It was only to be expected that the engineering challenges presented by the construction of the world’s tallest building would be considerable. Some, such as raising concrete to great heights or devising a strategy for all the lifts needed to move people around the finished Burj Dubai, were unprecedented in their scale.
So the contracting team that achieved it – a joint venture of South Korea’s Samsung, the local Arabtec and Belgium’s Six Construct with US architect Skidmore, Owings & Merrill and US construction manager Turner Construction International – deserves due recognition.
The usual thorough planning and preparation at the design stage was enough to mitigate many of the technical issues that could be anticipated. But as on any major building project, unanticipated problems arose while construction was in full swing. These included delays to the schedule brought about by design changes, the collapse of the cladding contractor, Switzerland’s Schmidlin, which went into liquidation in February 2006, and, perhaps most disruptive of all, the impact of the global credit crisis on the project’s suppliers and contractors.
The biggest design change was the decision to increase the height of the structure to 818 metres from the 700 metres originally planned. “The height was changed in 2007,” says one contractor on the project. “So completing the tower in 2008, as per the original schedule, was no longer possible. That, and a lot of other variations to the design, resulted in an official extension of the programme until the end of August 2009.”
But even that deadline slipped. As MEED went to press, the tower was due to officially open on 4 January. But the final fit-out of the building is not expected until mid 2010.
“There were always two milestones to the construction contract: the first for the levels up to level 40, which includes a hotel, and the second for completion of the rest of the works,” says the contractor.
Burj Dubai in numbers
31,000 – Tonnes of steel used in tower
12,000 – Number of people working on the tower site, in late 2009
600 metres – Height to which concrete was hauled
504 metres – Length of service elevator shaft
118 metres – Extra height added to original tower design, in 2007
Other contractors working on the internal fit-out of the building suggest that July is a realistic deadline for the final completion, but the developer, the local Emaar Properties, has not yet confirmed a completion date.
At the peak of construction activity in late 2009, more than 12,000 people were working on the Burj Dubai site. Only 2,000 of these were employed by the main construction contractor, with a further 10,000 employed by subcontractors working on everything from access roads and landscaping to the installation of security systems and interior decoration.
According to companies working on the project, cash flow problems are affecting many of these subcontractors and suppliers. They say the problem has been exacerbated by the client, Emaar, paying its bills as late as contract terms allow – as much as 90 days in some instances.
“Up until last year, we were paid on time and now they [Emaar] are paying at the last minute of the contractual conditions,” says one contractor. “It has not yet come to a dispute, but there is always a delay.”
The large number of changes to the design that had to be made following Emaar’s decision to increase the height of the building in 2007 have added to subcontractors’ cash flow problems. Whenever such changes are made, the contractor has to put in a claim to the developer for extra time and money, and reaching agreement on the terms for such contract variations can take time.
“It can be quite difficult to reach a compromise,” says the contractor. “The main problem occurs when it comes to decisions over extensions of time. Emaar will say there is no effect on time when the variation is small, but there might be 1,000 ‘small’ variations, so there are a lot of problems in resolving that.”
Contractors can even lose money because of disagreements about what the variations cost. “If you put in a request for payment for 100 per cent, it is agreed on account at 80 per cent, so you have lost 20 per cent,” says the contractor. “And again this is only paid after 90 days.”
Emaar would not discuss payment problems on the project. Even before the height of the Burj Dubai was changed, the project suffered a serious setback when Swiss cladding contractor Schmidlin declared itself bankrupt in February 2006. Its problems were not related to the Burj Dubai project but the construction programme suffered.
Consolidation in this sector of the construction industry over the previous decade had left a dearth of cladding firms capable of taking over the contract and it took Schmidlin’s partner on the project, the local Arabian Aluminium, 13 months to find a new partner – Hong Kong-based Far East Aluminium.
The scale of the building and the amount of work that needed to be done at extreme heights also presented challenges. Many, such as the effect of wind and the reduction of light entering the structure at lower levels, were overcome by the design.
Architect Skidmore, Owings & Merrill based the design on a concrete-framed structure with a Y-shaped footprint. The three wings of the building allow a large amount of light into the structure by maximising the number of outer walls, and at the same time enabling the structure to taper as it climbs, reducing the vertical loads on the base.
Other problems relating to logistics and construction methods had to be dealt with by the contractors. According to a brief on the construction planning prepared by Samsung, extensive testing of the concrete was carried out before any work began.
“Starts were staggered, with some at 5am, others at 6am, then 7am, and so on. Otherwise we would not all get into the lifts”
Contractor working on the Burj Dubai
With more than 430,000 square metres of concrete going into the structural walls of the tower, its performance was critical. Preliminary tests included trial concrete mixes and tests for durability, mechanical behaviour, pumpability, water penetration and curing performance. The tests resulted in a specification for high-strength concrete with additives to improve its flow.
The concrete used on the project was tested for shrinkage in particular. Over time, concrete shrinks or deforms to a greater or lesser degree, a phenomenon known as ‘creep’.
“Concrete will creep under a load in the long term,” explains tall buildings specialist Rob Smith, associate director in the advanced technology and research division of UK-based consultant Arup.
Smith sits on the Institution of Civil Engineers structures board and is editor of the ICE Structures & Buildings Journal. “Because of this you might even need to adjust your floor heights,” he says. “It is unknown how buildings of that height will perform, but in general it is mitigated at the design stage.”
Pump simulation tests were used to determine the behaviour of the concrete under site conditions. The concrete superstructure of the Burj Dubai finishes at just over 600 metres, and structural steel is used for the remaining 200 metres of the tower’s frame, so the team had to find pumps that were capable of pushing concrete higher in a single lift than had ever been done before, and at the same time choose a concrete mix that would flow well enough to travel such distances.
The tests taught the team that pumps made by Germany’s Putzmeister were able to move the concrete in a single lift, taking up to 25 minutes. As a back-up, a second pump was erected at level 124.
Samsung also gives credit to German access equipment specialist Doka for its innovative auto-climbing formwork system. Formwork is the temporary mould that supports the concrete while it is poured and cures. Using Doka’s system, contractors were able to build each floor in three days. “Never before had climbing formwork been deployed over such a long period, at such a great height and under such extreme conditions,” says a spokesman for Doka.
With the pumps and the formwork, the concrete lifting problem was solved, but people still had to be carried up the tower. “We have to be very organised,” says the contractor. “Starts on site were staggered, with some starting at 5am, others at 6am, then 7am, and so on. Otherwise we would not all get into the lifts. It can take 45 minutes to get to the highest areas of work.”
The lifts presented their own challenge to the Burj Dubai team. “Even on the most thoroughly planned building construction projects, there are always challenges that arise during construction,” says Rick Pulling, director of high-rise operations for the US’ Otis Elevator Company. “But Otis’s significant experience with high-rise construction projects enabled these issues to be identified early in the planning phase, and in many instances avoided altogether.”
Otis supplied 57 elevators to the project, including double-deck elevators serving the observation tower at the top of the Burj Dubai, which, according to Otis, are the highest-rising double-deck lifts in the world. A service elevator in the central core of the Burj Dubai is also a record-breaker, rising 504 metres and serving a total of 136 floors.
Throughout the building, Otis adopted a strategy of clustering the lifts. To reduce travel times, there are no elevators that travel to all 160 floors. Instead, express services take people to their destination while by-passing the floors they are not travelling to. For example, passengers travelling to the Armani Hotel will be grouped into the same elevators that go up to the 39th floor. Passengers travelling to the residential floors – 45 to 108 – will bypass the hotel.
With its 57 elevators, 230,000 cubic metres of concrete and 31,000 tonnes of steel, the Burj Dubai is now already the tallest structure in the world, currently standing at more than 800 metres. Following the downturn in real estate markets, it is likely to remain so for some time.
CAPTION – Nearing completion: Workers install the final panel at the top of the Burj Dubai in September 2009