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STRUCTURE

From a structural perspective the two towers are quite different from each other, but both have combinations of pile and pad foundations to suit the sloping rock-head levels. Both towers have a deep basement constructed from a contiguous piled wall which was chosen to take the traffic surcharge loads from above and to avoid the need to prop the basement wall during construction. The 27-storey residential tower has three levels of transfer structure: the first consists of transfer walls above the car park; the second of transfer walls over the eight-storey block of flats at the tower's base; and the third is a transfer plate below the penthouse pod.

The slender tower is constructed from a series of cross-walls and slabs, with trimmer beams incorporated within the depth of slab to increase the speed of construction, ease the passage of services and allow tunnel-form construction to be used. The tunnel-form construction involved reuseable shutters to form the cross-walls and slabs above, before being lifted along the floor plate to the next position. This form of construction was quick, gave the concrete a good finish and allowed all service/stair voids to be formed as part of the process. The penthouse pod cantilevers up to 5m over the main tower at the rear and was supported off a reinforced concrete transfer plate. The pod itself was constructed from steel with moment frames and bracing to provide stability. Various options were considered for the complex works until Laing O'Rourke decided to use a system previously used on bridge-repair contracts. This system was capable of taking the loads off the transfer slab, which weighed over 250 tonnes. The 16-storey office tower was constructed from slip-formed cores, post-tensioned flat slabs and reinforced-concrete columns. To give the maximum riser size around the lift cores, special nibs projected out from the cores and spanned vertically from floor to floor to support the post-tensioned slabs around the core. Careful consideration was given to the slab design. In order to maintain a shallow overall depth, three down-stand beams were used at each level. Around the main entrance, the edge of the slab was stepped back and supported off hanger posts from the slab above, allowing double-height columns around the entrance. The facades for both buildings were designed by specialist subcontractors, but we were involved in integrating the window-cleaning systems.

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