Typical Details
This article is to give architects, contractors and potential customers an idea of the sort of details used in typical fabric structures. It’s by no means conclusive, or fully comprehensive, but since we often find ourselves providing this sort of information, we hope it will be useful to anyone interested in this field.
Trying to list things logically, we will start with the bases, then consider the main structure, followed by some of the typical fabric and connection details, and finally we’ll cover the “rigging” hardware.
Foundations
Normally cast by the main contractor. We will have provided loadings so they know how big to make them. It’s generally preferable if they cast holding down bolts in for us, as they have exceptional holding capacity, and incorporate some tolerance for improved ease of installation. In other cases, we can drill the concrete ourselves and fit chemical anchors, but these don’t have the capacity to match holding down bolts.
Base Plates
These often require triangular stiffening plates. It’s important to know where finished ground level will be and what will be the make up of that level (eg bricks/paviors/slabs/tarmac). You may need to lower the foundations to make the installation of the finished surfaces easier.
Double Shear Connection
This is the strongest way to join a spar to a column with a pinned/bolted connection. The size of the pin/bolt determines the overall size of the connecting plates. If you were to use just two single plates, the pin/bolt would be in single shear and would need to be substantially larger.
Spigot Connection
A good way to join two tubes at full strength for a moment connection. An advantage is that internal drainage or cabling can be arranged through the connection.
Halving Joint
Another method to form a moment connection between two members in line with each other. This has the advantage of being installable in locations where a spigot connection would be impossible to fit.
Cleats or Lugs
These are designed and sized according to the loads they are subject to. Some may be subject to tension, some to compression, and some to both, so all structural specifications for the cleat size and amount of weld must be strictly adhered to during fabrication.
Telescopic Strut
These are usually used as either the main tensioning device, or to build in some tolerance to the whole structural design. The steelwork is accurately fabricated to enable correct installation without sticking so that tensioning and adjustment can be easily carried out on site.
Headring
Most conical shaped membranes are attached to a steel fabrication at the top called a headring.
As with all other components, it is sized to accommodate the high loads imposed on it by the fabric. The fabric is normally clamped to the outside face.
All sorts of designs and features can be incorporated into the details; in particular the use of a clear covering dome is ideal when full rain protection is required.
Boundary Pocket
This is the pocket formed on a curved scalloped edge of a membrane to house the boundary cable. They can be formed by either turning the fabric back on itself, or by adding a premade pocket strip.
Roped Edge
This could be the key to tensioned fabric. By turning the fabric back on itself, and welding in a rope, a perimeter detail is formed that can either be clamped or slid into a slot....
Extrusion
What a great invention for fabric, aluminium extrusions.....All sorts abound, but some standard ones prove the most versatile. They have calculable loading capacities so that structural engineers can use them confidently for high fabric tensions. Easily fitted on site, they can be powder coated to match the supporting steelwork. They generally need to be fixed at short intervals to a main structural element, such as a column, beam or strut.
Membrane Plate
The membrane plate is a vital component of nearly all tensile structures. It serves two main purposes: firstly, to clamp onto the corner of the fabric to enable the fabric to be pulled tight, and secondly, to hold the ends of the boundary cables that run in the fabric edge pockets. All of these combined loadings are effectively transferred through the membrane plate into the supporting steel structure.
Membrane plates can also act as drainage points, in conjunction with upstands on the fabric.
Extrusion Membrane Plate
The extrusion membrane plate serves the same purposes as a normal “free” membrane plate, but it is designed to fit into an extrusion to enable a sliding fit for tensioning. Smaller canopies use very simple plate details whereas larger canopies are likely to need additional supporting structure to prevent the membrane plate from pulling out of the extrusion.
Rigging Hardware
We use many traditional sailing type fittings in our canopies. They are all grade 316 stainless steel for outstanding durability and maximum resistance to corrosion.
Cables are typically terminated with either a threaded stud, or a fork end. The jointing process for stainless steel cables is known as swaging, which produces a very slimline and high strength connection that will exceed the breaking strain of the cable.
The swaging process involves fitting the cable into the terminal, then crushing the terminal onto the cable with a specialised swaging machine. All fittings are sized to ensure an equal or greater strength than the cable.
Turnbuckle tensioners (often known as bottle screws) are frequently used to adjust tie cables to the required length. They are available in a very large range of designs and sizes to suit all standard cable sizes. The highest quality tensioners incorporate brass elements into the threaded parts to avoid seizing.
They should be fitted with suitable locking nuts to ensure that they don’t become loose over time.
We hope that you have found this reference guide to the components of a tensile structure informative and useful. If it has left you with any of your own unanswered questions, we’d be happy to hear from you.
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