Abu Dhabi's towering achievement

Capital Gate tower defies gravity with its lean of 18 degrees that required engineers to come up with many answers to build this modern-day wonder.

The Capital Gate tower developed by ADNEC leans more than five times the angle of the Leaning Tower of Pisa.

To understand the epic engineering challenges of building the leaning Capital Gate tower in Abu Dhabi, you must first realise that nature wants it to collapse. "Everything about the tower makes it want to fall over," says Michael Johnson, who has been appointed to oversee the final phase of construction of the building by the Abu Dhabi National Exhibitions Company (ADNEC). "But it has been designed to stop."

The building leans westward at 18 degrees, more than five times the angle of the Leaning Tower of Pisa. The top 17 floors, out of a total of 35, hang over the edge, putting thousands of tonnes of pressure on the core of the building. Contractors and engineers devised a series of solutions to this problem, some of which have never been seen in the world of tower construction. In fact, this unique challenge is the reason that National Geographic is featuring the building along with Sheikh Zayed Mosque and the circular Aldar headquarters building as the "making of three modern day wonders" starting on Saturday.

"It's absolutely one-off," Mr Johnson says. "This will be in engineering textbooks for years to come." The piling of a normal tower goes straight down into the earth and holds the building steady. But for the Capital Gate engineers created two sections of piling, one deeper than the other. Together they create competing forces that keep the building upright. The core of the building was constructed with a slight lean away from the eventual slant of the building itself. As each consecutive floor was put in place, it too was pulled straight.

To make it even more solid, the core was threaded with thick bundles of steel cables that were pulled tight. The amount of cable was 44 times that used on the Golden Gate Bridge in San Francisco. The curving structure of the building is actually a web of steel "diagrids", which are "X" shaped pieces that are each designed on a computer and cut by a team in Sharjah to fit together exactly, says Werner Matyas, a senior project manager from Mace Group, who has worked on the project since the beginning. "One level included 18 nodes and required two weeks just to calculate," Mr Matyas says.

The heaviest diagrid weighed 16.65 tonnes. After each piece was swung into place, a team of welders would have to secure it before the crane could let go. The steel structure helps channel the forces acting against the building into the strong core. On the top half of the building is a second diagrid that creates an open atrium through the floors to bring light into the hotel suites. These were then covered by 700 diamond-shaped glass panes that were cut in a workshop in Musaffah. Part of that glass on the building will be cleaned by a mechanical platform, but the hard to reach areas can be accessed only by workers who will climb down with harnesses and ropes and clean them by hand.

Capital Gate would have been a challenge by itself, but it also underwent significant changes in design during construction that pushed the team to its limits. A year into construction, which began in September 2007, it was decided to add a swimming pool and restaurant suspended on a platform on the edge of the building facing the exhibition centre. Engineers had to secure it in place, while making sure it was consistent with the overall design.

The solution was a curving mesh skin that "splashes" against the building, culminating in the platform. The skin also shades the offices on the lower floors. Then word came down in the middle of last year that the building required a helicopter pad. Not only did this require recalculations of forces across the building, but advanced engineering just to make the pad safe for a helicopter to land. Wind tunnel tests showed large amounts of wind shear forming on the surface of the pad, so the team had to lower it substantially to reduce these forces.

And while these engineering challenges seem strenuous, Mr Johnson says the hard part is now under way. During construction, there are fewer teams at work: steel workers, concrete workers and structural engineers. Now, there are 29 separate missions to accomplish, from light fixtures and marble finishing to the link bridge that connects the building to the parking garage of the Abu Dhabi National Exhibition Centre. Each one of these tasks is made more complicated because no room or wall in the building is the same. Its curving structure makes every hotel room and office uniquely shaped.

"It's not symmetrical on any axis: X, Y or Z," Mr Johnson says. "Everything is different." This makes the final six months of the project perhaps the most difficult, he says. "It's all about communication and co-ordination now," Mr Johnson says. "I would say the hard part has just started." [e-mail]

How many people have been involved in the construction of the steelwork? An average of 240 people were on the job for a period of about 14 months for the execution of steel work. About 6.5 million man-hours have been spent in total up to April, since construction commenced in 2007. Does the steelwork provide the main support to the building and how does it do this? The frame is an exoskeleton with the diagrid carrying all the weight of the floors, except the internal diagrid which transfers its load to the concrete core at the 19th storey. How have you ensured that the steel work is in the exact place to ensure that you create the precise overall framework for the building? The steelwork is fabricated from a [three-dimensional, computer-assisted-design] model to very high levels of accuracy and this defines the overall framework of the building both for fabrication and construction What has inspired the interior design of the office areas? Sparkle and astrophysics. Imagine the earth or a meteoroid cut in half to reveal an interior never seen before. It would radiate warm colours to the core, starting with light yellow on the perimeter and finishing with strong red in the centre. RMJM's interior design idea on this basis was to take reference from the colour graduation across the sphere diameter but using a cool colour palette that is both elegant and more suitable for interior applications in warmer climates. Thus RMJM ID has planned light blue veins of colour graduating to darker blue veins in the white stone finishes of the tower lobbies. What are the main features of the design for the Hyatt Capital Gate hotel? Its Sky lobby and reception on the 18th floor will have spectacular views of the surrounding city and coastline. Luxurious and uniquely shaped guest rooms starting from the 20th floor will provide a sense of floating above the earth, where the convex curve of the structure seems to make the building disappear below on the north-west elevation. On the opposite side of the building facing south east, there will be even more dramatic bedroom views and gravity play where the 19th floor cantilevered swimming pool seems to be suspended in mid-air when viewed from above. On the ground floor we have an open plan food emporium, which will enable guests and visitors to ADNEC and the Hyatt to choose from a variety of outlets. The open tea lounge on the 18th floor is where visitors can enjoy afternoon tea while enjoying the views of the Grand Mosque and surrounding areas. The restaurant on the same floor with an inclined perimeter glass wall-facade allows visitors to see all of urban Abu Dhabi. At its outermost edges, the space will feel like it is floating above the ground.