Despite worldwide climate issues, fabric structures meet environmental challenges.
By Bruce N. Wright, AIA
With news of a 100-year “polar vortex” in the midwestern United States, increasing tsunami destruction in the Caribbean and coastal Japan and typhoons in Southeast Asia, one could begin to believe that “extreme climate conditions” is an oxymoron. It seems that the unusual is becoming the usual. This doesn’t deter fabric structure designers, however, because they know that with new durable fabrics and reliable, tested construction details, fabric structures can be made to meet any durability challenge in extreme environments.
Right here, right now
It all begins with understanding the local situation. All good architecture starts from a thorough knowledge of a building’s program and physical location—the climatic, and seasonal conditions that impact a building over time. This much all architects know, and they (usually) successfully design buildings that address these essential issues, whatever the climate or building material.
“The big issues are the same for all climates,” says structural engineer David Campbell, P.E., principal and CEO of Geiger Engineers in Suffern, N.Y. “Strength, durability, light transmission, appearance, finish, thermal performance and cost. With building envelopes, it is all about what you want to keep in and out and what you want to let in or out through the envelope.”
For fabric structures, there are many ways to accomplish this, according to Campbell. In this article, many of these methods and the materials (the fabric of fabric architecture) that apply in harsh environments will be addressed.
For architects, the most important thing to consider is the overall design of a fabric structure, to make sure it fits the climate for which it is intended. “When enclosing a [fabric] building,” says Samuel J. Armijos, AIA, principal of FabricArchitect.com, Fairfield, N.J., “determine what the priority is: thermal comfort or light translucency? Thermal comfort can be obtained without losing translucency. Light translucency can come in many forms, materials or hybrids.” As might be expected, there are significant differences in designs for hot versus cold climates, and this can sometimes come down to both design details and fabric choices.
“The main reason someone would want a fabric structure for hot, arid locations,” Armijos says, “is to provide shade and air movement. A perfect example is the Las Vegas Premium Outlet project with its central spine of conical fabric shade structures. It could have been done in a number of fabrics, but it is the design that allows air movement and temperature differences to occur.”
For cold climates, some experts suggest that architectural design and construction detail are more important than fabric selection. “Cold is not so much an issue,” says Mark Smith, director of Base Structures Ltd., Bristol, England. “All fabrics have lower temperatures below which their strength and durability will be affected, and in these cases weather issues need to be resolved through design and detail rather than fabric choice alone. Anything will fail if you pile enough snow on top of it.”
Design and climate
For starters, with both hot and cold environments, thermal issues set the construction detail agenda, as they do for all building construction, be it steel, brick, wood or fabric. However, for fabric structures, “generally there is much more freedom in detailing for hot, arid climates than for cold climates,” Campbell says. “In cold climates, the building enclosure should be detailed to avoid cold ‘bridging’ and condensation. For hot climates, thermal bridging is not as much of a concern.”
In hot climates, Campbell says, “a good design of the building envelope (especially the roof due to its exposure) has whatever visible light transmittance is desired (and no more than needed), as well as high reflectively, low absorption and low heat transfer.” There are many ways to accomplish this, especially when multiple layers can be employed. A small amount of thermal insulation can be very beneficial.
According to Campbell, good fabric structure design in cold climates employs materials, forms and systems that respond to snow and ice loads. For fabric building enclosures, designers must address the added issues of thermal performance of the building envelope and the attendant issues of water vapor.
With snow issues, it’s essential that designers address snow ponding, Smith says. “Snow cables or drains are used as necessary. In high winds, deflected forms need to be evaluated to ensure that fabric does not abrade against steel, and that cyclic loading does not lead to rigging screws or even steel connection bolts undoing and detaching.”
The durability of fabric
According to Michael Seidel in his book Tensile Surface Structures, “thermoplastics become brittle at low temperatures.” In contrast, “with increasing temperatures, they suffer, on account of their structure, a relatively large volume expansion.” That means that fabrics made from these materials have significant stretch, known as “creep,” due to temperature fluctuation. “PTFE-coated glass fabrics are considerably more prone to creep at low temperatures than PVC-coated polyester fabrics,” Seidel says. Therefore, when these fabrics are installed during low temperatures, it is possible to erect them only with considerably more effort than when installed at higher temperatures.
“The durability of a fabric membrane,” says structural engineer Craig G. Huntington in his book The Tensioned Fabric Roof, “is a complex performance parameter dependent on resistance to ultraviolet [UV] radiation degradation, damage resulting from wicking (the absorption of water along the length of yarn fibers due to capillary action), attack from algae or other organic matter, and the retention of seam strength.” Huntington notes that resistance to weakening of the fibers from long-term UV radiation exposure is the most common factor in fabric durability.
“Fabrics such as PTFE and silicon glass are not affected by UV,” although PVC is,” Smith says. “A good quality PVC will retain the majority of its tensile strength for a long time, but the PVC coating will lose its plasticizers, become brittle and get rather dirty—this is what will kill the PVC fabric over time. Also to be considered: ambient temperature within the canopy. If the temperature gets up to 60Ëš C (140Ëš F) or above, PVC deteriorates rapidly.”
Finally, with heat comes humidity. Prolonged exposure to humidity, according to Smith, will lead to mold growth, which can be extremely unsightly on a translucent fabric and is more of a problem with PVC.
“The life of some materials also may be shortened by wicking,” Huntington says, “which can lead to freeze-thaw damage of the material in cold weather conditions. Wicking is best prevented by adequate coating thickness over the entire area of the fabric scrim, says Huntington, as well as the design of structures that are adequately sloped throughout the membrane, that have seams laid out to ‘shingle’ water away from the cut ends of the material, and that are designed to avoid condensation.”