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 — a surface that possesses a high-point (positive) curvature along one principal axis and a low-point (negative) curvature along the other principal axis.
Anticlastic surface forms have double curvature in diametrically opposite directions, like a saddle, while synclastic surface forms have double curvature in the same direction, like a balloon. 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.
Determining Boundaries
The second step of the design process is to determine the boundaries of the tensioned fabric, which is referred to as the membrane. 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.
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 10 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.
Form-Finding
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.
Structural Analysis
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.
Materials
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.
Architectural fabrics can be manufactured to vary in translucency from one to 95 percent and, in thermal resistance, from that of a single pane of glass to that of a conventionally insulated structure, while still maintaining adequate daylighting. A fabric roof can be a source of interior light at night if artificial light is directed onto its highly reflective surface.
Fabric Selection
The performance of today's architectural fabrics depends on the weaving pattern, choice of substrate, and coating. Each composite has unique properties and characteristics that suit it to different applications. Most materials presented have a minimum of stretch and shrinkage in a wide range of temperature and humidity conditions, and coatings prevent mildew, staining, and streaking.
Choice of a material calls for understanding of its light reflectivity and light transmission. Reflectivity is the amount of light the surface of the material reflects. Transmission is the amount of light that penetrates the material. Most fabrics allow some amount of light transmission, but some materials come with a blackout scrim between layers and allow no light to penetrate, so light and heat from the sun can be controlled.
All the materials come in some shade of white; some are also available in a limited range of colors, depending on supply and demand. The proper selection of membrane material will be based on the proposed size, form, function, and desired longevity of the structure, and the economics of the project.
Membrane Fabrication
Membranes can be fabricated in a number of ways based on the material chosen and the orientation of the seams. All aspects of a fabric structure should be derived from the same computer model or full-scale mockup. Computer-generated patterns are the most widely accepted template for fabrication. Smaller structures, such as awnings, are patterned directly off a full-scale mockup.
Seams determine the appearance of joined panels. The seams can be sewn, glued, electronically welded, or heat-sealed. Seam styles can be parallel or radial to a mast. Butt seams are joints produced by placing two adjacent pieces directly beside one another and covering the joint with a strip of material. Lap seams are joints made by overlapping the edges of the material.
Reinforcements — multiple layers of material applied to specific areas of a membrane to strengthen it where concentrated tension loads exist — are also a part of the fabrication process and differ from project to project.
The book is available for purchase at Amazon.com.
Editor's Note: This is an excerpt from Fabric Architecture: Creative Resources for Shade, Signage, and Shelter by Samuel J. Armijos. Copyright 2008 by Samuel J. Armijos, with permission of the publisher, W.W. Norton & Company, Inc.