Tall Orders

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Down below, in the occupied floors, the building will incorporate a whole spectrum of new, post-9/11 defensive features. Exit stairways will be wider and will open directly onto the street. There will be dedicated stairwells for use by fire fighters. Air filters will block chemical or biological agents. The building "core"--the part surrounding elevators, stairwells and safety systems — will be solid concrete, not steel girders of the kind at the Trade Center that were easily sliced by the intruding planes. "There are many ways," says Childs, "that this building is responding to the exact event that caused the tragedy."

For its main structural support, the Freedom Tower will also employ an increasingly popular triangular-grid trusswork. From a defensive standpoint, structural strength, even more so than fire safety, is the most important consideration for tall buildings. "A square is not a geometrically stable shape," says Childs. "A triangle is stable because it has a diagonal." The Trade Center towers fell because intense fires eventually melted their interior steel. But their structural systems permitted both towers to remain standing after the initial impact of massive jetliners. So for the new 52-story headquarters of the New York Times, the construction of which will soon begin in Manhattan, the architect Renzo Piano agreed to reinforce the connections joining columns on lower floors to support structures above called outrigger trusses. If a blast severs the columns, the floors above could still hang from the trusses.