Printed headline: Building Safer Hangars
As American Airlines prepares to officially cut the ribbon on its new $215 million aircraft maintenance hangar at Chicago’s O’Hare International Airport in early 2019, maintenance staff have been working there comfortably since December 2018, despite the city’s chilly winter weather. Like other hangars around the world in areas prone to extreme climate or natural disasters, care needed to be taken to ensure that the hangar was designed so that workers, aircraft and the hangar’s structure could withstand the worst conditions that Mother Nature could throw at it.
The 614-ft.-long hangar’s dual sliding doors on both the north and south sides of the building can cause tremendous heat loss when opened in cold weather, so architects at Ghafari Associates looked to strategic air circulation to keep operating temperatures normal. According to Ted Oberlies, senior vice president of Ghafari Associates’ aviation group, the hangar uses a combination of direct-fired heating units—essentially, large furnaces that can quickly generate a lot of heat—and high-volume, low-velocity fans to circulate heat in winter. The fans also can provide a passive cooling effect in hotter weather, which is complemented by natural ventilation when both hangar doors are open.
Insulation of hangar buildings plays a large part in temperature regulation as well. Rubb Building Systems, which specializes in hangars composed of steel structures covered in heavy-grade PVC fabric, uses a patented insulation system called Thermohall. According to Chuck Auger, marketing manager for Rubb, Thermohall consists of two layers of PVC fabric stuffed with very-high-density appliance insulation to create an insulated “blanket” that is tensioned down around a building—which keeps the hangar’s steel framework warm.
Steel itself can be problematic in the coldest temperatures. “Steel doesn’t cope well with being made very cold,” says John Harrison, design office manager for REIDsteel. Harrison says steel can suffer from brittle fractures—a serious threat when the company was constructing a hangar for Eznis Airways in Mongolia, where temperatures can dip to between -20C and -30C (-4F to -22F). The steel needed to undergo various treatment processes to prevent such fractures, and during construction components such as hangar cladding and bolts needed to be warmed up before work was performed to prevent shattering.
According to Harrison, the Eznis hangar was built with a unique nose opening between its front doors that allows work to be done around an aircraft’s nose without the need to fully open the hangar door, which would lead to excessive heat loss. The nose opening features an inflated seal around the edge to prevent air from getting in or out once an aircraft’s nose is positioned in the opening.
Snow loading is another factor that needs to be considered when building hangars in areas with extreme winters. Oberlies says the weight of snow is an important part of structural design, particularly on wide-span hangars such as American’s. Roofing needs to be designed according to building codes based on the heaviest snowfalls anticipated in a region, and requirements can vary internationally. Although most countries accept the International Building Code, Oberlies says local requirements sometimes can be more stringent
“I think the interesting thing is that in the U.S., most of the code requirements for buildings are what we term as ‘prescriptive’—in other words, there is no debating or justifying deviation from those codes. It lays out exactly what you must do under certain circumstances,” says Oberlies. He adds that in other countries Ghafari works in, such as Turkey, academic institutions are often involved in studying structural applications for particular building locations to come up with an engineered solution backed up by scientific and engineering analysis.
Ghafari is currently working on the world’s largest aircraft maintenance complex for Turkish Airlines at the new Istanbul Havalimani Airport—an area that sees not only cold temperatures but major seismic threats as well. The MRO complex will use an air circulation system similar to the one at American’s hangar at O’Hare, but it will also feature a radiant in-floor heating system to help keep temperatures comfortable for workers. Because the location is built right along the Marmara Plate in the North Anatolian Fault Zone, the hangars for Turkish Airlines need to be built to the highest standard of seismic design.
Oberlies says hangars in seismic-sensitive areas like this require structural systems with greater flexibility or ductility to absorb energy in case of an earthquake. Essentially, this comes down to requiring more steel for bracing, which increases the complexity and cost.
“Aircraft hangars are not the worst structure in the world for seismic issues. They’re very big and are actually quite lightweight for what they are,” says Harrison. “The way seismic loading affects buildings is that the ground moves no matter what you say or do about it, so there are two ways of designing buildings to resist that.”
The first is designing a building to be flexible and move side to side, which is the case with Rubb’s fabric-based hangars. “The slight advantage to fabric structures over standard metal hangars is that our buildings can more easily accept settlement. That allows us a more flexible foundation,” says Auger.
The second method, Harrison explains, is designing a building to move with the floor and remain rigid during an earthquake. REIDsteel used this method with a hangar it created for Lockheed C-130 aircraft in Kandahar, Afghanistan. Harrison says the back of the hangar features a very wide bracing system that spreads the load that occurs when a building is moved side to side during an earthquake.
Bracing is also critical in withstanding high wind speeds such as those experienced during hurricanes. Harrison says that open doors during hurricane-speed winds can blow a hangar’s roof right off due to the tremendous volume of air trying to move through the hangar.
“Hangars have to be designed for a certain amount of uplift; so, for instance, if you open the hangar doors and there’s a wind incident, the forces actually kind of inflate the building like a balloon, and the roof structure and walls have to be designed to absorb that energy and to resist any structural failure,” says Oberlies. He adds that the reverse can also occur, where wind gusts act as a vacuum that causes deflection on the structure.
In areas prone to severe weather like this, REIDsteel uses its patented Union Jack bracing system, which transfers loads from the bottom of the roof truss down to the walls. Although steelwork can generally flex and redistribute forces around itself, cladding panels fixed on a hangar’s roof can be pared off sheet after sheet if one of the panels manages to be lifted. “If cladding takes a small amount of damage, it will very quickly look like an awful lot of damage is done to the structure,” says Harrison.
Although the fabric covering Rubb hangars is strong enough to hold people or even a vehicle, it can still be prone to tearing or ripping off in hurricane-force winds. Auger says the advantage in this case is that the hangar’s frame will be strong enough to remain standing, and the fabric can be quickly and easily replaced.
Another consideration in coastal and tropical environments is damage from sunlight and salt water. According to Auger, hot-dip galvanizing a hangar’s steel framework is the best corrosion protection to prevent salt from eating away at it. Meanwhile, Auger says the company’s PVC fabric—which can deflect sunlight and heat—will last for approximately 10-15 years before degrading and needing replacement. The fabric typically lasts 20-30 years in other environments, according to Rubb.
Harnessing the Sun is one way in which Auger says the industry is innovating to improve future aircraft maintenance hangars. According to Auger, Rubb is starting to incorporate solar panels into its hangars, which can reduce or eliminate heating costs if the building is facing in the right direction.
Ghafari is incorporating a multitude of energy efficiency features into the new Turkish Airlines complex, such as solar power, a highly reflective floor system and rainwater harvesting. Oberlies says continued advancements in smart building technology such as lighting controls for energy efficiency and intelligent building-management systems will provide even more opportunities in the future.
“The other thing that will affect the operations in hangars is better Wi-Fi technology and the ability to move data wirelessly,” says Oberlies. “That’s an improvement that will increase productivity and, frankly, all the aspects of the building—including safety—will be aided by advancements in wireless communication.”
Future innovations may also improve hangar design for severe weather such as hurricanes. Harrison says REIDsteel is using 3D modeling to evaluate wind forces on hangars, and the industry has advanced over the last decade with a practice called building information modeling, which allows different trades within the same industry to easily and freely share models across different types of software.
According to Harrison, 3D-modeling practices now allow REIDsteel to design strength specifically where it’s needed. “I think the advantage in 3D modeling we’re getting now to allow us to model airflows around hangars will make a huge difference in the near future and should see us build safer hangars at a reduced cost.”