The collapse of the World Trade Center occurred during the terrorist attacks of September 11, 2001, after the Twin Towers were struck by two hijacked commercial airliners.
One World Trade Center (WTC 1, or the North Tower) was hit at 8:46 a.m. Eastern time and collapsed at 10:28 a.m. Two World Trade Center (WTC 2, or the South Tower) was hit at 9:03 a.m. and collapsed at 9:59 a.m.
The resulting debris severely damaged or destroyed more than a dozen other adjacent and nearby structures, ultimately leading to the collapse of 7 World Trade Center at 5:21 p.m.
A total of 2,763 people were killed in the crashes, fires, and subsequent collapses, including 2,192 civilians, 343 firefighters, and 71 law enforcement officers as well as all the passengers and crew on the airplanes, which included 147 civilians and the 10 hijackers.
In September 2005, the National Institute of Standards and Technology (NIST) published the results of its investigation into the collapse.
The investigators did not find anything substandard in the design of the twin towers, noting that the severity of the attacks was beyond anything experienced in buildings in the past.
They determined the fires to be the main cause of the collapses, finding that sagging floors pulled inward on the perimeter columns, causing them to bow and then to buckle.
Once the upper section of the building began to move downwards, a total progressive collapse was unavoidable.
The cleanup of the World Trade Center site involved round-the-clock operations and cost hundreds of millions of dollars.
Some surrounding structures that were not hit by the airplanes still sustained significant damage, requiring them to be torn down.
Demolition of the surrounding damaged buildings continued even as new construction proceeded on the Twin Towers' replacement, the new One World Trade Center, which was opened in November 2014.
Background
Upon completion in 1973, the Twin Towers were briefly the tallest buildings in the world, and at the time of the terrorist attacks they were still in the top five.
One World Trade Center (WTC 1) the "North Tower" was, at , six feet taller than Two World Trade Center (WTC 2) the "South Tower", which stood  tall.
At the time of the attacks only the then recently completed Petronas Towers in Kuala Lumpur, Malaysia and the Willis Tower (known then as the Sears Tower) in Chicago were taller.
Built with a novel design that maximized interior space, the towers had a high strength to weight ratio as they utilized a new "framed tube" design that required 40 percent less steel than more traditional steel framed skyscrapers.
In addition, atop WTC 1 stood a  telecommunications antenna that was erected in 1978 bringing the total height of that tower to , though as a nonstructural addition, the antenna was not officially counted.
Structural design
right|thumb|Diagram showing floor truss system and concrete floor over steel pans The towers were designed as framed tube structures, which provided tenants with open floor plans uninterrupted by columns or walls.
The buildings were square and  on each side but had chamfered  corners making the exterior of each building roughly  wide.
Numerous, closely spaced perimeter columns provided much of the strength to the structure, along with gravity load shared with the steel box columns of the core.
Above the tenth floor, there were 59 perimeter columns along each face of the building spaced  on center.
While the towers were square, the interior cores were rectangular and were supported with 47 columns that ran the full height of each tower.
All of the elevators and stairwells were located in the core, leaving a large column-free space between it and the perimeter that was bridged by prefabricated floor trusses.
As the core was rectangular this created a long and short span distance to the perimeter columns.
The floors consisted of  lightweight concrete slabs laid on a fluted steel deck.
A grid of lightweight bridging trusses and main trusses supported the floors with shear connections to the concrete slab for composite action.
The trusses had a span of  in the long-span areas and  in the short-span area.
The trusses connected to the perimeter at alternate columns, and were therefore on  centers.
The top chords of the trusses were bolted to seats welded to the spandrels on the perimeter side and a channel welded to interior box columns on the core side.
The floors were connected to the perimeter spandrel plates with viscoelastic dampers, which helped reduce the amount of sway felt by building occupants.
The towers also incorporated a "hat truss" or "outrigger truss" located between the 107th and 110th floors, which consisted of six trusses along the long axis of core and four along the short axis.
This truss system allowed optimized load redistribution of floor diaphragms between the perimeter and core, with improved performance between the different materials of flexible steel and rigid concrete allowing the moment frames to transfer sway into compression on the core, which also mostly supported the transmission tower.
These trusses were installed in each building to support future transmission towers but only the north tower was ultimately fitted with one.
Evaluations for aircraft impact
thumb|Aviation fueled fireball emerges from north side of WTC 2 immediately after impact.
Smoke from fires can be seen coming from north side of WTC 1 at right.
Though fire studies and even an analysis of the impacts of low speed jet aircraft impacts had been undertaken prior to their completion, the full scope of those studies no longer exists.
Nevertheless, since fire had never before caused a skyscraper to collapse and aircraft impacts had been considered in their design, their destruction initially came as a surprise to some in the engineering community.
The structural engineers working on the World Trade Center considered the possibility that an aircraft could crash into the building.
In July 1945, a B-25 bomber that was lost in the fog had crashed into the 79th floor of the Empire State Building.
A year later, a C-45F Expeditor crashed into the 40 Wall Street building.
Once again, fog was believed to have been the contributing factor in the collision.
Leslie Robertson, one of the chief engineers working on the design of the World Trade Center, stated that he considered the scenario of the impact of a Boeing 707, which might be lost in the fog and flying at relatively low speeds while seeking to land at either JFK or Newark Airports.
In an interview with the BBC two months after the building collapses, Robertson claimed that, "with the 707, the fuel load was not considered in the design, I don't know how it could have been considered."
In the interview, Robertson stated that the main difference between the design studies and the event that ultimately caused the towers to collapse was due to the velocity of the impact, which greatly increased the absorbed energy, and was never considered during the construction process.
During their investigation into the collapse, the National Institute of Standards and Technology (NIST) obtained a three-page white paper that stated the buildings would survive an aircraft-impact of a Boeing 707 or DC 8 flying at .
In 1993, John Skilling, lead structural engineer for the WTC, during an interview conducted after the 1993 World Trade Center bombing remarked, "Our analysis indicated the biggest problem would be the fact that all the fuel (from the airplane) would dump into the building.
There would be a horrendous fire.
A lot of people would be killed", he said.
"The building structure would still be there."
In its report, NIST stated that the technical ability to perform a rigorous simulation of aircraft impact and ensuing fires is a recent development, and that the technical capability for such analysis would have been quite limited in the 1960s.The three-page white paper titled Salient points with regard to the structural design of The World Trade Center towers described an analysis of a Boeing 707 weighing  and carrying  of fuel striking the 80th floor of the buildings at .
It is unclear whether the effect of jet fuel and aircraft contents was a consideration in the original building design, but this study is in line with remarks  made by John Skilling following the 1993 WTC bombing.
Without original documentation for either study, NIST said any further comments would amount to speculation.
NIST 2005.
pp.
305–307.
In their final report on the collapses, the NIST stated that they could find no documentation that examined the impact of a high speed jet nor that of a large scale fire fueled by aviation fuel.
Fireproofing
Up until the mid 1970s, the use of asbestos for fireproofing was widespread in the construction industry.
However, in April 1970, the New York City Department of Air Resources ordered contractors building the World Trade Center to stop the spraying of asbestos as an insulating material.
After the 1993 bombing, inspections found fireproofing to be deficient.
Prior to the collapses, the owners of the towers, the Port Authority of New York and New Jersey, were in the process of adding fireproofing, but had only been completed on 18 floors in 1 WTC, including all the floors affected by the aircraft impact and fires, and on 13 floors in 2 WTC, although none were directly affected by the aircraft impact.
NIST concluded that the aircraft impact removed a significant portion of the fireproofing, contributing to the buildings' collapse.
In WTC 1 the impact stripped the insulation off 43 of 47 core columns on more than one floor as well as floor trusses over a space of .
In WTC 2 the impact removed insulation from 39 of the 47 columns on multiple floors and from floor trusses spanning an area of .
After the collapses, Leslie Robertson stated, "To the best of our knowledge, little was known about the effects of a fire from such an aircraft, and no designs were prepared for that circumstance.
Indeed, at that time, no fireproofing systems were available to control the effects of such fires."
September 11, 2001
thumb|right|250px|Impact locations on 1 and 2 WTC showing approximate angle of impact and alignment of the cores with the exterior of each tower.
Aircraft impacts and resultant fires
During the September 11 attacks, four teams of al-Qaeda terrorists hijacked four different jetliners.
Two of these jetliners, American Airlines Flight 11 and United Airlines Flight 175, both Boeing 767s, were hijacked after takeoff from Boston's Logan International Airport.
In its final moments, American Airlines Flight 11 flew south over Manhattan and crashed at roughly  into the north facade of the North Tower (WTC 1) at 8:46 am, impacting between the 93rd and 99th floors.National Transportation Safety Board, Office of Research and Engineering, https://www.ntsb.gov/about/Documents/Flight_Path_Study_AA11.pdf , Access Date, July 31, 2021 Seventeen minutes later, United Airlines Flight 175 approached from the southwest, over New York Harbor, and crashed into the south facade of the South Tower (WTC 2) at 9:03 am between the 77th and 85th floors at .September 11 Attack Timeline, 9/11 Memorial and Museum, https://timeline.911memorial.org/#Timeline/2 , Access Date, July 31, 2021National Transportation Safety Board, Office of Research and Engineering, https://www.ntsb.gov/about/Documents/Flight_Path_Study_UA175.pdf , Access Date, July 31, 2021
In addition to severing numerous load-bearing columns on the perimeter and inflicting other structural damage, the impacts ignited thousands of gallons of jet fuel, which in turn ignited office combustibles.
About one third of the fuel was consumed in the initial impact and resulting fireball.According to National Institute of Standards and Technology (NIST) estimates, Flight 11 was carrying  of jet fuel when it hit the North Tower.
were consumed in the initial impact when the aircraft hit and a similar amount was consumed in the fireball outside the building.
Approximately  burnt inside the office spaces igniting combustibles.
Similarly, Flight 175 was carrying around  when it hit the South Tower.
Up to  was instantly consumed in the initial fireball and up to  was consumed in the fireball outside the building.
More than  was burnt in the office spaces.
NIST estimated that each floor of both buildings contained around four pounds per square foot (60 tons per floor) of combustibles.
Some fuel from the impact traveled down at least one elevator shaft and exploded on the 78th floor of the North Tower, as well as in the main lobby.NCSTAR 1-5A, p 80 The light construction and hollow nature of the structures allowed the jet fuel to penetrate far inside the towers, igniting many large fires simultaneously over a wide area of the impacted floors.
The fuel from the planes burned at most for a few minutes, but the contents of the buildings burned over the next hour or hour and a half.
Emergency response and evacuation
thumb|The South Tower on fire Almost all the deaths in the Twin Towers occurred in the zones above the points of aircraft impact.
As the North Tower had been struck directly midway into the structure, the three stairways in the tower core were all damaged or blocked by debris preventing escape to lower floors.
In the South Tower, the impact was slightly off center to the central section of the tower and stairway A in the northwest portion of the central core was only partially blocked, and 14 to 18 civilians managed to escape from the point of aircraft impact and the floors above that.
The exact numbers of who perished and where in some cases is not precisely known; however the National Institute of Standards and Technology report indicated that a total of 1,402 civilians perished at or above the impact point in the North Tower with hundreds estimated to have been killed at the moment of impact.
In the South Tower, 614 civilians perished at the impacted floors and the floors above that.
Fewer than 200 of the civilian fatalities occurred in the floors below the impact points but all 147 civilian passengers and crew on the two aircraft as well as all 10 terrorists perished, along with at least 18 people on the ground and in adjacent structures.
All told, emergency personnel killed in the collapse included 343 members of the New York City Fire Department (FDNY), 71 law enforcement officers including 23 members of the New York City Police Department (NYPD), 37 members of the Port Authority Police Department (PAPD), five members of the New York State Office of Tax Enforcement (OTE), three officers of the New York State Office of Court Administration (OCA), one fire marshal of the New York City Fire Department (FDNY) who had sworn law enforcement powers (and was also among the 343 FDNY members killed), one member of the Federal Bureau of Investigation (FBI), and one member of the United States Secret Service (USSS).
The total death toll for civilian and non-civilians is estimated to be 2,606 persons.
Collapse
The destruction of the Twin Towers has been called "the most infamous paradigm" of progressive collapse.
They began with the local failure of a few structural components and progressed to encompass the whole of the structure.
Such collapses are characterized by "the separation of structural components (including non-load bearing elements), the release of gravitational energy, and the occurrence of impact forces."
The vertical impact force supplies the propagating action, the principal forces are parallel, and the primary load transfer is serial.
The key element in the structure that failed was constituted by the combined "vertical load-bearing members of one entire storey."
Excepting the top floors of the building, which would not have released sufficient gravitational energy to bring about a total collapse, the collapses could have begun with the failure of any story.
Under these conditions, the towers collapsed symmetrically and more or less straight down, though there was some tilting of the tops of the towers and a significant amount of fallout to the sides.
In both cases, the section of the building that had been damaged by the airplanes failed, which allowed the floors above the impact zone to fall onto the undamaged structure below.
As the collapse progressed, dust and debris could be seen shooting out of the windows several floors below the advancing destruction, caused by the sudden rush of air compressed under the descending upper levels.
During each collapse, large portions of the perimeter columns and possibly the cores were left without any lateral support, causing them to fall laterally towards the outside, pushed by the increasing pile of rubble.
Consequently, the walls peeled off and separated from the buildings by a large distance – about  in some cases – hitting neighboring buildings and starting fires that would later lead to the collapse of Building 7.
Some connections broke as the bolts snapped, leaving many panels randomly scattered.
The first fragments of the outer walls of the collapsed North Tower struck the ground 11 seconds after the collapse started, and parts of the South Tower after 9 seconds.
The lower portions of both buildings' cores (60 stories of WTC 1 and 40 stories of WTC 2) remained standing for up to 25 seconds after the start of the initial collapse before they too collapsed.
Collapse initiation
thumb|Simple representation of the collapse of the North Tower After the planes struck the buildings, but before the buildings collapsed, the cores of both towers consisted of three distinct sections.
Above and below the impact floors, the cores consisted of what were essentially two rigid boxes; the steel in these sections was undamaged and had undergone no significant heating.
The section between them, however, had sustained significant damage and, though they were not hot enough to melt it, the fires were weakening the structural steel.
As a result, the core columns were slowly being crushed, sustaining plastic and creep deformation from the weight of floors above.
As the top section tried to move downward, however, the hat truss redistributed the load to the perimeter columns.
Meanwhile, the perimeter columns and floors were also being weakened by the heat of the fires, and as the floors began to sag they pulled the exterior walls inwards.
"The ensuing loss in vertical load-carrying capacity was confined to a few storeys but extended over the entire cross section of each tower."
In the case of 2 WTC, the eastern face finally buckled, transferring its loads back to the failing core through the hat truss and initiating the collapse.
Later, the south wall of 1 WTC buckled in the same way, and with similar consequences.NIST report, p. 29 Total progressive collapse
Structural systems respond very differently to static and dynamic loads and, while the towers were designed to support enormous weight under normal conditions, they provided little resistance to the moving mass of the section above the damaged floors.
In both cases, the collapses began with the drop of the upper section through the height of at least one story (roughly three meters or ten feet), yet a fall of only half a meter (about 20 inches) would have released the necessary energy to begin an unstoppable collapse.
From there collapse proceeded through two phases.
During the crush-down phase, the upper block destroyed the structure below in a progressive series of column failures roughly one story at a time.
Each failure began with the impact of the upper block on the columns of the lower section, mediated by a growing layer of rubble consisting mainly of concrete from the floor slabs.
The energy from each impact was "reintroduced into the structure in [the] subsequent impact, ... concentrate[d] in the load-bearing elements directly affected by the impact."
This buckled the columns of the story immediately beneath the advancing destruction down to the next point of lateral support, usually the floor trusses of the given story.
After the columns buckled the block was once again unsupported and fell through the distance of that story, again impacting the columns of the story below, which then buckled in the same way.
This repeated until the upper block reached the ground and the crush-up phase began.
Here, too, the columns buckled one story at a time, now starting from the bottom.
As each story failed, the remaining block fell through the height of the story, onto the next one, which it also crushed, until the roof finally hit the ground.
The process accelerated throughout, and by the end each story was being crushed in less than a tenth of a second.
South Tower collapse
thumb|upright=1.5|right|Aerial view of the site after the collapse, with locations of the collapsed buildings outlined As the fires continued to burn, occupants trapped in the upper floors of the South Tower provided information about conditions to 9-1-1 dispatchers.
At 9:37 a.m., an occupant on the 105th floor of the South Tower reported that floors beneath him "in the 90-something floor" had collapsed.
The New York City Police Department aviation unit also relayed information about the deteriorating condition of the buildings to police commanders.
Only 14 people escaped from above the impact zone of the South Tower after it was hit (including Stanley Praimnath, who saw the plane coming at him), and only four from the floors above it.
They escaped via Stairwell A, the only stairwell which had been left intact after the impact.
Numerous police hotline operators who received calls from individuals inside the South Tower were not well informed of the situation as it rapidly unfolded.
Many operators told callers not to descend the tower on their own, even though it is now believed that Stairwell A was most likely passable at and above the point of impact.
At 9:52 a.m., the NYPD aviation unit reported over the radio that "large pieces may be falling from the top of WTC 2.
Large pieces are hanging up there".
With the warnings, the NYPD issued orders for its officers to evacuate.
During the emergency response, there was minimal communication between the NYPD and the New York City Fire Department (FDNY), and overwhelmed 9-1-1 dispatchers did not pass along information to FDNY commanders on-scene.
At 9:59 a.m., the South Tower collapsed, 56 minutes after being struck.July 31 2021, science.howstuffworks.com, Pumphrey, Clint: https://science.howstuffworks.com/engineering/structural/cause-world-trade-center-collapse.htm , Access Date July 31, 2021 North Tower collapse
After the South Tower collapsed, NYPD helicopters relayed information about the deteriorating conditions of the North Tower.
At 10:20 a.m., the NYPD aviation unit reported that "the top of the tower might be leaning", and a minute later reported that the North Tower, "is buckling on the southwest corner and leaning to the south".
At 10:28 a.m., the aviation unit reported that "the roof is going to come down very shortly" and indeed, the North Tower collapsed immediately thereafter, at 10:28 a.m., after burning for 102 minutes.
After the South Tower collapsed, FDNY commanders issued orders for firefighters in the North Tower to evacuate.
Due to radio communications problems, 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.
Three hundred forty-three firefighters died in the Twin Towers, as a result of the collapse of the buildings."
NY Firefighters attack Giuliani", BBC News, July 12, 2007 BBC.co.uk  No one was able to escape the North Tower from the impact zone or above, as all stairwells and elevator shafts on those floors were destroyed or blocked.Stairwell sign from the World Trade Center , National Museum of American History, Smithsonian Institution, 2002 The collapsing towers generated enormous clouds of debris and dust that enveloped lower Manhattan; light dust reached as far as the Empire State Building, located  away.
Building 7 collapse
left|thumb|250px|Portions of the outer shell of the North Tower lean against the remains of 6 WTC which suffered massive damage when the North Tower collapsed.
The remains of 7 WTC are at upper right As the North Tower collapsed, heavy debris hit 7 World Trade Center, causing damage to the south face of the building and starting fires that continued to burn throughout the afternoon.
Structural damage occurred to the southwest corner between Floors 7 and 17 and on the south facade between Floor 44 and the roof; other possible structural damage includes a large vertical gash near the center of the south facade between Floors 24 and 41.
The building was equipped with a sprinkler system, but had many single-point vulnerabilities for failure: the sprinkler system required manual initiation of the electrical fire pumps, rather than being a fully automatic system; the floor-level controls had a single connection to the sprinkler water riser; and the sprinkler system required some power for the fire pump to deliver water.
Also, water pressure was low, with little or no water to feed sprinklers.
thumb|Smoke from fires visible in WTC 7 at upper center before it collapsed.
Some firefighters entered 7 World Trade Center to search the building.
They attempted to extinguish small pockets of fire, but low water pressure hindered their efforts.
Fires burned into the afternoon on the 11th and 12th floors of 7 World Trade Center, the flames visible on the east side of the building.
During the afternoon, fire was also seen on floors 6–10, 13–14, 19–22, and 29–30.
In particular, the fires on floors 7 through 9 and 11 through 13 continued to burn out of control during the afternoon.
At approximately 2:00 pm, firefighters noticed a bulge in the southwest corner of 7 World Trade Center between the 10th and 13th floors, a sign that the building was unstable and might cave to one side or "collapse".
During the afternoon, firefighters also heard creaking sounds coming from the building and issued uncertain reports about damage in the basement.
Around 3:30 pm FDNY Chief Daniel A. Nigro decided to halt rescue operations, surface removal, and searches along the surface of the debris near 7 World Trade Center and evacuate the area due to concerns for the safety of personnel.
At 5:20:33 pm EDT on September 11, 2001, 7 World Trade Center started to collapse, with the crumble of the east mechanical penthouse, while at 5:21:10 pm EDT the entire building collapsed completely.
– View footage  of the collapse captured by CBS There were no casualties associated with the collapse.
When 7 World Trade Center collapsed, debris caused substantial damage and contamination to the Borough of Manhattan Community College's Fiterman Hall building, located adjacent at 30 West Broadway, to the extent that the building was not salvageable.
In August 2007, Fiterman Hall was scheduled for dismantling.
A revised plan called for demolition in 2009 and completion of the new Fiterman Hall in 2012, at a cost of $325 million.Fiterman is Funded  BMCC News, November 17, 2008Agovino T Ground Zero building to be razed  Crain's New York Business November 13, 2008 The building was finally demolished in November 2009 and construction of its replacement began on December 1, 2009.
The adjacent Verizon Building, an Art Deco building constructed in 1926, had extensive damage to its east facade from the collapse of 7 World Trade Center, though it was successfully restored at a cost of US$1.4 billion.
Other buildings
Many of the surrounding buildings were also either damaged or destroyed as the towers fell.
5 WTC endured a large fire and a partial collapse of its steel structure and was torn down.
Other buildings destroyed include St. Nicholas Greek Orthodox Church, Marriott World Trade Center (Marriott Hotel 3 WTC), South Plaza (4 WTC), and U.S. Customs (6 WTC).
The World Financial Center buildings, 90 West Street, and 130 Cedar Street suffered fires.
The Deutsche Bank Building, the Verizon Building, and World Financial Center 3 had impact damage from the towers' collapse, as did 90 West Street.
One Liberty Plaza survived structurally intact but sustained surface damage including shattered windows.
30 West Broadway was damaged by the collapse of 7 WTC.
The Deutsche Bank Building, which was covered in a large black "shroud" after September 11 to cover the building's damage, was deconstructed because of water, mold, and other severe damage caused by the neighboring towers' collapse.
Many works of art were destroyed in the collapse.
Investigations
Initial opinions and analysis
thumb|right|250px|Impact locations for 1 WTC (right) and 2 WTC (left)
In the immediate aftermath of the attacks, numerous structural engineers and experts spoke to the media, describing what they thought caused the towers to collapse.
Abolhassan Astaneh-Asl, a structural engineering professor at the University of California at Berkeley, explained that the high temperatures in the fires weakened the steel beams and columns, causing them to become "soft and mushy", and eventually they were unable to support the structure above.
Astaneh-Asl also suggested that the fireproofing became dislodged during the initial aircraft impacts.
He also explained that, once the initial structural failure occurred, progressive collapse of the entire structure was inevitable.
César Pelli, who designed the Petronas Towers in Malaysia and the World Financial Center in New York, remarked, "no building is prepared for this kind of stress."
On September 13, 2001, Zdeněk P. Bažant, professor of civil engineering and materials science at Northwestern University, circulated a draft paper with results of a simple analysis of the World Trade Center collapse.
Bažant suggested that heat from the fires was a key factor, causing steel columns in both the core and the perimeter to weaken and experience deformation before losing their carrying capacity and buckling.
Once more than half of the columns on a particular floor buckled, the overhead structure could no longer be supported and complete collapse of the structures occurred.
Bažant later published an expanded version of this analysis.
Other analyses were conducted by MIT civil engineers Oral Buyukozturk and Franz-Josef Ulm, who also described a collapse mechanism on September 21, 2001.
They later contributed to an MIT collection of papers on the WTC collapses edited by Eduardo Kausel called The Towers Lost and Beyond.
Immediately following the collapses, there was some confusion about who had the authority to carry out an official investigation.
While there are clear procedures for the investigation of aircraft accidents, no agency had been appointed in advance to investigate building collapses.Snell, Jack.
"The Proposed National Construction Safety Team Act."
NIST Building and Fire Research Laboratory.
2002.
NIST.gov  A team was quickly assembled by the Structural Engineers Institute of the American Society of Civil Engineers, led by W. Gene Corley, Senior Vice President of CTLGroup.
It also involved the American Institute of Steel Construction, the American Concrete Institute, the National Fire Protection Association, and the Society of Fire Protection Engineers.
ASCE ultimately invited FEMA to join the investigation, which was completed under the auspices of the latter.
The investigation was criticized by some engineers and lawmakers in the U.S.
It had little funding, no authority to demand evidence, and limited access to the WTC site.
One major point of contention at the time was that the cleanup of the WTC site was resulting in the destruction of the majority of the buildings' steel components.
Indeed, when NIST published its final report, it noted "the scarcity of physical evidence" that 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, although 95% of structural beams and plates and 50% of the reinforcement bars were recovered.Sylvie Boulanger and Sylvain Boulanger "Steel and sustainability 2: Recovery strategies" Canadian Institute of Construction.
March 17, 2004.
CISC-ICCA.ca
FEMA published its report in May 2002.
While NIST had already announced its intention to investigate the collapses in August of the same year, by September 11, 2002 (a year after the disaster), there was growing public pressure for a more thorough investigation.
Congress passed the National Construction Safety Team bill in October 2002, giving NIST the authority to conduct an investigation of the World Trade Center collapses.
FEMA building performance study
FEMA suggested that fires in conjunction with damage resulting from the aircraft impacts were the key to the collapse of the towers.
Thomas Eagar, Professor of Materials Engineering and Engineering Systems at MIT, described the fires as "the most misunderstood part of the WTC collapse".
This is because the fires were originally said to have "melted" the floors and columns.
: "The fire is the most misunderstood part of the WTC collapse.
Even today, the media report (and many scientists believe) that the steel melted."
Jet fuel is essentially kerosene and would have served mainly to ignite very large, but not unusually hot, hydrocarbon fires.
: "The maximum flame temperature increase for burning hydrocarbons (jet fuel) in air is, thus, about 1,000 °C—hardly sufficient to melt steel at 1,500 °C."
As Eagar said, "The temperature of the fire at the WTC was not unusual, and it was most definitely not capable of melting steel."
: "The temperature of the fire at the WTC was not unusual, and it was most definitely not capable of melting steel."
This led Eagar, FEMA and others to focus on what appeared to be the weakest point of the structures, namely, the points at which the floors were attached to the building frame.
: "It survived the loss of several exterior columns due to aircraft impact, but the ensuing fire led to other steel failures.
Many structural engineers believe that the weak points—the limiting factors on design allowables—were the angle clips that held the floor joists between the columns on the perimeter wall and the core structure."
NIST report
After the FEMA report had been published, and following pressure from technical experts, industry leaders and families of victims, the Commerce Department's National Institute of Standards and Technology conducted a three-year, $16 million investigation into the structural failure and progressive collapse of several WTC complex structures.
The study included in-house technical expertise, along with assistance from several outside private institutions, including the Structural Engineering Institute of the American Society of Civil Engineers, Society of Fire Protection Engineers, National Fire Protection Association, American Institute of Steel Construction, Simpson Gumpertz & Heger Inc., Council on Tall Buildings and Urban Habitat, and the Structural Engineers Association of New York.
The scope of the NIST investigation was focused on identifying "the sequence of events" that triggered the collapse, and did not include detailed analysis of the collapse mechanism itself (after the point at which events made the collapse inevitable).Bazant (2007), p. 2NIST final report (2005).
NCSTAR 1, p.
xxxvii. In line with the concerns of most engineers, NIST focused on the airplane impacts and the spread and effects of the fires, modeling these using the software program Fire Dynamics Simulator.
NIST developed several highly detailed structural models for specific sub-systems such as the floor trusses as well as a global model of the towers as a whole which is less detailed.
These models are static or quasi-static, including deformation but not the motion of structural elements after rupture as would dynamic models.
So, the NIST models are useful for determining how the collapse was triggered, but do not shed light on events after that point.
James Quintiere, professor of fire protection engineering at the University of Maryland, called the spoliation of the steel "a gross error" that NIST should have openly criticized.
He also noted that the report lacked a timeline and physical evidence to support its conclusions.
Some engineers have suggested that understanding of the collapse mechanism could be improved by developing an animated sequence of the collapses based on a global dynamic model, and comparing it with the video evidence of the actual collapses.
The NIST report for WTC 7 concluded that no blast sounds were heard on audio and video footage, or were reported by witnesses.
7 World Trade Center
thumb|right|300px|Plan view of collapse progression, with structural failure initiating on lower floors, on the east side of the building and vertical progression up to the east mechanical penthouse
In May 2002, FEMA issued a report on the collapse based on a preliminary investigation conducted jointly with the Structural Engineering Institute of the American Society of Civil Engineers under leadership of Dr. W. Gene Corley, P.E. FEMA made preliminary findings that the collapse was not primarily caused by actual impact damage from the collapse of 1 WTC and 2 WTC but by fires on multiple stories ignited by debris from the other two towers that continued unabated due to lack of water for sprinklers or manual firefighting.
The report did not reach conclusions about the cause of the collapse and called for further investigation.
In response to FEMA's concerns, the National Institute of Standards and Technology (NIST) was authorized to lead an investigation into the structural failure and collapse of the World Trade Center twin towers and 7 World Trade Center.
The investigation, led by Dr S. Shyam Sunder, drew not only upon in-house technical expertise, but also upon the knowledge of several outside private institutions, including the Structural Engineering Institute of the American Society of Civil Engineers (SEI/ASCE), the Society of Fire Protection Engineers (SFPE), the National Fire Protection Association (NFPA), the American Institute of Steel Construction (AISC), the Council on Tall Buildings and Urban Habitat (CTBUH), and the Structural Engineers Association of New York (SEAoNY).
The bulk of the investigation of 7 World Trade Center was delayed until after reports were completed on the collapse of the World Trade Center twin towers.
In the meantime, NIST provided a preliminary report about 7 World Trade Center in June 2004, and thereafter released occasional updates on the investigation.
According to NIST, the investigation of 7 World Trade Center was delayed for a number of reasons, including that NIST staff who had been working on 7 World Trade Center were assigned full-time from June 2004 to September 2005 to work on the investigation of the collapse of the twin towers.
In June 2007, Shyam Sunder explained, "We are proceeding as quickly as possible while rigorously testing and evaluating a wide range of scenarios to reach the most definitive conclusion possible.
The 7 WTC investigation is in some respects just as challenging, if not more so, than the study of the towers.
However, the current study does benefit greatly from the significant technological advances achieved and lessons learned from our work on the towers."
thumb|left|200px|7 World Trade Center on fire after the collapse of the Twin Towers on September 11, 2001 In November 2008, NIST released its final report on the causes of the collapse of 7 World Trade Center.
This followed their August 21, 2008 draft report which included a period for public comments.
In its investigation, NIST utilized ANSYS to model events leading up to collapse initiation and LS-DYNA models to simulate the global response to the initiating events.
NIST determined that diesel fuel did not play an important role, nor did the structural damage from the collapse of the twin towers, nor did the transfer elements (trusses, girders, and cantilever overhangs), but the lack of water to fight the fire was an important factor.
The fires burned out of control during the afternoon, causing floor beams near Column 79 to expand and push a key girder off its seat, triggering the floors to fail around column 79 on Floors 8 to 14.
With a loss of lateral support across nine floors, Column 79 soon buckled – pulling the East penthouse and nearby columns down with it.
With the buckling of these critical columns, the collapse then progressed east-to-west across the core, ultimately overloading the perimeter support, which buckled between Floors 7 and 17, causing the entire building above to fall downward as a single unit.
From collapse timing measurements taken from a video of the north face of the building, NIST observed that the building's exterior facade fell at free fall acceleration through a distance of approximately 8 stories (32 meters, or 105 feet), noting "the collapse time was approximately 40 percent longer than that of free fall for the first 18 stories of descent."
The fires, fueled by office contents, along with the lack of water, were the key reasons for the collapse.
The collapse of the old 7 World Trade Center is remarkable because it was the first known instance of a tall building collapsing primarily as a result of uncontrolled fires.
Based on its investigation, NIST reiterated several recommendations it had made in its earlier report on the collapse of the twin towers, and urged immediate action on a further recommendation: that fire resistance should be evaluated under the assumption that sprinklers are unavailable; and that the effects of thermal expansion on floor support systems be considered.
Recognizing that current building codes are drawn to prevent loss of life rather than building collapse, the main point of NIST's recommendations is that buildings should not collapse from fire even if sprinklers are unavailable.
Other investigations
In 2003, Asif Usmani, Professor of Structural Engineering at University of Edinburgh, published a paper with two colleagues.
They provisionally concluded the fires alone, without any damage from the airplanes, could have been enough to bring down the buildings.
In their view, the towers were uniquely vulnerable to the effects of large fires on several floors at the same time.
When the NIST report was published, Barbara Lane, with the UK engineering firm Arup, criticized its conclusion that the loss of fire proofing was a necessary factor in causing the collapses; "We have carried out computer simulations which show that the towers would have collapsed after a major fire on three floors at once, even with fireproofing in place and without any damage from plane impact."
Jose L. Torero, formerly of the BRE Centre for Fire Safety Engineering at the University of Edinburgh, pursued further research into the potentially catastrophic effects of fire on real-scale buildings.
Aftermath
Cleanup
The cleanup was a massive operation coordinated by the City of New York Department of Design and Construction.
On September 22, a preliminary cleanup plan was delivered by Controlled Demolition, Inc. (CDI) of Phoenix, Maryland.Post, Nadine M. and Debra K. Rubin.
"Debris Mountain Starts to Shrink."
Engineering News Record, 10/1/01.
Construction.com  Costing hundreds of millions of dollars, it involved round-the-clock operations with many contractors and subcontractors.
By early November, with a third of the debris removed, officials began to reduce the number of firefighters and police officers assigned to recovering the remains of victims, in order to prioritize the removal of debris.
This caused confrontations with firefighters.
Despite efforts to extinguish the blaze, the large pile of debris burned for three months, until the majority of the rubble was finally removed from the site.
In 2007, the demolition of the surrounding damaged buildings was still ongoing as new construction proceeded on the World Trade Center's replacement, One World Trade Center.
Health effects
The collapse of the World Trade Center produced enormous clouds of dust that covered Manhattan for days.
On September 18, 2001, the United States Environmental Protection Agency (EPA)  assured the public that the air in Manhattan was "safe to breathe".
In 2003 the EPA's inspector general found that the agency did not at that time have sufficient data to make such a statement.
Dust from the collapse seriously reduced air quality and is likely the cause of many respiratory illnesses in lower Manhattan.
Asbestosis is such an illness, and asbestos would have been present in the dust.United States Environmental Protection Agency Office of Inspector General.
"EPA's Response to the World Trade Center Collapse."
Report No. 2003-P-00012.
August 21, 2003.
EPA.gov  Significant long term medical and psychological effects have been found among first responders including elevated levels of asthma, sinusitis, gastroesophageal reflux disease and post-traumatic stress disorder.
Health effects also extended to residents, students, and office workers of Lower Manhattan and nearby Chinatown.
Several deaths have been linked to the toxic dust, and the victims' names will be included in the World Trade Center memorial.
More than 18,000 people have suffered from illnesses from the dust.
References
Explanatory notes
Citations
Bibliography
External links
NIST and the World Trade Center The National Institute of Standards and Technology's page on the collapse of the WTC.
Contains most recent developments in investigations and FAQs.
Video: The Collapse of World Trade Center 7: Why the Building Fell (NIST)
World Trade Center – Some Engineering Aspects Early suggestion by University of Sydney engineering instructor about how the towers might have collapsed.
Bill Biggart's Final Exposures contains a photo of the WTC Marriott severely damaged by the collapse of 2 WTC immediately before the collapse of 1 WTC in which the photographer was killed.
New light on 7 WTC collapse
World Trade Center Studies Abdolhassan Astaneh-Asl (Principal Investigator)
