It’s got the X-factor
An office block that “hovers” over Cannon Street railway station in central London borrowed a structural solution that was used on the Forth Railway Bridge. Stephen Cousins reports
Few city construction projects can have faced the complexity of site constraints experienced at the Cannon Place office block development in central London.
The eight-storey rectangular glass and steel block is perched directly above the rail tracks and platforms of Cannon Street mainline station. A Tube line runs just a few metres below one end of the site, and roughly half the remaining footprint is classified as a highly-protected scheduled ancient monument. Height restrictions, intended to protect views of St Paul’s Cathedral, prevented it being too high, while throughout construction the mainline station had to remain open for use by some 32,000 commuters a day.
With so few locations available to place foundations, architect, structural engineer and M&E engineer Foggo Associates devised a complex structural solution that would enable the block to partially levitate above the station floor.
The eastern elevation, showing one of the massive cross-bracing elements behind a stair-core
The result is a steel-framed “balanced cantilever” design based on the same principles as the pioneering Forth Railway Bridge in Scotland, designed by Sir John Fowler and Sir Benjamin Baker, opened in 1890. The idea was to suspend the northern 70m-wide by 21m-deep strip of office space above the station entrance, and balance this with the same arrangement at the opposite end.
The forces that keep the building upright can be seen in the exposed steel trusses, diagonal tension bars and giant steel “X-frame” structures, which all stand proud of the glass curtain wall facade. In a fundamental sense, the structure is the architecture, explains David Warrender, project architect at Foggo Associates: “We sometimes argue that there isn’t any architecture to this scheme, because the design celebrates a structural idea, not an architectural one. We chose to set the building’s skin back behind the steel exoskeleton so that the structural principles are on display and any viewer can stand on the street and understand what each structural element is doing and why,” he says.
The building occupies air rights above Cannon Street station and supplants a 1960s-built 15-storey office tower. Remarkably, Foggo has been involved with this project for more than 17 years, having finalised an initial design in 1997 — a fairly conventional plan comprising two financial trading floors and six traditional office floors.
When developer Hines acquired the site in 2002, it asked Foggo to make the design more flexible with office spaces suitable for a wider range of tenants.
The architect simplified the floor plate layouts to create basic, large rectangles split into three 21m-wide strips of office space, divided by two 12m-wide strips, which connect the spaces and also accommodate four lift and stair cores and two large atria.
The exoskeleton of Cannon Street station’s north facade shows off its huge cantilever, but forms only a quarter of the building’s complex structural diagram
Deciding how to support the north and south ends of the building proved tricky. The two 21m-wide open-plan office areas at each end required numerous columns spaced at 6m intervals around the perimeter, but extending all these columns down to ground level was impossible as they would found on areas considered too weak to support heavy structures — along the north end of the site the columns would stand on top of London Underground Circle and District line tunnels, further south a service road runs beneath the railway platforms, while at the southern end of the site the columns would found on either railway tracks or platforms. Added to this, the site’s scheduled ancient monument status would have meant going through a lengthy consent process to dig foundations through the archaeology.
The solution was to build a central structure around the four service cores inside the two 12m-wide zones, and stand it atop the existing tower’s foundations. The north and south blocks would then cantilever from either side of this structure and counter-balance one another.
The only option was to use the existing office’s concrete piles, which were underreamed with square footings that extended out at the base to about 6m2. “They were so large that digging them out was not an option and we had limited potential to put anything significant down around them,” explains Jim Fraser, structural engineer at Foggo Associates.
Although these piles were very strong the team decided to limit the forces exerted on them to match what they had been used to taking from the old office block and prevent increased settlement. At the perimeters, where there were increased loads, rows of deeply rooted settlement-reducing piles were installed to add stiffness. Meanwhile, underneath the railway arches in the central zones there was very little headroom to install extra piles, so mini-piles were installed in groups of 16-20 in an arrangement designed to mimic a single large pile.
The unusual design of the cantilevered structure required a different approach to the way construction was phased. “Most buildings are built from the ground upwards with the load gradually applied until the building is topped out,” explains Fraser. “Here we had to carry out a complex structural analysis to determine the sequence of how loads should be applied and supported during construction. The decision was made to build the central structure first and then extend out diagonal steel cables from the top of it to temporarily pick up the bases of the cantilevers while the floors above them were built.”
The cantilevers would only balance one another once the truss structures on the facades were installed and connected, so the steel cables provided support until the trusses were in.
The steel cables extend from lines of computer-controlled strand jacks positioned on the edges of the roof.
As the floors were built up the cables extended, by as much as 20-30mm per week, then at the weekends the strand jacks were activated to increase tension in the cables, reign in that load extension and accurately reset the floors to the correct level for construction to continue.
When the steel floors were completed the strand jacks were released, effectively transferring loads from the cables onto the giant steel X-frame structures on the east and west facades.
Steel beams within the cantilevers were “preset”, meaning they were manufactured into specific curvatures so that when the concrete floors were poured the structure would eventually settle into a level profile. This was critical to the project’s success or failure, says Alister Offiler CIOB, deputy project director at Laing O’Rourke: “The proof of the pudding came as we cast these floors — was the structure behaving how we wanted it to and would we end up with a level structure? After two or three floors were cast we were reasonably confident the structure would behave as we wanted and when casting was complete we finished within just 22-25mm of where we wanted to be,” he says.
The steel floor beams that connect to the exoskelton poke through the building’s recessed glass curtain wall at around 500 locations – a visual device intended to give the impression of a glass bubble suspended inside the frame – which put pressure on cladding supplier and installer Josef Gartner to design out potential issues with structural movement.
Although the curtain wall panels are suspended from the concrete floor slabs using standard cladding brackets, about 75 different gasket details had to be designed to enable the panels to move around the steel penetrations. The joints between cladding panels can also accommodate more movement than is typical, says David Warrender of Foggo Associates: “The bracketry is quite sophisticated — on most city office buildings, the joints between two cladding panels would measure
12.5-15mm, here we have 35mm joints.”
Cannon Place was completed last September and is already wowing commuters with its gravity-defying cantilevers. When seen from Cannon Street the northern cantilever really does appear to float with no visible supporting structure — a feat of structural engineering that surely even Fowler and Baker would be proud of. “The entire facade spans 70m, and the 21m-deep cantilever is very long, so combining the two on a frame that has no support below is unique,” concludes Warrender.
Huge X-frame structures stand proud of the glass curtain wall facade, allowing the glazing to be non-structural
The architect wanted to create the idea of a glass “bubble” within the structural frame, which meant close co-ordination between engineer and cladding contractor at their interfaces
Carry that load
The weight of the cantilevered floor slabs at either end of the building is carried by the 21m-long secondary I-beams that run in a north-to-south direction on each floor. Half of this load is transferred into the four service cores that are the “table legs” at the centre of the building and down into the foundations, while the other half is channelled into the steel trusses on the north and south facades.
These facade trusses are made up of horizontal and vertical steel members that absorb compression forces, and diagonal steel ties that absorb tension forces. The diagonal ties channel the forces to the corners of the building and onto the huge X-frame structures that run along the east and west elevation. The X-frames, in turn, transfer forces onto the foundations under the central structural zone.
In case of a partial failure of the structure, after a terrorist bomb for example, built in redundancy ensures that there are alternative load paths and the building does not collapse, explains Jim Fraser, structural engineer at Foggo Associates: “The building is over a transport hub so we had to build in this resilience. You could lose any one of the four arms on the X-frames and it would still work.
“Equally, on the facades you could take out any one of the columns and several of the diagonal ties and there would still be enough redundancy in the structure to carry the loads. There are so many load paths, it continues to work.”