FAQ
There are some questions that crop up quite regularly regarding membrane structures, so here are some answers...
How much will it cost?
Well the answer of course depends on the specification required, the type of fabric, and the design of the supporting structure. As a permanently installed external structure, it needs to be fully engineered, just like any building or other external structure in a public location. A typical cost per square metre of say £300-£400 for a standard PVC structure can be suggested, but of course this would vary hugely depending on a range of factors, including those mentioned above. To be able to give a price that is more useful, and of course more accurate, we can help you to establish a basic workable scheme. This also enables us to consider the project as a whole, from concept design right through to completion on site, and to offer assistance to the client/main contractor team throughout the planning and construction programme. So, in response to the original question, you would more than likely receive some simple questions in return, such as:
- What area are you covering?
- What life expectancy do you require for the structure?
- What is the supporting structure?
- Who provides the foundations? (Main contractor, normally)
- Who provides the steel framework? (Us, normally)
- What do you want to do with rainwater?
- What style/shape do you want?
With these basics established, we are much better placed to give you the reassurance of a considered estimate, together with a reliable and affordable outline design.
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How long does it last?
As a permanently installed and engineered structure, most of our structures are designed to last well in excess of ten years, subject to appropriate maintenance. If a longer life is required, we can provide a more rigorous specification.
The two most popular fabrics used in external canopy structures are PVC coated polyester and PTFE coated glassfibre.
PVC (poly vinyl chloride) coated polyester is the least expensive material of the two. The PVC coating includes various agents and pigments to help prevent degradation of the “high strength-low stretch” polyester base cloth, and the top coatings are of an especially high quality, to minimise dirt build up, to facilitate cleaning and to maintain the appearance of the fabric for many years. All but the most lightweight fabrics developed for canopy use would have a life expectancy of at least ten years.
PTFE (poly tetra flouro ethylene) coated glassfibre offers a truly long life material. The PTFE coating is resistant to virtually all chemicals, and will not degrade in the harshest of climatic conditions. The material has a life expectancy in excess of 20 years, although there are some structures in this fabric that have been in existence for even longer than this.
The steel framework is also expected to be highly durable, and this will depend on the finishes applied and on the local environmental conditions, as well as a regular regime of inspection and cleaning. In marine or coastal environments we would always recommend a galvanised finish or stainless steel for maximum durability. Galvanised finishes are extremely durable in their own right, and on thin sheet, the visual appearance is quite striking and attractive. However, on thicker, structural sections, the finish is not always as consistent, so in many cases, a top coat is applied in the form of a polyester powder coating or a wet paint system.
Cables and tensioning accessories for permanent external structures should be stainless steel grade 316. Although this might be subject to a superficial rusting in extremely aggressive or marine conditions, the lifespan for these components is almost unlimited.
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Will I need to clean it?
Fabric is a great diffuser of light. The top surface of the canopy may become quite dirty over time, but from the underside, the surface soiling is rarely noticed. Although the translucency of the fabric will have reduced accordingly, it’s quite surprising how little this affects the comfortable levels of light under the membrane.
In most cases, cleaning is a very easy operation to carry out. The purpose is primarily to remove built up areas of permanent dirt, on both the fabric and the steelwork which can act aggressively against the surface. A secondary benefit is improved brightness under the canopy.
The frequency the cleaning is required will vary from structure to structure.
Will it collect leaves/traffic pollution/bird droppings?
Will it get lots of sunshine, or is it more sheltered?
It would be good practice to clean most structures every couple of years. However, on large structures, access to all areas of the canopy is unlikely to be easy, so cleaning may well be less frequent. In many cases however, cleaning can be quickly and easily carried out by any cleaning contractor or caretaker using a mild detergent, soft brushing and lots of clean water to rinse.
We ensure that our structures are safe for cleaning by providing secure anchor points where required, and by providing the client with an O & M manual, including risk assessments and procedures for maintenance and cleaning.
By ensuring long term cleanliness, the benefits will be a longer life for the fabric, and a better looking, better maintained structure.
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What are the cables for?
Most of this answer can be found in the technical pages, but in general, cables serve one of two purposes – as a boundary cable, or as a tie cable.
A boundary cable is fitted into a pocket on a curved (scalloped) edge of a membrane. It effectively acts as a restraint, a curving edge to which the fabric is connected. The pull of the fabric against the cable puts some very high tension forces at each end of the cable. The membrane plates fitted at each end of a boundary cable are designed to transfer these loads into the supporting steelwork.
A tie cable is simply a straight cable linking two points. They are often fitted with a tensioner or adjuster to ensure the correct length. Their purpose is primarily to keep the supporting steel frame correctly positioned, particularly in high winds when some parts of the canopy can be in uplift, and others in download.
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Can it be repaired?
As you would expect, this depends on what the damage involves, and the type of structure affected. A small cut or puncture damage to the fabric can usually be patched on site without detensioning, although older fabrics can lose their “weldability” properties.
If the area of damage is more substantial, eg as result of a fire, or large tear, it may be deemed appropriate to take the canopy back to the fabrication shop and replace a whole panel or section.
However, it’s important to stress that all of the fabrics are extremely strong and durable. They are generally specified with a very high factor of safety over and above the maximum working loads they may be subject to during a hurricane or under a huge snow load. Accidental rips and punctures are not common occurrences, but it is a reassurance for all parties to know that small patches can be applied discreetly should this be necessary.
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What colours are available?
If you want to specify a colour for your structure, you have a vast range available to choose from in PVC on polyester.
Lighter weight fabrics are most frequently available in colours off the shelf, but these may not offer the life expectancy that the client requires.
To get a high performance architectural membrane in a specific colour, it may be necessary to produce a minimum run quantity, of several thousand square metres. This isn’t likely to be practical, so if you have a colour in mind, we can help you find the right material that’s available in stock.
The colour pigments in a PVC coated polyester protect the polyester filaments from the harmful effects of uv light. They also affect the light transmission properties of the fabric. Most fabrics are white because this allows the best light transmission (eg 10%) while still protecting the polyester base cloth. A darker colour such as green or blue will have much poorer light transmission – sometimes even less than 1%.
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Who provides the foundations?
In a normal contract, the client has their own “design and build” team of architect, structural engineer and main contractor. We would supply to the client loadings that our structure would impose on the clients building/ground. The clients engineer would then assess these loads and design a suitable foundation/supporting structure, according to local ground conditions/underground services. The main contractor usually excavates for and provides the foundation as designed by the client engineer.
In some cases the client requires us to design and supply the foundation, and we are happy to do 
this, once an on site inspection has been carried out to assess ground conditions and services. We can usually provide indicative loadings very early on in a project so that due consideration can be given by the client design team in the planning stages.
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Will it blow away?
In a nutshell, no. Our structures are designed to withstand a “one in 50 year” wind. This more or less equates to a hurricane. We then apply a safety factor on top of this. As long as the entire canopy, membrane and supporting framework has been fabricated and installed to the same standards as the engineering defines, these are truly permanent external structures.
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Can I make it demountable?
To withstand all possible wind and snow loads, it goes without saying that the canopy structure has to be appropriately tensioned and secured. Getting the canopy into position and correctly installed often requires the use of specialist tensioning equipment, such as ratchet straps, chain winches and hydraulic rams, and naturally the use of such equipment raises the issue of health and safety.
If the canopy is designed as demountable from the start, we can incorporate features within the supporting structure to make this as user friendly as possible, without compromising on the structures long term durability.
Other aspects to take into consideration may include the following:
Where will it be stored?
The fabric should be cleaned and dried before putting into a clean, well ventilated storage area.
How long will it take to put up/take down?
We can advise you on this during scheme development, but it may affect the number of times you would consider doing it.
Will it be safe during installation or detensioning?
Even the best designed and manufactured tensile fabric structure can be dangerous in gale force winds until installed unless properly restrained. You wouldn’t want to be trying to take down a structure if high winds are going to cause you problems.
Will it have a safe wind speed limit?
This is often a sticking point, because if we were to provide a structure that is designed to withstand say a 40km/hr wind, then you would have to make sure it’s taken down before the winds exceed that limit. And you would most likely need quite calm conditions in which to take it down safely. It is possible to design safety features into such a structure, but unless it really HAS to be demountable, we would always go for the safety and durability of a permanent fixed installation.
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How long does it take to design, manufacture and install?
For most bespoke structures, we would advise a period of 12-14 weeks from receipt of order to completion of installation on site. Most of the time, the basic scheme would normally have been established before the order is placed.
A typical breakdown would be:
- Site survey and general arrangement drawing for approval – 2 weeks
- Analysis and structural design – 4 weeks
- Steelwork and membrane manufacture – 6 weeks
- Finishing, delivery and installation – 2 weeks.
If you have a much tighter deadline, we are flexible and able to “fast track” projects when the need arises.
We provide our clients with a more detailed breakdown at an early stage in the project so that programming requirements can be built in by the construction site management team.
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Other Stuff
The headings below cover subjects we are currently researching and writing. Please contact us if you need any specific information.
Specification
We can provide a full specification for tensile membrane structures, covering the design of the steelwork, protective finishes, and manufacturing tolerances. Call for more details.
Environment
There are some big differences between the fabrics and their "green" credentials. It's not a black and white case, we will update this page in due course.
A History, tents around the world, the future
Who made the first tent?
What are the fundamental styles of fabric covered shelter?
Which is closest to our high tech tensile membranes of today?
How and when did tents go high tech?
Is it just a fad?........
Welding Fabric, Stitching & Lacing
How do you join PVC?
What about PTFE?
What are the traditional methods?
How strong are the joins?
Tensile Fabric Analysis
Lets take a simple membrane and look at the loads in the system.
How do we know what will happen in a hurricane?
Biaxial & Uniaxial Testing
Crucial elements for the correct design and patterning of fabric structures.
Biaxial - testing both directions at once, using the prescribed tension for warp and fill directions.
Uniaxial - test the fabric and welds to breaking point.
Why Bespoke?
Can't we just make one structure for all situations?
Look at the range of applications and consider the different wind loadings across the UK.
Internal Structures
Blinds, screens and ceilings. No wind or snow loadings. Just good design with high quality, fireproof materials manufactured to the most rigorous specifications.
Interiors
More about interiors
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