When he designed the twin towers, Minoru Yamasaki calculated the damage that could result from a plane striking the buildings. Air traffic controllers permitted planes to travel at 180 miles-per-hour within New York City’s air space. Using a Boeing 707 in his model, and factoring in the maximum speed of 180 mph, Yamasaki determined that seven floors on one side only would be demolished, but the building would stand. Why, then, did the towers collapse with so many people from so many countries trapped inside?
In the early stages of the investigation, engineers believed that fire and intense heat killed the buildings. Nothing, it was thought, could survive the temperature and heat generated when about 18,000 gallons of jet fuel exploded in each tower. The metal structures would have weakened and failed.
But it was more than the intense temperature and heat. By December of 2001, MIT Professor Dr. Thomas Edgar had a theory about the collapsing towers. When the jets crashed into the buildings, the force of the explosion blew off the insulation which protected the steel infrastructure from fire. Unprotected steel succumbed to the intense heat and gave way. A NOVA animated reconstruction, aired by PBS, depicts what likely occurred. (Click on the segment, "Deconstructing the Towers’ Collapse.")
Once the intense heat caused major structural failure of the towers’ central cores, they imploded with remarkable speed - almost in a free-fall condition. The steel lattice followed the rest of the structure as the buildings “pancaked” to the earth, one-quarter mile below.
Despite the towers’ collapse, their design and construction saved thousands of lives as people had time to exit the buildings before they fell. (Click on the segment, “World Trade Center Award.”) Keep in mind how large these towers were: They contained enough rentable space to fill 50 city blocks.
By the time the recovery and site-clearing (pictures by Digitial Globe) process officially ended on 30 May 2002, 1.8 million tons of debris had been removed from the disaster site. Regrettably, 1796 people were never recovered.