Assemble a 1500 tonne steel bridge on the fragile roof of a shopping centre while keeping the shops open and dodging rotten fruit? It's all in a day's work in the badlands of Shepherd's Bush …
In the contractor's site office, a safety officer briefs a new recruit. He tells him the site's rules, the location of the first-aid box and the type of footwear and clothing that must be worn. Then he pauses, looks out of the window at the two tower blocks that flank the north of the site, frowns, and says: "For your own safety, make sure you have a hard hat on when you're working near those two council blocks. The residents there have a nasty habit of aiming rotten fruit at workers on this site."

The site huts belong to joint-venture contractor Hochtief-Miller, which is undertaking the design-and-build refurbishment of the Concord Centre on Shepherd's Bush Green Road, west London. The aim is to turn the drab 1970s mall into a sleek leisure and shopping destination to be called "West 12".

Halfway through the 85-week programme, the protests of disgruntled tower block residents is just an unhygienic irritation for Hochtief-Miller. There is also the question of doing what nobody has ever attempted before in the UK: building a bridge over the shopping centre's rooftop car park and popping a 12-screen cinema on it. Then, the rest of the roof is to be demolished and rebuilt 1 m higher to allow an upper tier of shops to be created on both sides of the main mall.

When all the large-scale building work is completed, the existing malls and facades are to be refurbished to bring them into line with the centre's upmarket image. Oh, and the client, developer Tops Estates, had one extra demand: the shopping centre was to stay open throughout the works.

The commercial key to the whole development is the cinema; the toughest technical problem is to construct the bridge that will support it. "It's a challenge," says Peter Quaife, design co-ordinator for the contractor, rather understating the problems he faced.

From the window of his office, Quaife can see a team of riggers. They are completely dwarfed by the 50 m long, 11 m high steel truss they are bolting together. It is the sixth of the eight that will support the bridge.

The trusses are delivered as a kit of parts. Pat Hicks of structural engineer Knapp Hicks & Partners explains that his firm had to design the trusses so that they could be assembled like the world's biggest Meccano set from sections of steel weighing no more than five tonnes – the heaviest weight that the three tower cranes could lift on to the roof.

Once they are in position, the sections are bolted together using vast steel plates, peppered with bolt holes. These keep the junctions rigid between individual truss sections, and at the same time impart a Victorian railway aesthetic to the structure. "Forget hi-tec; this is more Victorian-tec" says Martin Pratt, Colman Partnership's project architect.

The task of assembling the 1500 tonne steel structure fell to contractor Bourne Steel. Its principal headache was the fact that the size and weight of the trusses meant that they had to be assembled in situ. Unfortunately, the car park roof was too weak to support the structure, so temporary props had to be put in place. Equally unfortunately, the supports could only be themselves supported by the columns of the shopping centre, most of which were in the wrong place. This meant that a series of steel transfer structures had to be constructed for the truss supports to stand on. Once again, the maximum lifting weight of the cranes meant these had to be designed from steel sections each weighing no more than five tonnes.

Once each truss was fully assembled, it rested on both its permanent supports – reinforced concrete columns at either end – and the temporary transfer structures. To allow the temporary supports to be removed, the truss had to be lifted. With the car park deck so weak, this posed Hochtief-Miller with yet another challenge.

Again, the centre's existing column supports had to be utilised. More temporary structures were constructed and a series of hydraulic jacks placed on these to raise the truss sufficiently to allow the temporary supports to be removed.

Each truss was then carefully lowered to its final position on the newly constructed reinforced concrete columns. "Each of the eight trusses is unique, but each had to be built with a 30 mm camber in its centre to compensate for the weight of the steelwork in the truss and the weight of the cinema," says Quaife.

Throughout the lowering process, the camber is carefully monitored to check the structure was behaving as the engineer had predicted. "The weight of the truss alone reduced the 30 mm camber to 15 mm," says Quaife. The final 15 mm is predicted to disappear once the weight of the cinema is added to that of the truss.

With the truss now secured in its final resting place spanning the full 56 m between column supports, it was time to give each bolt a final tighten. "There are more than 5000 bolts in one of these trusses," says Quaife, and each has to be tightened in sequence to a prescribed torque.

As with every other task on this project, even this was not as simple as it might appear. Because of the weak deck, special lightweight cherry-pickers had to be used to lift the riggers up to the bolts.

With the first five trusses successfully installed, construction of the cinema enclosure has begun. In the first two auditoria, the floor has already been installed, along with the concrete terraces that will support the seating.

Quaife is now anxious to start installation of the wall cladding but this will have to wait until the structure has been completed and the cambers have disappeared. He says: "We've got a few weeks more weeks of assembling the steelwork before that can happen." Perhaps the first film they will show on the finished product will be True Grit.

Getting the cinema off the ground

Structural engineer Knapp Hicks & Partners, novated to Hochtief-Miller, had the unenviable task of designing the bridge that would support the cinema. The bridge is directly above Safeway, the centre’s biggest retail tenant, and the developer had promised it that it could continue trading during the works. The first task was to find a suitable location for each of the reinforced concrete columns that will support the bridge. This was not a problem at the front of the supermarket. A row of eight columns was positioned in the shopping mall itself. But at the back of the store, plant rooms and fire escapes required that the remaining eight columns be slotted in wherever space and services allowed. The original scheme had the cinema located on top of the bridge. However, this would have blocked too much light from houses adjoining the site, so a way had to be found to reduce the height of the cinema extension. The designer’s solution was to incorporate the cinema into the bridge structure. This was accomplished by slotting the 12 auditoria between the supporting trusses, with the trusses forming their walls. Nick Barton, leisure director of developer Tops Estates uses the metaphor of “slices of toast in a toast rack” to describe this arrangement. Cinema operator Warner Brothers Citiplex insisted on precise internal dimensions for its auditoria and the location of the trusses had to be adjusted to accommodate these.

How the piling was undertaken

The 1500 tonne bridge structure was to stand on piled foundations. Originally, the plan was to install a series of large-diameter piles, each 33 m long, to carry the structure. But piles of this size can only be installed using a huge piling rig. To allow the shopping centre to remain open throughout the works, piling contractor Amec Piling opted instead to substitute the large diameter pile with a series of mini-piles. These were installed using a restricted height mini-piling rig with an auger assembled from 900 mm sections. This arrangement meant that the shopping malls could be kept open, and even allowed the piling rig to be used in some of the basement areas. Clusters of 9 to 16 piles topped with a pile cap were installed to support each of the cinema’s supporting columns. The piling contractor also had to take care not to disrupt services to the shops. In difficult locations, and where the results from site surveys were unclear, the contractor had to dig exploratory holes. In some instances, where it was not possible to reroute the services, the piles had to be redesigned. And of course, “where the piles go is where the services are”, says Peter Quaife, design co-ordinator for Hochtief-Miller.

Shopping malls get a new look – but residents don’t appreciate it

While the bridge was under construction on the roof of the centre, another team was busy transforming the shopping malls. As part of the centre's revamp, they were to be covered by a glazed roof, making the existing 2 m wide reinforced concrete canopy jutting out above the shopfronts redundant. The canopy’s removal had to be carried out at night to allow the shops to remain open. The task was given to specialist contractor Robocut, which opted to use a small hydraulic crusher to nibble away at the canopy. Work began after the supermarket closed at 10pm and was scheduled to finish at 1am. However, a series of noise complaints from residents in the tower blocks meant that this arrangement had to be modified. It transpired the residents were disturbed not by the crusher, but by the equipment being used to clear the debris after it had finished. As a compromise to the residents, Hochtief-Miller arranged for a team to start debris clearance at 6am each day. “This arrangement worked for the residents,” says Quaife. “But it meant that we had to limit demolition of the canopy to what the team could clear before the shops opened at 8am.”