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Concrete showcase

Airports Charles de Gaulle

At Paris Charles de Gaulle, pioneering architectural and structural design is being used to dramatic effect, discovers Andrew Mylius.

It seems perverse to deploy construction techniques developed for tunnelling on a building perched atop columns. But at Paris Charles de Gaulle airport, precast segmental elements more commonly seen lining tunnels have been used to fabricate the structural core of an elegant, tubular terminal building.

Charles de Gaulle is a showcase for three decades of structural and technical innovation that have been driven by the aesthetic adventuring of operator Aeroports de Paris' (ADP) chief architect Paul Andreu.

Sensuous curves, swooping rooflines, dramatic gashes of glazing that pour light into internal spaces and raw, untreated materials are characteristic of Andreu's palette.

At the arrivals and departures building of terminal 2E, now under construction, he wanted 'a space that was high and generous' without the interruption of supporting columns.

Because the terminal building faces south he ruled out a glazed facade - solar gain would be huge unless tinted glass was used, and 'I like to be able to see the proper colour of the sky.'

For thermal mass and to provide shading, Andreu homed in on one of his favourite materials, concrete, designing a 700m long, 32m wide vaulted structure.

Andreu is not a fan of composite architecture. 'I like concrete to be only concrete. I hate it being spoiled by steel or aluminium, ' he says. His austere aesthetic code dictated that, save for flooring, furniture and essential plant such as travelators and escalators, no alien materials would detract from the concreteness of the interior space.

Meanwhile, though he was aiming to create a building suffused with natural light, Andreu was adamant there were to be absolutely no windows. He felt large openings would destroy the smooth integrity of the concrete walls, and instead devised to perforate the vault with a regular grid of square piercings.

They are too small to break up the vault visually, but accentuate its vanishing perspective lines.

In plan the new terminal building is composed of 10, 68m long, straight sections, arranged into a gentle, banana-like curve.

But in section Andreu describes it as 'an onion', with a double storey structure at the centre, enveloped by the concrete vault which is in turn overlaid by services and an outer skin of glass and stainless steel.

When it set its brief for terminal 2E, ADP was aiming to reduce its construction costs in comparison with previous terminal buildings. Reconciling Andreu's unyielding aesthetic demands with the requirement for economy fell largely to ADP principal engineer PaulChristian Muller.

Andreu and Muller both worked on the previous Charles de Gaulle terminal, 2F, which, although it is twice as wide as 2E, bears a passing resemblance in having a vault of reinforced concrete overlaid by glass cladding. Terminal 2F's 60m concrete vault was cast insitu, and resulting falsework costs were high. Construction fell behind programme and quality was difficult to control, says Muller.

This, combined with Terminal 2E's smaller span - the vault is 27m wide at its base, opening up to 32m at mid-height - convinced Muller and Andreu to take a structurally unorthodox step.

They decided to break the vault down into three 4m wide, 300mm thick precast concrete elements - two wall elements and one for the vault's 'crown'.

This would remove the need for costly falsework and aid faster, more efficient construction.

Without any support from surrounding ground, however, the tunnel-like concrete vault would collapse, with its walls buckling outward and its crown sagging. Rather than add mass to the elements, Muller tackled the buckling problem by tying steel trusses to the outside of each wall. The trusses are bolted to connections cast into the top and bottom of the wall elements.

Eight struts between the truss and concrete element prevent deflection, keeping the truss in tension and the concrete walls in compression.

Each of the 10 straight sections of the vault has been erected sequentially. The contractor, a joint venture between Laubeuf and Eiffel, tackled construction in 4m 'rings', supporting two 35t wall segments and a 55t crown element on temporary steel frames before moving on to the next ring.

The whole building is raised on two rows of rectangular columns, founded on 600mm diameter, 15m deep concrete piles. Columns in each row are positioned at 8m centres. Once the vault elements were in place and supported by the temporary framework, massive longitudinal reinforced concrete beams measuring 600mm deep by 500mm wide were cast insitu atop the columns and tied into the wall elements. Roof and wall elements were joined using heavily reinforced cast-insitu concrete stitching.

Soon after the first longitudinal beams had been cast and the temporary supports removed, however, cracking was noticed in some of the column heads.

It emerged that the column reinforcement had not been placed high enough to give full structural integrity. Forces imposed by the beam threatened to shear off the columns' shoulders.

The problem is being solved by wrapping the top 300mm of the columns with carbon fibre bands: Eiffel/Laubeuf are jacking the beam 1mm to 2mm off the columns to relieve them before the wrappings are applied.

Deflection and differential movement in the structure have also posed a challenge. In section, the vault is expected to deflect up to 80mm under wind and snow loading, and 20mm as a result of expansion and contraction - temperatures in the void between the concrete vault and the steel and glass outer skin are predicted to reach 80infinityC at the height of summer. Expansion and contraction will also result in longitudinal movement.

Elastometric bearings have therefore been provided between the column heads and longitudinal beam. Differential movement between the concrete vault and cladding layer has been tackled by leaving the bottom of each cladding panel and one side edge free to move.

A different approach to movement has been needed for the terminal building's fully glazed end walls, however. Here, 300mm deep steel arches have been erected, from which deep steel stiffeners and sheets of laminated glass hang like a curtain. Though the arches are sandwiched between, and blend seamlessly with, the adjacent vault elements they are structurally independent, allowing the vault to deflect without impacting on the glass.

Three horizontal cables have been used to restrain the sides of the arch from bending under self-weight. Preloading was used to mimic the weight of the glass and progressively reduced as glazing advanced, ensuring the arch did not change shape.

Terminal building 2E is scheduled to go into operation in June next year.

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