Initiating Event
Structural Response and Collapse
␣ The buckling failure of Column 79 between Floor 5 and Floor 14 was the initiating event that led to the global collapse of WTC 7. This resulted from thermal expansion and failures of connections, beams, and girders in the adjacent floor systems.
␣ The connection, beam, and girder failures in the floor systems, and the resulting structural responses, occurred primarily at temperatures below approximately 400 C (750 F), well below the temperatures at which structural steel loses significant strength and stiffness.
␣ Thermal expansion was particularly significant in causing the connection, beam, and girder failures, since the floor beams had long spans on the north and east sides (approximately 15 m, 50 ft).
o Heating of the long beams resulted in proportionately large thermal elongation relative to the other components of the floor system, in effect, compressing the beams along their length. This led to distortion of the beams and breaking of the connections of the beams to the floor slabs. Furthermore, the simple shear connections used in the typical floor framing were not able to resist these axial compressive forces that developed as the floor framing was heated.
o At Column79,heating and expansion of the floor beams in the north east corner caused the loss of connection between the column and the key girder. Additional factors that contributed to the failure of the critical north-south girder were (1) the absence of shear studs that would have provided lateral restraint and (2) the one-sided framing of the east floor beams that allowed the beams to push laterally on the girders, due to thermal expansion of the beams.
o The fires thermally weakened Floors 8 to 14. As Floor 13 fell onto the floor below, a cascade of floor failures continued until the damage reached the massive Floor 5 slab, leaving Column 79 without lateral support for nine floors. The long unsupported length of Column 79 led to its buckling failure.
␣ Blast events did not play a role in the collapse of WTC 7. NIST concluded that blast events could not have occurred, and found no evidence whose explanation required invocation of a blast event. Blast from the smallest charge capable of failing a critical column (i.e., Column 79) would have resulted in a sound level of 130 dB to 140 dB at a distance of at least half a mile if unobstructed by surrounding buildings (such as along Greenwich Street and West Broadway). This sound level is comparable to a gunshot blast, standing next to a jet plane engine, and more than 10 times louder than being in front of the speakers at a rock concert. The sound from such a blast in an urban setting would have been reflected and channeled down streets with minimum attenuation. However, the soundtracks from videos being recorded at the time of the collapse did not contain any sound as intense as would have accompanied such a blast.
