Assessment of structural steel from the World Trade Center towers, part V: forensic photographic evidence of tower collapse initiation and progression

摘要

Photographic forensic evidence for the global collapse of the World Trade Center (WTC) towers was developed as part of the National Institute of Standards and Technology (NIST) WTC collapse investigation. The libraries accumulated during the Federal Emergency Management Agency/American Society of Civil Engineers study and NIST investigations were searched for images that showed gross building distortions preceding or during the collapse events. Frames or images showing draw-in of the cast face of WTC 2 and the south face of WTC 1 were analyzed to map the distortion field, which was tracked over time. The inward distortions increased in magnitude as the building burned, reaching a maximum at the time of collapse. Evidence of dropped floors within the towers before global collapse was developed, as was evidence of the kinking and rotation of the top portion of WTC 2 just before and during collapse.