Gherkin, anyone?

Vilified as the erotic gherkin, reinsurance firm Swiss Re's new London headquarters has been criticised for overshadowing the historic buildings in the city skyline. But like it or loathe it, the dramatic 180-metre high building strikes an impressive pose in the heart of the city's financial district, and the building methods employed on the project follow the latest models of best practice: strict pre-planning, just-in-time deliveries and off-site prefabrication being some of the methods used here.

Approval for the Fosters and Partners-designed building was gained in July 2000, with work on site beginning in December the same year. Rising from the site of the Baltic Exchange, which was destroyed by an IRA bomb in 1992, the building has 39 storeys and is topped off by a double-height executive restaurant. On completion it will be the second tallest building in the City, only Tower 42, NatWest's former hq, stands higher.

Perhaps more unusual for the City, the building has a round footplate, which bulges as you go higher, before narrowing to a domed top. The exterior glazing consists of triangular elements that have been lifted into position using floor-operated cranes and hooked into place with bespoke brackets. Six atria spiral the height of the building, created by triangular cut-out areas around the edges of the floor; the floor plan rotates on each successive floor, creating this staggered effect. These atria allow daylight to penetrate deep into the building and also act as part of the low energy services strategy, providing natural ventilation. Skanska Construction UK is main contractor on the project and also won the contract for the shell and core m&e services by competitive tender in a combined bid with its services branch Skanska Rashleigh Weatherfoil. This joint bid was a decision made by the firm rather than a prerequisite for the contracts. Roger Gellet, services director at Skanska Rashleigh Weatherfoil explains: "This was a high profile project, so the company thought 'let's get the best parts of the organisation to work together'." Rashleigh Weatherfoil worked on a domestic sub-contract to Skanska.

Senior project manager Steve Davenport elaborates: "It's an extremely fast-track programme. We're going to structural completion in November, so had only 22 months to take it all the way to the top. That's a lot of internal planning. And [the client] wanted to do sequential programming – Rashleigh Weatherfoil is able to support this with its offsite prefabrication plant."

In this inner city location space is at a premium, so everything had to be thought through with military precision. A large part of the planning was done before getting to site. Part of this process involved using the Navisworks Presenter software program to produce accurate 3D models of the building and its services from drawings produced by the individual project teams.

The architect produced drawings using Microstation, and on the services side the four-strong design team used AutoCAD's BS Link to produce pipework and ductwork details. These drawings were then automatically incorporated into the model using the software. "Navisworks Presenter basically pulls all the programs into one," explains Davenport.

Producing the computerised 3D models gave several benefits. Exact sizes and positioning of all the final elements of the building, from structural steelwork down to door lintels, are included in the model, as are temporary works. By doing this, the services routes could be accurately designed to suit the structure. A clash detection setting in the program warns of pipes hitting steel for example, so any rerouting could be worked out very early in the design process. "We probably designed out 250-300 main clashes," reports Davenport.

This meant that large sections of the services could be prefabricated and their installation carefully planned before construction began, enabling the services installation to follow the sequential programming of the building. "We're actually committed to manufacture the plantroom pipework straight from the 3D, deliver to site and install," Davenport explains.

  A further bonus was the ability to show site operatives the building and installation plans with a real perspective before they began working: "They can see it in 3D and get a feel of what they're going to be doing," says Davenport.

Setting up the models took around one year's work. Once on site, Skanska had a logistics crew that controlled deliveries and access to cranes throughout the construction. With storage space minimal, just-in-time deliveries were essential and materials could even be returned if windspeeds deemed the cranes unusable.

Servicing Swiss Re
Skanska Rashleigh Weatherfoil's contract covered the shell and core services, major plant and toilets. With few exceptions, this is concentrated in four main areas: the basement, a plantroom on level 35, the central core and a separate, adjacent building.

The basement holds most of the electrical plant, plus chillers and their associated equipment. This area is served by its own air handling unit; there is also an extract system for the small car park housed here.

There are two independent electrical feeds into the building, supplying hv and lv panels in the basement, which distribute to level 20. The top half of the building is served from the level 35 plantroom: an hv riser feeds a second set of transformers and switchgear here, which drop the supply to lv and feed down to level 20.

Also in this high-level plantroom are six water cooling towers, which are served by a condensed water distribution mains running from the basement level.

Primary distribution through the building is via four main risers in the central core. It was here that the prefabrication came to the fore, the mechanical pipework and ductwork being preinstalled on triangular steel frames in Rashleigh Weatherfoil's Slough facility. "The benefit to the programme has been substantial," stresses Gellett.

The installation of service risers was driven by the sequential programming of the construction, with mechanical risers being installed as close to the top of the building as was safe. As the plan for the construction schedule was to cast one floor per week, it followed that the risers were installed at the same rate. The physical installation of the risers was planned so that it would take only one day per floor; access to cranage being a factor as well as the construction cycle. Once craned into position, the risers hang down from the floors, with all connections carried out from the floor below at waist-height and below to minimise working at heights.

As the building footplate gets smaller towards the apex, a faster speed of installation was both possible and essential to ensure that everything was installed before the structure was closed. At the time of visit floor 33 was being cast; the mechanical risers had been installed to floor 31.

Electrical busbar was not included on the risers as the industry standard lengths of busbar do not match that needed to suit the floor heights and would have meant unnecessary cutting. The issue of waterproofing was also a consideration in the early stages of building – busbar is being installed as the floors become watertight.

The heating element of the services is contained in a two-storey plantroom in a new six-storey office building, which forms part of the construction scheme. This is connected to the mechanical risers by underground pipes. The decision to house boiler plant separately was made for two reasons: lack of space and a desire to maintain the clean shape of the structure. Davenport states: "Being a separate building you can take the boiler flues up to the roof and discharge them at roof level, whereas we'd have a problem here discharging from the boilers, we'd end up with a turret."

The building is due to complete in Autumn 2003 but it has already made an impact on the City of London, creating both a new skyline and a talking point.