freedom towers

Primers on high-security building design warn against basement garages. It’s a lesson learned from bitter experience: the 1993 truck bomb that exploded below the World Trade Center, killing six. But parking is a key commercial asset, and a large underground facility is planned for the Freedom Tower. Designers promise that vehicles will be screened and that blast-resistant materials will be used.

Running up the center of the building is a fortresslike tower whose walls, made of two-to-three-foot-thick reinforced concrete and steel, will provide structural support for the building and fire protection for the infrastructure it contains: elevators, stairways and utilities such as the pipes that carry water to the sprinklers.

If the World Trade Center attack had occurred at a busier time, it would have taken occupants four hours to get down the stairs—hours they didn’t have. The solution: emergency elevators. Surprising?

Immediately after 9/11, it looked like the age of the high-rise trophy building was over. But at the politically symbolic height of 1,776 feet (designated by master planner Daniel Libeskind), the World Trade Center's replacement will be among the three tallest buildings in the world upon its completion in 2008.

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.

The Freedom Tower’s designers had to contemplate the whole horsemen-of-the-apocalypse spectrum of possibilities: explosives big and small; fire; chemical, biological and nuclear attack. But the most obvious goal of the design team—headed by the architectural firm of Skidmore, Owings and Merrill—was to create a structure robust enough to avoid a reprise of the twin towers’ fate: catastrophic failure as the buildings buckled under their own weight, 110 stories pancaking down in 10 to 15 seconds.

Firefighters have trouble battling blazes in areas larger than 7,500 square feet. But the Freedom Tower will have the open plan favored by corporate tenants: 35,000 to 52,000 square feet (depending on the floor), broken only by a central corridor. Designers in China have an innovative solution to this conflict between safety and the flexibility businesses require: fireproof partitions housed in the ceiling that lower automatically in case of fire.

Up to 85 percent of injuries in bomb attacks are caused by flying glass—“knives and daggers,” in the words of blast engineer Tod Rittenhouse. But thanks to commercial pressure for views and a graceful exterior, the Freedom Tower’s skin will be mostly glass. Designers will use safety glass, but have not provided details. There are two ways to pacify glass: tinker with it chemically or keep it from traveling.

Diagonal columns wrap around the Freedom Tower. Connected to the central core by the floors, they share the job of supporting the building’s weight.

High manufacturing temperatures make blast-resistant glass strong but too heavy for an entire building. Laminated glass consists of glass layers sandwiched around plastic; upon breaking, glass fragments stick to the plastic. A futuristic solution&58; glass that’s been chemically treated so that it cracks from below the surface into sand-like grains, not shards.