The expansion of Jordan’s Queen Alia International Airport features a striking domed roof canopy that is clad in black in reference to the dark Bedouin tents of the region. © Nigel Young / Foster + Partners
The concrete used to create undulating roof domes above a new extension to the Queen Alia International Airport reflects the colors and textures of the local landscape.
May 14, 2013—“I think it’s important for us—for all of our projects—to make them unique to their place,” says Jonathan Parr, a partner of London-based Foster + Partners. “We very much believe in designs being ‘on the location’ as much as they possibly can be, and come out of either an inspiration of the place, and/or a relationship with the client, perhaps. It’s a unique process we go through.”
This is especially true for airports, he says, which serve as gateways to cities and countries. “With our airport in Beijing,” he continues, “that’s the amazing use of color, which is influenced by Chinese culture, and the reference to the dragon and the scales on the dragon’s back [in] the texture and the form of the roof of the building.”
So when the firm was selected to design an expansion of Jordan’s Queen Alia International Airport, located 35 km from the capital, Amman, it considered very carefully how best to capture the unique local culture. Its striking tessellated roof canopy—a rippling pattern of shallow concrete domes, framed by delicate X beams—is clad in black, a reference to the dark Bedouin tents of the region. The terminal serves two departure piers, one on either of side of the terminal but separated from it by courtyards that, according to a Foster + Partners report on the project, draw on vernacular Arabic architecture. The courtyards are filled with plants and trees that help to filter air before it enters the airport’s air-handling system.
The central terminal is flanked by vegetated courtyards that lead
to the departure gates, which will nearly triple the airport’s
capacity. © Nigel Young / Foster + Partners
But the airport’s chief connection to the land is the very concrete used to build it. According to Parr, designers settled on concrete as a primary building material because they determined that it could work well in a desert climate that sees extreme heat in the daytime and bitter cold at night.
“Concrete is a beautiful material in the sense that it can work with that climate, delaying the onslaught of heat buildup and retaining the warmth of an evening when the temperature drops outside,” Parr says. “You retain the heat within the building.”
The locally sourced concrete also possessed a “lovely warm tone” that spoke to the local environment itself. “It is not gray. It is beige. And with the qualities of the light in the Middle East, that further emphasizes that warmth, particularly in the late afternoon.”
However, the concrete also created its share of challenges, as designers crafted a complex and iconic geometry for the airport’s roof. “Over a period of time, it became very apparent that Foster was keen to do something a little more dramatic,” says George Keliris, CEng, a director in the London office of engineering consulting firm Buro Happold, which provided structural engineering on the project. “While the floor plates used were fairly traditional, we allowed the roof structure to be a little more dramatic.”
According to Keliris, the plans originally called for a 12 by 12 m grid of columns in the main terminal. When every second grid of columns was removed to open up the terminal’s interior, the roof spans grew from to 24 m, a distance that was resolved with a domelike roofing system.
The dome motif of the roof is continued to the exterior canopy.
Concrete was chosen in part for its ability to withstand the harsh
climate. © Nigel Young / Foster + Partners
While the foundations, pile caps, ground floor slab, and vertical columns were all made with cast-in-place concrete, the building also employed precast concrete throughout the terminal roof: both on the column heads that connect the columns to the X beams on the roof, as well as for the X beams themselves, which provide the framework for the concrete domes. (The X beams split as they approach the columns, creating a leaflike opening that admits light throughout the structure.)
The 127 roof modules—which comprise full domes, half domes, perimeter shells, and corner units—were also assembled out of precast concrete segments. The precast units worked both as a permanent formwork for the cast-in-place concrete that would later join the units together, as well as provided a finished surface that enhances views from below.
According to Foster + Partners, precast concrete was chosen for the domes because it could ensure the best surface quality for the roof as well as more easily achieve the roof’s complex geometry. Using cast-in-place and precast concrete together is not uncommon. “The idea is something which gives you a finished surface, which is able to span but couldn’t quite take the weight, which is supplemented with additional concrete,” says Parr. “It’s pretty straightforward.”
What was less straightforward was making the concrete work on such a large, geometrically complex project. Forming the X beams, for instance, required producing 308 pieces, each weighing between 19.5 and 19.9 tons. The domes themselves required 720 shell segments, each weighing around 23 tons. Even the column heads that connected the roof beams to the columns weighed 17 to 21 tons each.
Engineers and designers had to strike a balance with the precast: it is valuable because it is less expensive and lighter, which meant it was safer to transport and install. But it had to be strong enough to withstand the loads from demolding, transportation, and erection. According to its report, Foster used a high-performance steel fiber-reinforced mix with a maximum aggregate size of 3/8 in. “The design is complicated but not an order of magnitude more complicated,” says Keliris. But the actual practice of putting the cast-in-place and precast units together was, he adds.
The structure’s ground floor slab and vertical columns were all
made with cast-in-place concrete. Precast concrete was chosen
for the domes themselves to ensure the best surface quality to
achieve the roof’s complex geometry. © Nigel Young /
Foster + Partners
Self-leveling mortar and steel plates were used to ensure that the structure’s 110 columns were level, so that the column heads could rest on them evenly. A steel ring on the bottom of each column head was welded to a steel plate at the top of each column. After the X beams went up, the dome shell segments were placed—with curved steel supports in the dome apex to bring the pieces together—and then the cast-in-place concrete was poured to solidify the dome elements together.
“For construction reasons and for tolerance and for practical reasons, we did not take those segments to an absolute triangle point,” Parr says. “They would have been very vulnerable, they could have been difficult to cast, and the alignment between them could have been something of a logistical challenge.” The roof could deflect ever so slightly as the concrete was cast, and the square of the shell might not have been perfectly square. “If it was ever so slightly off square, then these eight points would not meet perfectly and you would see that quite noticeably,” Parr explains.
So crews plugged the top of each dome with an extra piece of concrete to provide some wiggle room. The plugs are visible at the drop-off at the exterior edge of the building, but inside they’re hidden by acoustic treatments on the underside of the roof.
Assembly of the roof required careful coordination with crews working below. It was tough, Parr notes, even just to keep the columns unmarred. “When you’re exposing concrete columns, people have a habit of drawing on objects around sites, to allow a conversation of how something should be built,” Parr says. “They’ll draw on the floor, or they’ll draw on the wall.” During construction of Hong Kong’s Chek Lap Kok Airport, for example, Foster tried to get contractors to mark the columns in wax crayons or chalk. At the Jordan airport, they also tried to defend against the columns being accidently hit by wrapping them in polyethylene and pieces of plywood.
Cast-in-place concrete connected the precast concrete dome
elements together, and these joints presented the greatest
challenges during construction. © Nigel Young / Foster + Partners
To help give the concrete in the roof a finished appearance, the crews fabricated the formwork from steel. “They gave a much cleaner expressed soffit,” Keliris notes. “A timber-type mold would have resulted in something less attractive.”
The formwork is made of 5 mm thick steel, and because the work of curving the formwork resembled ship construction, the steel molds were actually fabricated by workers in Athens with experience in shipbuilding. The 19.4 m long X beams were each made from two 9.7 m long forms, which had to be partially disassembled and transported from Greece to Jordan by ship. “The molds are quite monumental,” says Parr. “They’re truly heroic.”
The concrete was made at a batching plant built a few kilometers from the construction site. The close proximity had its advantages: “You can buy your aggregate from a single or limited [number of sources], so you have control over the quality,” Parr says.
The new terminal opened in March. According to Foster + Partners, the airport’s passenger traffic is expected to grow by 6 percent per year for the next 25 years, increasing capacity from 3 million now to nearly 13 million by 2030. The modular nature of the design will make future expansions relatively straightforward—Parr notes that Foster + Partners is already in talks for an additional expansion.