The hanging offices of rotterdam

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."