Spaces and forms interpenetrated. Sculptural concrete projections that housed the chambers and the mayor’s office protruded from a modular facade of offices. A brick amphitheater of stairs on the ground floor accommodated gatherings of citizens, even spontaneous sit-ins; the vast plaza in front, inspired by the Campo of Siena, Italy, anticipated the thousands of protesters of those riotous times.

It was a benevolent structure that took the side of the people, guaranteeing citizens free access through porous perimeters in that cradle of American democracy.

Twenty years Mr. McKinnell’s senior, Mr. Kallmann was best known for publishing an essay about two divergent trends in architecture: one in which buildings were composed rigorously within a controlling grid; the other, propelled by French and British Brutalists, in which designs in concrete embraced sometimes violent breakout forms. The Kallmann-McKinnell design synthesized the two approaches, the rational and the complex.

Both men had been educated in British architecture schools that taught that architecture’s mission was to build well in the service of a moral and social purpose. And both had been imbued with the belief that concrete was, as one theorist said, “the stone of our time.”

Built in concrete and brick, their robust City Hall design implicitly critiqued the thin commercial glass-and-steel buildings then chilling cities across America. It was a statement of protest against what Mr. McKinnell called the “degenerate frippery and surface concerns” of “cosmetic” architecture.