A recent conference on high rise structures revealed a creative shade solution.
By Mark Zeh
In June 2012, the Al-Bahar Towers, designed by the international architecture firm, Aedas of London, England, opened in Abu Dhabi, United Arab Emirates. These twin towers are located on a gateway site on the Eastern Ring Road, between Abu Dhabi and its international airport. The towers are the new headquarters for the Abu Dhabi Investment Council, providing working space for more than 2,000 employees.
A standout among its many interesting architectural features is its dynamic adaptive sunshade façade, potentially the largest adaptive architectural structure in the world if not the most-complex. The sunshades are composed of approximately 1,050 triangular fabric panels per tower. These are arranged in a tessellated hexagonal pattern on 2m standoffs around the sun-exposed portions of the glass façades of the buildings. Each of the hexagonal clusters acts as a single unit, controlled by an individual actuator, which allows the unit to be opened or closed in response to solar loading.
“The project brief was quite prescriptive,” says Peter Oborn, deputy chairman of Aedas. ”The client’s brief called for two 25-story towers on two adjacent sites, the use of modern materials, something which referred to the architectural heritage of the region, and an outstanding landmark building.”
The inspiration for the hexagonal pattern comes from the mashrabiya, a type of sunshade grille pattern found in Islamic vernacular architecture. The design team combined a basic origami pattern with the mashrabiya pattern to create this innovative sunshade concept. The result is a 50% reduction in solar gain on the glass façade and a 20% reduction in the amount of electricity that would be required to cool the building without the sunshade.
A key to successfully executing the sunshade concept was selection of an appropriate material for the triangular elements. Aedas collaborated closely with engineers and material specialists from the project engineering firm, Arup of London, England, to select the fabric.
“We looked at a range of materials including ETFE, polycarbonate, FRP [fiber reinforced plastic], a metal mesh, glass honeycomb sandwich and architectural fabric. There were a number of drivers and key characteristics, including light transmission, density, mechanical properties, UV light resistance, maintenance and obviously cost,” explains Konrad Xuereb, lead structural engineer for the project, from Arup of London. “Architectural fabric seemed to best fit most of the parameters, so we then looked more-closely at PVC/polyester and PTFE/glass fiber mesh fabrics, with the result that the PTFE/glass fiber mesh material was deemed the most-appropriate.”
“One of our goals was to preserve authentic views from the interior, even when the shades were closed,” says Oborn. “The fabric we’d selected [from Chukoh Chemical Industries Ltd, of Tokyo, Japan] is around 30 to 35% light transmissive, which, from our perspective in northern Europe, wouldn’t seem to offer much in the way of transparency. However, when you experience it in the strong sunlight of the desert in Abu Dhabi, particularly from a distance of 2m, it is like looking through a light net curtain: You don’t have a sense of being obstructed.”
Another key partner in final design, detailing, testing and construction of the mashrabiya structure was Yuanda China Holdings Ltd of Shenyang, China. Its engineering team at Yuanda Europe built an entire material evaluation and accelerated performance testing facility for the project in Basel, Switzerland.
Creating large structures with so many individual dynamic external components in a coastal desert environment presented the design team with serious challenges to resolve.
“The biggest challenge in building these structures, in my opinion, was to create a sympathetic structural solution to support the mashrabiya (which cantilever from the glass façade), without building something that is too cumbersome or visually intrusive,” says Xuereb. “Another key challenge was to conceive a solution that neatly resolved how adjacent triangles meet at the nodes to allow the releases required for the unitized panels to open or close. Each triangular panel has different end constraints to the ones adjacent to it.”
“The stubs that connected the cantilever arms supporting the nodes to the structure weren’t complicated assemblies, but they were critical assemblies,” adds Oborn. ”They had to be designed in a way that they could take up any dimensional tolerance.”
The Al-Bahar Towers project has won multiple awards for its innovative dynamic sunshade and certainly serves as inspiration for how buildings can be designed in a more sustainable fashion.
“I would hope that this building doesn’t so much provoke conversation around dynamic façades, but rather around the design of curtain wall buildings,” says Oborn. “The issue is really more that we can’t keep designing the way that we used to because it’s just too energy-inefficient.”