Scott is a director at structural engineer Adams Kara Taylor. He has lived and breathed this project for the past two years, ever since his practice, working with Hadid, won an international design competition for the centre in early 2000.
The scale of the challenge Scott has faced is soon apparent. "We had to use IT to the limit on this project – the geometry alone has stretched current 3D design packages," he explains. In fact, the building was so complicated that the structural analysis package had to be upgraded several times by the software's engineers.
But it is not just software that the engineers have pushed to the limit: the building's construction will stretch the boundaries of concrete technology through its use of complex curved and jagged forms, and its pioneering use of self-compacting concrete.
The building consists of a vast, irregular concrete box, which will house the centre's main exhibition space. This is lifted 7 m off the ground on a series of irregularly shaped conical legs. The architect has carefully positioned these legs to maintain access routes beneath the building.
"Zaha describes the building as a slab that has been lifted off the ground," explains project architect Christos Passes. "And within this slab are voids that drop to the ground – these become the structural legs on which the building stands." As well as allowing access to the first-floor exhibition space, the cones (or "voids", in Hadid-speak) will be filled with cafes, bars and shops; there is even an auditorium in one of them.
The cones will rise up from a flat, reinforced concrete raft foundation that will, eventually, form the floor of the building's basement car park. They are at their smallest cross-section at the point where they connect to this raft. "This puts a highly concentrated load on the foundation, but by using the raft we can spread the load more evenly over the ground," explains Scott. From the basement, the 10 cones rise through the building to support the ground-floor and first-floor slabs – the building has no internal columns. Six of the cones terminate at the first-floor slab, whereas the remaining four punch through it to support the steel roof above.
The geometry of the cones is complex. In plan they look like squashed rectangles with rounded corners, but each cone has a different form. Their shape was developed initially by Hadid's studio using a CAD package. This design was then imported into AKT's design software package for analysis so that the architecture and structure could be developed together. "The architecture is the structure, which then defines the space," says Scott, somewhat abstractly.
Just to add a bit of spice to the structural engineer's challenge, the architect inclined some of the cones and even sliced sections out of the walls to create openings for doors and lobbies. "This had a major effect on the design of the cones and the entire building," Scott adds.
Rather than just collide with the building's first-floor slab, the six cones that terminate at the first-floor level merge with the slab by mushrooming outwards to melt seamlessly into the horizontal floor. Scott describes the cones as "folding" into the exhibition floor. "Because they are inclined, there is not a hard definition between where the cone's wall stops and where the floor begins," he explains.
Just as the cones are positioned to channel visitors beneath the building, so the varying levels of the suspended first-floor slab guide people through the exhibition. The huge concrete floor plate heaves and falls over its 150 × 70 m surface to create an undulating landscape, within which the exhibits will be displayed. In places, holes in the slab will allow visitors to snatch a glimpse of the space below.
Standing beneath the building, this floorplate will appear to be constructed from a combination of recessed, trapezoidal-shaped indentations, known as waffles, and solid concrete. The waffles are orientated to reflect the main access routes beneath the building. Each waffle will house a light-fitting.
The architect has arranged the waffles in a cluster at the centre of the building to create a pool of light beneath the exhibition box.
Moving outward from the centre toward the edges of the building, the number of lights – and consequently the number of waffles – will decrease. Passos describes the pattern made by the lights as "confetti scattered beneath the building". Large blocks of polystyrene – called void formers – will be cast into the areas of solid concrete between the waffles to lighten the weight of the slab. Once complete, the floor structure will be post-tensioned.
This 900 mm deep slab, with its waffled indentations, appears to have been pulled up from beneath the building to form its southern facade – the "carpet effect". The theme of rectangular intentions that Hadid has developed beneath the building is continued in the precast concrete panels used to form this facade. In places, rectangular windows will fill the recesses where these have been pushed through precast concrete walls. On the building's remaining facades, the walls are constructed from insitu cast concrete, which acts like an edge beam to the floor, stiffening the structure.
The building's conical legs and the exhibition floor plate will be constructed as a single solid structural element. On their own, some of the cones would be unstable, but linked to the floor plate they distribute their loads through the remaining legs to form a stable, table-like structure. The building has no movement joints. "This is one reason why we've analysed the building as a single shell," explains Scott.
But analysing the structure as a single shell has pushed state-of-the-art software to the limit. Finite element analysis was used to calculate the stresses in the structure, which meant dealing with more than 17,000 elements. "I don't believe anything this complex has been done before," says Scott. "The problem was not the calculations, but the scale of the analysis resulting from the building's dramatic form. It was groundbreaking; five years ago I don't think we could have done it – the software had not been developed."
Scott is at pains to point out that throwing computer power at the building was never going to guarantee that it was structurally sound. "You still need an experienced structural engineer to ensure that the material can take the loads thrown up by the software, and to ensure that the structure can support them," he says.
The structural engineer considered the concrete legs and structure as a single element, and the steel roof as a separate single element. This roof is constructed from a series of intersecting braced trusses, supported in the centre on the tops of the four cones that rise up from the basement. The trusses rest on a beam running around the top of the perimeter walls – an arrangement that allows the roof to expand and contract without the need for movement joints.
"There were lots of possible ways of designing the roof structure," explains Scott. As it is visible from the exhibition floor below, the engineers have positioned the steel supporting trusses in a fan. In plan, the building looks like a triangle with a kink on one edge; in fact, it is a quadrilateral.
Construction is under way. The building's 15,000 m2 single-storey basement has already been excavated. With the appointment of concrete contractor Heitkemp in November 2001, construction of the cones is imminent – and with Adams Kara Taylor responsible for construction details, Scott's challenge has only just begun.
How the concrete legs will be constructedIt is not just the design of the cones that has presented the engineers with a challenge – they had to be sure their creation could be constructed. “Casting inclined cones is very complex,” says Paul Scott. Using traditional mechanically compacted concrete would have caused problems. First, it is notoriously difficult to control the quality of concrete compacted using vibrating pokers on inclined walls. And second, the architect demanded that the cones’ concrete walls were free of the horizontal joints that form between concrete pours. In fact, the architect demanded that the cones’ 7 m wall section from the ground floor to the underside of the first floor be completed in a single pour to avoid marring the finish. Scott's solution to the problem has been to specify self-compacting concrete. This type of concrete, which contains an additive to make it less viscous, is used on large civil engineering projects where its fluidity is used to allow the concrete to flow between densely packed reinforcing bars. This property will help the concrete fill the complex shuttering needed to form the cones. Fortunately for the engineers, self-compacting concrete also has an excellent finish, so the architect's requirement for a smooth, blemish-free finish is also satisfied. “The use of self-compacting concrete will be the key to achieving the finish,” says Scott. As far as he is aware, this is the first time self-compacting concrete has been used on a landmark building. Wooden formwork will be used to create the concrete moulds for the cones. “This will be hand-built and preassembled in the workshop, where its dimensional accuracy will be checked,” explains Scott. Then, once it has been reassembled on site, it will be surveyed again before any concrete is poured. The plan is to cast the cones in two sections: basement to ground-floor slab, and ground-floor slab to the underside of the first floor. Up to the point at which the slab links all the cones, they will be unstable and will need to be propped. Another pour will be required to construct the top of the four central cones to extend them up through the exhibition space to support the steel roof.
Client City of Wolfsburg Architect Zaha Hadid/Mayer Barhale Structural engineer Adams Kara Taylor Local partner (structure) Tokarz Freirichs Leipold Services engineer Buro Happold/NEK Cost consultant Hanscomb Concrete contractor Heitkemp