9-11/anomalies/collapse

Overview
Many of the objections to the official story of what happened on 9/11 center on the cause for the collapses of WTC1, WTC2, and WTC7. The objections can be summarized thusly:
 * Airplane impact plus fire is an inadequate explanation for the collapses (especially WTC7)
 * There is strong evidence that controlled demolition techniques were involved in all three collapses
 * WTC destruction comparison matrix

The WTC7 collapse has some markedly different characteristics from the collapses of WTC1 & 2 (e.g. it was not hit by a plane, which is the standard excuse for overriding expectations that the other two buildings should have remained standing), and its unambiguous resemblance to a standard controlled demolition is much more striking and obvious.

Pages

 * /debate: a structured debate about the possible causes of collapse
 * /temperatures: notes about what happens at different temperatures, and likely temperatures in the WTC buildings

inadequate explanation
It is often said that the 9/11 airplane impacts were so unusual, so unprecedented, that of course we should not be surprised if our pre-existing models break down. This claim basically dismisses the idea that rational investigation can be brought to bear; if our previous understanding was wrong, we need to understand how it was wrong so that we don't make the same mistakes again.

The major points:
 * If the planes are to be held responsible for the collapses, they were acting solely through a combination of damage plus fire – not the initial fuel explosion, and not the force of the impact.
 * The WTC buildings were specifically designed to withstand a jet impact.
 * The steel used was rated to 2000&deg;F for up to 6 hours; there is considerable evidence that the fires could not have burned anywhere near that hot, much less for 6 hours (WTC1 and WTC2 collapsed after 1h:41m:51s and 0h:55m:51s respectively; the WTC7 collapse is a special case)
 * max temp for open flame 1700&deg;F
 * structural steel loses half its strength at about 1200&deg;F
 * building was overengineered by a factor of 2 or 3
 * NIST claims temperatures up to 1800&deg;F, but NIST's metallographic analysis of the steel samples NIST chose based on proximity to hottest points showed only 452&deg;F
 * eyewitnesses (including a responding fireman, presumably trained to evaluate such things) who made it up to the impact area reported only normal or even minor levels of combustion. The fireman (Chief [something] Palmer) was of the opinion that "we can knock this out with two lines".
 * No structural steel building has ever, before or since 9/11, collapsed due to fire, despite many excellent opportunities.
 * Despite not having any historical reason to think that the buildings would collapse (having survived the initial impact), and despite the conclusion of the 9/11 Commission Report that "none of the [fire] chiefs present believed that a total collapse of either tower was possible", there is considerable evidence of foreknowledge that this was not only going to happen but imminent:
 * The OEM Issued a WTC Collapse Warning
 * video: Foreknowledge of 9/11 Twin Towers Collapse?: live British TV coverage after the first collapse but before the second collapse. According to announcers, the WTC web site had been updated with an explanation of the first collapse (including the questionable explanation which ignores the existence of the inner core) and a warning that the second tower would be collapsing as well (which it soon did). Also mentions a couple of other terrorist incidents not mentioned elsewhere: A car bomb outside the State Department in DC, and an explosion in a downtown DC shopping mall.

evidence of controlled demolition
There is considerable evidence that controlled demolition was used on all three buildings:
 * The explosive outward force of the twin tower collapses cannot be explained by the weight of the material above.
 * "Squibs" are seen in multiple videos and photos (individual expulsions of material several floors away from the current point of collapse)
 * Each floor is seen to explode in a sequence which seems very regular and even.
 * If this was due solely to gravity, shouldn't the explosions start small and get larger and faster?
 * If the collapse was due entirely to gravity:
 * why were the buildings so completely pulverized, except for a few vertical metal girders?
 * If the pulverization of the building was due to the grinding action of the falling block of building on the fixed lower block:
 * how did the falling block remain sufficiently intact to destroy all the remaining floors below it and then suddenly disintegrate into unrecognizable rubble at street level?
 * if the falling block was being abraded at the same time as it abraded the fixed block, why didn't the destruction stop after the approximate original height of the falling block?
 * if the falling block was somehow being abraded more slowly than the fixed block, how did it happen that it disintegrated just fast enough to completely destroy the fixed block and itself?
 * ...and what mechanism caused this unequal disintegration?
 * ...and even if the ratio happened to be just right in one tower, how was it also exactly right in the other one?
 * how did multiple steel girders end up embedded horizontally (thrown outward, not down) in neighboring buildings?
 * why were the outside structures, which were not as strong as the core, the ones which survived?
 * why did the collapse follow the path of most resistance, collapsing through the intact lower floors rather than toppling?
 * why did the resulting rubble look nothing like any other steel-frame building collapse in history?
 * how was such a relatively short, low-key fire well within the steel's fire-rating able to utterly destroy two steel-frame buildings when steel-frame buildings have previously been engulfed in flame for many hours (long past their fire rating) without collapse?
 * Explosions (other than the impacts and collapses) were both reported and captured on audio recordings and seismographs.
 * This is not proof, of course; but if they weren't explosive charges, then some plausible alternate explanation needs to be found for each one.
 * Witnesses to the Towers' Explosions
 * video (with sometimes-annoying background music) giving dozens of eyewitness, on-the-scene reports of both isolated explosions and sequential "bangbangbangbangbang..." or "like gunshots" controlled-demolition-type explosions; also account of firefighters finding "an incendiary device" in one of the towers and pulling out as a result: "There's a bomb in the building, start clearing out." The Chief of Safety for the NYC Fire Department Albert Turi (Turrey?) is quoted as saying there were additional explosions, he believed there were devices planted in the building. (also at least one description of the front of the WTC being "sheared off", which does not match any photographs or videos) Reports also of explosions and smoke in the lower levels of at least one building, including the entry level.
 * would be really nice to figure out who some of the witnesses are and find out what their current viewpoint is
 * Contradictory claim: ERROR: 'Seismic Spikes Preceded the Towers' Collapses': seismological data was widely misinterpreted
 * During at least one of the collapses, explosions are heard (and recorded) roughly in synch with the collapse of each floor
 * Just as WTC1 begins to collapse, smoke is seen at the bottom of the tower (in one video; are there any others with a view of the bottom?)
 * Evidence for use of thermite, often used in controlled demolition:
 * Molten steel (much too hot to be caused by oxygen fire) is seen pouring from one corner of WTC2 in the seconds before it began collapsing
 * Pools of molten steel were also found in the rubble
 * The chemical signature of thermite was found on the collapsed steel
 * The contents of the buildings were pulverized – normally you would find a few surviving artifacts
 * Circumstantial evidence: there are multiple reports of unusual activity in the twin towers during the weeks before the attacks.

counter-arguments
David Brin has suggested (albeit not as a refutation of the idea that controlled-demolition was used, as that idea was probably not yet extant at the time of writing) that the "clean" nature of the collapses was simply due to good engineering design intended specifically to minimize danger to those outside the buildings and prevent the feared "domino effect". No information confirming this has yet turned up, however, in the many discussions about the design of the twin towers.

evidence that fits both categories
The "free fall" nature of all three collapses fits into both the "inadequate explanation" and "controlled demolition evidence" categories.
 * The buildings collapsed in about the same amount of time as it would take an object in free-fall to drop from the point of impact.
 * Damaged buildings do not collapse in vertical free fall.

a related point

 * If the planes are to be held responsible for the collapses, they were acting solely through fire plus structural damage:
 * At the time of impact, there were three forces working towards destruction of the towers:
 * initial impact (the plane's inertia tending to push the building strongly in one direction)
 * the force of the explosion
 * structural damage from the impact and explosion
 * The first two forces were only in effect for a few seconds, so all that remained by the time of the collapse was structural damage.

Reference

 * Wikipedia:
 * Collapse of the World Trade Center

Investigation

 * Why the Towers Fell: Two Theories by William Rice, P.E. (Architects and Engineers for 9/11 Truth)

to be filed

 * 2008-05-27 Prominent Structural Engineers Say Official Version of 9/11 "Impossible" "Defies Common Logic" "Violates the Law of Physics": a substantial list of structural engineers with very good credentials, and some of the (pretty damning) things they have said about the collapses.