The undulating ETFE roof, breathing over the Mall of Africa, features individually shaped pillows in a precision-engineered steel framework.
The spectacular ETFE portion of the Mall of Africa roof is a highly specialised, precision installation. Consisting of steelwork that fits together like a puzzle and triangular air pillows in 846 different patterns, each piece of this meticulously engineered system had to be fitted perfectly in place.
Three air machines, guided by pressure sensors, maintain constant air flow in these pillows. During colder mornings, more air is required than at midday, when the air expands. It is a constant balancing act which makes the system seem as if it is breathing.
Andrew Spottiswoode, engineering manager at Novum, explains that each section of the roof is different. The Novum pillows were individually shaped and, based on extensive solar studies by MDS architecture and the professional team, some of the layers incorporate a shading pattern in order for the roof to achieve the optimum overall solar performance.
The sections for the steel frame were manufactured in the United States, but were assembled together with the ETFE cushions locally. The steelwork and ETFE pillows were installed by more than 40 workers from Novum Structures, Xel, TASS Engineering and Universal Steel.
Each node, which connects specific pieces of the steelwork, had to be installed in precisely the right place and be orientated in the right direction so that the right beam can be field bolted onto it. Should one piece be out of place, the rest of the puzzle wouldn’t fit.
A local success
Spottiswoode points out that the installation of the central portion was especially challenging due to crane lifting limitations and over 50m free span. This meant that the frame could only be installed three beams at a time. In order to accomplish this, 12 shoring towers were assembled and the installation team needed to continually survey the elevation of the structure to ensure it would land at the correct location on the other side of the opening.
Installing north to south, our team installed approximately 20 beams per day,” says Spottiswoode.
“The guys who worked on the installation were exceptional. They worked safely and every single beam and ETFE pillow was installed perfectly. Getting the complex clad grid shell installed with our local teams, to an international level of quality, was a very satisfying feat.”
Designing a freeform roof
To streamline the form finding of the ETFE membrane, Novum developed new software to design the pillow shapes and enable multiple patterns to be cut at the same time.
This execution also needed careful consideration since even tension in the ETFE membranes is crucial to avoid unwanted noise and leaks. With triangles, this is quite tricky to achieve in the corners of the pillow, so it required very exact engineering and fabrication to ensure that all the pillows fit flawlessly.
“Designing this roof was a lot of fun,” adds Spottiswoode. “The geometry was a key factor and every little piece needed to match up exactly to the next one in order for the roof to be installed successfully.”
Considering architectural intent
When incorporating a free form ETFE roof into a building design, it is imperative to keep in mind that once the design and budget are done, no changes can be made midway through. Because it is such a precision-engineered system, the outcome must be agreed upon from the start.
“MDS Architecture was a great firm to work with,” says Spottiswoode. “We had robust discussions at the beginning of the project to fully understand the architectural intent, and from our side convey the expected outcome. It was crucial to be involved from the design stage, because from there onwards the project ran very smoothly.”
Novum has two teams that are expertly trained to do complex steel and ETFE installations. To retain these skills, the teams work on projects locally and across the globe.
Tel: 011 462 5701