The 967-foot tower at 311 South Wacker Drive is the world's largest concrete building. Most of the 70 stories were formed with truss-supported flying deck units custom fabricated for the job. Work for the forming contractor began in a steel shell placed above each of the 44 column cassions of the tower foundation. The core wall area was excavated, braced, and formed conventionally, from the mat foundation to the core level. Once the core walls and columns had been poured to grade level and cured, the forms were removed. Top-down construction made it possible to get an earlier start on the tower forming.
After completing formwork on the nontypical second and third floors, the forming subcontractor started using flying truss forms at the fourth level and proceeding through the rest of the job. Because space at the jobsite was so limited, the trusses were built at the subcontractor's yard and barged by canal to the Chicago River, directly across from 311 South Wacker Drive. The truss panels were unloaded at night to minimize traffic disturbance and set directly into place by the tower crane. Approval for form stripping was based on strength development in the concrete, as determined by maturity meters embedded in the beams and slabs.