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Clockwork croissant

Airports Zurich airside centre

Swiss craftsmanship and British design skills have combined to produce a stunning new landmark building. Andrew Mylius reports.

Zurich airport's new airside centre is being crafted with the same sort of finesse as a Swiss watch. Its sleek, minimalist form is the embodiment of cutting edge structural engineering and fabrication technology. On site, construction tolerances have been exacting. And the construction programme is being run with the same kind of punctuality as the national railway.

'The Swiss do this kind of thing very well, ' sums up Arup senior engineer Holger Falter.

Zurich's private sector airport operator Unique is setting out to double passenger and freight capacity in the next 10 years with a budget of US$1.6bn. In 1993 it staged an international design competition, inviting ideas on how to manage the growth. Arup and UK architect Nicholas Grimshaw & Partners won with a proposal to demolish a clutter of buildings that had accumulated piecemeal over the years. In their place a new 'airside centre' would be built, unifying what was previously a disjointed site and providing the airport with an attention-grabbing signature building.

Grimshaw's final design is an aerofoil of steel and glass, 250m long, 50m deep and 25m high at its centre, from which passengers waiting for take-off can look out across the airport's aprons to distant hills and mountains.

At its core is a straightforward three storey mass reinforced concrete structure founded on concrete piles. 'Big, but not very exciting, ' Falter shrugs.

But the envelope has been all-absorbingly intricate.

Grimshaw project architect Kai Flender describes the airside centre's roof as 'a croissant on stilts'. It curves longitudinally, laterally and in plane, and developing the croissant into a form to which numerical, engineerable properties could be applied was a huge challenge, Falter says. It was finally represented on Arup's computers by bending a virtual tube in x and y axes, and then cutting out a section of the wall using two radii - 1,800m for the front and 240m for the back.

Structurally, the roof is an exposed lattice of tubular steel trusses, which create a space frame on a 7.8m grid. Its curvature means no two connection nodes are the same in either half.

Tubes are joined using snowflake-like plate steel junctions which, to achieve the necessary accuracy, had to be cut and fabricated robotically.

The flanges of these junctions were welded into vertical slots cut in the ends of each tube to create a quasi male-female coupling. Arup and Swiss roofing contractor Tuchschmidt looked briefly at welding tube to tube instead, but it would have created 'a very difficult stress and strain combination', notes Falter. Cutting the ends of the tubes to shape would also have presented a 'big geometry problem'.

To complicate things, Grimshaw was keen that the dominant longitudinal trusses should converge in sympathy with the curving front and back edges of the roof, running all the way to its chiselled tips.

Normally longitudinal trusses would run parallel, terminating wherever they hit an edge.

To save weight the diameter and wall thickness of tubes making up the trusses reduces as they draw together. Flender concedes the aesthetics-driven design requirement has bumped up cost, but he and Falter agree it is a defining feature of the new building.

There are no movement joints in the roof, which is unusual for so large a steel structure. It was expected to deflect 50mm to 60mm under dead load, which has been dealt with by tensioning the trusses' lower chords.

But the roof will also be subject to snow and possible seismic loading, and to considerable thermal movement.

Arup's solution has been to fix the roof down along only its front edge. The back edge is free to shift in all directions on 500mm diameter elastomeric pin bearings.

At its back, the roof rests on a robust forest of stubby steel and reinforced concrete columns on a 7.5m grid. But at the front the six groups of soaring columns appear incredibly insubstantial.

Two pairs of columns make up each group. Seen in front elevation each pair diverges from a shared footing to form a V. From the side, however, the two pairs of columns converge to form an A. This longitudinal and lateral triangulation holds the roof rigidly in place.

The columns range in height from 15m to 19m over the length of the airside building.

This subtly changes the angles at which they meet the ground and the roof, and meant that the 1.5m high, 600m2 mounting for each pair of columns had to be unique.

Massive 80mm thick steel plate has been used to manufacture the mountings, and the yokes into which the columns are pinned have been machined from solid steel blocks.

Falter is hugely excited by the columns, which are works of considerable beauty. They taper at the ends, and to achieve this Tuchschmidt bent 25mm thick steel plate to create columns with a finely faceted polygonal section, rather than a true cylindrical section. By making the facets trapezoidal rather than parallel, it was possible to reduce the radius of the resulting curvature, producing columns with a pleasing entasis - the subtle convex profile that ensures the sides appear parallel.

Erection of the roof was initially planned to take place in seven huge lifts, each filling the space between a cluster of columns. Tuchschmidt would build a section of roof on a disused area of apron about 1km from the site and the whole would be transported at night, across the runway, to be lifted by two of Europe's largest cranes. It would have been fantastically efficient, says Flender.

'But our horror was that steel could fall off the transport barge on its way across the airport - we would have had to close the airport down.'

Tuchschmidt decided instead to deliver large pre-assembled roof elements to site, join them into still larger sections at ground level, and then to lift these into position. Temporary supports were used while final welding was carried out.

Ways to fly Zurich airport is used by 20M passengers a year, and operator Unique wants to expand to handle 40M passengers without encouraging a major increase in car journeys. To manage this, Grimshaw and Arup are also been involved in US$200M redesign of the airport station, which will open later this year with concourse-level check in and airport baggage handling.

Grimshaw is also designing a 20 station light rail connection to the airport, the first section of which should be running by 2006, with phase two following in 2010.

Off the level

The airside building's challenging roof geometry affects the facade, which is inclined at 12degrees off vertical.

Geometrically it is like a section from the base of a cone, says Falter, which means there are no parallel lines.

Instead of cutting each pane of glass to fit an oddshaped hole it has been possible to accommodate rectangular panes within the tight 5mm construction tolerance. However, mullions are of varying length.

Glazing has been hung from cantilevers that project from the front and back of the airside building. Bearings at the facade's lower edge accommodate movement in the facade itself as well as the rest of the structure. To withstand fairly high wind loading of 1.53kN/m2, mullions are 850mm deep.

Swiss contractor Geilinger has carried out glazing with subcontractor Tobler.

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