Tensile Fabric Structures
 

     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.

 
The Theory Materials Performance Detailing Costs
 
1.The Theory
 
Resisting Loads
 

     Membrane structures rely on double curvature to resist imposed loads efficiently.

 

     Imagine a flat piece of fabric. An imposed download of snow can only be resisted by tension in the horizontal fibres – a bit like making the catenery cables on a suspension bridge horizontal and expecting them to still carry the weight of the road deck.

 

     In Fig 1, a classic Hyperbolic Paraboloid, any point on the membrane surface can be restrained by the corner points. The two high points pick up any downloads and the two low points resist the wind uplift.

Fig 1  Hyperbolic Paraboloid
 

     The flatter the fabric, ie the smaller height difference between the high and low points, the greater the resultant loads will be at the corners.

 
Other Forms
 

     Inflatable fabric structures are synclastic forms where constant air pressure balloons the fabric into shapes also exhibiting double curvature. Anticlastic forms like the Hyperbolic Paraboloid have opposing curvatures.

 

     Other common anticlastic forms are the cone (fig 2) and the arch form (fig 3)

 
Fig 2  Cone Fig 3  Arch (Barrel Vault)
 

     Nearly all tensile canopies are derivatives of these three shapes.

 
Pre-tension
 

     Pre-tension is the tensile forces introduced in the canopy during erection.

 

     The shape of a membrane surface is determined by the ratio of prestress in the two principal directions of curvature. These are established in the computer form generation process. The absolute values of prestress are calculated to be sufficient to keep all parts of the membrane in tension under any load case.

 

     Any imposed live load will be carried by redistributing the stresses within the membrane. If this results in any section going into compression, ie going slack, then creases will appear.

 

     Similarly if the pre-tension is not high enough a snow load could cause ponding.

 
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2.Materials
 
Construction
 

     A coated structural fabric consists of a woven base cloth stabilised and protected by a coating on both sides. The base cloth consists of warp threads running the length of the roll and weft threads running across the width.

 

     A mesh fabric is a coated cloth with a spacing between the thread bundles. With some meshes for interiors use the threads are coated before weaving.

 
Strength
 

     A typical structural fabric would have a tensile strength of 10 tonnes in the warp and weft direction. A factor of safety of 6 on maximum design loads is used to select a cloth although this may be reduced if the circumstances are well understood.

 

     ie. If the Maximum Strength of the membrane is 10 tonnes/linear metre the Maximum Permissible Load would be 1.7 tonnes/metre, and the typical Prestress Load would be 150-350kg/metre.

 
Stretch
 

     All fabrics will stretch under load although some exhibit different characteristics as a function of time. A structural fabric would not creep under load once it has reached full pre-tension.

 

     Each roll of fabric is tested in a biaxial rig to measure the stretch in both thread directions at load ratios derived from the form generation computer model.

 

     These figures would then be used as compensation percentages to be factored into the patterning software. The canopy is manufactured undersize so that when installed to its final dimensions it tensions out correctly.

 
External Fabrics
 
Coated Fabrics
 

     For external use there are two main choices - PVC (Poly Vinyl Chloride) coated polyester cloth and PTFE(Poly Tetra Fluoro Ethylene) [Teflon] coated glass cloth.

 

     The PVC coating contains additives that include UV stabilisers, fire retardants,colouring and antifungicides.

 

     There is a choice of protective PVDF(Fluorinated Polmer) lacquers that enhance the cleanability of the pvc membrane. With the Non weldable PVDF version we remove the lacquer before welding the seams. It will give a 15-20 year lifespan compared to 10-15 years for the Weldable PVDF type.

 

     Although a PVC/polyester fabric will have a structural lifespan in excess of 20 years it’s quoted lifespan is based on visual appearance. Plasticisers in the PVC will migrate towards the surface over a period of time making the surface harder to clean.

 

     The French fabric supplier Serge Ferrari will coat the fabric whilst keeping the warp and weft threads in tension known as the Precontraint method. This will result in more even stretch characteristics in both thread directions than a conventional coated fabric.

 

     The components of PTFE/glass are inert and are therefore the natural choice for permanent structures with a design life over 15 years. When new PTFE is a buff colour that will bleach white in strong sunlight in a matter of weeks. Weld discolourations will also disappear in a similar period.The anticipated lifespan of the membrane is 25 - 30 years.

 

     Meshes are available in both PVC/polyester and PTFE/glass. They are essentially shading fabrics but a version of the PTFE/glass mesh is available with a clear laminate on both sides giving a weatherproof fabric with a translucency of 50%.

 

     Other external fabrics are Tedlar coated polyester and Silicone coated glass both of which are currently not as technically reliable as those above.

 

     Etfe foil is not a coated woven cloth and is not covered in these notes..

 
Uncoated Fabrics
 

     Natural canvas is used where texture is important but is less stable than synthetics and is difficult to clean. A compromise is to use a fire rated modified acrylic canvas that has a similar texture but it is more dimensionally stable. Neither is suitable for large span canopies.

 
Comparison of fabrics suitable for external use
 
    PVC coated Polyester   PTFE coated glass
Life expectancy ** 10-15 years for weldable PVDF Laquers
15-20 years for non-weldable PVDF
lacquers
*** 25-30 years
Cost *** Very economical * 4-5 times the price of pvc off the roll (approximately half as much again as a pvc structure including the steel frame)
Strength *** Variety of weights/strengths available *** Variety of weights/strengths available
Cleanability ** Non-weldable stays cleaner than weldable *** Excellent at shedding dirt
Handling *** Easy to fold and handle * Handling procedures critical to avoid permanent damage
Manufacture *** Fast to assemble with High Frequency welders ** Slower to weld using hot plate welders
Translucency ** 7-18% depending on fabric weight. Blackout fabrics also available ** As PVC
Reflectivity ** Approx 75% ** Approx 75%
Fire retardancy ** Fire rated to BS 5867. Will not support combustion and will not produce flaming droplets *** BS 476 Part 7 Class 1 and Part 6 Class 0
UV resistance ** UV stabilizers limit deterioration *** Unaffected by UV radiation
Sound resistance * Virtually transparent to low frequency sound.
Effective as a mesh in front of acoustic quilt in reducing reverberation times
* As PVC
Heat conductivity * U value similar to single skin glazing at 4.6 W
Twin skin with an air gap of 200mm gives 2.6 W
* As PVC
Colours *** Wide range available ex. Stock for 500gm/m2 fabrics and in heavier structures fabrics with an area over 1800-3600 m2 * Usually white with limited colours available over 2000m2
Metalised fabric also available
Mesh versions *** Range of open mesh fabrics available *** Also mesh version with clear FEP film laminated both sides
(translucency 50%)
Erection *** Can be tensioned in one operation * Slower to tension and may need a subsequent retensioning
Appearance on erection *** As normal * Buff colour fabric and weld discolourations with bleach white
After 3 weeks in strong sunlight and longer on the underside
Repair ** Can be patched ** Can be patched
Relocateable *** Ideal for temporary/seasonal structures * Only re-erectable with great care
Environmental * PVC cheap raw product of oil industry. Careful manufacture avoids release of harmful byproducts eg dioxins.. Recycling becoming available with VINYLOOP process that converts fabric back to polyester strand and vinyl pellets * Fumes given off at 300degC can cause flu symptoms.
Fumes produced in laboratory conditions above 400degC are toxic. Disposal - incinerated in granule form or buried as landfill.
 
Internal Fabrics
 

     For Interiors there are three main fabrics.

 

     Cottons are the most economical and are available in a wide range of colours. Due to their susceptibility to staining and shrinkage they are ideally suited for short term use or where a softer and more natural texture is required.

 

     PVC coated glass mesh is very durable and acts like a theatre gauze or sunscreen.

 

     Polyurethene coated glass cloth which has the benefits of durability and a similar appearance to cotton.

 

     Silicone coated glass cloth is being used for its high fire resistance and low relative fire toxicity but tends to attract dirt.

 

     All these fabrics meet BS476 Part 7 class 1 and Part 6 class 0 which is a normal requirement for internal finishes. In some instances other fabrics with a lower fire rating such pvc/polyester, CS trevira and cotton lycra and silk have been approved.

 

     Pure glass cloth meshes can be used in exhibition halls when fire standards are very stringent. Some ceiling systems demand open meshes that allow water sprinkler systems to operate through them.

 

     Unfortunately other exciting fabrics such as rip stop nylons and mylars do not achieve an adequate fire rating.

 
Comparison of fabrics suitable for internal use
 
    PVC coated glass mesh   Cotton   Polyurethane coated
Life expectancy *** 10-15 years * 5-10 years ** 10 years
Cost ** 3 x cotton *** Very economical ** 3 x cotton
Strength *** 3 weights/strengths *** Wide range available ** Single weight
Cleanability ** Wipes clean * Best vacuumed
Shrinks if washed
Can stain
*** Easiest to wipe clean
Handling *** Simple ** Care required *** Simple
Manufacture ** Welded seams ** Stitched seams ** Stitched seams
Translucency ** From 10-40% ** From 10-30-% ** As PVC
Reflectivity ** Max 65% ** Max 65% ** Max 65%
Fire retardancy ***   *** Ditto *** Ditto
UV resistance *** UV stabilizers limit deterioration ** Some dyes may fade and fabric weaken * UV radiation may discolour PU coatings
Sound resistance * Virtually transparent to low frequency sound.
Effective in front of acoustic quilt in reducing reverberation times
* Ditto * Ditto
Colours ** Limited colour range *** Wide range available ex. Stock * Usually white
Graphics ** Ink jet and silkscreen *** Ink jet, vinyl and silkscreen *** Ink jet, vinyls and silkscreen
Environmental * See notes for PVC on external fabrics *** Biodegradable and recyclable ** Ideal as alternative for PVC
 
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3.Performance
 
Wind
 

     A frequently asked question is if ‘tensiles’ are suitable for windy sites. The answer is yes as long as the canopy is properly engineered. In the computer analysis of the different load cases wind uplift is usually as great as the live snow load. A factor of safety between 4 and 6 is then used to select the fabric weight. The detailing of the fittings and surrounding structure needs to take into account the maximum deflections of the membrane. Boundary details need to accommodate the oscillations that may be generated at the canopy extremities.

 

     Our conical canopy in Mauritius (see photo library on web site) is 32m in diameter and is designed for and has regularly experienced 150mph winds. In these extreme circumstances an annual retention/maintenance check is recommended.

 

     Designing for heavy snow loads requires more care as you have a greater risk of melt water ponding. The profiles would generally need to be steeper and spans smaller.

 
Sun
 

     PVC fabrics incorporate UV stabilizers which protect colour fastness and base cloth slowing the rate of degradation, however in high UV areas lifespan will be reduced. After 20 years the PVC will lose its flexibility and will become more brittle.

 

     In areas of high humidity regular cleaning will reduce the risk of mould growth on the surface of the fabric causing permanent staining.

 

     For a design life of over 10 years in areas of high UV, pollution or humidity, PTFE/glass becomes a better option.

 
Fire
 

     The fire performance of a membrane depends on the base cloth and the seam details. All membranes will de-tension under high temperatures. The speed of this process depends on the temperature and the pre-tension in the membrane.

 

     PVC/polyester will creep at around 70-80 deg C and seams will start peeling apart at around 100 degrees. At 250 deg C the PVC membrane will melt back from the heat source creating vent holes for heat and smoke. PVC has fire retardants in the coating so that it self extinguishes when the flame source is removed and therefore would not produce flaming droplets. With PTFE fabric the glass base cloth withstands temperatures up to 1000 deg C and the openings are limited to failed seams which would part at approx 270 deg C.

 

    The net effect in a fire can be quite beneficial as most canopy designs form a smoke reservoir which may well allow sufficient time for escape, and when sufficiently hot self venting will occur through a failed seam.

 

     Critical steelwork should be supported so that partial failure of a damaged roof will not cause collapse of the structure.

 

     The design should consider smoke generated by the membrane used. PTFE fabric used internally may require sprinkler systems or mechanical extraction to reduce toxic fume production at temperatures over 400 degC.

 
Thermal Insulation
 

     A single layer of either PVC/polyester or PTFE/glass with a typical weight of around 1200gm/m2 has a U Value of approx 4.5 W/m2K. In this respect it is very similar to glass so that a twin skin with a 200mm air gap will give a U Value of 2.6 W/m2K

 

     By suspending a quilt in the air gap you can get down to a U value to meet any building code requirements, but you obviously lose the some of the benefits of translucency.

 
Condensation
 

     As you would expect in some cold weather conditions condensation is likely to occur with roofs covering a sealed heated space. The design of the roof gradients and edge detailing can minimise any problems. Ventilation can obviously reduce the risk but if more control is required then it would be necessary to incorporate a second skin and possibly additional thermal quilt.

 

     Control of the air flow in the air gap is recommended to get the best environmental control. A sealed air gap is best in winter for insulation and a good air flow in the summer will help cooling. The design of roofs especially conic forms can make use of the passive stack effect ventilation with fans or louvers used to enhance the performance if required.

 
Acoustics
 

     A single fabric membrane is virtually transparent to low frequency sound due to its low mass. A double skin with an acoustic quilt interlayer will give you the absorption figures required.

 

     Reverberation times on the other hand can be very successfully reduced with tensile fabric linings with acoustic quilt behind either wall mounted or hung on drop wires.

 
Vandalism
 

     Unlike glass or brittle panels fabric is highly resistant to impact damage from blunt objects. It is however susceptible to sharp objects.

 

     Small cuts can be repaired with glue-on patches. Larger tears may need specialist repair with portable hot air welders. If an invisible repair is reqired then the membrane may need to be removed and a replacement panel inserted in the factory.

 

     Graffiti solvents may damage the PVC laquers so should be avoided. PTFE fabrics are highly resistant to abuse as paints won’t key to the surface.

 

     The sensible solution is to design out the problem as much as possible by putting the fabric out of reach and detail the masts accordingly to minimise the risk of climbing.

 

     In vulnerable areas a modular canopy with slide out panels may be a sensible precaution to minimise replacement costs. Some structures such as public bus stops in very exposed sites are probably not ideally suited for membranes.

 

     Advantages over overhead glass canopies are that thrown objects tend to bounce off a fabric canopy and the Health and Safety issue of falling glass is largely resolved.

 
Cleaning
 

     For the smaller structures this is done with long handled brushes and soapy water if necessary from cherry pickers or tower scaffolds. With the larger membranes trained rope access riggers abseil from high level anchorage points or traverse across the membrane with a ‘Latchway’ or similar restraint system using brushes and water filled back packs or pressure washers.

 

     Every structure has its own maintenance manual describing fixing points and cleaning procedures. Ideally canopies should be cleaned annually but PTFE/glass fabric would be the preferred option if cleaning is unlikely or impractical. This is because it’s inherent ‘non-stick’ surface resists pollutant adhesion and allows rain to clean off most dirt.

 

     Raw PVC is readily adhered to by pollutants so all membranes are treated with dirt resistant lacquers or surface foils. Careful cleaning maintains their life and the optimum appearance of the membrane.

 
Warranties
 

     The fabric manufacturer can issue a 5 year up to a 10 year warranty covering the structural strength and integrity of the fabric. The specialist subcontractor can be asked for a collateral design warranty.

 

     Architen Landrell are introducing an extended warranty of the complete structure in conjunction with a cleaning and maintenance contract.

 

     We recommend that some structures have annual inspections to ensure the ongoing integrity of each critical component.

 
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4.Detailing
 
Load resolution
 

     Canopies fall into two main types, those that transfer tensile loads into adjoining structures and those containing the tensile loads within their own frame.

 

     The first type may generate large lateral loads which may result in the need for additional reinforcement in existing structures.

 

     Likewise a typical ‘lightweight’ canopy with masts and cable tie backs to ground level will generally need large concrete foundations or screw anchors to resist the tensile loads. As part of the preliminary design process a provisional load analysis derived from a computer model will give typical loads directions and size of the design loads.

 
Boundary Detail
 

     The boundary of the membrane falls into two categories;

 

     Curved/scalloped edge - This generally consists of a cable slid through a pocket in edge of the membrane. In larger canopies webbing belts are added parallel to the edge to take out the shear loads. An alternative detail used for PTFE canopies is to have an exposed cable connected to the clamped edge of the membrane by series of stainless steel link plates.

 

     Straight edge - The membrane would have a bead/kedar edge formed by sealing a flexible pvc rod in a small pocket. This can then be trapped behind an aluminium clamp plate bolted directly onto the structural steel work or slid into an aluminium luff track extrusion.

 

     Canopies can be tensioned by hydraulically jacking up the mast with the base being housed in a sand pot - or the mast can be extended with a telescopic section.

 

     Corners can be pulled out with rigging screws, U bolts or by the shortening the perimeter mast tie back cables.

 

     Individual scallops can be tensioned by shortening the edge cable where the swaged studs connect onto the membrane plate.

 

     A very common detail is to pull panels into parallel lufftracks and tension by drawing out the corner plate that slides inside the luff track.

 
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5.Costs
 

     As a very rough rule of thumb fabric structures are nearly always cheaper than planar glazing and more expensive than polycarbonate solutions.

 

     They really come into their own in large span structures such as the Millennium Dome where the ratio of fabric to steel results in considerable savings over more conventional structures.

 

     For smaller structures under 100m2 they can be relatively design intensive, however there are often pre-engineered solutions which can fit a tight budget.

 
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