How to Design a Fabric Structure

This article gives you the information you need to put together a scheme design for a tensile canopy structure. Hopefully it will guide your thought process and point out the most essential aspects that you need to consider in order to develop an appropriate solution.

Let’s assume you’ve established that you want a permanent tensile fabric canopy to cover an entrance area or courtyard. You want the translucency and long term durability of fabric and your client is keen to create a bold visual statement to enhance the area.

How would you proceed?
When it comes to bespoke tensile canopies, you need to consider all the factors below, then prioritise and amalgamate ideas in order to achieve the best concept design for your canopy.

  • Main support
  • Canopy shape
  • Scheme concept
  • Engineering
  • Detail design

First fix bracketWhat will be used to support the canopy?
Will it be fixed to walls or to ground, or both?
Are there any services in the ground, and are there any structural elements of the building that could be fixed to?
Bear in mind it’s unlikely that suitable support can be derived from attaching directly to brick or blockwork, as the high loads imposed during severe weather can cause gradual failure of the supporting structure.

Are there pedestrian routes that need to be kept clear, or are there planting areas that can be used for the ground connections?
With some idea of what may be used for the main support points, you can give some thought to the general shape of the canopy. There are three main canopy forms – the cone, the barrel vault, and the hypar, but they all rely on the same overall principal – that is “the steelwork provides a rigid structure against which you can tension the fabric.” That is a little simplistic, or perhaps rather too brief, but all canopy structures follow this same rule and the best thing about fabric structures is that within this simple rule unlimited possibilities abound.

Barrel vault form and close upHypar form and close upConic form and close up

One of the biggest factors that may affect your options is “what happens to the rainwater?”
Hopper and outlet designThe natural path of water across the surface of the canopy can take many routes depending on the curvature of the fabric. It’s relatively easy to use the shape of the canopy to direct water to preferred areas, but it’s also worth knowing that there are ways of diverting water away from edges where necessary, and of collecting water at corners to discharge into downpipes. In fact there are many possible ways to deal with rainwater, but you still need to give it some thought at this early stage.

As well as hurricane force winds, the canopy will be engineered to support a full snow load. You need to make sure there are no areas of the fabric where built up Hopper and outletareas of snow will fail to drain away. This is known as “ponding” and must be avoided in all cases. This is checked properly during the engineering process, but at this stage, the risk of ponding is something that can normally be eliminated,  

What are the maximum and minimum canopy heights? In general, fabric structures need to have double curvature across the form in order to maintain stability in high winds. This is typically achieved by having high and low points on the canopy. In fact, for a given span of a given type of canopy, the optimum height from a structural point of view can be fairly well guessed at. For example, a well designed cone structure say 10m across, may well have a height in the middle of say 3- 4m above the eaves. Effectively it’s a matter of keeping the canopy shape in proportion, and sometimes a cone or hypar canopy may stand too tall to be practical, so alternative ways to derive double curvature can be investigated.

As for a minimum height, it’s generally recommended to keep the canopy eaves at a little over most people’s reach, partly to prevent damage, and partly to prevent people from being tempted to climb on the fabric (not a problem for the fabric, just a fall risk for anyone who would try). However in certain locations, where there is little likelihood of interference, the canopy can be brought down lower.

With all the above aspects considered, you have most likely formed a clear picture of the kind of structure that is going to fulfil all your requirements. However, in order to complete the scheme design fully, you need a pretty good idea of how you plan to build and tension the canopy. The following elements are the final pieces of the jigsaw, each reliant on each other, and all critical to a successful installation on site.

  • Seam orientation
  • Steelwork connections
  • Fabrication details
  • Site installation procedure
  • Tensioning method
  • Access for maintenance

Throw these into the mixing pot and then you will really be able to create and develop canopy structures for all situations and in all types. More realistically though, you would be discussing the project with a specialist tensile structure contractor with whom you could develop the final details to you and your clients satisfaction.

Designing a “proper” fabric structure is challenging and satisfying, and whilst it is undeniably quite a tricky discipline, we’d love to know if the information in this article has been useful to you. We’d love to hear from you!

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