Of all the typical issues that new airports commonly deal with, expansion was probably the most elemental at Stansted. From the very beginning this facility was intended to relieve the overburdened terminals at London’s Gatwick and Heathrow airports. Growth had to be planned for.
The distance of Stansted from the center of London added two important design considerations. First, the terminal would have to include a rail transit station with direct service from the city. This was essential to maintaining reasonable commuting times to Stansted and making it competitive with London’s Heathrow and Gatwick airports. Second, the relatively remote rural location led authorities to appoint Stansted as the designated landing field for hijacked flights. Antiterrorist measures heightened the need to incorporate security measures into the terminal scheme by controlling access and flow between the terminal areas. Ultimately, all of these restrictions conflicted with Foster’s intention of providing passengers with an easy stroll to the airplane.
Within the Stansted Mountfichet village community (1991 population of 5361 living in 2037 households) there were other issues to address. Quiet rural neighborhoods and noisy international airports are naturally not a good mix. Local authorities and the general populace of the sleepy Essex communities around Stansted had opposed the project on environmental grounds for years. It did not help that Stansted was also a large freight carrier and therefore featured frequent night flights with noisier cargo planes. Of course, as the airport grows and traffic increases, noise becomes an ever-increasing problem. In suburban Denver this noise problem led to environmental nuisance lawsuits and the eventual relocation of the airport.
Other challenges were presented directly by the client. Working for BAA, Foster Associates was dealing with an experienced group of experts who had detailed requirements. Here was a rare opportunity for BAA to plan a new airport from the ground up rather than expand an existing facility piecemeal. After enduring several years of waiting for the project to find a home and then two more years of public hearings, BAA was anxious to get Stansted completed. The client also expected everything to be optimized, efficient, and reconsidered from first principles. Moreover, after experiencing some “sticker shock” associated with expansion at Gatwick’s North Terminal, BAA was in no mood for cost overruns. Time and budget were established with very strict limitations. The project time frame was compressed, and thus construction started before design was complete. Further, the construction would have to be sequenced so that a dry enclosure would provide shelter for uninterrupted work to completion, regardless of the weather.
Airports are inherently complex programs, and Foster’s intention of dramatizing travel required clarity and simplicity. The model of Saarinen’s airport at Dulles was close to Foster’s vision of the pioneering days of commercial flight when passengers walked from their cars to their planes. But modern requirements and the escalating number of air travelers conspired against those romantic notions. The complexities of baggage handling, ticketing, security, high occupancy flows, concessions, retail shops, and the expansive dimensions of wide-body aircraft would have to be dealt with. Keeping the terminal simple and maintaining clarity in how passengers would move directly through it required a new prototype.
Mixed into the formal and experiential intentions was a response to the rural context. In agreement with the BAA, the terminal building would have a profile no higher than a mature tree: 49.2 ft (15 m). Given the resistance of local authorities who opposed the environmental threats of Stansted and the pressures imposed by its growth, Foster and the BAA would have to work diligently at creating neighborly relationships. An 800,000 ft2 (74,285 m2) terminal building with a 2 mile long runway and 8 million passengers per year was not going to blend easily into the village setting.
Appropriate Systems
The romantic precedents for Stansted come from Foster’s affection for small airports. Spencer de Grey, the director of Foster Associates in charge of the project, also likens the terminal to the vast skylit interiors of nineteenth-century iron and glass railway stations.
In regard to function, the design team looked at Dulles airport as the old prototype for direct passage through a terminal. Upon analysis, the economy of a mobile lounge solution was attractive enough, but its carrying capacity was too small for modern passenger movement and wide – body aircraft. The new prototype at Stansted would have to maintain the simplicity of Dulles, but substitute something else for the mobile lounge. A comparison of the two building sections shows the similarities of flow through the two- level terminal buildings. Both maintain simplicity by minimizing the distance passengers walk across the main concourse, and both banish all the mechanical and baggage functions to the understory.
As a continuum of Foster’s architecture, Stansted shares elements of its scheme with his earlier sheds for the 1977 Sainsbury Art Center and the 1982 Renault Distribution Center. There is also some mention of the steel masted Fleetguard factory by his former partner, Richard Rogers. The Renault building in particular reveals features that were important in the development of Stansted, especially its steel mast structure and square roof grids.
During the ten-year debate and two-year public hearing over where London’s third airport should go, ecological impacts on its setting became a focus issue. In 1980 a consulting ecologist firm, Penny Anderson Associates, was commissioned to survey the proposed development area at Stansted. Included in the firm’s study was a 100 acre (40 hectare) tract of ancient woodlands next to the development area, which had been officially designated as a Site of Special Scientific Interest by the Nature Conservancy Council. The airport site and the rest of the adjacent Essex County lowlands were mostly farms. Among them was an eroded network of woodlands, some small areas of grassy fields, a few ponds and streams, and some surviving hedgerow. Much of the natural woodlands environment had already been lost to postwar agricultural development.
Working with the planners and landscape architects, the ecologists helped to identify the areas of highest value to be preserved and worked with transplanting some of the hedges and grasses. This work had to be completed before construction was scheduled to start in the spring of 1986. A 2 acre (0.8 hectare) area was identified, and the ecologists set out to move the best of the transplanted vegetation and soils there. Some of this farmland area had been contaminated with enhanced nitrate and phosphate levels from fertilizer residue. After the topsoil was cleared away and replaced, the reserved plot was restored to a native wildlife setting. Other areas of the airport site were seeded with native grasses or plants, and a wetlands was developed on one end of the runway so as to attract dragonflies and frogs rather than birds. Maintenance programs such as mowing were established to match the growing cycle of the landscape. As the ecologists began working more directly with the landscape architects, Adrian Lisney & Partners, the restored wildlife area was used as a model for the landscape redevelopment of the entire site. Plantings were made according to the characteristics of the ancient woods and grasslands that are indigenous to Stansted. Eventually, 10 percent of the total land area was devoted to landscape and more than 250,000 trees and shrubs were planted.
Development of the terminal area and other structures on the site began with transportation links on both the landside and airside access. Passenger arrival is situated along the south edge of the terminal, where a drop-off and covered entry court are situated in front of the concourse level beneath an open bay of structural trees and covering roof canopy. Below that, in the undercroft level, is a 21,600 ft2 (2,006 m2) British Rail station. The south wall of the rail station doubles as a retaining wall for a large earth berm above the short-term parking lots. This wall is penetrated by a walkway leading from the surface parking to elevators and ramps up to the concourse. Passengers depart to aircraft boarding gates from the other side of the terminal via a tracked transit system to airside satellite terminals. The automated double-track transit system carries up to 100 passengers along 8900 ft (2713 m) of track and operates continuously in a four-stop loop. Four of the outlying terminals were planned, but only one was built in the first phase. An additional domestic flight terminal is so close to the terminal that it is connected by an enclosed walkway, and passengers walk to the jetways in the usual fashion.
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Figure 7.12 Site plan of Stansted Airport. |
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To keep the volume of the terminal below the 49 ft (15 m) height barrier, Foster selected a rolling area of the site and burrowed the building into it. More than 1.3 million yd3 (1 million m3) of soil were excavated and used as fill around the building. The rail station and the undercroft floors are about 10 ft (3 m) below grade and are 29 ft (9 m) high. A service road runs through the undercroft from east to west and divides the south mechanical services zone from a larger north baggage handling zone. Above, the concourse level extends south from the terminal floor to cover
the rail station and provide a forecourt entry under the extended roof. The forecourt deck is penetrated with thick glass skylights set flush into the concrete slab between the trunks of the structural trees. The skylights are centered over the British Rail station platform. On the north side of the terminal, the concourse floor is interrupted by the terminal rail transit system. A bay of structural trees and their canopy provide cover for the passenger boarding platforms. The east and west sides of the terminal are left blank so that the building section can be extruded in both directions to meet expansion requirements.
Overall, the undercroft is 26.3 ft (8 m) high and the passenger concourse level reaches another 39.4 ft (12 m). In plan the terminal is based on repetition of a 59 X 59 ft (18 m) grid of 11 X 11, or 121 equal bays. Thirty-six structural trees sit at the centers of alternate grid squares, 118 ft (36 m) on center and 59 ft (18 m) high from undercroft floor to roofline. Horizontal bay spacing between the trees was determined by the minimum bay size of the baggage handling system. Each tree rises as an 11.5 ft X 11.5 ft (3.5 m) trunk of four 18 in. (457 mm) diameter steel tubes from the building’s foundation to 13.1 ft (4 m) above the concourse floor. To this point, the frames are similar to the clustered column towers Foster used at the Hong Kong and Shanghai Bank (see case study #25). Above that point, the frame branches outward to pin connected tubular steel booms. The outrigger booms of 7.2 in. (183 mm) steel tube reach another 26 ft high and out to the corners of the 59 X 59 ft (18 m) bay to support the 12 in. (323 mm) diameter roof grid frame. Prefabricated roof canopies fill the span above the trees as well as the open bays between them.
Within the branching outriggers are two pyramidal frames for bracing. The first rests on top of the four vertical trunk members and converges 13.1 ft (4 m) above it, halfway to the roof. The second pyramid is inverted and consists of 1.6 in. (40 mm) steel tension bars prestressed from the top of the first pyramid out to the four corners of the roof frame.
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The shallow dome roof shells, or “bubbles,” are each prefabricated at grade level from a square frame, steel lattice supports, and a deck of simply curved aluminum panels. Once assembled, they are lifted by a crane to rest on the structural trees and connected by a gutter system. The decks are covered with 6 in. (150 mm) mineral wool insulation and sealed with PVC single-ply roofing. The canopies provide rigid bracing to the 59 X 59 ft (18 m) roof frame. Dimensional tolerances in the whole system are so close that no expansion joints are required. Expansion and contraction, as well as wind loads, are taken up by flexure in the steel frame. There is a scissors-like connection at the eave line between the roof structure and glazing wall that allows 7.9 in. (20 cm) of movement. The fascia is actually closed with a plastic membrane at the eave.
The concourse floor is a 12 to 24 in. (305 to 610 mm) thick waffle slab of precast and cast-in-place concrete. It is supported by concrete columns on a 20 ft (6.1 m) grid on the southern side, where the 29 ft (8.8 m) high undercroft area is used primarily for the mechanical and electrical plant. The northern half is supported on a 39.4 ft (12 m) square grid to allow minimum clearance for the baggage handling system.
Construction sequence was planned for uninterrupted speed. The strategy was to close in the envelope quickly and allow work to continue out of the weather. So the structural trees were erected first, followed by the roof canopies. Structural components were factory prefabricated, and all joints ultrasonically tested before delivery and assembly at the site. The tree trunk bases of 55.8 X 11.5 X
11.5 ft (17 X 3.5 X 3.5 m) came first. Their size was the largest that could be transported by truck. The towering bases were erected from the truck to their four foundation pads and bolted into place. Pin connections carry the connecting branches, and a custom casting ties tubular members to the tension bars. The final connection and pretensioning is via a “Jesus nut” that holds everything in place at the top of the upright pyramid frame where the four tension rods connect.
After the steel structure was in place, the roof canopies were assembled on the ground and lifted into the tree frame. Construction then proceeded with rain protection in place. Final excavation, drainage, and the at-grade slab for the undercroft were next. The concourse slab and the mezzanines followed.
The sheltering roof was planned to provide cover for construction work even before the on-grade and waffle slab floors were poured. The final solution resembles a field of 121 bubbles, each weighing 11 tons (10 metric tons) and crowning out 9.8 ft (3 m) above structure at the 49 ft (15
m) height limit above the concourse floor. Rainwater from the 421,000 ft2 (39,092 m2) roof is collected without interior drains that would interrupt the concourse space. Instead, drainage is accomplished by horizontal stainless steel pipes laid in gutters between the canopy modules. These are drained by siphon action to 16 exterior downspouts. Wind uplift across the roof is countered by an air foil section along the eaves. This foil was developed during predesign in wind tunnel tests at Bristol University.
Each of the 121 roof canopy sections incorporates four triangular skylights around a rotated solid aluminum square at its center. The 139 ft2 (13 m2) area of glazing cos – titute 4 percent of the total roof area. Daylight levels, based on a design daylight factor of 3 percent, provides 17 foot – candles on a clear winter day. Energy savings via reductions in lighting power are expected to amount to $500,000 per year. Beneath each skylight is a perforated metal reflector with a white finish. This reflector diffuses daylight and reduces glare; it also acts as a reflector for artificial uplighting from the service pods below.
Several materials were considered for the ceiling side of the roof canopies. Metal panels would be durable but acoustically reflective, creating a noisy interior. Fibrous surfaces would be quieter but hard to clean and less light reflective. Tensioned fabric panels were too expensive. The final solution meets all requirements: Perforated steel trays with fibrous mineral fiber backing provide both light reflection and sound absorption.
Glass encloses all four sides of the terminal at concourse level. The insulated tempered glazing units are argon filled and low-emissivity coated. An exterior (8 mm) pane, an air cavity, and a 0.25 in. (6 mm) inside pane make up each unit. Set in an aluminum frame, the window wall provides an effective R-3.5 insulation level. Each glass panel measures 11.8 ft (5.6 m) across by 6.6 ft (1.85 m) high, dimensions that open the view and limit heat losses through metal framing members. The solar orientation of the building is actually skewed so that the entry facade faces southeast. The reason is probably that the runway was long ago designed to face into the predominant southwest winds and the terminal building is aligned to look out onto the runway. On the unprotected northeast and southwest elevations, where there are no projecting structural trees or roof canopies, double pane laminated glass with a reflective inner layer reduces solar gain by more than 60 percent. The glazed walls are supported by a frame of 3.4 X 2.3 in. (86 X 58 mm) steel mullions. This frame is stiffened by a vertical truss of 3.2 in. (80 mm) steel tube running up the inside of the wall.
The undercroft is wrapped with 11.8 ft wide X 3.3 ft (3.6 m X 1.0 m) high insulated aluminum panels with an R-value of 20.0 and a polyester powder coat finish. Set in with them are double-glazed acrylic vision panels of the same size. Aluminum louvered panels are used where air intakes are required. Any of these panels can be removed and relocated within the vertical supports of 8 in. X 8 in. (203 mm) steel channel mullions that span the 26.3 ft (8.0 m) high undercroft. In order to allow service access for large equipment, there are no horizontal supports.
The services concept was another factor determined largely by the desire for an uncluttered roof that allowed for expansion and daylight. The major mechanical and electrical equipment would be housed below the concourse floor and feed through the structural trees.
Distribution of services was coordinated by reserving layers of space below the undercroft floor. Each layer is for services running in a specific direction. The lowest layer is
4.6 ft (1.4 m) deep and is dedicated to primary ducts and pipe work running north to south. This includes exhaust air ducts from kitchens, toilets, and smoke exhaust fans. Below that is a 2.3 ft (0.7 m) deep layer for east-to-west services. Air handlers, chillers, boilers, and electrical works fill the lower 13.8 ft (4.2 m) to the floor. Computer simulation was used to identify “interference” points where services were likely to clash.
Air is delivered through service pods integrated into the structural trees. Two sides of each of the 24 interior trees contain rectangular supply ducts that discharge air at the top of the pod through four adjustable drum diffusers. Return air is extracted through a register in the center of the pod’s roof and down through ducts similar to those used for supply air. Service access for each pod is reached by metal spiral stairs from the undercroft mezzanine.
Fire detection, sprinklers, and smoke exhaust are provided in the open-sided kiosks. Smoke extraction ducts draw air down to the undercroft and then exhaust it through chimneys on the east and west walls. The closed wall cabins handle smoke exhaust with the mechanical air conditioning system. In event of a fire the air handler goes to 100 percent outside air and 100 percent exhaust. A fire research computer program, “Jasmine,” was used to simulate fire spread and occupant exit patterns in the main concourse area. It predicted that 20 to 23 ft (6 m to 7 m) of air above floor level would still be breathable ten minutes after fire was detected and that evacuation could be safely accomplished in less than five minutes. A color animation of the smoke and egress calculations assured officials that no other measures were required in the main concourse.
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The concourse is arranged with all passenger movement on one level. Arrivals are on the east side and departures on the west. Overlaying this arrangement with landside functions to the south and airside on the north of the terminal creates a grid of four squares that can be extended laterally as the terminal grows. To board aircraft, passengers pass through the departures lounge and ride small rail transit cars to satellite terminals. This rail system was favored because it not only kept the terminal uncluttered, it also achieved the separation of arrival from departure areas so vital to security. BAA had already had a favorable experience with the same system at Gatwick airport.
To keep the concourse clear and open, Foster limited the height of all structures within the terminal to 11.5 ft (3.5 m). This maintained the desired sense of orientation that comes with seeing all the way through the building from landside to airside. You can see trees outside from the baggage claim and airplanes from the front door. The route is straight and clear both ways.
The lighting design by Claude Engle is intended to raise the apparent height of the ceiling by making it the brightest object in view. Each of the service pods has four luminaries mounted behind the diffusers. These fixtures hold four to six 400 W metal halide lamps, the number depending on their position in the concourse. Wide flood angle distribution and the long throw of light onto the high ceiling ensure uniform reflected brightness. The reflector panel below the skylights obscures the black surfaces at night. Step switching allows the lamps to be dimmed in response to available daylight, and battery powered backup provides emergency lighting from the same fixtures. Task illumination is provided at service desks with modular fluorescent lamps, and halogen fixtures are used in concession areas.
Finishes and furnishings in the terminal are meant to be background. Sufficient drama is created by the structural system, the floating roof, and the see-through enclosure; further sophistication was deemed as conflicting and potentially confusing. The white painted ceiling and structural trees along with tame interior color schemes diffuse interior light levels and balance the brightness of the view outside. This design scheme also allows the eye to be appropriately drawn to the bright information signage designed by the Pentagram group (see NMB, case study #27, for more on Pentagram).
The structural trees are the basis of interior organization in the terminal. The 13 ft (4.0 m) vertical bases below their pyramid branches are wrapped on all four sides with service elements. Along with directional signs and flight information monitors, the pods contain maps, clocks, alarms, public address (PA) speakers, security cameras, staff telephones, and fire hoses.
Foster designed the various freestanding functions in the terminal as demountable “cabins." They are kept below the 11.5 ft (3.5 m) height limit and constructed of steel frames with plastic composite panel cladding. The cabins cover about 25 percent of the concourse floor area.
Mezzanines are prohibited not only because they would restrict view, but also because they would be up in a smoke-affected zone in the event of fire.
Integration Highlights
The modular character of the structural trees facilitates expansion.
Extensive computer modeling was used to coordinate the routing of services through two independent layers below the concourse floor.
Placing services in the undercroft simplified the terminal environment and lowered the building scale.
The transparent structure allows views from the front door arrival point to waiting airplanes on the departures side, enhancing the expectancy of travel.
White painted surfaces and neutral finishes keep the terminal bright and clean but leave the emphasis on arrival and departure.
Integrated service pods provide multiple functions. The placement of mechanical equipment in the undercroft frees the roof for daylighting.
The lightweight roof structure allows long spans and uninterrupted floor space.
The high ceiling also acts as a smoke reservoir, so the main concourse can remain one large space.
The splayed branches extending from the tree columns reduce the roof span to 59 ft from the 118 ft (36 m) free width required by the baggage system in the undercroft.
The project was 15 percent under budget.
In January 2000 construction work began on Phase II of the airport, along with planning for an anticipated final capacity of 15 MPPA. The successes of the initial design are being repeated with more of Foster’s tree service columns.