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Ideas above one station: Cannon Place by Foggo Associates

Built above a railway terminus, Foggo Associates’ Cannon Place continues the long tradition of construction ingenuity in the City, writes Felix Mara

The City of London and its immediate hinterland are renowned as a rich archive of architectural history, but one of its greatest assets is structural engineering. From its Roman origins to Victorian pragmatism and Edwardian grandeur, it has been a showcase for audacious spans, resourceful load-path finding and construction ingenuity on difficult sites, garlanded by designers such as Christopher Wren, Horace Jones, Norman Foster and Richard Rogers. Foggo Associates’ Cannon Place development builds on this legacy. Its eight stories of commercial offices bridge across Cannon Street Station, which continued to operate throughout construction, supported on a structure which bears on the few available ground-bearing locations.

The original station, designed by John Hawkshaw and JW Barry and opened in 1866, had a glazed barrel vault, like a great engine shed with flanking towers at the end of a bridge across the Thames. Damaged by Second World War bombing, the vault was replaced by what Pevsner dubbed ‘a tall and dull curtain walled office slab’ crouching over the commuter station, designed by John Poulson, completed in 1965 and now replaced by Foggo Associates’ development. The adjacent Bush Lane House, completed in 1976 and designed by Arup Associates but without the involvement of Peter Foggo, who left to set up an integrated design practice of his own, is more sophisticated and has survived, its stainless steel lattice frame standing out against a background of dark grey tinted glass.

Although Foggo Associates’ design statement acknowledges ‘important’ buildings in the conservation areas to east and west, and the location of the Roman Governor’s Palace on the site, director David Warrender is adamant that context was not the project’s main driver. But viewing corridors of St Paul’s and a scheduled ancient monument on the site could not be ignored, and Cannon Place’s north facade is carefully aligned to emphasise the splayed corner of Bush Lane House. Nor was it initially approached as an exercise in virtuoso structural engineering, although this came later, almost as a by-product. Foggo Associates’ objectives were first to design high-quality flexible office space with maximum plot density and decent floor-to-ceiling heights, and second to improve the station environments and public realm.

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With underground tunnels, mainline railway tracks, a service road and archaeology zones, Foggo Associates didn’t have the option of plonking down columns willy-nilly. Most of these had to be located in a central area where there were existing foundations. Load paths could then be transferred above the station. So far so good, but supports at the office perimeter would still be required. Foggo Associates’ answer, inspired by the Forth Bridge, was to erect four colossal vertical trusses, two on the east elevation and two on the west. More monumental still, eight storey-high star-shaped configurations of steel wishbone trusses not only connect and brace these trusses, but also cantilever to the north and south. These in turn support the 67.5m-wide steel tensile cable-braced trusses of the north and south elevations, supported and adjusted by strand jacks during the construction phase.

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Principal steel beams spanning north to south between this exoskeleton and the inboard columns support composite slabs with lightweight concrete, avoiding deep transfer structures. With 21m spans, these beams are deep enough for 550mm-diameter openings for the meticulously co-ordinated services, which would usually occupy a deep ceiling void below. These are integrated with stacked service cores providing on-floor plant and aligned with service zones at station level. An elephant’s ear-shaped tower to the east accommodates additional plant and services, so they don’t have to be located on the roof, where they would restrict the available clearance for office accommodation. Foggo Associates even got away with glazed air vents and lift over-runs, which extend into the viewing corridor zone. Built when conservation was taken more lightly, Poulson’s development was higher and, as chief City planning officer Peter Rees observed, you’d need a strong pair of binoculars to spot these from Greenwich or Blackheath.

Result: tidy rectangular office plates with only eight freestanding columns per floor, each of which can be divided into four tenancies, with 2.8m-high ceilings and atria braced by horizontal diagonal members that bring touches of daylight into the deep-plan floorplates. In David Warrender’s words, a ruthlessly simple four-square diagram. Scenic lifts in the atria provide animation and are cleverly fitted with side-slung pulleys to reduce the required depths of pits, which are suspended above the station, where a minimum soffit level applies.

There’s a familiar and inevitable sense of infinity as you gaze across the space sandwiched between the office floor and ceiling planes. Imaginative fitting out may be the answer. Additional vertical penetrations, alongside the atria, would provide more relief, but the price would be reduced net area and flexibility, and a more complex fire strategy.

Cannon Place’s eight-storey star members are hardly dainty, and were never intended to be. They also throw the whole scale of the west facade into limbo, which is exhilarating, although their detailed design might have been more neatly engineered. The mainline and Underground stations now have more presence on the street, with better pedestrian circulation and access. There’s a legible glazed entrance hall to the offices and more appealing on-street retail, plus daylight in the Underground ticket office. Some of London’s most memorable railway termini are the ones you approach as you cross the Thames, but these aren’t always its most architecturally distinguished. Cannon Place is now more memorable as architecture, engineering and public realm.

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