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STRUCTURE

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The incorporation and re-use of existing basement structures, the significant cantilever of the building at level two, and the accommodation of the deflections between the main building and the bronze facade, were just some of the technical challenges that had to be solved within strict cost and time constraints at 60 Queen Victoria Street.

The articulated bronze facade on Queen Street and Queen Victoria Street combines panelised curtain walling technology with an external framework of bronze columns and beams. These columns support the high-level sunscreens and, in conjunction with the cantilevered beams, provide lateral restraint to the frameless apex of the chevron windows.

The large cantilever assembly beneath level two transfers facade loads carried by the external bronze columns back to the perimeter foundation, with the counter-balancing upward thrust principally carried by the level- two floor structure.

The building is founded internally on pile foundations, with the perimeter on the previous 1950s building's retained strip footing, reinforced with a capping beam and short piles connected to the foundation edge. For the combined foundation to work a proportionate distribution of load between the new pile and the existing footing was required. The pile deflection characteristics were matched to those characteristics predicted for the footing.

The typical floor structure consists of 100 thick slabs, 350 downstand beams at 2.5m centres spanning 8.45m between primary edge beams and a central spine beam. Internal columns are 550 diameter with 300 diameter on the facades. The building stability is provided by shear walls around the main core lifts with an additional wall in the north-east core to counter the torsional effects of loading on the main facades.

Concrete was chosen primarily because:

the site is highly irregular on plan and concrete is the most flexible material

the shaped perimeter of the typical floor slab edges determined by the facade design was more easily done in concrete

small-diameter expressed circular columns were required

the ability to shape elements to act as bi-planar transfer cantilevers at the second floor, where the facade is inset by 1.40m and the edge column spacing of 3.75m above transferred to 7.5m below, would be more easily achieved in concrete.

The main disadvantages associated with concrete structures are weight and size of vertical structure. In the case of 60 Queen Victoria Street it was important to minimise the weight not only to reduce load on the transfer cantilevers but also on the perimeter strip foundations. In the end the dead load of the floor structure was just about 4.0kn/m2 and the column sizes were reduced by increasing the concrete strength to 50n/mm2. Higher strengths were considered but it was not possible to procure these in the time available.

Ian Stephenson, Alan Baxter & Associates

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