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Diamond effort: Dubai's Al Mas tower

Chandra Puri is the project structural engineer on the 360m tall Al Mas tower, which is set to become the Dubai diamond exchange. He explains the challenges to Bernadette Redfern.

There were several structural challenges to overcome when Atkins engineers, including associate director for the structures team Chandra Puri, began turning the design for Dubai Multi Commodities Centre diamond exchange into a working structure. "The greatest challenge was dealing with the stress imbalance caused by the fact that one part of the building finishes 60m below the other," says Puri.Al Mas, meaning "diamond" in Arabic, takes the shape of two intersecting ellipses in plan, spanning 64m at maximum length and 42m at maximum width. "One part of the building ends below the other and creates a stress imbalance in the core, so we made one end of the core thicker by 150mm and had to monitor the placement of floor slabs very carefully. If these were not level this would have induced more vertical stresses," says Puri.

The building was carefully monitored during construction by Atkins, as supervising engineer, and the contractor, a joint venture between Japan’s Taisei and local Arabian Construction Company. Building above the 48th floor where the first part of the building ends, caused lateral deflections in the lower part of the tower. Although there were no codified design limits to apply here, the team wanted to ensure the building remained aesthetically balanced. To calculate the predicted movement it carried out an analysis based on total vertical loading and then adjusted it, taking into account the concrete strength, the effects of increasing the thickness of some of the core walls and the effects of creep. The final expected deflection was found to be 170mm.

The team wanted to ensure the deflection did not exceed 0.001% of the building’s height. So far movement has not exceeded 50mm, but the tower is not finished and therefore not fully loaded. Still to be completed is the 81m steel-framed lattice spire that tops off the tower. It is connected to the building with a 21m reinforced-concrete upstand and is clad with aluminium. The spire tapers so the higher and slimmer it gets, the more likely it is to vibrate. "Four, 2T-tuned mass dampers will be fitted when it’s finished," says Puri.

Click here for a cross section of the Al Mas Tower

To mitigate excessive vibration of the mast during construction a vortex suppression device was fitted to the top of the spire. This comprises a set of 250mm diameter plastic tubes that sit around the top of the spire interrupting air flow. To maximise the lettable floor space the building has enormous peripheral columns connected by 1m deep, 500mm wide beams. This minimises the need for interior columns. "We reduced the size of the columns by putting steel beams inside them so they act as composite structures. The concrete is also high strength, C70," says Puri. The columns are 1.1m in diameter up to the 28th floor. They then reduce to 900mm diameter reinforced-concrete columns above the lower levels.

The team opted for precast concrete floor slabs to accelerate construction. Using a 320mm thick hollowcore slab system with an 80mm structural topping that is cast insitu. Three service floors at the 9th, 28th and 48th levels have 450mm reinforced-concrete floors and to further increase stiffness, outriggers extend from the core at the service levels. "These engage with the perimeter frame and add lateral stability," says Puri.

Beneath the tower sits a fivestorey podium that incorporates the diamond exchange centre itself. The hall is the largest of eight triangular-shaped, steel-framed, glass-clad prisms that extend from the core. Supporting it is an exposed steel truss made of 500mm and 600mm diameter tubular steel members and raking columns. Design of the building began in 2004 and construction began in June 2006. The original completion date was September 2007 but client changes, including an 80m by 30m exhibition hall, meant this was extended to end 2008.

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