One of Melbourne’s tallest and most prestigious developments, this project is best known for its inspiration: Beyoncé's video ‘Ghost’, which features writhing dancers tightly shrouded in fabric.
The result is an elegant, amorphic form, designed by Elenberg Fraser, that sits on an island site opposite the city’s main train terminal. Expected to rise to 78 storeys, it will include no less than 1 million ft2 of space, comprising 780 one- and two-bedroom apartments and 180 hotel suites, as well as a range of leisure facilities.
Recreating those sinuous curves in glass, concrete and steel was no mean feat for the structural engineer, especially while minimizing intrusion on the usable floor area and internal layouts. “It’s in a prime location, and it’s got a very distinctive variable form,” says Peter Hindmarch, director at WSP in Melbourne.
Premier Tower is also a slender structure – the ratio of its height to structural width is 8.3 from the ground up, but a much more challenging 10.8 above the podium. To maintain the building’s movement in the wind within acceptable levels, mega-columns on the façade maximize the width of the stabilizing structure, and these are tied to the core by two- or three-storey outriggers concealed in party walls, and secondary outriggers at the mid-height plant floor. “The mega columns are sized to carry both gravity and wind loads,” explains Hindmarch. “The wind load forces can be equal to the weight supported by the column. Controlling the acceleration is the most important thing for a residential building, so we tested it extensively in a wind tunnel.” There is provision for a tuned mass liquid damper at the top of the building, to further slow the movement, and this will also double as the fire tank.
Post-Tensioned Concrete for Added Strength
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.