How hardware and fabric, combined in fabric structures, should be selected for good results.
By Bruce N. Wright, AIA
As any matchmaker will tell you, making the right connection can be the difference between a success that lasts, and a disaster that has long-lasting repercussions. This is especially true with tensile fabric structures where structure and fabric is often the same thing. Unlike more traditional construction systems of frame and cladding, with tensile fabric systems the fabric is the structure and cladding all in one. With that in mind, how should a designer match a fabric with a specific hardware, or does it matter? Climate and weather can play a big hand in this matchmaking as well.
More than anything with fabric architecture, the choice of material will determine the type of detail needed. Should the structure use glass fiber or polyester, mesh or solid membrane, and what hardware goes with what? There are at least two schools of thought on this question.
“Good detailing requires first looking at the material that you want to use and understanding the application and budget of the project,” says Nicholas Goldsmith, FAIA, LEED AP, senior principal of FTL Design Engineering Studio, New York, N.Y. “Yes, there are details designed for particular fabrics. For example, PTFE glass is very brittle and normally requires clamping and strap details, however if the catenaries [of the fabric edges] are wide and flat, a cable sleeve can be used. PVC polyester is more forgiving and normally is used with a cable sleeve. However the fabric slippage must be contained and can be done with membrane plates or web belts depending on the look of the detailing (sewn vs. clamping). The ePTFE fabrics can be detailed with both these approaches,” says Goldsmith.
“Flexible fabrics like HDPE [knitted mesh] require different detailing than rigid materials like PVC,” says Charles Duvall, principal of Duvall Design, Rockland, Maine, who most often creates sculptural shade structures in mesh fabrics. “With HDPE knitted fabric, I normally sew 2-inch webbing (folded in half) around the perimeter. Folding it allows it to go around the curves smoothly without buckling.”
Not everyone agrees entirely with the material-based approach. “I would say that application has more to do with the situation vs. fabric type,” counters Mark Welander, MFC of Fabricon LLC, Missoula, Mont. “Clamping plates are more applicable to point connections. We use either fixed or adjustable keder track depending on the application, not the fabric type. Other than systems used with ETFE, the above connections are interchangeable between fabric types,” Welander says. [See end of article for glossary of terms unique to fabric structures.]
Welander makes a good point in regard to the relatively new material ETFE, a transparent structural membrane that has seen a lot of use by European designers over the past five years, most notably in some high-profile projects in Spain and the UK. ETFE (ethylene tetrafluoroethylene) behaves more like a balloon when applied to architecture. As with a rope, it is strongest in tension. To ensure constant and equalized tension across the surface of the membrane, sheets of ETFE are clamped at all edges and given curvature in single, double or triple layers either through a framed single layer, or inflated pillows when used with double or triple layers. Unlike the structural fabric mentioned above, ETFE cannot sustain point loads so pin connections with cable edges do not work. Instead, the membrane works best with a framing system, somewhat like curtainwall extrusions with gaskets, and continuous edge clamps coupled with a self-tuning system of air inflation that constantly adjusts air pressure and tension.
Reduce, reduce, reduce
Another approach to matching fabric with hardware is to make the details as simple as possible with the goal of gaining repeatable results. “I typically make HDPE freeform structures,” Duvall says, “and try and keep the details very simple. So the only hardware I use are stainless steel welded rings at each point captured by the perimeter webbing that crosses at the point. A ring with 3/8-inch rod is useful as it makes a soft turn for the webbing. A 2.5-inch OD ring accepts the edge webbing nicely before it returns down each edge. I use extra-large stainless steel barrel type turnbuckles and the 3/8-rod fits into the clevis of the turnbuckle.”
Duvall prefers hardware that is a bit oversized. “An oversized turnbuckle will have less chance of binding or seizing under load, for example. An oversized ring is stronger and unlikely to deform under load, and less likely to cut through the edge webbing. An analogy is holding a ring inside one’s hand and choosing the size that fits most comfortably. All the various materials—fabric, webbing and stainless steel hardware—have to somehow work together nicely. When the webbing is turned or folded, a radius is less sharp and results in a stronger and safer connection.”
As you might expect, the size of a structure determines the size of the hardware and connections. “When a cable pocket isn’t strong enough for larger structures, we clamp the edge and mechanically fix it to the cable boundary,” says Jason Smith, business development manager with tensile structure fabricators Architen Landrell, Chepstow, Monmouthshire, U.K. “This is because as the load increases, the edge conditions change and the fabric can no longer take the load.”
For small-scale structures, the requirements come down to the choice of fabric when it comes to selecting hardware. “I would say requirements are based upon the specific fabric used,” Welander says. “Engineered fabrics, those tested for long-term stability in pre-stressed shapes, as opposed to the less stable fabrics like HDPE knit mesh, will dictate sizing. Often times, smaller structures require hardware as strong or sized similar to a large structure due to the pre-stress in the fabric. It can take the same forces to ‘dial in’ the loads required of the fabric, whether large or small.”
Hardware is available in various sizes, and larger structures require proportionally larger hardware, Duvall says. “However, I prefer a minimum threaded rod of 3/8 inch because of the amount of pretensioning required, even on small structures. The threads of the turnbuckle get worn out from adjusting under tension.”
“Besides the type of fabric, the cost of the project is reflected in the detailing,” Goldsmith says. “A top quality architectural project may use stainless steel machined details, whereas a more industrial project may use galvanized detailing or even belts and D rings. The key is to know the right detail for the right use.”
Bruce N. Wright, AIA, is an architect, design journalist and the former editor of Fabric Architecture magazine.
Glossary of terminology
Clevis = A two-pronged forked connection point at the end of a cable. Can be fixed—by means of mechanical crimping or molten metal spelter—or swiveled.
Come-along = A hand-operated ratcheted winch used to firm up cables and fabric installations until they are put into final position. Used for pulling connections and joints together so that more permanent pins or connections can be installed.
EPDM = Ethylene propylene diene monomer, a class M synthetic rubber, with a wide range of applications in construction.
HDPE = High Density Polyethylene, a popular knitted fabric type used for shading and in climates where high UV factors are critical.
Keder = Structural webbing used to transfer loads along the catenary curved edges of fabric roofs/canopies.
PTFE-coated glass fiber = Polytetrafluoroethylene (better known as Teflon®). Lasts 30+ years; light transmission: 11–14 percent.
ePTFE = expanded PTFE fabric. ePTFE does not need a protective coating unless used in harsh environments, where a surface coating such as TiO2 might be applied to combat pollution. Lasts 30+ years; light transmission: 38 percent.
PVC = Most often refers to PVC-coated (or PVC-laminated) polyester fabrics. Lasts 20 to 25 years; light transmission: 4–10 percent.
Rigging = All cables, hardware, masts and connections (in ensemble) used to support fabric structures. The terminology is adapted from sailboat technology.
Silicon-coated glass fiber = Lasts 30+ years; light transmission: 38–42 percent.