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Leadenhall Building by Rogers Stirk Harbour + Partners

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The latest addition to London’s skyline, RSHP’s Leadenhall Building, is striking not only for its height but also for the two significant voids it creates, writes Jay Merrick

From a cityscape and branding point of view, the tick-boxes associated with Rogers Stirk Harbour + Partners’ Leadenhall Building, London’s newest tower, are dreary: Cheesegrater, protected viewing corridor, leans away from St Paul’s, job done. But it’s what doesn’t exist in relation to the structure that gives the City’s newest vertical brandmark its most telling connection to the past - and to the future development of the Square Mile.

There is a small open space alongside the alley known as Undershaft. And if you stand in a certain spot under the north side of the Leadenhall Building in this lacuna (which is shown in Morgan’s 1682 map of London), a vividly instructive temporal narrative reveals itself.

On the left edge of your visual frame you see a vertical slice of the darkly glazed Miesian Aviva building from 1969; dead ahead, through the slim gap between the Aviva and the Leadenhall Building, the shining entrails of the 1986 Lloyd’s Building flash like a brilliantly designed steel hernia; the right-hand slice of your view is the north core facade of the Leadenhall Building, its colour-coded hi-res structural details shimmering through the glazed surface with a rippling jellyfish gleam that is Graham Stirk’s signature.

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The montage suggests that the 225m height of the Leadenhall Building, which has become the second tallest tower in London after the Shard, may not be that interesting - if it ever was - in terms of the decision relating to its visual effect on the profile of St Paul’s Cathedral. Fewer than 30 people turned up to opinionate at the decisive public meeting about the various impacts of the Leadenhall Building, whereas 200 pitched up to fret about RSHP’s proposals for the British Museum’s new exhibition gallery.

The critical planner’s verified viewing angle that apparently showed the only position from which the Leadenhall Building was visible from Fleet Street was captured from what seems a rather selective position: tight up against the wall of Pret and the Cheshire Cheese pub on the north side of the Street of Shame.

The skinny montage from Undershaft seems a more pertinently quotidian viewing corridor. The City is a hugger-mugger of architectural manners, sizes and incidental views, dominated by architecture from the 17th, 19th and 21st centuries in particular: small, profoundly historic buildings; medium-sized imperial palazzi; and tall, shiny real-estate confections which dominate the skyline. Most of the latter would look pretty mediocre in Chicago; the Leadenhall Building would not - though it might fail to attain the legendary status of the Hancock and Willis towers.

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Nevertheless, the new building’s treatment of the ground plane and its structural composition are of considerable interest (the latter involving exemplary synergy between engineer Arup and contractor Laing O’Rourke, and a unique super-crane capable of 42-tonne hoists).

I’ll summarise the grunt aspect first. The key design move - the sloping southern elevation - required a structural configuration with three linked vertical segments: the south-facing megaframe with 50 floors, including a six-storey plant attic; a slim intermediary secondary bracing frame with fire escape and ventilation cores at its edges; and the modular northern support core, which carries 24 lifts, 45 lobbies, 82 washrooms and building services plant.

There were four main issues for Arup’s associate director, Damian Eley. First, the connecting node points of the megaframe’s vast horizontal and triangulated members, where structural efficiency and aesthetics dominated the design processes. The engineer had to tackle the careful shaping of the megaframe’s 28m-long lowest members, the use of megabolts and sliding joints, the precisely resolved geometry of the welded plates, with all connecting parts recessed to achieve the articulated graphic and shadow effects that Stirk, Eley, and project architects Andy Young and Maurice Brennan sought.

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The second challenge was to ensure that the steel structure inside its glazed envelope would not affect the 25°C maximum inside the BREEAM Excellent-rated building, or move unpredictably in the space between the outer glazing and the inner double glazing due to temperature swings ranging from -10°C to 55°C.

Thirdly, an ‘active alignment’ procedure suggested by Laing O’Rourke was used to ensure that the southern megaframe subsided vertically against the rest of the structure - Eley says there’s ‘very little load’ on the the southern foundations. This shims-away procedure (more commonly used in bridge-building at key moments of load transfer) absorbed the extra loads that a 300mm ‘lean’ would have dumped on the rest of the structure.

And fourthly, the north core’s huge prefab ‘structural tables’ were designed so that the contractor’s super-crane could lift the 12m-deep modules of primary structure with floors and services already installed. These modules were critical to the building’s overall internal spatial efficiencies - 70 per cent, says Stirk, compared with Swiss Re’s 56 per cent.

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All very admirable commercially. But the most important thing about the architecture is the missing bit: the seven-storey void beneath the south-facing cantilever, which is equivalent to the volume of a chunky 10,000m2 building. This gaping shark bite out of the south elevation has eviscerated everything apart from two escalators, which Stirk likens to drawbridges; a double-jettied restaurant that hangs rather indecisively from the top of the void; and the dreary grey-worsted hard and soft landscaping that will eventually segue into similarly styled public realm in the pocket park at the corner of Leadenhall Street and St Mary Axe. This 84m-high void, originally encouraged by the late planning adviser Francis Golding and supported by the client, British Land, sets an international precedent in the relationship between the lower sections of tall buildings and the ground plane - and not only in historically significant locations, one hopes.

It’s impossible to say what the void’s vibe and effect on street life will be. At the moment it’s a rather stark volume, like the hold of an aircraft carrier. Apart from the finessed shaping and bolting of the angled steel members as they meet the ground, the volume is not architecturally beautiful in itself. By sentimental comparison, across the street, the complex undercroft of the Lloyd’s Building - all shadows, glints, grime and metallic doodads - seems terrifically atmospheric. But didn’t we think that was strange 28 years ago?

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The Leadenhall Building’s void is not the only significant big new space it creates. The slant of its south facade releases a twinned phantom wedge of vertical space that might otherwise have completed the building as a standard vertical oblong. This inferred extra void - and the way it prises opens the streetscape - is as significant as the piazza volume under the cantilvered eighth storey.

The City of London is almost unique (Copenhagen is another example) in the way that you can absorb its essential character without having to look up. In this braid of mostly narrow, off-kilter streets, the first six or seven storeys are enough. And more than enough on a bright autumn morning for Richard Rogers, green-shirted and ochre-trainered on the preview day. He was intrigued to have the view to the Lloyd’s Building through the gap between the Aviva and Leadenhall buildings pointed out to him; he hadn’t noticed it before. Out came his mobile phone, which he raised to his eye, framing and capturing this non-certified viewing corridor with the great care it deserved.

Jay Merrick is architecture critic of The Independent

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