PROJECT REPORT: TALL BUILDINGS 45


split into restaurant and amenities, and sitting above a colonnade. The rest of the floors above are offices, with further terraces at levels three and four. There are two other terraces to the north, adjacent to the core.


Structure


In order to free up floor space, the architects decided to push the core to the north of the footprint, a “pretty innovative” move in Chicago, says Kevin. “This gave us the ability to insulate that facade pretty heavily, which is where you want to do it here.” Crucially, the core performed “better as a cantilever” than being the lateral anchor, and enabled a consistent 60 foot span all the way up with no columns, plus the terraces at lower levels. With no real lateral system, SOM’s structural engineers designed external, high- waisted X-braced frames in steel, which represented a “continuation of the practice’s history of creating structurally expressive architecture.” They are far from mere formal expression, driven by no-nonsense practicality. The glass core of the building is suspended using this highly bespoke structural system, “enabling flexible, light- filled workspaces.”


In order to withstand the extremes of the Windy City’s harsh winters, and


ADF JUNE 2022


hot summers, the frames are designed to contract in cooler weather and expand as the structure warms.


Assisting the sustainability goals, the architects decided not to cover the bracing in aluminium; also it wasn’t required on fireproofing grounds due to being outside the envelope, and simply painted it. However, this exposed steel would be more susceptible to Chicago’s “massive swings in temperature,” says Rodenkirch. Having done something similar at 100 Mount Street in North Sydney, the structural team came up with a thin central hinge plate which “literally bends around 8 inches in each direction,” says Rodenkirch. “You can see in the morning when it’s cold it’s sitting kind of flush, and when it gets warm you see it start to bend out.”


The solution is a highly functional, lean use of steel. Rodenkirch: “I think structural engineers are the original low embodied carbon engineers by design, because everything they have done has been how do you do this in the most efficient design with the least amount of material. Post-tensioned beams were used elsewhere to reduce the structure’s thickness.


Programme


A triple height main lobby with a cantilevered staircase and mezzanine creates


WWW.ARCHITECTSDATAFILE.CO.UK


“It became clear that we needed to aggressively exceed the target, and go


for LEED Platinum” Kevin Rodenkirch


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