Designing simple fabric structures like tents, awnings, umbrellas and canopies so that they hold up under a variety of conditions can be a complex task. Each component is both visible and structural, and relies on all parts to function properly.
The first step in designing a fabric structure is to create a form with sufficient pre-stress or tension to prevent it from fluttering like a flag or sail. Lightweight structures with minimal surfaces optimally should have double curvature.
The degree of curvature depends upon the type and weave of the fabric as well as the type and direction of the loads. The three basic forms associated with tensioned fabric structures are the hypar (hyperbolic paraboloid), the cone, and the barrel vault.
The hypar, or simple saddle, is often a square or rectangular form in plan that in elevation is a series of high and low points. Mast- and point-supported structures are cone forms, arch- and frame-supported structures, in which the membrane is supported by a compression member, are barrel vaults.
The second step of the design process is to determine the boundaries of the tensioned fabric. Boundaries include frames, walls, beams, columns, and anchor points. The fabric is either continuously clamped to frames, walls, or beams or attached to columns and anchor points with membrane plates with adjustable tensioning hardware. 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. 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.
Once the primary points have been determined, the next step is form-finding, 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) direction 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. The panels are sized according to the width of the fabric being used.
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