cosmic,
thanks for asking, I'll post a couple of links to some credible sources on the WTC towers collapse. Putting aside politics, emotions, and hysteria, failure analysis on a mechanical system of this scale is intriguing.
--- here's a 2001 special report on the WTC collapse from the Journal of the Minerals, Metals, and Materials Society.
tms.org
"Why Did the World Trade Center Collapse? Science, Engineering, and Speculation"
THE COLLAPSE
Nearly every large building has a redundant design that allows for loss of one primary structural member, such as a column. However, when multiple members fail, the shifting loads eventually overstress the adjacent members and the collapse occurs like a row of dominoes falling down.
The perimeter tube design of the WTC was highly redundant. It survived the loss of several exterior columns due to aircraft impact, but the ensuing fire led to other steel failures. Many structural engineers believe that the weak points—the limiting factors on design allowables—were the angle clips that held the floor joists between the columns on the perimeter wall and the core structure (see Figure 5). With a 700 Pa floor design allowable, each floor should have been able to support approximately 1,300 t beyond its own weight. The total weight of each tower was about 500,000 t.
As the joists on one or two of the most heavily burned floors gave way and the outer box columns began to bow outward, the floors above them also fell. The floor below (with its 1,300 t design capacity) could not support the roughly 45,000 t of ten floors (or more) above crashing down on these angle clips. This started the domino effect that caused the buildings to collapse within ten seconds, hitting bottom with an estimated speed of 200 km per hour. If it had been free fall, with no restraint, the collapse would have only taken eight seconds and would have impacted at 300 km/h.1 It has been suggested that it was fortunate that the WTC did not tip over onto other buildings surrounding the area. There are several points that should be made. First, the building is not solid; it is 95 percent air and, hence, can implode onto itself. Second, there is no lateral load, even the impact of a speeding aircraft, which is sufficient to move the center of gravity one hundred feet to the side such that it is not within the base footprint of the structure. Third, given the near free-fall collapse, there was insufficient time for portions to attain significant lateral velocity. To summarize all of these points, a 500,000 t structure has too much inertia to fall in any direction other than nearly straight down.
----- here's the final reports on study by the NIST of the World Trade Center Towers I and II collapse. The main report is at the first link, NIST NCSTAR-1.
Chapter 6 describes in detail the likely sequence of events during structural failure... it is all explained. I doubt Rosie has ever read any of these reports....
wtc.nist.gov
---- here's the slideshow on the NIST report on WTC-7 collapse
wtc.nist.gov
---- here's Popular Mechanics article "Debunking the 9/11 Myths", including discussion of WTC-7 collapse
popularmechanics.com
** political disclaimer - I detest this White House. Their amateurish and incompetent management have brought this country closer to ruin.
But put politics aside when discussing the WTC collapse. Its not a conspiracy. I think the engineers and metallurgists can give a reasonable explanation of what caused the structures to collapse.
---This is not comparable to the endless debates still raging over exactly how the Titanic broke apart and sunk after it struck the iceberg, or whether there was another assassin besides Oswald in Dallas on Nov 22, 1963. |
