Once Sydney’s tallest building, the AMP Center was showing its age. The outdated structure of the 1970s was at the end of its life, and the tower’s owners wanted to replace it with something bigger, better and more energy efficient.
The resulting tower, dubbed the world’s first “recycled” tall building, opened earlier this year and was named World Building of the Year 2022 on Friday. The Quarter Tower retained more than two-thirds of the old structure and 95% of the original building’s core, including beams and columns.
“The tower was nearing the end of its life in terms of viability … but the structure and the ‘bones’ could actually last much longer,” said Fred Holt, partner at Danish architectural firm. design, 3XN, in a video interview. “You can’t always have everything on hand. But if you can maintain the structure – and that’s where most of your embodied carbon is here – then you’ve reduced your footprint.”
After the construction workers removed the parts of the old building that could not be salvaged, they erected a new structure next to it and then “grafted” over the remainder. A contemporary glass facade is wrapped around both to create a single skyscraper. The new design has doubled the building’s available floor space and in turn increased the number of people it can accommodate from 4,500 to 9,000.
The architects believe their approach has saved 12,000 tons of CO2 compared to tearing down the tower and starting from scratch – enough to power the building for more than three years. In addition to reducing the use of carbon-intensive materials such as concrete, the plan may have saved up to a year in construction time.
“The greenest building is the one that already exists,” said Holt, quoting Carl Elefante, former president of the American Institute of Architects.
The ambitious project, completed by 3XN together with engineering firm Arup and Australian architecture firm BVN, posed a series of design challenges. The first was simply to determine if the existing building fit the original design.
Tall buildings often shrink under their own weight, especially in the first few months after completion. As a result, the old AMP Center was “on a slightly different level than it was in the drawings,” said Holt, explaining that the concrete “spreads and falls” as it dries completely.
The design of the new tower consists of five volumes stacked on top of each other. Credit: Courtesy of 3XN
“There are always a lot of unknowns when you start robbing an existing tall building,” added Kim Herforth Nielsen, founder and creative director of 3XN. “Is the concrete really as strong as we thought? … This was crucial to figuring out how to ‘hang’ the new structure over the old one.”
It was only when construction began in 2018 that architects and engineers were able to assess the existing building more closely. Concrete samples were used to calculate how much – and where – the extra structural load could be supported.
The shortening of buildings over time brought with it another dilemma: What if the old and new buildings were out of alignment as the old ones gradually got smaller?
To counter this, engineers placed hundreds of sensors around the building to monitor even the smallest movements. This data was fed into what Holt describes as a “digital twin” – a dynamic computer model of the tower – used to make real-time adjustments and make sure “everything works, slides and shortens as it should.”
The building has elevated views of Sydney Harbour. Credit: Courtesy of Martin Siegner/3XN
Workers also left a 4-metre (13 feet) gap between new and old structures until the final stages of construction, allowing time for the new concrete to settle before the final “graft” was performed.
Paying for efficiency
From the outside, there are no obvious remnants of the building’s 1970s predecessor. Inside, too, the two parts of the tower “blend” seamlessly, Nielsen said.
When you go inside, you don’t think about where the old building is and where the new building is. “This was very important.”
The 676-foot-tall skyscraper has retained more than two-thirds of its original 1970s structure. Credit: Courtesy of 3XN
Meanwhile, the building’s eco-friendly identity garnered praise from the organizers of the World Building of the Year award given to the Quay Quarter Tower at the World Architecture Festival in Lisbon, Portugal last week. Paul Finch, program director of the event, praised the skyscraper as “an excellent carbon story” and “an example of adaptive reuse”.
For the tower’s owners, the design achieved another major achievement: It was much cheaper than building from scratch. Holt estimates that AMP Capital saved AUD 150 million ($102 million) by salvaging the original structure.
That’s why 3XN hopes the tower will serve as a case study not only for other architects and engineers, but also for building owners and corporate homeowners. Nielsen said the project demonstrates “how sustainability and value come together economically.”
“I don’t think many developers are looking at this as an option,” he added. “But it’s been done here now, and it could be a great example of how to do it in the future.”