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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.