When 875 North Michigan Avenue, formerly the John Hancock Center, opened on Chicago’s Magnificent Mile in 1969, it signaled a departure from the all-too-prevalent trabeated Miesian skyscraper.
Its subtly tapered 100-story form and iconic X-frame structure, designed and engineered by Skidmore, Owings & Merrill’s Bruce Graham and Fazlur Khan, respectively, demonstrated that beauty and structural performance need not be mutually exclusive. As if taking this lesson to heart, a new crop of expressively framed towers has sprung up around the country in recent years, each one upping the ante in exuberant form and structural daring.
In Seattle, a short walk from the city’s famed, OMA-designed central library, The Mark gamely cantilevers over its older neighbors. The 48-story hotel-and-office tower, designed by ZGF Architects and engineered by Arup, relies on a hybrid steel “megabrace” and concrete core structure to perform its acrobatic feat. Comprising multistory steel members, the triangulated megabrace—so termed by Arup—addresses the structural complexities of building a formally expressive tower in a seismically active region.
The architects constructed around two historic structures nestled below The Mark’s protruding midsection: the First United Methodist Church (now an event space) and the Rainier Club. Their efforts have resulted in an eclectic city block, complete with classically proportioned low-rise structures and a decidedly contrapposto tower.In San Francisco, the Heller Manus Architects–designed 181 Fremont employs a similar megabrace structure (again courtesy of Arup) capable of withstanding the city’s seismic activity and ever-present wind loads.
But the slightly tapering tower also deploys a structurally integrated damping system (rather than the more typical tuned mass damper), which enabled the architects to increase the tower’s height; at 802 feet, 181 Fremont is the tallest residential high-rise on the West Coast. And because the shocklike dampers work in-line with the megabrace, the design team was able to eliminate tons of steel from the project—3,000 tons, in fact. The envelope created additional efficiencies; calibrated to the angle of the sun, the “saw-tooth” glass facade reduces solar gain by 6 percent.