The unusual shape of the new office tower at 240 Blackfriars Road in the borough of Southwark, London, is dictated by its surroundings. © Allford Hall Monaghan Morris/The Neighbourhood
Designers use building information modeling in overcoming myriad challenges to create a dynamic, glass-clad office tower in the borough of Southwark, in London, United Kingdom.
October 15, 2013—Clean lines and seamless connections will make the new office tower at 240 Blackfriars Road in the borough of Southwark, in London, United Kingdom, appear straightforward and effortless. But achieving such a streamlined aesthetic for what will in fact be an elaborate glass-clad structure, featuring a large amount of visual concrete and multiple customized components, has been anything but simple. Designers relied on a great deal of building information modeling (BIM) and intricate detailing to translate the building’s complex geometry into a sleek, unified form.
The Great Ropemaker Partnership (GRP), a joint venture between London-based investment firms Great Portland Estates (GPE) and BP Pension Fund, submitted a proposal to the local planning authority in 2006 for a 14-story office tower on the Southwark site. Two years later, it submitted a revised application to install 17 floors within the same building height. But when the recession hit soon thereafter in 2008, the project was placed on hold, and during the following two years the developer and its design team reevaluated the plans to derive more value from the scheme and apply new building regulations for energy consumption. As a result, the developer submitted updated plans for the tower in 2010 that included two additional floors, resulting in a 19-story building. “We did a general refresh of the plans and added additional height to the building,” says Haydn Thomas, a senior architect for Allford Hall Monaghan Morris, a London-based architecture firm that had worked with GPE on several prior projects. “And, to be honest, the additional height enhanced the building’s elegance.” AKT II, a consulting structural and civil engineering firm based in London, is the structural engineer on the project.
The 22,000 sq m tower will have four distinct cuts or angles at strategic locations over its volume. In developing the building’s unusual shape and articulating its purpose, architects were inspired by natural geological forms. “These rather beautiful, slightly sculpted samples were a starting point for the idea of what this building could be,” Thomas says. From there, the building’s shape was determined by the Blackfriars Road site, which is restricted by a ramp that leads to an adjacent railway viaduct and by right-to-light ordinances that prevent new structures from shadowing existing adjacent buildings. As a result, the building’s footprint takes the shape of a parallelogram, Thomas says.
The office tower will have 19 stories, including a 3-story high
rooftop space. © Allford Hall Monaghan Morris/Smoothe
The placement of the building’s cuts was also determined by the site’s north-to-south orientation. The building’s north side will have the most prominent cut, responding to the view across the river Thames to central London. “The building is very much oriented toward the north,” Thomas says. “So that cut is deliberate in terms of acknowledging where the building is looking.” The cut at the top of the building will highlight the tower’s relationship with the sky and accentuate the triple-height, completely glazed room under the tower’s peak. The two final cuts will be at the base of the building, complementing Blackfriars Road—a main vehicular and pedestrian route—as well as the ramp leading to the railway viaduct. “The sculptural volume was informed by the context rather than being a form we could have generated and placed anywhere else in the city,” Thomas says.
Construction of the tower’s foundation and single-story basement began in January of 2012, before the rest of the design was completed. The client tendered the building’s substructure—a typical piled raft foundation—under a separate contract to get a head start on delivering the building as the economy began to emerge from recession, Thomas says. Around that same time, United Business Media (UBM), a business-to-business publishing firm currently headquartered in a building adjacent to 240 Blackfriars Road, agreed to move its operations to the top half of the new tower. GPR then committed to the tower’s construction, with targeted completion in February of 2014. Following a tender process, Mace, a London-based international consultancy and construction firm that also constructed the nearby Shard, was named the principal contractor on the project, and construction of the tower began in February 2012.
Due to site restrictions, the tower’s footprint takes the shape of a
parallelogram. © Allford Hall Monaghan Morris
The tower’s concrete core was erected ahead of the floor plates using a slip-form construction methodology. But before that work could commence, engineers conducted extensive analyses to determine how to keep the core, which is not in the middle of the building, from bending under the force of the unbalanced dead load to the west. “The uneven load forces the core to lean,” says Stelio Papastylianos, the director of AKT II. “That causes a number of issues, the primary issue being verticality for lift installation.” Engineers determined that if the core was angled slightly to the east, it would right itself once the dead loads were applied to the west. Concrete being a nonhomogenous material, its behavior is difficult to predict, so the team worked within a given tolerance envelope. “As the building was going up, we continually monitored where the core was and made necessary adjustments to ensure that once all of the floors were in place, it would actually become vertical,” Papastylianos says. “By the time we checked it at the final floor, it was within 10 mm of our predicted target, which was a very good result.”
The tower’s framing is primarily posttensioned concrete, allowing for spans up to 10 1/2 m long, with 275 mm floor plates. The concrete at the tower’s edges will be exposed and visible from both the interior and exterior, making it essential that the concrete have a smooth, seamless finish. Nowhere was this more important than at the tower’s ground-floor reception level, where a series of 9 m tall canted columns line the perimeter. “The architects were very particular about the look of those columns,” Papastylianos says. After considering several methods of forming the columns, the framing contractor developed an approach that involved pumping the concrete upward through steel formwork. “It was pressured in from the bottom rather than dropped in from the top,” Papastylianos says, adding that the method prevented the concrete from segregating for a consistent finish.
The tower’s unusual geometry posed several challenges. Among those is the fact that as the reception-level columns angle upward and the building’s north face angles downward, the entire load of the building—as much as 700 tons of horizontal thrust at the northwest corner—becomes concentrated at the point at which those elements meet at the second floor, causing the building to push westward. “Anything that you construct at an angle needs to be tied back somehow,” Papastylianos explains. To restrain the horizontal thrust created by the building rake, engineers designed the second floor as a precompressed system that will never go into tension. Then they outfitted the building’s west and north faces with 80 to 90 mm thick cast-in plates, tied to posttensioned cables that extend to the core. “The horizontal thrust is taken back into the core, which provides stability,” Papastylianos says.
Fritted glass pinstripes will reduce the tower’s solar gain while
also making light of its relationship to central London. © Allford
Hall Monaghan Morris
The cut on the tower’s north face also presented a challenge because as it becomes wider the columns begin to terminate, creating larger span floor plates. To keep the length of the upper floor spans to a minimum and maintain the 275 mm thickness, an inverted V-shaped column arrangement was designed to extend from the 11th floor up to the roof. The architects wanted that element to be steel so that there would be a different aesthetic compared to the concrete frame and to compliment the tower’s steel roof. But combining pure steel and concrete in that manner presented significant challenges with respect to the fast-track construction program, and after exhausting all of the potential options, the team opted for a composite concrete-filled steel sheath solution. “It just made it easier to deal with the connections at the floor plate, and it saved us time on the program,” Papastylianos explains.
The design team used BIM throughout the project, but perhaps nowhere was it more essential than for the tower’s steel roof. The roof is highly complex, featuring flat and sloped segments over the three-story room at the top of the building and an additional segment over the tower’s small triangular-shaped mechanical room. A 40 m long truss extends from the tower’s northeast corner to its southwest corner to support all of the segments. To achieve a streamlined appearance for the roof’s exposed steelwork, 20 m long sections—many with unusual trapezoidal shapes—were specially fabricated for the project and a great deal of detailing was carried out to conceal all of the connections. “The architects wanted it to look a certain way,” Papastylianos says. “They didn’t want any visible connections on the fabricated sections.”
The entire tower and roof will be covered in glass. While the building’s cuts will be smooth, its other sides will be pinstriped by fritted glass that is intended to serve two functions: one, reduce the building’s solar gain, and two, serve as an architectural reference to the tower’s relationship to central London, Thomas says. “The traditional image of the chap in the pinstriped suit was a trigger to emphasize proximity to the financial center and accentuate the height and elegancy of the form,’” Thomas says. At the 19th floor, the glass roof will have both a solar-control coating and the application of a fine chromium frit—a material that is both reflective and translucent. “The frit allows views out to the sky and is also very high performance in terms of solar gain reduction,” Thomas says.
Since construction began in 2012, the project has advanced at a steady pace. The building has been completely framed, the cladding installation is almost complete over the vertical parts of the structure, and crews are now working to clad the striking triple-height space at level 19. Despite myriad challenges, the project is on track to meet targeted completion in early 2014. In addition to being a fully occupied building and an enjoyable place for the tenants, Thomas expects that the tower will continue to inspire further redevelopment in Southwark. “This building becoming live is effectively what has started the further regeneration of the immediate area,” Thomas notes. “So it will be interesting to see this building finished and the regeneration of that area completed in the next 5 to 10 years.”