Oystein
Penultimate Amazing
- Joined
- Dec 9, 2009
- Messages
- 18,903
...Having a CD where collapse begins slowly and gradually builds up to freefall makes no sense. Had it been a CD going at freefall at any point, it should have been at the beginning to maximize gravitational momentum in the building's destruction.
If anyone in addition to C7 or MM wants to correct any of these observations, please do so. Tri? Shek? Oystein? You know me; I'll make corrections when I'm shown to be wrong.
I'd advise against any prescription of what characteristics of movement a building "should" display when it is being CDed. Such statements imply knowledge of the perpetratots' objectives, which we can't have.
Here is why:
A structure collapses if and when a sufficient number of its structural support elements fail.
The exact timing and dynamics of the collapse, as observed from the outside, are the result of structural members failing in a paricular temporal order.
This is true (a truism) for any kind of collapse, no matter what it's cause. For example, it is true when a structure collapses due to fire: Some elements fail first, and in sequence, other element failures follow.
A structure could be rigged with explosives such that they fail structural elements in the exact same chronological sequence as would happen if fire were the cause.
For example, imagine NIST had done their simulations before 9/11 already, and given the details of the FEA model to terrorists. These terrorists now go ahead and place explosive charges at all the elements that caused the collapse to start according to the NIST model (column 79, and the beams in its vicinity, etc.), and then perhaps also at all the elements that failed in the ensuing collapse propagation as per simulation. And then set them off in a times fasion precisely following the simulation. The result would be a collapse that looks exactly like the real thing: e.g. with <g onset during the 1st second of north wall release, 2.25 seconds of ~g, followed by decreasing acceleration.
In other words: It is, in theory, possible to make any CD look like any possible "natural" collapse scenario.(*)Which has this corrollary: The visually measurable dynamics of a building collapse do not allow us to rule out CD, ever.
(This doesn't go both ways however: It would be possible to CD a building such that the enduing collapse dynamics could not be attained by any "natural" causes. In the easiest case, just kill every joint (all the many thousands there are) from top to bottom at the same instance: everything will fall at freefall immediately to the ground, with no collapse progression)
Instead, to rule out CD, one has to look at other properties of it: For example, you can't have a chiefly explosive CD without there being insanely loud BANGs. You can't have it without the buckeled members in the debris pile showing particular failure modes. etc.
(*): Ok, not quite. Explosives sever steel members instantly, without any shortening before breaking, and that influences the dynamics. To account for that, the genius CD master would probably have to fiddle with ignition times and perhaps employ a few methods other than shaped explosive charges. My assertion still stands however: In principle, CD can, in principle, simulate a very wide range of natural collapses.