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Click to see video In regards to the Towers' immense solid steel construction, Frank A. DeMartini, Manager of WTC Construction & Project Management stated in an interview back in January 2001:

"The building was designed to have a fully loaded 707 crash into it. That was the largest plane at the time. I believe that the building probably could sustain multiple impacts of jetliners, because this structure is like the mosquito netting on your screen door - this intense grid - and the jet plane is just a pencil puncturing that screen netting. It really does nothing to the screen netting."

Frank DeMartini died on September 11th a hero. After accompanying his wife down 88 flights of stairs from his office in the North Tower, he went back up with fellow WTC worker Pablo Ortiz and rescued over 70 people. DeMartini and Ortiz - both 49 years of age - perished in the collapse of the North Tower.
More on Frank DeMartini:
WATCH Video of Frank DeMartini's interview 
READ Francis (Frank) Albert DeMartini, 49, New York, N.Y.   
READ Frank A. Demartini Tribute Page