|
|
 |
|
| Products & Markets |
|
|
See our affiliated websites
|
|
|
Construction Method
The design has been approved and the engineering calculations completed. Material has been ordered and is on site. The final task before inspection and being placed in service is construction of the anchor system (footings and foundations) and connections of the various components into an integrated whole. If constructed properly and according to plan, the tension cable structure will be able to provide years of maintenance free protection.
Footings
The type of footing to be employed for a shade canopy depends upon the engineering requirements that the structure is designed to meet. For example, a four post dome structure may require one type while a double cantilever will require another. Failure to use the proper footing will increase the likelihood that the structure will fail in the event of adverse, though expected conditions. This is another reason that each project should be reviewed by a competent, professional registered structural engineer.
Shade companies may employ epoxy anchor systems where steel bolts are driven into an existing concrete slab and fitted to a base plate or cast-in-place footings or cast-in-place footings with piers set in reinforced concrete to a depths of 9 to 12 feet. All concrete work should be in strict accordance with the latest American Concrete Institute Building Code (ACX II 318-89) with 28 day strength of 3,000 psi.
|
|
Epoxy Anchor
Cast in Place Footings
|
|
Connections
Metal to Metal
Whether prefabricated or built to specifications, a shade canopy is a unique combination of polyethylene fabric, steel, and steel components. The canopy usually includes several pieces of fabric that must be sewn together and sewn to the perimeter cable. Various steel members must be connected to form a frame. The connections between the various components are an integral element of structural integrity. If the connections are weak, the structure will fail.
Shade companies that employ prefabricated members usually rely upon a series of nuts, spacers, and bolts to connect pieces of the steel frame together in the field. In some cases, they utilize different sizes of tubing where the end of one piece is inserted into a second tube and secured with a shear pin. Other companies weld 100% of all steel connections relying upon experienced, certified Master welders in accordance with the latest specifications of the American Welding Society.
|
|
All Welded Connections
Nuts, Bolts & Spacers |
|
Use of Cable for Security
Steel cable or "wire rope" should be an integral part of the engineered structure and is generally used to connect the fabric top of a shade structure to the metal structure. There are a variety of grades of cable available, each with different strengths, fatigue resistance, stability, etc. depending upon the materials, method, and design of the cable. The cable used should be chosen to fit the needs and use of the project.
Some shade companies employ a single cable on the perimeter of the shade structure. They generally weld a metal stud to each corner of the steel frame, pre-sew the cable into a pre-cut fabric top, drape the cover over the steel frame, and tighten the cable to bind the top to the frame.
A more robust method uses multiple individual cables to attach the canopy to the steel structure, including independent cables to provide extra strength for snow and wind loads, and cables sewn into each seam to provide extra uplift stability. Heavy duty galvanized thimbles, aluminum impact sleeves, and steel plates with pre-punched holes welded to the steel frame are used at each connection point.
|
|
Some companies drop a cable over a metal stud to secure the top of the canopy.
The use of thimbles, impact sleeves, and plates welded to the steel structure is more secure |
|
Use of Cable for Strength
Most shade companies rely solely on the underlying metal frame for support, the sole cable being employed on the perimeter of the canopy slipped under a metal stud in each corner of the frame. Effectively, the canopy is connected to the frame at four (4) points.
A better design integrates the cable and its connections with the fabric and frame for greater strength and stability. Independent catenary cables are tensioned and attached to each support strut with heavy duty thimbles and sleeves. Additional cables sewn vertically into each seam are then connected to the catenary cables.
|
|
Cable integrated into each seam with multiple connection points to the steel structure. |
|
Cloth to Cloth
Since the polyethylene fabric material is received from the factory in rolls of ten foot widths, it is generally necessary to connect (sew) multiple pieces of fabric together to meet the job specifications. In addition, the perimeter cable must be sewn in the fabric to anchor the canopy to the steel frame.
Shade companies trying to cut costs may employ a variety of different thread types and seam configurations to make the fabric connections. The most popular seam, due to its economy and simplicity, is the Plain or "Lap" seam. Sewing a lap seam is simply putting one layer over the other and sewing. In some cases a second seam is used to "sew down" the connected parts.
Since the canopies may be subject to high wind stress and load factors, fabricators should employ a flat-felled seam exclusively to bind the pieces of fabric together. The appeal of the flat-felled seam is that it makes the joint stronger by running two lines of stitches through each piece of fabric, while simultaneously encapsulating the raw edges and preventing them from unraveling. Sometimes called a "Full French" or a "Double French", this seam is acknowledged to be one of the strongest connections possible between two pieces of cloth. It is used extensively in the manufacture of tents, jeans, and hot air balloons. It is more complicated to sew and more expensive since it requires extra manual labor to position the cloth.
Most shade companies measure, cut, and sew the canopy element in the shop and deliver it pre-sized to the field for attachment. A few companies tension and sew the canopy on site, attaching the perimeter of the canopy to the catenary cable on site with special hand-held sewing machines. This method ensures that the fabric canopy is perfectly tensioned every time and should never need re-tensioning.
|
|
A flat-felled seam is the strongest of all seams because the stress is shared by the fabric and the thread.
Attaching the perimeter of the fabric to the catenary cable by hand |
|
|
Airplane Canopies