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The first step in the design of spatial structures used as roof systems, is always to select the optimal roofing material or skylights. The type of roofing material used for spatial structures can affect the structural weight and purlin spacing. In addition, the roofing choice is visually important. There are a number of roofing materials available such as corrugated steel or aluminum deck, lightweight concrete, stressed skin, or translucent skylights. The most commonly used roofing is corrugated steel deck which can span up to about 15 ft. The aluminum decking is susceptible to cracking during the installation, and lightweight concrete is usually used when fire proofing or acoustical insulations are required.
It is usually more desirable to select the cladding for spatial structures from light materials since the structure is generally lightweight. Metal decking, wood, copper, lightweight concrete, acrylic, vinyl, fabrics, and glass have been used as cladding materials. Roofs are generally insulated. Depending on the type of the system used, the cladding and glazing can be either directly supported by the spatial structure members or by a secondary purlin system connected to the joints. In the first case, the members are subjected to local shear and bending in addition to axial forces, which are the only internal forces for the second case.
Also, camber or sloping of the main members are required to provide appropriate roof drainage and prevent ponding. When corrugated metal decking is used, it is usually placed on purlins, which are connected to the top layer nodes. The loads from purlins are transferred to the structure through the nodes. In this case, the purlin elevation can be varied by changing the heights of the connector stools to provide the required slope. For very large structures this may not be practical and cambering may still be required. Skylights can be placed within the openings of spatial structures.
Fabric Cladding
Fabrics have been widely used to cover spatial structures used as sport facilities, exhibit halls and shopping centers with the advantage of providing partial day-lighting.
There are a variety of different fabrics on the market. The most typical fabric used for spatial structures is a fire-resistant coated vinyl on polyester. The main advantages of this material are:
- high fire resistance
- high strength
- durability (typical life span of more than 15 years)
- availability in many different colors
- low cost to performance ratio
Usually, the fabric is either directly attached to the nodes or suspended from them. It can be translucent, opaque, or multi-layered to provide high insulation ( R=11 or more). One important advantage of using fabric is the possibility of having open or closed environments according to the weather conditions. This provides a more energy efficient design alternative as heating or air-conditioning systems may not be needed.
Generally the stretched membranes made of PVC-coated polyester or PTFE-coated fiber glass can span from 18 ft to 27 ft. For moderate span spatial structures (90 ft-120 ft), module sizes of about 9 ft can be easily used and the membrane can be supported at 9 ft or 18 ft spacing. If the membrane has a shallow depth, gutters can be placed on top of or below, the spatial structure. The membrane is usually either suspended from the top layer members or supported by additional components placed on top of the spatial structure connected to the nodes.
Glass Skylight
The most efficient modular size for pyramid or ridge skylights is 8 ft to 12 ft. When laminated glass is used on sloped roofs, the module size is usually limited to 25 square feet but can be increased to 40 square feet. Generally, the glass used for covering spatial structures is made of laminated and/or heat-strengthened (tempered) to account for accidental damage and large load capacity. Plastics (acrylic, polycarbonate or PVC), in the dome or barrel forms, are also used as a substitute for glass.
For safety reasons when glass is heat-strengthened, it must also be laminated for use as structural element. Use of laminated glass (without heat-strengthening) generally results in less cost but also is not as safe as heat-strengthened glass. The optimal size of a single glass panel is about 4.5 ft x 4.5 ft. Smaller units facilitate installation due to lighter weight but increases the total number of connections.
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