A complex project to rebuild office space above London’s Cannon Street station owes as much to bridge engineering design as it does to structural engineering.
Air rights projects are always complex. Building over railway lines invariably throws up constraints on foundations and loads. But Cannon Place, two 1960s office blocks currently being rebuilt as a one eight storey structure above London’s Cannon Street station is truly something else.
One of only two major buildings set to be completed in the capital this year, the project incorporates the redevelopment of two previous office buildings on the site and the renovation of Cannon Street’s main line and Tube rail stations - one of London’s busiest transport hubs serving 27M passengers a year.
Beating the recession
Developer is Hines, in joint venture with Network Rail and London Underground. Hines project director Mark Sweetman says beating the recession and just getting the building built is an immense achievement. And that’s before you consider the structure itself.
Making the development commercially viable was the first challenge: protected sight lines of St Paul’s Cathedral cap the height at 51.3m above Ordnance datum and a minimum clearance of 5.1m above track level had to be maintained. This gave just a height of 32m within which to plan the structure which occupies a 67.5m by 87m footprint. Moving the plant room to a satellite building - and therefore avoiding losing an entire floor of lettable space - was key here.”We finessed it,” says Sweetman.
“This building couldn’t have been built 10 years ago. The computing power simply didn’t exist”
Mark Sweetman, Hines project director
But the really remarkable stuff is how the project team has taken inspiration from the Forth Rail Bridge to build a structure in central London.
The development is above the north end of Cannon Street station, which stands on the north bank of the Thames, a terminus for commuter rail services from south London and Kent. When complete its 67.5m long northern façade will hang column free above the station entrance cantilevering 21m out from the building’s cores.
“The building itself represents a major feat of engineering,” says Sweetman. “We have adopted complex techniques and borrowed from the construction of the world’s major bridges to give the appearance of the building “floating” above the station.”
Cantilevered end sections
The building comprises five 67.5m long sections. Three of these are 21m wide and they are separated by two 12m wide sections containing the building’s lift cores. The cores anchor the two end sections which cantilever to the north and south respectively.
On each side of the building, external cross bracing in the form of massive eight storey high criss-crossing trusses tied together by steel perimeter beams, transfer loads from the cantilevered sections back to the cores. It is these trusses consisting of plated box sections with plate thicknesses up to 100mm that the give the building its distinctive appearance.
The steel frame at the north and south ends of the structure comprises a series of deep trusses with horizontal and vertical circular hollow sections and diagonal tie bars. These pick up the loads from the end of the 21m span beams on each floor and in turn transfer them to the cross bracing (see diagram).
“There are four very strong core pillars where we could get the foundations down”
Giles Fazan, Hines
More than 40% of the weight of the building is taken down through the crossbraces, so they are weighty beasts themselves - 23t each.
The loads are then transferred to the ground through four huge reinforced concrete thrust blocks - so-called “hour glass” structures.
Lack of column space
This load path was critical. The site was heavily constrained by an inability to found columns on top of the London Underground tunnels which run from east to west beneath the northern bay. There were also limitations on column setting out imposed by the mainline railway, a service road that runs beneath the station, and by the presence of a series of brick arches that are classified as an archaeologically sensitive Scheduled Ancient Monument. There were also significant existing foundations under the previous buildings, many of which had to be reused simply because the logistics of piling among them would have been horrendous.
“Put simply, this building couldn’t have been built 10 years ago,” says Sweetman. “The computing power simply didn’t exist.” As a graphic example, the builder of the scale model of the building in the developer’s showroom had to sneak in glass columns to make the model stand up.
Fundamentally, there were just four zones that could be used for piled foundations. These were used to create a base for the four huge reinforced concrete thrust blocks. “There are four very strong core pillars where we could get the foundations down,” says Hines construction director Giles Fazan.
“Having to feed in 21m long Fabsec beams through the side of the station in the dead of night was challenging”
Giles Fazan, Hines construction director
From these four, 12m2 central cores could be erected. Aside from the cores there are only eight columns in the entire building.
Building such a complex structure above a live railway station was never going to be easy. Partnership was key, says Fazan. All parties, including client representative Arup, architect and structural engineer Foggo Associates, design and build contractor Laing O’Rourke and cost consultant Gardiner & Theobald have played their part.
“We didn’t have the horror stories that you hear about on other air rights projects,” he says. “The key for us was getting all the designs that have an impact on Network Rail and London Underground done early and then getting their engineering team onto the site and into the offices.”
This meant any possible issues were raised early and talked through. “It was great to have the chance to be able to speak informally over a cup of coffee,” he says. These conversations included Laing O’Rourke from the earliest possible stage. “The design and build contract was signed in December 2007 but Laing O’Rourke was brought in on a professional services contract a year prior to that,” he says. More than £3M was spent on that advance contract within the £200M overall cost, and it more than paid a dividend, he says.
Tricky piling work
The piling works were unquestionably the trickiest task, carried out in 2009 by Laing O’Rourke subsidiary Expanded Piling (NCE 23 July 2009). Four hundred and twenty were needed in total.
There are four different types in all, up to 900mm in diameter and up to 38m deep, all bored with very little headroom because of the constraints of the site.
“The clever stuff was down in the ground. It really was,” says Fazan. But there’s been much clever work done since, starting with the challenge of erecting the deck for the first floor level that will double as the station’s new roof. “One of the awkwardnesses is that we had to construct the new station roof 1.5m beneath the soffit of the existing one. Having to feed in 21m long, 725mm deep Fabsec beams through the side of the station in the dead of night was challenging. We managed two or three Fabsecs a night. Laing O’Rourke put an awful lot of planning and logistics in there and it all went very smoothly.”
Once the beams were in, a 130mm deep lightweight concrete deck was cast allowing the existing building and station roof to be demolished. “That was probably one of the cleverest parts of the job,” says Fazan. From there, construction of the new building could race ahead, largely unnoticed by the thousands of passing commuters.
“We had people saying to us: ‘do you think this building is going to go ahead?’,” says Fazan. “I’d reply, ‘Well, hang on, I’ve got 200 guys in here every night’.
That construction like this can go unnoticed is a testament to how its been done.”
The 21m wide cantilevered strips front and back were installed using strand jacks. Twelve jacks per side were needed, each one capable of lifting up to 480t.
“The strand jacking was the blonde in the room that everybody noticed, but actually it was pretty straightforward,” says Fazan. Straightforward maybe, but care was still needed. To keep the building balanced the operation could not get more than two storey out of kilter front to back.
Once all the floor beams and exterior cross beams were installed to the exterior of the structure the stand jacks could be struck and concrete pours for the 150mm to 175mm deep concrete floor slabs begin, again with both front and back in balance.
Each beam was precambered to allow for sagging under selfweight, and each beam sagged as expected, explains Fazan. “That was quite a cause for celebration,” he says.
Once all 14,000t of concrete and 7,500t of structural steelwork was installed, one final operation remained. The entire structure had to be rebalanced using flat jacks and adjusting the external tensile braces as necessary.
The project, which began for Hines as far back as 2002, is now nearing completion, with the mainline station handed over in June. Fit out continues with the office building set to be ready to let from September and the newly renovated London Underground station opening in June 2012.
It’s a long gestation period for a developer, but in this instance, it’s worked. “When the recession hit, the banks phoned us up and asked ‘Do you still want to do this?’” says Sweetman. “But we’d already started and it was too late to stop. So we built through the recession. We’re still in the construction phase now, and so in a way, a long construction period was a blessing in disguise,” he says. “We should be finished just as the market picks up.”