Even veteran disaster investigators were stunned by the fall of the Twin Towers on September 11, 2001. The next thoughts of the researchers who probed the calamity, aside from outrage, were how and why it happened from an engineering perspective. Why did WTC 1 stand nearly twice as long as WTC 2 after the impact of the aircraft? How could World Trade Center Building 7, which a plane did not hit, stand for seven hours and then collapse? And could such building failures occur elsewhere? Answers to all these questions have not only solved mysteries but also have led to changes in international building codes that may help prevent future tragedies.
The greatest challenge that U.S. National Institute of Standards and Technology (NIST) investigators faced when they began in 2002 was the destruction of the main bodies of evidence that they would normally probe in such a disaster. "Most of the buildings and their contents, save the structural steel skeletons, was demolished to the point that they were unrecognizable and of no practical use in reconstructing the conditions prior to aircraft impact," says structural engineer Shyam Sunder, lead investigator of the NIST report on the World Trade Center disaster.
Instead, the team of more than 200 investigators gathered all the evidence they could to reconstruct the situation the buildings faced before and after the catastrophe. They analyzed 236 pieces of steel obtained from the wreckage, representing all grades of steel used in the buildings and including several pieces impacted by the aircraft or affected by fire. They obtained some 7,000 photographs and roughly 7,000 video segments totaling in excess of 150 hours from the media, public agencies and individual photographers. They compiled and reviewed tens of thousands of pages of documents and interviewed more than 1,000 people who had been on the scene or had been involved with the design, construction and maintenance of the buildings. They conducted lab tests involving large fires and the heating of structural components.
The investigators then developed computer models of how each tower was damaged upon impact, how the jet fuel dispersed, how the fires evolved across each floor, how the structures heated and how they ultimately failed. These simulations of the structures and the physical properties of their materials were extraordinarily complex, with the aircraft impact analysis requiring computations "that were accurate over microseconds," Sunder recalls. At times, researchers had to invent new modeling capabilities to get the simulations to work, such as mapping of fire-generated environmental temperatures onto the structural components of the buildings. Also, "new scientific understanding of spray-applied fire-resistive materials had to be developed," Sunder says.
It turns out that even a combination of high-speed collisions by two airliners and fires across multiple floors would not have destroyed the Twin Towers, according to NIST's final 2005 report on their collapse. The robustness and size of the structures helped them withstand the hits, and in the absence of damage, fires as intense as the ones the towers faced would likely not have led to collapse.
Unfortunately, the impacts dislodged fireproofing insulation that coated steel in the floors and columns, leaving the metal vulnerable to weakening under fire. The ceiling sprinklers also did not work, because the water supplying them was cut off by the collisions. Ultimately, WTC 2 collapsed more quickly than WTC 1 because it had more aircraft damage to the building core. Given how little time each tower had to evacuate, if both towers had been fully occupied with 40,000 people total instead of the estimated 17,400 present, about 14,000 occupants might have died instead of the 2,749 who did perish in the attacks.