United Airlines’ Membrane Hangar
NFPA Journal®, July/August 2008
By Timothy Hawthorne
At Boston’s Logan International Airport , United Airlines Corporation was using a hangar constructed in the 1950s and formerly owned by now-defunct Eastern Airlines. With the advent of the massive "Big Dig" construction project in Boston, however, the hangar was slated for demolition to make way for the Ted Williams Tunnel, an underground highway named after the Boston Red Sox Hall of Famer that connects East Boston, where the airport is located, with Boston, says James McGinty, Massachusetts Port Authority (Massport) Fire Marshal.
Since land is at a premium in East Boston and airport expansion is impossible, the need for a new hangar meant United had to work closely with the Massport officials to explore other concepts. To that end, the airline hired Whitney Atwood Norcross Architects, Inc. of Boston in 1992 to meet with McGinty and Maurice Pilette, P.E., Massport’s fire protection engineer.
The architects and United proposed building a simple, fabric-based, 38,000-square-foot (3,530-squaremeter) membrane structure to be produced by Rubb, Inc., of Sanford, Maine, a company specializing in membrane structures that provide low-cost shelter and natural lighting. The hangar was the first of two membrane structures in that location—one built in 1992 and the other in 1998. It complied with Federal Aviation Administration (FAA) requirements for providing protection from the weather when the airline performed light maintenance tasks, had a span of 175 feet (53 meters), large enough to accommodate the Boeing 757 and Airbus A320 series of aircraft United used in the region at the time. The hangar was also self-venting, meaning that the fabric would melt above a fire, releasing heat and smoke that built up inside the structure. An added benefit was the structure’s portability: It could be relocated easily.
According to McGinty, a membrane hangar was outside the purview of any adopted fire protection standard 10 years ago. In fact, a new chapter specifically addressing membrane-covered, rigid steel-frame hangars wasn’t added to NFPA 409, Aircraft Hangars, until the 2001 edition of the standard. "The Massachusetts State Building Code is used for all new construction projects uniformly throughout the state, but Massport can also use the newest editions of the NFPA’s fire codes for any design, at their discretion," says Pilette, a principal member of the NFPA Technical Committee on Airport Facilities.
Massport requires all structures on its property, regardless of size, to install a complete automatic sprinkler system that complies with NFPA 13, Installation of Sprinkler Systems. Since the membrane hangar could not support sprinklers, it was instead designed with an aqueous film-forming foam (AFFF) system that used four oscillating monitors installed 10 feet (3 meters) above the finished floor to provide coverage of the area under the aircraft. The foam system was activated with either infrared flame detectors attached to the steel supports or manual pull stations located at the exits. The tanks and proportioners were inside containers, and the fittings were connected through access ports in the containers’ sides. This design allowed the structure to comply with intent of Massport’s requirements for a temporary structure.
In 1998, United called upon the architect and Rubb to build a much larger, replacement membrane hangar that would serve as the airline’s main maintenance facility in the Northeast. The resulting design was 80 feet (24 meters) high, spanned a massive 253 feet (77 meters), and could house one Boeing 777, three Boeing 757s, or six Boeing 737s.
Inside the hangar was a 697-square-foot (65-square-meter) office area that provided a separately heated and air-conditioned interior work place for personnel and computers. The framework of the new hangar, which would be one of the largest structures of its type in the region, was composed of 850,000 pounds (385.5 metric tons) of hot-dipped, galvanized steel with a translucent white membrane roof. The hangar was equipped with a radiant heating system to take the chill out of the Boston winters and a 64-foot-candle, metal-halide lighting system that provided illumination for night work.
Once again, the design team met with representatives of Massport for permitting. Massport’s primary concern was developing the fire-safety plan for the new hangar, the size of which meant certain changes had to be made to the plan devised for the older membrane hangar. Low-level monitors had been acceptable in the smaller hangar, but they could not reach far enough in a hangar almost twice the size.
After careful review and discussion, the design team decided that the oscillating monitors of the older AFFF system could not discharge enough foam to reach the hazards without a fire pump. The original membrane hangar used AFFF foam with a higher water demand, but the new, larger hangar would be limited by a 12-inch (30-centimeter) water main for both domestic and fire protection purposes. Thus, the larger building would not have enough water to supply the volume required for a larger AFFF system.
As Logan International is located on a peninsula in Boston Harbor, AFFF runoff would also pose a serious ecological issue.
For these reasons, the team decided to use a ceilingmounted, high-expansion-foam-generating system rather than an AFFF system, says Gordon Collins, director of Marketing at Rubb. High-expansion foam is a cleaner agent, and it can dissipate much easier. The team also found that the high-expansion foam system would achieve the coverage area at a lower expense than an AFFF system.
The system provides a submergence time of 3 minutes and guarantees that every corner of the 65,877-square-foot (6,120-square-meter) floor area is protected. The system uses foam-generating apparatus from Ansul, Inc. and takes the air from inside the hangar, in accordance with NFPA 11, Low-, Medium-, and High-Expansion Foam.
The hangar’s state-of-the-art system has six foam generating apparatus at the ceiling fed by two 500-gallon (1,893-liter) foam-concentrate storage tanks with supply piping in a tree layout. The foam system, designed with one tank in reserve, contains high-expansion foam at a proportion of 2 3/4 percent. The system is activated by either manual pull stations at the exits or an automatic detection system that consists of eight cross-zoned infrared and ultraviolet detectors. With these cross-zoned detectors, the activation of one zone activates the audio-visual devices in the hangar as well as the alarm at the airport’s central dispatch.
Fire alarms from any of Massport’s four hangars result in an immediate response of the Massport Fire Rescue Department. Unlike many other airports, the layout at Logan allows responding apparatus to travel on vehicle service roads so that crossing active runways is not necessary.
The hangar is also protected by three 1 1/2-inch (4-centimeter) hose stations that are situated around the hangar for use in the event of a fuel spill and are fed by an AFFF low-expansion foam system. There are seven, 100-pound (45-kilogram), dry-chemical, wheeled extinguishers for initial attack. The hangar’s office areas are protected with an NFPA 13-compliant wet-pipe sprinkler system.
The new hangar was completed in 2001, one year after the original membrane hangar was demolished. The cost of the new membrane hangar was approximately $6 million dollars, a fraction of the cost of a conventionally built hangar.
Timothy Hawthorne is an NFPA fire protection specialist, is the staff liaison for NFPA 409, Aircraft Hangars. The author would like to thank United Airlines and Massport Fire Rescue Chief Robert Donahue, Assistant Chief James T. McGinty, and Mr. Maurice Pilette for assistance in writing this article.