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Collapse of the World Trade Centre

2007 Schools Wikipedia Selection. Related subjects: Recent History

                                                    Sept. 11, 2001 attacks
                                                                  Timeline
                                                        Background history
                                                                  Planning
                                                        September 11, 2001
                                                         Rest of September
                                                                   October
                                                            Beyond October
                                                                   Victims
                                                                 Survivors
                                                        Foreign casualties
                                                        Hijacked airliners
                                               American Airlines Flight 11
                                                United Airlines Flight 175
                                               American Airlines Flight 77
                                                 United Airlines Flight 93
                                                      Sites of destruction
                                                        World Trade Centre
                                                              The Pentagon
                                                 Shanksville, Pennsylvania
                                                     Effects and aftermath
                                                   World political effects
                                                    World economic effects
                                                                Detentions
                                                          Airport security
                                                Closings and cancellations
                                                 Audiovisual entertainment
                                                              Local health
                                                                  Response
                                                       Government response
                                                Rescue and recovery effort
                                                      Financial assistance
                                                   Operation Yellow Ribbon
                                                    Memorials and services
                                                              Perpetrators
                                                            Responsibility
                                                                Organizers
                                                             Miscellaneous
                                                             Communication
                                                            Tower collapse
                                                         Slogans and terms
                                                       Conspiracy theories
                                                              Opportunists
                                                                 Inquiries
                                                U.S. Congressional Inquiry
                                                           9/11 Commission

   On September 11, 2001, the two main towers of the World Trade Centre
   complex were each hit by hijacked commercial aircraft as part of the
   September 11, 2001 attacks. 2 WTC collapsed to the ground at 9:59 am,
   less than an hour after being hit, and 1 WTC followed at 10:28 am,
   causing massive damage to the rest of the complex and nearby buildings.
   In all, 2,595 people inside and near the towers were killed, along with
   the 157 people who were aboard the flights.

   The Federal Emergency Management Agency (FEMA) issued a performance
   study of the buildings in May 2002, declaring the WTC design sound and
   attributing the collapses wholly to extraordinary factors beyond the
   control of the builders. In its September 2005 report, the National
   Institute of Standards and Technology (NIST) concurred with this view,
   noting that the severity of the attacks and the magnitude of the
   destruction was beyond anything experienced in US cities in the past.
   It did add, however, that the towers' stairwell design lacked adequate
   reinforcement.

   7 WTC collapsed at 5:20 pm, with no casualties. No conclusive reason
   for the collapse has ever been established, however NIST's working
   hypothesis is that the collapse was the result of structural damage
   sustained during the collapse of Towers 1 and 2, combined with
   widespread fires in the building.

Construction of 1 and 2 WTC

Structural details

   Construction of the towers began in 1968 and was completed in 1970 (1
   WTC) and 1972 (2 WTC). 1 and 2 WTC were nearly identical structures.
   Both had 110 stories, although 1 WTC was 1,368 feet (417.0 meters) tall
   and 2 WTC was 1,362 feet (415.1 m) tall. 1 WTC also supported a 360
   foot tall (110 m) antenna. The core in 1 WTC was oriented with the long
   axis east to west, while that of 2 WTC was oriented north to south.
   Schematic of 1 WTC with assumed impact damage (the damage to the
   perimeter columns was observed, the damage to the core columns is
   estimated). Note narrow central core into which all internal columns
   are bunched. Adapted from NIST report "Baseline Structural Performance
   and Aircraft Impact Damage Analysis", October 19, 2004
   Enlarge
   Schematic of 1 WTC with assumed impact damage (the damage to the
   perimeter columns was observed, the damage to the core columns is
   estimated). Note narrow central core into which all internal columns
   are bunched. Adapted from NIST report "Baseline Structural Performance
   and Aircraft Impact Damage Analysis", October 19, 2004

   The towers were designed as framed tube structures, with columns
   grouped around the perimeter and within the core. The perimeter columns
   supported virtually all lateral loads, such as wind loads, and shared
   the gravity loads with the core columns. It was said that Live load on
   those columns could have been increased more than 2,000% before failure
   occurred. All columns were founded on bedrock, which unlike Midtown
   Manhattan, where the bedrock is shallow, is at 65 feet below the
   surface.[ ]

   Above the seventh floor there were 59 perimeter columns along each face
   of the building. The perimeter columns had a square cross section, 14
   inches on a side (36 cm), and were constructed of welded steel plate.
   The thickness of the plates and grade of steel were varied over the
   height of the tower, ranging from 36 ksi to 100 ksi, with the steel
   strength and plate thickness decreasing with height. The columns were
   connected with deep spandrel plates, which were typically 52 inches
   (1.3 m) deep. The spandrel plates were located at each floor, and
   served to transmit shear flow between columns, thus allowing them to
   work together in resisting lateral loads.

   The perimeter was assembled from prefabricated modules consisting of
   three columns and three spandrel plates. The spandrel plates were
   welded to the columns at the fabrication shop. These modules extended
   for two full floors and half of two more floors. Adjacent modules were
   bolted together, with the splices occurring at mid-span of the columns
   and spandrels. The joints between modules were staggered vertically, so
   the column splices between adjacent modules were not at the same floor.

   The core of each tower was a rectangular area 87 by 135 feet (27 by 41
   m) and contained 47 steel columns running from the bedrock to the top
   of the tower. The columns tapered with height, and consisted of welded
   box-sections at lower floors and rolled wide-flange sections at upper
   floors. All of the elevators and stairwells were located in the core.
   Impact locations for 1 and 2 WTC
   Enlarge
   Impact locations for 1 and 2 WTC

   The large, column-free space between the perimeter and core was bridged
   by pre-fabricated floor trusses. The trusses had a span of 60 feet
   (18.2 m) in the long-span areas and 35 feet (11.0 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.
   Approximately 10,000 of the trusses were also connected to the
   perimeter columns at their bottom chord through viscoelastic dampers to
   reduce wind-induced sway for occupant comfort. 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. The truss system 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.

Design innovations

   The outer skin of Tower Two remained standing and had to be demolished
   during the cleanup. WTC 3 is to the left.
   Enlarge
   The outer skin of Tower Two remained standing and had to be demolished
   during the cleanup. WTC 3 is to the left.

   The WTC towers were innovative in many ways, and were significantly
   different from earlier generations of skyscrapers such as the Empire
   State Building. One of the most innovative features is the tube
   structural system, which allowed large, open floor spaces uninterrupted
   by columns. This helped maximize the rentable space, and allowed
   tenants more flexibility in configuring their office space. The towers
   also made extensive use of pre-fabricated modules such as perimeter
   sections and floor trusses and extensively used light-weight materials.

   The use of express elevators also decreased the amount of space lost to
   elevator shafts. The express elevators took people to "sky lobbies" on
   the 44th and 78th floors, where they could board local elevators.

   Some of these innovative features have been cited as contributing
   factors to the collapse. These factors include:
    1. The large column-free area between the perimeter and core may have
       allowed the aircraft and the fuel they carried to penetrate deeper
       into the structure than they would have in a building with a more
       traditional grid-type column arrangement. This would spread the
       fire more rapidly through the building, and made damage to the
       stairwells and the fire proofing of the interior columns more
       likely.
    2. A structure with the columns grouped along the perimeter and within
       the interior core may be inherently less redundant and robust than
       one with the columns arranged in a grid pattern.
    3. The WTC used lightweight materials exclusively especially in the
       facade. Had the facade contained even minimal masonry elements
       and/or traditional heavy steel outermost column rows, it would have
       been less likely the aircraft would have cleanly penetrated to the
       core of each tower— a significant portion of debris and jet fuel
       would have remained outside, a much different scenario.
    4. The use of gypsum cladding instead of reinforced concrete to shield
       stairwells. Almost all skyscrapers, including those built since the
       WTC, shield stairwells in reinforced concrete. On September 11, it
       was the collapse of all stairways above the impact level that
       consigned all people above the impact zone in 1 WTC to death. 2 WTC
       had two of its three stairwells taken out above the impact area by
       the plane: some people above the impact zone survived by using the
       third stairwell. Computer models have shown that most of the
       stairwells in both towers would likely have remained usable until
       the general collapse had they been shielded in concrete.

   Fireproofing was added after a fire in 1975 that spread to six floors
   before being extinguished. Early tests conducted on steel beams from
   the WTC show they generally met or were stronger than design
   requirements, ruling them out as a contributing cause of the collapse
   of the towers.

Impacts of airliners

   Impact locations on 1 and 2 WTC
   Enlarge
   Impact locations on 1 and 2 WTC

   The towers were struck by hijacked Boeing 767 jet planes, American
   Airlines Flight 11 and United Airlines Flight 175. A typical Boeing 767
   is 180 feet (55 m) long and has a wingspan of 156 feet (48 m), with a
   capacity of up to 24,000 US gallons (91 m³) of jet fuel. The planes hit
   the towers at very high speeds. Flight 11 was traveling roughly 490 mph
   (790 km/h) when it crashed into the 1 WTC, the north tower; flight 175
   hit 2 WTC, the south tower, at about 590 mph (950 km/h). In addition to
   severing a significant number of load-bearing columns, the resulting
   explosions in each tower ignited 10,000 gallons (c. 40 m³) 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 buildings had in fact been designed to withstand the impact of the
   largest airliner of the day, the Boeing 707-320, in the event one was
   lost in fog while looking to land. The modeled aircraft weighed 263,000
   lb (119 metric tons) with a flight speed of 180 mph (290 km/h), as in
   approach and landing. As energy increases with the square of speed, the
   767s that hit the towers had a kinetic energy more than seven times
   greater than the modeled impact. Nonetheless, the impacts alone did not
   cause the towers to collapse.

The fires

   A New York City firefighter looks up at the remains of the World Trade
   Center
   Enlarge
   A New York City firefighter looks up at the remains of the World Trade
   Centre

   While the towers were designed to survive aircraft impact, and in fact
   did survive such impacts, little was known about the fires that might
   result from them.

   According to prevailing theories, it was ultimately the fires that
   brought the buildings down. The lightness and hollowness of the towers
   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 probably burned out in less than ten minutes, but
   the contents of the buildings burned over the next hour or hour and a
   half.

   It has been suggested that the fires may 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 and/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 strength of steel drops markedly with heat, losing half its
   strength at a temperature of 1,202°F (650°C). The heat from the fires
   quickly began to weaken the central steel columns, the longspan floor
   trusses, and cross trusses.

Collapse of the two towers

   The north tower, 1 WTC, was struck at 8:46 am and collapsed at 10:28
   am, standing for 102 minutes after impact. The south tower, 2 WTC, was
   struck at 9:03 am and collapsed about 56 minutes later, at 9:59 am.

   In both cases, the combined effects of the airplane impacts and
   subsequent fires caused the buildings to collapse. The impacts severed
   load bearing columns and dislodged fireproofing from the structural
   steel. Heat from the fires then gradually weakened the structures,
   causing the floors to sag and the perimeter columns to bow inwards. The
   towers collapsed abruptly when the perimeter walls finally buckled.
   Once the collapse was initiated, the enormous weight of the portion of
   the towers above the impact areas overwhelmed the load bearing capacity
   of the structures beneath them. As the floors above the impact point
   were relatively undamaged (save for fire), the upper portion fell as a
   single unit. The floors did not pancake. Instead, the upper portion
   stayed whole and smashed through the building like a sledgehammer. The
   buildings did not collapse: they were pulverized.

Physical features of the collapses

   In the case of 1 WTC, it took about 12 seconds to reduce the tower to
   rubble. NIST, however, also notes that elements of the cores of both
   buildings remained standing 15-25 seconds after the initiation of
   collapse (it was difficult to determine the total time of the collapses
   due to the obscuration of the scene by dust and debris). Both buildings
   collapsed symmetrically and more or less straight down, though there
   was some tilting of the tops of the towers. "The building section above
   came down essentially in free fall ... The falling mass of the building
   compressed the air ahead of it, much like the action of a piston,
   forcing material, such as smoke and debris, out the windows." The
   collapses also spread debris in a wide radius around the buildings,
   damaging other buildings nearby and producing enormous clouds of dust
   that covered Manhattan for days. These were composed mainly of
   pulverized gypsum cladding and dry wall, finely ground concrete from
   the towers' floors, glass particles, and lead (from the many computers
   in the buildings).

The collapse mechanism

   Ground Zero debris with markup showing building locations.
   Enlarge
   Ground Zero debris with markup showing building locations.

   Owing to differences in the initial impacts, the collapses of the two
   towers were found to differ in some respects, but in both cases, the
   same sequence of events apply. After the impacts had severed exterior
   columns and damaged core columns, the loads on these columns were
   redistributed. The hat trusses at the top of buildings played a
   significant role in this redistribution of the loads in the structure.

   The impacts also dislodged some of the fireproofing from the steel,
   increasing its exposure to the heat of the fires. This turned out to be
   crucial in bringing about the collapses. In the 56 and 102 minutes
   before the collapse of, respectively, 2 WTC and 1 WTC, the fires, and
   events associated with them, weakened the core, until it was unable to
   carry loads. The NIST report provides a useful image of the situation.


   Collapse of the World Trade Center

      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)


   Collapse of the World Trade Center

   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 am, 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. At 10:28 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 had
   their fire proofing partially upgraded.

Early attempts to understand the collapses

Unprecedented

   A woman stands in the gash in 1 WTC.
   Enlarge
   A woman stands in the gash in 1 WTC.

   On September 13, 2001, the cover of the New Civil Engineer in the UK
   consisted of a picture of 1 WTC during its collapse with a single word
   written across it: "unthinkable". "Just hours earlier, it had been
   genuinely inconceivable that structures of such magnitude could succumb
   to this fate."

   In subsequent efforts to make sense of the facts, a number of different
   collapse mechanisms were proposed, culminating in the NIST report of
   September 2005. Interviewed by the BBC in October 2001, the British
   architect Bob Halvorson correctly predicted that there would be "a
   debate about whether or not the World Trade Centre Towers should have
   collapsed in the way that they did." The autopsy would involve careful
   analysis of the plans of the WTC, its construction, eye witness
   testimony, video of the collapses, and examination of the wreckage.
   Emphasizing the difficulty of the task, Halvorson noted that the
   collapses were "well beyond realistic experience."

Total collapse

   The first scholarly work on the subject has also proved to be the most
   influential. On Thursday, September 13, Prof. Zdenek P. Bazant of the
   Department of Civil and Environmental Engineering at Northwestern
   University submitted a paper providing a 'simple analysis' of the total
   or 'global' collapse of the building. Bazant found that once the tower
   was brought to the point of local failure and the upper sections began
   to fall, the forces involved are an order of magnitude above the
   ability of the lower structure to withstand:


   Collapse of the World Trade Center

     the failure of the connections of the floor-carrying trusses to the
    columns is either accompanied or quickly followed by buckling of the
   core columns and overall buckling of the framed tube, with the buckles
    probably spanning the height of many floors (stage 5, at right), and
   the upper part possibly getting wedged inside an emptied lower part of
    the framed tube (stage 5, at left). The buckling is initially plastic
      but quickly leads to fracture in the plastic hinges. The part of
    building lying beneath is then impacted again by an even larger mass
   falling with a greater velocity, and the series of impacts and failures
                  then proceeds all the way down (stage 5).


   Collapse of the World Trade Center

   Other parts of Bazant's paper have since been superseded; notably his
   assumption that the point of structural failure was reached due "to
   sustained temperatures apparently exceeding 800°C". However, his
   analysis of global collapse has been highly influential in later
   engineering studies, notably for the NIST study, which, relying on
   Bazant's analysis did not study the reasons for global collapse at all,
   concentrating only on understanding the events which brought the
   structure to the point of collapse.

Pancaking floors

   Portions of the outer shell of the North Tower leans against the
   remains of WTC6 which suffered massive damage when the North Tower
   collapsed. The remains of WTC7 are at upper right
   Enlarge
   Portions of the outer shell of the North Tower leans against the
   remains of WTC6 which suffered massive damage when the North Tower
   collapsed. The remains of WTC7 are at upper right

   Bazant's work inspired the so-called "pancake" collapse theory, which
   was propounded by Thomas Eagar and popularized by PBS. On this view,
   when the connections between the floor trusses and the columns broke,
   the floors fell down, one on top of the other, quickly exceeding the
   load that any one floor was designed to carry.

   While the early efforts had exaggerated the temperatures of the fires,
   however, Eagar's theory would prove to underestimate the effect of the
   fires on the structural steel columns. In a paper published in December
   of 2001, he had focused on the joints between the floor assemblies and
   the perimeter columns, which, he argued, would be more vulnerable to
   the effects of the fires. On this assumption he proposed that "the
   joints on the most severely burned floors gave way, causing the
   perimeter wall columns to bow outward and the floors above them to
   fall".

   A similar line was taken by Tim Wilkinson, a civil engineer at the
   University of Sydney. In an "initial suggestion", written already on
   September 11, he outlined a range of possible effects related mainly to
   the effects of the fires.


   Collapse of the World Trade Center

       Eventually, the loss of strength and stiffness of the materials
   resulting from the fire, combined with the initial impact damage, would
    have caused a failure of the truss system supporting a floor, or the
       remaining perimeter columns, or even the internal core, or some
     combination. Failure of the flooring system would have subsequently
    allowed the perimeter columns to buckle outwards. Regardless of which
   of these possibilities actually occurred, it would have resulted in the
      complete collapse of at least one complete storey at the level of
                                   impact.


   Collapse of the World Trade Center

   Among a series of self-published accounts by structural engineers, HERA
   structural engineer Charles Clifton emphasized that a combination of
   factors led to the collapse. Clifton stated that the two towers
   collapsed in markedly different ways, which led some to suggest that
   there were two modes of failure. The north tower collapsed directly
   downwards, seemingly "pancaking" in on itself, while the south tower
   fell at an angle during which the top 20 or so stories of the building
   remained intact for the first few seconds of the collapse. Others, like
   Wilkinson, took these differences to be largely superficial. He argued
   that the "same mechanism of failure, the combination of impact and
   subsequent fire damage, is the likely cause of failure of both towers"
   While NIST did conclude that the collapses varied in their details,
   they proposed essentially the same "probable collapse sequence" for
   both towers and rejected "the pancake theory".

The role of the fires

   Portions of the outer shells of the South Tower at right and the North
   Tower at center left, as well as damage to all the other buildings at
   the WTC site are shown
   Enlarge
   Portions of the outer shells of the South Tower at right and the North
   Tower at centre left, as well as damage to all the other buildings at
   the WTC site are shown

   Many identified the fires as the key to the collapses. While NIST would
   eventually confirm this hypothesis, Thomas Eagar, an MIT materials
   professor, was nonetheless right to describe 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
   pointed out, "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. Nontheless, in 2003, three
   engineers at the University of Edinburgh, published a paper in which
   they provisionally concluded that the fires alone (without any damage
   from the airplanes) could have been enough to bring down the WTC
   buildings, while noting that 'A complete consensus on any detailed
   explanation of the definitive causes and mechanisms of the collapse of
   these structures is well nigh impossible given the enormous
   uncertainties in key data.' In this view, the towers were uniquely
   vulnerable to the effects of large fires on several floors at the same
   time.

   Even after the conclusions of the NIST study were public, at least one
   of these engineers, Jose Torero, is pursuing further research into the
   potentially catastrophic effects of fire on steel framed buildings.
   Moreover, when the NIST report was published, Barbara Lane, with the UK
   engineering firm Arup, criticized its conclusion that the structural
   damage resulting from the airplane impacts was a necessary factor in
   causing the collapses.

Core failure

   There had also been some visual evidence that the north tower's core
   had collapsed first: in videos, the large antenna on top of the core
   can be seen starting downward a fraction of a second earlier than the
   rest of the building. NIST, however, disputed this claim, stating "that
   observations from a single vantage point can be misleading and may
   result in incorrect interpretation. When records from east and west
   vantage points were viewed, it was apparent that the building section
   above the impact area tilted to the south as the building collapsed."

Other attempts

   Another early attempt, which included many of the elements already
   noted, came from MIT civil engineers Oral Buyukozturk and Franz-Josef
   Ulm on September 21, 2001.


   Collapse of the World Trade Center

     Some 60 tons or more of jet fuel could have easily caused sustained
      high temperatures of 1,500ºF and higher. Under these conditions,
   structural steel loses rigidity and strength. The resulting failure of
     the 2-3 floor system at the site of impact sent the 30 to 25 floors
       above free-falling onto the 80 to 85 floor structure below. The
   enormous energy released by this collapse was too large to be absorbed
     by the structure below. That impact may have ultimately caused the
   explosive buckling, floor after floor, of the WTC towers. Similar to a
   car crash in a wall, the towers crashed into the ground with an almost
                             free-fall velocity.


   Collapse of the World Trade Center

   They would later contribute to an MIT collection of papers on the WTC
   collapses edited by Eduardo Kausel called The Towers Lost and Beyond,
   published in May 2002.

   According to the NOVA documentary on PBS, "Why the Towers Fell", the
   core column structure of the south tower was momentarily intact after
   the floors had collapsed.

   Leslie E. Robertson, the lead structural engineer on the team that
   designed the towers, wrote that "The events of September 11 are not
   well understood by me . . . and perhaps cannot really be understood by
   anyone." As NIST would also conclude, however, Robertson conjectured
   that "the fires raging in the inner reaches of the buildings undermined
   their strength."

The NIST report

Design of the study

   The outer shell of the south tower (tower 2) of the WTC is still
   standing at right. The 22 story Marriott Hotel in the foreground was
   crushed when the adjacent tower collapsed.
   Enlarge
   The outer shell of the south tower (tower 2) of the WTC is still
   standing at right. The 22 story Marriott Hotel in the foreground was
   crushed when the adjacent tower collapsed.

   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:

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

Scope and limits

   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." 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. As stated in the report, it "includes little
   analysis of the structural behaviour of the tower after the conditions
   for collapse initiation were reached and collapse became inevitable."
   (p. xxxvii, fn2) 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.

   The publication of the NIST report did not end all disagreements about
   the collapses. The trade journal, the New Civil Engineer reported that
   some still believe 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." In August of 2006, NIST posted a
   webpage addressing frequently-asked questions.

7 World Trade Centre

   The WTC complex comprised seven buildings, three of which underwent
   total progressive collapse on September 11, 2001. At 5:20 pm, 7 WTC, a
   47-story steel-frame skyscraper across the street from the rest of the
   complex, became the third building to collapse.

   FEMA's provisional study was inconclusive and the collapse of 7 WTC was
   not included the final report of the NIST investigation into the
   collapse of the World Trade Centre when it was published in September
   of 2005. With the exception of a letter to the Journal of Metallurgy,
   which suggested that some of the structural steel had been exposed to
   temperatures sufficient to melt it, no studies of the collapse of 7 WTC
   have been published in scientific journals.

   The final report from NIST regarding the collapse of 7 WTC was due in
   July 2005, but its release date has been postponed. NIST released a
   progress report in June of 2004 outlining its working hypothesis. On
   this hypothesis a local failure in a critical column, caused by damage
   from either fire or falling debris from the collapses of the two
   towers, progressed first vertically and then horizontally to result in
   "a disproportionate collapse of the entire structure". In answer to the
   question of whether "a controlled demolition hypothesis is being
   considered to explain the collapse", NIST says that it "would like to
   determine the magnitude of hypothetical blast scenarios that could have
   led to the structural failure of one or more critical elements." At
   this point NIST is not committed to any one hypothesis, and new
   hypotheses may yet emerge.

Remarks by Osama bin Laden

   Although its authenticity was questioned, a videotape of Osama bin
   Laden that was verified by the Pentagon as indicating that Bin Laden,
   who is a trained Civil Engineer, had not believed that the buildings
   would collapse completely, but would collapse only above the levels
   where the planes struck:


   Collapse of the World Trade Center

    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.


   Collapse of the World Trade Center

Other buildings

   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 Centre (Marriott Hotel 3 WTC), South Plaza (4
   WTC), and U.S. Customs (6 WTC). The World Financial Centre buildings,
   90 West Street, and 130 Cedar Street suffered fires. The Deutsche Bank
   Building, Verizon, and World Financial Centre 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, known through images of it being
   covered in a large black 'shroud' after September 11th to cover the
   building's damage, is currently being deconstructed because of water,
   mold, and other severe damage caused by the neighboring towers'
   collapse.

Controlled demolition conspiracy theory

   Van Romero, an explosives expert in New Mexico, had seen videos on the
   day of the attacks and proposed that explosive devices had brought down
   the buildings. Though he quickly retracted his remarks, the idea has
   remained in circulation and the rapid collapses of the World Trade
   Centre buildings has been described as the " grassy knoll" of 9/11
   conspiracy theories. Prompted by inquiries from the public, NIST stated
   in its final report that it "found no corroborating evidence for
   alternative hypotheses suggesting that the WTC towers were brought down
   by controlled demolition using explosives planted prior to September
   11, 2001." The hypothesis has never been suggested in mainstream
   engineering scholarship and its proponents are considered "outsiders".
   In August of 2006, NIST posted a webpage addressing it among other
   frequently-asked questions.
   Retrieved from "
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   with only minor checks and changes (see www.wikipedia.org for details
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