Few people on the floors above where the planes hit the twin towers survived, in part because the stairs, sheathed only in drywall, were severely damaged. In the Freedom Tower, stairs will be housed in concrete enclosures within the central core, creating what SOM architect Carl Galioto calls “a core within the core.” The stairs will be pressurized to push out smoke. Photoluminescent strips will function in a power failure, and each stairway will branch into two street-level and two underground exits.

Not in the plans: Experts suggest that additional staircases be located near the building’s perimeter to provide alternative escape routes.