A new office tower planned for the eastern edge of London was inspired by the early-20th-century skyscrapers of New York City and Chicago. Featuring a series of vertical planes, the building will appear as a cluster of buildings rather than a single structure. © Dbox/Make Architects
The shifting vertical planes of a new London office building are designed to protect historic and strategic view corridors.
April 22, 2014—Inspired by the early-20th-century skyscrapers of New York City and Chicago, a multilevel office and retail tower planned for the eastern edge of London, the United Kingdom, already has an enchanting nickname: Gotham City. Although it is unclear whether the dark knight himself will set up operations in the building, it is definite that the new 154 m tall tower will have a defining presence near several existing high-profile buildings in the city’s Insurance District.
The tower will be located at 40 Leadenhall Street, rising from two adjacent lots on a block called Leadenhall Triangle. It will feature a series of shifting vertical planes or “slices,” giving it the appearance of a cluster of buildings as opposed to just a single structure. In addition to being inspired by American skyscrapers, the design was influenced by the project’s location within the protected view corridors of such significant sites as the Tower of London and directly adjacent to a Victorian-era masonry building that is listed as a culturally significant structure by English Heritage, an organization dedicated to preserving historic structures in the United Kingdom.
“Formed from a series of stepped vertical slices, the building terraces down to reduce its profile in response to key vantage points on the skyline, mediating between the scale of the adjacent towers to the north and west and the lower buildings to the east and south,” said Paul Scott, the lead architect on the project for Make Architects, the London-based firm that designed the tower, in written responses to questions posed by Civil Engineering online. “The tallest and widest slice is positioned closest to the center of the city, and [from there, the building] steps down towards the River Thames and the Tower of London.”
The 84,500 m2 building will have a central lobby as well as retail, cafe, and restaurant space on its ground floor. The tallest portion of the building, to the north, will have 34 office levels and three levels of mechanical space. To the south, the building will house 14 office levels and a rooftop terrace that will accommodate a two-story restaurant. Affording panoramic views of the city, the restaurant will be accessible via a dedicated elevator. The central portion of the building will have 10 office levels, while the building’s three basement levels will house a loading dock, parking for the disabled, bicycle storage, and lockers and showers for bicyclists.
Located on the western side of the scheme, the existing heritage building will be renovated and incorporated into the project. It will house cafes and restaurants within its first two levels and meeting and conference spaces within its two upper floors. A great deal of effort will be made to protect that building throughout the project, particularly during construction of the tower’s deep pile foundation, says Jane Richards, CEng, the director of the structural engineering department for WSP, an international professional services firm based in London and the structural engineer on the project.
Ground movement monitoring will be conducted during the foundation and basement construction to ensure that the pile rigs and pile installation do not damage the heritage building. Care must also be taken to preserve the integrity of the building when its rear facade is removed and its floors are linked to the new building. “I would say the protection of that listed building on the site is the greatest challenge of the project,” Richards says. “We’ll need to submit applications to English Heritage to show how we’re treating that building and how we’re protecting it.”
Although a site-specific geotechnical survey has not yet been conducted, the tower will likely be founded on 50 m long piles descending down to the area’s Thanet sands. Deep piles will reduce the number of piles that are needed overall, Richards notes. “We’ve gone with that option because we’re generating quite high loads on our columns, and that’s mainly driven by the fact that it’s a commercial building,” she says. “The building has quite a large structural grid and the commercial floor loading is such that the individual column loads are quite high, so we’ve taken a fewer number of piles down to the deep Thanet sands below.” An existing retaining wall leftover from a former building on the site will be used as temporary shoring for the new foundation, eliminating the need to relocate utilities near the site.
The building will be framed in fabricated steel beams, shear studs connecting them to a concrete slab on a profiled metal decking flooring system. Normal-weight concrete in the flooring system will enhance the dynamic performance of the building’s floors, some of which have spans as much as 16 1/2 m long. To keep the overall floor structure as thin as possible, the beams will have voids in them to accommodate the mechanical, electrical, and plumbing lines. “The fabricated steel option means that we can design every beam for its span and its load,” Richards says. “As a result, we can make the width of the flange size bespoke for the particular beam and the number of holes we’ve got in it.”
The building’s columns will end approximately 10 m above the ground level; cantilevers as long as 6 m will line the perimeter, giving the first level an open appearance. The building’s floor beams and columns will act as Vierendeel frames to support the cantilevers. “We don’t need to beef up any of the structure; we’re just making sure our connections between the beams and columns are stiff enough to give us that Vierendeel action,” Richards explains. “As the building goes up, that Vierendeel gets stiffer and stiffer to achieve the six meter long cantilevers.”
The building will be clad in a combination of metal panels and glass. The east and west facades will be covered by stainless steel panels and punctuated by windows, while the north and south elevations will be fully glazed. External louvers on the south facade will reduce solar gain as part of the building’s sustainable design, which is also includes a rainwater-harvesting system and carbon emission mitigation efforts. The project is designed to achieve a rating of excellent in the United Kingdom’s Building Research Establishment Environmental Assessment Method (BREAM) “green” rating system.
The project has received city planning commission approval and construction is expected to commence in 2015, completion anticipated in 2020. While the building will recall the towers of New York City and Chicago, its architectural design will be distinct, Scott said. “The project allowed us to design a world-class twenty-first-century building that breaks new ground and will join the city of London’s iconic portfolio of modern buildings.”