Introduction
I provide a short computation, focused on World Trade Center building 7. Based on very favorable assumptions for achieving a fast fall, including ignoring resistance due to intact steel columns, I could only get the building to fall in about 8.3 seconds, whereas the observed roof-fall time is approximately 6.5 seconds. The problem is the large number of floors and conservation of momentum in a collision. Some of the “official” explanations about progressive collapse are evocative but they do not explain the difficulty in the rapid fall of the building along with what is evidently taking place when the video of the falling building is observed.
