Article Written by Giorgio Buffoni
Located in the financial centre of Abu Dhabi, Al Bahar Towers represent one of the most impressive examples of adaptive architecture and integrated design completed in recent contemporary architecture. The building is the result of an international competition held in 2007 by the Abu Dhabi Investment Council, and won by the Londonbased architect Aedas supported by Arup as multidisciplinary engineering designer. The construction began in early 2009 and was substantially completed by late 2012.
Fig.1 – Traditional shading screen used in vernacular Islamic architecture
One of the key design drivers was to develop an energy efficient and iconic building, inspired by the local vernacular Islamic architecture. Since the early stages of the competition it was evident how the envelope design and performances would have played an essential role in the whole process. The idea was simple. Control the solar gains introducing an external shading system rather than just relying on the glass to filter the solar radiation. The inspiration came from the Mashrabiya, the traditional lattice shade used to protect from the sun and create comfortable and sustainable buildings in one of the harshest environment. The challenge was to re-interpret this concept in a modern language and apply it to 150m high tower.
Design – performances and materials
The aspiration was to achieve a highly glazed building, allowing a great view from the inside and providing the best possible level of internal visual and thermal comfort. It was obvious to the design team that a heavily tinted or reflective glazed envelope was not the answer.We all agreed that a more transparent glass than typically used for similar glazed building in the region was the best approach to take, as this would bring a series of benefits such as enhanced daylighting within the building, a reduced use of artificial lights and associated energy savings.
The team challenged conventional thinking and managed to turn a great intuition into reality, enabling a paradigm shift in the design of tall buildings and setting a benchmark to be followed for many years to come.
Fig.2b – Early 3D models of the primary structure, facade and shading system (source Aedas)
But how to keep the sun out? We needed an adaptable shading system.Due to the almost cylindrical shape of the building, we soon realized we had to develop something slightly more sophisticated than standard horizontal or vertical louvres. The geometry of the tower led the design team in the direction of a triangulated shading system, opening and closing according to the sun position, similar to a big ‘umbrella’, creating a pattern strongly related to the local tradition. A few paper models and hand sketches were developed during the competition phase to describe how the panels could have been operated and how these would interact when grouped together. The final result five years later, after an intense research and development process, looks pretty much the same.
As a first step, extensive thermal modeling and 3D solar analysis of the building were carried out on selected areas of the towers at various times throughout the year, to identify the required extent of the shading system and its regime of opening and closing. This helped derive the precise portion of the North facing facade where the shading was not required. The vision area consists of floor to ceiling high double glazed units, with laminated inner and monolithic outer panes, both heat-strengthened.
In a similar climate the primary issue is clearly controlling the solar radiation, while conductive gains due to temperature difference are less of an issue. On buildings heavily shaded with reduced gains due to solar radiation, however, the conductive component becomes more and more relevant. For this reason particular attention was paid to enhance the thermal performance by specifying Argon filled double glazed units, not common in the Middle East five years ago, and introducing thermal breaks where the brackets supporting the Mashrabiya penetrate the thermal line.
The building physics analyses were also essential to define the right balance between solar control and light transmission performances of the glazing and shading components, and the effect of the combined systems. A clear glass with a high performance coating has been selected, achieving a g-value (solar control) of 0.26 and a light transmission of 44%. Both values are significantly higher than any other similar building in the Middle East. The crown was the subject of a separate 3D solar and thermal analysis and additional measures were required to control the solar gains, due to the architectural desire not to extend the shading to the very top of the building and to the gap between shading and façade line.The glass performance in this area was improved by applying fritting with a variable pattern according to the level of solar exposure.
These exceptional levels of transparency and consequent increased daylighting have been possible thanks to the movable mesh sitting in front of the façade, providing shading only when/where required. Different options were investigated to select the most appropriate fabric for the triangular panels, and PTFE-coated glass fibre mesh was identified as the most durable and bestperforming option. PTFE fiberglass coating is capable of withstanding high temperatures and it is a ‘self-cleaning’ fabric, which helps reducing cleaning and maintenance time. The final fabric presents an open area of 15% and a light transmission of 25%.
Façade design process
The façade is a unitised curtain wall system, with floor to floor high panels of variable width, as the towers are barrelled in elevation and the floor plate dimension varies across the high of the tower. Simplifying the building geometry and achieving the architectural intent was an important step of the design process. The design and overall shape of the building were optimized to improve panel repetition, and to limit rectangularity deviation and warping. This approach helped to significantly reduce the system’s complexity and ultimately the costs.
The façade to floor plate ratio was different on every floor for half of the tower, and this required a high level of coordination between the Architect and our Façade and Building Services teams, managed by maximizing the use of 3D design modeling. Another critical aspect of the façade design was the interface between the unitised curtain wall panels and the cantilever stainless steel arms supporting the shading panels. The spandrel area of one every four panels had to be shaped to allow the units to be installed around the bracket from both sides. Façade panels can be cleaned and replaced via a BMU basket supported by a roof crane and running within the cavity between the curtain wall and the shading system. The curtain wall framing has been designed to accommodate local restraint pins required to keep the basket adjacent to the façade even where the envelope is leaning outward, in the bottom half of the tower. The same basket can run outside the cavity for any maintenance and replacement required by the shading system.
Mashrabiya design process and testing
The most iconic element of the all building is clearly the movable shading, which wraps most of the envelope and modifies its shape, appearance and performance any time of the year, reflecting natural daily and seasonal rhythms. The shading becomes the real envelope and defines the building, with form and function working in close harmony. This incredible effect is the result of a thorough design process.
Hand sketches, paper models, small scale physical models and 3D digital models: these were the preliminary steps required to refine the initial idea and to prove to the design team and to the Client that the concept was feasible. Moving the design forward, we realized a key challenge was the interface between the Mashrabiya and the towers’ superstructure. Various structural arrangements were assessed early in the design, and the creative input from various disciplines including structure, façade, lighting and architecture helped to develop the final option. The Mashrabiya is conceived as a unitised system cantilevering 2.8m from the primary structure, so that each element can be replaced without affecting the structural stability of the all system. The supporting arms allow connection from the tapered ends of six adjoining panels, and each shading device presents different releases at each of three supporting nodes.
Each tower features 1049 Mashrabiya panels, 4.2m high, each weighting about 600kg, 1.5 tonnes including the supporting brackets. No precedents of movable shading systems on this scale were available, and most of our efforts during the design have been focused to de-risk the process, making sure that this unique and unconventional shading system was able to protect the building from the solar radiation and could also operate reliably in an aggressive environment.
As part of the Façade Technical Performance Specifications, a thorough testing regime was specified besides performance targets and materials requirements. The key moment of the whole design period was the construction of a fully functional 1:1 scale prototype by the Façade Contractor responsible for the postTender Design Development phase.
The design was then refined and tested in a wind tunnel facility and in a climatic chamber. More than 30,000 opening and closing cycles were simulated at different temperature conditions, varying from 240C to 600C, and at different levels of relative humidity. Sand and salted water were applied regularly throughout the testing process on all the critical joints to prove the required durability life of actuators, bearings and mechanisms.
The Mashrabiya elements are grouped in sectors and operated by a sun tracking software controlling the opening and closing sequence according to the sun’s position. It is possible to override the system to control individual panels. The control system is linked to an anemometer and a solar radiation sensor, to adjust the position in case of extreme wind speed or prolonged overcast conditions.
The final result of this unique and exciting journey is extraordinary. An enlightened Client, an inspired Architect and more than 300 Engineers across 14 different disciplines worked together delivering a truly integrated design and achieving something never done before. The team challenged conventional thinking and managed to turn a great intuition into reality, enabling a paradigm shift in the design of tall buildings and setting a benchmark to be followed for many years to come. The project secured the 2012 Council for Tall Buildings & Urban Habitat’s (CTBUH) Innovation Award, and it was listed amongst its 20 most Innovative Tall Buildings of the 21st Century. It also featured in the November 2012 Time as one of the ’25 best inventions of the year’. Al Bahar Towers recently won the 2013 Society of Façade Engineering Award.
Project Credits
• Client: Abu Dhabi Investment Council, Abu Dhabi, United Arab Emirates.
• Architect: Aedas, London.
• Multidisciplinary Engineering Designer: Arup, multiple offices (Façade, Structure, Building Services, Civil Engineering, Geotechnics, Lighting, Acoustics, Fire, Wind, Security, Traffic, Vertical Transportation, IT and Comm, Catering Consulting).
• Façade A&M: Reef, London.
• Architect and engineer of record: Diar Consult, Abu Dhabi, United Arab Emirates.
• Cost Consultant: Abu Dhabi office of AECOM (formerly Davis Langdon).
• Project manager: Mace, London.
• Main Contractor: Al-Futtaim Carillion LLC, Abu Dhabi, United Arab Emirates.
• Façade Contractor: Yuanda China Holdings Limited, Shenyang, China.Project
Awards
• 2012 Council for Tall Buildings & Urban Habitat’s (CTBUH) Innovation Award, and it was listed amongst its 20 most Innovative Tall Buildings of the 21st Century
• It featured in the November 2012 Time as one of the ’25 best inventions of the year’.
• 2013 Society of Façade Engineering Award.
• 2013 Middle East Architect Award – best overall building and best commercial buildingAwards
• 2012 Council for Tall Buildings & Urban Habitat’s (CTBUH) Innovation Award, and it was listed amongst its 20 most Innovative Tall Buildings of the 21st Century
• It featured in the November 2012 Time as one of the ’25 best inventions of the year’.
• 2013 Society of Façade Engineering Award.
• 2013 Middle East Architect Award – best overall building and best commercial
Image credits
1, 3, 4a-4d, 5, 6a-6d, 8a, 10c-10d, 13a-13c Arup; 2b, 7b, 9d, 11b, 12a-12e, 14c Aedas; 14a Yuanda.