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. To make the Freedom Tower structurally stronger and more fire-resistant, its designers are fortifying the building’s core with a thick concrete-and-steel wall and have chosen a web-like structure for the exterior that should help redistribute weight if some support columns are damaged. Only the first 70 or so floors will be occupied; SOM architects acknowledge it might be hard to find businesses willing to rent any higher.

The Freedom Tower’s developer has promised to go beyond the safety measures required for a New York City high-rise. The building will have biological and chemical filters and two sprinkler systems (to provide a backup if one is destroyed). But what passes for innovation in the U.S. is often old-hat when compared with high-rise design in Asia, Israel and the U.K., places long accustomed to earthquakes and terrorism threats. The Freedom Tower will include some measures common abroad, such as upgraded elevators for use by emergency responders. But there are numerous other features–such as fire-resistant steel, which can endure temperatures up to 800