Melbourne’s construction industry is predisposed to concrete. This is always post-tensioned, reducing floor-to-floor heights and adding strength – spans of 8m x 8m are typical for apartments. At Premier Tower, the flat plates are 200mm bonded post-tensioned slabs, allowing floor-to-floor heights of 3m, with ceiling heights of about 2.7m. The shape of the floorplates varies throughout the building, so walking columns were used to transfer loads to different parts of the structure. At the deepest curves, the slab edge on the line of the corner columns varies by 5m. The internal edge of the columns remains in the same place, while the external is stepped to support the longer cantilevers. The result is that the corner columns change in section from 800 x 800mm to 3,950mm x 300mm in six steps. Along the shallower curves, the internal columns walk in and out to fit around internal layouts and doorways.
Below ground, there are four basement levels with car parking. These are broader than the tower, so the columns transition horizontally as they reach the podium, by approximately 4.5m over a height of 18m, or six storeys. “If we’d kept the column locations the same as in the tower above, they would have clashed with the aisle in the car park,” says Hindmarch. “By transitioning them, we can locate them between parking bays instead. This also increases lateral stiffness, and it meant we didn’t use any transfer beams, which reduced structural height and construction time and cost.