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Burj Dubai reaches for the skies

DUBAI

In a state where building tall has become a national obsession in recent years, one project is set to stand head and shoulders above the rest. Alasdair Reisner visited the site of Burj Dubai, where developer Emaar is reaching towards a world record

AT THE moment it is hard to work out what everyone is getting so excited about. Bearing in mind that there are currently more than 100 tall towers in various stages of construction across Dubai, the 21 storeys of concrete standing unassumingly on land near Defence roundabout seem little to get excited about. Right now this structure is dwarfed by neighbours such as the 34-storey Burj Residences.

But go anywhere around Dubai and this is the project that everyone is talking about. Billboards nearly 100 m tall advertise its arrival, taxi drivers have video screens heralding its coming in the back of their cabs and you can already buy a souvenir T-shirt sporting a picture of it.

That is because the site in question is home to Burj Dubai. Over the next three years this tower is set not just to beat the record for the world's tallest building, but to absolutely annihilate it. The current height record is held by Taipei 101 at 509 m. Can Greg Sang, Burj Dubai project manager for developer Emaar, say how much taller Burj Dubai will be?

'You can ask that question but I'm afraid that I'm not going to be able to tell you the answer, ' he laughs.

It's not that Mr Sang does not know the answer. The reason he is staying tight-lipped on the subject is a result of the fiercely competitive nature of the construction industry in Dubai.

Although Burj Dubai is set to win the world's tallest building title by a comfortable margin ? Mr Sang confirms that it will be taller than 700 m ? the final height is a closely guarded secret. Emaar knows that rival developer Nakheel is also preparing a tall tower for Dubai and fears that the earlier it shows its hand, the better chance Nakheel has of ensuring its building is designed to be taller, stealing the glory from Burj Dubai. Understandably, when you are spending £550 million to build the tallest building in the world, you want to keep hold of the title for a while.

'They are not telling us how tall there's is so we aren't saying how big our's is, ' grins Mr Sang, admitting that the Burj Dubai designs even include the possibility of increasing the height during construction if Emaar gets wind that their rival's scheme is planned to be taller.

If such a fluid approach to the design appears risky it merely reflects the way the whole project has been put together. Despite already being around 100 m tall, the final design work for Burj Dubai is only now being wrapped up with final drawings, expected to be complete within the next few weeks.

The reason for this approach is the speed of construction demanded by Emaar, which appears to be common to most construction projects in the Em irate.

'This is a very fast-track project so we started building it before we finished all of the design, ' says Mr Sang. 'We have our contractor, Samsung, on board and we need to feed them all of the designs continually so that they have something to build. One of the main challenges is getting drawings and specifications out of the door to feed Samsung so they can keep going. It's like a monster with a big appetite!' The speed of construction means work never stops on the tower. A two-shift system is in operation to ensure that construction can carry on through the night, while the design team takes advantage of global time differences to keep work ticking over.

'With this project you consider how unique it is as you have specialist consultants from 45 different countries working around the clock. Someone somewhere is working on it at any given point during the day, ' says Mr Sang.

Night time working will become essential for another reason as the temperature rises in the summer. With midday temperatures averaging 34 deg C, there is a danger that any concrete poured will go off too quickly.

'You've got to be pretty careful doing your concreting operations during daylight in the summer. You are better off doing them at night.

If concrete gets too hot while it is curing you can get cracks. Even when we are working at night we still need to add ice to the concrete to keep it cool enough to prevent cracking, ' says Mr Sang.

The height of the project presents a second problem for the concrete pourers. The project is expected to use about 250,000 cu m of concrete, all of which needs to be delivered to the workface, no matter how high that is.

Mr Sang says: 'We have had to bring in Putzmeister pumps that have been specially developed to pump concrete to 600 m. They develop pressures of around 350 bar. That's probably the highest pressure for a concrete pump anywhere in the world. For conventional construction the pressures would be more like 100-150 bar. Hopefully we can get all of the concrete in without a transfer pump.

'That would mean building a staging point half-way up the tower to relay the concrete, which would have a knock on effect on our programme.

We'll have to see what the pump can actually do on site; we'll see what we can squeeze out of her.' The first concrete was poured in 2004 by Middle East Foundations, a local firm that installed 200, 1.5 m diameter bored cast in-situ friction piles.

These were then tied together with a 3.7 m-thick concrete raft across the footprint of the whole site, poured by joint venture Nasa/Multiplex.

The walls are built up with a jump form system by main contractor Samsung in joint venture with Belgians Besix and local outfit Arabtec. Currently the team is achieving a floor every four or five days but Mr Sang hopes to get that down to th ree as workers get more efficient with the equipment and the size of the floor plates decrease up the tower.

By prefabricating sections of reinforcement cage up to 8 m sq on the ground before lifting them into place in the jump-former the team can save time and also reduce the number of workers that need to be working at the top of the tower. With up to 5,000 people working on the structure, any saving in the numbers trying to get through the bottleneck at the bottom at shift changeover can have a dramatic impact on productivity.

The concrete used on the lower levels of the tower is of a slightly higher strength (C80) than normal to deal with the load on it. But the design of the building itself also helps to prevent overloading.

Mr Sang says: 'The building is designed to distribute the gravity loads throughout the perimeter. Normally, the load just goes straight down the walls and the columns. We are moving them sideways so that when you get to the bottom you don't just get these huge great concentrated loads. It is achieved by the ar rangement of the walls and the use of transfer devices.' Yet, surprisingly for a building that pushes the envelope in terms of what can be achieved in construction, Mr Sang says that the project features relatively little in the way of innovative materials and techniques. But he says there is a very good reason for this.

'We've stuck with pretty straightforward techniques, albeit for a big-shot application. We are not inventing or reinventing anything. They are all triedand-tested methods although we are pushing them to the lim it. We didn't want to take the risk associated of inventing a new technique only to find out it doesn't actually work. The programme doesn't allow for that.' It is only where the nature of the project makes doing so unavoidable that the team has had to improvise and come up with new solutions.

'Ou r tower cranes had to be specially modif ied because cranes have never been up to this height before. They have had to have more than 800 m of cable installed on the drum. That is not normal.

You have to get drums and engines capable of lifting that far specially made and upgraded to enable the tower crane to serve the building.

'We also have high-speed hoists; they are pretty much the fastest in the world, travelling at speeds of up to 120 m a minute. You need that kind of speed in order to keep turnaround times down, ' says Mr Sang.

The project is even too big to be measured using conventional methods. While sites across the world use lasers to carry out surveying work, the Burj Dubai will outst rip this capability.

Mr Sang says: 'It would be quite convenient to use lasers but they are only really accurate up to a few hundred metres so we have had to develop a GPS-based system. The problem with GPS is that it doesn't normally need to be that accurate.

If you are using it to find your way somewhere in a car it is fine for it to be accurate to about a couple of metres. For construction it has to be accurate to the millimetre. We have to use special software to achieve those results with our GPS.' But no matter how you measure it, the killer question still remains. Just how tall will it be when it is finished? The capabilities of the cranes and concrete pumps seem to suggest a height of somewhere around 750 m.

Yet, intriguingly, in the sales office for the scheme there is a mock-up of the lifts in the tower. The button for the top floor is numbered 189. Given that ? as its name suggest ? Taipei 101 has 101 floors, Emaar may just have something up its sleeve that will keep competitors for the title of world's tallest building at bay for a long time to come.