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Change in slab spec blamed for Burj Dubai reinforcement

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LAST-MINUTE design changes are to blame for progressive slab deflection at Burj Dubai, set to become the world's tallest building, it has emerged.

An engineer on the project told NCEI that the floor design had been changed from posttensioned slabs with a depth of 200mm to conventional reinforced concrete, but the depth of the slab had not been increased to compensate.

Carbon bre reinforcement has been added to at least 10 or slabs of the reinforced concrete tower as they were deflecting more than anticipated.

'We couldn't ignore those deflections. They were increasing with time ? it was a continuous process, ' said the engineer.

The floors span 9m between walls and cantilever 3m at the edge. Deflections were occurring in the middle of the floors and on the cantilevers, the engineer said.

A UK-based high-rise specialist contacted by NCEI calculated that a conventional reinforced concrete floor slab spanning 9m would need to be 325mm thick.

But the engineer on the project said: 'The thickness [of the slab] wasn't enough and the amount of steel inside wasn't enough ? rebar was at large distances.' After the deections were noticed on floors six to 16, the slab design for subsequent floors was 'completely changed', with extra rebar added.

The designer couldn't increase the depth of the floor slabs because that would add a large amount of weight to the building which would limit its height, ' the engineer said. 'Extra steel was put in to locally stiffen the floors, ' he explained.

Client Emaar has been secretive about the final height of the structure, but it is believed to be more than 800m. The concrete structure currently stands about 400m tall and will be just over 600m tall when complete.

This will be topped with a steel superstructure.

Carbon fibre strips can be bonded to the concrete surface to provide external reinforcement. The material's high tensile strength helps reduce deections and cracking in the concrete.

Emaar has issued a statement denying that there were any strength problems with the slabs, but saying: 'During the course of design renement and construction co-ordination, it was decided to further increase the stiffness of the slabs to reinforce the long-term performance of the tower.' sructural engineer and architect for the Burj Dubai is Skidmore Owings & Merrill.

Hyder is the client's engineer.

The contractor is a Samsung/ Besix/Arabtech joint venture.

More reinforcement has also been required on the structure where holes have been cut to allow installation of mechanical and electrical services.

'There were changes in electrical and mechanical co-ordination after floors had been constructed, ' said the engineer working on the project. 'Holes had to be drilled in [floor] slabs, which had to be strengthened.' It is understood that Emaar invited tenders for a ú35,000 contract to supply carbon bre fabric last year and that Swiss firm Sika won the job. Sika has also carried out reinforcement design.

Sructural expert Stuart Alexander, WSP group technical director, said that: 'A bit of cracking in a concrete oor slab isn't terribly signicant.

Most of the time you wouldn't need to fill them. Things have to be pretty bad before you start putting carbon fibre on.' WSP is not involved in the project.

It is also believed that different reinforcement techniques were considered by Emaar before carbon fibre reinforcement was selected.

'To convince the main consultants and the owner that carbon fibre was the best option three to four slabs were reinforced using different systems and then loaded, ' the engineer said.

'Carbon fibre performed the best ? it's been used on the underside of slabs to correct deection in the middle of the floor, and to the tops of the slabs to cope with deection of the cantilevers.' But both project designers and the client played down the reported cracking.

Andy Davids, chief structural engineer for Hyder, the client's engineer, said there were 'no serious problems' with the structure. He would not confirm that carbon bre reinforcement is being used but said that all 'sorts of leading edge technologies are deployed in a building of this size'.

Emaar assistant project director Greg Sang denied carbon fibre is being used 'anywhere'.

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