With valuable land in city CBDs around the world getting smaller, as well as scarcer, how do you maximise their development values and are skyscrapers viable on these compact sites?
As well as sites getting smaller, so are apartments. But do they need to? How do we maximise floorspace to improve returns without compromising design and comfort?
Incredible shifts are happening in technology and building practices. We have a new generation of technology pushing the limits of concrete and steel like never before.
But for engineers to continue to lead, they must provide compelling answers to the question: ‘Can we take what you want, and give you more?’
For developers, it’s more sellable area. For architects, it’s more space with less structure. For builders, it’s increased speed of construction, while at the same time ensuring health and safety.
At their very core, structural solutions for skyscrapers have to be simple, innovative and cost competitive.
Melbourne is in the midst of a skyscraper construction explosion. Three of the five tallest under construction are nearing completion; the 72-storey Prima Pearl on the South Bank, the 68-storey 568 Collins Street; and the 58-storey Abode 318 on Russell Street.
Abode 318 and 568 Collins Street both sit on compact sites.
With a height to depth ratio of 9:1, Abode 318 on Russell Street in Melbourne’s CBD, could almost be classified as super skinny or pencil thin skyscraper (this is usually defined at a ratio of 11:1)
To achieve the requirements for 58-storeys on such a small site, optimising the building’s stability solution was critical. Wind tunnel testing and Finite Element Analysis were therefor conducted early in the project.
This early analysis highlighted the challenges associated with wind acceleration and revealed an overall building natural frequency on the borderline for occupancy comfort.
“We incorporated provision for a tuned mass damper to be located at the top of the building, while at the same time optimising all of the building’s primary elements and lateral mechanisms, including car park ramps, lift cores and shear walls, in a computer model,” explained Vincent Amato, Senior Structural Engineer.
“When the structure was 80% complete the actual building frequency was measured using a ‘drop test’. One of the building cranes safely swung a weight above the structure and the building response was measured.”
This test revealed the dynamic response of the building to be acceptable without the need of the costly tuned mass damper, which could have cost the client $100k-$150k.
“It was also important to not underestimate the importance of floor systems,” added Amato. “When you have literally acres of the same footprint, it’s a significant cost to the overall project.”
The horizontal and vertical wave form of the Abode façade meant that every floor had a different geometry to the floor below and the floor above.
An economic floor plate design was achieved by seeking order in the waves and adopting a 10 floor repetition of the floor plate with the majority of the floor plate remaining consistent and variation being limited to the edge. This allowed the contractor to achieve economies of repetitive design with a bespoke outcome
The 68-storey, mixed-use tower at 568 Collins Street meanwhile is set on a plan dimension of only 30m x 40, which is very small for a building of this height.
Traditionally in the Melbourne market high rise towers get their lateral stability solely by the central service core. On this site, however with the limited space the client wished to maximise the sellable floor space whilst not impeding the views with bulky structure.
By working closely with the architects and client in the early stages of the project a transfer-free structure has been created. This is rarely achieved in a mixed-use tower due to the floor plates requiring different functional grids for various uses.
“We were able to achieve this feat by use of advanced long term shortening and shrinkage
modelling to predict and compensate for an uneven loading pattern to the structure,” explained Amato.
“We called on our experience in the Middle East and Asia and adopted an outrigger system, which alleviated the core of all the work and kept the façade free of bulky structure. With an outrigger system in two locations (top and bottom), we were able to shrink the core and give more sellable space back to the client. “
“In addition, a retention system without temporary ground anchors accelerated construction and avoided potential damage to surrounding services.”
The whole approach also reduced the building’s overall height while keeping the desired number of floors and reduced the cost to the developer.
Located across from the Crown Casino, the prestigious 72-storey Prima Pearl apartment tower is nearing completion. The mixed-use building includes an indoor swimming pool, sauna, spa and gymnasium, private cinema, lounge and library, 8 –level podium car park, virtual golf driving range and level-67 sky lounge.
Unlike Collins Street, this tower has the core doing all the work. Nevertheless, there are still opportunities to optimise space.
“We considered the impact of small internal core walls on the overall building stiffness,” said Amato. “And despite receiving calls from the contractor’s steel scheduler to check what we had submitted was correct, we were able to reduce the required reinforcement and wall thickness of the main external core walls.”
At the top of the building, on the penthouse floors, the client demanded even more.
“We have changed the shape of the floor plates above level 30 from concave to convex, which has created larger floor plates for the premium apartments,” said Amato. “This structural floor plate design, which we developed, easily accommodated the changing building footprint without compromising a typical design philosophy for fast construction.”
This is clearly where the benefit to the project translates to added value back to the client.
It is literally possible to grab real estate out of the sky.
Vincent Amato is Associate – Structures in the Melbourne office