Construction World March 2017

PROJECT PROFILE

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• Where possible 60% of all reinforcing steel and steel products should have a high-recycled content. • The building will have post tensioned concrete slabs, as opposed to conventionally reinforced concrete slabs. Post tensioned slabs are substantially thinner and have less reinforcing steel in them than conventional RC slabs. This not only reduces the material used in the slabs but also reduces the size of the columns, foundations and the concrete core walls. This will reduce the amount of concrete and reinforcing steel used in the project by more than 10%. • The same principle applies in reducing the size of the core wall through parametric optimisation. Façade Performance glass is used to minimise the mechanical plant requirements and reduce operational costs. Infrastructure and wet services • Rainwater harvesting is future enabled. Rainwater will be drained from the building roof using a Pluvia type system. The downpipes will run vertically in the core and connect in a network in the basement levels to storage tanks that could be filtered and used to supplement the supply of water for flushing of toilets and urinals in future. • A sump in the lowest basement level collects ground water seepage through the retaining walls and is pumped out by means of a submersible type pump to irrigate the PwC gardens and the neighbouring park. This water would have otherwise connected directly to the storm water system and been lost. • All excavated material has been locally reused on the rest of the Waterfall City site. Quantifiable time, cost and quality Optimisation and collaboration The tower has complex geometry and various options have been reviewed, with the help of parametric modelling, to identify the most efficient way of supporting the structure that complies with the architectural intent. “Close alignment with the façade design has allowed a seemingly difficult form to be achieved with a reasonable degree of repetition and coordinated detailing. Buildability and maintainability have also influenced the structural and façade design,” adds le Roux.

Construction programme reduction Programme reduction was taken into account when completing the design of the project. To reduce lead time before breaking ground on site, Arup issued earthworks as a separate package ahead of the rest of the building. For a tall building the programme is governed by the floor to floor cycle time. Slip forming the core adds an early activity to the programme but then allows each floor to be completed more quickly, saving time overall. The use of bonded PT slabs also allowed the floor cycle time to be reduced without compromising the future flexibility of the building. Façade The curtain wall is constructed using the unitised principle in which complete glazed panels are made in a factory and then installed onto pre-set brackets on the building. This means that: • No scaffolding is required, which would have been very costly for the almost 100 m height and 20 m overhang of the top floor over the ground floor. • Curtain wall installation can proceed before all concrete floors are cast • Quality is better controlled in a factory than on site. • Panels can be installed quickly, typically 20-30 panels per day. • Panels are watertight as soon as they are interlocked in position, allowing following trades to commence immediately. Risk management Concentrated solar reflections Le Roux comments: “The PwC façade is concave and twisting. Considering several high profile cases beset with concentrated solar reflection problems (notably those at 20 Fenchurch Street in London – the ‘Walkie-Talkie’ – where plastic fittings on a Jaguar on the street melted in 2013), concentrated solar reflections were identified as a risk for the tower and needed careful study. At the time of design, no general analytical software existed for calculating the intensities of solar reflections. A purpose-built script was developed for this analysis, with the results benchmarked against other Arup studies globally.” Using this script, Arup was able to calculate the magnitude of the solar concentrations in the open areas surrounding the tower and provide feasible and practical solutions to mitigate the impact of the solar reflections. Solutions considered were sunshades on the

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CONSTRUCTION WORLD MARCH 2017