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CONSTRUCTION WORLD
MARCH
2017
34
PROJECT PROFILE
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
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
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
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.