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Innovative design

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CHARLES DE GAULLE'S $800M Terminal 2E set out to be architecturally and structurally innovative.

The mould-breaking structure was designed by client and airport operator Aeroports de Paris (ADP), although detailed design was carried out by contractor GTM's consultant Ingerop.

It consists of a 700m long concrete vault, oriented northsouth, broken down into 10, 68m long sections. These enclose a conventional, two storey concrete portal frame concourse structure.

Concourse and vault are structurally separate.

ADP chief architect Paul Andreu described the vault in 2002 as 'a tunnel on columns'.

Andreu's tunnel analogy is misleading, however. Unlike a tunnel, the vault has no invert, gains no support from surrounding earth pressure, and is subject to considerable dynamic loading from thermal and wind-induced movement. It must also cope with snow loading.

Parallel rows of rectangular reinforced concrete columns at 8m centres carry the vault. Columns are founded on 600mm diameter, 15m deep piles.

The vault itself is constructed from 300mm thick, 4m wide precast concrete segments. Its circumference was broken down into three elements: a 35t element for either wall and a 55t element for the 'crown'.

Crown elements are solid, while wall elements are latticed, with a regular grid of windows.

Vertical load is estimated to be 100t and horizontal load 20t.

Arches are normally a highly efficient structural form, but at Terminal 2E the vault swells from 27m wide at its springing point to 35m wide at mid-height.

This introduces huge buckling forces in the vault's walls. These are resisted by external steel trusses which have bolted connections at the tops and bottoms of the vault walls. Struts between the outer chords of the trusses and the concrete shell keep the walls in compression.

Wall elements were delivered to site complete with their stiffening steel trusses.

Atkins chief structural engineer Mike Otlet and WSP group technical co-ordinator Stuart Alexander both described the vault as behaving like a four pin arch, with the designed for 80mm of wind-induced movement accommodated through flex and rotation at the edge beams and stitch joints.

Consecutive vault rings were aligned using locator bolts.

However, these perform no structural function.

The only known location at which structural connections were made between vault rings was in the area of the collapse, and at an identical point on the other side of the terminal's centre line.

At these two points the land-side wall of the vault is pierced by three pedestrian walkways, linking the concourse structure with an intermediate terminal hall. The square-section steel tubes are nonload bearing.

Paris-based structural engineer Henry Bardsley suggested that they may have been suspended from the vault.

Loads from the vault elements above the walkway apertures were directed into neighbouring vault walls bearing onto the edge beam.

It is not known whether bolted, shear key or in-situ concrete connections were used.

The interconnection of the six vault rings is thought to be key to the collapse of such a large section of the vault.

During construction, the walls and crown element of each arch were supported on temporary props while insitu concrete stitch joints and longitudinal edge beams were cast in-situ.

Longitudinal edge beams bear onto rectangular columns at 8m centres. Silicone bearings were installed to accommodate movement in the vault resulting from wind, snow and temperature change.

During construction of the terminal building, cracking occurred in the heads of 20 columns as a result of unexpected bursting forces.

Consequently, the vault was jacked 1mm to 2mm off the columns while carbon fibre reinforcement was wrapped around the top 300mm.

Carbon fibre wrapping was also carried out on all of the vault's remaining, undamaged columns, as a precaution, in advance of vault erection.

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