The "official version"
What is the "official version" of the collapse of the Twin Towers?
It is incorrect to speak of an "official version": it gives the impression that it is a government handout. Instead, there is a very detailed technical reconstruction of the events, carried out by experts from the National Institute of Standards and Technology or NIST, signed by structural engineers and metallurgy experts and accepted by their colleagues worldwide. This technical reconstruction documents the following.
The impact of the airliners pierced the structure, which was entirely made of steel (with no structural reinforced concrete elements), and severed a significant number of load-bearing columns, stripped the fireproofing from the steel columns in the impact area, and poured more than 9,0000 gallons of jet fuel into each of the towers.
The Towers withstood the impact, but not the subsequent fire. The fuel ignited and triggered the combustion of the contents of the towers across multiple floors, also flowing down the elevator shafts and igniting fires further below. The fuel itself was consumed in just over ten minutes, but the fires continued because they were fed by the flammable materials inside the Twin Towers: furnishings, finishes, paper, people.
The fires (which normally, in residential or office buildings, reach temperatures of 1000°C, according to the fire standard ISO 834) did not melt the load-bearing columns: steel melts at higher temperatures (from about 1300°C upward, depending on the type). They caused them to soften, making them lose much of their load-bearing capacity: unprotected steel, in fact, begins to weaken at around 400°C and loses about 50% of its strength at 600°C. At 980°C it has less than 10% of its initial strength. Temperatures of 500°C are considered "critical" for structural steel by the Italian Fire Brigade. The fires were able to act directly on the steel because the aircraft impacts had stripped away its fireproofing.
The structure began to fail exactly at the locations of the fires and impacts. Here, the heat from the fires deformed the floors, causing them to sag downward, and the floors pulled the façade structural columns inward (facts documented photographically). The deformation of the facade columns exceeded one meter. The bent and overheated columns could no longer support the load of the floors above and failed, triggering the collapse.
Images show that the floors did not stack up before the collapse, triggering it, as initially hypothesized, but only after and only partially.
The upper part of the towers fell onto the lower part, initially acting like a hammer but progressively disintegrating and turning into an avalanche of debris.
The lower floors offered resistance, lengthening the collapse time compared to free fall (videos document a duration of at least 16 seconds, compared to about 9 seconds for an equivalent free fall). However, the structure, designed to withstand a static load, could not withstand the dynamic load of the falling mass of debris (initially 14 and 29 floors for WTC1 and WTC2 respectively), and the collapse continued to the ground.
The central columns did not collapse immediately together with the rest of the building: they remained standing for several tens of seconds after the collapse, forming the so-called "spires", about 200 meters tall.
As shown by the distribution of the debris, the Twin Towers collapsed by opening like a flower, not perfectly vertically, but with a lateral component produced by the outward push of the falling mass of debris, projecting pieces up to 170 meters away. by Paolo Attivissimo and Leonardo Salvaggio, with contributions from all the authors of Undicisettembre. The article has been updated after its initial publication. The publication date is fictitious for blog reordering purposes. Last update: 2024/02/12.
These are the concise answers to the most recurring doubts about the attacks against the Twin Towers of the World Trade Center (WTC) on September 11, 2001. For further details, you can read the articles indicated in the links below or search this blog using the search box on the right.
Background events
The Empire State Building was hit by a huge B-52, yet it did not collapse—why?
It was not a B-52 (which is a strategic bomber with eight jet engines and up to 220 tons of takeoff weight), but a B-25: a twin-engine propeller aircraft weighing about ten tons, much smaller, lighter, and slower. At the time of the Empire State Building incident, in 1945, the B-52 did not even exist (it entered service ten years later). So anyone claiming this is making a gross error and showing they have done no basic verification before speaking and know nothing about aeronautics (details; more details).
In terms of impact energy, it makes no sense to compare the crash of a 10-ton aircraft flying at 400 km/h, as in the Empire State Building incident, with that of a 120-ton Boeing 767 traveling at over 700 km/h, as happened at the WTC. The drawings below show the dimensions of a B-25 and those of a Boeing 767 like the ones that struck the Twin Towers. The perimeter of the square represents, to scale, the footprint dimensions of the Twin Towers.
Moreover, the Boeing 767s that struck the Twin Towers caused damage not only due to impact energy but also because of the amount of fuel they carried, which ignited massive fires. Each Boeing 767 carried about 40,000 liters of fuel; a B-25 carries at most 3400.
The buildings themselves are also hardly comparable: the Empire State Building has a dense lattice structure of steel and masonry, whereas the Twin Towers had a structure entirely of steel with large internal spaces lacking load-bearing columns.
Is it true that an Israeli company at the WTC was warned in advance of the attacks?
No. Checking newspapers from the time is enough to find that the company in question, Odigo, did not have offices at the WTC: it was located four blocks away. Moreover, the “warning” message was so vague that it was only understood after the attacks, and it was received at the company’s office in Israel, not in New York (details).
Were bomb-sniffing dogs withdrawn from the Towers shortly before September 11?
They were not withdrawn at all: they were in service even on September 11. This is confirmed by the very Newsday article of 9/12/2001 that conspiracy theorists cite as proof of their withdrawal, selectively omitting the part that explains the canine units had been reinforced after a series of telephone threats. It is also confirmed by one of the survivors, Greg Trevor, who recalls that one of these dogs, Sirius, died in the collapse of the Twin Towers (details).
Why were there a blackout and an evacuation in the Towers shortly before September 11, and why did technicians enter to "perform maintenance"?
Nothing of the sort happened; it is a conspiracy myth. A Port Authority employee, who would have been responsible for managing and reporting such events, explicitly denied that anything like this occurred (details). The only person who claims to have witnessed these events is one Scott Forbes, but he cannot provide a single document to confirm it. Yet such an operation would have left significant documentation: notices, circulars, plans, permits; and people working in the Towers would remember it.
There would also have been media coverage, since the image of the Twin Towers completely empty and shut down would have been striking. But no one has produced any articles or documents describing such an event.
Even if what Scott Forbes claims were true, it should be noted that Forbes actually says that the blackout would have affected only half of one tower: the other half, the other tower, and WTC 7 were not involved. Therefore it cannot be used as a pretext to secretly place explosives in those other buildings.
Was the fireproofing insulation in the Towers full of asbestos that was extremely expensive to remove, and was that why they were demolished?
Those who claim this are suggesting that because removing asbestos from the Towers would have cost a fortune, it was preferable to stage the largest and most complex hoax in human history, kill three thousand people, destroy half of Manhattan, and crash the global economy—just to save on remediation costs?
Aside from its obvious absurdity, the claim is not supported by the facts: Tower 2 was built without using asbestos. Tower 1 was initially constructed using asbestos in the Blaze-Shield Type D cementitious mixture used for sprayed fireproofing of the columns, but only up to the 38th floor. Above that, a non-asbestos product was used (details).
Is it true that FEMA, the government agency for emergency management, was already strangely in place in New York the day before the attacks?
No. The rumor is based solely on a television interview with a single FEMA employee, Tom Kenney, conducted on September 13, 2001. Kenney said he had arrived Monday evening (the attacks occurred on Tuesday), but it was a slip of the tongue. There is no other indication or documentation that FEMA was already on site. A document request later confirmed that Kenney’s activation order is dated September 12 (details).
Is it true that 4,000 Jews working at the WTC did not show up for work on September 11?
No. Aside from the implausible idea that the Jewish community could perfectly keep a secret and spread it to thousands of people without anyone leaking it even to a non-Jewish friend to save their life, journalistic research shows that at least 119 Jews died at the WTC and another 72 victims were probably Jewish; according to the Jewish Anti-Defamation League of New York, at least 400 Jews died. There are no official figures because U.S. authorities do not record religious affiliation.
The origin of this rumor is that a few hours after the attacks, the newspaper Jerusalem Post reported that it was believed that about 4,000 Israelis (not "Jews", but "Israelis") lived or worked in New York and Washington. The newspaper did not say they had died or were missing or had not shown up for work: it simply said they were people who might have been near the attacks.
Within a few days, the story was picked up in Beirut by al-Manar, a satellite channel affiliated with Hezbollah, and by the Syrian daily al-Thawra, which altered it by saying that 4,000 Israelis working at the WTC were absent on the day of the attacks. The distorted claim spread across many Arab countries through word of mouth on antisemitic websites, evolving further: the 4,000 Israelis became 4,000 Jews who had been tipped off by Mossad.
The rumor even appeared on Pravda (which later withdrew the article) and in Italy in La Stampa (October 9, 2001, in an interview with an imam), Il Tempo (October 3, 2001, interview with an expert from the Pontifical Institute of Arabic Studies), and Cuore (cartoon, October 2001).
Sources: BBC, The Conspiracy Files; Der Spiegel, 2003; The Legend of the 4,000 Jews, in September 11 War Legends, Paolo Toselli, Avverbi, 2002, p. 104.
Why did Israeli Prime Minister Sharon cancel at the last moment his visit to New York scheduled for September 11?
This claim is common among conspiracy theorists (9/11 Research; Information Clearinghouse; Rense; 9/11 Studies), but in reality Ariel Sharon’s visit to New York was scheduled for September 23, not the 11th, and was understandably canceled on September 12 (9/11 Myths; Jewish News Weekly of Northern California; Jewishfederations.org).
Is it true that the Twin Towers were sold a week before the attacks?
No. The Towers were not sold, but leased for 99 years by the government agency Port Authority of New York and New Jersey to Silverstein Properties, together with other buildings in the complex (except WTC 7, which was already owned by Silverstein), as part of the privatization of the World Trade Center launched by the Port Authority in September 1998.
Moreover, there is no single date for the transfer: an initial contract was signed on April 26, 2001, four and a half months before September 11 (Real Estate Weekly), and the final lease agreement was signed on July 24, 2001, six weeks before the attacks (Washington Times).
The temporal proximity of these events might be seen as suggestive of conspiracy, but it should be considered that in a hypothetical plot meant to remain absolutely secret, it would make no sense to attract attention by having the destruction of the Twin Towers occur so shortly after a change in management. by Paolo Attivissimo and Leonardo Salvaggio, with contributions from all the authors of Undicisettembre. The article has been updated after its initial publication. The publication date is fictitious for blog reordering purposes. Last update: 2024/03/06.
These are the concise answers to the most recurring doubts about the attacks against the Twin Towers of the World Trade Center (WTC) on September 11, 2001. For further details, you can read the articles indicated in the links below or the cited books, or search this blog using the search box on the right.
Aircraft maneuvers and impacts
How did inexperienced hijackers manage to find the Towers?
According to technical data, to reach New York they used the aircrafts’ automatic navigation system, as they had learned to do during the professional flight lessons they attended in the United States to obtain a commercial pilot license. This is relatively easy: you just enter the destination’s geographic coordinates and the aircraft does the rest.
Once they arrived within sight of the Towers, they manually controlled the aircraft. Spotting the buildings was extremely easy: they were the only two rising 400 meters high, at the tip of Manhattan (photo below), and after the first impact they were also marked by a plume of smoke so large that it was visible from space (details).
An airliner cannot fly at 900 km/h at 200 m above the ground: why does the official account say it did?
So far, no one has presented technical documents confirming this alleged impossibility.
In fact, airline pilots consulted on the matter confirm that an airliner can do it just fine, but normally does not, because high-speed flight at low altitude consumes an excessive amount of fuel and can damage the structure if sustained for long.
Airliners can do things many people do not imagine, including low passes just a few meters above the ground, steep near-vertical climbs, and even inverted flight (Tex Johnston, Boeing 707, 1955; video). Of course, these maneuvers are normally not performed with passengers on board to avoid motion sickness and serious damage. On September 11, the hijackers had no such concern. The 900 km/h flight lasted only a few seconds.
What is that unusual device, a “pod”, seen in some footage and photos under one of the aircraft?
After all, who would be foolish enough to place a conspicuous device under the aircraft that was not supposed to be there, hoping no one would notice or photograph it? (details; more details; further details; additional information)
What is that mysterious flash seen in some videos of the aircraft impacts?
At present, it is not known exactly what it is. Some proponents of alternative theories claim it is a missile, but it would make no sense to fire a missile just before striking the target with a 120-ton object traveling at very high speed. It would be like putting a razor blade on a cannonball thinking it would make it more effective.
It could be, for example, a sudden discharge of static electricity accumulated in flight, or the very rapid combustion of oxygen tanks located in the aircraft’s nose. In any case, a very similar flash can be seen in a famous impact test of a military jet against a concrete wall conducted by Sandia in 1988, so it is not an impossible event or necessarily a sign of anything mysterious (details).
Why did the planes that crashed into the towers have no windows or markings?
This was stated by only one person, Marc Birnbach, who was about 4 km away from the Towers. From that distance, windows and markings would not be visible to the naked eye, as can be verified by standing 4 km from an airport and observing parked airliners. The photographs, taken with telephoto lenses and in some cases from shorter distances, clearly show the livery of the second aircraft.
Moreover, various fragments of the aircraft were found among the debris of the Towers, some of which were fuselage pieces with windows.
A fragment of fuselage from United Airlines Flight 175 found on the roof of WTC5.
Detail from figure 2-29 of the FEMA report, chapter 2.
Detail from figure 2-29 of the FEMA report, chapter 2.
Why are there testimonies saying that the first plane was not from American Airlines?
In reality there is only one, from a woman (who remained anonymous) saying something of this kind, in the video "In Plane Site". She was at least one and a half kilometers from the impact point: a distance from which identifying the livery would have been very difficult.
Why do some videos on the Internet, such as September Clues, claim to have proof that the Twin Towers were not hit by any aircraft and that TV footage of the impacts was faked?
Those making these claims should explain what the tens of thousands of people who were on the streets that morning in New York, looking up, actually saw, since they were eyewitnesses to the impacts. They should also explain the photographs of the second aircraft taken by passersby.
In reality, these are amateur interpretations of grainy, second- or third-generation video images taken from sites like YouTube. No video expert supports these interpretations, which contain numerous glaring errors and demonstrate ignorance of the basic rules of optics, perspective, and photography. Anyone interested in these claims can consult, for example, the document Debunking September Clues (notably written by conspiracy theorists themselves).
A group of Israelis was caught filming and celebrating the attacks in New York: isn’t that suspicious? And why were they released?
The so-called "dancing Israelis" were actually interrogated by the FBI and CIA for months before being released. Investigations indicated that they were most likely Israeli agents monitoring the activities of radical Islamic propaganda centers in New York, and that they were in no way connected to the attacks and had no prior knowledge of them. The testimony of the person who reported them to the FBI clearly states that they arrived and began filming the attacks after they had already started, so they were not already positioned in advance waiting.
How is it possible that aluminum planes cut through skyscraper steel like butter?
It is simply a matter of basic physics. Material hardness is not the key factor: what matters is the kinetic energy imparted to the material. Indeed, there are machines that cut steel using a jet of water propelled at extremely high speed. This technology is called waterjet or water cutting (details; examples). Moreover, in 2002 a small aircraft managed to penetrate the Pirelli Building in Milan, which is a reinforced concrete skyscraper.
Why did the planes not leave debris? Where did the extremely strong engines go?
In fact, the planes did leave debris (details and photos).- Part of an engine from flight UA175 was found at the intersection of Church and Murray Street (photo beside).
- A fragment of fuselage from flight UA175 was found on the roof of WTC5, as shown above.
- A landing gear wheel from flight AA11 was found at the intersection of West and Rector Street.
- Various fuselage fragments were found in the streets adjacent to the Twin Towers.
- A landing gear wheel was found lodged in a fragment of façade that had fallen onto West Street.
- An airplane seat cushion was found in the street.
- A life vest was found on a nearby building.
- Various fragments of mechanical aircraft parts were recovered from rooftops of buildings near the WTC.
- A jump seat (folding seat) was found together with the charred and bound body of a flight attendant (details).
What happened to the black boxes?
The black boxes of UA175 and AA11 were not found. Initially there were reports that they had been recovered, but these were later denied. Their disappearance is not implausible; they are robust, but not indestructible, and the impact was extremely violent. Moreover, the search took place under extremely difficult conditions, in a pile of hundreds of thousands of tons of metal, concrete, and remnants of the Towers’ interiors. Even if they had survived the initial impact, they would also have had to withstand the destructive force of the Towers’ collapse and then be found in an enormous mass of debris.
How many videos exist of the impact of the first plane?
There are three (details):
- the one by the Naudet brothers, which is the most famous
- the one by Czech worker Pavel Hlava
- the one by German artist Wolfgang Staehle
The fires
How could the steel melt if jet fuel does not burn hot enough to reach steel’s melting point?
The steel of the Twin Towers did not melt before the collapses: those were journalistic exaggerations spread initially, before technical investigations had been conducted. The technical reconstruction states that the steel softened due to the heat, becoming unable to bear the loads, and this led to the structural collapse. Secondly, the jet fuel started the fire, but the fire continued fueled by the contents of the Towers (furniture, paper, carpets, etc.): therefore the burning temperature of jet fuel is irrelevant, while the combustion temperature of the buildings’ contents is what matters.
Unprotected steel loses about 50% of its strength at 600°C and at 980°C retains less than 10% of its original strength, according to data provided by Farid Alfawakhiri, chief engineer for building standards at the American Iron and Steel Institute (Debunking 9/11 Myths, p. 39).
How come no steel building had ever collapsed from fire before 9/11?
That is not true. In fact, there are cases of steel structures collapsing solely because of fire before 9/11 (details; more details):
- the McCormick Center in Chicago;
- the Sight and Sound Theater in Pennsylvania;
- the Kader factory in Thailand;
- the Mumbai High North oil platform;
- the paper mill near Malvern, United Kingdom.
a) first struck by 120 tons of aircraft traveling at over 700 km/h
b) then set ablaze by approximately 32,000 liters of jet fuel
c) and finally left to burn (the WTC fire suppression systems were disabled by the impacts).
Firefighters around the world are fully aware of the dangers of fires in steel buildings and know that fire can trigger structural collapse.
In Madrid a skyscraper burned for twenty hours and did not collapse. Why did the Twin Towers collapse in less than two hours instead?
The Madrid skyscraper (the Windsor Tower or Torre Windsor) actually did collapse: its entire steel section failed. Yet it had not been struck by any aircraft and had a completely different structure from the Towers (reinforced concrete, not pure steel).
It makes no sense to compare a steel-framed skyscraper that was first pierced by a large aircraft, then soaked with about 32,000 liters of fuel and finally set on fire and left burning, with a largely reinforced-concrete skyscraper in which only construction materials burned and firefighters were able to intervene (complete series of articles on the subject).
Conspiracy theorists often cite this building, but it should be stressed that in the Spanish tower the entire steel portion collapsed, without even a drop of additional fuel: the fire of the building materials alone was enough.
How come the Twin Towers steel was allegedly certified by UL to withstand temperatures above 1000°C for 6 hours, yet it failed in less than two hours?
This certification story is an invention by conspiracy theorists, who have never been able to document it. In fact, in the United States steel itself is not certified; rather, structural assemblies are certified according to standards such as ASTM E 119 (details; more details; further details), and the Twin Towers’ structures were so innovative for their time that they were never formally certified (details).
Why did the Towers collapse if the fires were small and not very hot?
The premise is wrong: if the fires really had been small and not very hot, then why did more than two hundred people choose to commit suicide by jumping from the Twin Towers (details)?
Looking at the image below, it is very difficult to describe the fire as small, considering that each façade measured 64 meters.
If the fires were extremely hot, how is it possible that there are photos of a person standing in the impact hole?
The impact hole area was never affected by major fires (this can be seen in photos taken a few minutes after impact): the impact had dragged the aircraft fuel inside the tower, distributing it forward along the disintegration path of the plane. Therefore the impact area was the least affected by burning fuel.
Moreover, the person in question, often identified as Edna Cintron, was photographed 45 minutes after the impact against the North Tower. The area around the breach therefore had time to cool down.
Furthermore, the gash acted as an air intake for the fires, so it experienced a strong inflow of fresh air drawn from outside by the chimney effect caused by the flames inside the tower. If there was one ventilated spot in the entire devastated area, it was precisely the impact breach. Using this photograph as “proof” of low temperatures in the rest of the burning zone therefore shows serious incompetence regarding fire dynamics in general (details).
Jet fuel could not generate fires hot enough under those conditions even to reach 500–600 degrees, and it would have burned out in 15 minutes: how is it possible that the fires caused the structural failure and lasted much longer?
The WTC fires did not burn only jet fuel. The contents of the buildings themselves burned. The Towers housed offices, with everything normally found in such environments: computers, paper, panels, desks, plastics, chairs, cabinets. All of this burns. A fully developed fire in such an environment easily reaches temperatures around 1000°C, as confirmed by the Italian Fire Department, and burns fiercely, as can be seen in this video.
The jet fuel did not sustain the fire for over an hour: the fire was fueled by the materials inside the towers. The jet fuel merely ignited the fire by spreading, already burning, throughout the structure.
If some people managed to pass through the fire zone, how is it possible that the fires were so intense?
Those people (Brian Clark, Stanley Praimnath, and others) did not pass through the fires: they went around them by using the corner opposite the impact zone (let us not forget that each WTC floor measured 64 by 64 meters), using the only fireproof staircase left intact in the South Tower, Stairway A. It is called a “fire staircase” precisely because it protects from fire.
At the level of the impact floors, that staircase deviated from the center of the structure to go around the elevator motors (twelve motors weighing 24 tons each), which acted as shields against the aircraft impact. That is precisely why it remained usable.
This episode cannot be used to prove that the fires were modest, because it does not apply to the North Tower: in fact, above the impact floors in the North Tower, nobody survived. Furthermore, only sixteen people escaped from the South Tower thanks to that staircase (details).
In an audio recording a firefighter said, “We’ve got two isolated pockets of fire, we should be able to knock them down with two lines”. Doesn’t that mean the fires were small?
No. The statement was made by firefighter commander Oreo Palmer, who died in the collapses, while he was on the 78th floor of the South Tower, just minutes before the building collapsed. Palmer was simply describing the situation at the specific spot where he was, not the fire as a whole. The 78th floor of the South Tower was the lower boundary of the impact zone, since the aircraft struck between floors 78 and 84, so it was at the edge of the damaged area.
Besides, if the fire had truly consisted of “two isolated pockets”, it could not possibly have produced images like the one shown above.
There is a thermographic video on YouTube recorded on 9/11 during the fires. The maximum temperature shown on the scale is only 120°C. How is that possible?
The thermal camera that recorded the video was too far from the towers to measure the fire temperatures accurately. Furthermore, at the time of the recording several camera parameters had not been properly configured. In particular, the temperature range had been set to 15–120°C, so any temperatures above 120°C were still displayed as the maximum value of 120°C. More details can be found in the interview with James Seffrin.
Why weren’t helicopters used to rescue the people trapped on the upper floors from the roof?
For many reasons. The roof access doors were locked, and nobody above the impact zone had keys. Only one of the roofs (that of the South Tower or WTC2, which was open to visitors) could even be approached by helicopter, but it did not have a suitable landing area; the other roof was crowded with antennas (one large central antenna plus many smaller ones scattered around), making approach impossible. Helicopters would have had to hover above a burning building, despite smoke and air currents (already extremely strong at 400 meters even without fires), lower someone down to open the access doors, and then hoist people up one at a time with a winch: an extremely dangerous operation.
The attempt was nevertheless made by the New York police, but had to be abandoned because of the smoke and the excessive heat (NIST NCSTAR-1 report, p. 26; Staff Statement no. 13, pp. 20-21).
The collapses
New York Mayor Rudy Giuliani was warned in advance about the collapse of the first Tower (he says so himself in a video), but he did nothing to save the firefighters inside. Why?
Giuliani received the warning only a few minutes before the collapse, when the news was already spreading. The firefighters and rescuers outside the Towers already knew that collapse was imminent, but they could do nothing to warn their colleagues: the radios were notoriously unreliable. Moreover, it was not Giuliani’s responsibility to give evacuation orders to firefighters.
For example, John Peruggia, chief of emergency medical services, knew of the imminent collapse risk and informed emergency medical technician (EMT) Richard Zarrillo, fire marshal Steve Mosiello, and Commander Ganci. But they learned of it only moments before the collapse itself (details).
If the Towers were designed to withstand the impact of an airliner, why did they collapse?
The design documentation shows that the Twin Towers were conceived to withstand the crash of a commercial airliner while landing: that is, an aircraft flying at reduced landing speed, not at maximum speed, and carrying very little fuel because most of it would already have been consumed. This is a very different situation from that of September 11, when the aircraft were still full of fuel and were flown at high speed into the Towers (details).
Furthermore, the buildings’ ability to withstand such an impact was only presumed, based on approximate calculations impossible to verify with the engineering simulation tools available at the time.
It should also be clarified that the Twin Towers did withstand the impact: it was the fire that caused them to collapse. Police helicopter pilots, engineers, architects, and firefighters on site who had an overview of the damage recognized the risk of collapse (details).
The designer stated that the Towers could withstand even multiple airliner impacts: so why did they collapse?
This statement was not made by a designer, but by Frank DeMartini, who was not involved in the structural design of the Twin Towers. Moreover, it was made during an interview for a documentary, not in a technical report (details).
DeMartini was construction manager at the World Trade Center and took that role only after the 1993 bombing. He died in the collapse of the Towers on September 11, 2001, after radioing that the express elevators in the South Tower were at risk of collapsing: a sign of severe structural problems (102 Minutes, p. 150).
As far as is known, none of the actual designers ever stated before the September 11 attacks that the Towers could withstand multiple impacts.
What did the designers say about resistance to aircraft impacts?
In November 2001, Leslie Robertson, the principal engineer responsible for the structural design of the World Trade Center, stated that the Twin Towers had been designed to withstand the impact of a Boeing 707 during landing, therefore at reduced speed and carrying little fuel, and that it had not been possible to take into account the effect of the fuel. On September 11, 2001, the Twin Towers were struck by Boeing 767s, slightly heavier aircraft than the 707, traveling at extremely high speed (between 700 and 900 km/h) and full of fuel. Therefore, the September 11 impacts are not comparable to those envisioned in the design.
Robertson said: “The energy input into the building is proportional to the square of the velocity: double the velocity and the energy quadruples [...] With the 767 the fuel load was enormous compared to that of the 707; it was a fully fueled aircraft, whereas the 707 was a landing aircraft. There is absolutely no comparison between the two.” (details).
In 1993, shortly after the first terrorist attack against the Twin Towers (the truck bomb in the underground parking garage), John Skilling, chief structural engineer of the World Trade Center, stated regarding the hypothesis of an aircraft impact that “Our analysis indicated that the biggest problem would be the fact that all the fuel (from the airplane) would dump into the building. There would be a terrible fire. A lot of people would be killed.” Skilling added, perhaps optimistically, that “the building structure would still be there.” Which in fact happened, at least partially: the North Tower remained standing for 102 minutes, the South Tower for 56 (details).
Why do the collapses of the Twin Towers resemble a controlled demolition?
They do not resemble a controlled demolition at all; the only common aspect is that a building collapses. The differences are extremely important and obvious:
- a controlled demolition using explosives starts from the bottom, whereas the collapses of the Towers began at the top, at the impacted and burning floors;
- a controlled demolition with explosives produces extremely loud bangs audible blocks away just before collapse, whereas such sounds were absent during the collapses of the Towers (some witnesses spoke of explosions at the WTC, but before the collapses, which is normal during a fire and was also reported by witnesses of the Pentagon attack);
- the facades of the Towers bowed inward just before collapse, something that does not occur in controlled demolitions;
- in a controlled demolition, air bursts appear along the entire perimeter of the building and not as small localized puffs as at the World Trade Center;
- in a controlled demolition, all windows are removed beforehand to prevent the shock wave from hurling them outward in a deadly shower of shards, but obviously no such preventive removal occurred at the WTC, so if there had been internal explosions there should have been a massive and widespread outward projection of window fragments.
What are those puffs seen during the collapses? Are they explosions?
In skyscrapers such as the World Trade Center, over 90% of the volume enclosed by the structure is usable “empty” space, meaning it is filled with air (details). Therefore, when the structure collapses, all the air inside it is compressed and seeks escape routes. The puffs are simply the visible manifestation of this venting.
An explosion, on the other hand, would have produced a clearly visible shock wave that would have shattered the windows of the Towers and nearby buildings; it would have generated sharp and extremely loud booms, absent from the audio and video recordings; and it would have caused a much faster and more violent projection of debris.
Moreover, these puffs were observed in the Twin Towers only after the collapse had begun; in controlled demolitions, by contrast, puffs appear before the collapse begins, because the collapse starts only after the explosives are detonated.
It is true that some images show slow and small puffs well before the collapse: but they are still too few and too slow to have been produced by explosives, which always cause extremely rapid air displacement. They are instead compatible with air displacement produced by a local structural failure.
A demolition technique without explosives is used in France and produces smaller puffs than those caused by explosives, therefore more similar to those visible during the collapse of the Twin Towers, as can be seen here.
In the technique used, called verinage, buildings are completely destroyed by mechanically breaking the load-bearing columns of a single floor using large hydraulic jacks and allowing the upper block to fall onto the lower one (details).
How is it possible that the Twin Towers were completely pulverized without explosives?
It is not possible, and in fact it did not happen. Even one of the main proponents of alternative theories, Steven Jones, said that the “complete pulverization” claim is nonsense (details).
The steel of the Towers was not pulverized at all: one only has to look at the pile of debris. The relatively small amount of concrete in the Towers crumbled into pieces of all sizes: “we found large pieces of concrete (of irregular shape and dimensions, one measured about 5 x 3 x 3 cm) as well as medium-sized fragments of drywall (with the binding paper still attached)”, says Jones himself (source).
Why was the collapse so vertical?
Because the greatest force acting on the Twin Towers was gravity, and gravity acts vertically. There was no lateral force capable of causing the entire tower to fall sideways: the force produced by the aircraft impacts was vastly insufficient.
A skyscraper is not a tree: it is designed to stand only when upright. As soon as it tilts beyond a certain limit, it breaks apart. For example, the Leaning Tower of Pisa is at risk precisely because if it exceeds a certain tilt it will fragment, not because it will topple over as a single block.
In the Twin Towers, the block above the impact zone began to tilt sideways, but then shattered under its own weight and internal stresses, and came down like an avalanche of debris.
Why did the Twin Towers fall so quickly, in 8-10 seconds?
Doubts about the collapse time concern the conspiracy theorists’ claim that the time taken by the Towers to collapse is comparable to the free-fall time of an object dropped in a vacuum from the height of the Towers, and that this would be suspicious because it would supposedly prove that the building offered no resistance. First of all, this argument makes no sense for at least two reasons. First, one cannot explain a phenomenon in which masses change during observation (a collapse) using the physics of rigid bodies (the fall of an object). Second, it is impossible to calculate the collapse time precisely from videos without knowing how many format conversions they underwent (an example of this phenomenon is the so-called PAL Speedup), especially because the videos never show the collapse all the way to the ground since the bases of the Towers are obscured by other skyscrapers and by the dust cloud. Moreover, conspiracy theorists have never explained what they mean by collapse time. Is it the time taken by the first debris to reach the ground? Is it the time taken by the last parts of the structure to reach the ground? Precisely because this was not a rigid body, there is no unambiguous definition.
In any case, even if we wished to address the issue, the NIST reports indicate about 11 seconds for WTC1 (North Tower) and about 9 seconds for WTC2 (South Tower) (report NCSTAR 1-5A; NIST FAQ no. 6, August 2006). However, these are the times taken by the first debris to reach the ground, falling in free fall along the sides of the structure (“the time for the first exterior panels to strike the ground after collapse initiation in each tower”, says the NIST FAQ). They are not the total durations of the collapses, because (as can also be seen in the videos) the primary collapses continued even after the first debris had reached the ground and the collapse front lagged behind that debris.
In reality, the collapse itself lasted much longer. Simply listening to the audio of the footage makes it clear that the roar of the collapse lasted at least 16 seconds, and some footage shows that after ten seconds the North Tower was still halfway through collapsing.
Furthermore, none of the supporters of alternative theories are able to say, calculations in hand, what percentage of the free-fall time from the Towers’ height would make the collapse suspicious; nor has anyone ever demonstrated that controlled demolitions are faster than collapses due to structural failure, thereby proving that the two phenomena could be distinguished based on the time taken.
Is it possible that the Towers were demolished using a substance called “thermite”?
No. Thermite is an incendiary mixture: it works by molten flow, not by explosion. To use it to cut the Towers’ columns would have required enormous quantities (130 g for every kg of steel to be melted), contained in extremely bulky containers resistant to the very high temperatures of the chemical reaction and equipped with igniters and chutes to pour it onto the columns. None of the Towers’ occupants saw anything of the sort; nothing of the sort was found in the debris.
Moreover, the reaction and cutting times of thermite are not instantaneous (many tens of seconds to cut a single beam) and are therefore incompatible with the collapse of the Towers, which lasted about 30 seconds in total.
In August 2009, National Geographic carried out a practical experiment: 70 kg of thermite failed to sever a steel column much smaller than those of the Twin Towers (details).
Besides, just think about it for a moment: if thermite could really be used to cut large vertical columns in a few moments, why did rescuers instead use the extremely slow oxy-fuel cutting torches to bring down the remaining column stubs of the Towers? (details)
Why did the Towers fall within their own footprint?
The Towers did not collapse within their own footprint. During the collapses, numerous pieces of debris were hurled against or even over nearby buildings, destroying or severely damaging them to the point that demolition became necessary.
The four lower buildings of the WTC complex and St. Nicholas Church were destroyed precisely because they were struck by the collapse of the Towers, and WTC7 was so badly damaged that it caught fire and collapsed a few hours later. As shown in the following chart from the FEMA report, very large debris damaged buildings well beyond the Plaza.
The Winter Garden, part of the World Financial Center complex, was also destroyed by debris from the Twin Towers.
Why are there scientific studies by engineers claiming that the collapse was impossible?
These “studies” are scientific in name only: they all contain gross technical errors, simply because the people who wrote them are indeed engineers, but not in fields related to skyscraper design. They are not structural engineers: for example, they are mechanical engineers (Gordon Ross). Would you ask an electrical engineer for an opinion on the structure of a dam?
In reality, scientific studies by actual specialists confirm that the collapse was not only possible, but inevitable. One may consult, for example, the work of Zdenek Bazant, professor of civil engineering and materials science at Northwestern University in Illinois (details), or that of engineer Keith Seffen, Senior Lecturer in the Structures Group of the Engineering Department at the University of Cambridge in the United Kingdom (details). These FAQs also include a list of specialist articles devoted to analysis of the World Trade Center disaster.
What are the odds that three towers, two struck at different levels and one (WTC7) that supposedly suffered no significant structural damage, would all collapse perfectly into themselves?
For any structure, the primary acting force is gravity. And gravity always acts downward. Regardless of where and how a tall, slender building is struck or damaged, it will therefore always fall predominantly vertically. It is not a matter of probability, but of certainty. Furthermore, probability calculations are not applicable to complex events of this kind, and anyone who applies them is making an amateurish error.
How is it possible that a structure over 400 meters tall, clearly capable of supporting its own weight, could collapse like a house of cards... under its own weight?
Every building is designed to support its own weight, or more properly, the static load generated by its own structure, but not the dynamic load (movement) generated if that same structure begins to move.
In the Twin Towers, the part of the structure below the impact zone was capable of supporting the weight (static load) of the upper block of floors while they were stationary. It was not capable of supporting the force of that same block once it was moving, when it began to fall because of the failure in the impact zone.
To understand the enormous difference between static and dynamic load, imagine placing a crate of beer on your head. You can support it without problems, right? That is a static load. Now imagine the same crate suddenly falling onto your head from a height of 10 centimeters. That is a dynamic load. Do you think you could withstand the impact without injuring your head? It is at least more difficult to support, right? Exactly.
The fact that failure of the load-bearing structures of just one floor is enough to trigger total collapse is demonstrated by the demolition of two 20-story buildings in February 2009 in Belfort, France, using the verinage technique: hydraulic jacks bent and broke the load-bearing columns of a single floor, so that the falling upper floors demolished the building. The destruction was total and the collapse continued all the way to the ground without the aid of explosives (details).
In France there was a demolition of a large building without using explosives. Could this be the technique used at the WTC?
No. The demolition in question, carried out in 2007 in Vitry-sur-Seine, was one of many performed using powerful hydraulic jacks placed against load-bearing structures to destabilize them (verinage). It would have been impossible not to notice preparatory work of this kind in an open-space building like the Twin Towers (details).
Why are there witnesses who speak of explosions before the collapses?
The opposite would be surprising. In a large office building undergoing a major fire, there are many possible sources of extremely violent bangs. Some examples:
- the fuel tanks of cars and trucks near and in the underground parking garages of the Towers (testimony of firefighter Thomas Donato);
- the large fire suppression systems of the computer centers of banks and other companies located on various floors of the Twin Towers, which used large high-pressure cylinders that a fire or impact could suddenly rupture, producing extremely powerful blasts;
- the emergency generators of those same computer centers, powered by fuel tanks and likewise at risk of exploding;
- the bodies of people who jumped from the Towers, striking cars and rescue vehicles at high speed, produced violent noises terribly similar to explosions (this can be heard in the Naudet brothers’ film 9/11, 39 minutes from the beginning);
- WTC5 and WTC6 contained 1.2 million rounds of ammunition, grenades, and explosives stored by U.S. Customs, the Bureau of Alcohol, Tobacco and Firearms, and the Secret Service. Both buildings were devastated by fire. Fire battalion commander Richard Picciotto recounts the explosions in his book Last Man Down.
If the reported explosions had been the cause of the collapses, the Twin Towers would have collapsed immediately after the explosions. Instead, all the testimonies speak of explosions occurring long before the collapses. How could a building have remained standing after explosions that supposedly (according to conspiracy theorists) destroyed its structure? by Paolo Attivissimo and Leonardo Salvaggio, with contributions from all the authors of Undicisettembre. This article has been updated since its initial publication. The publication date is fictitious.
These are the brief answers to the most common questions regarding the attacks against the Twin Towers of the World Trade Center (WTC) on September 11, 2001. For more in-depth information, readers can consult the articles linked below or search this blog using the search box on the right.
Events Afterward
How is it possible that one of the hijackers’ passports was found intact in the rubble, surviving the impact and fire?
The passport of one hijacker, Satam al-Suqami (not Mohammed Atta as some claim; photo at right), was not found under the rubble, but on the street, and before the collapse of the Towers.
There is nothing anomalous about this discovery: in fact, not only a hijacker’s passport was found in New York, but many personal belongings of ordinary passengers were also recovered:
- an in-flight magazine
- a passenger’s passport
- ID cards
- three letters from the mail carried aboard the aircraft
- a seat cushion
- pieces of fuselage and landing gear (photo above)
- a life vest
- a pilot’s headset.
Another fragile type of object, more macabre, was also found after the attacks: fragments of the passengers’ bodies.
Fragile objects sometimes survive violent impacts, as can be seen in many other non-suspicious air disasters, and they do not burn if they are thrown far from the site of impact and fire or protected by more resistant objects. For example, the crash of the plane carrying the Grande Torino football team against the Basilica of Superga (May 4, 1949) left some of the victims’ documents intact.
In the Space Shuttle Columbia disaster, a videotape recorded by the astronauts survived both the shuttle’s disintegration and atmospheric reentry at over 29,000 km/h (details). In the vertical crash of Pacific Southwest Airlines Flight 1771 in 1987, a paper airsickness bag survived on which the suicidal hijacker had written the motives for his act (details).
On September 11, the hijacked aircraft pierced through the Towers and some parts emerged from the opposite side of the buildings, before the fuel in the aircraft tanks ignited. The hijacker’s passport therefore did not need to survive any fire, because the fire did not yet exist. Like the other fragile objects recovered, it presumably survived the mechanical impact because it was protected by other, more resistant objects (aircraft parts).
Why were pools of molten metal found beneath the rubble? Don’t they prove that thermite was used?
The documentation of this molten metal remains anecdotal: there are no images showing it. The images presented by conspiracy theorists turned out to be crude fakes. However, it is possible that pools of molten metal formed without this proving the use of thermite.
It is documented that temperatures beneath the WTC rubble remained high for weeks because of fires smoldering after the collapse, fueled by the fuel in the WTC’s technical tanks and flammable material in the underground levels. If this molten metal existed, it is not known whether it was steel or instead aluminum, which melts easily at the temperatures of such a fire.
It could not have been thermite, because thermite burns in a few seconds and cannot keep metal molten for weeks. To keep it molten, there must be a continuous heat source. Furthermore, thermal images of Ground Zero show that the heat shifted over the course of several days, which would be impossible for masses of molten metal (details).
Why was the Towers’ steel quickly sold to the East? Was someone trying to get rid of the evidence?
There was no rush and not all the steel disappeared. Removing the rubble took eight and a half months: the last column was removed on May 30, 2002. The steel was taken to Fresh Kills, New Jersey, where it remained for months to be sifted through by the FBI, recovering about 4,200 human remains (details).
It is true that over 350,000 tons of WTC steel were sold to China, Malaysia, Korea, and India (details). 1,350 pieces were stored for years at Hangar 17 of New York’s JFK Airport, some weighing over 30 tons (details; photos), and were later transferred; most are now at the 9/11 Memorial Museum, opened in 2014 beneath the new World Trade Center, while smaller pieces are in museums, military bases, and embassies (details).
Additionally, 7.5 tons of WTC steel were used for the bow of the warship USS New York; they were part of a 24-ton shipment sent to Louisiana (details).
There are also large pieces of steel from the Twin Towers in Padua, publicly visible in a monument located at Porte Contarine (photo above).
In 2011, sculptor Antonio Paradiso received about 20 tons of Twin Towers steel to create a sculpture, Globalized Last Supper, exhibited in Matera (details).
In short, it would be foolish to make the evidence disappear while leaving all these samples around to be examined.
Why was the Towers’ steel never examined?
It absolutely was examined. The NIST technical reports examined it in detail: for example, report NCSTAR 1-3, entitled "Mechanical and Metallurgical Analysis of Structural Steel" (184 pages). There are also FEMA technical reports entitled "Structural Steel and Steel Connections", "Limited Metallurgical Examination", and "Steel Data Collection" (another 54 pages). These reports describe 236 pieces of steel, with a total weight of 500 tons.
Why were cut columns found in the rubble? Isn’t that proof the skyscrapers were demolished?
No. Those columns were cut by rescue workers, because after the collapse, extremely tall remnants of the façades were still standing. Photos of the rescue operations clearly show the torch-cutting work (details).
If the collapses were genuine, why did nothing change in skyscraper building regulations and why don’t civil engineers discuss them?
In reality, they discuss them extensively, and regulations absolutely did change. For example, Local Law 26/2004, which came into force in 2004, changed building regulations in New York City. Ongoing projects were modified (New York Times, 2003; Boston Globe, 2006), and the skyscraper replacing WTC Building 7 follows new regulations (New York Times, 2003).
The International Building Code, the main U.S. building code, was revised and updated specifically based on the World Trade Center collapses (details) and on the basis of NIST recommendations.
NIST published its technical recommendations for updating regulations based on the WTC collapses, and the NIST report on the fire-induced progressive collapse of WTC7 introduced additional recommendations to account for the dangers posed by thermal expansion in long beams.
Italian structural engineers also studied the collapse without finding anomalies. Here are some specialist articles written by Italian researchers:
- Robustness of core walls against deliberate aircraft impact: lessons from the WTC collapse. V. De Rosa, R. Landolfo, F. M. Mazzolani, 2nd International Conference "Collapse and Reliability of Civil Structures", University of Naples Federico II, Department of Structural Analysis and Design, Naples, May 15-16, 2003.
- Ballistic Limit-Based Design Criteria and Finite Element Simulations for the Core Walls under impact of Fast-Flying Commercial Aircraft, Vincenzo De Rosa, European Cooperation in the field of scientific and technical research - Cost Action 12 - Improving Buildings’ Structural Quality By New Technologies - WG2 - Structural Integrity under exceptional actions, December 16, 2002.
- The collapse of the WTC twin towers: preliminary analysis of the original design approach, De Luca A., Di Fiore F., Mele E., Romano A., STESSA 2003, 4th International Conference on “Behaviour of Steel Structures in Seismic Areas”, Naples, Italy, June 9-12, 2003, pp. 81-87.
- The collapse of WTC Twin Towers: general aspects and considerations on the stability under exceptional loading of columns with partial-strength connections, De Luca, A., Mele, E., Giordano, A., Grande, E. (2004). COST C12 Final Conference, January 2005, Innsbruck, Austria.
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