Twelve years after 9/11, design and construction continue over the 16-acre site, and an essential part of this reconstruction is the new world-class Transportation Hub. The landmark feature of the new hub is the magnificent Oculus, which will rise to a height of 170 feet above ground level. The Oculus is supported by the new transit hall, a reinforced and composite concrete structure founded 70 feet below grade.

The concrete substructure has several unique and important design and construction features. The lower levels house the mechanical systems that serve the entire complex. The next two levels are for bus parking and car parking. The transit hall pedestrian level is immediately above the mechanical and parking levels. It is anticipated that the transit hall will provide access for up to 250,000 daily commuters, visitors, and tower occupants. The space will connect eleven New York City Subway lines. Additionally, there will be world-class retail and dining options throughout the World Trade Center. One of the key architectural features of the transit hall is the grand staircase, a circular- shaped reinforced concrete structure that will provide connection to the new PATH Station, One World Trade Center and the World Financial Center.

Special features of the transit hall concrete design include several massive shear walls and concrete diaphragm slabs at the two lower levels. These structural elements will share all earth, subway and seismic forces with the adjacent Tower 2 to the north and Tower 3 to the south. The next two upper levels are composite concrete and steel and will transfer these forces directly through the floor diaphragms into the adjacent Tower slabs and core system.

Close design coordination between the designers of the Hub and towers was required. Based on comprehensive temperature and load transfer analysis, it was determined that connecting the concrete slabs of the transit hall and the adjacent Towers 2 and 3 were preferred to providing expansion joints. This resulted in a continuous 1000 feet concrete plate for several levels of these structures. A second order indeterminate structural analysis was performed to determine the reinforcing steel and concrete strength needed to meet these demands and to prevent cracking.

As a result of these requirements, performance-based concrete mix requirements were developed with the owner and included in the contract documents. These requirements limited the maximum allowable long-term shrinkage of the concrete to less than .03%. These criteria enabled a time savings of six months by eliminating shrinkage strips in addition to minimizing shrinkage throughout.

Mass concrete placement requirements based on latest ACI Committee 301 requirements were incorporated for constructing the large shear walls. Concrete strengths ranged from 5,000 psi to 8,000 psi for slabs and columns, and as much as 12,000 psi for major shear walls. Additional provisions were made in the concrete design as part of protective design for public safety and securing key mechanical systems. LEED recommendations in regard to concrete design mixes were incorporated.

The next phase of work, the above-grade Oculus superstructure, is currently being erected. The unique concrete designs and mix criteria utilized in the transit hall will help this facility serve the public for many years to come.