Editor’s Note: Sign up for CNN’s Life, But Greener newsletter. Our seven-part guide helps you minimize your personal role in the climate crisis and reduce your eco-anxiety.

CNN  — 

Once Sydney’s tallest building, the AMP Centre was showing its age. The outdated 1970s structure had come to the end of its lifespan, and the tower’s owners wanted to replace it with something bigger, better and more energy-efficient.

But demolishing high-rises comes with significant environmental costs, from construction waste to the CO2 emitted by heavy machinery. So in 2014, Australian investment firm AMP Capital launched an architectural competition with an unprecedented brief: To build a new skyscraper without demolishing the old one.

Dubbed the world’s first “upcycled” high-rise, the resulting tower opened earlier this year and, on Friday, was named World Building of the Year 2022. Standing at 676 feet tall, the vastly expanded 49-story skyscraper, now known as Quay Quarter Tower, retained more than two-thirds of the old structure, including beams and columns, as well as 95% of the original building’s core.

“The tower was coming to the end of the end of its life, in terms of viability… but the structure and the ‘bones’ can actually last a lot longer,” said Fred Holt, a partner at the Danish architecture firm behind the design, 3XN, in a video interview. “You can’t always retain everything. But if you can retain the structure — and that’s where the majority of your embodied carbon is — then you’re lowering your footprint.”

After removing unsalvageable parts of the old building, construction workers erected a new structure beside it that they then “grafted” onto what remained. A contemporary glass facade was wrapped around them both to create a single skyscraper. The new design doubled the building’s available floor space and, in turn, the number of people it can accommodate, from 4,500 to 9,000.

The architects believe their approach saved 12,000 tons of CO2 when compared to demolishing the tower and starting from scratch — enough to power the building for over three years. As well as reducing the use of carbon-intensive materials like concrete, the scheme may have saved up to a year in construction time, too.

“The greenest building is the one that already exists,” Holt said, quoting former president of the American Institute of Architects, Carl Elefante.

‘A lot of unknowns’

The ambitious project, which 3XN completed alongside engineering firm Arup and Australian architecture practice BVN, posed a slew of design challenges. Among the first was simply determining whether the existing building matched its original design.

High-rises often shrink under their own weight, especially in the first few months after completion. As a result, the old AMP Centre “was at a slightly different level than was on the drawings,” said Holt, explaining that concrete “spreads and drops” as it fully dries.

The new tower's design comprises five stacked volumes.

“There are always a lot of unknowns when you start stripping an existing high-rise,” added 3XN’s founder and creative director, Kim Herforth Nielsen. “Is the concrete really as strong as we think it is? … This was crucial to working out how to ‘hang’ the new structure onto the old one.”

Only when construction work began in 2018 could the architects and engineers more closely assess the existing building. Concrete samples were used to calculate how much — and where — extra structural load could be supported.

The fact that buildings shorten over time also presented another predicament: What if the old and new structures became misaligned as the former slowly shrank?

To counter this, engineers installed hundreds of sensors around the building to track even the tiniest movements. This data fed into what Holt described as a “digital twin” — a dynamic computer model of the tower — that was used to make real-time adjustments and ensure that “everything worked, shifted and shortened the way it was supposed to.”

The building enjoys towering views over Sydney Harbour.

Workers also left a gap of 4 meters (13 feet) between the new and old structures until the very last stages of construction, giving the new concrete time to settle before the final “graft” was carried out.

Making efficiency pay

3XN’s striking design, which is part of a wider 1 billion Australian dollar ($670 million) redevelopment project, features five stacked volumes that twist toward the sky. Described by the architects as a “vertical village,” Quay Quarter Tower contains retail space and offices overlooking the Sydney Opera House, as well as a series of rooftop terraces.

Viewed from the outside, there are no obvious remnants of the building’s 1970s predecessor. Inside, too, the tower’s two parts have been seamlessly “blended,” said Nielsen.

“When you’re in there, you don’t think about where the old structure is and where the new one is,” he added. “That was crucial.”

The 676-foot-tall skyscraper retained more than two-thirds of the original 1970s structure.

The building’s green credentials have meanwhile attracted praise from the organizers of the World Building of the Year prize, which was awarded to Quay Quarter Tower at last week’s World Architecture Festival in Lisbon, Portugal. In a statement, the event’s program director Paul Finch praised the skyscraper as an “example of adaptive re-use” with “an excellent carbon story.”

For the tower’s owners, the design achieved another important feat: It was significantly cheaper than building from scratch. Holt estimates that AMP Capital saved 150 million Australian dollars ($102 million) by salvaging the original structure.

As such, 3XN hopes the tower can serve as a case study — not only for other architects and engineers, but for building owners and corporate landlords. Nielsen said the project demonstrates how “sustainability and value come together, economically.”

“I think many developers didn’t look at it as an option,” he added. “But now it has been done here, and can be a great example of how to do it in the future.”