Passengers flying out of Madrid's Barajas airport glimpse an extraordinary sight alongside the recently completed runway. A forest of 22 red and white painted tower cranes mark the spot of the new terminal which is almost a year into construction.
More cranes will appear shortly as work gets under way on a second site for a satellite terminal connected by tunnel.
The spectacular new terminal building, a 1km long slender wing shape, will more than double the potential capacity of Spain's main airport when it comes on stream in 2004 as part of a £1.86bn expansion programme.
Passenger growth of 10% has increased throughput to 35M annually in the existing and recently extended terminal. The new facility will provide another 35M passenger capacity with a further 15M in the satellite.
'We think we could become the main gateway airport for Europe from South America, ' says a spokesman for airport authority and client Aeropuertos Espanoles y Navigacion Aerea (AENA).
'With completion of the new runway we have discovered a lot of hidden demand, ' he adds.
Plans for the new terminal were made in the early 1990s and the project got under way seriously with an international competition in 1997. Renowned British architect Richard Rogers teamed up with Spanish firm Estudio Antonio Lamella for the project bringing in UK structural consultant TPS (Carillion) and Spanish firm Inetec, the latter having the legal powers to sign off the drawings.
'It was almost certainly the roof which won it for us, ' says Leonart Grut at Richard Rogers.
The 1km long lightweight steel cover with a curved profile is likened by AENA to a giant aircraft wing.
Specialist steel consultant Anthony Hunt was brought on board for this part of the structure.
But the layout of the terminal also impressed, in particular the full height atria or 'canyons'.
Stretching the length of the three and four storey high central building, they act as giant light wells, with a glazed roof and free standing lifts and escalators.
The building is made up of modules each 54m wide and 72m long. Along the airside length these form a continuous structure, while a second row and shorter third and fourth rows form the central retail and check-in areas and the landside transport interchanges.
The satellite will be predominantly a double row of modules.
The two will be connected by a 2km long people mover in a huge underground tunnel, already in place below the runway, which will also have sections for baggage transport and service vehicles, and links to existing terminals.
'The main terminal structure is all in concrete, ' says TPS project manager John Veale.
'We were told that in Spain steel was ruled out.' Steel will be used for the roof, however.
Richard Rogers wanted an 18m by 18m grid, says Veale, although this proved too difficult. 'The architects always want no columns at all while the engineers want plenty, ' he says.
The compromise was an 18m span longitudinally with 9m column spacing laterally. The central column is a double square section with links at each floor forming a ladder. This carries the main loadings from the roof on 'horns', four steel tubular struts angling out to 9m centres.
Circular columns stand in rows either side to make up three 9m wide bays. The 18m longitudinal links are formed with post-tensioned beams, chosen because steel was ruled out and reinforced concrete too thick.
The floor slab is essentially a one way span, with concrete planks bridging the 9m gaps.
This structure allows gaps to be left, creating voids.
'It gives framework discipline within which the architect can play tunes, ' says Veale, 'though there are limits.'
The module's floor beams are stressed during construction in 36m lengths to avoid excessive column deflections. Each module is jointed to the next with expansion joints slightly offset from the grid.
'There are kilometres and kilometres of post-tensioned beam, ' says Veale, 'probably more than Spain has ever seen or have even been made in one place anywhere in Europe.'
Above the concrete frame sits the steel roof. Les Postawa, Anthony Hunt's project manager, says this is fairly straightforward comprising a steel frame with the main steel girders fabricated in the curved shape.
The girders are at 9m centres supported by the four tubular 'horns' which splay out like branches. More slender tubular struts support the ends of the roof.
'And then we have the kipper trusses, ' says Postawa. These are tension elements, in the shape of a fish, with curved catenary rods separated by short horizontal compression rods.
These put a load of about 50t on to the roof which aids stiffness, but their main function is to support the set back glass facade, cutting the need for heavier columns. 'We had an extra uplift to allow for on the frame too, ' adds Veale.
TPS has also acted as specialist planner, analysing passenger paths and traffic flows of baggage, personnel, and the retail and services spaces in between.
All this has been set against a fraught timescale. The project was delayed after go ahead, and then suddenly pushed forwards at great speed to meet the end date. One significant outcome was a very tight design schedule.
In fact construction got under way with last detail design still on the drawing board. It is now the job of the contractor, a joint venture of FCC, ACS, Ferrovial, Necso and Sacyr, checked off by the engineers.
Complications at the site include the sheer scale of the work, with 2,500 workers already on the site and at least double that number expected at peak.
A major issue for AENA as site project manager is security, where workers must move between landside to airside. It is also concerned about safety, a growing issue in Spain, particularly on such a prestige site.
Away from the main terminal contract, Dragados is at work building the large multistorey carpark. And a 1,770m extension is being built to the 840m underground section of the M11 highway linking Barajas to Paracuellos del Jarama.
Four two lane horseshoe shaped tunnels are formed side by side using a special steel formwork frame. The overall width of 54m fits in a 16m deep trench which when backfilled, gives 8m depth to the crown.