To a certain extent, friendly competition has been around since the platform foundation was invented in Chicago in the late 1800s, making it possible to build taller than a few stories. But in an era when “tall” means supertall (or megatall), things start to get more interesting, especially from a logistics point of view. It took six years to build the Burj Khalifa, the 830m reigning world champion. It took eight years to build Abraj Al Bait, the second tallest, and seven to build One World Trade. The construction of a supertall is slow and inefficient — and largely economically unsustainable.
But, in China, a company called Broad Sustainable Building Corporation — led by an eccentric and reportedly brilliant CEO named Zhang Yue — claims to have solved the problem using prefabricated parts. You might remember Broad from its astonishing demonstration, from 2012, when the company built a 30-storey building in fifteen days. “Traditional construction is chaotic,” Yue said in a fascinating Wired profile from last year. “We took construction and moved it into the factory.”
Last year, Broad announced a plan to build the world’s tallest building, Sky City One, in no more than a few months. Using 100 per cent prefabricated parts, the company claimed, the building would rise at a rate of five stories per day. And, last week, Broad renewed its claim, releasing a new rendering and announcing that construction was about to begin. Broad’s General Manager in the United States, Sunny Wang, told me over the phone that construction is still a few months out. “It’s still in the phase of applying for the construction permission,” he said. “Hopefully we’ll start in September.”
Sky City will be 150 storeys taller than the current tallest prefabricated building in the world. That’s because prefab buildings aren’t especially great at handling lateral forces (or those that move from side-to-side), like the wind. Prefab towers are usually made up of stacks of steel-framed modules. That’s great for vertical loads, but when the wind blows, there’s very little structure to stop the whole thing from toppling over. Anyone familiar with the laws of physics will be sceptical of Broad’s plans — and experts are critical, too. “When I look at the drawings of the building of Sky City One I can’t see any provision for structural elements,” Bart Leclercq, the Head of Structures Design at WSP Middle East, told me over email. “I see no cores or shear wall. I see no clear column locations.”
North America’s tallest prefab building, the B2 Tower, which is slated to rise in Brooklyn later this year, will only reach 30-odd storeys. Curious to know what the experts behind the project would say about the Sky City proposal, I got in touch with Tony Colonna, VP of Prefabrication Operations at Skanska, the construction company handling B2. “What’s fundamentally different about B2 is that we designed the structural module first,” he said. “Often what we’ve seen is architect designs a building where prefab is an afterthought, and what you’re left to do is take an architect’s traditional design and try to break it up and modularise it.” Instead, with B2, Skanska and structural engineering firm Arup designed a steel “spine” that stabilizes the individual modular blocks. When I asked him about Broad, he was cautiously optimistic, adding, “the challenge of going vertical is really just a matter of design.”
Let’s suspend disbelief for a second and assume — as Colonna suggested — that reaching 220 stories in two months is simply a matter of planning. Here’s how it would happen. Broad prefabricates its modules in a factory, including everything from HVAC ductwork to floor tiles. Those Lego-like blocks, which are roughly 15m long by 3m wide, are packed along with boxes of the hardware needed to secure them, including the vertical supports, which are a unique Broad innovation: the steel columns sprout diagonal “legs” at each end, creating the building’s cross-bracing. Those packets of structure and flooring are lifted by crane into place, where worker crews fasten them into place with maddening speed. Will it be enough to withstand winds and earthquakes at 800 odd metres above the earth? “From a technical point of view, it’s totally safe, and it’s been approved by the technical team,” Wang says.
Even if Sky City never rises above 30 storeys, it might not matter. The real radicality of Broad’s plan lies in its logistics. Zhang Yue has described his mission as one of sustainability — a quest to make construction less wasteful — and it’s hard to argue that he hasn’t. The company estimates that constructing a traditional supertall generates roughly two million kilogramas of waste material, while Broad’s prefab method generates about 50,000. As Wang explains to me, there’s also an urban logic to the idea. “Putting a whole city in a skyscraper saves a lot of energy,” he says, adding that Broad is franchising its methods in countries outside of China. “It’s not difficult to learn, and it’d be easy to copy. I think it’s the model of the future for buildings.”
It’s easy to dismiss Sky City based on precedent, but it’s hard not to be charmed by the audacious plan. And after all, didn’t people say the same thing in the 1880s, when the first five- and six-story building rose terrifyingly high above the Victorian streetscape? As for the structure, only time will tell. “At the moment it is not clear to me how they intend to make this work, but I am very interested to find out,” Leclerq added after discussing the plan. “We are never too old to learn.”