Assembling the Assembly

The GLA building is part of the much larger "More London" scheme for developer and project manager CIT (see "What is 'More London'?", page 63). Its prime south-bank site, directly across the Thames from the Tower of London and adjacent to Tower Bridge, meant the building's design had to be a "knockout blow", says Ken Shuttleworth, a partner at architect Foster and Partners. To gain planning approval "the scheme had to be so bloody good no one could stand up and say, 'We don't like it'".

The architect's response to the site is an ellipsoidal structure that looks like a giant, lopsided egg on the verge of toppling over. Its south side sweeps outward and steeply upward in a series of steps to reach the roof, and the north side sweeps back from the river in a bold but gentler curve. Behind this riverside facade nestle the council's main assembly chamber and a ramp that spirals up from the ground floor to the rooftop viewing gallery on the ninth-floor. Hidden from the river, beneath a public piazza, is a massive undercroft of debating chambers and public spaces. The architect's ambition is that the GLA building should be an icon for London. "Hopefully, it will become London's answer to the Sydney Opera House," says Shuttleworth.

The challenge for Smith and his team is to ensure that the building is completed on time and to budget. The building's unique shape, its differently sized floor plates and the interaction of the construction elements make this a difficult task. "The complex shape and design create a web of interdependency in terms of planning and control – it's comparable to a Rubik's Cube," says Smith.

On site, the completed skeleton of structural steelwork now surrounds the building's twin concrete spines. The distinctive primary steelwork consists of 10 angled, faceted tubular columns 600 mm in diameter, which follow the building's curved perimeter as they rise. A further four columns surround the atrium. Viewed from the riverbank, the angled columns tracing the building's perimeter contrast sharply with the perpendicular concrete cores as they rise from the raft foundation 6 m below ground to the rooftop, 42 m above high tide.

At each of the nine floor levels, a web of I-section steel beams, spanning up to 14 m, tie the leaning structural columns firmly back to the building's concrete core. As well as transferring horizontal loads from the supporting columns, his web also supports the irregularly shaped, composite concrete floor slabs.

Construction of floors one to four is complete. On the fifth floor, the final sections of steel reinforcing mesh are being tied in place to form a rusty carpet over the gleaming ribbed metal deck before the building is covered in concrete. On the upper floors, a team can be heard hard at work clattering sheets of profiled metal decking into position to form the lower surface of the floor, ready for its reinforcing blanket.

Meanwhile, on the finished floors the advance guard of the cladding contractor's army of installers is at work marking the positions where the cladding-support brackets will be fitted. Smudges of bright green paint now ring the perimeter of the freshly concreted lower floors to highlight precisely scribed marks that will locate these fixings.

However, by far the biggest cluster of activity is taking place in full view of the day-trippers lining the decks of pleasure boats plying the river. Just inside the building's northern perimeter, a tepee of steelwork 30 m high temporarily fills the void that will be the building's full-height atrium. Prefabricated ramp sections are being welded together around this temporary frame to form the spiral ramp to the viewing gallery. Outside the atrium, specialist steelwork fabricator Seele is hard at work craning in huge sections of the diagonal grid of tubular steel to carry the clear-glazed cladding panels that will span this void. The completed spiral will be hung from anchors on the tubular steel's support brackets.

Getting to this stage in the 18,000 m2 building's construction has not been easy. Smith uses the analogy of "a tightly wound spring" to describe the pressure on the construction team, mostly the result of the complexity of the building's shape. Less than a year into the 23-month construction programme, the project has already had to be pulled back on programme following a setback. "I don't think the programme quite reflects the complexity of the job," explains Smith. "If one thing goes, the whole programme could go." The setback in the programme occurred during the construction of the building's reinforced concrete cores. Work on the piling was begun in May last year, and the first section of the building to rise above the ground was the central concrete core. Concrete contractor Geoffrey Osborne Civil Engineering used a single jump-form system of movable shuttering supplied by Doka to form the cores. "There is nothing unusual about the concrete core except the walls are slightly thicker than normal to withstand the horizontal loads from the angled perimeter columns," says Malcolm Turpin, project engineer with structural consultant Arup. However, high winds put the tower crane out of action for days on end, which delayed lifting the jumpform rig to the next level so that concrete could be poured in. Construction of the core was eventually completed in mid-January, five weeks later than programmed.

As the cores' movable shuttering system jumped skyward it was the turn of steelwork contractor, Wescol Glosford to begin the complex task of erecting the angled structural frame. The building's amorphous shape and the absence of a regular structural grid made positioning the steelwork a tricky business. This task was all the more critical because of the out-of-balance loads created by the leaning columns.

"If a connection node on a column [the point where the I-beams tie the column back to the concrete core] is out of position by as little as 75 mm, the lateral load would be increased by up to 15%," explains Turpin. For this reason, Arup specified a tolerance of 15 mm at each connection node.

To ensure that this tolerance was achieved, a sophisticated surveying system using 3D co-ordinates was used. CIT employed surveyor Warner Land Surveys to check the setting-out on site and encouraged the specialist contractors to use the same outfit for their works. Wescol Glosford even used the surveyor to check the dimensions of the angled perimeter columns before they left their fabrication shop. "Geometric control is the key to the project," explains Smith. The fact that the building has no horizontal or vertical reference points meant that every component had to be positioned using a series of 3D co-ordinates.

The columns were supplied in three-storey lengths to minimise setting-out time. The theory was that once the top and bottom connections of each column had been accurately positioned, the two intermediate floor connections would automatically be in the correct location. "Wescol Glosford did have some problems setting out the steelwork," says Mace's Smith. David Glover, Arup's project director, admits that in "a few circumstances the columns were out". But he makes light of any problems this may have created. "It was a steep learning curve on the lower floors," says Glover, "but we sat down as a team and worked it out." Finally, the columns were tied back to the concrete core by I-section beams before the intermediate steelwork to support the floor plates was added.

Sitting in the site office with an uninterrupted view of the building's unclad skeleton, Smith acknowledges the contribution made so far by the specialist contractors on the project. "The uniqueness of the product meant design development relied on an input from key trades," he says, "so we had to go to the premier league of specialists." Technical input from many of the specialists was needed early in the programme.

"Because each specialist used sophisticated computer tools, managing the interface between each package was less of a problem," says Smith.

To ensure that the difficult project ran smoothly and the interfaces between trades were controlled, Mace employed a full-time programmer. "It's not normal on a job this size, but the complexity of this project required it," explains Smith. The programmer was needed despite having a comprehensive construction programme because the detail design stage was "more dynamic". Mace also convinced the client that a full-time design presence would be required on site. "This was a key part of the project – problems need to surface quickly and be solved quickly, hence the co-location," he explains. The move was, Smith says, "a bit of balls on the part of the client because it required them to invest a large sum of money".

It is not just pressure from the client to keep the scheme on time and to budget that makes this one of the toughest jobs Smith has ever been involved with; London's mayor, Ken Livingstone, is enough of a magnet for the media to ensure that Smith's every move is closely scrutinised. "It's the third big millennium project after the London Eye and the dome, and people are looking at this one to fail too," says Smith.

However, there is an upside to all this attention. Everyone involved is determined that the scheme should not fail, a factor that Smith says has "galvanised the project team and stopped people being too precious about their design". But George Kyriacou, CIT's commercial director, is less fazed by the prospect of dealing with Livingstone: "It could be Jesus Christ himself moving in: it's our building, our risk, and we'll make sure it comes in on budget," he says.

Having come so far without a significant slip-up, the construction team is under pressure as the scheme progresses towards completion. The programme has reached what Smith describes as "the most risky part of the job" – the ramp and the installation of the cladding. "Once this is completed, I'll be a relieved man," he says.