Mile-high tower could soon become a reality

The mile-high tower has long been a dream of architects and developers. Sean Cronin talked to three of the top high-rise designers in the world, who say it is a question of not ‘if’ but ‘when’ the dream will become reality.

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By  Sean Cronin Published  October 9, 2005

It has existed as an architectural fantasy since the 1950s when Frank Lloyd Wright first proposed a 528-storey building called Mile-High Illinois. Since then several other architects have built their own castles in the sky, including US-based Pickard Chilton, which has an ongoing research project into the idea. Taisei Corporation of Japan also drew up plans for a 4000 m-high structure called X-Seed 4000, which at 800 storeys, was the highest building ever envisiged. But the ‘Mile High Tower’ has never managed to make it off the architect’s drawing board. That could be about to change as our revelations about Nakheel’s plans for a 1600 m structure prove that the concept may soon become a reality. It was, at least, being taken seriously enough for the developer to appoint international heavyweight consultants to draw up plans for the project earlier this year. But super-tall construction carries with it a host of engineering problems that need to be overcome, according to Arup director Mohsen Zikri. He says: “As the building becomes taller, you encounter more and more problems and some of these are to do with wind. When the building sways, you need to counteract it, because if you are not careful you will feel discomfort. “The top of a super-tall building can move by a metre or more, but what is more important than sway is acceleration.” As a result, one of the biggest challenges facing designers of super tall buildings is limiting building sway, and several techniques are used to do this. “You have to dampen this by mechanical means,” says Zikri. “If you imagine a cylinder and at the top, you have a piece of wire hanging down with a weight at the bottom. The weight sinks in a tank of water at the bottom or another viscous liquid, so the movement of the building is counteracted by the resistance of the liquid. “We have used this approach in Hong Kong, using liquid silicon,” he says. Taipei 101, currently the world’s tallest tower, also has a tuned mass damping system. The use of tuned mass damping systems ensure that lateral movement and acceleration in a building does not cause excessive discomfort or feelings of nausea for residents or office workers. The movement of the building also needs to be accommodated by the cladding system on the exterior to ensure it is robust enough to stay in place, says Kamran Mouzami, one of the world’s leading authorities on super tall construction and a director at WSP — the designer of the Freedom Tower. “When building taller, the cladding-structural interface becomes a major issue, so when the building moves, the cladding has to be able to cope with lateral drift. It means you need to have proper jointing on the cladding and curtain walling,” he says. Dennis Poon is managing principal at Thornton-Tomasetti Group and led the structural design of both the New York City World Financial Centre and the Taipei 101 in Taiwan — currently the tallest tower in the world. He also highlights wind-loading as one of the major challenges of super high-rise construction. He says: “Over 2000ft or so, you have to look at local wind data collection because the wind might be blowing in one direction at the base and in a completely different direction a few thousand feet up.” Poon also points to the impact that the lateral movement of a building can have on the interior and exterior finishes. “The lateral movement of the building can also have an adverse effect on the cladding and curtain walling — your exterior finish won’t be happy and your door frames won’t be happy. “Another very important issue with super high-rise structures that you need to be careful about is differential axle shortening between the tower core and the exterior columns.” But several of the most persistent problems encountered in super-tall construction relate not only to the structure, but also to the lift used — and many designers believe elevator technology will be the single most important factor in the development of super-tall buildings. Designers need to consider the issue of discomfort not only in relation to the lateral sway of the building but also in relation to the acceleration of the lifts. Lifts need to be fast enough to transport people up and down the structure as efficiently as possible, but cannot speed up or slow down too quickly. Currently the fastest lifts can move at a rate of around 12 m per second — but even at that rate, they would take at least two minutes to go from the bottom to the top of a mile high tower. This creates particular problems for super-tall office buildings where the lifts need to cope with massive numbers of workers arriving and leaving the building at roughly the same time, according to Arup’s Zikri. “The taller the building, the more people you have to move up and down it. “With taller buildings you need a bigger core for all the lifts that are required so it is not very efficient because your net to gross area is lousy, as the core has to get bigger to allow enough lifts for people to get in and out.” The designers of the Petronas Towers in Malaysia tried to overcome this problem with the use of double-decker lifts — but even this approach may not be sufficient to cope with the number of people living or working in a mile-high structure. Two of the biggest problems associated with lifts in super-tall buildings are cable stretch —where steel lift cables stretch under their own weight and cable whiplash — where rapid lateral movement of the cables increases with the height of the buildings. As the lifts move up and down air is also compressed in the shaft, which needs to be released in a controlled way to avoid mechanical problems. Wind loading is one of the most significant factors that super tall building designers need to think about, according to Zikri. “Where you have severe winds, the design has to be stiff enough to counteract that; the cladding also has to be able to withstand that pressure. “You need to be careful about windows; specifically opening windows on opposite sides of the building, because you have very positive pressure on one side of the building and the reverse on the other, which could cause windows to smash,” he says. While the building super-structure needs to be robust enough to handle these lateral forces, its foundations also need to be strong enough. “Obviously, the taller you go the more load there is on the foundations, so you have to make sure you have adequate foundations to carry the loads. That means you have to look at the soil structure. In New York you have rock, in London you have clay and in Dubai you have sand,” says Mouzami of WSP. “Foundations are not an issue in New York, but in London and Dubai, you need large diameter bored piles. “Secondly, as you add weight to the top of the building, the columns need to become much larger, so the idea is to place them in [so] that they take a common share of loading.” Finally, Zikri says that the construction process itself is a crucial consideration for any project of this scale. “You need to make sure that construction is unimpeded, so you have to plan it carefully. “You need to have enough materials but you cannot store all the materials you would need on the site itself, so you have to look at just in time delivery and the use of pre-fabricated elements where possible.” Zikri, Poon and Mouzami believe that there is no reason why a mile high tower cannot be built in the future; and where would be a more likely location than the world capital of mile high ambitions — Dubai.

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