Where do you build if you don't want to use up your valuable land? In mid-air, of course … We found out how it was done at the latest wonder of the construction world – the gravity-defying De Brug office block in Holland
The residents of Rotterdam have seen some amazing buildings erected in their city, but for sheer spectacle, nothing compares to De Brug. This four-storey office block straddles a factory complex on the bank of the River Maas and resembles a giant trestle table with architects' models stacked underneath. But what makes this 7500-tonne building unique is the story of how it arrived at its extraordinary location.

A clue as to how De Brug was constructed lies in its name. "Brug" means bridge in Dutch and it was the utilisation of bridge-building techniques that helped the team to pull off its audacious design. The steel superstructure was constructed in one area of the site on a temporary steel structure, moved on huge trailers, and then slid down a track into position over the Unilever factory roof.

First-time visitors to the site will wonder why the offices weren't built on the large, empty, seemingly ideal site next to the Unilever factory. In fact that was the original brief from Unilever two years ago, but architect JHK's radical proposal made better financial sense. By erecting the office in the factory's airspace, more land on the quayside could be freed for lucrative housing (see "Getting permission", page 42).

"It's an expensive construction, but financially it works on this site," says JHK's Chris de Jonge.

As soon as JHK came up with its bridge concept, contractor–developer Dura Vermeer asked steel contractor Hollandia to find out whether the design could actually be built. Hollandia started out as a bridge-builder and Dura Vermeer knew that it had experience of tackling difficult structures – one of its more challenging projects was the technically fraught London Eye.

The first thing Hollandia did was call in Dutch lifting and transport firm Mammoet to confirm that the structure could be moved after it had been built. "It's basically a bridge that, by itself, is a standard building," says Cees van Rouijen, commercial manager at Hollandia. "Transportation was the most difficult part." Once the engineers had analysed the project from every conceivable angle, the project team pronounced that De Brug could be built quickly, safely and at a reasonable cost. What's more, the whole operation would only require the factory to be closed for one day, ensuring that the continuous manufacture of Unilever Bestfoods products such as Cup-a-Soup and I Can't Believe It's Not Butter.

A key decision the team had to make was whether to build the structure at ground level or 25 m above the quayside at the same height as its final height. The team chose the latter and decided to build a temporary steel structure on which De Brug would be constructed. If the structure was built on the ground, expensive heavy lifting gear would have been required to raise it onto a temporary steel structure before it could be moved.

After the temporary steel structure was constructed, the central beams of De Brug were placed, followed by the rest of the central framework. To ensure that no unwanted force developed in the steel construction, jacks were used to ensure the suspension points on the structure were at the correct level.

The steel decking of the Corus Slimdek flooring system (see "Corus of approval", right) arrived on site already mounted on the floor joists, so contractors had a secure floor they could work on immediately. The specification of the decking was important, says de Jonge, because it had to be light to make transportation possible. To keep the weight to a minimum during transportation, the concrete floors were poured once De Brug had arrived in its final position. The glass and the rest of the interiors will also be installed in situ.

The project provided massive challenges for architect JHK and Dura Vermeer. The presence of the existing factory meant that De Brug could not simply be rolled into place: instead a skid track 16 m high had to be piled and built over the southern side of the factory.

Poor ground conditions also presented the team with problems. JHK's de Jonge describes the ground as akin to "thick water" and its lack of solidity meant the steel skeleton had to be as light as possible. The project team also had to account for gusts of wind and the possibility of ships hitting the quayside causing vibrations that could topple the steel structure. The old quayside had to be reinforced, so it was covered with 20 mm steel plates to ensure it could take the combined 3160 tonne load of the structure and means of transport.

The operation to move De Brug from its temporary steel support to its permanent home appeared fraught with danger, but at no point did Mammoet, the lifting firm, show any concern. It has moved far heavier structures in its four decades of experience, such as oil rigs, and has also lifted much more awkward loads, most notably stricken Russian nuclear submarine The Kursk (see "Mammoet task", below). "The size and the weight of the object is not really special for us," says Mammoet's communications manager Larissa van Seumeren. "I know it sounds arrogant, but we've experience of moving these kinds of loads. We calculated everything in advance and recalculated it. There is such good preparation – but it is always exciting when the structure starts moving."

For the first 100 m, De Brug's weight was borne by two pairs of trailers known as self-propelled modulator transporters, SPMTs, which are vertically adjustable so they can lift and lower their loads. The SPMT at the riverside consisted of two lines of 28 axles each with four wheels and a load-bearing capacity of 1400 tonnes. On the factory side there were four lines of 35 m long SPMTs with four sets of 24 axles and a load-bearing capacity of 2400 tonnes. The wheels were driven and controlled hydraulically by an operator who communicated with the SPMTs' computers by remote control. The operators were aided by supervisors who monitored the route of the SPMTs – which in De Brug's case included taking a right-hand bend at the river side to ensure the building ended up at the correct angle across the factory.

The trickiest part was moving the factory end of De Brug from the SPMTs onto the skid track, along which it would slide into place.

To ensure accuracy, Mammoet used a laser system to guide the structure onto the track. The building sat on eight 600-tonne skid "shoes", which each had jacks that could pick up the load and put it down again. These glided on Teflon pads through a duct on the skid track and were pushed forward by horizontal jacks towards the three permanent supports.

If all that weren't enough, putting up those supports was far from easy, too. The points at the front and back consist of two V-shaped steel columns each with a diameter of 1.20 m, and in the centre is a 20 × 30 m steel core that will eventually house the lift shaft and staircases. The difficulty was that the Unilever Bestfoods factory had to remain open throughout the construction programme. This meant that not only were there few available locations for support piles E E but that, when they were being sunk, the construction team had to ensure the vibrations did not interfere with the factory's sensitive production processes. The pile limitations resulted in the whole structure being built at an angle to the factory and offices, with the central steel core situated in the clock square and the rear support actually bursting improbably through the roof of the factory.

However, architect de Jonge says De Brug's angled position happens to offer the best views across the river and towards the famous De Hef railway bridge nearby. He is also pleased with the contrast between De Brug and the factory. "We wanted to emphasise the new building by having it positioned next to the old, which is 111 years old," he says.

At the end of its journey, with the permanent support structure completed, De Brug was lowered into position. The operation went so smoothly that the three-day transportation programme finished a day ahead of schedule. Over the next two years, the De Brug offices will be completed and fitted out from the inside, and a car park and warehouse will replace temporary offices next to the factory.

As well as impressing the local residents who gathered in the early morning gloom to watch the start of the lift, De Brug has also galvanized the Unilever workforce, according to Anthonie Stal, chairman of Unilever Bestfoods. "The energy and motivation within the company in the last two months is amazing," he says. "It is the excitement of making the impossible possible."

Mammoet task: Raising The Kursk

Transport and lifting company Mammoet has nearly 40 years of experience lifting colossal structures weighing many thousands of tonnes. It recently moved a 9500 tonne concrete railway bridge into place in Northfleet, Kent. But the firm’s most momentous job has to be the raising of Russian submarine The Kursk (pictured right), which sank in August 2000 in the Barents Sea off the north-west coast of Russia. Mammoet, which is Dutch for mammoth, was the only salvage company that said it could raise the Kursk by the end of 2001, thereby fulfilling President Putin’s pledge to the Russian people. The salvage plan was based on Mammoet’s Strand Lifting system, which consisted of 26 lifting units each with a lifting capacity of 900 tonnes. The lifting units were installed on a 140 m pontoon, which then travelled from Holland, round Scandinavia, to the Barents Sea. The system’s 26 lifting hooks were guided down into 26 pre-cut holes in the inner and outer hull of the submarine. Once in place, the lifting hooks hydraulically opened up inside the inner hull – rather like a plasterboard Rawlplug. A bundle of 54 cables was fixed to each gripper and the 9000-tonne Kursk was slowly raised. To ensure that the cables’ tension was constant and to eliminate the risk of snapping in rough seas, the strand jacks were specially designed and fabricated with heave compensators, which acted like shock absorbers. Once the submarine was raised to just below the pontoon, it was safely towed to the Russian port of Murmansk.

Corus of approval: De Brug’s flooring system

Corus Slimdek (see diagram) is a composite steel and concrete flooring system consisting of asymmetrical beams, cold-rolled steel sections and poured concrete. The Conflor steel deck is placed on the bottom flange of asymmetrical Slimflor beams (ASBs) before concrete is poured on top to give Slimdek its composite strength. As the decking is fixed to the bottom flange rather than the top flange the Slimdek system is shallower than other systems, which means it is lighter, and that the overall height of the building can be reduced. It was important to ensure that the flooring system was as light as possible because flooring makes a substantial contribution to the overall weight of the structure. Even with the light Slimdek, Corus supplied 500 tonnes of ASBs and 17500 m2 of Conflor steel decking. So, to minimise the weight during transportation, the concrete flooring was not poured until De Brug reached its final destination.

Getting permission for a flying building

Unilever invited four developers to submit proposals for a 15,000 m2 office and about 150 dwellings on the site of the old Oranjeboom factory. Three of the teams came back with plans for a mixed development of offices and housing on the quayside, whereas architect JHK and Dura Vermeer came back with the radical idea of freeing up more land for housing by locating the offices on top of the factory.

Unilever fell in love the idea, but there was one problem:

the local planning office had restricted the height of the building to four storeys. The architect managed to convince the planners to allow their design by explaining that if the project got the go-ahead, 100 extra, much-needed new homes would be built. It also had to reassure the officers that all the health and safety regulations would be met. “All those procedures for getting a building approved were stretched,” says de Jonge.

The extra 100 homes provided Dura Vermeer made financial sense. The final element of the business plan was persuading an outside company to invest in the project. Unilever didn’t want the risk of developing the land itself and so will lease the office from the investor, Siemens’ pension fund, when it is complete.