The John Hancock Center, finished in 1969, was Fazlur Rahman Khan and Bruce Graham's answer to a hard question: how do you build 100 stories on a narrow site without filling the floors with columns? Khan's trussed-tube design moved the structure to the skin, which is why the X-braces are visible — they are the building, not decoration. It was designed with apartments above offices.
Source: Skidmore, Owings & Merrill, John Hancock Center project record; Council on Tall Buildings and Urban Habitat, The Skyscraper Center, 875 North Michigan Avenue building record