It's easy to see why the spectacular new building at 15 Alice Lane, Sandton, was nicknamed "Dancing Towers” by the team responsible for its design and construction. Its curved silhouette, combined with the striking black and white glass of its façade, make this new Sandton landmark look like it is dancing on the skyline. It's this unique façade – a design first – that makes the team at Aveng Grinaker-LTA Façades proud of their achievement.15 Alice Lane Towers, designed by Paragon Architects, consists of two curved towers linked by an atrium. Each tower has glazed wings which cantilever 2.5m past their gable ends. "It is a building with a dynamic form," says Croydon Schmidt, managing director of Aveng Grinaker-LTA Façades. The company rose to the challenge of manufacturing and installing a curtain wall to meet the architects' requirement for a glazing effect, using black and white glass, to simulate falling paper. Challenges abounded on this project, but Schmidt says that his team consistently came up with innovative solutions. One particularly testing element was the need to cater for the building’s curved "underbelly" on the lower levels, while the upper levels act almost as a roof light.The unitized glazing system was selected as it is erected without the need for scaffolding. “Conventional post-and-panel systems would not have worked because they wouldn’t have got around the curved belly of each façade,” says Schmidt. “By using the unitized system, we were able to complete this testing installation within the demanding timeframe.”
Schmidt reports that the first unit was installed on 1 October 2009, and the last on 24 July 2010. A total of 9 569m² of glass incorporated into 2 997 curtain wall units make up the glass façades. Each curtain wall unit weighed around 600kg and most are over 2m wide. Aveng Grinaker-LTA Façades' glass supplier (GSA) invested in state-of-the-art machinery to handle the fritting of such large pieces of glass.Every single row of glass is a different size, with varying combinations of glass type – in pattern and colour. This presented a significant challenge in manufacturing the curtain wall units and quality checks and balances of the highest order were required. The use of bar-coding assisted Aveng Grinaker-LTA Façades in achieving this end.
A further challenge for the curtain wall team was the fact that the starting point for the installation was actually in mid-air, above the atrium entrance level. The atrium is 10 storeys high; the towers are joined with link bridges; and each floor is at a different angle to the bridge below. This creates a fan-like effect. With each bridge link being different, interfacing to the interior required ingenuity in terms of glass alignment and closures.On the east and west elevations, dark grey glass has been used to create slashes across the façades. This was achieved by introducing 400mm-deep composite panelled boxes – so-called "pillows". Each one is a different shape and size and is mechanically fixed to the unitized curtain wall. Here, the curtain wall incorporating the angular "slashed" glazing acts as the weather line. Aveng Grinaker-LTA Façades worked closely with the façade design engineers, Pure Consulting, and the project team to assist in the design and development of this innovation. The blocks restrict the heat transfer on these elevations that are most exposed to the sun and the slot windows are recessed by 400mm. The original vertical slot windows developed into cut-outs in the façade.
Cleaning and maintenance are important considerations for the façades of all high rise buildings. At Alice Lane Towers, specially designed, concealed guide rails have been incorporated into the unitized glazing system to carry a glass replacement carriage and cradle. Cleaning of the façades will be done by abseiling cleaners using a rope-rigging system combined with the guide rails.The curtain wall is also energy-efficient and will ensure operational cost savings for the occupants. This is a key consideration for Aveng Grinaker-LTA Façades whenever its input is sought on design and development for new building projects.
Schmidt says that during the initial design stages of Alice Lane Towers the newly launched pilot version of the GBCSA’s Green Star rating tool to enhance energy-efficient design for office buildings was used. 15 Alice Lane outperforms most conventional buildings (300 kWh/m²) in terms of its energy efficiency, though it falls 25 kWh/m² short of the South African National Standard 204 requirement of 200 kWh/m². Several factors contributed to achieving this low level of energy consumption, including the use of low-E double glazing in the façades. Fritting the façade panels to create shading, the use of well-insulated spandrel panels, and minimal glazing in deep-set windows on the east and west façades, also helped."This is a project that we are very proud to be able to stand back and admire now," says Schmidt. "We learnt a lot: from the complexities of doing a curved façade, to the intricacies of putting a 'suit of clothes' onto split high-rises joined by a large glazed atrium. We worked with the project team in the design and development of the façades and we rose to the challenges presented. I am confident that we are well-equipped to tackle the more complex curtain walls that we foresee becoming more widespread in the industry," he concludes.