This initial conceptual design shows the most visible characteristics of the tower design: its generous ground-level, center, and rooftop terraces, cantilevered exterior fire stairs, and megabracing, which extends up the sides of the building. Mirvac Group
A building boasting two high-rise sections—one on top of the other—has topped out in Sydney, Australia.
April 9, 2013—Engineers can often make the most complicated of architectural designs appear effortless. And they can do this even when the engineering is a visible part of the structure, on display for every visitor or passerby. Such is the case with a new tower in Sydney, Australia—called 8 Chifley—that has recently topped out. Comprising what is essentially two high-rise sections stacked one atop the other, the building is characterized by airy, multistory cutouts at the ground, middle, and top levels, as well as by a concrete core on the south side, or back, of the building.
“The structural design is technically demanding to achieve its simplicity,” said Andrew Johnson, CPEng, a principal in Arup’s Sydney office, in written responses to questions submitted by Civil Engineering online. Arup was responsible for the structural engineering, building services, sustainability features, and fire safety engineering of 8 Chifley. “Our biggest challenge on buildings such as this is to make the legible structure appear effortless—combining all of the systems in a complementary natural order.”
The 34-story building—which is owned jointly by Sydney-based Mirvac Group and Singapore-based Keppel REIT—begins with a 6-story plaza, referred to as a reverse podium, that is open to the elements and is backed by a small, glass-enclosed lobby. The sides of the podium feature diagonal braces and the first tower technically begins at the sixth floor—above these braces and the lobby—and rises an additional 12 stories. At level 18, a three-story void provides space for a second plaza, this one offering views of Sydney’s harbor. The second high-rise section then commences, rising nine stories before being topped by another three-story plaza with expansive views. The uppermost plaza includes a roof structure supported by bowstring trusses and box rafters that will both provide shade and support a photovoltaic array. Exterior cantilevered fire stairs that are tied to the rear core descend the full height of the building.
The building’s interiors are arranged in a collection of individual multistory spaces referred to as villages. Each village has been organized around a central atrium and contains its own circulation system. The design team was able to maximize the interior space of the building by using an innovative gravity stability system, described below, and by placing the fire stairs outside. Because the fire stairs are considered “open,” they do not need to be counted as floor area, according to Johnson.
The foundation conditions in Sydney are excellent, Johnson noted, and so the building is located atop pad footings on sandstone with a bearing capacity of 8 MPa. The building’s 1.7 by 1.7 m megacolumns, one at each of the structure’s four corners, can carry loads of up to 80 MN and are founded on pad footings measuring 4.5 by 2.5 m.
The building contains posttensioned-concrete floors with interior concrete columns located on an 18 by 12 m grid, according to Johnson. Expressed steel and concrete composite raker columns transfer loads from the interior columns at both the midheight plaza and at the ground-level “reverse podium” to the megacolumns. Despite the building’s height, the raker columns enabled the design team to reduce the size of the interior columns to those of a 12-story building.
The exterior megabraces for the building work in both tension and
compression. A central node serves as an articulation joint to
release the elastic and time-dependent shrinking and creep of the
columns, which reduced the size of the steel necessary for use in
the bracing by approximately 50 percent. Arup
The building’s stability system includes steel megabracing located along its eastern and western facades that controls loading in the north–south direction, along with eccentrically located reinforced-concrete shear walls that control in the east–west direction, Johnson explained. Since the core is located at the rear of the building, the megabracing is crucial for preventing twisting movements induced by wind or seismic forces.
The exterior megabraces work in both tension and compression. At each central node, an articulation joint “serves to release the elastic and time-dependent shortening of the columns (shrinkage and creep) from loading the steel braces with gravity load,” Johnson said. Each node “also releases thermal movements” of the concrete and steel. The nodes, which are unique in a building of this type, according to Johnson, reduced the required size of the steel for the bracing by approximately 50 percent.
“The systems work together seamlessly, with all elements combining to achieve the overall function of resisting gravity and vertical loads,” said Johnson, who pointed out that “the system has been proportioned such that no tensions are transmitted to the foundations.”
One unusual feature is that the megabracing has not been covered in plating or clad for fire protection. “A structural and fire engineering strategy considered in detail [the potential] fire locations, intensities, and spread within the building, allowing us to look at the opportunity to alleviate this protection,” Johnson said. The design team was able to use a simple, economical paint system instead, he noted.
Sustainability is also of fundamental importance to 8 Chifley’s owners, according to Andrew Partridge, a project architect with the London office of Rogers Stirk Harbour + Partners, which designed the building with Sydney-based Lippmann Associates. Partridge also wrote in response to questions submitted by Civil Engineering online. The building is designed to achieve a six-star rating in the Green Building Council of Australia’s Green Star program, which is approximately equivalent to platinum certification in the U.S. Green Building Council’s Leadership in Energy and Environmental Design (LEED) rating system, according to Johnson.
To reduce cooling costs, the external shading system was optimized to augment the shading provided by adjacent tall buildings, according to Partridge, and the windows will feature the lightest possible tint to maximize the use of daylight. Additionally, the building will have a trigeneration system, which in addition to generating electricity will provide heating and cooling. These energy-saving measures, together with the rooftop photovoltaic arrays, are expected to give the building sufficient capacity to export energy to other buildings for their benefit. Additionally, 8 Chifley will include a water-recycling system that will rely on wastewater and water from a cooling tower as its sources.
“The main thing about the building is that it’s responsive to its environment—it’s engaging,” Partridge said. The views of Sydney harbor are spectacular, he said, and the open-air elements provided on the ground, at the midpoint, and at the top of the building will make it easy for building occupants to engage with the outdoors.