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Top notch

Burj Dubai - By 2008 the Burj Dubai will be the world's tallest building.Bernadette Redfern scaled its already considerable height.

What is immediately obvious about the $1bn (£567M) Burj Dubai is that it is going to be an extremely slender structure.

Even at its current height of around 150m it has a certain soaring grace, despite the presence of bright yellow concrete jump forming kit.

'It'll be extremely slender. As it goes higher we have reduced the size of the floor plate so that it steps back with a series of terraces, ' explains architectural and structural designer Skidmore Owings & Merrill (SOM) associate partner Eric Tomich.

Incredibly, the concept structure was designed in just four weeks during March 2003 when developer Emaar held a competition to design the world's tallest building.

'This was by far the best option in terms of efficiency, buildability, construction cost and maintenance. It was also the most elegant and well articulated structure, ' says Emaar project manager Greg Sang.

'The core of the structure is high performance concrete and the building is going to break some records even as it's being built, ' says supervising engineer Hyder's project manager Rob Pickering. 'Not only will it be the tallest structure on earth but pumping concrete beyond 500m has never been done before, ' he says.

Beyond the 152nd floor the concrete structure will be topped with steel. 'Effectively we will place a 50 storey steel structure on top, ' says Pickering.

The transition fulfils a number of purposes. 'It's really driven by the structure's geometry. As the floor plate steps back the plan eats into the hexagonal core, ' explains SOM structural partner Bill Baker.

Introducing steel elements frees up floor space. But the transition also introduced the possibility of increasing the height of the building, should Emaar wish to do so. Although it is set to be at least 700m, it could end up much higher, although the team on site will not say by how much. 'Initially it was going to be slightly taller than Taipei 101 [which stands at 509m] but it has grown a lot since then, ' laughs Baker. The steel structure will be jacked off the concrete core, rather like a self-climbing crane, enabling new steelwork to be slotted in beneath that already assembled.

The rush to get the structure up has meant that design is still under way while the building is being constructed. Although this is standard practice on design and build projects, the speed at which the Burj is being erected is breathtaking. The construction period is just 47 months.

And design is not straight forward. Reaching higher and higher has made wind modelling a critical part of the design process. 'Design codes were just not written for something like this, ' says Baker. Canadian consultant RWDI created a number of models which were used to test force-balance and aero-elastic behaviour as well as the performance of cladding - for this tests were conducted at full scale. Results were verified by the University of Ontario.

'The building shape changes around its circumference and it tapers vertically. Every section of the surface has a different aerodynamic profile and air flow rate. This reduces the wind loads and prevents vortex shedding [turbulence in the lee of a structure which buffets it and, at the right frequency, can cause oscillation], ' says Baxter.

The testing has led to some design changes. 'We were using the models to shape the structure, ' says Baker. The Burj has three buttress-like legs.

Which ever direction the wind blows from, one or two of these will become the tower's 'leading edges', parting the air flow.

Turbulence peeling off the rear side of the buttresses will bash into the tower's central spine.

The design team found that, as the building steps back asymmetrically over its height, vortex shedding was particularly severe over one section. But 'weather data showed that wind here doesn't really come from the south, so we turned the whole structure around', says Baker.

Because of the iterative nature of the design process and the possibility that the height will change, the substructure has been designed to take additional loading if required.

The tower is supported by 200 1.5m diameter reinforced concrete friction piles. These are 50m long and are capped by a 4m thick concrete raft.

'There is reserve capacity in the foundations. It pays to spend extra money, as on a complex project like this there are bound to be changes, ' remarks Tomich.

The person charged with making these changes a reality is contractor Samsung's project manager KJ Kim. As the man responsible for building Malaysia's Petronas Towers, Kim has twice put constructing the world's tallest structure to his name.

'The construction method is pretty straight forward but we have had to use a special concrete mix to ensure that it stays fluid as it is being pumped.

We also had to get a specially modified pump to transport the concrete. The one we are using can get concrete up 600m off the ground and operates at 350bar, ' he says. Typical concrete pumps operate at around 100bar. 'We have 2,000 men on site now but this will increase to more than 5,000 during the peak construction period, ' he remarks.

Work began in February 2004 and today the building stands 27 floors high, increasing by a floor every four days. 'Soon it will be a floor every three days, ' anticipates Kim. By the end of the year the building should stand 100 floors tall, with completion planned for December 2008.

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