As anyone with knowledge of the history of tension structures will tell you, there is a logical and long-standing connection between marine hardware and tension structure hardware. One came from the other. Or more accurately, one (tension structure hardware) is an adaptation of the other (marine hardware). And just as the extreme environmental conditions of marine applications of turnbuckles, toggles, clamps, cables and shackles can dictate the details or choice of materials (stainless steel vs. galvanized?), the same issues hold for land-bound applications of hardware for tensioned shade structures.
“The galvanized (or painted) hardware used on fabric structures is generally adapted from industrial rigging hardware, including swaged and speltered cable ends, clevises, chain and anchor shackles, eye nuts, swaged sleeves, wire rope clips, turnbuckles and so forth,” says Craig G. Huntington, SE, F.ASCE, president of Huntington Design Associates Inc., Oakland, Calif., a consulting structural engineer who specializes in designing for tensioned fabric structures. “The most commonly specified supplier is Crosby, but the geometry and material properties of this hardware are pretty well standardized, such that they can be purchased from any number of sources. The clever and nice-looking stuff is pretty much all stainless (steel).”
Starting from the ground up, there are numerous considerations to account for in selecting the proper fittings and anchors for any shade structure. These all depend on a few key factors that will affect the choices: permanent vs. temporary or deployable; location and climate; standardized design vs. custom fabrication. The choice of hardware would also be different if a project were located within a coastal area (where corrosive salt water is prominent), forcing a choice of stainless steel fittings, versus inland where normal weather conditions would not require extra protection, and either a galvanized or powder-coated finish would be adequate.
“Both a small shade structure and a large stadium roof are [fabric] structures,” says Huntington, “and their form and details properly reflect structural considerations. The form of the membrane in a shade structure is (like that of a soap bubble) largely a function of the equilibrium shape of the membrane under prestress forces—it cannot be manipulated to suit architectural wishes in the manner of a ‘conventional’ structure with relatively deep members that can resist loads in bending.”
Temporary structures (up for fewer than 180 days) and permanent structures have distinct needs. When it comes to anchors, there are several choices, from screw-in augers (for temporary structures) to poured-in-place foundations with piles (permanent installations). But some foundations can be as simple as helical anchors placed strategically. “I’ve always been a big fan of augers,” says Samuel Armijos, AIA, architect and principal of Fabric Architect LLC, Fairfield, N.J. “But I don’t get to use them often because they are frequently misunderstood. They can be used for small canopies, clearspans and tensile structures, and can work both in tension and compression with minimal disturbance of soil.”
For temporary structures, up for 180 days or less, a system of rope and stakes is one way to go, says Mark Welander, MFC, owner of Fabricon, Missoula, Mont. “Or footings with tie-backs of cable, rods or columns, and these can be adjustable or fixed.”
A key factor in all cases is correctly estimating the angles of entry of the augers into the soil and the depth of anchorage based on the actual conditions of the soil, shear forces that will be in play, and scale and weight of the structure resting on the foundations.
The right connections
As 20th-century architect Ludwig Mies van der Rohe said about making good architecture, “God is ever in the details.” This can be especially true for tensioned shade structures, as the entire assembly—fabric, tension cables, compression masts or struts—is dependent on all elements working together and in equilibrium to withstand natural forces and external elements. Clevis ends, pin ends, eye ends, shackles, plates—the list of unique connecting elements can sound like an arcane language to the uninitiated. Each has its purpose and appropriate application.
Not all hardware or connection methods can be used on all shade structures, and some are inappropriate. “Many shade sails use webbing reinforcing around the perimeter and at tri-ring attachments,” says Welander. “These can be suitable for temporary applications, but are not advisable for permanent ones, especially if snow loading is an issue. HDPE is also not as stable as PVC or PTFE, causing it to stretch or creep over time and loosen up.”
The range of connections, says Welander, can be as simple as grommets and rope along fabric edges (a method linking back to the history of awning construction) to sophisticated clamping plates with built-in adjustability, uniquely designed for a particular structure. “Edge tensioning is best suited to cables that can be adjusted [turnbuckles or threaded ends], or of a fixed length that fabric stretches into. Webbing can be used, but more as a reinforcement against failure than as a tensioning element.”
“For knitted fabrics, we use a simple D-ring and webbing to finish the corner of the sail to connect it to the adjustable hardware that goes to the column,” says Ryan Chism, estimator and project manager, The Chism Company, San Antonio, Texas. “For structural fabric like coated polyester or PVC, we use a custom set of plates at the corners to contain the fabric, exterior cables and to accept the fittings.”
In most cases, according to Chism, the type of fitting is not an issue. “The structural function requires a certain fabric with certain connections,” says Chism. “For example, with a hypar structure that has various elevations, you need corner plates made to a certain size that will flare the cables and enforce the shape that you want. If you just used a D-ring, you’d have no way to control the cables. In a covered parking application, we use a standard awning termination bar or a slot tube to contain the fabric, and connect to the steel on two sides with a catenary cable fixture on the front and rear.”
In all cases, the equalization of tension/compression forces needs to be accommodated or the materials will suffer undue stress and risk failure. A prime concern should be allowing full swivel articulation at every connecting point that has not been precisely calculated, and there are many forces that need not be precisely calculated if the major structural elements are sized and attached correctly. “For rigging, I’m seeing a lot more webbing belts and chain hoists,” says Armijos, “in addition to wire rope and come-alongs. Installers have a better understanding of the loads and are also taking better care of the material finishes.”
Design to the finish
Galvanized, powder coated and stainless steel—all have been discussed as the three most common finishes chosen for shade structure hardware, and the relevant costs must be factored into the overall cost of a project. The ongoing trade-off is mostly between galvanized and stainless when considering cost. “Galvanizing is a protective coating of zinc applied by hot dipping steel into molten zinc to create a bonded finish that is resistant to scratching and wearing off,” says Welander, “and is more economical than stainless, which is why it is used in electrical towers, fencing and such where long wear resistance is required. Powder coating should also be noted because of the excellent and more aesthetic look it can give at a similar price. Stainless is generally my first choice for visible hardware because of its aesthetics, strength and greater rust resistance, but it is more expensive than galvanized.”
Armijos agrees: “Galvanized is safe, but stainless is always my first choice,” he says. “However, there are some nice galvanized fittings, and remember there is a scale of quality for both finishes.”
Stainless steel is a mix of chromium and nickel (the 300 series has more than 10 percent chromium; a common architectural grade is 316 with more than 14 percent) combined with carbon steel to create a metal that is highly resistant to corrosion and rust. However, says Welander, “One should recognize the threads are more subject to galling if appropriate measures aren’t taken to avoid this. And it also should be noted that either [galvanized or stainless] are subject to rust if installation, application and the proper care isn’t taken.”
Architects and fabricators: blind dates?
“Architects who do not have a good sense of membrane behavior,” says Huntington, “can choose shapes that cannot be achieved in fabric without adding cables or beams or other members to manipulate the shape. These added elements, of course, make the structure visually heavier and more expensive as well.”
“Working with an architect that understands fabrics, design processes and codes is always a preference,” agrees Welander. “Unfortunately, not many do when it comes to fabric. More often it involves educating the architect in the types of fabrics, design options available and structural forces with consequences that appear over time yet often are overlooked for economic reasons. Too often, architects come to us late in the process with a design that is not feasible either for economic or practical reasons, and it gets scrapped or becomes economically inefficient because they can’t back out of a design they have already invested in.”
“More and more we see very conceptual drawings in the plans,” says Chism, “with notes reading ‘Fabric structure, engineered by manufacturer.’ In a situation like that, once we have the award, we’ll submit engineered shop drawings matching the overall concept in the plans. What follows is usually a brief back-and-forth regarding finish appearance and placement of the hardware that we detail.”
“Whether the shade structure designed is conceived by the architect, the engineer or the fabricator,” says Huntington, “it must reflect some understanding of how membranes carry load.” Frequently the design brought to a fabricator will include several three-pointed pieces of fabric because these produce pleasing shapes that go well with many architectural styles. However, going back to basic geometry, and knowing that by definition three points in space produce a flat plane, understand that no amount of stretching
a piece of fabric will produce curvature in that plane. This is the reason most in the industry call these types of shade structures “sails.” Like sails on a sailboat, a three-pointed (cornered) sheet of fabric
is meant to catch wind and pull strongly at its attachments, moving a boat forward with wind power.
Unfortunately, unlike sailboat sails, shade sails are not meant to pull out of their attachments and need to include some sort of detaching mechanism if wind gusts reach a certain high level that could cause failure in materials. The simplest way of minimizing this problem is to introduce a fourth corner in the fabric and raise two opposite ends of the fabric plane to introduce double curvature. This reduces stresses on connections and produces a safe, fixed form that endures more readily than tri-corner sheets.
“So, shade structures don’t necessarily require consulting by a structural engineer,” says Huntington, “but they need to be both conceived and detailed with a good understanding of their structural behavior. Architects by themselves are generally not capable of this. Sophisticated fabricators, working with them, can fill in this gap, and the owner and architect should take this into consideration in choosing a fabricator.”
Structural engineer Craig G. Huntington, president of Huntington Design Associates Inc., describes the challenge of designing and sizing the elements that make up a shade structure: “Shade structures are generally intended to be economical structures, and, as such, it’s often difficult to include a proper membrane analysis in their budget. There is a common expedient taken by designers of small shade structures I have seen which results in extremely unconservative assumptions about the loads that act on the supports. Consider, for example, an uncurved, three-sided canopy supported by a post at each of its three corners. Let’s say it has a plan area of 300 square feet, and wind acting up or down against its surface of 20 pounds per square foot. This suggests a total vertical load of 300 x 20 = 6,000 pounds, which is then assumed to distribute equally to the three corners, or 2,000 to each post. So far, these assumptions are reasonable.
“But then these designers go astray by assuming that the horizontal load acting at the top of each of the three posts is equal to this vertical load; that is, 2,000 pounds acting horizontally at the top of each post. Remember, though, that the fabric (and cables at the edge of the fabric) are pulling in a nearly horizontal direction against the top of the post. The loading on the tops of the posts is directly in line with these cables, and the 2,000-pound vertical load at the post tops is therefore associated with a much larger (typically 3-5 times) horizontal load at the top of the post. Taking this simple approach, the designer is grossly underestimating the horizontal load at the top of the post, and seriously undersizing his posts or any tieback cables.”
Huntington suggests that shade structures don’t necessarily require a structural engineering consultant, but they do need to be conceived and detailed with a good understanding of their structural behavior.