Construction World May 2017

PROJECTS & CONTRACTS

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 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. 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.

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 is the construction material of choice for twisted buildings, as it can create the desired twists and turns while providing the required structural strength.

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CONSTRUCTION WORLD MAY 2017