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A head in the clouds

As the Burj Dubai tops out at a reported height of 818m this month, SOM partner Bill Baker looks back at the groundbreaking project

When the Burj Dubai is complete, it will soar above the current tallest building – the 509m high Taipei 101 building in Taiwan. However early proposals would have meant it only just topped the Taipei building.

“Our original was only slightly taller than Taipei,” says Skidmore Owings Merrill partner in charge of structural and civil engineering Bill Baker. Baker is the structural engineer who came up with the conceptual design for the Burj. “We didn’t want to promise tallest. We went 10m or 20m higher.”

This design originated from SOM during a two week ideas competition for client Emaar in March 2003. Its entry was the winner. “We had done a similar structural system in Korea which was very efficient,” says Baker. “We thought with a few changes here and there, we could really go high. Then the client asked us to go higher. We were cautious and said we’d do our best. Even after the foundations went in, we went higher. Emaar changed the brief several times. We refined, redesigned and reduced loads.”

The management of wind is key when it comes to tall buildings and the careful evaluation of how the building would respond to wind helped SOM create the first real mega-tower in the Middle East. “People take wind as a given but you can manage how the building reacts just by changing its shape,” says Baker. “We took our design into a wind tunnel. We made multiple tops, keeping the shaft the same. We were experimenting, just trying to understand it.”

The changes to the profile helped it to react well to wind loads. The alterations broke up the wind flow around the building meaning that the wind forces could not accumulate and create large motions. “When wind hits an object sometimes it goes round it in an unstable manner,” explains Baker. “When that happens, it puts small forces on the tower, which turn into large dynamic forces [when that small force occurs along the height of the tower]. The Burj has 24 step changes which reduce the forces because of the different reactions [of each section] to the wind. We left room for dampers but the building was within normal criteria [so dampers weren’t needed].”

The weight of a building can also be used to counteract the wind forces. “Gravity is amazingly reliable,” says Baker. “An empty can is easier to knock over than a full can. You can use gravity to cancel out wind forces.”

Baker explains that by spreading the feet of a building out, the counter moment of the building’s self weight can be maximised. This can lead to cost savings, as the weight of the concrete is doing the work rather than the more expensive rebar. “If piles are on the neutral axis, they steal the gravity load,” says Baker.” “You need to spread the feet of a building. In reinforced concrete, the most expensive is the rebar and the next is the formwork. By pushing loads where wind wants to go, you can minimise rebar.”

Even after the foundations went in we went higher. We refined, redesigned and reduced the loads

Bill Baker, Skidmore Owings Merrill

At the centre of the Burj floorplate is a hexagonal core with high torsional stiffness. Three wings abut the core in a tripod like formation, resisting twist. “The whole building acts as a beam,” says Baker. “The high torsional stiffness of the core helps minimise the twisting of the building. Three wings buttress the core. When wind blows, one wing buttresses the other two.”

The building is now reported to have reached its final height of 818m and the spire will be launched onto the top of the building by jacking it up from the inside. “It’s an exciting time,” says Baker. “I’m guessing the Burj will hold the record for four or five years. We started the super structure in April 2005 and it’s taken nearly four years to get this high.”

The sky is the limit for Baker, who continues to work on skyscrapers in China and the Middle East. “There’s not a structural limitation [to how high we can go],” says Baker. “Only an economic limitation.” The final completion date for the Burj is April 2011.

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