Inside and near the towers, 2,753 people were killed, including all 157 passengers and crew aboard the two airplanes. The collapse of the twin towers also caused extensive damage to the rest of the complex and nearby buildings. At 5:20 p.m. 7 World Trade Center collapsed as well.
The Federal Emergency Management Agency (FEMA) completed its performance study of the buildings in May 2002. It declared the WTC design sound and attributed the collapses wholly to extraordinary factors beyond the control of the builders. While calling for further study, FEMA suggested that the collapses were probably initiated by weakening of the floor joists by the fires that resulted from the aircraft impacts. According to FEMA's report – and subsequently contradicted by NIST's findings – the floors detached from the main structure of the building and fell onto each other, initiating a progressive "pancake" collapse.
FEMA's proposed explanation was rejected by a later, more detailed investigation by the National Institute of Standards and Technology (NIST), which was completed in September 2005. Like FEMA, NIST vindicated the design of the WTC, noting that the severity of the attacks and the magnitude of the destruction was beyond anything experienced in U.S. cities in the past. NIST also emphasized the role of the fires, but it did not attribute the collapses to failing floor joists. Instead, NIST found that sagging floors pulled inward on the perimeter columns: "This led to the inward bowing of the perimeter columns and failure of the south face of WTC 1 and the east face of WTC 2, initiating the collapse of each of the towers."
The cleanup of the site involved round-the-clock operations, many contractors and subcontractors, and cost hundreds of millions of dollars. The demolition of the surrounding damaged buildings continued even as new construction proceeded on the World Trade Center's replacement, the Freedom Tower. Of the destroyed buildings, only 7 World Trade Center has been replaced .
The floors consisted of 4 inch (10 cm) thick lightweight concrete slabs laid on a fluted steel deck. A grid of lightweight bridging trusses and main trusses supported the floors. The trusses had a span of 60 feet (18.2 m) in the long-span areas and 35 feet (11 m) in the short span area. The trusses connected to the perimeter at alternate columns, and were therefore on 6 foot 8 inch (2.03 m) centers. The top chords of the trusses were bolted to seats welded to the spandrels on the exterior side and a channel welded to the core columns on the interior 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 trusses supported a 4 inch thick (10 cm) lightweight concrete floor slab, with shear connections for composite action.
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 some load redistribution between the perimeter and core columns and supported the transmission tower. It was found to play a key role in the collapse sequence.
The designers had considered the effects of the impact of a passenger jet, and believed the structures would remain standing in such an event. But NIST found reason to believe that they lacked the ability to properly model the effect of such impacts on the structures, especially the effects of the fires. (For more, see history.)
Hijackers flew two Boeing 767 jet airliners, American Airlines Flight 11 (a 767-200ER) and United Airlines Flight 175 (a 767-200) into the towers. One WTC was hit at 8:46 a.m. by Flight 11 between the 99th and 93rd floors. Two WTC was hit at 9:03 a.m. by Flight 175 between the 85th and 77th floor.
A Boeing 767-200 is 48.5 m (160 ft) long and has a wingspan of 48 m (156 ft), with a capacity of up to 62.2 (767-200) or 91 (767-200ER) m³ of jet fuel (16,700 or 24,000 US gallons). The planes hit the towers at very high speeds. Flight 11 was traveling roughly 700 km/h (440 mph) when it crashed into the 1 WTC, the north tower; flight 175 hit 2 WTC, the south tower, at about 870 km/h (540 mph). In addition to severing a number of load-bearing columns, the resulting explosions in each tower ignited 38 m³ (10,000 gallons) of jet fuel and immediately spread the fire to several different floors while consuming paper, furniture, carpeting, computers, books, walls, framing, and other items in all the affected floors. The force of the explosion from the initial impact in 1 WTC traveled through at least one express elevator shaft all the way down to the lobby floor, blowing out all of the windows and leaving a number of people injured.
In Building One (the North Tower), jet fuel ran down at least two elevator shafts to the basement, and two or more elevators plummeted to the lower levels. Fire continued to burn in the shafts, which may have helped weaken the core.
NYPD helicopters report deteriorating conditions of the North Tower.
With dispatchers overwhelmed, there was minimal communication with the NYPD, and the FDNY were experiencing problems with faulty radios. Firefighters inside the towers did not hear the evacuation order from their supervisors on the scene. 343 firefighters died in the Twin Towers, as a result of the collapse of the buildings.
At 9:59 a.m., the south tower collapsed, 56 minutes after being struck. The north tower, struck at 8:46 a.m., collapsed at 10:28 a.m., 102 minutes after impact. The collapses produced enormous clouds of dust that covered Manhattan for days. In both cases, the commonly accepted process is that the damaged portion of the buildings failed, which allowed the section above the airplane impacts to fall onto the remaining structure below. Both buildings 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. As the collapse progressed, dust and debris could be seen shooting out of the windows several floors below the advancing destruction.
The impacts also dislodged some of the fireproofing from the steel, increasing its exposure to the heat of the fires. In the 102 minutes before the collapse of 1 WTC, the fires reached temperatures that, although well below the melting point, were high enough to weaken the core columns so that they underwent plastic deformation and creep from the weight of higher floors. The NIST report provides a useful model of the situation.
At this point, the core of WTC 1 could be imagined to be in three sections. There was a bottom section below the impact floors that could be thought of as a strong, rigid box, structurally undamaged and at almost normal temperature. There was a top section above the impact and fire floors that was also a heavy, rigid box. In the middle was the third section, partially damaged by the aircraft and weakened by heat from the fires. The core of the top section tried to move downward, but was held up by the hat truss. The hat truss, in turn redistributed the load to the perimeter columns. (p. 29)
The situation was similar in 2 WTC. In both towers, perimeter columns and floors were also weakened by the heat of the fires, causing the floors to sag and exerting an inward force on exterior walls of the building.
At 9:59 a.m., 56 minutes after impact, the sagging floors finally caused the eastern face of 2 WTC to buckle, transferring its loads back to the failing core through the hat truss and initiating the collapse; the section above the impact area then tilted in the direction of the failed wall. At 10:28 a.m., 102 minutes after the impact, the south wall of 1 WTC buckled, with similar consequences. After collapse ensued, the total collapse of the towers was inevitable due to the enormous weight of the towers above the impact areas.
A combination of three factors allowed the north tower to remain standing longer: the region of impact was higher (so the gravity load on the most damaged area was lighter), the speed of the plane was lower (so there was less impact damage), and the affected floors had received partially upgraded fire proofing.
Analysis of video footage capturing the initial collapse and analysis of seismic data from Palisades, New York shows that the first fragments of the outer walls of the collapsed north tower struck the ground 9 seconds after the collapse started, and parts of the south tower after 11 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. These times are approximate because dust obscured the view.
The NIST report analyzes the failure mechanism in detail. An early analysis explains that the kinetic energy of the upper portion of the building falling onto the story below exceeded by an order of magnitude the amount of energy that the lower story could absorb, crushing it and adding to the kinetic energy. This scenario repeated with each successive story, crushing the entire tower at near free-fall speed.
The WTC complex comprised seven buildings, three of which completely collapsed on the day of the attacks. At 5:20 p.m., 7 World Trade Center, a 47-story steel-frame skyscraper across the street from the rest of the complex, became the third building to collapse. Unlike the Twin Towers, the collapse of 7 WTC had been anticipated for several hours and the building had been evacuated. A transit (or theodolite) was used to measure the extent of a visible bulge.
FEMA's provisional study was inconclusive and the collapse of 7 WTC was not included in the final report of the NIST investigation into the collapse of the World Trade Center when it was published in September 2005. NIST released a progress report in June 2004 outlining its working hypothesis which was that a local failure in a critical column occurred, caused by damage from either fire or falling debris from the collapses of the two towers, and progressed first vertically and then horizontally to result in "a disproportionate collapse of the entire structure".
On August 21, 2008, NIST released a draft of its final report on the collapse of 7 World Trade Center. In the report, NIST explains that fire was the main reason for the collapse, along with lack of water to fight the fire. The fire continued to burn throughout the afternoon on the lower floors, and eventually at 5:20 pm., a critical column on the 13th floor buckled and triggered a progressive collapse of the entire structure.
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.
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. This provided the authority for the NIST investigation, which published its results in September 2005.
One question that investigators tried to answer was whether the buildings had been designed to survive events like those on 9/11. The WTC towers were, of course, designed to survive foreseeable fires and, while "no building code in the United States has specific design requirements for impact of aircraft", the scenario was considered by the structural engineers. NIST, however, found it difficult to document how the buildings were designed to anticipate aircraft impact.
Two accounts of the designers' anticipation of aircraft impact have been suggested in the course of the investigations. FEMA described the modeled aircraft as weighing 263,000 lb (119 metric tons) with a flight speed of 180 mph (290 km/h), both of which were exceeded in the actual impacts of 9/11. This description follows Leslie Robertson's remarks in the days immediately following the collapses. Robertson, who had participated in the structural design of the towers, recalled he "addressed the question of an airplane collision, if only to satisfy his engineer's curiosity". NIST was unable to document this reported study; Robertson could not find a copy and it was not an official study done by the WTC engineers. Journalists investigating the issue were also unable to find anyone to verify Robertson's recollection.
NIST found a three page white paper from 1964 summarizing an extensive WTC structural study that included the effects of a Boeing 707 weighing 336,000 lb (152 metric tons) and carrying 23,000 US gallons (87 m³) of fuel impacting the 80th floor of the buildings at . While this suggests planes that were faster than those that crashed into the towers on 9/11, the study found that the buildings would not collapse on such a scenario. This study is more in line with remarks made by John Skilling's after the 1993 bombing. When asked about this study by The New York Times Magazine, Robertson insisted that he had no knowledge of this study and that it had not actually been carried out.
It is unclear whether the effect of jet fuel and aircraft contents was a consideration in the original building design. "One view," writes NIST, "suggests that an analysis was done indicating the biggest problem would be the fact that all the fuel would dump into the building and there would be a horrendous fire. Another view suggests that the fuel load, and the fire damage that it would cause, may not have been considered. Without the original calculations, which were used to render its conclusions, NIST said, any further comment would amount to speculation.
As in the theory which is currently accepted, the fires were taken to be the key to the collapses. Thomas Eagar, an MIT materials professor, had 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. As Eagar said, "The temperature of the fire at the WTC was not unusual, and it was most definitely not capable of melting steel." Jet fuel is essentially kerosene and would have served mainly to ignite very large, but not unusually hot, hydrocarbon fires. 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. Once these connections failed, the pancake collapse could initiate. The NIST report, however, would ultimately vindicate the floor connections; indeed, the collapse mechanism depends on the strength of these connections as the floors pulled the outer walls in.
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, $24 million investigation into the structural failure and progressive collapse of several WTC complex structures. The study included in-house technical expertise and drew upon the knowledge of several outside private institutions for aid to include:
The scope of the NIST investigation was limited to "the sequence of events from the instant of aircraft impact to the initiation of collapse for each tower" and "includes little analysis of the structural behavior of the tower after the conditions for collapse initiation were reached and collapse became inevitable. In line with the concerns of most engineers, NIST focused on the airplane impacts and the spread and effects of the fires, modeling these at a very high level of detail. 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.
Some engineers have suggested that our 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. In October 2005, the New Civil Engineer reported criticism of NIST's computer modelling. Colin Bailey at the University of Manchester and Rober Plank at the University of Sheffield called on NIST to produce computer visualizations of the collapses in order to correlate the collapse models with observed events.
We calculated in advance the number of casualties from the enemy, who would be killed based on the position of the tower. We calculated that the floors that would be hit would be three or four floors. I was the most optimistic of them all. (...Inaudible...) Due to my experience in this field, I was thinking that the fire from the gas in the plane would melt the iron structure of the building and collapse the area where the plane hit and all the floors above it only. This is all that we had hoped for.
The entire WTC complex was destroyed on September 11, 2001, and many of the surrounding buildings were also either damaged or destroyed as the towers fell. 5 WTC suffered a large fire and a partial collapse of its steel structure.
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, Verizon, and World Financial Center 3 suffered 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, is currently being deconstructed because of water, mold, and other severe damage caused by the neighboring towers' collapse.
The colossal pile of debris left on the site burned for three months, resisting attempts to extinguish the blaze until the majority of the rubble was finally removed from the site. The cleanup was a massive operation coordinated by the City of New York Department of Design and Construction.
On September 18, 2001, the Environmental Protection Agency issued a statement assuring the public that the air in Manhattan was "safe to breathe". In a report published in 2003, however, the EPA's inspector general found that the agency did not at that time have sufficient data to make such a statement. Also, it found that the White House had influenced the EPA to remove cautionary statements and include assuring ones, in part motivated by the desire to reopen Wall Street. In fact, the collapse of the World Trade Center resulted in serious reductions in air quality and is likely the cause of many respiratory illnesses among first responders, residents, and office workers in lower Manhattan.