As some 40,000 or so individually shaped slabs of dazzling white Carrara marble are fitted into place on the new Oslo Opera House this autumn, residents of the Norwegian capital are beginning to see for the first time the impact that the building will have on the centre of their city.
The tapered wedge-like structure is already eye-catching, not least because of its location.
The NKr3300M (£263M) opera house sits at the heart of a new waterfront area, replacing old docks and factories that were the core of the city in medieval times.
A competition for the prestigious building was launched in 1999. The design, by Norwegian architect Sn°hetta - famous for its work on Egypt's Alexandria library - will certainly be eye-catching, with the building set on the shore with its shallow sloping roof running down not just towards the sea, but into it.
Projecting through the sloped roof is a dark-glazed main entrance hall. Behind that the square grey form of the fly tower for the main stage area looms, topping the building out at 32m height.
'It is intended to be as abstract as possible, ' says project architect Simon Ewings, 'to leave visitors with as blank a canvas as possible for the 'suspension of disbelief' that is a critical part of an opera experience inside.' It also echoes the white slopes of Norway's snow-covered mountains.
The dense marble surface should also be extremely durable, fulfilling a major requirement of the brief, that the building last a minimum of 300 years.
Raking columns hold up the roof, some 28 in all with varying lengths to a maximum of 21m.
Load is also taken by structural concrete walls, explains Tore Steigen from structural and civil engineer for the project Reinertsen.
The rm works in joint venture with Per Rasmussen for electrical work and Erichsen & Horgen for the ventilation.
The façade of the foyer is high-transparency glass allowing the interior to connect visually with the white slopes outside.
More glass fills in the angle between the two different roof slopes on the land side.
'The foyer spaces are intentionally connected together with the exterior made to be as public as possible, ' says Ewings. It was part of the brief that the building should provide a significant public space for the city as well as an opera theatre.
The roof will be an open space. People will be able to walk about its 20,000m 2 area, which is kept at a deliberately shallow angle at about 12.5% - an 18% gradient over the stage area.
Timber is used inside the 1,350 seat main auditorium that lies at the centre of the building.
Its acoustics, designed by Arup Acoustics, had a significant impact on the structure (see box).
The gleaming white marble of the roof surface is textured and shaped with a series of gentle transitions between the various flat slopes, and shallow steps appearing and running off to form an edge here and there. There are five different surface textures, changing abruptly at various boundaries.
Three sculptors worked with the architect to devise the various forms.
This means that every one of the 40,000 slabs is different.
Each one is individually cut, textured and polished, and then carefully fitted into place like a giant jigsaw puzzle. Each piece had to be designed in 3D, with the CAD drawings then sent to Italy for cutting.
The marble roof slabs, currently being carefully installed by contractor Naturstein, are not light and when a sandwich of layers beneath it is built up it makes for an exceptionally heavy structure. Besides the 100mm thick stone itself there is a 100mm thick basic concrete slab and above that a reinforced screed. A three-layer asphalt membrane goes above that and then 50mm of mineral wool acoustic insulation.
Then comes thermal insulation of extruded (closed cell) polystyrene and then a specially developed mortar is used to bed in the stone.
A separating sheet ensures that the mortar doesn't seep down and damage the thermal insulation membrane.
The high loads imposed by this unusually thick 'walking roof' have been one of many challenges faced by the structural engineer. 'That has been combined with a need to keep the profile of the whole building as low as possible, ' says Steigen. The building is also highly irregular in its shapes, especially in the main theatre area and the foyers. 'There is hardly a right angle to be found, ' says Steigen.
Steel beams for the roof are at least 'straight between their bearings, ' says Steigen, although they are set at various angles on plan. The roof does include a complicated 'crown' of radiating steel elements over the main theatre area.
'And the grid of the building's superstructure does not really follow the grid of the piles below, ' adds Ewings, 'which means the load paths are more complex.'
Foundations, completed about two years ago, were a complex story in themselves (see box).
Other difficulties involve the full height glazing of the foyer, which uses no metallic elements at all in support.
Instead 'buttresses' of high strength glass are set at right angles to the main glass, shaped to follow the load imposed by the glass.
Foundations The opera house sits on land and sea with a piled platform at about mean sea level, supported on driven cased steel piles. There is aggressive ground here with contamination in the seabed including heavy metals.
Steel piles are keyed into rock 30m down through harbour and soft clay. A major structure was the deep central well for the main theatre and its rising stage, which required a 21m deep excavation within a sheet piled wall; an inverted dome plug of mass concrete sealed this.
Four huge 2.5m diameter bored piles with casings cut into the granite beneath the glacial clay are used to support the stage and the fly tower - the highest point of the structure.
A submerged piled platform has also been built to one side of the theatre. This will provide collision protection against the ferries and boats that will continue to use nearby terminals.
Acoustics The 1,350 seat main auditorium was designed with input from Arup Acoustics, with modifications to the shape and materials following the analysis. 'This is a relatively conventional horseshoe layout, which is proven for opera, ' says Rob Harris, project engineer at Arup. 'But we widened the top end a little, which helped increase the reverberation.' The increased size demanded an additional 2m on the height of the building, despite the architect's wish for as low a profile as possible. This led to some structural complications because the wall at the top of the space was pushed back somewhat. 'We have an unusual cantilever at that point, ' says Steigen.
But the volume was critical. A key issue has been achieving a balance between the 'dry' sound needed to hear words and the richer reverberation of music. 'Italian buildings, where they understand the lyrics, tend to be 'drier' than northern theatres, ' says Harris.
Materials and fittings have been a key part of the work, with combinations tested using both simulation software and a physical acoustic site model. Arup nowadays uses the equivalent of visual virtual reality simulation in a 'SoundLab' where design changes can be heard in ambiosonic surround sound.
'Traditionally, timber is the best material, ' says Harris, 'but it needs to be quite thick to ensure the proper reflectivity of the lower notes.' Seat padding is critical too.
Reflectors have been kept to a minimum, although above the orchestra pit they help give the on-stage performersa better sound.
There are movable column reflectors at the side of the proscenium arch too.
A ignificant amount of the work is in testing light fittings and the underfloor ventilation to ensure as quiet a background as can be achieved.
'Lights are particularly important, ' says Harris, 'especially so you avoid those that 'ping' as they cool, which can be a disaster in the middle of a tense pause in the music.' Modern light-emitting diodes are a great help here.