Perhaps somebody here can explain why ~20 floors of wtc 7
Eight or nine, actually.
just collapsed in 2.3 seconds!!
Well, yes, many of us can, but it's debatable whether it's worth doing so to someone who, we suspect, will choose either to disbelieve the answer, misunderstand it, or both.
First of all, the floors themselves had already collapsed, as had the core columns that supported them. We know this because the mechanical penthouses had already fallen into the building, indicating that there was no intact structure beneath them. With no core columns holding up their inner ends, the floor beams cannot still have been in place, because each was now only supported at one end.
Next, we know that there was an initial period of collapse that was significantly slower than freefall. Since, during this period, gravitational potential energy was being converted into some other form than kinetic energy (that's implicit in the definition of freefall as the acceleration at which all the GPE is converted to KE), we know that some GPE was being converted into structural deformation energy. In other words, bending and breaking of the supports. We know this because there was no other form for it to be converted into, and energy cannot disappear.
Finally, we know that, after steel has been deformed through a certain amount, it fractures. The deformation of a steel column typically takes the form of a multi-hinge buckle, in which a section of the column bends sharply at its top, bottom and a point roughly mid-way between. When these three points fracture, the two separated sections of column are laterally displaced from the load above, so they will cause no resistance to its collapse. And, if the fracture takes place when the top and bottom bends are about eight floors' worth of height apart - which can easily happen if the initial buckle is longer than this - then there is nothing to stop the load above from falling at freefall acceleration until it strikes the remaining structure at the bottom of the buckle. So, what we expect if this happens is the form of collapse that you describe:
That leaves no resistance from "bending" of support columns, or any walls, or anything of the sort. There was NO RESISTANCE WHATSOEVER on ~20 floors.
Except for a minor arithmetical error on your part, which is that the building actually fell more like eight or nine floors' worth in that time.
A building that wasn't even hit by a plane, mind you.
But which was allowed to burn for seven hours, during which time no significant firefighting effort was possible because the water main had been severed by the collapse of WTC1 and so insufficient water pressure was available. And it's also interesting to note that the two buildings that were hit by planes collapsed in a very different way.
What's the cause of NO RESISTANCE WHATSOEVER on ~20 floors, on all sides and all corners of these ~20 floors?
Eight or nine floors, not twenty. s = ut + 1/2 at^2. The velocity at the beginning of the freefall period was small enough to be neglected for a first approximation, so the collapse distance for 2.3 seconds was about 9.81 * 2.3^2 / 2 = 25.9 metres. 20 floors would mean a floor-to-ceiling spacing of less than four feet, which I've never seen outside of
Being John Malkovich.
It's almost as if those 20 floors just weren't even there.
Eight or nine, and, as we know, the floors
weren't there.
Dave
