Aircraft impact

That day, Al-Qaeda suicide bombers took control of two Boeing 767 planes covering the Boston-Los Angeles route, shortly after their departure from Logan International Airport. In its final moments, American Airlines Flight 11 flew south towards Manhattan and crashed at 710 km/h on the north facade of the North Tower, between the 93rd and 99th floors, at 8:46. 17 minutes later, at 9:03, United Airlines Flight 175, approaching from the southwest, impacted the south façade of the South Tower at 870 km/h, between the 77th and 85th floors.[8] In addition to causing the destruction of numerous load-bearing beams on the perimeter and core of the towers, as a result of the impact, approximately 38,000 liters of aircraft fuel were burned, causing fires in office areas.[note 2]​[5]​ Some of the fuel went down the shaft of at least one elevator in each building, exploding on the 77th and 22nd floors and on the west side of the reception.

Fires

The first plane hit the 93rd to 98th floor of the North Tower at 8:46 a.m. on September 11. The second plane hit the South Tower just a quarter of an hour later, at 9:03. Like its twin, the tower was on fire in less than 60 minutes. The terrible attack caused almost 3,000 deaths and tens of thousands of injuries. The terrorists died in the impact of both planes.

South Tower Collapse

As the fires continued to burn, occupants trapped in the upper floors of the South Tower provided information via 9-1-1. At 9:37, an occupant on the 105th floor reported that below him, on the "ninety-something" floor, a floor had collapsed.[18] The air unit also reported the deteriorating conditions of the buildings to police commanders, who issued orders to evacuate their personnel from the towers.[19] At 9:52, the New York Police Department (NYPD) air unit reported by radio that "they may large fragments are falling from the top of WTC 2. There are large fragments hanging."[18]​ With the warnings, the Police issued orders to withdraw all their personnel from the tower. During the emergency response, there was minimal communication between the Police and the New York Fire Department (FDNY), and 9-1-1 call takers were overwhelmed and failed to pass information to FDNY commanders on the scene. At 9:59, the South Tower collapsed, 56 minutes after being hit. After being hit, only fourteen people managed to escape from the impact zone of the South Tower, and only four people from the floors above it, including Stanley Praimnath and Brian Clark "Brian Clark (9/11 survivor)"), escaped through Stairway A, the only staircase that had remained intact after the impact.

North Tower Collapse

After the collapse of the South Tower, NYPD helicopters relayed information about the situation in the North Tower. At 10:20, the NYPD air unit reported that "the top of the tower may be leaning," and a minute later that the North Tower "is sagging at the northwest corner and leaning toward the south." At 10:27, the air unit reported that "the roof is going to collapse shortly."[18] The North Tower collapsed at 10:28, after burning for 102 minutes.

After the South Tower collapsed, FDNY commanders issued orders to evacuate firefighters in the North Tower. Due to radio communication problems, the firefighters inside the towers did not hear the evacuation order from their supervisors on the scene, and most were unaware that the other tower had collapsed.[20]​ 343 firefighters died in the Twin Towers, as a result of the buildings collapsing.[19]​[21]​[22]​ No one in the impact zone or above it in the North Tower was able to escape after being hit, as all stairs and elevator shafts on those floors were destroyed.

7 World Trade Center Collapse

The WTC was made up of seven buildings, three of which completely collapsed on the day of the attacks. At 5:21 p.m. after its roof collapsed at 5:20 p.m.,[23] 7 World Trade Center, a 47-story building, became the third building to collapse. Unlike the Twin Towers, the collapse of WTC 7 had been anticipated several hours earlier and the building had been evacuated.[24]​.

FEMA's interim study was inconclusive regarding the 7 World Trade Center collapse.[25] The WTC 7 collapse investigation was conducted separately from, and subsequent to, the Twin Towers collapse investigation. In June 2004, NIST published a report with the results of its working hypothesis for the 7 World Trade Center, which was that a column failure, caused by damage from either fire or falling debris from the collapses of the two towers, progressed first vertically and then horizontally resulting in "a disproportionate collapse of the entire structure."[26][27]​.

On November 20, 2008, NIST published its final report on the fall of WTC 7.[28] In the report, NIST explains that fire was the main reason for the collapse, along with the lack of water to fight it. The fire continued to burn all afternoon on the lower floors. At 17:20, a critical column buckled, causing the 13th floor to collapse, causing a chain failure of all floors that culminated in the collapse of the entire structure. The entire façade above the region of the bowed column collapsed downward as a single unit, until the global collapse sequence was completed.[29]​.

In the immediate aftermath of the attacks, numerous structural engineers and experts spoke to the media, describing what they thought had caused the towers to collapse. Hassan Astaneh, a professor of structural engineering at the University of California, Berkeley, explained that the high temperatures in the fire weakened the steel in the beams and columns, causing them to become "weak and doughy" until they eventually failed to support the structure above. Astaneh also suggested that the fire protections came off during the initial impact of the aircraft. He also explained that, once the first structural failure occurred, the progressive collapse of the entire structure was inevitable. César Pelli, who designed the Petronas Towers in Malaysia and the World Financial Center "Brookfield Place (New York)" in New York, commented that "The building was not prepared for this type of stress."

On September 13, 2001, Zdeněk Bažant, a professor of civil engineering and materials science at Northwestern University, published a draft paper with the results of a simple analysis of the fall of the World Trade Center. Bažant suggested that heat from the fires was a key factor, causing the steel columns, both at the core and perimeter, to weaken and deform before losing their load-bearing capacity and warping. Once warped more than half of the columns of a story, the upper structure can no longer be supported and collapses completely. Bažant later published an expanded version of his analysis.[32] Another analysis was carried out by civil engineers Oral Buyukozturk and Franz-Josef Ulm of the Massachusetts Institute of Technology (MIT), who also described a collapse mechanism on September 21, 2001.[33] They later contributed to a collection of MIT papers on the collapse of the WTC edited by Eduardo Kausel, called The Towers Lost and Beyond.[34]​.

Immediately after the attacks, there was some confusion over who had the authority to conduct an official investigation. While there are clear procedures for investigating aviation accidents, no agency had been designated in advance to investigate building collapses. The Institute of Structural Engineers of the American Society of Civil Engineers (ASCE) quickly assembled a team. Also participating were the American Institute of Steel Construction, the American Concrete Institute, the National Fire Protection Association (NFPA), and the Society of Fire Protection Engineers. ASCE ultimately invited FEMA to join the investigation, which was completed under the latter's auspices.

The research was criticized by some engineers and lawmakers in the US. It had little funding, no power to compel testing, and limited access to the WTC site. One of the main points of controversy at the time was that the cleanup of the WTC site was resulting in the destruction of most of the buildings' steel elements.[36] In fact, when NIST published its final report, it noted "the paucity of physical evidence" it had had at its disposal to investigate the collapses. Only a fraction of a percent of the buildings remained for analysis after the cleanup was completed: some 236 individual pieces of steel, despite having recovered 95% of the structural plates and beams and 50% of the reinforcing bars. disaster) there was growing public pressure for a more thorough investigation.[38] Congress passed the National Construction Safety Team Act in October 2002, giving authority to NIST to conduct an investigation into the collapse of the World Trade Center.[39]​.

Ambos edificios se derrumbaron simétricamente y más o menos en línea recta hacia abajo, aunque hubo cierta inclinación de la parte superior de las torres y una gran cantidad de escombros desprendiéndose por los lados. En ambos casos, la parte del edificio que había sido dañada por los aviones falló, causando que la sección que estaba por encima de los puntos de impacto cayera sobre la inferior sin dañar. Los primeros fragmentos de los muros externos que cayeron de la Torre Norte golpearon el suelo 11 segundos después del comienzo del derrumbe, y los fragmentos de la Torre Sur tras 9 segundos de iniciarse. Secciones significativas de los núcleos (60 pisos del WTC 1 y 40 pisos del WTC 2) permanecieron en pie hasta 25 segundos después del inicio del derrumbe, antes de colapsar ellos mismos.[14]​.

Pese a que estaban diseñados para soportar cargas estáticas enormes, ofrecieron poca resistencia a la masa en movimiento de las secciones por encima de los pisos donde se inició el colapso. Los sistemas estructurales responden de forma muy distinta a cargas estáticas y dinámicas, y puesto que el movimiento de la sección que caía fue una caída de al menos un piso (unos 3 m), la estructura inferior fue incapaz de detener los colapsos una vez iniciados. De hecho, una caída libre de sólo medio metro habría bastado para liberar la suficiente energía para iniciar un colapso imparable.[40]​.

Start of collapse

After the impact and before the collapse, the cores of both towers could be divided into three different sections. Above and below the impacted floors, the cores consisted of what were essentially two rigid boxes; The steel in those sections was intact and had not undergone significant heating. The section between them, however, had suffered significant damage and, although they were not able to melt it, the fires were weakening the structural steel. As a result, the core columns were being slowly crushed, sustaining plastic deformation and creep by the weight of the upper stories. When the upper section attempted to give way, however, the stabilizing truss redistributed the load to the perimeter columns. Meanwhile, the perimeter columns and floors were also being weakened by the heat of the fire, and as the floors began to sink, they pulled the exterior walls inward. In the case of WTC 2, this caused the east face to bow, transferring the load back to the core, which had already failed, through the stabilizing lattice, thus initiating the collapse. In the case of WTC 1, the south wall warped in the same way and with similar consequences.[41]​.

Progressive total collapse

The collapse of the World Trade Center has been called "the most infamous paradigm of progressive collapse").[42] In the case of both towers, the upper sections leaned toward the side where the side walls had buckled, basically behaving as solid blocks separate from the rest of the buildings. They fell at least one story in free fall and impacted the lower sections with a force equivalent to more than thirty times their own weight. This was enough to bend the columns of the stories immediately below, making the blocks They fell again one more floor. Total collapse was inevitable as the process was repeated along the total height of the lower sections. The force of each impact was also much greater than the horizontal moment of each upper section, which prevented their inclination from growing significantly. They remained intact throughout the collapse, with their center of gravity within the perimeter of each building.

Muchos de los edificios colindantes también fueron dañados o destruidos cuando las torres cayeron:.

• - El World Trade Center 5 sufrió un gran incendio y un colapso parcial de su estructura de acero.

Otros edificios dañados o destruidos fueron:.

• - Iglesia ortodoxa griega de San Nicolás.

• - Marriott World Trade Center (WTC Marriott Hotel 3).

• - Sur Plaza (4 WTC), de Aduanas y de los EE. UU.

• - World Trade Center 6.

• - World Financial Center "Brookfield Place (Nueva York)").

Otros edificios de las calles:.

• - 90 West Street, y 130 Cedar Street sufrieron incendios.

• - El Deutsche Bank Building, Verizon, y el World Financial Center 3 sufrieron daños por el colapso de las torres.[43]​.

• - Al igual que la calle 90 West.

• - Plaza de la Libertad ha sobrevivido estructuralmente pero sostenido daño superficial incluyendo las ventanas destrozadas.

• - 30 West Broadway fue dañada por el colapso de WTC 7.

Cleaning

The cleanup was a massive operation coordinated by the New York City Department of Design and Construction. On September 22, a first cleanup plan was delivered by Controlled Demolition, Inc.) of Phoenix, Maryland. Mark Loizeaux, president of the CDI, highlighted the importance of protecting the mud wall (or "the tub"), which kept the Hudson River from flooding the basement of the WTC.

The large pile of rubble at the site burned for three months, resisting attempts to extinguish the fire until most of the rubble was finally removed from the site.[46][47]​

In early November, with a third of the debris removed, officials began reducing the number of firefighters and police officers assigned to recovering victims' remains, in order to prioritize debris removal. This led to clashes with firefighters.[48] In 2007, demolition of the damaged buildings was still ongoing in parallel with construction of the new World Trade Center complex.

The collapse of the World Trade Center produced huge clouds of dust that covered Manhattan for days. On September 18, 2001, the Environmental Protection Agency issued a statement assuring the public that Manhattan's air was "safe to breathe."[49] In a report published in 2003, however, the EPA's inspector general found that the agency did not have sufficient data to make such a statement. He also considered that the White House had influenced the EPA to eliminate cautionary warnings, ensuring that they were included in the report, partly motivated by the desire to resume Wall Street activity. In fact, the collapse of the World Trade Center "World Trade Center (1973-2001)") resulted in serious reductions in air quality and is the likely cause of many respiratory illnesses, with residents and workers in offices and businesses in Lower Manhattan being the first affected.[50]​.

According to a 2006 survey, 16 percent of American adults believe that the World Trade Center could have been destroyed by "controlled demolition", rather than as a result of plane impacts.

• - Wikimedia Commons hosts a multimedia gallery on World Trade Center Collapse.

• - Wikimedia Commons hosts a multimedia category on Black Tuesday.

• - Wikimedia Commons hosts a multimedia category on World Trade Center.

Contenido

El diseño de las torres era de tubo enmarcado, con plantas diáfanas, sin interrupciones de columnas o muros. Numerosas columnas perimetrales con un estrecho espaciado proporcionaban gran parte de la fuerza de la estructura, compartiendo la carga con las columnas del núcleo. A partir del séptimo piso había 59 columnas perimetrales a lo largo de cada cara de la construcción y había 47 columnas de carga en el núcleo. Todos los ascensores y las escaleras se encontraban en el centro.

Los pisos consistían en losas de hormigón ligero de 4 pulgadas (10 cm) de espesor, vertido sobre una cubierta de acero estriado. Los pisos eran sujetados por una retícula de vigas puente ligeras de celosía y vigas principales, con conexiones anguladas a la losa de hormigón para un efecto combinado. La vigas se extendían 60 pies (18 m) en las áreas anchas y 35 pies (11 m) en las áreas cortas. Las vigas conectaban con el perímetro en columnas alternadas, y por lo tanto, estaban a 6,8 pies (2,1 m) unas de otras. Los extremos de las vigas se fijaban con pernos; en el lado exterior, a unos asientos en L soldados, y en el interior, a un canal soldado a las columnas centrales. Los pisos estaban conectados a las placas anguladas mediante amortiguadores viscoelásticos, lo que contribuía a reducir la percepción de balanceo que sentían los ocupantes del edificio.

Las torres también incorporaban una "celosía sombrero" o "celosía estabilizadora", ubicada entre las plantas 107 y 110, que consistía en seis vigas a lo largo de la longitud axial del núcleo y cuatro a lo largo del eje corto. Este sistema permitía optimizar la redistribución de cargas de los diafragmas de los pisos entre el perímetro y el núcleo, mejorando el comportamiento entre los diferentes materiales de acero flexible y hormigón rígido, permitiendo a la estructura exterior transferir la carga del balanceo al núcleo. También hacía de soporte de la Torre de Transmisión.[7]​

[8]​.

Impact Safety Concerns

Structural engineers working on the World Trade Center (1973-2001) considered the possibility that a plane could crash into the building. In July 1945, a B-25 bomber that was lost in fog crashed on the 79th floor of the Empire State Building.[9]​ A year later, another plane crashed on the 58th floor of the 40 Wall Street building.[10]​ During the design of the World Trade Center, Leslie Robertson, one of the chief engineers, considered the impact hypothesis of a Boeing 707 airplane that could be lost in fog and flying at relatively low speeds, trying to land at JFK or Newark Airport.[10][11][9]​.

NIST found a three-page article that mentioned another analysis of the impact of a Boeing 707 at 600 miles per hour (970 km/h), but the original documentation of the study was lost when the Port Authority offices were destroyed in the collapse of the World Trade Center (1973-2001). The biggest problem would be that all the fuel from the plane would spill over the building. There would be a horrible fire. Lots of people would die," he said. "The structure of the building would still be there."[13]​ In their investigation, NIST found reason to believe that they lacked adequate ability to correctly model the effect of these impacts on structures, particularly the effects of fire,[14]​ although they offer no evidence for this consideration.[note 1]​.

Fireproof insulation

Fireproof insulation was incorporated in the original construction, and more was added after a fire in 1975 that spread 6 stories before being extinguished. After the 1993 attack, inspections revealed that the fireproofing insulation was poor. The Port Authority was in the process of replacing it, but only 18 floors had been completed in the World Trade Center's North Tower, including all floors affected by the plane impact and fires,[15] and 13 floors in the South Tower, although only three of these floors (77, 78, and 85) were directly affected by the impact.[16] Although the thickness of the replacement insulation was specified at 1.5 inches (3.8 cm) thick, NIST found that the average thickness was 2.5 inches (6.4 cm).[17] NIST concluded that "neither the condition of the insulation before the planes hit the WTC, nor the thickness of the insulation applied to the flooring system, played a significant role."

Aircraft impact

That day, Al-Qaeda suicide bombers took control of two Boeing 767 planes covering the Boston-Los Angeles route, shortly after their departure from Logan International Airport. In its final moments, American Airlines Flight 11 flew south towards Manhattan and crashed at 710 km/h on the north facade of the North Tower, between the 93rd and 99th floors, at 8:46. 17 minutes later, at 9:03, United Airlines Flight 175, approaching from the southwest, impacted the south façade of the South Tower at 870 km/h, between the 77th and 85th floors.[8] In addition to causing the destruction of numerous load-bearing beams on the perimeter and core of the towers, as a result of the impact, approximately 38,000 liters of aircraft fuel were burned, causing fires in office areas.[note 2]​[5]​ Some of the fuel went down the shaft of at least one elevator in each building, exploding on the 77th and 22nd floors and on the west side of the reception.

Fires

The first plane hit the 93rd to 98th floor of the North Tower at 8:46 a.m. on September 11. The second plane hit the South Tower just a quarter of an hour later, at 9:03. Like its twin, the tower was on fire in less than 60 minutes. The terrible attack caused almost 3,000 deaths and tens of thousands of injuries. The terrorists died in the impact of both planes.

South Tower Collapse

As the fires continued to burn, occupants trapped in the upper floors of the South Tower provided information via 9-1-1. At 9:37, an occupant on the 105th floor reported that below him, on the "ninety-something" floor, a floor had collapsed.[18] The air unit also reported the deteriorating conditions of the buildings to police commanders, who issued orders to evacuate their personnel from the towers.[19] At 9:52, the New York Police Department (NYPD) air unit reported by radio that "they may large fragments are falling from the top of WTC 2. There are large fragments hanging."[18]​ With the warnings, the Police issued orders to withdraw all their personnel from the tower. During the emergency response, there was minimal communication between the Police and the New York Fire Department (FDNY), and 9-1-1 call takers were overwhelmed and failed to pass information to FDNY commanders on the scene. At 9:59, the South Tower collapsed, 56 minutes after being hit. After being hit, only fourteen people managed to escape from the impact zone of the South Tower, and only four people from the floors above it, including Stanley Praimnath and Brian Clark "Brian Clark (9/11 survivor)"), escaped through Stairway A, the only staircase that had remained intact after the impact.

North Tower Collapse

After the collapse of the South Tower, NYPD helicopters relayed information about the situation in the North Tower. At 10:20, the NYPD air unit reported that "the top of the tower may be leaning," and a minute later that the North Tower "is sagging at the northwest corner and leaning toward the south." At 10:27, the air unit reported that "the roof is going to collapse shortly."[18] The North Tower collapsed at 10:28, after burning for 102 minutes.

After the South Tower collapsed, FDNY commanders issued orders to evacuate firefighters in the North Tower. Due to radio communication problems, the firefighters inside the towers did not hear the evacuation order from their supervisors on the scene, and most were unaware that the other tower had collapsed.[20]​ 343 firefighters died in the Twin Towers, as a result of the buildings collapsing.[19]​[21]​[22]​ No one in the impact zone or above it in the North Tower was able to escape after being hit, as all stairs and elevator shafts on those floors were destroyed.

7 World Trade Center Collapse

The WTC was made up of seven buildings, three of which completely collapsed on the day of the attacks. At 5:21 p.m. after its roof collapsed at 5:20 p.m.,[23] 7 World Trade Center, a 47-story building, became the third building to collapse. Unlike the Twin Towers, the collapse of WTC 7 had been anticipated several hours earlier and the building had been evacuated.[24]​.

FEMA's interim study was inconclusive regarding the 7 World Trade Center collapse.[25] The WTC 7 collapse investigation was conducted separately from, and subsequent to, the Twin Towers collapse investigation. In June 2004, NIST published a report with the results of its working hypothesis for the 7 World Trade Center, which was that a column failure, caused by damage from either fire or falling debris from the collapses of the two towers, progressed first vertically and then horizontally resulting in "a disproportionate collapse of the entire structure."[26][27]​.

On November 20, 2008, NIST published its final report on the fall of WTC 7.[28] In the report, NIST explains that fire was the main reason for the collapse, along with the lack of water to fight it. The fire continued to burn all afternoon on the lower floors. At 17:20, a critical column buckled, causing the 13th floor to collapse, causing a chain failure of all floors that culminated in the collapse of the entire structure. The entire façade above the region of the bowed column collapsed downward as a single unit, until the global collapse sequence was completed.[29]​.

In the immediate aftermath of the attacks, numerous structural engineers and experts spoke to the media, describing what they thought had caused the towers to collapse. Hassan Astaneh, a professor of structural engineering at the University of California, Berkeley, explained that the high temperatures in the fire weakened the steel in the beams and columns, causing them to become "weak and doughy" until they eventually failed to support the structure above. Astaneh also suggested that the fire protections came off during the initial impact of the aircraft. He also explained that, once the first structural failure occurred, the progressive collapse of the entire structure was inevitable. César Pelli, who designed the Petronas Towers in Malaysia and the World Financial Center "Brookfield Place (New York)" in New York, commented that "The building was not prepared for this type of stress."

On September 13, 2001, Zdeněk Bažant, a professor of civil engineering and materials science at Northwestern University, published a draft paper with the results of a simple analysis of the fall of the World Trade Center. Bažant suggested that heat from the fires was a key factor, causing the steel columns, both at the core and perimeter, to weaken and deform before losing their load-bearing capacity and warping. Once warped more than half of the columns of a story, the upper structure can no longer be supported and collapses completely. Bažant later published an expanded version of his analysis.[32] Another analysis was carried out by civil engineers Oral Buyukozturk and Franz-Josef Ulm of the Massachusetts Institute of Technology (MIT), who also described a collapse mechanism on September 21, 2001.[33] They later contributed to a collection of MIT papers on the collapse of the WTC edited by Eduardo Kausel, called The Towers Lost and Beyond.[34]​.

Immediately after the attacks, there was some confusion over who had the authority to conduct an official investigation. While there are clear procedures for investigating aviation accidents, no agency had been designated in advance to investigate building collapses. The Institute of Structural Engineers of the American Society of Civil Engineers (ASCE) quickly assembled a team. Also participating were the American Institute of Steel Construction, the American Concrete Institute, the National Fire Protection Association (NFPA), and the Society of Fire Protection Engineers. ASCE ultimately invited FEMA to join the investigation, which was completed under the latter's auspices.

The research was criticized by some engineers and lawmakers in the US. It had little funding, no power to compel testing, and limited access to the WTC site. One of the main points of controversy at the time was that the cleanup of the WTC site was resulting in the destruction of most of the buildings' steel elements.[36] In fact, when NIST published its final report, it noted "the paucity of physical evidence" it had had at its disposal to investigate the collapses. Only a fraction of a percent of the buildings remained for analysis after the cleanup was completed: some 236 individual pieces of steel, despite having recovered 95% of the structural plates and beams and 50% of the reinforcing bars. disaster) there was growing public pressure for a more thorough investigation.[38] Congress passed the National Construction Safety Team Act in October 2002, giving authority to NIST to conduct an investigation into the collapse of the World Trade Center.[39]​.

Ambos edificios se derrumbaron simétricamente y más o menos en línea recta hacia abajo, aunque hubo cierta inclinación de la parte superior de las torres y una gran cantidad de escombros desprendiéndose por los lados. En ambos casos, la parte del edificio que había sido dañada por los aviones falló, causando que la sección que estaba por encima de los puntos de impacto cayera sobre la inferior sin dañar. Los primeros fragmentos de los muros externos que cayeron de la Torre Norte golpearon el suelo 11 segundos después del comienzo del derrumbe, y los fragmentos de la Torre Sur tras 9 segundos de iniciarse. Secciones significativas de los núcleos (60 pisos del WTC 1 y 40 pisos del WTC 2) permanecieron en pie hasta 25 segundos después del inicio del derrumbe, antes de colapsar ellos mismos.[14]​.

Pese a que estaban diseñados para soportar cargas estáticas enormes, ofrecieron poca resistencia a la masa en movimiento de las secciones por encima de los pisos donde se inició el colapso. Los sistemas estructurales responden de forma muy distinta a cargas estáticas y dinámicas, y puesto que el movimiento de la sección que caía fue una caída de al menos un piso (unos 3 m), la estructura inferior fue incapaz de detener los colapsos una vez iniciados. De hecho, una caída libre de sólo medio metro habría bastado para liberar la suficiente energía para iniciar un colapso imparable.[40]​.

Start of collapse

After the impact and before the collapse, the cores of both towers could be divided into three different sections. Above and below the impacted floors, the cores consisted of what were essentially two rigid boxes; The steel in those sections was intact and had not undergone significant heating. The section between them, however, had suffered significant damage and, although they were not able to melt it, the fires were weakening the structural steel. As a result, the core columns were being slowly crushed, sustaining plastic deformation and creep by the weight of the upper stories. When the upper section attempted to give way, however, the stabilizing truss redistributed the load to the perimeter columns. Meanwhile, the perimeter columns and floors were also being weakened by the heat of the fire, and as the floors began to sink, they pulled the exterior walls inward. In the case of WTC 2, this caused the east face to bow, transferring the load back to the core, which had already failed, through the stabilizing lattice, thus initiating the collapse. In the case of WTC 1, the south wall warped in the same way and with similar consequences.[41]​.

Progressive total collapse

The collapse of the World Trade Center has been called "the most infamous paradigm of progressive collapse").[42] In the case of both towers, the upper sections leaned toward the side where the side walls had buckled, basically behaving as solid blocks separate from the rest of the buildings. They fell at least one story in free fall and impacted the lower sections with a force equivalent to more than thirty times their own weight. This was enough to bend the columns of the stories immediately below, making the blocks They fell again one more floor. Total collapse was inevitable as the process was repeated along the total height of the lower sections. The force of each impact was also much greater than the horizontal moment of each upper section, which prevented their inclination from growing significantly. They remained intact throughout the collapse, with their center of gravity within the perimeter of each building.

Muchos de los edificios colindantes también fueron dañados o destruidos cuando las torres cayeron:.

• - El World Trade Center 5 sufrió un gran incendio y un colapso parcial de su estructura de acero.

Otros edificios dañados o destruidos fueron:.

• - Iglesia ortodoxa griega de San Nicolás.

• - Marriott World Trade Center (WTC Marriott Hotel 3).

• - Sur Plaza (4 WTC), de Aduanas y de los EE. UU.

• - World Trade Center 6.

• - World Financial Center "Brookfield Place (Nueva York)").

Otros edificios de las calles:.

• - 90 West Street, y 130 Cedar Street sufrieron incendios.

• - El Deutsche Bank Building, Verizon, y el World Financial Center 3 sufrieron daños por el colapso de las torres.[43]​.

• - Al igual que la calle 90 West.

• - Plaza de la Libertad ha sobrevivido estructuralmente pero sostenido daño superficial incluyendo las ventanas destrozadas.

• - 30 West Broadway fue dañada por el colapso de WTC 7.

Cleaning

The cleanup was a massive operation coordinated by the New York City Department of Design and Construction. On September 22, a first cleanup plan was delivered by Controlled Demolition, Inc.) of Phoenix, Maryland. Mark Loizeaux, president of the CDI, highlighted the importance of protecting the mud wall (or "the tub"), which kept the Hudson River from flooding the basement of the WTC.

The large pile of rubble at the site burned for three months, resisting attempts to extinguish the fire until most of the rubble was finally removed from the site.[46][47]​

In early November, with a third of the debris removed, officials began reducing the number of firefighters and police officers assigned to recovering victims' remains, in order to prioritize debris removal. This led to clashes with firefighters.[48] In 2007, demolition of the damaged buildings was still ongoing in parallel with construction of the new World Trade Center complex.

The collapse of the World Trade Center produced huge clouds of dust that covered Manhattan for days. On September 18, 2001, the Environmental Protection Agency issued a statement assuring the public that Manhattan's air was "safe to breathe."[49] In a report published in 2003, however, the EPA's inspector general found that the agency did not have sufficient data to make such a statement. He also considered that the White House had influenced the EPA to eliminate cautionary warnings, ensuring that they were included in the report, partly motivated by the desire to resume Wall Street activity. In fact, the collapse of the World Trade Center "World Trade Center (1973-2001)") resulted in serious reductions in air quality and is the likely cause of many respiratory illnesses, with residents and workers in offices and businesses in Lower Manhattan being the first affected.[50]​.

According to a 2006 survey, 16 percent of American adults believe that the World Trade Center could have been destroyed by "controlled demolition", rather than as a result of plane impacts.

• - Wikimedia Commons hosts a multimedia gallery on World Trade Center Collapse.

• - Wikimedia Commons hosts a multimedia category on Black Tuesday.

• - Wikimedia Commons hosts a multimedia category on World Trade Center.