As SOM architects neared the end of their planning stage, they realized something. At ninety-five stories, the Sears Tower would only be about ten stories short of the World Trade Center (WTC). The two towers of the WTC were still being built in New York City. When they were finished, their height would be 1,368 feet (417 m). Then the twin towers would become the world’s tallest buildings. SOM architects changed their plans and added another fifteen floors to the Sears Tower. When finished, it would stand at 1,450 feet (442 m). That was the highest the tower could go. If higher, it would interfere with Chicago airplane traffic. Workers broke ground for the Sears Tower in August 1970.
To build the foundation, they built a wall 3 feet (1 m) deep around the outside of the site. Inside the wall, they dug a hole 50 feet (15 m) deep. The workers then drilled 114 holes another 50 feet into the ground until they hit bedrock. The holes were lined with steel and filled with concrete. These holes anchored the foundation of the building. After the foundation was complete, workers began building the tubes and trusses. For the exterior walls, trucks hauled in prefabricated steel pieces. Each piece was made of a 25-foot (7.5 m) vertical beam crossed by two 15-foot (4.5 m) horizontal beams. Workers called these Christmas trees. Each Christmas tree was welded to the next to form the tower’s steel frame. Other workers constructed the floors by pouring concrete into forms built over the steel truss beams.