The M.I.T. professor added that about 30 percent of the collapse energy was expended rupturing the materials of the building, while the rest was converted into the kinetic energy of the falling mass. The huge gray dust clouds that covered lower Manhattan after the collapse were probably formed when the concrete floors were pulverized in the fall and then jetted into the surrounding neighborhood. "Of the kinetic energy impacting the ground, only 0.1 percent was converted to seismic energy," he stated. "Each event created a (modest-sized) magnitude 2 earthquake, as monitored at Columbia University's Lamont-Doherty Observatory, which is located about 30 kilometers away from New York City." Kausel concluded that the "the largest share of the kinetic energy was converted to heat, material rupture and deformation of the ground below."
Despite the expert panel's preliminary musings on the failure mechanisms responsible for the twin towers' fall, the definitive cause has yet to be determined. Reportedly, the National Science Foundation has funded eight research projects to probe the WTC catastrophe. The American Society of Civil Engineers is sponsoring several studies of the site. Meanwhile the Structural Engineering Institute of the American Society of Structural Engineers has established an investigative team to analyze the disaster and learn from the failure. W. Gene Corley, senior vice president of the Construction Technology Laboratory in Skokie, Ill., is said to be heading the ASSE study team through its initial phase of data gathering, and then William Baker, a structural engineer at the Chicago-based firm of Skidmore Owings & Merrill in Chicago, will lead the following analysis phase. The Structural Engineering Institute is to partner with the American Institute of Steel Construction, the National Fire Protection Association and the Society of Fire Protection Engineers. The Federal Emergency Management Agency has been invited to join as well.
How the Towers Fell
Given the lack of firm conclusions regarding how the collapses occurred, the M.I.T. panel participants asked their audience to consider various theories they put forth. In general, it was agreed that as the structure warped and weakened at the top of each tower, the frame, along with the concrete slabs, furniture, file cabinets and other materials, became an enormous consolidated weight that eventually crushed the lower portions of the structure below. The details of how the frame members failed remain under contention.
Professor Connor's theory focused on weaknesses in how the vertical and horizontal structural members were tied together. During construction, he explained, each prefabricated floor system was lifted into place by a crane and "supported at the ends like a hinge, where they were bolted and welded to the inner and outer framing tubes" so that part of the gravity load went through the core and the other part through the exterior structure. "The floor trusses sat on beams and were tied down so the core was locked to the exterior," he said. "It was an unusual system and very lightweight. If you lose the connection between them, however, you lose the ability to carry the floor loads and allow the floors to slide back and forth under stress. If a damaged floor system were to fall, it would break the end connections in the lower floors and down and down the floors would go."
"In my theory, the hot fire weakened the supporting joint connection," Connor continued. "When it broke, one end of a floor fell, damaging the floor system underneath, while simultaneously tugging (pulling) the vertical members to which it was still attached toward the center of the building and down." This phenomenon started a parasitic process that accelerated until total failure and the structure fell in on itself, he said.