London's Mayor Ken Livingstone can finally start thinking about moving into his £50M new home next spring. With a year of construction gone, topping out of the Greater London Authority HQ will be complete by the end of the month, leaving the building on schedule for its spring 2002 opening.
To the casual observer, constructing the 42m tall landmark building in just two years appears to be no mean feat. The glass clad building, with its distinctive ellipsoidal shape, has been likened to everything from a giant fencing mask to an egg about to topple over, and conjures up images of a design and construction nightmare.
However, Arup, engineer for the structure, services, facade, fire and acoustics, insists nothing could be further from the truth.
'It is an unusual project and you have to think outside the normal routine, ' explains project director David Glover. 'But little actually is outside the normal routine. It is just an ordinary building with crazy geometry.
'We spent much time persuading people not to be scared of the building, ' he says.
Despite Glover's candour, there are significant engineering challenges to be overcome.
The angle of the structure means the exterior wall meets the ground at an angle of up to 22degrees, generating lateral 'out of balance' loads five times that of wind loading. These loads are transmitted to ten 600mm diameter tubular steel primary columns and a cantilevered reinforced concrete box core from nine superstructural composite floors (NCE 14 October 1999).
A 175mm deep composite deck slab was chosen, supported by I-section floor beams that also anchor the primary columns to the core. The glass cladding is fixed directly to the concrete floor slabs, supported at the bottom and restrained at the top.
With much time invested at design stage in making the structure work, there have been few surprises during construction.
The real challenge has been achieving extremely tight tolerances.
The tolerances are vital for two reasons, says Arup project engineer Malcolm Turpin. 'The location of the nodes is important structurally, but with every trapezoidal glass panel unique the location of the edge is important for the cladding.'
Turpin explains that if a node in the steel frame was out of position by 75mm lateral loads would be increased by 15%. To prevent this happening a tolerance of +/-15mm in three dimensions has been specified.
A tight quality assurance loop has been put in place to ensure that this tolerance is achieved.
All columns are surveyed in shop before leaving for site, and each column is placed in three, threestorey lifts to minimise the setting out required.
After each lift a thorough survey is made by a full-time site surveyor - employed both by client CIT Markborough as checking surveyor and by steelwork contractor Westcol Glosford - before the works can progress.
Any failures not rectified at this stage can then still be corrected in the floor beams and slabs. 'This is our get out of jail strategy, ' says Glover, 'which we've used a couple of times.'
The structure has been closely monitored throughout construction using 3-D drawings, without which the project could not have got off the ground: 'A structure like this would be impossible to understand if it wasn't drawn in 3-D, ' Glover claims, 'These tools weren't there five years ago.'
The use of cutting edge software has been vital in keeping the project on schedule. Clever sub-routines written by architect Foster & Partners have allowed data to be immediately translated and transferred between the different software packages used by the many parties involved in the project, reducing the lead in time for trades from two weeks to half a day.
Other savings have been made by applying a rational approach to construction. The 175mm floor slab was chosen ahead of a 135mm slab, meaning a higher capital cost but shaving four weeks off construction time by reducing the number of intermediate beams from three to two.
Construction began in May 2000, with the foundations completed in a 2,000m 3concrete pour one Sunday in July. The two central cores were poured in single storey lifts over 12 weeks.
Attention now turns to the cladding and the 'diagrid' (see box), before the building is fitted out ready for the mayor.
He will be blessed with a good view. Standing in the shell of the mayor's penthouse suite, overshadowing Tower Bridge and looking across the Thames to the Tower of London and beyond, Turpin sums up the project: 'Ken cannot ignore the city of London anymore.'
Client: CIT Markborough Architect: Foster & Partners Structure, services, facade, fire and acoustics engineers: Arup Construction manager: Mace Concrete contractor: Osorne Piling contractor: Expanded Steelwork contractor: Westcol Glosford Facade contractor: Schmidlin Spiral walkway contractor: Waagner Biro Diagrid contractor: Seele
On the spot
Name: Malcolm Turpin Age: 29 Qualifications: MEng, MIStructE Job title: Project engineer Best thing about current job:
The chance to design an unusual building with the architect and then to see it happen on site.
And the worst: The weather and the mud.
Best project ever: Darlaston swimming pool - a really interesting structure using 26m long laminated veneer timber roof beams.
And the worst: Projects that don't go anywhere!
Worst thing about being an engineer: You can moan about the pay - but you have to balance it against what you get out of the job.
And the best: The feeling that there will always be another GLA.