The world’s tallest hybrid timber commercial building is challenging high-rise convention and encouraging engineers to rethink how towers carry load.
With its suspended floor plates, shifted structural loads, 50 MN of force channelled through two locations at the building’s base, giant steel nodes, cantilevered upper floors and the world’s tallest, self-climbing screens, the Atlassian Central tower seems an engineering fantasy.
Under construction next to Sydney’s Central Station, it has already become the world’s tallest hybrid timber commercial building. Beneath its striking diagrid facade is a complex series of engineering challenges stacked one on top of another, all above Australia’s busiest live transport hub and within arguably the nation’s most constrained construction site.
The unconventional, 186 m commercial tower is designed for 50 per cent less upfront embodied carbon and to operate on 100 per cent renewables. Its scale and ambition is as grand and daring as the business that has given the building its name.
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Rising above the rails
Atlassian Central is located on the edge of Central Station, directly above the historic Parcels Shed, itself currently being carefully dismantled, refurbished and reinstated.
A 137-room youth hostel will take up the first five levels, with 59,000 m2 of lettable commercial office space beginning above that.
Its diagrid exterior, coming together to a narrow base that creates a popsicle-like silhouette, will bring numerous benefits to the finished building and its occupants. These include an integrated facade photovoltaic system that simultaneously provides shading, plus an automated operable facade for natural ventilation of landscaped areas and adjacent offices.
For engineers delivering the build, this translates to fascinating challenges. Perhaps the greatest is the site itself.
“We are building on top of Australia’s largest railway station in Australia’s biggest city,” Jahaan Rowdah, Construction Manager at Built, told create.
The tightly constrained land parcel next to Central Station is bordered by live rail infrastructure and existing development.
Project Director Peter Morley, of Dexus, described it as “Sydney’s most challenging development site”.
These complex constraints have left their mark on every aspect of the project’s design and construction methodology, beginning with a 20 m turntable for trucks.
Following that was an extraordinarily collaborative design process that brought 17 companies together across three continents and involved, according to official documentation, more than 1800 Zoom meetings, 160 cost planning drafts, 55 embodied carbon studies and more than 50 design submissions.
After all of that, the final design remained true to the original concept. “I think that’s a reflection of a very strong vision as to what we were looking to achieve,” Morley said.
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Unique and dynamic
“When compared with other structures in the world, we do not believe there’s anything that compares to this building,” Rowdah said.
That is for several reasons, including the visual signature of the structure, characterised by a lattice of diagonal steel tubes that wrap the exterior.
This type of diagrid assembly is not new. What is unique is what happens to the forces carried by these structures on this building.
“The Atlassian Central’s building structure is very unique and dynamic for a series of reasons,” Rowdah said. “What is unusual on this project is that the load does not go down to the ground.
“At level seven, it transfers through diagonal columns into two locations, the concrete core and two mega columns. The load that is going through one of these inclined columns into the core exceeds 50 MN of load, which is an extreme amount of force.”
To safely manage those forces, engineers developed one of the build’s most remarkable structural elements: the level-seven tie floor.
At the base of the tower’s visible diagrid but seven floors up, this post-tensioned structure is heavily reinforced and performs numerous roles. It redistributes loads from the tower above into the mega columns and concrete core below and, at the same time, becomes a suspension point for the six levels below.
This design affected construction sequencing as 12 temporary perimeter columns were erected to support the lower floors until level seven was built and fully stressed. Only then could the temporary columns be removed.
“Once level seven is poured, with a huge, multi-strand, concrete tie floor, then level one to level six is hung and suspended off the level seven floor plate,” Rowdah said. “Above level seven is the commencement of the diagrid, resulting in a very dramatic cantilever across the perimeter of level one.”
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Creating balance
Above level seven, Rowdah said, “the concept of the building is that there are seven mega-floors, one every 15.5 m, supported by a series of diagrid steel tubes and a structural steel floor plate with a composite concrete slab on top”.
Between levels seven and 11 there is also a six-metre cantilever on two of the four elevations, resulting in extreme, out-of-balance loads.
“On level seven, there is greater load on the eastern nodes ahead of the cantilever occurring,” Rowdah said. “And then on level 11, that shifts over to the western elevation.”
The nodes are large, steel transfer points that connect the diagonal members of the diagrid. They gather forces from above and redistribute them downward.
“The diagrid on the cantilevered elevation was a 65 mm thick plate, which took us about nine days of constant welding to complete,” he said.
The largest nodes, weighing more than 28 t, required dual-crane lifts to manoeuvre into place.
“The structure between level seven and level 11 was without a doubt the most complex activity logistically – and from an engineering perspective – that I think I’ll ever come across again in my career.”
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Timber challenge
Timber has been used across the build to achieve a lofty sustainability goal.
The team decided early on to shoot for a 50 per cent reduction in embodied carbon as compared to a conventional commercial tower build. Part of the solution was a habitat concept.
“The hybrid structure essentially breaks the building up into a series of what we call habitats, but which are essentially four-storey buildings stacked on top of each other,” Morley said, adding that each is anchored by a structural steel and concrete floor, with three timber office floors suspended between. As a result, about 75 per cent of the commercial floors are timber, enabling a significant reduction in embodied carbon.
Extensive fire testing was carried out, as was performance modelling and peer reviews.
The inclusion of timber demanded changes in construction methodology, particularly as those elements needed protection from the weather. This led to the timber being installed behind the facade.
“The work that’s gone into the finalisation of the screens, the mobilisation of the screens, the climbing of the screens and the subsequent dismantling of the screens at the end of the project is phenomenal,” Rowdah said.
“This complex and remarkable project would never have been possible without the vision of the co-owners, Dexus and Atlassian, and the delivery partner Built in joint venture with Obayashi Corporation. Thank you to all the subcontractors, consultants and everyone else who’s played a part in getting us to where we are now on the project.”
By the numbers
The self-climbing screens, which stand up to 34 m tall and weigh around 12 t each, have enclosed the building perimeter as it rises. They don’t just protect the timber, but also provide greater comfort and safety for workers.
Custom-made to suit the geometry of the tower, the screens are a reminder, for Rowdah at least, that much of the most challenging engineering is invisible.
“I’ve spoken about the permanent structure of the building being difficult, but there is a large amount of temporary works required for this building to perform, and for us to be able to construct it,” he said.
“While you won’t see it, the work associated with finalising that temporary work is arguably as difficult as the work that goes into the permanent part of the building.”
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Future vision
Morley said Atlassian Central’s greatest achievement is not related to its engineering at all, but to the fact “the vision for the building has been delivered from concept to construction”.
That has required innovation and creativity around approaches to engineering, logistics, sustainability and construction methodology that will continue to have their own effects long after the building is delivered.
“I’m extremely proud,” Morley said. “It’s going to make a really meaningful difference as to how office buildings are imagined and developed in the future.”
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