The triangular design of the roofs was deliberately chosen by the architect, Richard Rogers Partnership, to break up the building's impact on its surroundings. "One of the most important elements is the skyline. If we had a single skyline the building would look massive," says Rogers. "Instead it's a butterfly building, it's light, it moves, it's biological rather than inert. It's a bridge where the country meets the built environment."
As an architectural vision, it has panache. But engineering it into reality was a huge task for the team. After winning the competition, it took the joint venture – made up of Richard Rogers Partnership, VK Studio and Arup – 18 months to find the ideal formula for the roofs. "The toughest part was sorting out the geometry, integrating the services and enclosing the spaces – it makes me shudder to think about it," says Rogers site architect Kevin Gray.
After multiple design changes, the team finally settled on a geometric form called a hyperbolic paraboloid for its structural simplicity (see the box over the page for more on this). In simple terms, a hyperbolic paraboloid is a double curved surface – similar in shape to a saddle. "On plan it is a simple grid, you then pull up each corner to the height you want to create for the double-curved surface," explains Gray.
Each "tooth" of the roof is made up of four sections, each one a hyperbolic paraboloid. These are bolted together on site to form the finished structure. Two of these are higher than the others to create an offset effect. The gap between the high and low roof is glazed. "The idea is to get as much natural daylight as possible into the court," explains Gray. "The rooflight faces north and is set back for solar shading to reduce the need for mechanical plant. It works really well and also provides quite ecclesiastic spaces."
However, at one stage it looked as if the roof design would have to be scrapped because it was too expensive to build using conventional timber construction techniques. Rogers' design called for the roof, which is open to the interior, to be assembled from timber. This would provide a visual link to other timber-lined public areas, creating a coherent, natural feel. Only an innovative solution by timber contractor Merk, ensured that the architect's vision could be realised.
Already, several finished roofs are peeking over the top of the precast concrete frame. Each roof caps a square courtroom that is 15 m across and consists of a tall, north-facing triangular peak swept back at an angle – just like a sail. Avtar Lotay, project architect, explains the inspiration for the roofs. "Flanders is very flat so the trees lean to one side, responding to their environment," he says. "In the same way, the roofs lean to the north for lighting reasons. Also in a lot of Flemish paintings you see these very distinctive triangular sails but not the canals they are sailing in."
Each roof is clad in stainless steel, which sparkles under a bright blue sky. The exposed undersides of the roofs provide a more dramatic visual experience and brings light into the courtrooms. When the building is finished there will be 26 short roofs 8 m high, and six that will be 23 m tall. The tall roofs will sit on either side of the Salle des Pas Perdus, which runs through the centre of the building under a glazed roof, forming the main public circulation space.
The roofs are peculiar not only in shape, but also in the way they have been assembled. In keeping with Antwerp's status as Europe's second-largest port, the roofs are made on a production line in a shipyard upriver, floated to the site on barges and craned into position. The roof arrives in sections complete with insulation, cladding safety access points and even the guttering and walkways all finished.
"It would have been hell to produce the roofs in situ," says Gray. "We wanted to assemble a building made from beautiful pieces. It was important for us that there was a manufacturing process that was well organised to control quality. It is much easier to build the roofs using pre-assembly and more economic as we are building more than 30 of the same thing."
At the shipyard, three separate contractors churn out four sections, enough for one roof, every six days. Roofing contractor Lemants brings in steel frames for each section and welds them together on a jig, then passes them into a giant spray booth for painting. The section goes to German timber specialist Merk, which constructs the main central area of the roof section in wood. Once it has finished, roof covering contractor ME Construct installs a waterproof membrane, the insulation and finally the stainless steel outer cladding, guttering and walkways.
Gray is full of praise for the specialists: "We always try and push the technology to its limits, that is why it is important for us to work with people who know what they are doing. The project relies on that level of craftsmanship to pull it together."
So far, 15 of the small roofs have been completed and the tall roofs will start being erected on site in November. The entire roof should be finished by spring next year and the building is due for completion in May 2005 – by then Antwerp's residents will be familiar with the flotilla on the edge of the city.
What is a hyperbolic paraboloid, anyway?A hyperbolic paraboloid is a simple geometric shape. The best way of visualising it is to take a square or rectangular piece of lined paper and lift up two diagonally opposite corners to create a curved surface. This curved surface can now be defined by the lines when viewed from above. At Antwerp these lines have been expressed by the parallel seams of the stainless steel cladding of each roof section and by the rectangular grid of timber forming its central structure.
In theory it should be possible to make the structural grid using straight sections of timber that are simply bent in one direction to create the curve. However, the theory only holds true for thin straight lines. The minute the straight lines acquire a thickness the theory crumbles. Because the timber sections are 120 mm thick they have to be twisted to ensure they sit flush against the roof’s curved covering.
This means each structural piece of timber has to be twisted if it is to follow a straight line when viewed from above, and follow the curve when seen from the side. Arup’s original idea was to take short sections of glulam and plane each one to form the twist. Steel fixings would be used to join the sections together to create the structural grid. This would then be covered with LVL, a sort of overgrown plywood. This solution was so expensive it looked at one point as if the timber part of the roof would have to be scrapped and replaced with steel. German structural timber specialist Merk saved the day by suggesting an alternative way of constructing the roofs. “We completely rethought the way the roofs should be built,” says Michael Keller Merk’s overseas client development executive. “The only thing remaining from Arup’s design is the outer steel frame of each roof section. All the steel connections are gone, and this saved enough to keep timber in the project.” Merk’s solution has introduced a new word into construction’s vocabulary, namely “screwlam”. Each structural grid is progressively built up by screwing together lengths of spruce that are 120 mm wide and 27 mm thick. These are individually flexible enough to twist and follow the curve, but once seven are screwed together they form a solid structure. Merk also saved money by covering the grid with two layers of standard timber laid perpendicular to each other instead of more expensive sheets of LVL.