RMJM’s Capital Gate tower in Abu Dhabi leans at a stomach-lurching 18º, making it a project that largely consisted of geometrical and structural problems – not least of which was finding out where the building had moved to each morning

We’ve applied to go into the Guinness Book of Records for the world’s most steeply inclined building,” says Tony Archibold, associate director of RMJM and the man responsible for the design of what must rank as the most extraordinary creation ever dreamed up by the architect. “The average inclination from the ground floor to the top is 18º. To get that into perspective the leaning Tower of Pisa inclines by just 3.9º.”

This 160m-high building feels as bizarre as RMJM’s recent decision to hire Fred Goodwin, the reviled former banker, as an adviser. Called Capital Gate, the 160m high tower in Abu Dhabi is shaped like a shallow S: it starts off fairly straight, kinks about a quarter of the way up, then straightens out again towards the top. The building also twists as it goes up and the top overhangs the base by a gravity-defying 54m. As you would expect, it was difficult to make such an irregular form stand up physically and financially: inside, the building is stuffed with 13,200 tonnes of steel and the massive core necessary to hold it up, which in turn means tiny floorplates and premium rents.

What’s more, the budget has escalated by 25% and sorely tested the team responsible for delivering the building. It was meant to have been finished in time for last November’s Abu Dhabi Grand Prix but now won’t be ready until October 2010.

So why go for such an extravagant structure? Archibold says the client, the Abu Dhabi National Exhibition Company (ADNEC), wanted a landmark building to indicate the presence of its exhibition centre in the middle of medium-rise Abu Dhabi. It also had to shout “technical prowess” to draw attention to the cutting-edge exhibits showcased below. “It was a very nice brief,” says Archibold, adding that RMJM was also the structural engineer on the building. He adds that the building’s tilt is a response to the wind. “The prevailing winds come off the sea so the idea was that the building should look as if it was sculpted in some way by the wind.”

The building is also a tribute to Sheikh Zayed Bin Sultan Al Nahyan, the late president of the UAE. It has a canopy – nicknamed “the Splash” – that sweeps down the side of the building and continues until it reaches a nearby grandstand, one of the oldest buildings in Abu Dhabi and a favourite of the sheikh’s.

How the building stands up

How did RMJM go about ensuring the building wouldn’t fall over? The core, which is about the only vertical element in the whole building, is hard up against the north-west perimeter at the base and actually touches the south-eastern side at the top.

The overturning force generated by that 54m overhang meant a standard concrete core wouldn’t do: this one was post-tensioned on the side opposite the lean. The core was also pre-cambered, which means it was built leaning in the opposite direction to the building; as the structure went up, it was gradually pulled into a vertical position. “It was necessary because of the extreme eccentricity of the forces on the core,” says Archibold. “We think it’s a world first.”

The conventional columns couldn’t be used to support the floorplates at the building’s perimeter, so a steel diagrid was used. This would be able to follow the complex shape of the building, and form a shell that could help to support the floors. However the top section of the building cantilevered out so far that another way had to be found to support that part. The answer was six huge trusses 8m high and 8m deep spanning levels 17 and 18, which transferred the loads back to the core. The building also has a 60m-high atrium beginning at level 19 and stretching to the floor 33 at the top. “One of the reasons for this is it takes the dead loads out of the overhang,” says Archibold.

This is all fine in theory but someone had to realise it. This was the job of contractor Al Habtoor Engineering, which also had to cope with the fast-track programme typical of the region. The original two-year build-time started in August 2007 with the design developed as work progressed. Luckily Al Habtoor Engineering wasn’t on a fixed-price deal. The FIDEC contract was used which relies on bills of quantities with rates that are re-measured as the building is constructed. “If it had been design and build it would have bust the company,” observes site manager Craig Rooney.

The project got off to an inauspicious start. The design included an 84m-long basement extending in the opposite direction to the lean. This had tension piles at the far end to stop the whole basement being ripped out of the ground by the overturning forces generated by the tower. Unfortunately the main sewer for Abu Dhabi was discovered underneath the proposed basement. As the sewer was pressurised, the ground couldn’t be dewatered near it because there was a risk the pipe could burst. The solution was to shorten the basement by 6m and reduce the entrance canopy, but agreeing and implementing it added six months to the project.

The core

Incredibly the pre-cambering and post-tensioning of the core were variations in the contract; originally it was going to be straight. The core was built so it leaned from the vertical by a theoretical 350mm at the top. In practice the steel diagrid and cladding were added as the core was constructed, progressively straightening out the building. Rooney likens it to shooting at a moving target. “Every time you added a section of structural steel, it had an effect; every time you added a cladding panel, it had an effect,” he says. “People couldn’t get their heads around it.”

The core was built in 4m high lifts with each section staggered 20mm to create the pre-camber. Every four lifts, 36 steel tendons were inserted into the ductwork in the concrete and post-tensioned.

Les Fairchild, the construction manager responsible for the core says the construction process was a “nightmare”.“On the lower levels we were using 250 tonnes of rebar for every lift, which is two-and-a-half times more than a normal core,” he says. “There were 250 men working on the core day and night which means there was a steel fixer every square metre.” The only way to ensure there was enough space for everyone was to cast each lift in three sections. There were constant detail design changes including a requirement to add an extra 35,000 rebar links per lift. The core took over a year to build and cost more than twice its estimated price.

All the heavy steel in the core meant lots of lifting, but the shape of the building meant only two tower cranes could be used. “Probably the biggest argument on the job was allocating the tower cranes,” says Rooney. “We had a meeting every morning with the suppliers to thrash out who was getting how much crane time.” The cranes had to be fixed inside the core because the shape of the building meant freestanding ones couldn’t be used – the ties needed to link it with the core would need to be up to 32m long. Putting in cladding panels once the ties were removed at the end of the job wasn’t on either because the cladding had to be fixed sequentially.

The answer was to put one crane on the core, the other on the steel perimeter and have the cladding team work at night. As the Grand Prix deadline approached, Rooney brought in a 192m long crawler crane to help speed things up on the lower part of the building.

The steel diagrid wasn’t any easier to fabricate and erect than the core. It is made up from 600 x 400mm box sections with a wall thickness of 80mm at the base decreasing to 12mm at the top. Each of the squares making up the diagrid is 8 x 8m with 18 ringing the perimeter at each level.

“There are 702 of these and not one is the same,” says Rooney. This is because the building twists as it goes up, which means the shape of each floorplate is different. The angle of incidence where the steel members meet varies, and the box sections are twisted at various angles too.

Steel specialist Eversendai was responsible for steel fabrication and erection. The diagrid was brought to site as X-sections that were welded together. Each section had to be held in position by the crane while it was welded to its neighbour; but as a complete weld would tie up a crane for 72 hours, each section was tack-welded using brackets. Even this took up to four hours per section.

The cladding

The cladding was done by Austrian firm Waagner Biro. Each section divided up into smaller triangular panels. These were brought to site and joined together to form one large panel which was craned into position. Each large panel is fixed in just two places at each side; a special sliding joint was developed to locate the top and bottom of each panel and allow for vertical building movement. The minimal number of fixings meant steel was used for the cladding frame rather than the more usual aluminium.

Great care was needed to ensure the diagrid was in exactly the right place so the cladding panels would fit. Because the building was constantly moving, this was done by measuring the building position against a 3D model. Although the diagrid ended up in exactly the right place, there was the issue of thermal movement; and as temperature could reach 50º in the summer, these were not negligible. “At 10 in the morning, when you were about to install a panel, you didn’t know where the building was,” says Rooney. “You could find a panel that didn’t fit one day would fit the next.”

Despite these challenges the team managed to get the structure and cladding completed for the November Grand Prix so it at least looked finished from a distance. Now that the structurally difficult phases are out of the way the fit-out is in full swing. From levels 18 to 33 will be a Hyatt hotel with 189 rooms; below this will be offices. Whether the project gets into the Guinness Book of Records remains to be seen but one thing is sure, it will be a long time before anything else like this is constructed again.

Project Team

Client ADNEC
Architect, structural engineer and M&E engineer RMJM
Hyatt interior design RPW
Project manager Mace
Contractor Al Habtoor Engineering
Structural steel Eversendai
Facade Wagner Biro
M&E installation ETA