Construction World March 2017

PROJECT PROFILE

JOHANNESBURG’S NEW ICON The horizon of Midrand, north of Johannesburg, is set to change with the construction of the new high- rise head office for PricewaterhouseCoopers (PwC). The R1,5-billion project will be a focal point

It is set to become a structure of iconic proportion due to its distinctive twisted form designed by LYT Architecture for Attacq Waterfall Investment Company and their developer Atterbury, and realised through close collaboration with consulting engineers Arup. The structure is a concrete frame with post-tensioned (PT) floor plates and each floor of the 28-storey office tower rotates 1,2 degrees relative to the floor below. Many of the design challenges and innovations on this project relate to this twist. Richard Lawson, project manager and associate at Arup adds: “As the 26 th tallest twisted building in the world, and the only twisted building in Africa, the PwC Tower has 26 twisting office floor plates, five basement levels and two plant floors. Our scope on the project is for structural, civil, façade, wet services and transport design”. Construction innovation technology Slip forming The core of the building was constructed using a method called slip forming, where the concrete is poured continuously within steel shutters – creating a structure with no joints - which moved up at a pace of 2,5 m a day (+- 100 mm an hour). With this particular project, the sliding team worked continuously in two 12 hour shifts, 24 hours a day. At any given time, there was a minimum of 62 workers on the slide shutter itself, and the sliding team had their lunch/dinner on top of the slide. The core wall was constructed in 43 days with an average of 90 m³ concrete placed per day (roughly 3,7 m³ per hour). Due to the duration and intensity of the contract, there were three planned weekend stops to give the sliding teams a break. “By erecting the core in one operation early in the project, construction time was reduced as the floors and columns could be built without waiting for construction of the walls. It also allowed more time to install the lifts and central services. Internationally this construction method is relatively common for buildings over 10 storeys, but in South Africa it has most recently only been used for forming chimneys and cooling towers,” explains Lawson. Façade details The Arup façade engineers worked closely with the façade contractors Geustyn & Horak on a new suite of extrusions specifically for this project, which is the first curtain wall in South Africa with twisted unitised panels. The twist caused design challenges at the façade intersections as the frames needed to be warped, but the glass needed to be flat. An aesthetically pleasing solution was achieved by adjusting the setting out of each mullion precisely to create parallel straight profiles on a warped surface. With this approach, any geometric abnormalities are hidden between levels below the sills. Cleaning and maintenance gondola At some points along the façade, the curvature causes overhanging in the up and coming Waterfall City development and comprises 45 000 m² of modern offices, designed to house 3 500 PwC employees in an efficient and optimally designed workplace.

floors, which means that 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. Arup thus 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. Design innovation Parametric design Parametric modelling can be described as modelling a structure or object in an n-dimensional space, where certain chosen parameters of the structure are adjustable. In other words, it makes it possible to explore the impact of any of the input parameters on the design and cost of a structure. Explains Lawson: “The biggest structural challenge was that the spiralling form of the tower causes the gravity loads to create naturally a clockwise torsional load on the building. The traditional

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CONSTRUCTION WORLD MARCH 2017