New York City's World Trade Center was destroyed during the terrorist attacks of September 11, 2001, after being struck by two hijacked commercial airliners. The North Tower was hit at 8:46 am and collapsed at 10:28 am. The South Tower was hit at 9:03 am and collapsed at 9:59 am. The resulting debris severely damaged or destroyed more than a dozen other adjacent and nearby structures, ultimately leading to the collapse of Seven World Trade Center at 5:21 pm.
As a result of the attacks to the towers, a total of 2,763 people died including 2,192 civilians, 343 firefighters, and 71 law enforcement officers as well as all the passengers and crew on the airplanes, including 147 civilians and the 10 hijackers.
The scale of the destruction initially puzzled engineers, who had expected the buildings to survive airplane impacts. In September 2005, the National Institute of Standards and Technology 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, 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 Twin Towers' replacement, One World Trade Center, which was opened in November 2014. As of July 2019, five new buildings had been erected on the site; the last one, Two World Trade Center, is scheduled for completion in 2022.
When they were completed in 1973, the Twin Towers were the tallest buildings in the world. Though they used a novel design, they were thought to be among the strongest structures ever built relative to their weight. The design engineers had even considered the possibility of aircraft impacts. Fire had never before been known to cause the collapse of a skyscaper, and much effort was subsequently devoted to understanding what happened.
The towers were designed as "framed tube" structures, which provided tenants with open floor plans uninterrupted by columns or walls. 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, and there were 47 heavier columns in the core. 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.
The floors consisted of 4-inch-thick (10 cm) 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 60 feet (18 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 6.8-foot (2.1 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 interior box 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 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.
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, and there was a near-hit at the Empire State Building. Leslie Robertson, one of the chief engineers working on the design of the World Trade Center, has since said he personally considered the scenario of the impact of a Boeing 707 or another jet airliner, which might be lost in the fog and flying at relatively low speeds while seeking to land at either JFK Airport or Newark Airport. However, in an interview with the BBC, 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."
During their investigation into the collapse, the National Institute of Standards and Technology (NIST) found a three-page white paper that mentioned aircraft-impact of a Boeing 707 at 600 miles per hour (970 km/h). The original documentation of the study, which was part of the building's 1,200-page structural analysis, was lost when the records in the Port Authority of New York and New Jersey's offices were destroyed in the collapse of the 1 WTC; the copy was lost in 7 WTC. In 1993, John Skilling, lead structural engineer for the WTC, recalled doing the analysis, and 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.[note 1]
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. Fireproofing was incorporated in the original construction and more was added after a fire in 1975 that spread to six floors before being extinguished. After the 1993 bombing, inspections found fireproofing to be deficient. The Port Authority was in the process of replacing it, but replacement had been completed on only 18 floors in 1 WTC, including all the floors affected by the aircraft impact and fires, and on 13 floors in 2 WTC, although only three of these floors (77, 78, and 85) were directly affected by the aircraft impact.[note 2]
During the September 11th 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 767's, 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 440 miles per hour (710 km/h) into the north facade of the North Tower (WTC 1)) at 8:46 am, impacting between the 93rd and 99th floors. 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 540 miles per hour (870 km/h).
In addition to severing numerous load-bearing columns on the perimeter and inflicting other structural damage, the resulting fires in each tower ignited 10,000 US gallons (38,000 L) of jet fuel, which in turn ignited office combustibles. According to National Institute of Standards and Technology (NIST) estimates, Flight 11 was carrying 10,000 US gallons (38,000 L) of jet fuel when it hit the North Tower. 1,500 US gallons (5,700 L) were consumed in the initial impact when the aircraft hit and a similar amount was consumed in the fireball outside the building. Approximately 7,000 US gallons (26,000 L) burnt inside the office spaces igniting combustibles. Similarly, Flight 175 was carrying around 9,100 US gallons (34,000 L) when it hit the South Tower. Up to 1,500 US gallons (5,700 L) was instantly consumed in the initial fireball and up to 2,275 US gallons (8,610 L) was consumed in the fireball outside the building. More than 5,325 US gallons (20,160 L) 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. Jet 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.
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. It has been suggested that the fires might not have been as centrally positioned, nor as intense, had traditionally heavy high-rise construction been standing in the way of the aircraft. Debris and fuel would likely have remained mostly outside the buildings or concentrated in more peripheral areas away from the building cores, which would then not have become unique failure points. In this scenario, the towers might have stood far longer, perhaps indefinitely.
The fires were hot enough to weaken the columns and cause floors to sag, pulling perimeter columns inward and reducing their ability to support the mass of the building above. The final fires at the World Trade Center complex were extinguished on December 20, exactly 100 days after the attacks.
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 New York City Fire Department (FDNY); and 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.
Both of the Twin 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 from the 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. The result was that the walls peeled off and separated away from the buildings by a large distance (about 500 feet in some cases), hitting other 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.
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. In the case of 2 WTC, this caused the eastern face to buckle, transferring its loads back to the failing core through the hat truss and initiating the collapse. In the case of 1 WTC, the south wall later buckled in the same way, and with similar consequences.
The collapse of the World Trade Center has been called "the most infamous paradigm" of progressive collapse. Structural systems respond very differently to static and dynamic loads and, while the buildings were designed to support enormous weight under normal conditions, they provided little resistance to the moving mass of the section above the damaged floors. 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.
The 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. 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.
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.
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. 343 firefighters died in the Twin Towers, as a result of the collapse of the buildings. 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. After the collapse, light dust reached as far as the Empire State Building, located 2.93 miles (4.72 km) away.
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 face between Floor 44 and the roof; other possible structural damage includes a large vertical gash near the center of the south face 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.
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. 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. 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.
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.
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. 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.
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 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. Jet fuel is essentially kerosene and would have served mainly to ignite very large, but not unusually hot, hydrocarbon fires. As Eagar said, "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.
The large quantity of jet fuel carried by each aircraft ignited upon impact into each building. A significant portion of this fuel was consumed immediately in the ensuing fireballs. The remaining fuel is believed either to have flowed down through the buildings or to have burned off within a few minutes of the aircraft impact. The heat produced by this burning jet fuel does not by itself appear to have been sufficient to initiate the structural collapses. However, as the burning jet fuel spread across several floors of the buildings, it ignited much of the buildings’ contents, causing simultaneous fires across several floors of both buildings. The heat output from these fires is estimated to have been comparable to the power produced by a large commercial power generating station. Over a period of many minutes, this heat induced additional stresses into the damaged structural frames while simultaneously softening and weakening these frames. This additional loading and the resulting damage were sufficient to induce the collapse of both structures.
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). 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.
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."
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.
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.
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. 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, 1 World Trade Center.
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. Significant long term medical and psychological effects have been found among first responders including elevated levels of asthma, sinusitis, gastroesophageal reflux disease and posttraumatic 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.
al-Qaeda leader Osama bin Laden appeared in a new message aired on an Arabic TV station Friday night, for the first time claiming direct responsibility for the 2001 attacks against the United States.
The 23rd Street Fire was an incident that took place on October 17, 1966, in the New York City borough of Manhattan, when a group of firefighters from the New York City Fire Department responding to a fire at 7 East 22nd Street entered a building at 6 East 23rd Street as part of an effort to fight the fire. Twelve firefighters were killed after the floor collapsed, the largest loss of life in the department's history until the collapse of the World Trade Center in the September 11 terrorist attacks.Sally Regenhard
Sally Regenhard is an American activist who has become one of the leading voices for the families of the victims of the September 11 attacks. A former long-time resident of Co-op City in The Bronx in New York City who has degrees in behaviorial sciences and gerontology and has worked in the nursing home industry for over 20 years, Regenhard became an advocate for skyscraper safety after the death of her 28-year-old son, Christian, a probationary firefighter with the New York City Fire Department, who perished in the collapse of the World Trade Center on September 11, 2001.Shyam
Shyam is a name of Krishna and an Indian masculine given name and surname. Notable people with this name include:
ShyamShyam (actor), a Hindi film actor
Shyam (composer), an Indian music composer from Kerala
Shyam Benegal, film director
Shyam Satardekar, Indian politicianShyam Sunder (economist) (born 1944), accounting theorist and experimental economist
Shyam Sunder, protagonist in books written by Madhu Babu
B. Shyam Sunder (1908–1975), Indian intellectual and public figure
S. Shyam Sunder, lead investigator of the Collapse of the World Trade Center
Shyam Sundar Chakravarthy, Indian revolutionary
Shyam Sunder Goenka (born 1932), Indian entrepreneur
Shyam Sunder Kapoor (born 1938), Indian nuclear physicist
Shyam Sunder Patidar, Indian politician from Madhya Pradesh
Shyam Sunder Rao, Indian volleyball player
Shyam Sunder Sharma, Indian politician
Shyam Sunder Surolia, Indian freedom fighter
Shyam Sunder Vyas (1922–2009), Indian freedom fighter, journalist, and philanthropist
Karam Shyam, Indian politician
Shyam P, Indian ,Nightie of shru, PS, DW, Sponge, HM, EM, FM, CM, MC etc.SyamSyam Pushkaran, scriptwriter in the Malayalam cinema
Syam Sudhakar (born 1983), Malayalam-language poetThe Guys
The Guys is a play by Anne Nelson about the aftereffects of the collapse of the World Trade Center. In the play, Joan, an editor, helps Nick, an FDNY captain, prepare the eulogies for an unprecedented number of firefighters who died under his command that day. The play debuted off-Broadway at The Flea Theater on December 4, 2001, directed by Jim Simpson and starring Sigourney Weaver and Bill Murray.
Since 2001, The Guys has been presented in 48 US states and in the Czech Republic, Argentina, Japan, Italy and Poland. Tim Robbins and Susan Sarandon presented it at the Edinburgh Festival. It enjoyed a commemorative rerun at the Flea Theater in 2006 on the 5th anniversary of 9/11. In parts of the theatrical run the two roles were played by Anthony LaPaglia and Sigourney Weaver. They also star in a 2002 film adaptation, for which Weaver was nominated for a Golden Satellite Award for best performance by an actress. The play was published by Dramatists Play Service and Random House in 2002. The audible.com recorded version, which featured Bill Irwin and Swoosie Kurtz, won an Audie Award for best recorded play of the year.The NIST World Trade Center Disaster Investigation
The National Construction Safety Team Act (NCST Act), signed into law on October 1, 2002 by President George W. Bush, mandated the National Institute of Standards and Technology (NIST) to establish the likely technical cause or causes of the three building failures that occurred on September 11, 2001 at the World Trade Center as a result of a terrorist attack. NIST issued its final report on the collapse of the World Trade Center Twin Towers in September 2005. It issued its final report on 7 World Trade Center in November 2008.
NIST concluded that the collapse of each tower resulted from the combined effects of airplane impact damage, widespread fireproofing dislodgment, and the fires that ensued. The sequence of failures that NIST concluded initiated the collapse of both towers involved the heat-induced sagging of floor trusses pulling some of the exterior columns on one side of each tower inward until they buckled, after which instability rapidly spread and the upper sections then fell onto the floors below. World Trade Center Building 7 (7WTC), which was never directly hit by an airplane, collapsed as a result of thermal expansion of steel beams and girders that were heated by uncontrolled fires caused by the collapse of the North Tower and failure of the fire-resistive material.The World Trade Center Tapestry
The World Trade Center Tapestry was a large tapestry by Joan Miró and Josep Royo. It was displayed in the lobby of 2 World Trade Center (the South Tower) in New York City from 1974 until it was destroyed in 2001 by the collapse of the World Trade Center.
Saul Wenegrat, former director of the art program for the Port Authority of New York, had suggested to Miró that he could make a tapestry for the World Trade Center, but the artist declined as he would only make the work with his own hands but had no experience of making a tapestry. However, after his daughter recovered from an accident in Spain, Miró agreed to make a tapestry for the hospital that had treated her as a token of his gratitude. Having learned the technique from tapestry maker Josep Royo, Miró made several other tapestries with Royo, including one for the World Trade Center, Woman for the National Gallery of Art in Washington, DC, and one for the Fundació Joan Miró.
The work was an abstract design, with bright blocks of colour, red, green, blue and yellow, with black elements and a light brown background. Made of wool and hemp, it measured 20 × 35 feet (6.1 × 10.7 m) and weighed 4 tonnes. It was completed in 1973, and displayed at a retrospective at the Grand Palais in Paris before being installed in New York in 1974.William M. Feehan
William M. Feehan (September 29, 1929 – September 11, 2001) was a member of the Fire Department of New York who died during the collapse of the World Trade Center during the September 11 attacks.William M. Feehan (fireboat)
William M. Feehan is a fireboat built for and operated by the New York City Fire Department (FDNY). Her namesake, William M. Feehan, was the oldest and most senior FDNY firefighter to perish in the line of duty on September 11, 2001. Her nameplate is carved from a steel plate salvaged from the collapse of the World Trade Center. The vessel's $4.7 million cost was largely covered by a FEMA Port Security Grant Program.The boat is designed to operate in shallow waters, including bay areas close to Laguardia and John F. Kennedy airports. She also has systems to protect those onboard from chemical, biological, radiological and nuclear hazards using a pressurized and filtered cabin.She was built in Kingston, Ontario, at MetalCraft Marine. Wellwishers gathered to welcome her as she transited the Oswego Canal, Erie Canal, and Hudson River.The fireboat entered service on November 20, 2015.World Trade Center controlled demolition conspiracy theories
World Trade Center controlled demolition theories contend that the collapse of the World Trade Center was not solely caused by the airliner crash damage that occurred as part of the September 11 attacks, and the resulting fire damage, but by explosives installed in the buildings in advance. Controlled demolition theories make up a major component of 9/11 conspiracy theories.
Early advocates such as physicist Steven E. Jones, architect Richard Gage, software engineer Jim Hoffman, theologian David Ray Griffin, and Dutch demolitions expert Danny Jowenko, proposed that the aircraft impacts and resulting fires could not have weakened the buildings sufficiently to initiate a catastrophic collapse, and that the buildings would not have collapsed completely, nor at the speeds that they did, without additional energy involved to weaken their structures.The National Institute of Standards and Technology (NIST) and the magazine Popular Mechanics examined and rejected these theories. Specialists in structural mechanics and structural engineering accept the model of a fire-induced, gravity-driven collapse of the World Trade Center buildings, an explanation that does not involve the use of explosives. NIST "found no corroborating evidence for alternative hypotheses suggesting that the WTC towers were brought down by controlled demolition using explosives planted prior to Sept. 11, 2001." Professors Zdeněk Bažant of Northwestern University, Thomas Eagar, of the Massachusetts Institute of Technology and James Quintiere of the University of Maryland, have also dismissed the controlled-demolition conspiracy theory.
In 2006, Jones suggested that thermite or super-thermite may have been used by government insiders with access to such materials and to the buildings themselves, to demolish the buildings. In April 2009, Jones, Dane Niels H. Harrit and seven other authors published a paper in The Open Chemical Physics Journal, causing the editor, Prof. Marie-Paule Pileni, to resign as she accused the publisher of printing it without her knowledge; this article was titled 'Active Thermitic Material Discovered in Dust from the 9/11 World Trade Center Catastrophe', and stated that they had found evidence of nano-thermite in samples of the dust that was produced during the collapse of the World Trade Center towers. NIST responded that there was no "clear chain of custody" to prove that the four samples of dust came from the WTC site. Jones invited NIST to conduct its own studies using its own known "chain of custody" dust, but NIST did not investigate.
World Trade Center