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.