Fabric options, and the design-build process.
By Samuel J. Armijos, AIA, president, Fabric Architect LLC
The most important quality in choosing a 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 a 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 building envelopes respond to heat and light much differently than previous generations of fabric. They offer features and benefits different than conventional construction materials. Architectural fabrics are manufactured to vary in translucency from 1 to 95 percent and, in thermal resistance, from 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.
The proper selection of membrane material is based on the proposed size, form, function and desired longevity of the structure and the economics of the project. The project can be fabricated a number of ways based on the chosen material 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.
PTFE-coated fiberglass is the preferred material for large-scale permanent structures or structures requiring long life and specific construction code compliance. PTFE has excellent weather, temperature and chemical resistance, as well as durability and strength. Its life span is more than 30 years, and it is noncombustible. PTFE varies in translucency from 7 to 15 percent and is heat-sealed at the seams. Today, there are versions of PTFE fabric with thermal qualities that conform to energy standards required for LEED points and rebates.
Silicone-coated fiberglass is an alternative to PTFE fiberglass, and shares many of its attributes. It has high tensile and tear strength and is more flexible than most other materials. The seaming process requires an adhesive that takes less time to cure completely than PTFE, which reduces labor cost. It is long lasting, flame resistant, dimensionally stable and available in a range of colors and translucency.
Woven PTFE is a 100-percent fluoropolymer fabric made with high-strength PTFE. It offers durability, strength and flexibility. It transmits up to 40 percent of light and combines good light and water resistance with the ability to withstand repeated flexing and folding. The material is pliable enough for retractable and deployable structures.
PVC-coated polyester is the most cost-effective membrane material and is ideal for both temporary and permanent structures. The material is soft, pliable and less expensive than PTFE. It is available in a variety of weights to meet a wide range of structural requirements and is fire retardant. This material is sealed quickly with a radio-frequency (RF) welder or hot-air sealer. PVC material has a life-span range of 15 to 20 years depending on the topcoat chosen. It comes in a variety of colors and translucency. It is available in a perforated mesh.
High-density polyethylene (HDPE) is manufactured and used primarily as a shading material. HDPE comes in a variety of styles, colors and shade factors from 50 to 95 percent. It provides high tensile strength, UV stability and high UV absorption. HDPE is 100-percent recyclable and its expected life is 10 to 12 years. The material is sewn together with industrial machines and long-lasting PTFE thread.
ETFE foil is a polymer resin from the same family as PTFE. It is produced in very thin sheets and is manufactured to be installed in single layers or more commonly as inflated pillows. It is an alternative to structural glass and, because of its light weight, can reduce the size of the primary structural system significantly. ETFE pillows are supported by a constant airflow supplied by an inflation system consisting of a centrifugal fan unit and emergency backup, with humidity controls and filters to prevent moisture and dirt from getting inside the pillows. The material has low tear propagation, is UV resistant, inert to chemicals and 100-percent recyclable. Multiple layers of ETFE can provide an effective thermal enclosure and active sun control. It can be used in a single layer for smaller structures such as awnings and canopies. ETFE is joined together using RF welding techniques.
Getting started, and the process
The best way to get started is by researching what’s already been done and looking for manufacturers of each component or a one-stop shop for the entire structure being built. Most important is to understand the process.
Designing simple fabric structures like tents, awnings, umbrellas and canopies so that they hold up under a variety of conditions requires an understanding of the structure and the materials involved. 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 three basic forms associated with tensioned fabric structures are the hypar, 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 pocket or attached away from the membrane with cable straps.
Once the primary points have been determined, the next step is form-finding, which is the art and engineering of creating 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.
Today, fabric structures are primarily done with computers using membrane software in combination with programs like AutoCad and SketchUP. These programs allow designers to create a 3D model that can be viewed at various angles and modified easily.
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 and equipment. Structural analysis identifies areas of possible ponding and shows where high stresses are located on the structure. The analysis enables the designer to determine reaction loads for foundations and building anchors, size structural members and cables, determine the appropriate fabric and create computer-generated cutting patterns according to the width of the fabric being used.
A fabric structure cannot be fabricated or completed without site conditions verified to match the theoretical geometry used to analyze and detail the membrane and structural components. Much like “barn raising” or post-and-beam construction, understanding the process by which the fabric structure will be installed is important.
Developing a means and method statement and knowing how the structure will arrive, be installed, tensioned and ultimately maintained over time are all critical to the process.
Samuel J. Armijos, AIA, is an architect and the author of “Fabric Architecture: A Visual Resource for Shade, Signage and Shelter.” He is director of the consulting firm FabricArchitect.com.
Part 1 of this article on the role of fabric structures in architecture appeared in the May issue of Fabric Architecture.