The Mall of Africa is a 131 000m2 development and is currently South Africa’s biggest single-phase shopping mall development.
The central skylight is located at the heart of the mall and consists of 4,420m2 of ETFE air-filled pillow (AFP) cladding supported by a Novum free form (FF-System) steel grid. The structure is 170m long in the North-South direction and varies in width from approximately 12m to 50m (in the central portion).
The complex free-form steel grid was achieved through the use of 1357no. unique painted hollow steel sections which were bolted together to 507no. custom machined steel nodes. Each steel beam had steel casting that was shop welded onto each edge that assists in creating this geometry.
Due to the length of the structure, the skylight was divided at the building expansion joints into three structurally isolated portions. In each section, the steelwork transferred load via bending and arching action into the perimeter concrete parapet (design by Aurecon). Due to the large spans in the central region, four tree columns were introduced to ensure all the roof steelwork could have the same outer dimensions.
Steelwork was selected for this skylight due to its high strength to weight ratio and the ability to shop fabricate all the components so they could be bolted together in the field.
Hollow rectangular tubular sections were used for architectural, detailing and structural reactions. Hollow sections enabled us to use hidden bolted connections which also served as a means to hide lighting conduit. Rectangular tubes were used due to their good torsional resistance, high strong axis bending capacity and short weak axis dimension – this helped to reduce the size of the nodes at connection points.
Field welding was only required at the perimeter of the skylight (for tolerance reasons), so the vast majority of the connections were bolted together using a calibrated torque wrench and a custom made chain drive. This not only reduced the installation duration, it also ensured the geometry of the skylight would not change during installation. This is very important as not only is all the cladding is pre-made based on this geometry but if the form of the skylight changed during installation, it could change the stress distribution within the grid shell.
From the architect’s original design intent, it is important that the geometry of the skylight undulated in both plan and section view. Between MDS and Novum we explored numinous grid and surface options before the final grid was selected.
ETFE cladding was selected as it enables the use of a large triangulated grid as well as provided the desired thermal and visual performance properties. A mixture of two layer clear/printed and opaque white ETFE pillows were selected and distributed over the skylight to avoid glare at the shopfront, while still letting in sufficient light into the space. The pressure in these pillows is maintained by three air machines located on the roof, adjacent to the skylight, and through hundreds of meters of flexible airlines.
The steelwork required for this project was fabricated both internationally and locally and required a high degree of precision. In addition, due to the large number of unique components (some of which appeared to be very similar), the logistical requirements of making, transporting, sorting and correctly installing all these components required special attention from the site and project management teams.
The steelwork was installed using a combination of a small self-erecting crane and a 8 tonne mobile crane. Due to weight limitations, the steelwork was installed in small “spider” sections consist of a few beams bolted to a node. A series of shoring towers were erected to provide temporary support during Due to the open span of the central portion of the skylight (approximately 52mx51m) this section was the most difficult and time-consuming to install. This was further complication by site limitations restricting the use of our cranes, so a few tubes needed to be installed by hoisting from the partially installed frame.
To accommodate the tight installation tolerance, the elevations of numinous nodal points needed to be continuously checked as the structure was installed from one end to the next. If this was not done, the support node on the far end of the structure would not be correctly aligned over the perimeter embed plate. the installation.
In the preliminary design phase, Novum carried out various different costing models based on different grid sizes to try to reduce the overall budget. If the grid was made smaller, it would have driven up the cost of the skylight as it would have resulted in significantly more material, connections and a longer installation time. If the grid was made larger it would have appeared too segmented and lost the free-form appearance.
The combination of a free-form structure with triangular ETFE pillows on this size is highly unique. As Novum Structures, this was our largest ETFE projects that we have installed in South Africa and our first free form project. As a result, we needed to develop a number of custom details specifically for this project and carried out offsite testing to ensure these details worked adequately.
The steelwork was installed by a fully local crew and the ETFE was installed by a local team with some assistance from one of our UK based ETFE supervisors. Netting was used for the installation of the ETFE pillows.