Rising 1,127 feet above Chicago, the John Hancock Center stands as a monument to structural engineering innovation. Designed by architect Bruce Graham and structural engineer Fazlur Rahman Khan of Skidmore, Owings & Merrill, the skyscraper introduced the trussed tube system—a concept that radically changed tall building construction in the late 1960s. Instead of relying on a dense internal skeleton or massive central core to resist wind loads, Khan recognized that moving structural support to the exterior perimeter would maximize efficiency.
The defining feature of the building is its striking dark-exterior X-braces. These diagonal steel members cross five vertical bays, connecting perimeter columns and spandrel beams into a single, cohesive structural facade. This creates a rigid outer tube that acts like a hollow cantilever beam fixed to the ground.
When powerful winds off Lake Michigan strike the tower, the X-braces convert bending moments into simple axial forces—tension and compression—distributed evenly across the outer columns. This tubular rigidity dramatically reduces building sway while utilizing roughly half the steel per square foot required for traditional braced frames of similar height. Furthermore, the diagonal bracing permitted vast column-free interior floor plans, proving that expressive structural logic could simultaneously drive architectural beauty and financial economy.