The "sky garden" on the top three storeys of 20 Fenchurch Street – the controversial tower in the City of London – presented its own unique construction challenges. Tom Ravenscroft explains.
“Greedy architecture…” “It feels like you’re trapped in an airport…” “Bloated, inelegant, thuggish…”
Negative reviews from the mainstream press had left Construction Manager with low expectations. However, it was a pleasant surprise when I arrived before sunrise at the “sky garden” on the 35th floor of London’s latest and most controversial skyscraper, 20 Fenchurch Street. Looking outwards offers sweeping views of London in all directions, while looking up, the roof’s steel structure is proudly on display.
Built for Land Securities and Canary Wharf Group, the unusually shaped skyscraper, nicknamed the “Walkie-Talkie”, which flares outward from its base to maximise the available high-value office space on upper storeys (see below), is undoubtedly a success for its developers.
However, the tower, which became infamous in the autumn of 2013 when sunlight reflected off its convex south facade and melted parts of a car that was parked at street level, has been criticised for its impact on the skyline and detachment from the City of London’s cluster of towers. The planning process that allowed it to be built has also come under fire.
Promoted as the “building with more on top”, the three floors at the summit of 20 Fenchurch Street are home to “London’s highest roof garden”, which opened to the public last month. Although “sky garden” may be slightly misleading description – an observation deck, viewpoint or rooftop restaurant may be more appropriate – the provision of a public space was an obligation of the building’s planning consent and the end result is a quasi-public space with views to rival any in London.
The “sky garden” is a 3,364 sq m vaulted column-free greenhouse with panoramic views
This developer contribution is not only an asset for the city, but also an impressive structural feat. Designed by Rafael Viñoly, with Adamson Architects as executive architect, the sky garden is a 58m by 58m (3,364 sq m) vaulted greenhouse with 360-degree views across the city.
It houses a bar and two restaurants split over three levels. Constructing this fully enclosed, column-free space 150m above the ground, while maintaining the architect’s vision, was a challenge, but one that was successfully overcome by the engineers and contractors.
"The biggest challenge was that every piece of steel was unique in length, curvature and twists at different angles."
Derek Hill, Josef Gartner
To create a unified external appearance, Viñoly’s desire was for the fins that rise up the east and west facades of the building to continue horizontally across the roof. In what Paul Walters, project director at structural engineer CH2M Hill, describes as a “fairly conventional solution”, a space frame was initially proposed to support the aluminium fins that would in turn support the glazing. However, there was a more elegant solution. Using steel portal frames that match the profile of the fins maintained the impression of architectural unity while creating a cleaner-looking, more efficient and affordable structural solution.
“The fins that extend across the top of the building are actually 1,200mm-deep fabricated steel box sections – they are the structure and they support the glazing,” explains Walters.
Showing its metal
Sitting like a cap on top of the building, the 980 tonnes of geometrically complex steel used to enclose the roof garden was manufactured separately from the steelwork for the tower’s main frame. While the building’s main structural steel was the work of fabricator William Hare, the steelwork for the garden summit section was designed and manufactured by the building’s overall cladding coordinator, Josef Gartner, which is part of the Permasteelisa Group.
Because of the unique shape of the sky garden’s floorplate, all 34 of the main structural members have an individual geometry. Each flattened arch comprises two ice hockey stick-shaped pieces of steel attached to the tower’s main structural steelwork, one on the east and one on the west facade, which support the curved steel beams that span up to 58m across the sky garden.
“It’s unparalleled in London,” says Derek Hill, commercial manager of Josef Gartner. “It’s the largest use of bespoke curved steel the company has ever undertaken. The biggest challenge was that every piece of steel was unique in length and curvature, and twists at different angles.
Assembly of the steel structures took place on the roof, 150m above the streets of the City of London
“The steel sections, while acting structurally, are fabricated within reduced tolerances compared with traditional steelwork to accommodate the fact that all of the building services are integrated within the steel sections.”
Lighting and fire-detection sensors were placed within the steelwork, and photovoltaics were fixed to the side of the beams, in accordance with the architect’s requirement that no services would be visible from the sky garden.
With assembly taking place 150m above the ground, there was no room for error. The bespoke steel elements were fabricated in Germany, and trial erected there, before being transported by truck to the site. The traffic of heavy goods vehicles is restricted in central London, so the elements – some weighing up to 9 tonnes – had to be hoisted and fitted at night.
A high standard of finish was also essential because of the high visibility of the steelwork. Any imperfection or depression in the steel would have been on show for sky garden visitors to see.
Around 4,800 sq m of laminated glass panels on the roof, and an additional 1,000 sq m of steel-framed “stick” curtain wall, were individually fitted and sealed into the steelwork to complete the sky garden.
Plant life
Topping the building with this enclosed space also created a challenge for the service engineers, as the obvious place to locate the plant required to service 34 floors of office space was already occupied. Andrew Ashfield, divisional director at Hilson Moran, the building services engineer for 20 Fenchurch Street, explains that to “maximise the available space for the roof garden, the plant was relocated down the building”.
A complex plant room and air-handling plant are hidden in the wedge-shaped space beneath the sky garden’s floor, which rises from the south to the north. The building’s cooling towers are also in this space. Louvred vents are aligned with the upper north-facing terrace.
1. Sky Garden 2. Upper terrace 3. Level 35 “Skypod” cafe 4. Open-air terrace 5. Level 36 “Darwin” restaurant 6. Cocktail terrace 7. Level 37 “Fenchurch” bar
Other elements of the HVAC system were moved outside the building entirely. A four-storey annexe opposite its southern entrance, clad with a green wall, houses back-up generators and dry air coolers.
“Ideally, these would have been on the roof,” says Ashfield. “Or they could have been fitted on to a floorplate, but there would have been replacement issues.”
Because of the complex and unusual form of the building, BIM was a key tool in designing and modelling 20 Fenchurch Street’s steelwork.
“BIM came into its own when trying to model the shape,” says CH2M Hill’s Walters. “At the end of every week the architects and engineers uploaded our models to a central location.” This model was exported into analytic software to test the structural capacity of the design. When the form was complete CH2M Hill provided William Hare with 2D drawings and a copy of the Revit model to fabricate the building’s main steelwork.
The effort taken to realise the architect’s vision for a light, airy, column-free space at the top of 20 Fenchurch Street has certainly been rewarded. Although opinions may be divided about the building’s aesthetic and its impact on the London skyline, the scale and purity of the fully enclosed space are as dramatic as the views from it.
Structural challenges of ‘top heavy’ design
The fins across the top of the building are actually 1,200mm structural steel box sections that support the glazing
The unique “top heavy” form of 20 Fenchurch Street was conceived to provide the developers with 60% more lettable office space on the 34th floor than the fourth. This design principle, combined with the fact that the north and south facades are concave, while the east and west facades are convex, so no two floorplates in the building are the same, meant the structure of this tower was far more complex than that of a traditional skyscraper.
Once the form of the building had been determined, the engineer, CH2M Hill, had to design a structure capable of supporting it. The first key decision was to move the core, which was originally placed in the centre of the ground floor. Although a natural position in many towers, this skyscraper bows to the south, placing more weight on that side of building. So that the core was not placed under a continuous turning force, it was moved around 3m and aligned with the centre of the building’s mass.
The building’s steel frame had to be extensively modelled in BIM. Whereas in a traditional skyscraper with identical floorplates steelwork can be optimised for one floor and then repeated for every storey, at 20 Fenchurch Street the engineers had to optimise the steelwork for each individual floor.
As far as possible the steelwork follows the architectural intent that all columns should follow the curved profile of the building’s facades. However, fabrication of curved columns is expensive, so straight beams have been used that are faceted every four storeys on the lower floors and every two storeys further up the building, where the curve is tighter.
This faceted steelwork matches the facade on the lower 22 floors. However, to maintain a constant structural depth for each floorplate, it was necessary to move the columns inside the building on upper floors because of the facade’s increasing distance from the core.
Read more about the use of BIM at 20 Fenchurch Street at our BIM+ website
