BSB’s ‘Sky City’ is a 220-storey, 838m tower in Changsha, Hunan Province, with construction penciled to start in November 2012 and finish in January 2013.The project has reportedly received backing from the local authorities and is pending ‘final approval from the government’, according to information on the company’s website.BSB claims the secret to building fast lies in its use of its ‘modular technology’ which features ‘95% factory prefabrication at a five-storey per day construction speed’.This method was used in the company’s successful construction of a 30-storey hotel in 360 hours in December 2011, following its previous completion of a 15-storey building in six days.According to Bart Leclercq, head of structures Middle East for WSP - the engineering firm behind The Shard in London - prefabrication is certainly a concept that makes sense. “I absolutely love the idea that they are looking at ways to speed up construction.“To prefabricate, plan ahead and assemble units before they are hoisted into place and connected together - I think that’s innovative. You can see that happening more and more. I think that’s brilliant and I love the ambition.”He continues: “From an investor’s point of view they want to build this as quickly as possible. There are always booms and busts, so I imagine they really want to speed things up [before the next downturn].“BSB has got a lot of people thinking about prefabrication. It’s great because you are able to manufacture everything under good circumstances in factories. You can make sure the conditions are right, which is difficult on site. I think that is definitely the way forward.”Kevin Brass, public affairs manager and journal editor for the Council on Tall Buildings and Urban Habitat (CTBUH), also notes that prefabrication is a method worth exploring more. “There is a lot of innovation in tall building design. Prefabrication is not new but it is an idea worth examining to create an efficient building in more efficient timelines, with fewer materials and a lower cost. That is really worth taking a look at, especially in China, where there is a huge demand for tall buildings.”Yet Brass adds that building a super tall is very different from a mid-rise tower. “They [BSB] have certainly shown they can build quickly, but this new proposal is a different scale. The prefabrication process and the sheer quantity of materials necessary is a real challenge. A small building doesn’t prove that they can build at that height.”Leclercq agrees: “By trial and error they have established that this is quite possible. But don’t forget, this is a 220-storey tower - this is no joke.”After thoroughly scrutinising the details of the Sky City ‘blueprint’ document on BSB’s website, Leclercq highlights the absence of a wind load strategy. He continues: “What I find strange is they are talking about safety and ‘magnitude nine earthquake resistance’.“But anyone designing a tall building will know that once you go over 30 or 35 storeys, earthquake is not the governing load - it’s wind. Especially when you are talking about a building this kind of shape.“You are talking about an enormous wind load. This document makes no mention of wind. There will be an enormous horizontal load all the way to the bottom of the tower.”CTBUH’s Brass concurs: “There are forces working on a building that tall, including the wind. It is not a minor thing at that height.”Leclercq continues: “By just using these simple units all put together, you’re not going to get enough stiffness; this building will have an enormous storey drift and it will sway. It would be interesting to see their concept for the structure and how they are going to deal with the stiffness, the strength and the size of the columns.“As you go down the structure, the building will have to carry the enormous load of an almost one-kilometre high tower on top of it. You need big structural elements to deal with that. If it got built I don’t think I would feel safe in this tower, unless I had a really good understanding of how they put it all together.”
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