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.