It was also its biggest gamble. With the City's office market approaching the kind of frenzy last seen at the peak of the 1980s, Wates City decided to sink £160m into Britannia House, the drab megalith behind Moorgate Station that used to house BP's headquarters.
The plan was to reincarnate it as a gleaming 36-storey tower, complete with seven-storey annexe, extended floor plates and state-of-the-art services.
Unfortunately, this meant that Wates City would see no return on its investment for 33 months. "With capital tied up in the scheme, we had to pressurise its construction," says Rodney Clutton, joint managing director of Wates City, putting it mildly.
To gain some certainty that there would be a return at the end of that time, Wates City pre-let almost 60% of the scheme. All of the low-rise annexe and levels one to eight of the tower have been taken by international law firm Simmons & Simmons; US lawyer Simpson Thacher & Bartlett has taken floors 20 to 22.
To call what was planned for Britannia House a refurbishment draws a pained expression from Clutton. He prefers the term "rebuilding". Ideally, Clutton would have redeveloped the site altogether, but as City planners tend to maintain strict height restrictions in the Square Mile, "redevelopment would have meant starting from scratch – and they'd probably have disallowed it". The task facing the construction team was formidable. First, it had to strip the original building back to its concrete skeleton. Then, all 36 floor plates had to be extended outward on the east and west elevations by 1 m, with the result that they now finish flush with the main reinforced concrete columns.
The building's flat southern elevation was also to be extended by 7 m and remodelled so that it sported a rounded, glazed prow. And the floor plates on the northern elevation were to be extended outward by 3 m and joined to the main elevations by way of gently curved corners.
The effect of these changes would be to soften the original building's rectilinear profile, so, once the cladding was on, the tower would have a more rounded, contemporary appearance.
While this was going on, the seven-storey office was to be built at the base of the tower on a barren, windswept plaza that formed the approach to the main entrance. The tower and its diminutive partner were to be linked by a sweeping glazed roof, and the main entrance to the offices relocated to the smaller block in the striking form of a giant glazed eyelid.
The commercial impact of adding the low-rise office and extending the tower's floor plates was that the scheme's lettable floor area was increased by an incredible 75% to 110 000 m2.
When planning permission was granted two years ago, John Shreeves, director of the eponymous project manager, had less than three months to devise a programme before work started on site.
He had to arrange it so that the team met the first handover deadline of November 1999 for the Simmons & Simmons floors, and the second deadline of March 2000 for the handover of floors 20 to 22.
Access for men and materials is notoriously difficult in a tower, so, in a bid to speed up construction to meet the tenants' occupancy deadline, Shreeves took a lead from the computing industry and decided to adopt a system of parallel working on the site. One team of contractors would work on the tower while another would be busy in the building's basement.
To-ing and fro-ing up the tower
Now the shell of the tower is almost complete, and cladding contractor Permasteelisa is about to install the building's distinctive glazed crown. Below, the rattle and hum of hoists fills the air as cars move up and down the outside of the tower hurrying men and materials skyward.
Oliver Simm, project director of co-ordinating contractor Mowlem, describes the work as "a production line", with teams of contractors yo-yo-ing up and down the tower after each other. The procession was led by Weathershield in May 1998. Its task was to remove the tower's existing cladding. Starting at the top, it moved slowly down the outside of the building, working from a giant access platform that circled two floors at a time.
As the cladding started to disappear from the outside, enabling contractor J&J City began work on the interior. J&J workers followed the cladding removal team down the building, stripping out the existing 100 mm concrete floor screeds as they went. Removal of the floor screed served two purposes, explains Mark Dillon of project architect Sheppard Robson. It created an extra 100 mm floor-to-ceiling height, which was enough to squeeze in a raised-access floor to accommodate cabling. It also reduced the load on the office floors by 24 lb/ft2, allowing the landlord to offer the tenant higher floor loads.
Meanwhile, 125 m above the heads of the public, the tower's top two floors were gently demolished. These have since been replaced with two new floors, strengthened to carry the rooftop air-conditioning plant. "It's an element of the work that worried me stiff," says Simm, "but fortunately the task was finished without incident."
Hitting rock bottom
While work continued apace at the top of the tower, demolition contractor Keltbray had a team hard at it in the basement. "The basement is one of the biggest in London," says Shreeves. "Its five floors extend below the original entrance plaza and some of the surrounding buildings."
However, the existing structure was not strong enough to support the weight of the new low-rise office. New foundations were needed. Structural engineer Bunyan Meyer & Partners opted for a monolithic piled raft to stop differential settlement occurring between the tower and the new office. The new raft will link the tower's existing raft foundations to the basement's diaphragm wall with a gigantic concrete slab.
Keltbray began to dismantle the basement floor by floor, starting at ground level. As the floors were removed, huge props were installed to stop the retaining walls collapsing. In the gloom, four floors below, another team was busy removing the lowest basement slab prior to steel and concrete contractor O'Rourke starting work strengthening the huge beams supporting the basement's walls from the tower's foundation.
When the upper basement floors had finally been removed, piling contractor Cementation took possession of the basement void to install a ring of more than 1200 anti-settlement piles around the tower's raft foundation and beneath the floor extensions on the north and south elevations. The piles were threaded between the newly strengthened sections of the basement slab.
"It was all going at 100 miles an hour," says Shreeves. Then disaster struck. The piling contractor encountered what Sheeves describes as "the biggest block of concrete ever". In fact, the contractor had stumbled across a 5 m3 section of the foundation from a previous building and it "couldn't have been in a worse place", explains Shreeves. The block was situated next to the building's southern elevation where the piles to support the tower's soon-to-be-constructed rounded prow were to be drilled.
The piles were essential – they would carry the weight of the facade and floor extensions for all 36 storeys of this elevation. "The structural engineer was unhappy about trusting the block to support the extension," says Shreeves, "particularly after polystyrene was discovered beneath it." It had to be removed.
Keltbray lowered a handful of small mechanical breakers into the basement and began the slow process of chipping the lump into manageable pieces. "It delayed us for some considerable time," says Shreeves. "Worst of all, the works were on the programme's critical path."
Getting the project back on target was going to be difficult. Out-of-hours working was not an option because the site had to meet strict noise criteria during the day, and the site's proximity to the nearby Barbican residential development prohibited site noise before 8am and after 6pm. This effectively limited the noisy work to six hours a day: 8–10 am, 12–2 pm and 4–6 pm.
Weekends were no better. On Saturday, the site was restricted to work between 9am and 2pm, and no noise was permitted on Sunday. "It was the worst of all worlds," moans Shreeves. "It added days to the programme."
With the tenant handover date looming, it was the structural engineer that came to the rescue. "He saved the day by devising a quicker method of erecting the new supporting structure at this end of the building by changing the design from a cast insitu concrete structure to a mixture of reinforced concrete and steel," says Shreeves. Once the obdurate lump of concrete had been removed, the piles could be installed, and O'Rourke was able to press on with the structural frame for the low-rise office and the floor extensions in October 1998.
Putting in the guts
Despite the problems in the basement, work inside the tower continued unabated. Once the floor screeds had been removed, services contractor Matthew Hall began threading pipework up through the service risers in the tower's core to connect the basement plant to the rooftop equipment.
Following close behind was lift engineer Kone, whose mission was to install three lift-shafts at the south end of the tower. Behind them was a team from Swift carrying out builder's work to the tower's structural core. The last of the teams chasing skywards was also from Swift; it was fitting the slab extensions to each floor.
As work was progressing inside the tower, cladding installer Permasteelisa was racing up the outside of the tower fitting the cladding. Installation started at the beginning of April, and by the November handover date, the contractor had managed to complete installation as high as the ninth floor. When the project is complete, more than 42 000 m2 of cladding will have been installed.
Back in the basement, engineers from Matthew Hall are busy installing the pipework connecting the basement plant with the service risers. Testing and commissioning works on some M&E systems have already started in preparation for the second phase of tenant handover. The basement has also been let to a leisure operator, and fit-out – including the installation of a swimming pool – is scheduled to begin in August.
Now, 24 months into the contract, the project is starting to look complete. Simmons & Simmons' fit-out is under way, and the effort now is focused on completing the entrance before the first of the occupants arrive in August. At the same time, the remaining first phase handovers for the upper tower floors are still in progress. Final completion for the entire scheme is set for December 2000.
Client Wates City of London Properties Architect Sheppard Robson Co-ordinating Contractor John Mowlem Project Manager John Shreeves Quantity Surveyor John Shreeves Structural Engineer Bunyan Meyer & Partners Services Engineer Foremans