|Featured December 2013|
|Gate Tower was originally featured as a case study in CTBUH Journal 2013 Issue IV, available as a PDF download. Gate Towers was recognized as a "Best Tall Building Middle East & Africa" Finalist in the 2013 CTBUH Awards Program.|
|Other Featured Tall Buildings|
|“The introduction of the ‘horizontal’ – the potential urban plane – into the increasingly ‘vertical’ of our cities, as seen here at Gate Towers, is an urgent urban imperative.”|
- Antony Wood, Awards Juror, CTBUH
The Gate Towers allude to a future in which tall buildings appear at such density that many of the amenities of urban habitat on the ground can and should be replicated at height. While clearly symbolic as an ageless gesture of welcome, the towers also afford a sophisticated experience for inhabitants. Moments of conviviality not normally experienced at height, such as hanging gardens, are cross-pollinated with experiences that could only come from pioneering engineering at height, such as spectacular skybridges.
|The Shams Abu Dhabi district is a newly-created land mass, formed as an extension of the Central Business District of Abu Dhabi, UAE, on what was formerly exposed tidal sands within a fringe of mangroves. Set at five to seven meters above sea level, Shams is gradually developing at a high density and growing in prominence. The Gateway complex occupies a narrow strip of land at the far side of Al Reem Island, which forms the neck of the isthmus leading to the wider Shams development.|
The Gateway forms one of the Middle East’s largest developments, signifying the entrance to Shams. The developer, Aldar, set forth a challenge: create an unmistakable introduction to a larger development, that would also serve as a landmark on its own. The brief called for a unique mixed-use development on a complex site. This involved demanding standards for innovation, quality, and schedule.
|Figure 1. Rendering showing Gate Towers (front) within the Shams development|
|In August 2005 the architects, Arquitectonica, began designing the project. The concepts were developed in close cooperation with the developer under a tight schedule, and were approved in March 2006. The project was officially launched on April 2, 2006. The engineering was prepared in parallel with the architecture to facilitate an early construction start and save costs.|
The complexity of the project required phased construction. The first phase began with the Sky and Sun towers, a pair of elliptical buildings; the second involved the Gate, a three-tower group with a connecting, curved skybridge. The economic crisis in 2008 slowed construction slightly, but the modest downturn was put to good use in value engineering and refining the construction process.
|Figure 2. Site plan|
The Gate’s architecture announces the arrival at, and forms a welcoming statement for, Shams Abu Dhabi, Reem Island.
A series of residential gateway towers act as pillars supporting the skybridge, a large-scale lintel that creates a monumental portal, defining the threshold to the island. The lintel, containing penthouses, glows at night and serves as a marker, visible from miles away. An oval courtyard containing residential and hotel towers, sits behind the gateway towers and serves as a foyer; this contained space acts as a monumental room. It provides an unexpected event on the road, intended to calm both traffic and the human psyche, similar to how a square amidst the rush of the city does. The first half of the oval has been built; the second half will be built at a later date.
The role of the site is to act as the gateway to the larger development of Shams Abu Dhabi. The design was enacted to carry that message. The composition seeks to assign meaning to forms, telling a story about the purpose and location of the buildings.
The Gate’s structural design is a reinforced concrete system. The foundation is a continuous raft extending over the full plot area of 67,447 square meters and sits on top of 4,076 piles.
The typical flooring system is made of reinforced concrete for the podium and post-tensioned flat-plate slab system for the upper floors. Beams have been provided where necessary for preventing critical punching of the slabs and for controlling the long-term deflection of long spans. This choice of system has given maximum flexibility to the MEP system, maximized the floor-to-floor height, and resulted in a clean soffit, thus eliminating the necessity to provide false ceilings. Reinforced-concrete columns and shear walls have been used to transfer floor loads to the foundation.
The lateral-load-resisting system is a moment-resisting frame system composed of columns and shear walls framing together and coupling with beams and/or flat slabs. The frames have been designed and detailed as intermediate moment-resisting frames in accordance with UBC97/ACI 318.
Glazing and Cladding Systems Engineering
The repetitive pattern of the towers’ curtain wall and the scale of the project lend themselves to be more suitable to unitized systems. Ventilation screens are integrated into the curtain wall at the parking areas.
High-reflectivity glass gives a solid, lifeless look to a building, despite its superior thermal properties. Due to the residential function of The Gate Towers, low-reflectivity glass was placed to the outside and faces the interior. A light transmittance, above 40%, was used to allow for natural lighting. The outer panes are heat-strengthened, to minimize the possibility of spontaneous breakage. The inner panes are fully tempered, as specified by ANSI Z97/1 requirements for safety glazing.
The glass has a U-value of 1.9W/m2K and 0.30 shading coefficient, which satisfies UAE authorities’ requirements. Insulation of building glazing by using low-e coating has reduced the heat gain, and subsequently the cooling loads of the building.
Additionally, for the first 20 floors of the towers, the glass has been treated and increased in thickness to reduce street noise transmission to the lower part of the towers.
|Figure 3. Tower 3 typical section|
One of the most distinctive elements of the project, the skybridge, connects all three towers, housing 21 breathtaking duplex penthouses. Sixteen of the penthouses have an indoor pool, overlooking the city.
The skybridge length is approximately 300 meters, extending beyond the edge of the East Tower (Tower 5), which was placed from above by travelling gantries.
|Figure 4. The two-story skybridge|
|Two main trusses, one on each face of the bridge, carry loads in the circumferential direction of the structure. Each of the two main trusses is composed of complex chords, formed by trussed members connected the main diagonals spanning between the chords. Roof and floor trusses carry loads in the radial direction between the main trusses and around the oval openings in both radial and circumference directions. Extensive computer models and thorough checks were made for structural movement, mode shapes, block work movement, cladding, and glazing movement, in order to study every possible combination of wind loads and seismic movements. This was then checked by two third-party accredited reviewers.
The portion of the skybridge above the towers was built in place above Level 63, whereas the portion between the towers was assembled on a platform extending from the podium at the fifth level and was then lifted into place.
To complete the skybridge, the infill structure, weighing 750 metric tons, measuring 40 (W) x 31 (D) x 12.5 (H) meters, was lifted by strand jacks to Level 64, at a height of 238 meters. This was the heaviest and highest lift ever attempted for a real-estate project to date. The lifting of each section took approximately three days at a speed of 18 meters per hour. All strand jacks were synchronized to ensure the safety of the lift. Extreme precision and accuracy of survey were the key factors in ensuring that the lifted portion of the skybridge fit exactly into place.
Once the bridge section was in place, the Dubai rope-access company Megarme installed an innovative system of tension cables to provide an underside working platform for installing soffit panels and abseiling (rappelling) hooks. Tension cables were strung between two towers at three-meter centers, with periodic connections into the structure above, and nylon nets were stretched between them.
|Figure 5. Construction photo series showing installation of skybridge spans|
|The Gate Towers has introduced the concept of skybridges to real estate development in the UAE, as well as the region, through proven construction technology and methodology. The payoff is most obvious when taking in the view from one of the skybridge’s 21 luxury penthouses. A panoramic view from Abu Dhabi Island to the Arabian Gulf is revealed from each room.|
The Gate Towers were recognized as Finalists in the Best Tall Building, Middle East & Africa category of the 2013 CTBUH Awards. The jury statement in the Awards Book captures the value of both the aspiration and the execution of the project: “The Gate Towers allude to a future in which tall buildings appear at such density that many of the amenities of urban habitat on the ground can and should be replicated at height. While clearly symbolic as an ageless gesture of welcome, the towers also afford a sophisticated experience for inhabitants. Moments of conviviality not normally experienced at height, such as hanging gardens, are cross-pollinated with experiences that could only come from pioneering engineering at height.”
|Figure 6. Overall view of the Gate Towers complex
CTBUH Skyscraper Center Profile:
Gate Towers was recognized as a Best Tall Building Middle East & Africa Finalist in the 2013 CTBUH Awards Program.
CTBUH Journal 2013 Issue IV:
Download the Paper
|The CTBUH would like to thank Aldar Properties and Arquitectonica for their assistance with this article.|
Photography © Aldar Properties PJSC, Arquitectonica