The sky is the limit
Oliver Ephgrave speaks to construction experts about China's plan
Oliver Ephgrave speaks to construction experts about China’s plan to build the world’s tallest building in 90 days
In July, Chinese company Broad Sustainable Building (BSB) stunned the construction industry with its audacious proposal to build the world’s tallest building in a mere 90 days. BSB’s proposed Sky City is a 220-storey, 838m-high tower in Changsha, Hunan Province, with construction pencilled to start in November 2012 and finish in January 2013.
The project has reportedly received backing from 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 at 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 is innovative. You can see that happening more and more. I think that is brilliant, and I love the ambition.
“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 is 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 at a lower cost. That is really worth taking a look at, especially in China, where there is a huge demand for tall buildings.”
However, Brass adds that building a super-tall tower is very different from a mid-rise tower. “[BSB] has certainly shown it can build quickly, but this new proposal is of a different scale. The prefabrication process and the sheer quantity of materials necessary is a real challenge.
A small building does not prove that they can build at that height.” Leclercq concurs: “By trial and error, it has established that this is quite possible. But do not forget that this is a 220-storey tower - that 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 is wind, especially when you are talking about a building of 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 are not going to get enough stiffness; this building will have an enormous storey drift, and it will sway. It would be interesting to see the concept for the structure and how it is 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 1km-high tower on top of it. You need big structural elements to deal with that. If it got built, I do not think I would feel safe in this tower, unless I had a really good understanding of how they put it all together.”
Whereas the Burj Khalifa cost around $1.5bn, BSB claims its project will only come to $628m. Leclercq asserts: “If you are doing it for half of the cost, it suggests you are only putting half of the materials in, which means you will only have half the stiffness and half the strength. I think this is not going to fly.
“The Burj Khalifa is all about its stiffness and the strength. The skyscraper is a structural element - it is a pin stuck in the ground. If you are saying you can do it for half the money, I do not believe that is the case.
"You have to pay for every cubic metre of concrete that goes in there – that is what your price is based on. I am very sceptical about this project, and I do not know how serious it is.
"It looks like it could be a marketing ploy rather than a real scheme, otherwise there would be much more factual information in there, such as talk about the wind load. It does not look right at all. I do not think an actual architect or structural engineer has looked at it.
“Maybe the blueprint is conceptual and they will hand over to the architect. But when you are doing the design of a skyscraper, the first thing you look at is the stiffness, the structural elements. An architect and a structural engineer are in constant dialogue and working on the floor plans. This is not going to work.”
Brass adds: “It is hard to get accurate information out of China. A building of this height is such an involved science; it is not a simple thing. Architects like Adrian Smith [architect of the Burj Khalifa] are the foremost experts in this field. It is up to NASA levels [of design and engineering] and is not something that can be handed over a lunch napkin.”
Yet if BSB is working with the architects of the Burj Khalifa, as claimed in its blueprint, is it technically possible to build a 220-storey tower in three months? Leclercq believes that the Burj Khalifa’s construction timeframe of five years cannot be beaten at the current time. “I think they did a tremendous job to build the Burj Khalifa in five years,” he says.
“With all the construction methods and knowledge we have, they managed to squeeze the maximum speed on that particular project. I really believe they did it as quickly as was technically possible. I do not see any other way they could build it faster.
In order to get stiffness in your building you need lots of areas of concrete and steel. And in order to get that all in place, you need an enormous amount of time to bring it up, put it in place, put it all together, pour the concrete and put the reinforcement in place.
“You need high-strength concrete for a building like this, and to make sure you can work with it, you put in plasticisers so it takes longer to harden. The really high-strength concrete goes off really quickly, and you cannot pour it in place any more.
For the concrete to harden, you can only take the shuttering away after several days. You can only pour a few metres height each time. So no, I do not think it is possible to build a 220-storey tower as quickly as the company claims. If they manage to build this structure in three months then I will give up structural engineering. I will hang my hat and retire. I will be eating humble pie as well.”