Broadgate's crowning glory

Broadgate was one of the City of London's biggest developments, a huge construction project spanning the late 1980s and much of the 1990s.

One of the last phases planned was a major office building above the tracks leading into Liverpool Street station. Work on the supporting deck started back in 1998, but client British Land delayed building the offices because of a dip in demand.

Finally, two years ago, after a surge in the office market, the project was given the green light. Bovis Lend Lease was appointed as construction manager for the £200m scheme - though its work was cut out accommodating the final design on the supporting deck.

"The deck wasn't built with this scheme in mind," explains project director Colin Willats. "It had been designed for multiple building types, but not for a tower this size, so it had to be adapted."

The Broadgate Tower he refers to is a 160m-high wedge-shaped steel structure, which will be clad in glass and stainless steel. Its 35 storeys will dwarf the adjacent 12-storey, 60m-high office block, 201 Bishopsgate, which is part of the same development.

The 2.5m-deep transfer deck is effectively a giant sandwich panel, comprising two precast slabs with a steel girder in between. The upper slab was designed to support building loads, the lower slab to act as a 'crash deck' to protect the railway lines below, ensuring construction activity above could proceed without the need for track possessions.

"To take the extra load of the tower, we constructed additional transfer steelwork, weighing 800t, inside the interstitial space,"explains Bovis's project manager for the tower Rob Dudley.

The deck was also given a bit of extra "beef", adds Willats, to take the weight of the tower cranes, and to save using temporary works. There are six in total on the project, and the biggest of these, on the tower, is the tallest in the UK at 160m. "It's got a capacity of 30t, mainly because of the weight of the plant going on the roof - 90% is prefabricated," he says.

The unusual wedge shape of the tower means greater loading on the west side of the raft, so eight new ground-bearing piles had to be constructed adjacent to the railway lines. These giant piles are 16m deep from the basement and the biggest has a 3m-diameter shaft, belling out to 9.25m at the bottom.

"We had to dig them out out using mini-excavators," explains Dudley, "installing pre-cast rings as we went down - rather as if we were constructing a manhole - before pouring in the concrete."

Cross-braces

The tower structure itself is supported by six giant A-frames, which stablise the tower up to level five. Above this, the structural steel is characterised by giant diagonal cross-braces, which will give the building a distinctive criss-cross façade when complete.

The original tower design, by Chicago-based architect and structural engineer Skidmore Owings & Merrill (SOM), was modified following recommendations by Bovis and its key trade contractors.

"Previously, the steel had been designed to be 'outboard' of the cladding, giving an exposed steelwork effect," explains Willats. "This would have made it very difficult to attach the cladding to the steelwork, as well as being awkward for future maintenance.

"So we proposed making the steelwork 'inboard', using a very simple dove-tailed bracketry system devised by our cladding contractor Gartners, which would also make the tolerances much easier to control.

"The exterior of the structural steelwork will eventually be clad with stainless steel, which sits flush with the glass cladding, so the original effect SOM intended will be pretty much the same."

"It's one advantage of construction management," adds Dudley, "that we were able to get our key trade contractors - Gartners and William Hare - to help with the value engineering. With a lump-sum job they wouldn't have touched the job till the design was finished."

The lower sections of the steelwork are the biggest, at around 25t. Above the fifth storey, they are no bigger than 15t and 12.5m long, spanning three storeys.

"The floor plates are fairly small so we wanted to keep the section sizes down," explains Dudley. "However, this has meant employing a team of welders every third floor, and even when they are double-shifted, meaning a crew of 25 to 30, it still takes two and a half weeks to weld the sections on each band of floors."

When CJ visited last month, the structural steel was roughly three-quarters of the way to the top, with the cladding going on to the lower storeys of the tower.

It is being installed using another innovation of Gartners: a monorail bolted to the outside of the structure, which carries the panels around the building façade on a winch.

"It's a very simple but effective system," says Willats. "A support fitting is cast into the floor slab, a bracket bolted on, and the monorail is attached to the brackets.

"A cladding panel is then brought out on to a launching platform, tilted and lifted upwards by the winch on the monorail, swung across to its appropriate position, and dropped into place.

"There's no need for any bolting the dovetailed bracket system designed by Gartners does the job. A final neoprene seal completely secures each panel."

Once one band of floors is done, the monorail is moved up to the storey above. Before being used, it is tested as rigorously as for any other crane you would use, says Willats.

"The logistics of this job is driven by tower cranes," he adds, "and obviously the bad weather this winter has meant we haven't always been able to use them, so if we can save on tower crane time using innovations like this, so much the better. "

The panel installation process requires a six-man crew: one to operate the monorail winch by remote control two others to guide the panel down from the storey above (the monorail is generally two or three storeys above the floor where the panel is to be installed) and three more to guide it into position. A gang would do between 12 and 14 panels a day, reckons Willats.

"Not all the panels can be installed as smoothly as this," he adds. "Most are 4m high and 1.5m wide, weighing about half-a-tonne, but those that fit around the diagonals are a more awkward shape, as are the corner panels, which are also much heavier."

The remaining steelwork and cladding should be finished by the autumn. Still to be constructed is an enormous glass and structural steel sloping canopy roof spanning the space between the two buildings, supported by ties suspended from seven cables attached to the 11th storey of the tower. There's also the main entrance, a five storey atrium, featuring another canopy roof that tapers with the A-frame down to level two. The exposed legs of the A-frames, which protrude outside the tower, will be clad in stainless steel, and the anchor blocks finished with stone.

The project is scheduled for completion midway through 2008, and while Bovis hasn't been offered the fit-out contract, the team is optimistic it will stay on site to finish the job off.

Project Fact File

Project: Broadgate Tower & 201 Bishopsgate (shell & core)

Client: British Land

Budget: £200m

Start date: October 2005

Scheduled end date: Summer 2008

Construction manager: Bovis Lend Lease

Architect and structural engineer: Skidmore Owing & Merrill

Services engineer: Jaro Baum & Bolles

Steelwork: William Hare

Cladding: Gartner

Concrete: John Doyle

Electrical: T Clarke

Ductwork: FCS (Tower) Gardner (201)

Pipework: Axima

Lifts: Kone

Toilet fit out: Swift Horsman

Tower cranes: HTC