LAST-MINUTE design changes are to blame for progressive slab deflection at Burj Dubai, set to become the world's tallest building, it emerged this week.
An engineer on the project told NCE 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.
NCE revealed last week that carbon fire reinforcement had been added to at least 10 floor 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. Defl ctions were occurring in the middle of the floors and on the cantilevers, the engineer said.
A UK-based high-rise specialist contacted by NCE calculated that a conventional reinforced concrete floor slab spanning 9m would need to be 325mm thick.
The engineer on the project added: 'The thickness [of the slab] wasn't enough and the amount of steel inside wasn't enough - rebar was at large distances.' After the deflections 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.
Emaar issued a statement this week denying that there were any strength problems with the slabs, but saying: 'During the course of design refinement and construction co-ordination, it was decided to further increase the stiffness of the slabs to reinforce the long-term performance of the tower.' Structural 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 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 deflection in the middle of the floor, and to the tops of the slabs to cope with defl ection of the cantilevers.'