ANALYSIS OF CAPITAL GATE
Capital Gate, Abu Dhabi, holds the title of “World’s furthest man-made leaning tower” with a dramatic 18-degree westward lean. Many of us might relate it to The Leaning Tower of Pisa, known for its nearly 4-degree lean due to an unstable foundation, but Capital Gate has a built-in lean, making it stand out from the rest of the structures. Its design has several innovations, making it one of a kind. Let us dive deeper and see what is so amusing about this structure.
What makes The Capital gate so unique?
It is the first building in the world to use vertical post-tensioning of the core to counter movement and support stresses created by the building’s overhang. The tower’s curvaceous shape is inspired strongly by two main geological elements, one being the sea and the other the desert. These two elements resonate strongly with the essence of Abu Dhabi.
Low-lying sandy deserts dominate the landscape of Abu Dhabi; with the type of soil present in the region having a poor bearing capacity, designing a suitable foundation was a challenge as “It is the foundation that will stand the test of time”. Luckily, a layer of bedrock was beneath, which served as the anchor for the foundation used. A deep foundation was laid by running piles of varying depths to provide proper anchorage. Half of the piles run to only about 20 meters of depth and are under compression. In comparison, the other half runs to the bedrock at 30 meters of depth in order to anchor tightly to the soil, and these are under tension due to the massive eccentric overhang of the building.
The superstructure consists of a post-tensioned concrete core surrounded by two steel truss diagrid systems (external and internal diagrid systems). The core is the most exciting part of the structure; during construction, it was poured slightly off the vertical and straightened by the load due to the tower floors added in the opposite direction (jump forming technique).
One of this building’s distinctive features is the facade, consisting of a steel diagonal framing system (Diagrid) and a diamond-shaped glass module. The building has a solid bracing system as the interlocking part, called a diagrid node, and a horizontal tie beam connects all nodes horizontally through the circumference of the building. The diamond-shaped glass modules are triangular panels connected by hinge joints, forming a diamond-shaped panel spanning two floors. The structure is waterproof as a two-layered seal connects the diamond modules.
The internal diagrid cruciform consists of hollow circular sections, and it supports the total weight of the building and has the shape of a funnel. The composite floor system spans from the central core to internal and external diagrids, and the concrete core is joined to the floor beams through embedment plates already cast in the core walls.
The Capital Gate’s architects added a novel design element known as cantilevering from the framework during construction. The design required a two-storey extension 90 meters above the ground, without visible supports, containing a pool and restaurant. The water in the pool was presumed to weigh about 150 tonnes, and support was required from below; 22 underpinning steel struts were used to provide the support, also satisfying the architects’ aesthetic requirements.
Introducing a helipad on the roof was a last-minute design addition and was a challenge. The difficulty originated from the erratic wind, which could lift the helipad or produce a suction that would pull the helicopter off course; later it was planned to lower the helideck height to 2 meters over the roof for safety.
The ‘splash’, a sun-shading device also an aesthetic element adds a touch of beauty to the structure; stretched across a metal frame, it comprises a layer of woven metallic mesh. The canopy at the entryway is supported by column structures in the shape of trees. The splash harmonizes and highlights Capital Gate’s distinctive form.
The wind generally blows in the area with a predominant direction of the northeast to the southeast and a speed range of 5 to 15 mph. The diagrid system serves as this structure’s primary lateral load resisting system. However, the thickness of the diagrid system’s members varies depending on the structural stresses placed on each part. The illustration demonstrates how the tower’s funnel shape reacts aerodynamically to the prevailing winds.
The grand structure was constructed with a budget of 233 million dollars with a sprawling built-up area of 53,100 square meters. Six hundred people from different countries worked in the labour force, with around 400 subcontractors involved in this project. It is the Capital centre and is worth a whopping 2.2 billion dollars. ADNEC is developing a 23-tower business and residential micro-city.
Capital Gate has won several awards; some of them are Guinness World Record — The furthest-leaning man-made tower with an inclination of 18 degrees, ‘Best Commercial/ Mixed Use Project’ at the International Property Awards in 2015, Sustainability Award at the AEO Excellence Awards, Venue of the Year for ExCeL London at the AEO Excellence Awards.
The capital gate structure is distinct and built using the most innovative construction techniques. It comprises an inner atrium in a tilting mass, a cantilevered restaurant, and foundation piles of varying depths to provide proper stability. The edifice embodies the atmosphere of Abu Dhabi and portrays its culture, making it a construction marvel.
This article has been written in collaboration with Anurag Dhote, Kamini Venkata Sravani, Laasya S V, Mekala Pavan and Minnikanti Ajay Teja.