The mechanics of the collapse are really much more simple than conspiracy theorists would like you to believe. The heat expanded the steel in the truss in all directions. As a result they also expanded into the columns. The trusses/floor system, sagged in the middle because the columns were preventing the trusses from expanding in their direction. That led to the bowing of the exterior columns.
Listen to the NIST lead investigator explain the leading hypothesis for the towers collapse.
http://www.pbs.org/wgbh/nova/wtc/sund-fl...
Computer simulation of expanding truss pushing out on a perimeter column and sagging
After the columns bowed, the weight was no longer going straight down. Like taking a straw and bowing it in the middle, it no longer can hold the same weight as it did when it was straight. The building tried to transfer the load to the core columns and massive hat truss on the roof. The weakened core, weakened by fire and impact, couldn't hold the massive weight from tilting. As with the perimeter column, the massive load on the deformed core columns gave way.
Now look at what a REAL Controlled Demolition looks like...
You can hear the explosives clearly over the helicopter engine and rotating blades.
The NIST and "Pancaking"
The massive weight easily caused a "Pancaking" effect but unlike the original hypothesis, the pancaking didn't cause the collapse. It was a result of the collapse.
Update:
Conspiracy theorists are taking the above out of context in an effort to mislead readers into thinking the NIST and I are in disagreement. We are not. As I mentioned above, the pancaking happened AFTER the building was on it's way down and therefore NOT part of the NIST investigation. The NIST only studied the collapse until "Global collapse was inevitable". Any conspiracy theorist that tells you the NIST said the building NEVER pancaked is lying. The building didn't pancake CAUSING the collapse but evidence is strong the building pancaked AFTER the collapse was "inevitable".
What the NIST observed:
Failure of the gusset plate welded to the top of the truss chord was again almost exclusively observed regardless of location. This may be a result of overloading the lower floors as the floors above were "pan-caking".
NIST NCSTAR 1-3C Sect 3.5.3
Of course this will be labeled a contradiction by conspiracy theorists. As if the NIST can't observe something without studying the behavior of it.
Let me make this really easy for them...
1) The NIST said, the heat from the fires sagged the trusses which bowed the columns inward CAUSING the collapse. Pancaking did NOT cause the collapse. The evidence I see agrees with this conclusion.
2) The evidence on the ground strongly indicates, after the collapse began, the building pancaked spreading the debris as we see below. The NIST never studied this so how could we be in disagreement?
I recently E-mailed the NIST to verify this. Here is their response:
NIST did not describe the specific sequence of events after global collapse initiated. The progression of global collapse was induced by the failure of the supporting structure (columns carry vertical loads; floors hold columns together, they do not carry vertical loads). NIST's investigation focused on the factors that led to the initiation of collapse, rather than the sequence of events after the collapse initiated.
Sincerely,
WTC Investigation Team
From the NIST report, page xxxvii:
The focus of the Investigation was on the sequence of events from the instant of aircraft impact to the initiation of collapse for each tower. For brevity in this report, this sequence is referred to as the “probable collapse sequence,” although it does not actually include the structural behavior of the tower after the conditions for collapse initiation were reached and collapse became inevitable.
So while they did investigate the squibs issues and other parts of the global collapse like the gusset plates failure, they never studied the "structural behavior" of the global collapse, as they did "the sequence of events from the instant of aircraft impact to the initiation of collapse". And why should they? Every paper which passed peer review by respected scientific journals (The Journal of 911 Studies is not one of them.) on the collapse of the towers calculates the massive weight of the top sections would crush the buildings as seen. There are mathematical calculations which are shown in Bazant's paper (See below) and are peer reviewed. Others have passed similar papers. The idea that these buildings could not fall as they did flies in the face of these facts.
Conspiracy theorists are clearly lying about this in order to paint this site as unreliable. The irony of their claims seem to be what's most reliable in their movement.
--------------------------------------...
Below we see the top of the south tower fall behind the perimeter columns at an angle. The top falls behind the perimeter columns and on the floors. (This happened at both buildings as this evidenced by these video screen shots of the north tower.) The angle at which it falls pushes the perimeter columns in front of the falling top section outward, giving them distance.
Conspiracy theorists use the original hypothesis - which wasn't created by the government, and which was wrong - to say the NIST can't be trusted, but that's the way all science is. They look at the evidence and create a hypothesis, test the hypothesis against the evidence and if new information comes out, they change the hypothesis accordingly. You would think if the NIST was going to lie, they would just build the lie around the first hypothesis. That they changed it only shows independence. Ironically, Professor Jones has changed his paper numerous times, yet the conspiracy theorists don't cast doubt on whether his paper is correct. Proof of the pancaking effect is the core columns, which can be seen collapsing seconds after the perimeter columns hit the ground.
If the building didn't pancake, what happened to the trusses? Assuming they didn't just fly away it's obvious they fell straight down. More evidence of pancaking is on ground zero.
Note what's left of the core with most of the debris from inside the building around it. This photo was taken in October 3rd so many of the columns were already picked up but the major debris is located closest to the core.
Below is another interesting photo. It shows the perimeter columns laid out as if they simply tilted over. The only explanation is that the floors went straight down and the unsupported perimeter columns pivoted over in large sections.
Below is the Bankers Trust building.
As you can see, the building never caught fire so it was never in any danger of collapse. The evidence points to a combination of unfought fire and damage which brought down the WTC 7. That's what the NIST draft said as well. The Bankers Trust was also constructed differently, with a web column design. The interior columns were not pushed out to the perimeter.
Note the WTC columns laid out as if there were a path to the building. There are no concrete slabs attached to columns.
With the floors pancaking straight down, the perimeter walls were free to lean over in tall sections before breaking off and coming down. That's what gave them distance.
The above photo illustrates just how far the perimeter columns could have gone. If the columns didn't break up as they leaned out they could have made a path to the Hudson river. Reaching Building 7 and the Winter Garden would have been well within reason.
The last piece of evidence is the standing perimeter columns. You can see what's left of the steel plates which hold the floors up. The tremendous weight stripped them off as the floors were on their way down.
As generally accepted by structural engineering and structural mechanics experts (though not by some laymen and fanatics seeking to detect a conspiracy), the failure scenario, broadly proposed by Bazant (2001), and Bazant and Zhou (2002), on the basis of simplified analysis, and supported by very realistic, meticulous and illuminating computer simulations and exhaustive investigations by S. Shyam Sunder's team at the National Institute of Standards and Technology (NIST, 2005), may be summarized as follows:
1. About 60% of the 60 columns of the impacted face of framed-tube (and about 13% of the total of 287 columns) were severed, and many more were significantly deflected. This caused stress redistribution, which significantly increased the load of some columns, near the load capacity for some of them.
2. Fire insulation was stripped during aircraft impact by flying debris (without that, the towers would likely have survived). In consequence, many structural steel members heated up to 600±C (NIST 2005) (the structural steel used loses about 20% of its yield strength already at 300±C, NIST 2005, and exhibits significant visco-plasticity, or creep, above 450±, especially at high stresses that developed; see e.g. Cottrell 1964, p. 299; the press reports right after 9/11, indicating temperature in excess of 800±C, turned out to be groundless, but Bazant and Zhou's analysis did not depend on that).
3. Differential thermal expansion, combined with heat-induced viscoplastic deformation, caused the floor trusses to sag. The sagging trusses pulled the perimeter columns inward (by about 1 m, NIST 2005). The bowing of columns served as a huge imperfection inducing multi-story buckling. The lateral deflections of some columns due to aircraft impact and differential thermal expansion also decreased buckling strength.
4. The combination of six effects
a) overload of some columns due to initial stress redistribution,
b ) lowering of yield limit and creep,
c) lateral deflections of many columns due to sagging floor trusses,
d) weakened lateral support due to reduced in-plane stiffess of sagging floors,
e) multi-story buckling of some columns (for which the critical load is an order of magnitude less than it is for one-story buckling), and
f) local plastic buckling of heated column webs finally led to buckling of columns (Fig. 1b). As a result, the upper part of tower fell, with little resistance, through at least one floor height, impacting the lower part of tower. This triggered progressive collapse because the kinetic energy of the falling upper part far exceeded the energy that could be absorbed by limited plastic deformations and fracturing in the lower part of tower. (Bazant, Verdure, 2006)
http://www.civil.northwestern.edu/people...
/ProgressiveCollapseWTC-6-23-2006.pdf
For a detailed account of the collapse, do yourself a favor and READ ALL the NIST FINAL reports. Any conspiracy site which gives you the old preliminary reports are being dishonest. There is incredible detail which mirror the evidence of the event in each of the reports below.
Media_Public_Briefing_040505_final.pdf
NISTNCSTAR1CollapseofTowers.pdf
NISTNCSTAR1-1.pdf More detail here: http://wtc.nist.gov/oct05NCSTAR1-1index....
NISTNCSTAR1-2.pdf More detail here: http://wtc.nist.gov/oct05NCSTAR1-2index....
NISTNCSTAR1-3.pdf More detail here: http://wtc.nist.gov/oct05NCSTAR1-3index....
NISTNCSTAR1-4.pdf More detail here: http://wtc.nist.gov/oct05NCSTAR1-4index....
NISTNCSTAR1-5.pdf More detail here: http://wtc.nist.gov/oct05NCSTAR1-5index....
NISTNCSTAR1-6.pdf More detail here: http://wtc.nist.gov/oct05NCSTAR1-6index....
NISTNCSTAR1-7.pdf More detail here: http://wtc.nist.gov/oct05NCSTAR1-7index....
NISTNCSTAR1-8.pdf More detail here: http://wtc.nist.gov/oct05NCSTAR1-8index....
http://wtc.nist.gov/reports_october05.ht...
The Collapse of the Twin Towers Part V: Can the Conspiracy Theorists debunk this?
I applaud all the research and work you went to, to try and show people like Rosie O Donnell how this happened. Sad to say, the world has Jim Jones followers that will fall for anything, believe anything, and spread anything. And then there are American haters that are more then happy to spread evil.
Reply:Conspiracy theorists are nothing more than extreme Bush bashers. As a conservative, even I know Bush hasn't got the I.Q. to coordinate such an event.
Not to mention, he was zig-zagging all across the contintent in Air Force 1 at the time trying to figure out what to do.
Reply:Whether or not the US government was involved in the actual 9/11 attack is less relevant than what they did in response. Which was to pursue a long held and carefully detailed Republican agenda of a neverending war on so called 'terrorism' - read our civil liberties. Liberal western democracies all over the world followed suit, beefing up police and defense spending and immigration laws, watering down citizen's rights and the very foundations of liberal democracy - the right to be assumed innocent until proven guilty for example, all in the name of September 11. No matter who was behind it September 11 was a bonanza for governments everywhere interested in gaining more control over their citizens.
The worst insult has been the way it's been framed in terms of a religious war - the forces of evil muslim fundamentalism against enlightened western christianity
Reply:Their hatred of Bush trumps facts and logic. The answer by one above is a cut and paste from another question that was asked. It all comes down to hating Bush.
Reply:Did you ask yourself why did all the so-called "investigators" are ALL appointed by the Bush Administration to "investigate" ? No conflicts of interest there.
Do you remember the Cheney involvement in distorting and covering up global-warming ?
Maybe something will add up in your mind ;)
Reply:Wow, impressive 5.
Reply:And your point is!the guy in the video said NOTHING!
