Utilization of fabric structures in arid environments

November 1st, 2010 / By: / Continuing Education

Learn strategies for designing for climates where scarce water, large temperature swings and delicate ecosystems require careful intervention.

Arid climates like Las Vegas and much of the Southwestern United States, Mexico and many other areas of the world are very severe. They are characterized by a scarcity of water, high evaporation rates, large temperature swings and very delicate ecological systems. As such, they offer designers challenges and, at the same time, opportunities in logically responding to substantial variations of temperature, water, air movement and an inherent unique beauty of the landscape.

Historically, there have been two basic design strategies for dealing with such climatic extremes. The first uses high mass construction, adobe or other masonry, that tempers the pulse of thermal energy through a fixed form building envelope so that heat can be used at night in the winter or flushed back to the outside in the summer. The second utilizes low mass structures, such as a tent or a tipi that can be adapted in response to changing climatic conditions. Traditionally, tipis were set up on open mesas or plains to take advantage of summer winds. Cooling ventilation was controlled by opening and closing areas of the skin along the bottom edge and at a central apex flap. In the winter, the tipis were moved to more sheltered landscapes and fur insulation linings were added to the interior to retain heat, which could be supplemented with a central fire that was also managed with air flow controlled by adapting the structural skin.

Unfortunately, many designers have viewed these two basic strategies as an either/or proposition. It is my belief that these strategies can be advantageously combined in many situations. Generally speaking, thermal mass can be used as a heat sink to store warmth or cool, and low mass fabric elements can be designed as “selective filters” to control warmth or cool, producing elements such as sunlight, air movement, water and other energy sources.

Another historical example, the Apache Tribal Council meeting ramada in San Carlos, Ariz., embodies principles that can be beneficially utilized with fabric structures. From a general or macro environmental viewpoint the ramada was planned to shade from the high overhead summer sun, while letting air move under and through the structure. In the winter, low sun radiating under the ramada provides warmth. From a more specific or micro perspective, the shadow cast by the ramada moves throughout the days and the year. So, in response to the specific climatic conditions at any given time, users can choose to sit in the shade or sun as desired. This is especially useful during varying conditions of the spring and fall. Another, perhaps even more important purpose of the ramada is that it provides a setting open to the natural environment and symbolically, by being visible, invites broad-based participation in governmental discussions.

In arid regions, and in particular urban environments in desert areas, overheating is the primary design problem; on the other hand, there are periods when heating is desired. Strategies to create comfortable environments in exterior spaces need to do more than simply provide shade or admit sun. They should be planned in consideration of the fourth dimension of time—the daily cycle with large diurnal temperature swings, the seasonal cycles and the longer-term continuum of time—growth, change and adaptation. As appropriate, integrated strategies should consider:

  • Provision of shade throughout the day, in the summer; solar access in the winter; choices of sun/shade in the spring/fall.
  • Forming and shaping of buildings to create advantageous self-shading or solar gain/day-lighting.
  • Unique characteristics of various exterior micro-environments, i.e., building/exterior interfaces on the south, southeast, east, northeast, north, northwest, west, southwest and the roof; north/south, northeast/southwest, east/west and southeast/northwest alley ways; adjacent to natural features—foothills, arroyos, hill tops, valleys, open desert, tree covered, etc.
  • Advantageous mean radiant temperatures of surrounding surfaces, i.e., warm in the winter and cool in the summer.
  • The influence of interrelated site and landscape elements.
  • Provision of cooling sources, i.e., fountains, water harvesting, landscape irrigation/misting, waterwalls, natural and/or assisted air movement, flow through or exhaust cooling from adjacent buildings, cool towers; and heating sources, i.e., passive solar gain and re-radiation, outdoor heaters, fireplaces or exhaust heat from adjacent buildings.

Fabric structure cases

Three recent tensile membrane structures in Arizona are all shaped so that the downward or restraining curvature is oriented east/west to parallel the high arc of the summer sun across the sky. However, each varies in response to different architectural design objectives and the local climatic context.

At the Hualapai Nation Visitors Center at the Grand Canyon (fig. 1 in the box above) the author served as tension structure consultant to The Architecture Company, Richard Fe Tom, AIA. In this project, the south edge of the upward or supporting curvature is raised to provide an inviting entry and to allow direct and reflected sunshine under the canopy to provide warmth by direct radiation and re-radiation of heat absorbed by the rocky sitescape environment. The side profile of the structural form alludes to a big horn sheep, which is an important figure in Hualapai culture. In addition to shading in the summer, the form of the canopy collects and directs prevailing wind currents that rise from the canyon floor across the site ridge plateau through the visitor seating area.

At the “Summer Invitation” on the Phoenix Civic Plaza (fig. 2), which was a temporary exhibit focused on demonstrating possibilities for comfort during the hottest time of the year, the south edge was lowered and undesirable solar radiation was screened further by surrounding landscaping on the south, east and west and a supporting trailer structure on the north. The upward curvature rose from south to north. Warm air rose by convection along this surface drawing in ambient air along the south so that a natural cross ventilation air movement was generated. When it was desirable, fans at the top edge on the trailer could be turned on to enhance the airflow.

At the new entrance for the Tucson Zoo in Reid Park (fig. 3), the tensile membrane together with an arcing ticket booth wall provides shade for a ticketing and entry area on the north side. During the day as the sun drops toward the west and temperatures become warmer, the extent of the shading increases toward the northeast, which is the main entry approach area. The lowest point of the membrane, supported by a “V” shaped mast, is oriented to the west-northwest, the direction of the most intense late afternoon summer sun. The configuration of the membrane and this edge point directs rainwater run-off into a cistern. From there, it is used to irrigate and sustain xeriscape desert landscaping, which further enhances shading of the area. Inside the entry on the south, heat is generated by solar radiation toward and re-radiated from the surrounding surfaces. This will be advantageous in the winter and occasionally in the spring and fall. In the summer, more shading will be desirable in this area. For this and the related following project, the author was a tensile fabric consultant to the architects Burns Wald-Hopkins.

The xeriscaped area on the northwest side of the zoo entry will be extended further to engage an adaptive recreation center (fig. 4), which is currently being developed for the City of Tucson Parks and Recreation Department. Initially, this project was studied as a traditional enclosed building. Preliminary analysis indicated that the mechanical system costs for indoor swimming pools would be prohibitive. Much like the Apache Tribal Council Ramada, the project was re-envisioned as a pavilion in the park with a 485m2 fabric structure covering a recreational swimming pool, a lap pool and a resistance current pool. Locker rooms and a therapy pool with a roll-up door that opens to the outdoor deck area are in adjacent building elements. The fabric structure is formed so that air will be drawn in about the perimeter and rise up to exhaust vents at the top of the 24m central mast. During the day the shadow of the pavilion roof form will move across a generous landscaped and pool deck area, always providing users with a choice of staying in the shade or sun. More shade will occur when the sun is high overhead in the summer; and more lower winter sunlight will penetrate under the southeast, south and southwest edges of the pavilion.

In the mid 1980s the University of Arizona Environmental Research Lab Solar Oasis (fig. 5) began to study possibilities of an integrated approach to site/landscape development and built form that would reflect sustainability considerations of land, energy and water use and air quality.

A major component of this project, which continues today, is a south facing tensile membrane green house roof. Several fabrics were tested for this structure over the years. With any given fabric shading/translucency the problem for plant growth was too much shade in the winter and not enough in the summer. The current solution is a sky blue chain link mesh that serves as an arbor for deciduous vines. Leaves are lost in the winter allowing direct sunlight to the plants. In the summer, a thick canopy of leaves grows out over the arbor. Like a ramada, this provides shade but permits warm air to rise up through the canopy. When the vines are irrigated with a misting system, air dropping through the canopy is cooled providing an “evaporative cooling” effect for the greenhouse area and other terrace areas of the solar oasis beyond.

At the Seawater Farm in Massawa, Eriteria, East Africa, the project is focused on sustainable development and enhanced food production. A key component is a series of 80 20m-diameter shrimp cultivation ponds. Each requires a ribbon of shade around its perimeter. Criteria for a prototype structure (fig. 6) that the author helped develop on site were to maximize the use of local labor and materials. The alternative developed used the matt weaving abilities of local women to fabricate membrane segments, native ropes, posts fabricated from plentiful eucalyptus limbs, and anchors and other elements made from materials found at the main recycling market in the capitol Asmara.

Concepts studied and/or explored at the University of Arizona solar oasis and the “Summer Invitation” at the Phoenix Civic Plaza were developed further in a design (unfortunately unrealized) for a permanent Arizona solar oasis (fig. 7) at that site in Phoenix. The plaza, like many public places built in the 1960s, was pleasant in the winter, but in the summer surface temperatures were often over 540C, a classic example of the urban heat island effect out of control. To remedy this, it was proposed to re-introduce biology into the city by removing a third of the plaza deck and parking garage below and replace it with nutrient rich desert soil, in which a xeriscape desert garden could be cultivated. The garden would have been sustained by water harvested from roofs of the surrounding fabric structures and the Convention Center and Symphony Hall buildings.

The fabric pavilions were designed to shade from the high overhead summer sun at mid-day and take advantage of the long shadow cast in the afternoon by the Hyatt Hotel on the west. However, certain south oriented edge areas of the pavilions were raised to permit winter solar gain through the openings of the adjacent street corridors and over the Symphony Hall. Within this shaded environment, cooling was to be provided by new and existing fountains, a long water wall between the two plaza levels, landscape irrigation and cool towers. The towers would supply cool airflow through the pavilions into the garden, where a large feature tower would flood the garden entry with a sea of cool air. The garden tower was proposed to be a fabric structure that visitors could ascend to a top platform for an overview of citywide environmental and resource issues, and then descend down through the cool air inside the tower to the garden below. The tower concept was developed in a study model (fig. 8) and tested with a one-quarter-scale mock-up (fig. 9) set up at the Arizona state capitol on a recent Earth Day. The author was the design architect for the proposed Arizona Solar Oasis and NBBJ was the architect of record.

Conclusion

I would like to suggest that as issues of resource availability, environmental impact and society’s recognition of the need for a more sustainable and regenerative approach to design and planning become more critical, fabric structures have a wonderful opportunity not to be planned as singular objects but as integrated elements and components of built environments designed with a comprehensive focus on such issues.

R. Larry Medlin, an architect with extensive experience in tensile structure design, teaches at the School of Architecture, University of Arizona, Tucson, Ariz.

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