a design life of 15 to 20 years due mainly to ultra violet attack. Besides cost it has the advantage of being very robust, easier to ship and install and can be de-mountable. It also comes in many colours.

Silicone coated glass cloth and Teflon coated fabric has a higher tensile strength than PVC coated polyester, but being glass it is slightly more brittle therefore it can be subject to more damage from repeated flexing. This fabric is not subject to ultra violet attack and as a result has a 30+ year design life.

Silicone 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 retardant, most commonly used is hybrid fabric such as PVC coated glass cloth.

Internal fabrics

All types of fabric can be used if suitably fire retardant. The most commonly used is PVC coated glass cloth due to its easy maintenance and very good fire resistance. Untreated cotton can be used which will burn out in a flash and will not drop hot plastics on anyone below. Lycra materials can be used for awkward or ad-hoc shapes.

An open system has a fabric perimeter supported by an integral cable. The loads can be substantial on open systems therefore they require larger foundations than a closed system. Open systems are considered the most sculptural form of tensile structure. The open system support structure can be edge masts, corner tri-pods masts or central masts with 'push-ups'.



A closed system structure consists of rigid members around the edge and closed systems require smaller foundations. In a closed system the fabric edge will be supported continuously by a 'keder' fed into an extruded section, normally fabricated from aluminium.



Open system structures, by their very nature, move in heavy weather conditions. Trying to join rigid walls to a free standing open system structure requires a flexible jointing system between roof and walls can be considered as untidy. However controlling water is much easier in a closed system as is its simple boxy framework lends itself well to integral drainage. The masts, tripods and booms of the supporting structure can be used as down-pipes in order to allow water to be led away without other visible appendages.

Trying to successfully control water from an open system structure is more complex and requires the addition of guttering (as with a conventional building) however this can look unsightly and loses the advantage of the beautiful free form provided by fabric.

Alternatively, the addition a foam-filled cylindrical ‘dam’ attached to the edge of the fabric can direct water towards a desired location (ideally a membrane plate), which incorporates a drainage point, normally leading back to the mast along the boom.

Interface Connections – fitting fabric to framework

The final part of the design process considers how to attach the fabric to the support system. Care must be taken not only to provide a path for the load to easily flow into the support structure but to allow flexibility in the connection for displacement and rotation.

During the installation process the whole structure will probably experience loads greater than the snow and wind effects during its working life span. This is because of the uneven loads imposed as the structure is assembled and tensioned.

Some fabrics can develop creep or elongation due to the type of weave or coating on the weave, heat and moisture. This should be considered during analysis (which we are not covering today) and has a direct effect on the connection system.

Creep will induce a loss of pre-stress tension in the fabric, which will in turn mean that it can develop ponds of water on its surface and will flap in the wind. This loss of tension is dangerous for the stability of the structure and if not regularly maintained will lead to a failure of the structure.

Connections from the fabric to the support system should always be adjustable. Teflon coated fabrics require re-tensioning once the fabric has settled over a period of a few weeks.
Most of the time one simply orientates the canopy form so that the main ‘traffic’ routes occur beneath high points or edges that will not shed water.

An open system structure looks best when used as an independent stand-alone statement. If walls are required these need to be designed separately using the tensile structure as a primary roof.

If an open system structure is to be joined to a building then care needs to be taken with loadings. Often the building will need to be reinforced to take the pre-stress load. It is important to take this into early consideration and undertake a load analysis, which may incur a fee at the pre-tender stage.

Basic Support Systems Ideas

Conventional structures have an internal rigidity in order to be stable. Fabric structures, being mainly fabric and cables, have little or no internal rigidity therefore they must rely on their form and internal pre-stress to perform the same function.

To resist these loads we have to put equal loads into the fabric for it to remain stable, this is called the pre-stress.

Because fabric structures rely on internal tensile forces to remain stable, their behaviour is more complicated than their conventional counterparts and therefore they are more difficult to design.

The significant changes in their geometry means they are non-linear even though the fabric remains more or less stable. If properly designed this is a desirable quality that increases their ability to carry load as they deform from the effect of live loads.

Fabric Structures are more capable in this respect due to their very high strength weight ratio than equal spans of concrete or steel.

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.

These loads have to be transmitted into our support structure. A support structure might be edge tripods, central masts or 'push-ups', or it could be agreed to suspend the fabric in some form to give a very large mast free area.

In a closed system the edge will be supported continuously by a 'keder edge' inside of an extruded section of aluminium.

As a building medium, fabric requires a different approach to that of conventional roofing materials, and offers limitless possibilities of three-dimensional forms.

A tensile structure should be viewed as an integral part of a building, rather than a last minute add-on, even though it is categorised as an optional extra. It should be included in the design process from the concept stages, and the following points considered:

Key Design Factors:

• Determine the loads that will be exerted on connecting buildings and/or ground
• Calculate and locate foundation pads
• Locate services adjacent to foundations and re-route if required
• Management of rainwater/ rainwater run-off
• Find out whether there is a need for add-ons such as lighting, protection or bird netting

Site considerations:

• Location (wind and snow loads)
• Foundations
• Drainage
• Access

Canopy considerations:

• Acoustic performance
• Fire resistance
• Thermal values
• UV attack
• Condensation
• Translucency
• Reflectivity
• Lighting

Life Cycle Costs:

• Maintenance
• Fabric Longevity
• Steel protection
• Bird netting or security
• Vandalism