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Realised through Arup and designed by LYT Architecture, the 28-storey PwC tower was the result of a design brief for an iconic building that would ignite the Waterfall City development as a new top tier business destination.
The building’s twisting shape posed a variety of design challenges for our structure and façade teams which they solved using parametric modelling. Our wet services and site development design were integral to achieving LEED silver rating and our transport planning was key to the upfront development.
Achieving the twist
As each floor of the tower rotates at 1.2º relative to the floor below, the biggest structural challenge was the spiralling form that caused a gravitational torsional load on the building. Our parametric modelling quickly and effectively explored a number of design solutions to find one that best achieved balance between structure, aesthetics and floorplate plan. The final scheme incorporated structural columns on the façade of the building. These columns slope in a counter clockwise direction around the core, balancing the gravity loads on the corner columns and reducing the torsion on the core of the tower.
Project Summary
87% of gravity induced torsion
15unique geometries
20%+ reduction in core wall thickness
Wrapping flat glass around a twisting structure
We also used parametric modelling to consider a number of façade solutions and we made real-time adjustments to spacing and angles that could be displayed instantly in 3D. Our model integrated factors such as glass utilisation, aesthetic integration and integration of blinds with a sloping curved façade. The twist meant façade intersections were complex. The frames needed to be warped, but for economy the glass needed to be flat. Our modelling provided an aesthetically pleasing solution by adjusting each mullion (a vertical element that forms a division between units) precisely to create parallel straight profiles on the warped surface, with geometric abnormalities hidden between levels.
Concentrated solar reflections
Because the façade is concave and twisting, we identified that concentrated solar reflections were a potential risk. As at the time of design, no software existed for calculating the intensity of this, we developed a purpose-built script for this analysis. The script helped us to calculate the magnitude of the solar concentrations in the open areas surrounding the tower and then test practical solutions to mitigate the impact of the solar reflections in problem areas.
“The team at Arup brings international experience to the project, although they are all local professionals. The kind of systems and thinking that they were able to apply to the design really made that building possible, so it’s as much their design as it is ours ” Guy Steenekamp Director, LYT Architecture
Cleaning and maintenance gondola
At points along the façade, the curvature causes overhanging floors, meaning a conventional cleaning gondola could not be used. Rope access may have been possible for cleaning, but for glass replacement, a suspended gondola is necessary. We worked with specialists, Riggers Steeplejacks, to develop an integrated system where a guide channel is incorporated into the mullion extrusions to guide a cleaning gondola over the height of the building.
Slip forming the core wall
The core was constructed using slip forming. Internationally this construction method is relatively common for tall buildings, but recently in South Africa, it has generally been used for forming chimneys and cooling towers. By erecting the core in one operation early on in the project, construction time was reduced because the floors and columns of the building could be built without waiting for the construction of the walls. It also allowed for more time to install the lifts and central services.
