Every day over the past weeks, huge 30m long steel plate girders have been appearing on the City of London landscape just behind Liverpool Street Station. Each element has been swung into place overnight so as not to disrupt the busy financial centre and together they make up the rib cage of a beast that will support the capital's newest skyscraper ? the 35 storey wedge-shaped Broadgate Tower and its 12 storey sidekick at 201 Bishopsgate.
The ú292M ($555M) offices will increase the Broadgate estate, which houses financial giants Lehman Brothers and UBS, by 20% when completed in mid-2008. Contractor for the scheme is Bovis.
The steel girders form the diagonals of six, five storey high A-frame structures and bear onto a 2.4m deep concrete and steel-framed 'raft' that covers most of the site and bridges over four railway lines coming out of Liverpool Street station.
This raft was built seven years ago by contractor Bovis (see box) at a time when the City office development market had plummeted.
At the time landowner British Land decided to delay building the superstructure until the market was more favourable.
Fast forward to 2006 and the 8,000m 2 site is once again a frenzy of activity.
Architect and structural engineer Skidmore Owings & Merrill (SOM) designed the raft without knowing what the future superstructure would look like. So the brief at the time was to make it flexible enough to accommodate a single tall building or a group of buildings.
'Although we never contemplated having a 35 storey building on it, ' admits British Land head of construction Richard Elliot.
SOM is still the designer on the job and has had to detail 800t of extra steelwork to be shoehorned into the raft to suit the new superstructure loads ? steel fabricator is William Hare.
The configuration also means that the east side of Broadgate Tower lies over the edge of the raft, while the west side bears onto a line of eight new ground bearing piles adjacent to the railway lines. Since the tower load on the raft is so grossly eccentric, SOM has transferred most of the tower's weight through a series of six colossal steel A-frames, the toes of which straddle the pile and raft (see diagram).
The A-frames will form part of the gallery that connects the two buildings, the 25t diagonals of which are tapered from 1m by 1m square sections to 0.8m by 0.8m sections to appear less bulky. 'The floors up to the fifth level are hung off the A-frame, so most of the steelwork you can see is temporary until all the A-frames are up, ' says Elliot.
Being situated in the valley between the two buildings, the gallery will be subject to high wind loads and uplift in particular.
So steelwork connections are designed as much for their loadcarrying capacity as for tying the structure down.
Both buildings are steel framed with composite floor slabs to keep their weight as low as possible and each block sits on rubber bearings. This ensures that the buildings are isolated from railway borne vibrations or the effects of railway accidents.
The Broadgate Tower has been designed as a steel-framed 'tube' structure where the main stability elements are contained in the building's perimeter. These are expressed in the cross bracing detailing on the cladding that also gives the wedge-shaped building its character. The tube design makes the structure extremely stiff with a high level of redundancy, so that steelframed lift cores only nominally contribute to the building's stiffness. The main job of the cores is to help direct loads to the piles on the western side of the site, away from the raft.
'The braced steel core has the same characteristics as the World Trade Center, but is protected using additional reproong and strengthened dry [partition] walls, ' says Elliot.
Headds that as part of the 'extreme events analysis', stairwells have been designed to allow the complete evacuation of the building. 'Steel plate is also used in the core [for additional robustness], ? but it's difcult to create a tower building that can support the load from a plane ying into it, ' says Elliot.
Both buildings have been designed to be energy efcient.
So as well as optimising heating and cooling equipment, 15% of the building materials will come from recycled products.
The raft is a twin-deck steel and concrete sandwich whose upper deck has been designed to support building loads, while the lower deck acts as a crash deck to protect the railway lines below. It was constructed during engineering hours and under railway possessions in 1999 involving close coordination with Railtrack. The provision of the crash deck ensures that the Broadgate tower and Bishopsgate building can be built free from the constraints of a live railway.
'The 2m void between each deck [of the raft] will eventually be used for lift, escalator and tree pits, ' says Bovis project manager Rob Dudley.
201 Bishopsgate: little brother
Construction of 201 Bishopsgate begins this week.
It sits squarely on the raft and is of more conventional steel-framed and compositeoor design with two concrete stair cores. The building also contains two externally braced walls that are detailed using the same detailing as Broadgate Tower.