Abu Dhabi, the mighty capital of the UAE is well-recognized for its striking skyline of mesmerizing skyscrapers. One of its prestigious buildings is the Capital Gate tower which catches one’s eyes due to its organic form. The curvaceous capital gate is an eye-catching and iconic building, making a dazzling identity through its gravity-defying inclination of 18 degrees. Being situated at the confluence of sea and desert, the building form symbolizes a swirling spiral of sand while the curved canopy represents a splash.
With its construction completed in 2011, the building was able to acquire much recognition. The 18 degree westward leaning tower has a Guinness world record for the “world’s furthest leaning manmade tower”. The tower is one of its kinds due to an exceptional structural system that uses a pre-cambered core and vertical post-tensioning of the core.
Derivation of the Design Concept:
Abu Dhabi National Exhibitions Company (ADNEC) appointed RMJM architects to design a state-of-the-art exhibition center for the promotion of Abu Dhabi’s event sector. The idea was to create a signature tower with futuristic design aesthetics with world-class facilities for live events. The structural and technical aspects should be such that it honors human achievement and resonates with Abu Dhabi’s dynamism.
The design idea is splendidly reflected in the tower’s form. The two main natural elements of Abu Dhabi – sea and desert form the foundation for the building’s striking form. The tower form signifies a swirling spiral of sand. The parametric canopy, aka splash, running over the grandstand creates a wave-like effect owing to the tower’s proximity to the sea. The amalgamation of the traditional and modern style is reflected by integrating the tower with Abu Dhabi’s historic structure- the National Day Grandstand.
Due to the complete asymmetric shape of the building, each floor plate has a unique shape and size. The form of the floor plans changes from ‘curved triangular’ to ‘curved rectangular’ as the height of the building increases. To give the twisted and lean form, the floor plates are staggered giving a different look to the building from all directions. All the rooms have different shapes and areas.
The 160m tall tower consists of 35 floors and offers over 15,000 sqm. of mixed-use commercial purposes including offices, retail spaces, and the Hyatt hotel. The building comprises 30 meeting rooms, a conference area, a business center, food courts, and a helipad on the rooftop. A swimming pool placed 80m above the ground is built on the 17th and 18th floor. A gigantic atrium invites natural light inside the structure and also contains a tea lounge. The external lighting uses smart technology to minimize energy consumption and adjust according to the availability of natural light.
There were some challenges faced during the construction period as new features were introduced at a later stage. A two-storied development including a pool and restaurant was introduced 90 meters above the ground. The weight was estimated to be 150 tonnes and the cantilevered structure was then supported by 22 underpinned steel struts. Further, merely 6 months before the construction initiation, a helipad was introduced as a design element. The unpredictable wind loads were a potential threat that could uproot the helipad upwards. However, the issue was eventually resolved by reducing the height of the helideck to 2 meters above the roof.
Making history with its ‘never been used before” technologies:
The twisted and leaning form of the structure was made possible with the help of a post-tensioned concrete core surrounded by a steel-truss diagrid system. The steel beams run throughout the building span holding the diagrid system at extreme ends together while supporting the concrete floor and metal deck. The building core and external bracing are connected via horizontal trusses called outriggers.
Foundation: To compensate for the wind, seismic and gravitational forces caused by the leaning building feature, the tower is built on reinforced concrete rectangular raft footing on 490 concrete piles. Half of the piles which run 20 meters deep are under compression while the other half running 30 meters deep are under tension due to the building’s overhang.
Core: The building core was pre-cambered (tilted) away from the overhang and consequently straightened by the dead load of the tower. Jump forming technique was used for the construction of the core and concreting was executed at night as day heat could dry out or crack the concrete. The core was post-tensioned with the help of 146 tendons, each 20m long, spanning through 5 floors.
Floor plate: Composite flooring is used to connect the main core to the diagrids. The pre-casted embedment plates in the core are joined to the floor beams for supporting the floor plate. While being stacked on over the order till the 10th floor, the floor plates stagger by 800-1400mm on the floors above it till the 27th floor. The remaining floor plates have an inclination from 300-900mm.
Diagrid system: 600mm x 600mm hollow steel sections make the external diagrid system. Each member has a different length, curvature, thickness, and angle. The thickness of steel sections ranges from 80mm on the lower floor to 40 mm on the top floors. A strong bracing system is maintained by connecting all the diagrid nodes with tie beams. The internal diagrid consists of hollow steel sections which are 80mm in thickness. The internal diagrid supports the weight of the 18 floors which consists of an atrium. The funnel-shaped internal diagrid is connected to the building’s core via structural members on the mechanical floor.
Wind bracing: The Capital Gate tower resists the winds by a combination of core walls and outriggers inbuilt on the mechanical floor. The aerodynamic form provides resistance to torsion. Further, the thrust due to the diagrid is transferred to the foundation with the help of a concrete ring beam on the ground floor.
Sun-shading: The splash-shaped metal mesh works as the sun shading for the tower. At every five floors, the mesh is braced to the building with cross-bracing rods at a diagrid node.
It is suspended from the 18th and 19th floor where the terrace is cantilevered by 12 meters. The cantilever is connected to the external diagrid with the help of steel sections placed diagonally on the mechanical floor. Basically, on the 18th floor (mechanical floor), all the structural elements of the tower are braced together.
Making of the iconic façade:
Due to the organic shape of the building, all the 12,500 glass panes and 8,250 steel sections are of different dimensions. Additionally, all 800 diagrid nodes have different configurations. One of the challenges was to manufacture, install an asymmetric diamond shape façade which should be weatherproof and energy-efficient as well.
A curtain wall and glazing system accounts for the cladding system of the Capital Gate tower. The curtain wall is made up of diamond-shaped panels which are two-floors high. Bracing the panels at the middle floor slab ensures resistance against wind. The glass used in the curtain wall has low-emissivity, good thermal insulation and glare reduction properties.
Sustainable strategies used in the making of Capital Gate Tower:
ADNEC, RMJM architects and the engineers were committed to integrate passive and active design solutions to achieve sustainable building accreditations.
Double skin façade, energy-efficient glazing, sun-shading mesh, low-flow water fixtures, reduced construction material, green roof, smart HVAC and energy control systems are some of the elements which contributed significantly to creating a sustainable building whilst enhancing the building’s performance.
The splash-shaded sun shading metal mesh envelops the building from the south to avoid the direct sunlight. The mesh retracts 30% of heat gain, consequently reducing the load on air conditioning.
The internal and external diagrid system forms a double skin façade at the upper hotel floors which reduce the solar heat gain. The air from the guest rooms is sent to the façade cavity where an insulating buffer is created between the cool indoor air and hot outdoor wind. This recycled cool air is then recirculated in the rooms. The transparent façade glass has low-emissivity which keeps the indoor cool while avoiding the glare.
The organic and curved form saves a significant amount of construction material including steel, glass and concrete, therefore reducing the carbon footprint of the tower. The vegetated roof uses native plants that use less irrigation and simultaneously increase thermal insulation. Instead of installing a chiller plant on the site, district cooling is used as an energy-efficient means for air conditioning. Smart building systems for lighting, HVAC and power consumption also reduces the overall energy consumption by the building.
Capital Gate tower is the first of its kind of building which has set new benchmarks due to its structural innovations. Besides having a world record of the furthest leaning manmade tower, it is the first building to use a pre-cambered core that straightens up as the upper floors are added. It is also the first structure to use the concept of vertical post-tensioning of the core and dual diagrid system (external and internal). The tower is an excellent example of a structural marvel that uses modern technologies to create an identity of its own and a set example for the futuristic buildings to come.