Temporary works specialist contractor Efco UK operations manager Nick Williams says the cantilever support system currently carrying access scaffolding for the cladding of the distinctive residential 'pod' is the first of its type the US-owned company has ever done.
Key to the system is the full moment bolted connection between Efco's standard steel formwork panels. This allows different sizes of plate girders to be built up piece by piece to match the needs of each location. On Unity the largest structural girders are 2.4m high, made up of panels measuring up to 2.4m by 3.6m, bolted together back to back to create a form of I section.
'These carry the biggest loads at the corners where the slab cantilevers in both directions, ' Williams explains. 'A maximum load of 120t is transferred to the 24th floor at the corners. Steel props are needed to distribute this load into the main structure of the building over two oors below.' Assembling the grid of girders was relatively straightforward. The inboard sections were built up insitu between the 24th and 25th oors. Outboard sections were prefabricated on the ground and craned up into position before being bolted onto the inboard sections just inside the building line. Dismantling it all will be trickier, with crane access severely restricted by the pod structure. A complex arrangement of counterweights will be needed to allow sections of main plate girder to be slid out beyond the line of the pod and lifted away.
To pour the slab Efco's aluminium EZ-Deck system was built up off the main girder grid. This has now been removed, but the exterior access provided by the grid is still vital to the completion of the pod.