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Wed 6 Jan 2010 04:00 AM

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Lean on me

RMJM's Tony Archibold teamed up with Eversendai to bring to life his Capital Gate design.

Lean on me
Tony Archibold, associate director, RMJM.
Lean on me
Capital Gate leans 18 degrees, compared to the Leaning Tower of Pisa’s four degrees.
Lean on me
Capital Gate leans 18 degrees, compared to the Leaning Tower of Pisa’s four degrees.
Lean on me
Capital Gate has used 13,200 tonnes of steel.

RMJM's Tony Archibold teamed up with Eversendai to bring to life his Capital Gate design.

Capital Gate is part of the Abu Dhabi National Exhibition Centre's development in Abu Dhabi. The 160 metre, 35 storey tower leans 18 degrees to the west and forms the ‘gateway to Abu Dhabi'. The project is being developed by Abu Dhabi National Exhibitions Company (ADNEC) and was designed by RMJM. Capital Gate forms the centerpiece of the Capital Centre development, a business and residential micro city being constructed around the ADNEC district.

In December 2009, RMJM and Eversendai completed the steelwork on the building and are looking to 2010 for the official opening. MEA caught up with Tony Archibold, associate director of RMJM, and Pitchaimuthu Baskaran, project manager with Eversendai Engineering, to talk about some of the specifics of a building that is as pioneering as it is bizarre.

  How does Capital Gate compare to the Leaning Tower of Pisa?

Capital Gate leans 18 degrees to the west, the leaning Tower of Pisa leans only four degrees. An application to the Guinness Book of World Records has been submitted for the category of The World's Furthest Leaning Manmade Tower, and is scheduled to be judged following the completion of Capital Gate's external facade toward the end of 2009.

The structure has now topped off its central core and has reached its final height of 525 feet. From either an architecture or engineering perspective, is this significant?

The topping out of the central core is a major milestone in the building's construction and we are moving forward swiftly with other elements as well.

December 2009 saw the completion of the steelwork on the external facade and diagrid structure, which provides a large percetage of the building's support, of course, along with the central core.

The first phase of the Splash, which sweeps down from the 17th floor of the tower over the top of the main ADNEC Grandstand, is expected to complete before the end of the year. Fitting out and final additions to the building's overall structure will occur throughout 2010.

How long has it taken to top off the core since the design was taken off the drawing board?

Work started on site in August 2007 so the shell and core have taken approximately 27 months to complete.

What challenges has RMJM faced and continue to face?

The challenges are numerous on such a pioneering building. These include the pre-cambered core, which meant building the steel frame and facade whilst the building itself was continuously moving; the unique nature of each piece of steel diagrid, which required custom fabrication of each one to very precise tolerances; managing the logistics and planning of the façade installation which is effectively like a huge jigsaw puzzle with each piece designed to fit in one specific location; not to mention the world record breaking incline and the resulting difficulties of contraction with such a pronounced overhang.

How does one top off a core that slants in the opposite direction of the building's lean?

The central core is a vertical structure that provides the central strength, but that also houses what will be the lift and emergency stairwell systems within the building. Post-tensioning was used to improve the stiffness of the building.

At the same time, the core was built at an angle so that as the weight of each concrete slab is added the centre of gravity of the building pulls the core into its final vertical position.

The result is a much stronger and stiffer core, but this construction technique affected the building design in profound ways, such as accommodating the resulting movements in the facade, the installation of the lifts installation and the coordination of the service risers.

Has this been done before?

No. Capital Gate is the first building of its kind to be developed in the world.

How wide is the base of the building and how much reinforced steel did it take to form the mesh?

The base of the structure is a concrete raft which is 2m deep solid concrete slab packed with reinforcement to allow it to resist the enormous forces acting on it.

How many piles were drilled? How did you go about accommodating gravitational pressure, wind speed and seismic pressure which will have been caused by the lean?

There are 490 piles that descend on average around 30m into the ground. Conventional piles are compression piles and are designed to resist descending vertical load. Due to the lean of the building some of the piles are in tension and are designed to resist ascending forces that would otherwise pull the pile form the ground. Is the leaning tower the biggest challenge RMJM has undertaken?

This is certainly one of RMJM's more challenging projects, but as one of the world's largest and leading architectural firms we have been involved in many notable and challenging projects. One example would be the Scottish Parliament Building in the UK which won the Sterling Prize. What was RMJM's reason behind choosing the diagrid structure?

The diagrid approach was driven by the unique architectural form as well as by the varying core position in relation the floor plate and also by the need for clear internal floor space particularly in the hotel floors at the upper level of the tower. The diagrid approach being a gravity and lateral load resisting system is really an obvious choice in this context over the conventional approach which would not be suitable.

Where did you source the steel?

The steel was fabricated in Sharjah and Dubai, then transported to Abu Dhabi.

How many tonnes of steel has it taken to build Capital Gate?

Right now, the total tonnage of steel is approximately 13,200 tonnes.

Does the steelwork provide the main support to the building?

The frame is an exoskeleton. That means that the diagrid carries all the weight of the floors, with the exception of the internal diagrid, which transfers its load to the concrete core at level 17.

How does the steelwork interact with the other materials?

The floor steel spans between internal and external diagrid, and between external diagrid and the core. The floor beams are connected to the concrete core via embedment plates which are cast directly into the core wall when the concrete is poured.

How much steel is in the diagrid?

There is approximately 7,000 tonnes of steel use to create the diagrid system.

What are the dimensions of the Capital Gate's podium footprint?

The footprint for Capital Gate is essentially elliptical. The diametre of its major axis is 50m while its minor axis diameter is 35m.

Do floor plate dimensions vary with height? If so, by how much?

The floor plates are all more or less the same size throughout the tower. The floors up to 10th level are stacked vertically directly on top of one another.

Between levels 10 and 27, the floor plates stagger over each other, in relation to the lean and twist of the shell, by between 800 to 1400mm and then back to 900mm.

Between Level 27 and 34 the range is between 900mm and 300mm in relation the line of the facade.

Does the lean of the building create an overturning moment?

The lean does create an overturning moment.  The maximum axis tension on one diagrid is 8500 KN.

How well is the overturning moment resisted in the foundations? Are there tension piles?

There is a large podium footprint which means the piles are not predominantly in tension. The piles however, have been designed for tension as well as compression. In total there are 287 1m piles of 25 to 30m deep and another 193 600mm piles which plunge to 20m deep.

Does the building have any unusual energy use/production features? If so, what are they?

The hotel space, in particular, will feature a double facade. This will create a thermal cushion in the intervening space which should significantly reduce building energy demands. There are also energy recovery measures integral within the building's MEP systems.

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