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Simple answers to museum complexity

CONCRETE QUARTERLY - BUILDING STUDY

Zaha Hadid's vast Phaeno Science Centre in Wolfsburg, Germany, blurs the traditional boundaries between structural elements.Engineer Adams Kara Taylor therefore had to take a new approach to the supply of information The Phaeno Science Centre in Wolfsburg, northern Germany, appears, in the words of its designer, as 'a mysterious object, giving rise to curiosity and discovery'.

The museum building occupies a high-profile site at the end point of a chain of important cultural buildings - by Aalto, Scharoun and Schweger - in the town and also provides new public spaces both within and outside its walls.

According to the designer, Zaha Hadid Architects, 'the visitor is faced with a degree of complexity and strangeness', which is ruled by 'a very specific system of structural organisation'.

In simple terms, the building consists of a basement car park out of which rise 10 reinforcedconcrete 'cones', flaring out to support the main exhibition space two storeys above. Each cone is of a different geometric shape, and they all change shape as they rise. Four of the cones continue through the exhibition concourse to support the steelframed, metal-clad roof.

It is a vast building, measuring 150 x 80m and encompassing 12,000m 2of exhibition space. The cones, main floor slab and much of the facade are made from in situ reinforced concrete without a single movement joint.

Although there might be 'very specific structural organisation', it is not a traditional hierarchical system of primary and secondary structural elements. Instead, the cones, slab and facade act together as a single structure. 'The facade is sometimes being supported by the slab, and sometimes it is supporting the slab, ' explains Julian Birbeck, design engineer with the building's structural engineer, Adams Kara Taylor (AKT).

'The cone walls are inclined up to 45infinity, which blurs the boundaries between walls and floors.'

This blurring also occurs in the relationship between the cones and the slab. Although the cones are the main support for the building, they also depend on the slab for restraint. As Paul Scott, AKT's project engineer, says: 'The building is fooling itself a lot of the time.'

The single life The only way for AKT to approach the structural design of the building was to treat it as a single entity, rather than breaking it down into traditional elements, and then analyse the whole building for gravity loads, thermal loads and shrinkage in one model. 'It meant we could break free from the traditional engineering approach of beams and columns, ' says Birbeck.

This is particularly evident when considering the cones, all of which have openings in them to house functions of the museum - such as WCs, gift shop and auditorium - and entrances and public spaces at ground-floor level. 'Structurally they are not quite one thing, and not quite another, ' says Scott. 'I suppose they are part arch and part beam, but we had to get away from the traditional idea of thinking of it in terms of structural elements and naming them.'

The next challenge was to work out how to build the structure. All the concrete was to be placed using traditional hand-built timber formwork, so the engineer had to provide construction information in a format that the concrete contractor could easily understand and use.

Zaha Hadid's concept was of a floor space melting down into the 10 cones with their geometry undefined. AKT redefined the cones so that each was either a triangle or quadrilateral in plan with rounded corners of fixed radii. Although the shapes will change from ground floor to concourse level as the cones flare out, they will continue to be either triangles or rectangles.

The underlying principle of construction is that setting-out details were provided for the cones at the two levels, and the line of the walls could then be interpolated linearly between the two. A triangular cone could therefore be set out using 12 setting out points: the six tangent points between the curve radii and straight lines on the two levels. 'That was the key to allowing the design process to continue, ' says Scott. 'If we didn't appreciate that need for simplicity at the early stages, we would have had problems later on.'

Having refined the geometry of the cones, the engineer then had to find a way to create the irregular openings that are sliced into each one. Initially it designed the plane of each slice, then used Microstation to create the slice and define all the key points, such as the centre points and radii for each arc.

These details went back and forth between architect and engineer until both reached what Scott describes as 'a structural and geometric comfort point'.

Once everyone was happy, the information was translated into setting-out information for the formwork, which was supplied by defining the angle of each cutting plane, and then giving setting out information on that plane.

Having established how to set out and build the formwork, AKT then had to devise a system that would enable the steel fixers to install the reinforcement in the irregularly shaped cones. The subcontractor opted to erect the forms for the outside surface of each wall, and then to allow the steel fixers to work inside the formwork. To enable them to set out the reinforcing bars, AKT had to present them with an 'unfolded' version of each inside face. As Birbeck says: 'We were collapsing the 3D world into 2D.'

Scott adds: 'In Germany there are quite traditional ways of producing drawings. We had to step out of that for this project.

Drawing traditions were questioned a lot, as were materials and the way things were built.'

Although the basic construction method is traditional - reinforced concrete the equivalent of C50 in timber forms - the material is not. The engineer specified concrete with a selfcompacting admixture for the cone walls and parts of the concourse slab. There were two main reasons for this: the height of the pours - up to 8m in one pour - and the inclination of some walls.

The external walls of the cones are only 300mm thick, and are very heavily reinforced, making it difficult to use a traditional poker to compact the concrete. In addition, the walls slope at angles of up to 45infinity.

'The self-compacting admixture gels the mix together, ' says Scott. 'It has given us a finish that would have been impossible to achieve through general construction techniques.'

Continuous concrete supply was crucial for the big pours, as the admixture has a tendency to accelerate the concrete going off.

'If the mix didn't come to site quickly, the surface would start to harden, ' explains Scott. 'But the benefits far outweighed trying to use traditional mixes.'

The walls are quite heavily reinforced, partly to control the cracking that is inevitable in such large expanses of concrete with no movement joints. 'We know it is going to crack, ' says Birbeck, 'but it's about having enough reinforcement to control those cracks so they're invisible.' The result is a reinforcement schedule that includes small bars fixed close together, with horizontal bars at 75 or 100mm spacings.

Since the structure was designed as a single entity, and the cones and slab are so dependent on each other for support, the whole structure had to be propped until the entire concourse slab had been poured. A forest of steel props gave temporary support to the slab, while more props held up the inclined cone walls. Those props have all now been removed, as the steel roof structure is now being erected. Later this summer the main glass facade will start to go up.

For AKT the whole experience has been liberating, both in design terms and the way it has presented the construction information. As Scott says: 'If we can't free ourselves from convention, we are going to find it hard to move forward with complex forms.'

Zaha Hadid will present the 2004 BCA Berthold Lubetkin Lecture, sponsored by The Concrete Centre, on 17 November at the RIBA, London. For further details and registration visit www. concretecentre. com, email seminars@ concretecentre. com, or tel 0700 4 500 500

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