Take the 2-minute tour ×
Skeptics Stack Exchange is a question and answer site for scientific skepticism. It's 100% free, no registration required.

There have been a lot of claims made about the circumstances surrounding the events of 9/11 and Building 7 in particular.

The central claim regarding Building 7, that I have heard personally, is that Building 7 fell in such a perfect fluid motion that its collapse had to be due to a controlled demolition. Furthermore since Building 7 wasn't hit by a plane directly, it is claimed that there can be no other explanation for how the building collapsed is such a seemingly controlled fashion.

Is there any evidence to support the claims that Building 7 was destroyed in a directly controlled fashion?

share|improve this question
debunking911.com has a nice section on WTC7. –  Sjoerd C. de Vries Mar 28 '11 at 22:24
This claim is usually accompanied by the assumption that '9/11 was an inside job'. Chomsky, who was certainly no friend of the Bush administration provides some good arguments against that assumption. –  naught101 Nov 5 '12 at 4:24

2 Answers 2

up vote 104 down vote accepted


Fires ignited by falling debris (from WTC 1) are the sole cause of the WTC 7 collapse.

Short answer:

  • Debris from the collapse of WTC 1, about 350 ft. to the south, impacted WTC 7, igniting fires on at least 10 floors.
  • Other than starting the fires, the debris impacts were not a contributing factor.
  • Fires on six of those floors grew from the time of the debris impact (10:28:22 a.m.) and lasted until the building collapsed (5:20:52 p.m.).
  • After several hours, the heat absorption ability of the fire-proofing began to fail. The building's structural steel began to soak up heat from the fires.
  • A ''seated connection'' was used for some of the girders framed to interior columns. The girder was held by four "erection bolts" (''girders'' span between columns; ''beams'' span between girders).
  • A 13.7 m (540 in.) long W33x130 beam heated uniformly to 600 °C will expand along its length by 4.5 inches.
  • In doing so it will produce a lateral force of 6.9 million pounds (30.7 million newtons).
  • The four erection bolts resisting this force had a total shear capacity of 0.180 million pounds (i.e., 2.6 percent of the possible force).
  • By the time the girders reached 164 °C the four bolts had failed (i.e., torn in two, ripped apart, cleaved in twain, etc.).
  • With the failure of the bolted connection, further thermal expansion of the floor beams pushed a 13th floor girder, between Columns 79 and 44, off its seat.
  • The flooring system on Floor 13 subsequently failed, collapsing onto the floors below, resulting in the collapse of floors 12 through 6.
  • The floor collapse left more than 150 ft. of Column 79 without lateral support.
  • Without structure transferring load laterally, Column 79 buckled and failed.
  • The transfer of load, once supported by Column 79, overwhelmed the structural capacity of the other columns. They too failed.
  • After burning for 6 hours and 52 minutes, WTC 7 suffered a fire-induced progressive collapse. The building was completely destroyed.

Long answer:

1. Erection bolts are not structural connections

In a seated connection, the beam or girder was supported by the seat, which was welded to the column. Bolts were installed that fasten the beam or girder to the seat for erection purposes These erection bolts did not carry any gravity load; rather, they were installed to insure that the beam or girder was held in place during erection. NIST found no evidence that the girders or beams in WTC 7 were welded to the seats. In a similar way. an angle or plate was bolted to the top flange to prevent the beam or girder from twisting, but there was little restraint to bending in the plane of the beam. NCSTAR 1-9 Figure 8-21 Page 349 Consider the girder that spanned between Column 79 on the interior of the building and Column 44 on the exterior. Thermal expansion of this girder would have loaded the erection bolts in shear, since (1) there were no shear studs anchoring the girder to the slab (and thereby restraining elongation), and (2) the columns were prevented from lateral movement because they were embedded in the floor slabs which had considerable in-plane stiffness. Additionally, the expansion of floor beams that framed into this girder, because the framing was asymmetrical, tended to add additional shear load to the erection bolts. The combination of these two shear loads could have failed the bolts in shear. If the erection bolts were to fail, then there would be no positive attachment preventing the girder from being pushed off the seat. Source: (NCSTAR 1-9) VOLUMES 1 and 2, page 348.

2. Thermal expansion will break stuff

The first failures observed were of the shear studs, which were produced by axial expansion of the floor beams, and which began to occur at fairly low beam temperature of 103 °C. Axial expansion of the girder then led to shear failure of the bolts at the connection to Column 79; and, at a girder temperature of 164 °C, all four erection bolts had failed, leaving that end of the girder essentially unrestrained against rotation. Continued axial expansion of the floor beams pushed the girder laterally at Column 79, as shown in Figure 8-26, in which failed shear studs and bolts were evident. When the beam temperatures had reached 300 °C, all but three shear studs in the model had failed due to axial expansion of the beams, leaving the top flanges of the beams essentially unrestrained laterally. Continued axial expansion of the girder caused it to bear against the face of Column 79, generating large axial forces that led to failure of the bolts connecting the girder to Column 44. When the girder temperature had reached 398 °C, all four erection bolts at Column 44 had failed, leaving the girder essentially unrestrained against rotation at both ends. After failure of the erection bolts in the seat at Column 44, continued axial expansion of the floor beams pushed the girder laterally, where it came to bear against the inside of the column flange. Axial compression then increased in the floor beams, and at a beam temperature of 436 °C, the northmost beam began to buckle laterally. Buckling of other floor beams followed as shown in Figure 8-27(a), leading to collapse of the floor system, and rocking of the girder off its seat at Column 79 as shown in Figure 8-27(b). The collapse process took time to occur in the LS-DYNA analysis, during which the temperatures had ramped up to their maximum values in the simulation. Source: (NCSTAR 1-9) VOLUMES 1 and 2, page 352.

3. Collapse Initiation

The simple shear connection between Column 79 and the girder that spanned the distance to the north face (to Column 44) failed on Floor 13. The connection failed due to shearing of erection bolts, caused by lateral thermal expansion of floor beams supporting the northeast floor system and, to a lesser extent, by the thermal expansion of the girder connecting Columns 79 and 44. Further thermal expansion of the floor beams pushed the girder off its seat, which led to the failure of the floor system surrounding Column 79 on Floor 13. The collapse of Floor 13 onto the floors below-some of which were already weakened by fires-triggered a cascade of floor failures in the northeast region. This, in turn, led to loss of lateral support to Column 79 in the east-west direction over nine stories (between Floors 5 and 14). The increase in unsupported length led to the buckling failure of Column 79, which was the collapse initiation event. Source: (NCSTAR 1-9) VOLUMES 1 and 2, page 611.

4. "Free fall" is a myth

To further clarify the descent of the north face, NIST recorded the downward displacement of a point near the center of the roofline from first movement until the north face was no longer visible in the video. Numerical analyses were conducted to calculate the velocity and acceleration of the roofline point from the time-dependent displacement data. The instant at which vertical motion of the roofline first occurred was determined by tracking the numerical value of the brightness of a pixel (a single element in the video image) at the roofline. This pixel became brighter as the roofline began to descend because the color of the pixel started to change from that of the building façade to the lighter color of the sky.

The approach taken by NIST is summarized in NIST NCSTAR Report 1A, Section 3.6, and detailed in NIST NCSTAR Report 1-9, Section 12.5.3.

The analyses of the video (both the estimation of the instant the roofline began to descend and the calculated velocity and acceleration of a point on the roofline) revealed three distinct stages characterizing the 5.4 seconds of collapse:

  • Stage 1 (0 to 1.75 seconds): acceleration less than that of gravity (i.e., slower than free fall).
  • Stage 2 (1.75 to 4.0 seconds): gravitational acceleration (free fall).
  • Stage 3 (4.0 to 5.4 seconds): decreased acceleration, again less than that of gravity.

:This analysis showed that the 40 percent longer descent time—compared to the 3.9 second free fall time—was due primarily to Stage 1, which corresponded to the buckling of the exterior columns in the lower stories of the north face. During Stage 2, the north face descended essentially in free fall, indicating negligible support from the structure below. This is consistent with the structural analysis model, which showed the exterior columns buckling and losing their capacity to support the loads from the structure above. In Stage 3, the acceleration decreased as the upper portion of the north face encountered increased resistance from the collapsed structure and the debris pile below. Source: Questions and Answers about the NIST WTC 7 Investigation.

NCSTAR 1-9 Figure 12-77 Page 603

Claims, assertions and questions :

Many of the claims and assertions by WTC 7 conspiracy theorists can be found at WTC 7 Draft Reports for public comment. The submission by Richard Gage of Architects & Engineers for 9/11 Truth is a good representation of how 911 conspiracy theorists rely on a lack of evidence to somehow draw conclusions (see: argument from ignorance).

Investigation by the National Institute of Standards and Technology*:

The National Institute of Standards and Technology (NIST) is responsible for conducting fact-finding investigations of building failures that resulted in substantial loss of life or that posed significant potential of substantial loss of life.

Final Report on the Collapse of World Trade Center Building 7 (The results of all NIST WTC investigations can be found at : NIST and the World Trade Center)

NIST: Questions and Answers about the NIST WTC 7 Investigation

How did the fires cause WTC 7 to collapse?

The heat from the uncontrolled fires caused steel floor beams and girders to thermally expand, leading to a chain of events that caused a key structural column to fail. The failure of this structural column then initiated a fire-induced progressive collapse of the entire building.

According to the report's probable collapse sequence, heat from the uncontrolled fires caused thermal expansion of the steel beams on the lower floors of the east side of WTC 7, damaging the floor framing on multiple floors...

How hot did WTC 7's steel columns and floor beams get?

Due to the effectiveness of the spray-applied fire-resistive material (SFRM) or fireproofing, the highest steel column temperatures in WTC 7 only reached an estimated 300 degrees C (570 degrees F), and only on the east side of the building did the steel floor beams exceed 600 degrees C (1,100 degrees F). However, fire-induced buckling of floor beams and damage to connections-that caused buckling of a critical column initiating collapse-occurred at temperatures below approximately 400 degrees C where thermal expansion dominates. Above 600 degrees C (1,100 degrees F), there is significant loss of steel strength and stiffness. In the WTC 7 collapse, the loss of steel strength or stiffness was not as important as the thermal expansion of steel structures caused by heat.

Some people have said that a failure at one column should not have produced a symmetrical fall like this one. What's your answer to those assertions?

WTC 7's collapse, viewed from the exterior (most videos were taken from the north), did appear to fall almost uniformly as a single unit. This occurred because the interior failures that took place did not cause the exterior framing to fail until the final stages of the building collapse. The interior floor framing and columns collapsed downward and pulled away from the exterior frame. There were clues that internal damage was taking place, prior to the downward movement of the exterior frame, such as when the east penthouse fell downward into the building and windows broke out on the north face at the ends of the building core. The symmetric appearance of the downward fall of the WTC 7 was primarily due to the greater stiffness and strength of its exterior frame relative to the interior framing...

Graphic showing the buckling of WTC 7 Column 79 (circled area), the local failure identified as the initiating event in the building's progressive collapse

Did investigators consider the possibility that an explosion caused or contributed to the collapse of WTC 7?

Yes, this possibility was investigated carefully. NIST concluded that blast events inside the building did not occur and found no evidence supporting the existence of a blast event...

Is it possible that thermite or thermate contributed to the collapse of WTC 7?

NIST has looked at the application and use of thermite and has determined that its use to sever columns in WTC 7 on 9/11/01 was unlikely.

Thermite is a combination of aluminum powder and a metal oxide that releases a tremendous amount of heat when ignited. It is typically used to weld railroad rails together by melting a small quantity of steel and pouring the melted steel into a form between the two rails...

...Analysis of the WTC steel for the elements in thermite/thermate would not necessarily have been conclusive. The metal compounds also would have been present in the construction materials making up the WTC buildings, and sulfur is present in the gypsum wallboard used for interior partitions...

An emergency responder caught in the building between the 6th and 8th floors says he heard two loud booms. Isn't that evidence that there was an explosion?

The sound levels reported by all witnesses do not match the sound level of an explosion that would have been required to cause the collapse of the building. If the two loud booms were due to explosions that were responsible for the collapse of WTC 7, the emergency responder-located somewhere between the 6th and 8th floors in WTC 7-would not have been able to survive the near immediate collapse and provide this witness account.

*NIST is an agency of the U.S. Department of Commerce.

share|improve this answer
@jon_darkstar The main towers did not fail from side impact. They failed because the truss hangers for the concrete floors seperated from their perimeter supports and the floor started dropping....bang, bang, bang...all the way down. Why straight down ? Gravity. –  Rusty Mar 26 '11 at 22:34
@jon_darkstar I can understand you might think that. Makes perfect sense. But the towers are not big poles. They are big boxes (an inner box and an outer box) and they are mostly hollow. The floors are basically just hanging between the boxes and when one comes off its hangers it can only go down. Down the inside of the box. –  Rusty Mar 26 '11 at 23:03
Gentle but firm reminder: comments are not the place for discussion, use the chat for that. Thank you. :-) –  Ebenezer Sklivvze Mar 26 '11 at 23:09
we can argue finer points of that forever, so lets not. return to the question at hand - building 7. can you propose any reasonable explanation for it that does not involve explosives? also, try to explain why the 911 Commission Report NEVER mentions building 7 in over 400 pages. Not a footnote. Nothing! @Sklivvz - sorry, read it right after –  jon_darkstar Mar 26 '11 at 23:10
You may want to show pictures of the SOUTH side of WTC7 prior to its collapse. Most conspiracy nutters neglect the fact that nearly 25% of the first ten floors were missing on the south side (they only, conveniently for themselves, show the north side of the building which faced away from the collapse). Check pages 59-66 of 911myths.com/WTC7_Lies.pdf –  Larian LeQuella Sep 17 '11 at 3:39

Yes, World Trade Center building 7 fell due to controlled demolition, there are many reasons why we can expect this to be so.

1.) Free fall speed was achieved after global collapse was initiated, indicating complete failure of all support structure.

This is an important point because it proves without any doubt that the failure of the central column could not account for the speeds attained by the failure of the rest of the building.

If any of the other columns in the building resisted gravity to any measurable degree, then the building could not have fallen at free fall speed, by definition. That would necessarily mean that the other 23 interior columns stopped offering any resistance to gravity after the single column under the east penthouse collapsed, and the collapse of all exterior walls initiated (The entire perimeter collapsed simultaneously).

To clarify, even if you accept that there was only free fall in "Stage 2" as NIST claims (Because they start counting when the penthouse falls.), everything I have said is just as valid because once global collapse begins, nearly every column that supported the perimeter walls must have been entirely (not partially) destroyed. Even only a handful of undamaged columns would have prevented free fall and likely would have resulted in asymmetric collapse.

In his video showing free fall collapse, David Chandler shows a margin of error of 0.456, or about 5%. This number I think is very pessimistic, but if we take it at face value, this means that if any combination of columns were able to withstand 6% of the force of gravity, this would have been measured. There were 42 columns in the building which account for the resistance to gravity. We will assume that every floor is at full load bearing capacity, that each has a safety factor of at least 2, and that they are all equally burdened.

Under this scenario, if half of the columns were destroyed, there would have been no collapse. If ~6% of the remaining 21 columns were undamaged, this would have resulted in asymmetrical collapse and/or a measurable decrease in the acceleration of collapse. That number comes out to 1.26. So it is beyond any shadow of a doubt that 3 or so undamaged columns would have prevented free fall to a measurable degree and/or an asymmetric collapse.

the New York Building Code of 1892 specified a safety factor of 4:1. -Pg. 172 Rise of the New York Skyscraper: 1865-1913

2.) The central column(s) failed first and rapidly.

It is clear from the fall of the penthouse that central columns in the building failed before global collapse. This is suggestive of controlled demolition for two reasons;

a.) It is common to destroy the central columns first to avoid collateral damage, and in this case would be done to avoid visibility.

b.) Falling debris did not damage the central structure of the building, and fires could not have destroyed, or targeted, column 79.

The initiating local failure that began the probable WTC 7 collapse sequence was the buckling of Column 79. -NIST NCSTAR 1A

Additionally, the column could not have failed rapidly. At best the central column could have been weakened by the fires, and initiated a slow collapse of the penthouse. (Though fires have never caused this to happen in history.)

This is because a hydrocarbon based fire will take about 7.5 minutes to get a room to 1000 degree's Celsius (Because of fireproofing, and steel's high thermal conductivity, it would take much longer to heat the steel to the temperature, if it happens at all). This gradual increase in temperature corresponds to a gradual reduction in strength and therefore a gradual collapse.

3.) Larry Silverstein was talking to his insurance company about destroying Building 7 in a controlled demolition, according to second hand witness Jeffrey Shapiro.

This would have been impossible to prepare that day, and suggests it had already been prepared before 9/11.

4.) Thermitic material was found in the dust of the World Trade Center building collapses, as well as iron spheres.

Iron spheres could only have been made by temperatures far in excess of the capabilities of diesel, jet fuel, or office fires (Except under highly controlled conditions). The evidence has been published in a peer reviewed journal seven years ago by professionals in their fields and has to this day never been refuted by another peer reviewed paper. In fact it has been corroborated by an independent expert Mark Basile.

5.) The building fell symmetrically.

The damage to building 7, if done by fire must have been stochastic which would not permit simultaneous global nearly instantaneous failure, and if it is postulated that the damage to the corner of the building initiated the collapse, then the building should have collapsed toward the direction of the damage.

Symmetrical collapse rules out either damage from fire or building debris as an explanation for building collapse.

6.) Buildings 3,4,5, and 6 did not collapse from either debris or fire.

This is despite being closer and taking significantly more damage. With the one exception that the core of building 6 collapsed. It is relevant to note that the center collapsing did not result in global collapse.

7.) When a debate was attempted no respectable scientists was found that would defend the official story about how the WTC fell.

On the side stating that the official story is physically impossible is David Griscom PHD with a Hirsch Index of 52. Harvard, Yale, MIT, CalTec, Stanford, Colgate, and the American Physical Society were contacted to come and debate him. No one accepted.

Since pre-meditated controlled demolition is currently the only theory that can explain free fall symmetrical collapse, the conversation by Silverstein, the iron spheres, and the thermitic material, one can safely say, at the very least, that controlled demolition of building 7 is currently the scientific consensus.

share|improve this answer

This post does not cite any references or sources. Please help improve this post by adding citations to reliable sources. Unsourced material may be challenged and removed.

I can see I have a hostile audience here. I enjoy the challenge, but I do wish you would let me know the reason for your down votes. Thanks. –  user2699942 May 9 at 23:34
This is based on argument from ignorance. The fact that you or someone else can't explain some things, it does not mean they are impossible –  sashkello May 10 at 1:00
I just downvoted, because I happened to read the last sentence. You are claiming to have scientific consensus? Bwuhahahaha! –  Oddthinking May 10 at 1:23
You don't provide a reference for "Even only a handful of undamaged columns would have prevented free fall and would have resulted in asymmetric collapse." –  user5582 May 10 at 4:09
"The rest of the criticism here is ad hominem." No, pointing out argument from ignorance is not ad hominem. No, pointing out the distinction between opinion and direct facts is not ad hominem. No, pointing out that the trustworthiness you implied by "peer-reviewed" was unwarranted is not ad hominem. No, pointing out that your claim of scientific consensus was laughable was not ad hominem (although, admittedly, merely gainsaying, not an argument). –  Oddthinking May 10 at 12:35

Your Answer


By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.