Tilt to last
Advanced analysis methods have given rise to the development of a luxury off-kilter tower in the heart of Abu Dhabi
The Capital Gate Tower is 35 storeys of gleaming glass and steel in the middle of the Abu Dhabi National Exhibition Centre. Its curvy, faceted walls betray the presence of a supporting diagrid — familiar to British observers from London’s Gherkin building, or St Mary Axe to give it its proper name, but a first for the desert kingdom. But it’s not the height of the building that draws the eye or its surface texture. It’s the fact that it leans over so much that it makes you wonder if you’re standing up straight.
While Pisa’s leaning tower might be the iconic face of off-kilter buildings, Capital Gate has it beat on two counts. It leans over further (18° to Pisa’s 3.9° off the vertical) and its incline is deliberate, rather than a Renaissance accident.
Working out how to build the Leaning Tower of Abu Dhabi, and how to stop it toppling over once it was built, was a job for the Advanced Engineering Analysis team of Gifford, a UK civil engineering company that is now part of Danish contractor Ramboll. The team was brought in by architectural practice RMJM Dubai, which designed the tower, team leader Carl Brookes explained to a recent Ramboll seminar in London. ’Predicting the final position of the tower was a complex business,’ he said. ’RMJM estimated horizontal drift of between 0.5m and 1.2m, and it decided to obtain specialist help.’
A variety of techniques had to be used to keep Capital Gate up, some of them for the first time in this type of construction
The tower is a mixed-use building, with offices and shops in the lower floors and a luxury hotel at the top; it also has a swimming pool half way up. Its structure is based around two diagrids — metal frameworks where girders form a series of triangles that wrap around in a 3D, complexly curved shape. One of these forms the outer skin of the building and the other is internal, forming an atrium measuring 60m in height to provide light to the upper floors.
But while the diagrids provide strength for the building to resist wind and seismic pressure, it isn’t what holds Capital Gate up. The main supporting structure is a concrete service core, which also holds the building’s lifts and other machinery. This core is a vertical tube, an ellipse in cross section, inside the building. At the base, it sits towards the side of the building that leans out. As the tower climbs around it, the floors gradually stack further and further across the core, half way up, in the centre of the floor. At the top floor, the core sits against the side opposite the lean, between the outer wall of the tower and the inside wall of the atrium diagrid. This means that as well as supporting the vertical load of the building — as conventional concrete cores in symmetrical towers do — it also has to support the sideways load of the leaning tower, constantly trying to topple the building’s upper floors over. Not only that, but the floors twist around the core as well as leaning outwards, meaning that the loads on the final core were extremely complex.
While concrete is immensely strong (under the correct loads), it’s not immovable; in fact, it’s an elastic material. ’Concrete creep, shrinkage, maturity and cracking have to be considered,’ said Brookes. This, combined with the lateral load of the lean, meant a variety of building techniques had to be used to keep Capital Gate up, some of them for the first time in this type of construction.
One technique was pre-cambering, Brookes explained. This means the concrete core was actually built wonky, leaning over in the opposite direction to the finished building. As the building was constructed by pouring the concrete core in stages and building the floors around it, this meant that the accumulating tower gradually pulled its supporting core to the vertical as it rose.
“Concrete creep, shrinkage, maturity and cracking have to be considered”
Carl Brookes, Gifford
Another technique was vertical post-tensioning, a method of reinforcing concrete that involves compressing the concrete after it has set by tightening long screws that run vertically through it. ’Vertical post-tensioning is common in certain types of civil engineering structures, such as bridge piers and containment structures, where lateral loads tend to be more significant,’ said Brookes.
The compression from the tightening bolt reduces tensile stress in the concrete, caused by lateral loads pulling them sideways, which concrete is not good at resisting alone. However, the team believes that Capital Gate is the first tall building to use vertical post-tensioning in the core, because the loads it had to withstand, both as it was built and once it was complete, were so untypical. Even the amount of pre-tensioning needed differed; for example, the post-tensioning had to overcome a variety of tensile strengths, from 6MPa at foundation level to level 18, 8MPa from level 18 to level 27 and 4MPa from level 27 to the roof.
Working out how to apply these techniques was difficult enough, but the diagrid provided yet another complicating factor. Because of the building’s lean and twist, it had no symmetry whatsoever, and therefore every joint on both of the diagrids was unique, with different geometries and forces. Brookes’s team had to use finite element analysis (FEA) to define and balance all the forces — mechanical and thermal, constant and changing. The engineers had to develop a variety of FEA models, analysing the core, the two diagrids, the beams supporting the floors and the floor plates themselves, and then put all of these together to get a complete picture of the building. The fully assembled FEM model contained 44,000 nodes, 23,000 beam elements and 264,000 degrees of freedom.
The FEM analyses helped the Gifford team to determine at which point in its construction the building would begin to pull hard enough on its core to start deforming it. This helped to define aspects of the construction process, such as how far the core construction had to be ahead of the floors and at what point the post-tensioning had to be added.
’The building has achieved recent attention for becoming recognised by the Guinness book of world records as the world’s furthest-leaning man-made tower,’ said Brookes. ’This accolade is the culmination of many months of design development and advanced analysis to make the lean not only possible, but also predictable and manageable.’
The building has been recognised by the Guinness Book of World Records as the further-leaning man-made tower
- Height: 160m
- Floor area: 53,100m2
- Lean angle: 18°
- Minimum floor-to-floor overhang: 300mm
- Maximum floor-to-floor overhang: 1,400mm
- Core composition: concrete 15,000m3; steel 10,000m3
- Foundation depth: 20-30m
- Foundation cross section: 90 x 60m
- Foundation composition: 490 concrete piles