On 20/09/18 12:06, z9wahqvh wrote: [...]
in WTC 1 and 2, posit a mechanism for building collapse
The mechanism is of course simply that the heavy concrete floors of the top twenty or ten stories, partly held together by the outrigger truss system in the roof, fell one or two stories onto the concrete floor below when the fire softened and buckled the steel structure supporting them. The fallen-on floor gave way almost at once, and the concrete fell on the floor below that, now with increased weight. And so on. The remaining intact shell and the speed of collapse mostly kept the falling bits together, though as the concrete floors fell the shell also broke apart and fell.
that, whatever else you think about it, has never been seen before or since,
While rare, Progressive Collapses, where the collapse happens story by story, are hardly unknown. Most commonly, progressive collapses progress from the ground upwards, with floors falling because their support has previously fallen: well-known examples include the Oklahoma City Bombing. Slightly less common is when the collapse starts higher up and progresses downwards, with floors falling because higher floors have fallen on them and overloaded them, as in WTC1 and 2. The term Pancake Collapse is sometimes loosely used for these, although the distinction is not always rigidly applied. Apart from WTC examples of downward-progressing pancake collapse include the collapses of Plasko and L'Ambiance Plaza, and the partial collapses of Skyline Towers and of Ronan Point. Actually Ronan Point was both a pancake and a progressive collapse - the collapse of floors 18 to 1 progressed downwards in pancake fashion, then later the collapse of floors 19 to 22 progressed upwards. Pancake Collapse is also known in some types of ten story residential buildings whose construction makes them prone to fail in this way, especially in fires - where the term originated (long before 9/11). There have been about 12 progressive collapses in highrises or skyscrapers caused by other means than fire since 1950, the majority of which were in steel framed buildings. There have been about 30-40 major or floor-wide fires in highrises or skyscrapers since 1950, about four of which pancake collapsed either wholly or partially. Apart from WTC, none of those fires were unfought, none of the buildings were of lightweight tube-framed structure, and none had such significant damage pre-accompanying the fire. -- Peter Fairbrother