Full project details
Performing the impossible to provide London with an elegant and flexible space for business and travel.
Every so often, we are required to combine our areas of expertise to deliver a one-of-a kind development. Cannon Place is one such example. Because of its sheer complexity, many believed it couldn’t be built.
Work comprised the demolition of a 16-storey 1960s office building and much of the former mainline railway station, the construction of a new eight-storey ‘air rights’ office block and the reconfiguration of both the mainline and underground stations. It posed several unique challenges – not least in delivering without disrupting rail operations.
A jewel in the City
Above ground, the striking new office development stands where its predecessor once blemished the streetscape. Incorporating 7,800t of steel, the exposed lattice of its exoskeleton protects a fully glazed ‘bubble’ within – an impressive facade in which the structure is the architecture of the building.
Indoors, 500,000ft² of virtually column-free offices offer a flexible, open-plan workspace and spectacular views of St Paul’s Cathedral. Elsewhere, a dramatic new forecourt welcomes City workers, while the redeveloped mainline and underground stations provide some of London’s best rail and retail amenities for the 27 million passengers that pass through yearly.
The safety requirements and risk assessments associated with a job of this magnitude – which took place literally above the heads of the public and over a ‘live’ operational railway – were enormous. It was therefore vital we insisted on strict safety assurances throughout the entire process.
Technology vs. history
The unusual ‘inside-out’ design of the building is a direct response to an almost prohibitive list of site restrictions, particularly the limited locations in which foundations could be established. Many obstructions hindered the foundation teams, including:
- The inability to found any vertical structure over the railway tunnels on the northern edge of the site
- Limitations on column setting imposed by the platform and running tracks of the mainline railway
- The previous Cannon Street facing slab was founded on shallow 6m² under-ream pile foundations, the bases of which cover a large plan area
- A Victorian brick mainline viaduct remains over much of the site
- The southern half of the site is a scheduled ancient monument – part of a priceless Roman Governor’s palace
A value engineered solution was arrived at following an iterative process of consultation and design within the project team. We also discussed this in detail with the English Heritage inspectors and the senior planning and archaeology officer at the Corporation of London (CoL). We developed a novel and cost-saving foundation design claimed not only to be a world first but also to be quicker and safer to construct than any alternative; the team conceived, tested and created a series of rectangular foundation blocks formed of mini-piles to mimic a single large pile.
X marks the cantilever
The structural scheme balances a cantilevered 21m-deep ‘strip’ of office space to the north with the equivalent to the south through the use of deep-transfer structures. This removes the need for columns in impossible areas over the platforms, and the transfer structures – by virtue of their location – do not eat into the development zone and reduce floor-to-ceiling heights. The 21m cantilevers are supported via six large ‘X-frames’ back to the cores, which in turn are tied together via similar X-frames to enable the whole structure to balance. These exposed steel trusses are supported in turn on four composite steel and concrete thick-plate structures which load to the foundations of the building.
Keeping on track
Improvement works to the mainline and underground stations increased connectivity between the mainline platforms, the street and underground departure areas including the installation of new shops. The underground part of the station was also to receive new accommodation, improved disabled access, a new ticket hall and improved presence on Cannon Street.