A fire in a simulated office of the WTC devours virtually everything; workstations, paper, walls, carpet. Until in reality, both towers dissolved into 110-story chimneys.
But new tests at the National Institute of Standards and Technology (NIST) show it was not the jet fuel that kept those flames alive.
Dr. Frank Gayle, Metals Expert: "Your gut reaction would be the jet fuel is what made the fire so very intense, a lot of people figured that's what melted the steel. Indeed it didn't, the steel did not melt."
Dr. Shyam Sunder, Lead Investigator: "The jet fuel probably burned out in less than 10 minutes. And what did burn over the next hour, or hour and a half, was much of the contents of the buildings."
Investigators have rebuilt several WTC offices. Dissecting insulation, stress-testing computer models of the aircraft-battered girders. Many of which are still remarkably intact at their Maryland lab.
Dr. Frank Gayle: "This is a particularly valuable piece for us -- the upper part of the fuselage. ... Hit these three legs of this panel, and kicked them into the building."
They are simulating everything. For instance using a high-speed test ramrod to hit the steel as fast as the planes hit the buildings. They're discovering which parts of the buildings failed, so future towers don't.
Kevin McGrattan, Mathematician: "A common practice in fire-protection engineering is to separate different parts of the building using rated walls. A two-hour rating will keep a fire from penetrating it for roughly two hours. But this tower was designed so that each floor was fairly open."
The investigators have found that the windows between the columns blew out, fueling the fire and bringing so much oxygen in that it weakened the structure underneath, which then collapsed.
Dr. Shyam Sunder: "When we put the basic temperatures in this model we start to see sagging of the floor system, as well deflection of the (support) column."
And while the planes' wings and engines weakened some areas, scientists don't think that was enough to make the towers fall. They're now focusing on the floor truss supports where white foam fireproofing may not have been as strong as builders believed.
The reality is that more than two years later, investigators are still uncertain why the towers fell. No one has ever attempted a simulation this large.
Engineers insist it is worth the trouble. The strength of new skyscrapers, the fire-proof emergency elevators and stairwells, can help ensure that more people survive any fire.
The National Institute of Standards and Technology will report what they've learned at a public hearing here in New York Next week
