Hi stockman scott; This is a link to an engineering analysis of the WTC collapse, with comments written by someone who is obviously uneducated in the subject. The comments are a hilarious read for an engineer:
911research.wtc7.net
But the above guess, that is, that it was the columns that failed, is undoubtedly wrong. Instead, what happened is that a floor failed, dropped to the floor below, which then failed, etc. The response to this theory is:
Let's accept Dr. Eagar's zipper scenario (despite the clear evidence that fires did not cover a whole floor in either tower) and imagine that all the trusses of a floor failed in rapid succession and the whole floor fell. Then what? It would fall down about ten feet, then come to rest on the floor below, which was designed to support at least five times the weight of both floors, the fall cushioned by the folding of the trusses beneath the upper floor. But let's imagine that the lower floor suddenly gave up the ghost, and the two floors fell onto the next, and that failed, and floors kept falling. Then what? The floor diaphragms would have slid down around the core like records on a spindle, leaving both the core and perimeter wall standing.
911research.wtc7.net
Okay, let's assume that the fall was "cushioned by the folding of the trusses beneath the upper floor". My own guess is that the trusses would do a rather poor job of cushioning a slab of concrete. The reason for this is that it is unlikely that the trusses would stay aligned under the concrete. So they'd end up flattened under the load. And if they did stay aligned, the shock of the upper floor would be transferred through them with little cushioning to the floor below. But we can make a best case calculation for the effect of cushioning.
Let "M" be the mass of a floor. Then the force applied by a floor due to gravity is "ma = Mg" where "g" is the acceleration of gravity. Let's say that a truss is 12 inches tall, and that it collapses to zero height in a smooth and continuous manner (best case for "cushioning"). What is the force that the floor applies on the floor below during that cushioning? The energy acquired during the drop was "E = 9Mg", where "9" is the distance dropped in feet. This (kinetic) energy must be converted into crushed trusses over a distance of 1 foot. Let "F" be the force applied by the floor on the trusses (and on the floor below). Equating energies, we get "1F = 9Mg". Thus, the force applied on the floor below is about 9x the weight of a floor. The total weight that is applied to the trusses of the floor below is about 10x the weight of a floor. And that's probably enough to guarantee the collapse.
Another way of looking at it. Have you ever tried to catch something that was dropped from a height? The higher it is, the harder it hurt, didn't it. And if it was heavy enough for you to feel its weight, its weight seemed larger didn't it.
As far as the floors dropping off like "records on a spindle", this might have gone on for a floor or two, but as the vertical members were bent by having their trusses ripped off, and lost the ability to transfer stresses to the rest of the building, they too would collapse. Probably after only a few stories of floors dropped.
You're just caught up in a miasma of bad engineering. Hey, it was probably generated by the same people who hate engineers, hate modern technology, etc., so why do you think that they know how these things work?
-- Carl
P.S. Do you know who the author is of this tripe? |
