On the morning of September 11, 2001, two planes flying to Los Angeles were hijacked by terrorists and then flown directly into the twin towers of the World Trade Center (WTC). Within two hours, both skyscrapers collapsed. Total number the victims of the terrorist attack approached 3,000. The site where the WTC stood became known as Ground Zero.

Not everyone knows, but in addition to the northern (1 WTC) and southern (2 WTC) twin towers, the 7 WTC skyscraper, which was also part of the World Trade Center complex, was completely destroyed. Buildings 4 WTC, 5 WTC and 6 WTC, as well as the Marriott Hotel, partially collapsed. Thus, the entire complex was a terrible sight. It is not for nothing that this place began to be called Ground Zero - a place on the surface of the earth - the epicenter of a nuclear explosion.

Twin Towers now

Since 2001, there has been a long process of creating a new complex consisting of skyscrapers, a memorial, a museum, and a transport hub. As of 2017, skyscrapers 7 WTC, 1 WTC and 4 WTC have been completely built. The remaining skyscrapers are under construction.

Initial stage construction of tower 7 WTC

The 9/11 Memorial was dedicated on the 10th anniversary of the terrorist attack on the site of the World Trade Center's twin towers.

The memorial covers an area of ​​2.5 hectares. On its territory, two large mirror pools were built, with water flowing down the inner walls. Both pools are located exactly on the spot where the destroyed skyscrapers stood and follow their contours. Water, falling like a waterfall from the walls, goes into large holes located in the center and symbolizing the abyss. All this makes a strong impression. The names of the 2,977 people who died here during the terrorist attacks of 1993 and 2001 are written on the outer walls of the pools.

More than 100 white oak trees are planted nearby. There should be even more of them in the future. One of the trees is known as the Survival Tree. Planted back in the 70s of the 20th century, this pear tree was found under the ruins in a severely damaged state. Part of the tree remained alive and was saved.

The memorial took a long time to create. Despite the political decision to perpetuate the tragic events, construction was delayed due to the long bureaucratic approval procedure. The concept of the memorial itself was known already in 2004. Then a professional jury, having reviewed over 5,000 applications, chose the work of architects Michael Arad and Peter Walker, which was called “Reflective Absence.”

You can enter the memorial territory for free by receiving a pass at the entrance. To enter the museum you must purchase tickets in advance www.911memorial.org

What is on the site of the Twin Towers

Now, on the site of the destroyed Twin Towers, a complex of seven new skyscrapers is being built. Towers 1, 4 and 7 are completely ready. The rest are in varying degrees of completion, for example, Tower 3 of the World Trade Center is almost ready.

The 9/11 Museum is located on the same site and is surrounded by the remains of the original World Trade Center. The largest hall of the museum is Foundation Hall. There you can find a wall built to hold back the Hudson River and the remains of columns that once formed the outer structure of the Twin Towers. The museum contains artifacts, photographs, videos and other materials about the history of the WTC and the tragedy.

New skyscraper built on the site of the destroyed Twin Towers

The new World Trade Center building opened in New York. The new 104-story skyscraper was built on the site of the twin towers destroyed on September 11, 2001. The new building, with a height of 541 meters, became the tallest structure in the United States.

Western media reports that tenants have already begun to move to their offices in the new World Trade Center building, for example, employees of the Conde Nast publishing house. In total, 60% of the skyscraper's area has already been commissioned. But the observation deck at the top of the building can be visited freely; numerous tourists have already flocked there.

However, no official ceremonies have yet been held in connection with the opening of the World Trade Center. This is due to the fact that the date of the ceremonies is still being agreed upon by the secretariats of the governors of New York and New Jersey.

"New York's landscape has been restored," said Patrick Foye, executive director of the city's Port Authority, which owns the building and the 6.5 acres of land on which the center was built.

Construction of the new World Trade Center building cost $3.9 billion. Construction lasted eight years. The building is currently the tallest in the United States. On the territory of the 541-meter-high skyscraper there is a memorial in memory of the victims and a museum that opened this year.

According to Foye, the World Trade Center "sets new standards in construction, design, prestige and integrity." Also, according to Foye, the building is the safest office center in the entire United States.

The Twin Towers of the World Trade Center opened in 1973. During the terrorist attack on September 11, 2001, when two hijacked planes crashed into the skyscrapers, the towers were destroyed. Almost three thousand people died as a result of the terrorist attack. After this, it was decided to build a new skyscraper on the site of the destroyed towers.

Few people know that during the terrorist attack of September 11, 2001, not only the famous New York Twin Towers were destroyed. The explosions affected a huge area. Several more skyscrapers that were part of the World Trade Center complex and the Marriott Hotel collapsed. The place where towers and other buildings used to rise was nicknamed “zero mark” by the townspeople. Despite the fact that the whole of America was having a hard time experiencing the September attack, work to restore this site began already in 2001. To date, new majestic structures have appeared on the site of the destroyed Trade Center.

Freedom Tower

One of the initial tasks set by the New York administration for engineers and city planners was the creation of a new World Trade Center building. After much debate, Daniel Libeskind's project was chosen from among all the options. The architect proposed placing the offices of the World Trade Center in a mirrored skyscraper topped with a thin spire. Construction began only in 2006, and in 2014 the tower was inaugurated. Today, the Freedom Tower is the tallest building in America. Its height is 541 meters or 1776 feet (according to the architect, the number 1776 should symbolize the year the US Declaration of Independence was signed).

In the spring of 2016, a new transport hub, Oculus, was opened in front of the Freedom Tower, connecting 11 metro lines, an above-ground train station and a ferry crossing. According to the architect Santiago Calatrava, the station building resembles a dove taking off, spreading its wings.

In the center of the structure there is a huge oval hall - the “Oculus”, which gives the name to the entire structure. Along the circumference of the hall there are steel piles converging at a height of 120 meters. The majestic building has already been called the most expensive metro station in the world. According to experts, over 250 thousand people will pass through Oculus every day.

9/11 Memorial and Museum

Discussions about the construction of a memorial began in the fall of 2001. A special commission examined thousands of proposed projects. By early 2004, architect Michael Arad was declared the winner of the competition. The memorial, called Reflective Absence, features two huge square pools paved with granite. The swimming pools are located strictly in the places where the foundations of the destroyed towers were located. The names of all 2,977 victims are engraved on the outer granite walls, and water flows endlessly along the inner walls, flowing into square holes in the center. The memorial is surrounded by white oak trees, and the famous “surviving tree” also grows here - a pear that was found under the rubble of the World Trade Center.

Next to the memorial there is a small, bright building - the 9/11 Museum. Here you can see fragments of structures and cars destroyed by the explosion, personal belongings of the victims, photographs from the scene of the tragedy. The remains of still unidentified victims are kept in the same building.

Finite element model of the World Trade Center floor to evaluate the building's structural system

The dimensions of the structure in plan are 63.4 x 63.4 m, the rigidity core is 26.8 x 42.1 m. The WTC towers were designed as a “pipe in pipe” (stem structural scheme) in order to create a continuous space on the floors, not divided walls or columns. This was achieved due to the fact that the outer wall of the tower, in fact, was a set of columns installed side by side, taking on the main vertical load, while the wind load fell mainly on the power columns located in the center of the tower (stiffness core). Starting from the tenth floor, each wall of the tower was composed of 59 columns, 49 load-bearing columns were installed in the center of the tower. All elevators and stairwells ran in the rigid core, leaving a large free space between the core and the perimeter of the tower to accommodate offices.

The floor structure consisted of 10 cm of lightweight concrete laid on permanent formwork made of profiled decking. The corrugated sheeting was laid on secondary (auxiliary) trusses (beams), supported by the main trusses, transmitting the load to the central and peripheral columns. The main trusses had a length of 11 and 18 meters (depending on the span), and were laid in increments of 2.1 m, and were attached from the outside to lintels connecting the peripheral columns at the level of each floor, and from the inside to the central columns. The floor was secured through elastic dampers designed to reduce the impact of building vibrations on the people working in it.

This truss system allowed optimized load redistribution of the floor diaphragms between the perimeter and core, with improved performance between the different materials of flexible steel and rigid concrete allowing the moment structure to transfer influence into compression on the core, which also mainly supported the transmission tower.

The towers also included an "outrigger truss" located between the 107th and 110th floors, which consisted of six trusses along the longitudinal (long) axis of the core and four along the short (transverse) axis, which served to redistribute the load and increase the overall stability of the building, and also to support the antenna spire, which was installed on only one of the towers. NIST determined that this structure played a key role in the sequence of events that led to the complete destruction of the towers.

Possibility of resisting fire and aircraft hits

Like all modern high-rise buildings, the WTC towers were designed and built to withstand a normal fire. Many fire safety features were incorporated during the design and construction phase, and others were added after a 1975 fire that consumed six floors before being contained and extinguished. Tests carried out before the disaster showed that the steel structures of the towers met or even exceeded current fire resistance requirements.

The structural engineers who designed the World Trade Center considered the possibility that a plane could collide with the building. In July 1945, a B-25 Mitchell bomber became disoriented in fog and crashed into the 79th floor of the Empire State Building. A year later, a twin-engine C-45 Beechcraft collided with the skyscraper at 40 Wall Street, and another plane came close to another collision with the Empire State Building.

NIST stated that “American Building Standards do not include requirements for the stability of buildings when struck by an aircraft. … and thus buildings are not designed to withstand the impact of a fully fueled commercial airliner.” , nevertheless, the designers and architects of the World Trade Center discussed this problem and recognized its importance. Leslie Robertson, one of the chief engineers of the World Trade Center, recalled that the scenario considered was that a Boeing 707 jet would hit the building, disorientated in fog and flying at relatively low speed in search of JFK Airport or Newark Liberty Airport. John Skilling, another WTC engineer, said in 1993 that his subordinates carried out an analysis that showed that most big problem in the event of a collision between the WTC towers and a Boeing 707, all the aircraft’s fuel will fall inside the building and lead to a “terrible fire” and many casualties, but the building itself will remain standing. FEMA wrote that the World Trade Center buildings were built to accommodate a collision with the Boeing 707 jet airliner, which weighs 119 tons and has a speed of about 290 km/h, much less in weight and speed than the aircraft used in the September 11 attacks.

NIST found in the archives a three-page report summarizing a study that simulated a Boeing 707 or Douglas DC-8 hitting a building at 950 km/h. The study showed that the building should not collapse as a result of such a hit. But, as NIST experts noted, “the 1964 study did not model the effect of a fire caused by jet fuel spraying into a building.” NIST also noted that in the absence of initial calculations used to model the situation, further comment on this topic would be largely “speculation.” Another document found by NIST was a calculation of the period of oscillation of a building if a plane struck the 80th floor of the World Trade Center tower, but it made no assumptions about the fate of the building after the impact. A property risk assessment report prepared for Silverstain Properties considered the possibility of an aircraft hitting the World Trade Center towers as unlikely but possible. The report's authors cited WTC structural engineers who said the towers should have withstood a collision with a large passenger plane, but burning fuel flowing from the plane to ground level would have damaged the building's skin. Some of the documents related to the analysis of the scenario of an aircraft hitting the towers were lost as a result of the destruction of WTC 1 and WTC 7, which contained documents from the Port Authority of New York and New Jersey and Silverstain Properties.

Aircraft hitting towers

The terrorists flew two Boeing 767 jet airliners, American Airlines Flight 11 (767-200ER) and United Airlines Flight 175 (767-200), into the towers. The North Tower (1 WTC) was hit at 8:46 am by Flight 11, between the 93rd and 99th floors. Flight 175 crashed into the south tower (2 WTC) at 9:03, between floors 77 and 85.

The Boeing 767-200 airliner has a length of 48.5 m, a wingspan of 48 m, and carries on board from 62 tons (-200) to 91 tons (-200ER) of aviation fuel. The planes crashed into the towers at very high speed. Flight 11 was traveling at approximately 700 km/h when it struck the north tower; Flight 175 crashed into Southern at a speed of about 870 km/h. In addition to the fact that the hits caused severe damage to the supporting columns, they caused the explosion of approximately 38 tons of aviation fuel in each tower, which led to the almost immediate spread of a large fire on several floors containing office furniture, paper, carpeting, books, and other flammable materials. The shock wave from hitting the north tower spread down to the first floor, along at least one high-speed elevator shaft, and broke glass on the first floor, causing injuries to several people.

Fire

The lightweight construction of the towers and the lack of solid walls and ceilings led to the fact that aviation fuel spread throughout a fairly large volume of buildings, leading to numerous fires on several floors close to the impact areas of the aircraft. The aviation fuel itself burned out within a few minutes, but combustible materials in the building itself maintained intense fires for another hour or hour and a half. It is possible that if more traditional structures had been in the path of the aircraft, the fires would not have been so centralized and intense - aircraft debris and aviation fuel could have remained primarily in the peripheral area of ​​the building, rather than penetrating directly to its central part. In this case, the towers would probably have survived, or in any case, would have stood significantly longer.

Development of the situation

• 9:52 - A fire department helicopter radios that “large pieces of the building may fall from the upper floors of the south tower. We are seeing large parts of the building in limbo.”

• 9:59 - a helicopter reports that the south tower is falling.

Helicopters also reported the development of the situation with the north tower of the World Trade Center.

• 10:20 - Fire Department helicopter reports that the upper floors of the north tower may be unstable.

• 10:21 - it is reported that the south-eastern corner of the tower is warped, and that the tower is beginning to lean towards the south.

• 10:27 - it is reported that the roof of the north tower may collapse at any minute.

• 10:28 - The fire department receives a report that the north tower has collapsed.

Overload of dispatchers and bad job Communications led to the fact that the fire and police departments of New York had great difficulty in ensuring timely communication, both with their departments and with each other. As a result, the fire brigades located in the towers did not receive orders to evacuate and 343 firefighters died when the buildings collapsed.

WTC towers collapse

The south tower collapsed at 9:59 a.m., 56 minutes after impact. The north tower stood until 10:28, 102 minutes after the plane hit it. The collapsed towers created a huge cloud of dust that covered much of Manhattan. In both cases, a similar process occurred, the upper damaged part of the building collapsed onto the lower floors. Both towers fell almost vertically, although there was a significant deviation from the vertical of the top of the southern tower. Debris and dust were also observed flying from the windows of the building below the rapidly advancing collapse zone.

Tower collapse mechanism

The NIST investigation found that because the planes hit the towers in different ways, the process of destruction of the north and south towers was also slightly different, although it was generally the same in both cases. After being hit by the planes, the internal power columns were seriously damaged, although the external columns suffered relatively little damage. This caused a serious redistribution of the load between them. The upper power structure of the towers played a significant role in this redistribution.

Aircraft that struck buildings stripped fire-resistant coatings from large portions of steel structures, exposing them to direct fire. In the 102 minutes preceding the collapse of the north tower, the temperature of the fire, although well below the melting point of the metal, had reached a sufficient magnitude to cause the weakening of the strength columns in the center of the building, which began to deform and bend under the weight of the upper floors. The NIST report describes this situation as follows:

A similar situation occurred in the south tower (the internal power columns were seriously damaged). The peripheral columns and floor structures of both towers were weakened by the fire, causing the floors on the damaged floors to settle and placing significant stress on the peripheral columns towards the interior of the building.

At 9:59, 56 minutes after impact, the settling floor caused severe inward bending of the outer columns on the east side of the south tower, the upper load structure transferred this bending force to the central columns, causing their failure and the beginning of the collapse of the building. upper part At the same time, the tower deviated towards the damaged wall. At 10:28 a.m., the south wall of the north tower buckled, causing approximately the same sequence of events. As a result of the subsequent collapse of the upper floors, the complete destruction of the towers became inevitable, due to the enormous weight of the part of the buildings that were located above the damage zone.

The reason that the northern tower stood longer than the southern one was a combination of the following three factors: the area where the aircraft hit the northern tower was higher (and the weight of the upper part of the building, accordingly, less), the speed of the aircraft that hit the tower was lower, in addition, the aircraft hit floors whose fire protection has previously been partially improved.

The theory of complete progressive collapse

Ruins of the South Tower (right) and North Tower (left), as well as other World Trade Center buildings

A huge cloud of dust obscured the collapsing towers, making it impossible to determine the exact duration of the destruction based on visual evidence.

Because the NIST report focuses primarily on the mechanisms of the initial collapse, it does not address the subsequent complete collapse of both WTC towers. Initial analyzes explained the collapse by saying that the kinetic energy of the falling upper floors was much greater than what the floors could withstand, which also collapsed, adding kinetic energy to the falling building. This scenario was repeated with increasing speed until the towers were completely destroyed. Although this is the most widely accepted view among civil engineers, it has been criticized for not taking into account the resistance of the underlying structures, which should have slowed, or even stopped, the collapse of the towers.

7 WTC building collapse

FEMA's initial study was inconclusive, and the collapse of 7 WTC was not included in the final NIST report released in September 2005. Except for the letter published Metallurgical Journal, which suggested that the building's steel frame could have melted in a fire, no other studies of this issue have been scientific journals was not published. The 7 WTC crash was investigated separately from the 1 WTC and 2 WTC crashes, and in June 2004, NIST released a working report that contained several hypotheses about what happened. One hypothesis was the destruction of one of the building's critical support columns, caused by fire or large debris from falling towers, leading to "disproportionate collapse of the entire structure."

NIST diagram showing the bending of Column 79 (outlined in orange) that began the progressive collapse of the building.

The order of destruction of 7 WTC in the diagram from the 2004 NIST preliminary report. Column 79 is marked with a circle in the center of the red zone.

NIST's 7 WTC fall model. In the first part of the video, columns 81, 80 and 79 are depicted from left to right.

On November 20, 2008, NIST published the final report on the fall of 7 WTC. The NIST report cited fire as the main cause of destruction, along with a lack of water for firefighters and the automatic fire suppression system to fight the fire. NIST reconstructs the sequence of events as follows: At 10:28 a.m., debris from the falling 1 WTC caused damage to the nearby 7 WTC. Fires also broke out, likely from burning debris from 1 WTC. Firefighters immediately arrived at 7 WTC, but at 11:30 they discovered that there was no water in the fire hydrants to fight the fire - the water came from the city water supply system, destroyed as a result of the falls of towers 1 WTC and 2 WTC. New York City Fire Department ( English), fearing for the lives of firefighters in the event of the destruction of 7 WTC, at 14:30 he recalled the firefighters and stopped fighting to save the building. The fire was observed on 10 floors from the 7th to the 30th, and on floors 7-9 and 11-13 the fire got out of control. Thermal expansion of the beams heated to approximately 400 °C around column 79 in the eastern part of the building in the area of ​​the 13-14th floor led to the failure of fire-weakened floors adjacent to column 79 from the 13th to the 5th floor. The destruction of the floors deprived column 79 of horizontal support, and it began to bend, which was the immediate cause complete destruction buildings in a few seconds. The bending of column 79 led to the transfer of the load to columns 80 and 81, which also began to bend, as a result of which all the floors associated with these columns were destroyed up to the top of the building. The falling floors destroyed truss 2, which led to the fall of columns 77, 78 and 76. As a result of the increased load transferred from the bent columns, the falling of fragments of the floors from above, and the lack of horizontal support from the destroyed floors, all internal columns from east to west began to bend sequentially. Following this, in the area of ​​floors 7-14, the outer columns began to bend, to which the load was transferred from the lowered inner columns and the center, and all floors above the bent columns began to fall down as a single whole, which completed the final destruction of the building at 17:20.

Some authors criticized the city authorities' decision to locate the 7 WTC headquarters on the 23rd floor Office of Emergency Situations(English) Office of Emergency Management ). It was believed that this could have been a significant factor in the destruction of the building. Particularly noted was the placement of large diesel fuel tanks in the building, which were supposed to serve to power emergency power generators. NIST concluded that diesel fuel did not play a role in the destruction of the building, but the rapid evacuation of the Office of Emergency Management was a contributing factor to the poor communication between various agencies and the loss of control over the situation. The main cause of the building's destruction was fire; damage from debris from the falling 1 WTC accelerated the fall of the building, but NIST calculations showed that 7 WTC would have collapsed from an uncontrolled fire alone.

Progress of investigations

First reaction

The destruction of the World Trade Center towers came as a surprise to construction engineers. “Before September 11,” the magazine wrote New Civil Engineer“We quite sincerely could not imagine that a structure of such a scale could suffer such a fate.” Although the damage from the aircraft was severe, it only affected a few floors of each building. The challenge for engineers was to figure out how such local damage caused the complete progressive collapse of three of the world's largest buildings. In an October 2001 BBC interview, English architect Bob Halvorson aptly predicted that there would be much "debate about whether the World Trade Center could have collapsed the way it did." A full analysis would include architectural and structural plans of the World Trade Center, witness statements, videos of the destruction, debris survey data, etc. Underscoring the complexity of the task, Halvorson said the destruction of the World Trade Center towers is "far beyond the realm of normal experience."

Research credibility

Immediately after the disaster, there was a situation of uncertainty as to who had sufficient authority to conduct an official investigation. Contrary to the practice found in air crash investigations, clear procedures for investigating building collapses simply did not exist.

A short time after the disaster at the base Institute Construction Engineers (English) Structural Engineers Institute (SEI)) American Society of Civil Engineers(English) American Society of Civil Engineers ASCE), a working group was created, which also involved specialists American Institute of Steel Construction(English) American Institute of Steel Construction ), American Concrete Institute(English) American Concrete Institute ), National Fire Protection Association and Fire Protection Engineering Society(English) Society of Fire Protection Engineers ) . ASCE also invited the Federal Emergency Management Agency (FEMA) to join the work of this group, which later received joint ASCE-FEMA control. This investigation was subsequently criticized by American engineers and lawyers, however, the authority of the above-mentioned organizations was sufficient to conduct the investigation and to provide access to the disaster site for the group's experts. One of the most poignant aspects of the investigation was that clearing the disaster site actually led to the destruction of the remaining components of the building. Indeed, when NIST published its final report, it noted a “lack of physical evidence” as one of the main problems of the investigation. Only a fraction of a percent of the building's remains remained available for investigation after the clearance work was completed, with a total of 236 individual steel fragments recovered.

FEMA released its report in May 2002. Although NIST had already announced its involvement in the investigation in August of that year, in October 2002, under increasing public pressure for a more detailed investigation, Congress passed a bill creating new group administered by NIST, which published its report in September 2005.

FEMA's "stack of pancakes" theory

In its early investigations, FEMA developed a theory to explain the collapse of the World Trade Center towers, called the "stack of pancakes" theory. pancake theory). This theory was defended by Thomas Iga and was widely covered by PBS. According to this theory, the connection between the lintels supporting the floor and the columns of the building failed, causing the floor to collapse on the floor below, placing loads on its structure that it was not designed for. Some individual publications have proposed other sets of factors that caused the collapse of the towers, but overall this theory has been accepted by most.

Fires remained the main key factor in this theory. Thomas Iga, a professor of materials science at MIT, described the fires as "the most baffling part of the collapse of the World Trade Center towers." Although the fires were initially thought to have "melted" the steel structures, Iga stated that "the temperatures of the fires in the WTC towers were unusually high, but still certainly not sufficient to cause melting or severe softening of the steel." Aviation kerosene combustion typically results in extensive fires, but these fires do not have very high temperature. This led Iga, FEMA and other researchers to believe that there was a weak point, and this point was identified as the floor anchorage to the building's supporting structure. Due to the fire, these fastenings weakened and, when they collapsed under the weight of the floor, collapse began. On the other hand, the NIST report states completely and unequivocally that these fasteners were not destroyed. Moreover, it was their strength that was one of the main reasons for the collapse, since through them a force was transmitted to the peripheral columns, bending the columns inward.

At temperatures above 400-500 °C there is a sharp decrease in the tensile strength and ductility limit (3-4 times), at 600 °C they are close to zero and the bearing capacity of the steel is exhausted.

NIST report

Organization of the study

As a result of mounting pressure from experts, construction industry leaders and family members following the release of the FEMA report, National Institute of Standards and Technology The Commerce Department's NIST conducted a three-year, $24 million investigation into the destruction and collapse of the World Trade Center towers. The study included a series of experiments, in addition, leading specialists from many third-party organizations were involved in it:

• Structural Engineering Institute of the American Society of Civil Engineers (SEI/ASCE)
• Society of Fire Protection Engineers (SFPE)
• National Fire Protection Association (NFPA)
• American Institute of Steel Construction (AISC)
• Council on Tall Buildings and Urban Habitat (CTBUH)
• Structural Engineers Association of New York (SEANY)

Scope of study and its limitations

The NIST study's scope was limited to examining the "sequence of events from the moment the aircraft was hit until the collapse of each tower began" and also included "a small analysis of the structural behavior of the tower structure once the conditions for its failure had been reached and collapse was imminent." Like many other engineers working on this issue, NIST experts focused on aircraft hits on towers, simulating the effects of hits such as structural failure, fire spread, etc. in a very high degree of detail. NIST has developed several very high-detail models of various building components, such as floor-supporting lintels, and entire buildings have also been modeled, but at a lower level of detail. These models were static, or quasi-static, and included deformation simulations, but did not include simulations of the movement of structural elements after they were separated from each other. Thus, NIST models are useful for understanding why towers begin to collapse, but do not provide a way to model the collapse itself.

Parallel investigations

In 2003, three engineers from the University of Edinburgh published a report suggesting that the fires alone, even without taking into account the destructive effects of aircraft strikes, were enough to completely destroy the WTC towers. In their opinion, the design of the towers made them uniquely vulnerable to large fires covering several floors at once. When the NIST report was released, Barbara Lane and the English engineering company Arup criticized its conclusion that the destruction caused by aircraft hits was a necessary factor for the collapse of buildings .

Criticism

Some engineers have offered insight into how the towers collapse by developing animated collapse sequences based on dynamic computer models and comparing the resulting results with video footage of the disaster site. In October 2005, the magazine New Civil Engineer relative to a computer model created by NIST. In response, NIST enlisted Colin Bailey of the University of Manchester and Robert Planck of the University of Sheffield to create the computer visualizations needed to work to correct models of tower collapses and bring those models into full agreement with observed events.

Other buildings

Parts of the outer wall of the north tower opposite the remains of the 6 WTC building, which received very serious damage as a result of the fall of the north tower. In the upper right corner are the remains of the 7 WTC building.

On September 11, 2001, the entire World Trade Center complex and the small Greek Orthodox Church of St. Nicholas, which stood on Liberty Street opposite the south tower of the World Trade Center, were destroyed. In addition, many buildings surrounding the complex were damaged to one degree or another.

Consequences

Clearing the disaster site

The huge pile of rubble at the World Trade Center site continued to burn and smolder for another three months, with efforts to control the fire unsuccessful until a significant amount of debris and debris had been removed. The clearance was a very large-scale operation, coordinated by the Department of Construction (DDC). A preliminary clearing plan was prepared on September 22 by Controlled Demolition Inc. (CDI) from Phoenix. Mark Lozo, president of CDI, particularly emphasized the importance of protecting the clay wall (or "bathtub") that protected the WTC foundation from being flooded by the waters of the Hudson. The cleanup was carried out around the clock, involving a large number of contractors, and cost hundreds of millions of dollars. In early November, after about a third of the debris had been cleared, the city government began reducing the involvement of police and firefighters in the cleanup as they searched for the remains of the dead, and shifted priorities to trash removal. This caused objections from firefighters. As of 2007, demolition of some of the buildings surrounding the WTC was ongoing, amid construction of a WTC replacement. memorial complex and Freedom Tower.

Controlled demolition versions

There is a theory that the WTC towers could have been destroyed as a result of a planned, controlled demolition, and not as a result of being hit by aircraft. This theory was rejected by NIST, which concluded that no explosives were involved in the collapse of the towers. NIST stated that it did not perform tests to look for explosive residues of any kind in the debris because it was not necessary:

12. Did the NIST investigation look for evidence of the WTC towers being brought down by controlled demolition? Was the steel tested for explosives or thermite residues? The combination of thermite and sulfur (called thermate) "slices through steel like a hot knife through butter."

NIST did not test for the residue of these compounds in the steel.

The responses to questions number 2, 4, 5 and 11 demonstrate why NIST concluded that there were no explosives or controlled demolition involved in the collapses of the WTC towers.

In a 2008 report, NIST also analyzed the WTC Tower 7 explosion hypothesis and concluded that the explosion could not have caused the observed effects. In particular, the smallest amount of explosive that could destroy Column 79 would produce a noise of 130-140 decibels at a distance of 1 km from 7 WTC, but such noise was not noticed by either sound recording equipment or witnesses. This theory became a prominent part of most of all "conspiracy theories" that arose as a result of the events of September 11.

Notes

1. Relatives gather at ground zero to mark 9/11, The Associated Press/MSNBC(September 9, 2007). Retrieved November 3, 2007.
2. PartIIC - WTC 7 Collapse (pdf). NIST Response to the World Trade Center Disaster. National Institute of Standards and Technology (April 5, 2005). Archived from the original on March 4, 2012. Retrieved November 1, 2006.
3. Hamburger, Ronald, et al.(pdf). Federal Emergency Management Agency. Archived
4. Snell, Jack, S. Shyam Sunder NIST Response to the World Trade Center Disaster (pdf). National Institute of Standards and Technology (November 12, 2002). Archived from the original on March 4, 2012. Retrieved July 27, 2006.
5. Chapter 1 // . - NIST. - P. p. 6.
6. National Construction Safety Team Final Report on the Collapse of the World Trade Center Towers. - NIST.
7. Barrett, Devlin Steel type in WTC met standards, group says. The Boston Globe. Associated Press (2003). Retrieved May 2, 2006.
8. Glanz, James and Eric Lipton. The Height of Ambition (English) , The New York Times(8 September 2002).
9. Adam Long. PILOT LOST IN FOG; SCENE OF PLANE CRASH LAST NIGHT AIRPLANE CRASHES INTO SKYSCRAPER Ceiling Reduced by Fog, The New York Times(24 May 1946).
10. (pdf). NIST NCSTAR 1-1 Pages 70-71 Archived
11. Leslie E. Robertson. Reflections on the World Trade Center (English) // The Bridge. - National Academy of Engineering, 2002. - Vol. 32. - No. 1. Archived from the original on February 28, 2010.
12. Fahim Sadek. NIST NCSTAR 1-2. Baseline Structural Performance and Aircraft Impact Damage Analysis of the World Trade Center Towers. - NIST, September 2005. - pp. 3-5, 308.
13. Nalder, Eric. (English) The Seattle Times (27-02-1993).
14. Ronald Hamburger, et al. World Trade Center Building Performance Study. - Federal Emergency Management Agency. - P. 1-17.
15. NIST's Working Hypothesis for Collapse of the WTC Towers (Appendix Q). NIST (June 2004). Archived from the original on March 4, 2012. Retrieved December 21, 2007.
16. NIST was informed by the Port Authority that the documents cited were destroyed in the collapse of WTC 1 and the WTC owners documents held in WTC 7 were also lost.
17. Lew, H. S.; Richard W. Bukowski and Nicholas J. Carino Design, Construction and Maintenance of Structural and Life Safety (pdf). NIST NCSTAR 1-1 Page 71. National Institutes of Standards and Technology (2006). Archived from the original on March 4, 2012. Retrieved October 15, 2007.
18. Jane's All the World's Aircraft Boeing 767. Jane's (2001). Retrieved August 19, 2007.
19. Field, Andy A Look Inside a Radical New Theory of the WTC Collapse. Fire/Rescue News (2004). Retrieved July 28, 2006.
20. Gross, John L., Therese P. McAllister Structural Fire Response and Probable Collapse Sequence of the World Trade Center Towers (pdf). Federal Building and Fire Safety Investigation of the World Trade Center Disaster NIST NCSTAR 1-6 Archived
21. Wilkinson, Tim World Trade Center - Some Engineering Aspects (2006). Archived from the original on March 4, 2012. Retrieved July 28, 2006.
22. Lawson, J. Randall, Robert L. Vettori. NIST NCSTAR 1-8 - The Emergency Response P. 37. NIST (September 2005). Archived from the original on March 4, 2012.
23. McKinsey Report - Emergency Medical Service response. FDNY/McKinsey & Company (August 9, 2002). Archived from the original on August 26, 2011. Retrieved July 12, 2007.
24. McKinsey Report - NYPD (August 19, 2002). (inaccessible link - story) Retrieved July 10, 2007.
25. NY Firefighters attack Giuliani. BBC News, July 12, 2007 http://news.bbc.co.uk/2/hi/americas/6294198.stm
26. Bažant, Zdeněk P.; Yong Zhou (2002-01-01). "Why Did the World Trade Center Collapse? - Simple Analysis". J Engrg Mech 128 (1): pp. 2-6. DOI:0.1061/(ASCE)0733-9399(2002)128:1(2) . Retrieved 2007-08-23.
27. Bažant, Zdeněk P.; Mathieu Verdure (March 2007). "Mechanics of Progressive Collapse: Learning from the World Trade Center and Building Demolitions." J Engrg. Mech. 133 (3): pp. 308-319. DOI:10.1061/(ASCE)0733-9399(2007)133:3(308) . Retrieved 2007-08-22.
28. Cherepanov, G.P. (September 2006). "Mechanics of the WTC collapse". Int J Fract(Springer Netherlands) 141 (1-2): 287-289. DOI:10.1007/s10704-006-0081-8. Retrieved 2007-10-07.
29. Hayden, Peter WTC: This Is Their Story. Firehouse Magazine (April 2002). (inaccessible link - story)
30. Observations, findings and Recommendations (pdf). World Trade Center Building Performance Study, (Chapter 8.2.5.1). Federal Emergency Management Agency. Archived from the original on March 4, 2012. Retrieved July 28, 2006.
31. Barnett, J.R.; R.R. Biederman, R.D. Sisson Jr. An Initial Microstructural Analysis of A36 Steel from WTC Building 7. Feature: Letter. The Journal of Materials (2001). Archived from the original on March 4, 2012. Retrieved May 12, 2006.
32. Key Findings of NIST's June 2004 Progress Report on the Federal Building and Fire Safety Investigation of the World Trade Center Disaster. Fact sheets from NIST. National Institute of Standards and Technology (2004). Archived from the original on March 4, 2012. Retrieved July 28, 2006.
33. Interim Report on WTC 7 (pdf). Appendix L. National Institute of Standards and Technology (2004). Archived from the original on March 4, 2012. Retrieved July 28, 2006.
34. NIST Releases Final WTC 7 Investigation Report. National Institute of Standards and Technology (November 20, 2008). Archived from the original on March 4, 2012. Retrieved August 28, 2009.
35. Robert MacNeill, Steven Kirkpatrick, Brian Peterson, Robert Bocchieri. Global Structural Analysis of the Response of World Trade Center Building 7 to Fires and Debris Impact Damage. - November 2008. - pp. 119-120.
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37. Barrett Wayne Grand Illusion: The Untold Story of Rudy Giuliani and 9/11. - Harper Collins. - ISBN 0-06-053660-8
38. Replying to Giuliani
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40. Whitehouse, David WTC collapse forces skyscraper rethink. BBC News (2001). Archived from the original on March 4, 2012. Retrieved July 28, 2006.
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Twins Tower or twins - this is how New Yorkers lovingly called the tragically famous towers of the World Trade Center. The tragedy of September 11, 2001 went down in US history as a national tragedy, and the tragic news spread throughout the world in a matter of minutes.

It was not without reason that the terrorists chose the New York skyscrapers as the target of their attack, which were not just the pride of the Americans, but a symbol of their exorbitant pomp and love for gigantism. The “Project of the Century” turned into a tragedy at the end of the century.

How did the miracle begin? In the 60s, the prestige of American democracy was shaken. In order to awaken people's pride in their country and restore optimism and faith in the future to the people of the United States, an all-American project was needed - something grandiose that would have a stunning effect on the minds and feelings of millions of people.

There have been many projects put forward that could capture the imagination of the American people. One of the architects actively involved in the project was the American architect Japanese origin Minoru Yamasaki (1912 -1986), already well known for his works, including the airport building in St. Louis, the complex of buildings of the Institute of Arts and Crafts in Detroit, and the American Concrete Institute.

Minoru Yamasaki conceived the World Trade Center project in 1962; in January 1964, the architect commissioned by the Port Authority created drawings of the buildings; a little later that year, he presented for discussion a 1:130 life-size model, and two years later (5 August 1966) powerful excavators began to dig a foundation pit.

Before the twins, skyscrapers in New York were built on a natural stone base. Manhattan is truly made of stone, it has stone and granite under a layer of earth, this can be seen when you watch the construction of new houses: pits here are not dug, but cut down, gnawed out with the steel teeth of loggers.

The first problem that the engineers encountered was that there was no footstone in the place where the twins were supposed to stand. Instead, they discovered artificial, alluvial soil that previously “belonged” to the Hudson River. This soil consisted of a large amount of artificial earth, mixed with layers of cobblestones, sand, gravel, pebbles, even old ships were found in artificial soil. The builders were in despair: additional difficulties, additional costs, additional concrete.

This was not the only problem that beset the architect and engineers. The next problem that befell them was the 164 large and small, narrow and wide buildings, most often made of stone, that stood on the site of the future World Trade Center and had to be demolished. It was not difficult to take them down, but the problem still remained after that. It was much more difficult to leave it intact, and then to transfer the saturated and complex system underground utilities, fire alarm system, multi-core telephone and electrical cables, gas, heat, pneumatic and water pipes, do not affect the nearby expressway and preserve numerous pedestrian roads and crossings.

Another problem was the underground station railway, from here starting an underwater route to New Jersey that carries hundreds of thousands of people to and from work. If the road were closed, New York and the entire United States would face inevitable economic problems. The subway transported people until a new underground station was built in the lower tier of the complex.

This is not to say that the work of the builders was easy. What are they worth? 1.2 million cubic meters yards of land which had to be dug up and removed. Instead, under the twins, the so-called Plaza was created - an underground space where there were numerous restaurants and banks, travel agencies, airline ticket offices, shops, a new station of the New Jersey road, much better than the previous one, warehouses, technical workshops for servicing the twins and an underground garage for two thousand cars.

Faced with the challenge of constructing a building of unprecedented height, engineers adopted an innovative structural model: a rigid "hollow tube" of closely spaced steel columns, with floor trusses extending out toward the center. Along the outer surface of each of the four sides of the building, 61 steel beams ran along the entire height, between which cables were also stretched along the entire height. The columns, clad in silver aluminum alloy, were 476.25mm wide and set just 558.8mm apart, making the towers appear from a distance to have no windows at all. The load-bearing walls were assembled from prefabricated steel blocks, each weighing 22 tons, height 36 feet (4 floors high), width 10 feet. The steel embedded in Gemini weighs two hundred thousand tons in total.

As the twins grew, the installers laid interfloor ceilings from special, pre-prepared corrugated steel and durable concrete slabs. The ceilings were attached to the external load-bearing walls from the outside and to the internal, the only steel columns in the twins with a purely useful function - they were erected to attach internal elevators.

The elevator system used in the buildings is also. The Twin Towers were the first supertall buildings designed without masonry. Concerned that the intense air pressure generated by high-speed elevators could bend standard shafts, engineers developed a solution using a "dry wall" system anchored in a reinforced steel base. Elevators with a standard configuration to serve 110 floors could require half the area of ​​the lower rooms to accommodate the shafts. Otis Elevators developed a fast and compact system in which passengers would take turns in "sky lobbies" on the 44th and 78th floors, cutting the number of shafts in half. In total, the World Trade Center complex had 239 elevators and 71 escalators, which were controlled by a computer center from the Port Authority. Each elevator with a lifting capacity of 4536 kilograms could lift 55 people with a lifting speed of about 8.5 meters per second.

Construction proceeded quickly, despite the difficulties with financing that arose every now and then. New York City Budget 1965 -1970 was 6 billion dollars. To raise money to invest in the construction of the Trade Center, the city issued bonds with a guarantee of their repayment. But in 1970, New York suffered a financial crisis. The deadline for paying off the bonds has also arrived. The construction was almost frozen. To save the situation, new, increased taxes in the business sector had to be introduced. Another source of money was found: they began to rent out future Gemini premises for offices. And they were expected to be huge - 100 thousand square meters. m. We finally managed to “get out” of all the difficulties. The north tower was completed in 1971, the south tower in 1973. The grand opening of the World Trade Center took place on April 4, 1973.

The twins soared to a record height of 450 m. The towers were square in cross-section, with a side of 65 m. Each tower was 110 floors. The foundations of the structures went 23 m underground. 200 thousand tons of rolled steel were spent on building frames, and cables electrical networks, with a total capacity of 80,000 kilowatts, stretched 3 thousand miles - half the distance from New York to London, across the Atlantic. Listing quantitative data can hardly get boring, because each new figure or number speaks of the unprecedented scope of what has been created.

The structure of the buildings was simple and reasonable. The facades are made in the form of steel frames and modular aluminum sections mounted on them measuring 3.5x10 m, manufactured by factory stamping. This design is earthquake-resistant and can withstand very strong wind pressure at great heights. The World Trade Center building could even withstand a collision with an airplane or other flying object. But the unfortunate Gemini could not resist the violent explosion and fire of 5 thousand liters of aviation gasoline from the tanks of the aggressor aircraft.

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