Cutting through the Bull

Birmingham's concrete heart has been ripped out. The Bullring, the city's legendary 1960s shopping centre, with its graffiti-scarred walls and labyrinth of subterranean passageways, has been knocked down. And in its place, a new icon is emerging that many fervently hope will bring something altogether more human back into the city.

Five tower cranes act as temporary signposts for the construction site where the Bullring's replacement is rapidly taking shape. Scores of shoppers bustling past the site can already catch, through gaps in the site's hoarding, glimpses of the building's completed steel frame, walls of diverse material and texture, and the first of the spectacular glazed facades. Now, a team of scaffolders is noisily dismantling the forest of tubing that, until recently, supported the platform from which workers installed the glass roofing of the centre's four malls.

It is another year before the centre's September 2003 opening, and already the contractor is confident the centre will open on time. But getting to this stage has not been easy. The new Bullring is Europe's largest retail regeneration scheme. Plumb in the centre of Birmingham, the site is hemmed in on all sides by busy commercial activities. Contractor Sir Robert McAlpine has had to overcome the site's considerable logistical difficulties, both in the demolition of the original centre and in programming construction of its swanky replacement. The contractor has had to design and build two pedestrian bridges to connect the new centre with the existing streets, and suspend two levels of shops beneath them.

It has had to level large areas of the sloping site and work above Victorian railway tunnels – all without disrupting services. And as if the technical challenges were not hard enough, planners have insisted that a pedestrian route running through the middle of the site is kept open 24 hours a day.

Understandably, therefore, the unveiling of the pedestrian-friendly centre is eagerly awaited. Unlike its impenetrable forerunner, the new Bullring has been designed to knit into the fabric of the city. Forty years ago, the planners kowtowed to the motorcar. The result was a development isolated from the city by a concrete collar of the inner ring road. It was accessible to pedestrians only through a series of torturous underpasses. "People were pushed underground, and traffic had priority – we've struggled to break free of that," explains Nick Guy, a director at the scheme's concept architect Benoy.

The new centre has been designed with people in mind; Benoy has composed a scheme on three axes. A new open street will link the City's drum-shaped landmark, the Rotunda building, in the north to St Martin's Church at the lower, southern end of the site. A covered street running east-west will traverse the development. Two smaller streets, also covered, will tie the east-west corridor back to the Rotunda to subdivide the development into a series of blocks (see site plan, overleaf). The inclusion of two new public squares in the development – one in front of the Rotunda, and one next to St Martin's Church in the middle – further emphasise the scheme's pedestrian-friendliness.

The development is being funded by the Birmingham Alliance, a three-way partnership between developers Hammerson, Henderson Global Investors and Land Securities. The scheme is a massive undertaking: the site covers a total of 10.5 ha or the equivalent of 15 football pitches. Retail giants Debenhams and Selfridges will occupy the scheme's two anchor stores, with more than 100 shops, cafes and restaurants filling the remainder. The needs of the motorist have not been ignored, however – the site will also have parking for more than 3000 cars.

Contractor Sir Robert McAlpine is working under a design and management contract for a guaranteed maximum price of just over £240m. Peter Carruthers, McAlpine's project manager, says it is only the second time this form of contract has been used for the construction of a major retail development – the first was West Quay in Southampton where Carruthers again headed McAlpine's team. "This form of contact gets rid of any conflicts of interest," he explains. "The client gets cost certainty, we get to control the design and, if the project costs come in below the GMP, we get to share the margin."

The team won the contract on a competitive tender based on a set of employer's requirements. That was in spring 2000. By May, the contractor was informally appointed. And by June of that year, the contractor had started on site.

Despite the unwelcoming atmosphere of the original centre, it was home to the city's thriving markets – and the market traders are a powerful lobby. So, before McAlpine could start demolition, purpose-built markets had to be constructed adjacent to the site, under a separate enabling works contract, for the "indoor market" and the "rag market".

But even though the markets had disappeared from the old centre, the traders' influence was as strong as ever. They insisted that the contractor maintain a pedestrian access route through the centre of the site to their new location. This right of way is a legacy that has dogged McAlpine throughout the project. The demolition programme had to be carefully staged to keep the path open. At one stage, while the site was still being excavated, the contractor even had to install a 230 m long bridge, 15 m above the groundworkers, to preserve the crossing. It is only now, with the centre nearing completion, that the path can follow the final route of the centre's main street.

At the same time as the demolition contractor was pulverising the old Bullring's concrete structure, subcontractor Balfour Beatty was hard at work remodelling the road system around the site. One of the contractor's first tasks involved lowering a major road beneath the centre's main pedestrian thoroughfare so that shoppers could approach the centre from the high street without having to battle their way through subways.

But lowering the road was another tough assignment. For part of its route, the road follows the path of two 1830s brick railway tunnels that provide access to the nearby New Street Station for mainline and local train services. To achieve clearance for double-decker buses, the road had to be lowered so that it was almost sitting on top of the arch of the two tunnels. Before work could begin, the operation had to be modelled on a computer to see how the tunnels would respond to temporary loads as well as under final load conditions. "It's hard to believe how much modelling had to be done," laughs Carruthers, before returning to his more matter-of-fact manner: "It was obviously a very sensitive operation."

With excavations for the road complete, Balfour Beatty's next task was to install 20 bearing piles between the two road tunnels, to support the bridges from which shops were to be suspended over the road. "It all had to be done with the agreement and vetting of Railtrack," recalls Carruthers. Throughout the operation, the tunnels were monitored for movement and vibration.

With the piles in place, four 50 m long bowstring trusses were then installed over the road. These support two levels of shops, which hang beneath the trusses and connect the new centre with the existing streets in one continuous line of shops, cleverly designed to draw people into the centre. The trusses, which weigh 120 tonnes each, were delivered in sections and welded on site before two cranes hoisted them into position. This had to be done at night because the trusses were swung out over the railway. "It was a fairly horrendous task," says Carruthers, with uncharacteristic expressiveness.

The next major element of the groundworks involved the construction of what Carruthers calls "a contiguous piled wall". The site slopes down steeply from north to south, 20 m higher at the foot of the Rotunda than at the St Martin's Church end. To enable the north of the site to be excavated to the same depth as its southern boundary, and the basement car park to be constructed, the contractor had to install a wall of 280 abutting concrete piles.

The wall held back the earth so that the excavations could proceed without the Rotunda and various other buildings collapsing into the excavation. A series of huge temporary props had to be used to hold the wall in position until the Bullring's new structure was complete. "Some of the most complicated civil engineering work took place under the first part of this project," says Carruthers.

With demolition of the old Bullring under way and the enabling works in hand, McAlpine had just 15 months to press on with the scheme's detailed design. Architect Benoy had developed the scheme with three levels of detail: an outline scheme for planning approval, a more refined version of the design used at tender stage, and, finally, what Benoy's Nick Guy calls a concept design, which was handed to McAlpine.

"There were so many important decisions to make, for example, on the structural steel, the lifts and some of the m&e plant – all of which had to be resolved quickly because of their long order times," says Carruthers. And the programme dictated the time each decision had to be made if the handover was to be met. "It needed big resources from our design team," Carruthers explains. "Particularly as the developer had already signed up tenants."

One of the more innovative aspects of the centre's design is the building's structural frame, which is a mix of concrete and steel. "We couldn't have completed the job on time if we had used all concrete or all steel," explains Carruthers. A concrete frame forms the structure of the two lower levels, whereas the three upper levels have been constructed in steel. Between levels four and five, the entire structure has been swung through a 45° angle so that the centre's superstructure is aligned with the city's existing street pattern. To enable this rotation, a series of steel transfer beams allow the loads from the upper floors to be conveyed to the concrete frame.

With the building's structural frame complete and the glazed roof of the shopping mall also finished, Carruthers' attention is now focused on the development's facades. This is mammoth task in itself, because each facade will be different. The architect has selected a palette of brick, stone and glass to blend the centre into the surrounding urban landscape. "There is just about every type of cladding you could imagine," says Martyn Reeve, McAlpine's design manager on the project.

One facade outside the contractor's scope of works, however, is that of the new Selfridges. This building's sculptural curving exterior with its dressing of shiny metal discs, designed by architect Future Systems, is being carried out under a separate contract by Laing. The McAlpine team has been a little thrown by the architect's request that the concrete terraces surrounding the buildings are finished in blue concrete – they have still to figure out how to achieve this. "We're debating what form that will take," admits Carruthers.

For the city's shoppers the clock is ticking – in a year's time the city's heart will start to beat once more.