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Protecting those who serve

Features | November 1, 2015 | By:

The barrel vault design of the 25,000-square-foot cover for the Silverbell Army Heliport increases its durability while enhancing the look of the structure. Photo: International Tension Structures
The barrel vault design of the 25,000-square-foot cover for the Silverbell Army Heliport increases its durability while enhancing the look of the structure. Photo: International Tension Structures

A fabric cover at an army heliport shields against the Arizona sun.

The Silverbell Army Heliport, located near Marana, Ariz., about 30 miles northwest of Tucson, is a training site for the Arizona Department of Emergency and Military Affairs (DEMA). There, aviation training is conducted using several different types of helicopters, which are stored and maintained on site.

When DEMA acquired UH-72 Lakota helicopters, it needed a new facility from which it could move the aircraft in and out efficiently, as well as provide space for maintenance.
Maintenance management wanted an open-air structure with the height and expanse that could provide the clearance they needed. The heliport already employed such structures in other cases where they needed to expand the capacity of their hangars. They also preferred it for the cost savings and lower maintenance requirements.

Shade: a top priority

The critical need is shade. The area, known for its striking desert landscapes, has more than 250 days of sun a year on average. Coupled with summer temperatures that can top out at 104 degrees Fahrenheit or more, that means sustained months of sunny, hot weather.

This presents a challenge when it comes to storing and maintaining helicopters that have to be ready to go at a moment’s notice, according to Lt. Col. Frederic Edquid, the maintenance battalion commander for the Western Army Aviation Training Site. “Certain instruments like flare sensors and some fire indicators degrade from prolonged exposure to the sun. It takes them two or three hours to recover,” Edquid explains.

DEMA called on Caliente Construction Inc., a Tempe-based general contractor that had worked with them on other projects. Caliente in turn contracted with International Tension Structures (ITS) of Fountain Hills, Ariz., a specialty contractor of fabric and tensile membrane structures, to design and build the facility.

Justin Miller, Caliente Construction project manager, said they worked with DEMA through several rounds of concepts to land on the solution that would fit the airbase’s needs.

The structure had to meet specific requirements for clearance, capacity and durability. First, of course, it had to provide shade for helicopters and the crews working on them. It also needed to be large enough to provide significant spans between columns to accommodate the helicopters and the maintenance work that was to be done.

“They needed more than a simple shade structure,” Miller says. “We were able to give them a larger span that would enable them to work on multiple aircraft within one bay rather than smaller, segregated bays.”

The proposed structure called for a cover that measured 25,000 square feet. Designed by ITS, it features a barrel vault design with center rafters that bend to create arches. The barrel vault repeats, creating seven bays under the one cover, architecturally defining separate workspaces. Each bay spans 65 feet.

Form follows function

“We provided a creative solution by spanning even farther than they thought possible,” says Jacob Schwartz, director of engineering and construction for ITS. “We take form and aesthetics into consideration. We brought creativity to the structure beyond the function the army required.”

Schwartz said his team took inspiration for the design from the helicopters themselves, drawing on the actual components of the aircraft and creating a sense that the structure could almost take flight. The arching rolled beams of the barrel vault give an impression of the helicopter’s canopy. Compression struts resemble helicopter rotors and drive shafts, while the beams and bracing have the appearance of landing skids and boom bracing that makes up the helicopter’s tail.

The cover is made of Commercial 95™ shade cloth from Gale Pacific USA. It was chosen for its ability to provide shade, hold up to spans and loads and, at the same time, meet budgetary restrictions.

The heavy-duty architectural fabric is made from UV-stabilized HDPE monofilament and tape yarns. It blocks up to 98 percent of UV rays and is manufactured to resist tears, cracks, rotting and fading. It keeps things fresh and airy with a lockstitch knit that creates raised channels to catch and pull air underneath.

All in the details

The fabrication of the cover’s panels is key to its durability, according to Schwartz. “The barrel vault design will enhance the life of the fabric as opposed to a design in which the material lays flat,” he says. The panels were stitched together using patterns that strengthen the hold. Reinforcement was added at stress points.

Built to withstand 115-mph, 30-second wind gusts, the cover can handle high winds common to the area, as well as the gusts of the helicopters in flight. The framework is made of structural steel.

Putting it together

The timeframe for a project of this magnitude was short—about six months from conception to completion. Field construction took roughly two-and-a-half months.
Installing the structure had its challenges. The infamous desert winds kicked up on the first day, causing a delay. Plus, the airbase was open for business, so work had to be stopped and started throughout construction for helicopters moving in and out of the area.

In addition, says Miller, when construction began, they discovered that the existing concrete they were building on had elevation inconsistencies. They compensated by adjusting the concrete base heights, as well as some column heights and arches. “We made modifications in a couple of areas,” he says. “We went back to the engineers and made changes without dramatically affecting our schedule.”

A seven-person crew assembled the structure using boom lifts and cranes. The sheer size of the components created a challenge as well. The steel beams that fashioned the vaults were long enough to create a 70- to 80-foot arching radius. Together, the seven panels that made up the 25,000-foot cover weighed 2,400 pounds.

The cover is attached to the framework by common beams. A sail track was used to grab the fabric, which is folded down to the super structure. Cantilever beams at the corners have cable pockets and points to connect the fabric. ITS chose this hybrid of point and continuous connection for its architectural look.

Job well done

The shade structure was completed in February 2015 and has now been through its first Arizona summer. DEMA is satisfied both with the structure and with the experience of working with Caliente Construction and ITS. Says Schwartz, “The client and the end users felt this was a home run. They were extremely impressed that we were able to exceed DEMA’s clearances and program requirements.”

Julie Swiler is a freelance writer and editor based in St. Paul, Minn.

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