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CONSTRUCTION WORLD
MAY
2017
20
PROJECTS & CONTRACTS
Concrete is the construction material of choice for twisted
buildings, as it can create the desired twists and turns while
providing the required structural strength. It also suits the stylistic
intentions of these buildings, which are to achieve a more organic
shape; looking more like a sculpture than a traditional block shape.
Visible from a 30 km radius, the R1,5-billion PwC Tower will
comprise 45 000 m
2
of office space and house 3 500 employees.
Apart from its unique design, the building is also required to
contribute to environmental sustainability by consuming less energy
and having a minimal impact on the environment.
Structural challenges
The biggest structural challenge was that the twist causes the
gravity load to naturally create a clockwise torsional load on
the building. To counteract this load, the designers Arup made
unique use of parametric modelling software to establish the
optimal solution.
This solution comprises five structural columns on the façade
of the building every five floors. The structural columns slope in a
counter-clockwise direction around the core and span from floor to
floor (3,74 metres in height and 750 mm in diameter). The concrete
strengths of these columns range from 50 MPa (from the basement
levels to level four) to 40 MPa at the mid-levels (level 5 to level 14)
and 30 MPa at the higher levels (level 15 to level 26).
Arup’s structural solution was aligned with the aesthetics of
the façade design. The five structural columns along the slab edge
resulted in a favourable perimeter span of eight metres; the decision
to repeat the column series every five floors allowed the relationship
between the columns and the façade units to remain constant. In
other words, each of the 25 possible column positions relative to the
slab edge align to the centre of a façade panel; if one follows any
column up the elevation of the building, it will always be central to a
façade panel at each floor.
Concrete supply
As the concrete supplier to this complex project, AfriSam designed
19 project-specific and special application mixes with an average
ordinary Portland Cement replacement of about 30%. Also, each
slide concrete mix had normal and retarded versions to satisfy the
different applications; the trial mixes were produced at AfriSam’s
Jukskei laboratory.
Each of these mix designs used a unique eight digit code,
specifically created for the PwC project in order to make sure that
the correct concrete was ordered during each stage of the daily pour.
Concrete was delivered from AfriSam’s Jukskei A dry readymix plant
backed up by Jukskei B wet batch plant, both situated at the Jukskei
Quarry in Midrand, close to the PwC Tower site.
Both readymix plants have fully automated command batch
computerisation, so it was possible to modify concrete designs
remotely and at any time as the project demanded. Batch
accuracy was within 1% on all materials batched, so there was
no room for errors.
The close proximity of the readymix plants to the site made the
planning and deliveries to site more convenient. AfriSam’s Jukskei
A Plant was the primary supply plant while its Jukskei B Plant acted
as the back-up, allowing for concrete to be delivered to site around
the clock.
During certain construction phases, concrete was supplied
24 hours a day, with three readymix trucks on a permanent
turnaround basis. These readymix trucks had the ability to efficiently
mix the concrete at the Jukskei A plant and again on site.
CONCRETE DOES THE TWIST
The iconic PriceWaterHouseCoopers (PwC) Tower in
Midrand – Africa’s first twisted building – placed a
number of unusual demands on concrete during its
construction; AfriSam and CHRYSO Southern Africa
delivered the concrete solutions.
Concrete is the construction material of choice for twisted buildings, as
it can create the desired twists and turns while providing the required
structural strength.
The five structural columns along the slab edge resulted in a favourable
perimeter span of eight metres; the decision to repeat the column series
every five floors.