Rainwater harvesting and fabric structures.
By Samuel J. Armijos
Fabric structures are often designed to protect one from harmful UV rays; however, they are more often designed to protect one from inclement weather (rain, wind and snow). Fully enclosed fabric roofs are used in Northern climates to increase their use where snow can cancel many events. Wind screens or openings in a membrane designed in humid or arid climates are designed to move air or screen light in ways to improve human comfort. Rain, on the other hand, can be diverted away from the covered space or left to cascade off the edge much like a waterfall. Where does the water go? A moving and sustainable trend in the industry is to design fabric structures to collect the water coming off the membrane much like a funnel and put it to good use.
The tension fabric structure at Arizona State University (ASU) in Scottsdale is a fine example of creating not only a shade structure but designing the structure to collect water as well. Designed by FTL Design Engineering Studio and constructed by FabriTec Structures, SkySong uses approximately 45,000m2 of PTFE membrane. The design consists of a series of high and low points built around a circle. The low points, or inverted cones, work like a funnel to collect water. Using engineer Martin Brown’s NDN software for tensile structures, we are able to determine the amount of “discharge” coming off the membrane so that one can properly design drainage details. In Arizona, where the annual rainfall amount is about 20cm a year, the four inverted cones of the SkySong structure collects close to 303,000 liters (80,000 gal.) of water to irrigate flower beds, landscaping and gardens. In comparison, if the same structure were built in New Jersey where the annual rainfall amount is 104cm a year, one could collect 1.5k liters (400,000 gal.) of water a year!
Calculating the potential for harvesting rainwater with a fabric structure can play a huge role in any sustainable project. Take these simple steps to determine the amount of water that may be collected in your current design.
Figure the square feet of the collection area (A). This will be the area from which rainwater will be collected.* For SkySong, the approximate area of the four inverted cones was 16,000 sq ft. (1,486m2).
Multiply that number by the average rainfall per year for your location (B). For Arizona, it’s about 8 in (20cm).
Multiply A times B to get C. Divide the multiplied numbers (C) by 12, which represents inches per foot. For SkySong, 16,000 x 8 = 128,000/12 = 10,667. This will determine the cubic feet of water per year which can be collected (D).
Convert the cubic feet of water (D) to gallons. Multiply total by 7.43 to convert to gallons per year. (10,667 x 7.43 = 79,253 gallons.) [299,972 liters per year].
Fabric structures that collect water? Why not.
Regular contributor Sam Armijos, AIA, is Vice President for FabriTec Structures, a brand of USA Shade and Fabric Structures. He is author of Fabric Architecture: A resource for creative solutions in Shade, Signage and Shelter.
*Area in this case refers only to the area of the fabric contributing to the rainwater collection function, not the total area of fabric in the structure.