Tensile Fabric Structures
We hope that the following page will answer some common, initial questions you might have about the feasibility of a tensile fabric solution you are considering. If you have more detailed questions or would like to arrange a CPD presentation on the technical side of fabric architecture pleased don’t hesitate to get in touch with us here.
What shapes can be achieved?
Nearly all tensile canopies are derivatives of three basic shapes. This is because they rely on double curvature of the fabric to create strength. These 3 dimensional shapes are necessary because fabric in a 2 dimensional (flat) form can’t take significant perpendicular forces for example from wind and snow. By tensioning fabric into a doubly curved shape it gains sufficient strength to withstand loads and create a stable structure that will have a design life of 20+ years using modern architectural fabrics.
The 3 basic shapes or forms are:
- Conical A cone shape, characterized by a central peak
- Barrel Vault An arched shape, usually characterized by a curved arch design
- Hypar A twisted freeform shape
Every canopy uses the curves and principles of these three forms and they may be combined on a single canopy.
What about load resolution?
The tensile load of a Tensile Fabric Canopy can be transferred either to adjoining structures or to the canopy’s integral frame. The first type may generate large loads which may result in the need for additional reinforcement. The second type, or more typical lightweight canopy will usually need masts and cable tie backs to ground level plus concrete foundations with screw anchors to resist the loads. The most significant load being the overturning moment caused be wind under the canopy.
For either structure type we would undertake a load analysis early on in the design process, using a computer form finding model, so that all of the necessary loads, directions of loads and structural requirements were understood before progressing to final design.
What is the strength of a tensile fabric structure?
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.
I.e. 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.
What if the proposed site for the Tensile Fabric Structure is prone to high winds?
A correctly engineered tensile fabric structure would be entirely feasible. We would simply add the predicted wind loads into the computer analysis to calculate the fabric weight, detail of fittings and size of attaching structure required to withstand the predicted deflections. A safety factor would then be added.
Designing for high snow loads is slightly more complex because the ponding of melt water must be considered. This usually results in a design with a steeper profile and smaller spans to allow the melt to slide off.
Will a tensile fabric structure meet fire regulations?
The fire performance of a tensile fabric structure depends on the membrane fabric chosen since different architectural fabrics have different fire properties. All fabrics have fire ratings under British Standards and EU rating classifications.
The coatings of PVC fabric contain fire retardants so that it self extinguishes when the flame source is removed and does not produce flaming droplets. At 70-80 degrees C it will creep and seams will part at 100 degrees C. The membrane will melt back creating vent holes at 250 degrees C.
PTFE fabric has a glass base cloth which will withstand temperatures up to 1000 degC however seams would part at about 270 degree C.
What thermal insulation can I expect?
A single layer of either PVC/polyester or Ptfe/glass with a typical weight of around 1200gm/m’ 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.
What about 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 an 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 louvres used to enhance the performance if required.
Will damage be a problem?
Unlike glass or brittle panels fabric is highly resistant to impact damage from blunt objects and thrown objects tend to bounce off so that the Health and Safety issue of falling glass is largely resolved. Fabric 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 required then the membrane may need to be removed and a replacement panel inserted in the factory.
Graffiti solvents may damage the coatings on a PVC fabric so should be avoided however PTFE fabrics are highly resistant to abuse as paints won’t key to the surface.
We would always recommend that the problem of vandalism is designed out 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.
What budgets should I plan for?
As a very rough rule of thumb, fabric structures are nearly always lower cost than planar glazing and similar in cost to 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 100m’ tensile fabric structures can be relatively design intensive, which is why we recommend choosing a pre-engineered (‘off the shelf’) solution so that you do not have any design costs to account for. It is for this reason that we have developed our range of modular canopies which you can find here.
We hope that you have found this more technical information useful but please do call us to discuss your project whether you are at initial concept stage or are looking to value engineer an existing idea. We will be pleased to answer any questions and lend our 20+ years of experience to your project.
Call us on 02392 001 322.