Articles

NEW AIRPORT TERMINAL, BARAJAS, MADRID
POST-TENSIONED CONCRETE COMES INTO ITS OWN
By John Veale, TPS Consult - published in Concrete Engineering International Autumn 2004

Background
In 1996, TPS Consult teamed up with two Spanish companies, architects Estudio Lamela and engineers Initec, to enter an international design competition for the new terminal area (known as NAT) at Barajas Airport, Madrid. The signature UK architects, Richard Rogers Partnership (RRP), were approached and completed the joint venture (JV). In the face of stiff international competition, the JV was successful with a design that combined distinctive architecture with a modular layout to allow for future expansion.

The design brief was developed with the client, Aena (the Spanish airport and air navigation authority), who had sponsored the design competition. An initial scope of work comprising half the main terminal building and transport interchange, with a design period of two and a half years, increased to include the whole of the main terminal building and transport interchange plus a satellite building. The design period was reduced to one and a half years, a reflection of the pressures imposed by Spanish political considerations.

Description
The new Barajas terminal includes a 470,000m² main building, a 315,000m² satellite building, a 350,000m² multi-storey car park for 9,000 vehicles and a transport interchange linking cars, taxis, buses, metro and mainline train services to the automated people mover (APM). A 2.1km service tunnel connects the main and satellite buildings and accommodates vehicular, baggage-handling and APM movements. Figure 1 shows the overall NAT layout and reveals the linear form of the two main buildings, each almost a kilometre long, made up of 72m long modules. The characteristic shape of the roof structure is clearly apparent in Figure 2.

Structural design
The project has been carried out in accordance with Spanish construction industry practice. The design was developed in two stages - concept design (Proyecto Basico) and tender design (Proyecto Constructivo). The tender documents, comprising drawings, specifications, calculations and quantities, represented the final design and were completed by the end of 1999. The contractor's scope of work covered design and construction but, unusually in Spain, included responsibility for the design. This acknowledged the severely curtailed pre-tender design period.

Design was carried out in accordance with Spanish National Codes. Estudio Lamela appointed two Spanish structural sub-consultants to help with the design work and to provide essential advice on Spanish construction practice - OTEP Internacional and HCA, both based in Madrid. At the time of the design competition, Anthony Hunt Associates (AHA) were appointed as structural sub-consultant to advise on the distinctive roof design.

In the early stages, the overall structural design approach for the NAT buildings was developed by the architectural and structural design team. The modular system, which was a notable feature of the winning design, applied to the main terminal and satellite buildings. The car parks, elevated road structures and underground transport interchange adopted different structural arrangements, albeit within the context established by the modular system.

Main terminal and satellite buildings
The main terminal building falls into two main parts, the finger pier approximately 0.8km long, providing access to the aircraft stands around the building perimeter, and the central entrance area where passengers check in. The satellite building is similar. The finger pier consists of 72m long modules laid end to end. The typical cross section is shown in Figure 2. The central entrance area is made up of 72m long modules laid both end to end and side by side (at 72m centres). In general, the entrance area modules have two floors above and below ground level (at levels +1 and +2 and at levels -1 and -2). These floors are continuous across the 54m module width above ground level, with two additional 9m spans between modules below ground level. This arrangement creates an 18m gap, two storeys high above ground level, known as "canyons" (cañónes). These form a noteworthy feature of the layout and accommodate stairs, escalators and bridges to suit the pedestrian movement requirements. The roof is wider than the typical module structure and is extended to create a continuous cover over the central terminal area, while preserving the distinctive wing shape centred on each line of modules. In the longitudinal direction, the floor spans are 18m. Figure 3 shows the shallow floor structure and long spans, as well as the longitudinal articulation of the wing-shaped roof.

The module structure was developed from investigations into possible floor structures. Column grids of 9m x 9m and 9m x 18m were considered in conjunction with concrete, steel or composite beams and floor slabs. Long spans and shallow floor structures were preferred, in order to give the greatest possible uninterrupted areas and minimum building height. Flexibility in the provision of floor openings was also desirable. Concern was felt, however, that the use of only one structural material for the whole of the building works would impose unacceptable pressure on the construction industry, given the very large size of the project. The design team concluded that a composite floor structure, using steel beams acting in conjunction with a composite steel and insitu concrete floor slab, would achieve the optimum balance between steel and concrete content. It would also be structurally efficient and capable of adaptation to accommodate floor openings. The Spanish construction industry, however, is accustomed to using concrete for the great majority of building structures and had no doubts about its ability to handle the great size of the NAT project. As a result, concrete beams and precast concrete floor slabs were adopted, despite the fact that the only way for the main longitudinal beams to meet design criteria was by the use of post-tensioning. Above ground level, these beams were limited to a structural depth of 800mm for spans of 18m. At ground level and below, with higher design loads, a structural depth of 900mm was used for spans of 18m. The beams were 1.8m wide, leaving a 7.2m gap for the precast concrete floor planks. The reinforced concrete beam section was capable of achieving adequate strength, but required post-tensioning to remove dead load deflection. Creep and live load deflections were within acceptable limits.

Lateral stability is achieved by framing action, to avoid the need for bracing or shear walls interfering with usable space within the buildings. Reinforced concrete columns are circular in section, apart from the roof support columns on the module centreline. These are rectangular in section, arranged in pairs, as can be seen in Figure 4. The modules are 72m long, corresponding to four 18m spans. The modules are separated by movement joints, offset 2.7m from a column line to simplify the joint detail.

The use of post-tensioned concrete beams in the structural module can be regarded as an innovative technique in building construction, especially when it is applied to the many kilometres length of beams on the NAT project. It has had the beneficial result of imposing a clear-cut discipline on the building layout, which nevertheless allows considerable flexibility in accommodating local variations. The longitudinal beams are clearly "inviolate", but the longitudinal gaps between them can be left open for baggage-handling conveyors, stairs, lifts and services with considerable freedom, provided that enough transverse structural connections are made overall to maintain lateral stability. The resulting structure has clean, simple lines as seen in Figure 5, which also shows the clearly defined gaps between precast concrete floor units to accommodate ceiling fixings. Further flexibility is easily achieved by adding beams and columns to create a walkway along a finger pier at level +2, for example, or to create the extra levels and continuous floor areas needed within the main terminal entrance area.

Car parks
The multi-storey car parks are completely separate from the main terminal building and required no special structural consideration. An early proposal to adopt long spans, in order to improve the car-parking layout, was not pursued. The Spanish design loading of about 4kN/sq.m is higher than the UK loading of 2.5kN/sq.m and makes the longer spans uneconomic. The car parks are of insitu concrete construction, using circular columns on an 8m x 8m grid with waffle slab floors.

Miscellaneous structures
The project includes much underground construction, not only in the main terminal and satellite buildings, but in the transport interchange also. The underground structures and the elevated access roads are predominantly concrete structures. There are, however, many access structures associated with pedestrian movements within, into and out of the terminal and satellite buildings that are predominantly of steel construction. There is also a large amount of steel in the modular roof.

General comments
The structural design of new terminal buildings for Barajas Airport was not intended to be innovative for the sake of innovation. In fact, there was probably an unspoken preference for tried and tested materials and techniques, in order to minimise the pressures on contractors undertaking a very large and politically sensitive project. It is, therefore, ironic that the large-scale use of post-tensioning in building structures should have been suggested by the very contractors who might have been expected to demur at its use! With hindsight, the adoption of post-tensioned concrete beams has been of considerable benefit to the project, well in keeping with the distinction already conferred by the architectural and structural quality of the roof.