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 prestress.
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 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
1.PVC (polyvinylchloride) coated polyester, 2. Silicon coated glass and 3. 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 Ltd.
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 struct
Basic Support Systems Ideas
Conventional structures have an internal rigidity to be stable. 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 same function.
To resist these loads we have to put equal loads into the fabric for it to remain stable, 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, we might decide 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 “Kader” in an extruded section.
16m x 13m conic with 3m dia. headring Industrial galvanised finishes
12m x 6m double conic on staircase at Olympic Centre Dundee
Fabric to Support System Connections
The final part of this design process is 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 erection process the whole structure will probably experience loads greater than the snow and wind effects during its working lifespan. 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.
Head of tripod with catenary cables
Extruded section with membrane plate and catenary cable
Tensioners in form of dolphin
Headring with separate fabric clamps
Understanding Fabrics
Fabric, being flexible and normally woven, is a ‘live’ medium that stretches across the diagonal to a greater extent than in the direction of the weave, one can most easily see this with a piece of netting.
Certain fabrics can 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.
There are many advantages and disadvantages with different fabrics, which is a seminar in its own right, however, there are three basic external fabric types most commonly used in fabric structures.
External fabrics
These are:-
- PVC coated polyester (polyvinylchloride),
- Silicon coated glass and
- Teflon coated glass P.T.F.E. (polytetrafluroethylene)
The polyester and glass are the woven element which we call sub-strait, the PVC, Silicon and Teflon are coatings 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 erection can be demountable. It also comes in many colours.
Silicon glass has a much higher tensile strength than polyester; however, being glass is brittle. To over come this, the fibres are made to a very small diameter but are still subject to damage from repeated flexing. The advantage of glass is that it is not vulnerable to ultra violet attack which gives it a 30+ year design life. Both silicon and Teflon are almost completely chemically inert, resistant to moisture and micro-organisms and have good self cleaning properties. Silicon Glass is less expensive than the Teflon glass, however the Teflon has a better “self cleaning” properties. One other point about Teflon coated glass fibre is that it is difficult to handle and transport in large pieces because when sharply folded the Teflon coating visibly “bruises”.
Internal fabrics
All types of fabric can be used if suitably fire retarded. 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.
Chase Manhattan, Bournemouth. Triangular sails stretched across atrium and hypers suspended from ceiling.
Fabric concealing columns, on 4 No. elliptical frames at top, middle and bottom, supported off column, lit from within.
Internal ceiling on flexible frame.
Appropriate Applications
I will start this from a negative standpoint of what does not work particularly well.
Open system structures by their 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 which usually looks untidy.
Trying to successfully control water from an open system structure is difficult and requires guttering as a conventional building which again looks unsightly and loses the advantage of the beautiful free form. One can use a foam filled cylindrical thickening attached to the edge of the fabric to direct water towards the membrane plate, which incorporates a drainage point leading back to the mast along the boom. Most of the time one simply orientates the canopy form so that the main routes under it occur beneath high points or edges that will not shed water. This does not effect a closed system structure which is relatively easy to drain.
Water drainage via membrane plates, booms and down through mast.
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.
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 abut a building then care needs to be taken with loadings. Often the building will need to be reinforced to take the pre-stress. It is important to take this into early consideration and undertake a load analysis which will cost money at pre-tender stage. The only other disadvantage is thermal values which limit use. This can be overcome with thermal lining and double skins at the cost of translucency.
The disadvantages are far outweighed by the advantages, their sculptural forms, environmental sensitivity and magical luminosity. Tensile fabric structures engender and incorporate all the balance of nature, covering areas using the minimal of materials in a most cost-effective way.
Cost implications
Early involvement of a Contractor will help greatly in maintaining cost, however, designing within a budget is essential for Architect and Contractor alike and nearly all projects are put out to closed tender so the advantage a Contractor has in knowing the budget does not help in this competitive industry. It is worth noting that main contractors largely regard tensile structures as complicated tents and having little knowledge of them, will try and avoid them.
Life cycle costs:-
The most economic material to use is nearly always going to be PVC coated polyester if maintenance and renewal schedules are in place. The additional initial cost of the glass fabric approx. 50% more expensive for their extended life of approximately 10 years is difficult to justify unless a decision is taken not to maintain a structure.
Current Case Study
- Car show room and surrounding area
- Objective: To create a cost effective highly visual sales showroom and surrounding point of sale areas.
- Their initial budget for the tensile structure over the sales showroom was £80,000.
- The ground area to be covered was 660m². This gave a budget cost per square metre £121.
- The actual cost excluding foundations was calculated as £145m².
The sequence of events on this project was fairly typical:
- Architect rings to discuss.
- Sends whatever he has; normally a plan with an area shaded in and an idea of the form he wants.
- We advise on ideas against costs over the phone to evaluate budget restrictions.
- He accesses web page to review existing structures for ideas.
- Together we firm up some basics and then we normally visit the site, 1½ hour.
- We pull together some quick visualisations for them to present to the client and planning authority.
- If the price and scheme are feasible a decision will be taken on whether to award us the contract or to put it out to tender. Up to
- this stage no fee is chargeable.
If the decision is to go to tender we will help with specifications or for a fee write the tender documents. If we perform this function we cannot tender but will help co-ordinate the tenders and sit on the tender committee.
This particular project is presently in the planning stage and will probably not be tendered due to the competitiveness of our pricing.
The tensile structure is being used as a primary roof over a secondary flat roof.
The structure will be erected in 8 days and will allow the secondary structure to be built under it saving down time due to the weather.
The fabric will be lit from the underside making a highly visual statement.
If successful after a trial, several such sale rooms could be commissioned, with savings on the design and engineering costs.
It was decided to use PVC material due to last 15 years, as their corporate image will probably change during that time.
It is worth noting that life cycle expectancy is very important in choosing the correct materials. Often it is more expedient to use a PVC rather than a teflon and replace every 15 years.
PVC Coated Polyester
- Sound Transparent
- Reflectivity 70%
- U Value 4.5w single layer of fabric
- Translucency 12%
- Fire rating PVC will self extinguish and will not produce flaring droplets (BS5867)
- U.V. PVC will deteriorate between 15-20 years
- Silicon & Teflon glass are not affected; assume 25 + years.
- Costs PVC structure 50% less than Teflon structures.
- Strength Structural fabric will have tensile strength of approximately 10 tonnes/linear metre.
- Safety Factor on loads of 4 to 6 times
- Vandalism Design around the problem, connections above arm-reach etc
- Cleaning Once a year
- Condensation Control of ventilation and or flow if second skin is used.
- Colour PVC & Silicon glass can be made to any colour. Teflon glass white only.
- Repair All fabrics can be patched
- Lighting Up-lighting recommended
Considerations in designing initial concept.
Site
- Location (Wind and snow loads)
- Foundations
- Drainage
Canopy
- Acoustic Performance
- Fire resistance
- Thermal values
- UV attack
- Condensation
- Translucency
- Reflectivity
- Lighting
Lifespan Costs
- Maintenance
- Access
- Vandalism
How to design a tensile fabric structure”.
Thank you for attending this presentation, which I am sure, will give you a valuable insight into the world of tensile fabric structures. This presentation can only be an overview, but will be helpful in illuminating this unique building medium and give you at least the basic knowledge to appreciate where you can use tensile fabric structures. Please feel free to ask questions during the presentation particularly if you have a project in mind, as actual case studies help to bring these presentations to life. There will be a question and answer session at the end.
16m wide barrel vault fabric ceiling in burn out material with sprinklers above and 9m flying saucer feature beneath. Click picture to view
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.
- Daylight translucency
- Night-time luminosity
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 practises, 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.
Double conic with tripod edge supports in Kuwait. PVC membrane subject to intense UV radiation, salt corrosion and regular very heavy sandstorms. Support structure is galvanised mild steel as 316 stainless steel corrodes badly in these conditions. Click to view
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 prestress.
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 marvellous 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 Demonstation
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.
