Designing Fabric Structures
Membrane plates are custom-designed plates used to link the membrane and edge cables to the structural supports. In most cases the fabric forms a curved edge, or catenary, between connection points, requiring a cable, webbing belt, or rope to carry loads to the major structural points.
Catenary describes the scalloped edge shape of the boundary of a uniformly stressed fabric structure attached only at specific end points or nodes. Catenaries are usually curved inward ten to 15 percent of the total length of the span.
The cable, belt, or rope is usually inserted in a cable cuff, an edge treatment created either by folding the edge of the material over itself to form a pocket, or by attaching a ready-made pocket along the edge. The shallower the curve along the perimeter, the more tension there is in the cable and ultimately in the overall structure and foundation.
Very high tension loads require a cable-strap treatment, which consists of a continuous clamping of the edge with a series of steel or aluminum straps spaced at specific intervals to support a cable that cannot be inserted in a cable cuff. Cable straps can increase the cost of a fabric structure substantially.
Once the primary points have been determined, the next step is form-finding, or the art and engineering of ascertaining the most efficient structure that can be fabricated with as little waste as possible. In form-finding, it is just as important to design a structure that can be easily transported and installed.
There are two methods of form-finding: physical modeling and computer-aided design. Fabric structures may be visualized with physical models or full-scale prototypes, depending on the complexity of the design. Models are created by stretching nylon stockings over wire frames. Working with physical models or prototypes enables the designer to view the structure from any angle.
However, most fabric structures today are modeled with sophisticated computer software programs. These programs allow the designer to create a three-dimensional model that can be viewed at various angles; they also allow customization to provide information for facilitating fabrication and installation.
The programs can calculate the amount of fabric required; the dimension of each fabric piece; the size and length of structural members; the size, length, and tension of cables; and the necessary hardware. With a software program, the designer can modify the shape more easily than with a physical model.
The last step in the design process is analysis of the structure's response to loads, including dead loads and live loads, such as snow, wind, people, and equipment. Structural analysis identifies areas of possible ponding (water collecting on a flat area) and shows where high stresses are located on the structure. The analysis enables the designer to determine reactions, size structural members and cables, determine the appropriate fabric, and create computer-generated cutting patterns.
Computer patterning is the process of developing a two-dimensional representation of a three-dimensional membrane surface. Patterns are created after receiving results of a biaxial test of the specified materials done by the fabricator or provided by the manufacturer to determine the compensation factors required for the specific project. A biaxial test is the testing of a membrane in both the warp (threads running the length of the roll goods) and fill (threads running across the width) directions to calculate the expansion of the material under a given loading condition.
Compensation factors are the reduction made to a cutting pattern to allow for the expansion of the membrane once in tension. In some cases, decompensation — addition made to the length of a piece of the membrane that was shortened by compensation — is required to meet certain geometric conditions, such as fixed points, where there is no access for tensioning. The panels are sized according to the width of the fabric being used.
Today's architectural fabrics are composites of woven substrate fiber protected by an applied coating or polymers of films and laminates. New fibers — primarily nylon, polyester, polyethylene, and fiberglass — have been developed to meet the need for materials with high strength, long life spans, and a high modulus of elasticity. The woven substrate provides the basic tensile strength of the material and its resistance to tearing. The finish coating applied to the substrate material seals the fabric against weather and dirt, provides resistance to UV light, serves as a medium for joining panels, and incorporates fire-resistant properties.
The most important quality in choosing material for a fabric structure is its fire resistance. National Fire Protection Association (NFPA) 701 is the most common fire test for textiles and films. The American Society for Testing and Materials (ASTM) is another recognized standard for a wide range of materials, and ASTM E-84, 108, and 136 are common tests related to fabrics for membrane structures.
The latest architectural fabrics used for a building envelope respond to heat and light much differently than previous generations of fabric did. They also offer features and benefits different than conventional construction materials.
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