Before it happened

some damage

1988 :: Structural engineer Charlie Thornton explains that a fully loaded 747 — a three hundred ton element — crashing into a building that has been designed to carry thirteen thousand tons would probably not do anything to the major building. It could affect localized structural elements, knock out a column, and there could be some damage, but as far as a plane knocking a building over of that type, that would not happen. [Source]

the building structure would still be there

February 27th, 1993 :: John Skilling, one of the towers' structural engineers, is quoted by Seattle Times saying "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. The building structure would still be there. However, I'm not saying that properly applied explosives — shaped explosives — of that magnitude could not do a tremendous amount of damage. I would imagine that if you took the top expert in that type of work and gave him the assignment of bringing these buildings down with explosives, I would bet that he could do it." [Source]

just a pencil puncturing a steel netting

January 25th, 2001 :: The towers' construction manager, Frank deMartini, explains the building was designed to have a fully loaded 707 crash into it. He believes the building could probably sustain multiple impacts because the structure is like the mosquito netting on a screen door — an intense grid, and the jet plane is just a pencil puncturing that screen netting, really doing nothing. [Source]

noone thought

September 11th, 2001, 9:05 a.m. :: In the lobby of the north WTC tower, just after the South Tower is hit, Fire Commissioner Thomas Von Essen speaks briefly to Fire Chief Ray Downey. According to Von Essen, Downey — who is a highly respected expert on building collapses — says to him, “You know, these buildings can collapse.” Von Essen later recalls, “He just said it in passing, not that these buildings will collapse in 40 minutes and we have to get everybody out of here, or not that they’ll collapse by tomorrow, or not that they necessarily will collapse at all. Just that they can collapse.” But other firefighters do not appear to have shared this concern. According to the National Institute of Standards and Technology (NIST), the Fire Department command officers who are planning for operations inside the Twin Towers expect that there will “be localized collapse conditions on the damaged fire floors,” but do “not expect that there [will] be any massive collapse conditions or complete building collapse.” At the end of its three-year investigation of the WTC collapses, NIST will report, “No one interviewed indicated that they thought that the buildings would completely collapse.” [National Institute of Standards and Technology, 9/2005, pp. 72 and 75-76] Deputy Fire Commissioner Lynn Tierney will meet up with Downey and others—including Von Essen—slightly later, on the south lawn of the WTC complex, where a new command center is set up. At that time, according to Tierney, Downey will only be concerned that the 360-foot antenna atop the North Tower will fall, and “No one ever thought the towers were going to come down.” [Pittsburgh Tribune-Review, 9/11/2006] However, shortly before the first tower comes down, EMT Richard Zarrillo will be asked to relay a message to some senior firefighters that the mayor’s Office of Emergency Management “says the buildings are going to collapse”. And later in the day, Mayor Giuliani will recount that around the same time, he had been told “that the World Trade Center was going to collapse”. The 9/11 Commission will state, “The best estimate of one senior [fire] chief, provided to the chief of the department sometime between 9:25 and 9:45, was that there might be a danger of collapse [of the South Tower] in a few hours, and therefore units probably should not ascend above floors in the sixties.” The Commission does not state, however, whether this fire chief was referring to a total building collapse or just a localized collapse. [Source]

When it happened

oh my god

September 11th, 2001, during and seconds after South Tower collapse :: Eyewitnesses urgently invoke their deities, reporters are at a loss of words, speak of a "huge explosion", compare the collapse to a controlled demolition, speculate a large part fell off or that the towers were reduced to a pile of rubble. [Source]

a witness

September 11th, 2001, minutes after North Tower collapse :: Mark "Harley Guy" Walsh witnessed both collapses, which happened mainly due to structural failure; because the fires were just too intense. [Source]

we believe

September 11th, 2:40pm :: Asked at a press conference about the cause of the explosions that brought the two buildings down, mayor Rudy Giuliani answers "We believe that it was caused by the after effects of the planes hitting the buildings." [Source, transcript] :: For the collection of oral histories by journalists who covered the attacks "Running Toward Danger - Stories Behind the Breaking News of 9/11" (July 16th, 2002), Beth Fertig, WNYC reporter, will remember: "I spent the rest of the afternoon at the mayor's command center. The reporters were trying to figure out what happened. We were thinking that bombs had brought the buildings down. The mayor [Giuliani] talked to us and said he had no evidence of bombs." (p. 203) [Source] (thx Mo!)

The first reactions

lucky design choice

September 12th, 2001 :: New Scientist says a lucky design choice saved lives. Most buildings would have come down immediately, but the towers remained upright for nearly an hour. Eventually raging fires melted the supporting steel struts, but the time delay allowed hundreds of people to escape. It's a good thing they didn't fall over. [Source]

shredding downward

September 12th, 2001 :: Arizona Wildcat reports that according to experts, although the impact of the jetliners was strong, it was the heat from the explosion that most likely caused the buildings to collapse and cites Richard Ebeltoft, a structural engineer and University of Arizona architecture lecturer, who speculates that flames fueled by thousands of gallons of aviation fuel melted the buildings steel supports. Once the top of the building lost balance, the floors appear to have begun shredding downward. [Source]

so predictable

September 13th, 2001 :: BBC depict the towers' core as a single steel reinforced concrete column, explain that the tower's ultimate collapse was inevitable because once the steel frame on one floor had melted, it collapsed downwards, inflicting massive forces on the already-weakened floor below. Structural engineer Chris Wise says "it was the fire that killed the buildings. There's nothing on earth that could survive those temperatures with that amount of fuel burning. The columns would have melted, the floors would have melted and eventually they would have collapsed one on top of each other" and "the top was acting like a huge piledriver" :: The buildings' construction manager Hyman Brown says "Steel melts, and 24,000 gallons of aviation fluid melted the steel. Nothing is designed or will be designed to withstand that fire" and John Knapton, professor in structural engineering at Newcastle University, UK, knew the building would fall within two hours: "The eventual collapse of the twin towers was so predictable that the order should have been given to withdraw emergency services within an hour." [Source]

doomed

September 13th, 2001 :: Professor Z.P. Bažant circulates first drafts of a simple analysis of the likely scenario. Assuming a collapse time of ~9s, with the stated aim to prove that the towers must have collapsed and do so in the way seen, approximates that under most optimistic assumptions, the towers were doomed because the percentage of the kinetic energy dissipated plastically is of the order of 1%. The reason is the dynamic consequence of the prolonged heating of the steel columns to very high temperature. The heating caused creep buckling of the columns of the framed tube along the perimeter of the structure, which transmits the vertical load to the ground. Since the plastically dissipated energy W[p] is, optimistically, of the order of 0.5 GN m, and the total release of gravitational potential energy is W[g] = mg 2h = 4.2 GN m, even under the most optimistic assumptions by far, the plastic deformation can dissipate only a small part of the kinetic energy acquired by the upper part of building. :: The paper, co-authored by Yong Zhou, is published in the Journal of Engineering Mechanics January 1st, 2002. [Source]

limit state conditions

September 17th, 2001 :: G. Charles Clifton, HERA Structural Engineer, for tenlinks.com explains that the gravity and lateral load-resisting systems were designed to deliver the strength and stiffness required from a 110 story building with minimum dead load. This was achieved very well, with a steelwork weight of only 44.5 kg/m² floor area. The very light and open structure, superbly engineered to meet the design serviceability and ultimate limit state conditions on a building of this height and size, probably made the buildings more vulnerable to collapse from the aircraft impact than would have been the case for a more inefficient and heavier structural system. In his opinion the fires had a less important role to play in the collapse of both towers than the damage from the initial impact. It took both to cause the collapse, however the fire was in no way severe enough to have caused the collapse on its own. [Source]

no clue

September 17th, 2001 :: According to the enr.com cover story " Massive Assault Doomed Towers - Terrorist Attacks Brings Down World Trade Center" by Nadine M. Post with Sherie Winston, Jon D. Magnusson, chairman-CEO of Skilling Ward Magnusson Barkshire Inc., Seattle, one of the successor firms of Skilling Ward Christiansen Robertson, structural engineer for the original World Trade Center, says: "From what I observed on TV, it appeared that the floor diaphragm, necessary to brace the exterior columns, had lost connection to the exterior wall." When the stability was lost, the exterior columns buckled outward, allowing the floors above to drop down onto floors below, overloading and failing each one as it went down. :: Mark Loizeaux, president of Controlled Demolition Inc., says: the 1,362-ft-tall south tower, which was hit at about the 60th floor, failed much as one would fell a tree. That is what was expected. But the 1,368-ft-tall north tower, similarly hit but at about the 90th floor, "telescoped". It failed vertically, rather than falling over. Regarding the cause of the telescoping, he says "I don't have a clue." [Source 1, Source 2]

strong enough

October 2001 :: Tufts Journal quotes Masoud Sanayeis, professor of civil and environmental engineering, saying the buildings were strong enough to survive the impact of the airplanes but collapsed as a result of the ensuing fire. The buildings were designed using a tube-in-tube structural arrangement, creating a system that is very stiff and strong and able to resist lateral loads such as wind or an earthquake. The towers survived the initial impact. But the large amount of jet fuel delivered to the towers followed by explosions and subsequent fire weakened the floor systems and the columns. He suspects the fire created temperatures higher than what is normally experienced in office building fires caused by burning furniture or rugs or paper. The building still had to carry the massive loads of higher floors above the plane crash location. The structural columns could no longer carry the gravity loads of the floors above the fire. In addition, the heat could have weakened the floor systems above, as well as the floor-to-column connections. This combination caused the top-down collapse, producing a domino effect. The exterior columns that formed the outside tube of the World Trade Center buildings guided the self-contained collapse within these buildings. It looked like a designed and time-delayed implosion that took only a few seconds to bring each tower down. If there had been no fire, potentially the buildings could have survived. [Source]

performed admirably

November 7th, 2001 :: Scientific American reports on a panel of Boston area-based civil and structural engineers who convened to discuss the fate of the superskyscrapers on the campus of the Massachusetts Institute of Technology. Robert McNamara says that the World Trade Center was probably one of the more resistant tall building structures. The buildings displayed a tremendous capacity to stand there despite the damage to a major portion of the gravity system. The lateral truss systems redistributed the load when other critical members were lost. It's a testament to the system that they lasted so long. According to him, the planes hit at just the right place. The truck bomb attack showed that the explosion at the bottom had little effect and that it's much easier to collapse a building from the top than the bottom. If they had hit the very top of the building, the fire damage wouldn't have had such a catastrophic effect. At the bottom, the columns are much heavier and stronger and so they would have taken a much larger load. :: Eduardo Kausel, M.I.T. professor of civil and environmental engineering, says the World Trade Center was never designed for the massive explosions nor the intense jet fuel fires that came next — a key design omission. Kausel says the towers performed admirably; they stood long enough for the majority of the people to be successfully evacuated. He approximates that about 30 percent of the collapse energy was expended rupturing the materials of the building, while the rest was converted into the kinetic energy of the falling mass, concluding that the the largest share of the kinetic energy was converted to heat, material rupture and deformation of the ground below. He goes on to explain that the intense heat softened or melted the structural elements—floor trusses and columns—so that they became like chewing gum, and that was enough to trigger the collapse. The floor trusses are likely to have been the first to sag and fail. As soon as the upper floors became unsupported, debris from the failed floor systems rained down onto the floors below, which eventually gave way, starting an unstoppable sequence. The dynamic forces are so large that the downward motion becomes unstoppable. He determines that the fall of the upper building portion down onto a single floor must have caused dynamic forces exceeding the buildings’ design loads by at least an order of magnitude and performed computer simulations indicating the building material fell almost unrestricted at nearly the speed of free-falling objects. According to his hypothesis, the towers' resistive systems played no role. Otherwise the elapsed time of the fall would have been extended. Asked why the top did not tip over, he answers: "A tree is solid, whereas building is mostly air or empty space; only about 10 percent is solid material. Since there is no solid stump underneath to force it to the side, the building cannot tip over. It could only collapse upon itself." :: Unnamed, but well-informed correspondents are quoted with the suggestion that the aviation fuel fires burned sufficiently hot to melt and ignite the airliners' aluminum airframe structures. Aluminum, a pyrophoric metal, could have added to the conflagrations. Hot molten aluminum could have seeped down into the floor systems, doing significant damage. [Source]

hasn't fallen yet

December 6th, 2001 :: Blair Kamin, architecture critic, for The Chicago Tribune interviews senior vice president at Construction Technology Laboratories in north suburban Skokie, W. Gene Corley: "I said: 'At least it hasn't fallen yet. But if they don't get the fire out, it will.' " "The aircraft hits wounded the buildings, but they could have recovered if it weren't for the fire" :: The article also states that no one has authoritatively explained why One World Trade Center remained standing for 1 hour and 40 minutes while Two World Trade Center crashed to the ground just 56 minutes after impact. Another mystery is why smaller, nearby structures remained standing even though fire and steel from the twin towers rained down on them and even ripped huge chunks out of their facades. :: It also names three possible collapse scenarios: exterior column, interior column or floor truss failure. [Source]

The next years

would have collapsed without fire

January 9th, 2002 :: Richard Gewain, a senior engineer at Maryland-based Hughes Associates and a member of the federally designated panel of engineers probing the collapses, says the major insult was the plane knocking out the columns, and destroying the connections to the floors. Conceivably, the building would have collapsed without the fire. The impact was so tremendous that each building's fireproofing was rendered irrelevant. [Source]

as a result, the building collapsed

March 6th, 2002 :: Funded by the Federal Emergency Management Agency, Dr. Abolhassan Astaneh-Asl, professor of civil and environmental engineering at the University of California, Berkeley, under the Directorate of Engineering of the National Science Foundation (NSF), is one of five expert witnesses invited to testify before the Committee on Science of the U.S. House of Representatives at the hearing "Learning from 9/11 – Understanding the Collapse of the World Trade Center". Dr. Astaneh-Asl’s presentation includes a movie of a simulated crash of a passenger aircraft onto a generic steel structure created with MSC.Dytran and MSC.Marc Nonlinear Software. While the impact of the crash substantially weakened the structure, "the remaining steel beams and columns melted because of the extreme temperatures. Also buckling in the trusses was possible, especially after the damaged crossbeams were unable to provide lateral support. As a result, the load could no longer be distributed and the building collapsed." [Source]

one after another

April 30th, 2002 :: NOVA (PBS), for "Why the Towers Fell", create an animation showing how the columns were a loose bundle of straws, gaining all their stiffness solely from the floor slabs, and then of a pancake collapse that leaves the core standing up. Matthys Levy (author of "Why buildings fall down") explains: "As the steel began to soften and melt, the interior core columns began to give. Then you had this sequential failure that took place where it all pancaked — one after the other." [Source, transcript]

proper parameters

June 2002 :: Lu Xinzheng and Jiang Jianjing present a LS-DYNA FEA "Simulation for the Collapse of WTC after Aeroplane Impact". They say the simulation results are very close to the real situation, which means that such type of special damage process can be recurred on the computer with proper parameters and numerical model. The results show that the direct reason for the collapse is the softening of steel under fire and the chain reaction damage of floors under the impact load of upper floors. If the fire resistance and the ductility of the structure are improved, the collapse may be avoided. As the fracture of steel is considered in this analysis, the failure plastic deformation is discussed as parameters, whose value is set as 0.5%, 1% and 5% respectively. If the fracture plastic strain of steel structure is 0.5%, the chain collapse will take place entirely. However, if the fracture strain is improved to 1%, the impact energy of upper floors will be absorbed by the lower structures and the chain collapse will be stopped at about 100m under the airplane impact zone. When the fracture strain is improved to 5%, only part of the structure near the airplane impacting zone will be damaged, and no chain collapse will take place. Hence, if the structure has enough ductility to absorb the energy of upper floors' collapse, the chain damage will be controlled. Even though the influence of heap load is considered, the towers still have much larger chance to escape from the entire collapse. [Source]

FEMA: rapidly converted energy

September 1st, 2002 :: FEMA publishes the "World Trade Center Building Performance Study" and finds that the structure was able to successfully redistribute the building weight after the plane impacts to the remaining elements and to maintain a stable condition, and that the structure may have been able to remain standing in this weakened condition for an indefinite period. However, fires heated floor framing and slabs, causing expansion, which developed additional large stresses in some members. As the temperature of column steel increased, the yield strength and modulus of elasticity degraded and the critical buckling strength of the columns decreased and initiated buckling, a significant effect in the failure of the interior core columns. Once collapse initiated, much of the potential energy was rapidly converted into kinetic energy. As the large mass of the collapsing floors above accelerated and impacted on the floors below, it caused an immediate progressive series of floor failures, punching each in turn onto the floor below, accelerating as the sequence progressed in a pancake-type collapse of successive floors. [Source]

no biggie

September 11th, 2002 :: For Naples Daily News, Ralf Kircher interviews Bernard Panto, who worked in various positions as an engineer on the World Trade Center construction project. He had heard that a plane had hit the World Trade Center, but had thought to himself that it was nothing to worry about. "No biggie," he said to himself. "Can't knock that building down." [Source]

bolts popped and fell apart

October 28th, 2002 :: Jaime Holguin for CBS reports that the single-bolt connections in the framework of the World Trade Center popped and fell apart, causing the floors to collapse on top of each other. According to an analysis conducted by a team of researchers from the Massachusetts Institute of Technology, the bolts did not properly secure the towers' steel floor trusses. The bolts were pulled toward the center of the buildings while the floor trusses sagged. [Source]

vulnerable point

November 24th, 2003 :: The Sunday Times, in an article called "Kamikaze attackers may have known twin sisters' weak spot", cite a top British structural engineer, Gordon Masterson, who says the airplanes crashed into the Twins at their most vulnerable spot, which suggests the suicide attackers may have known where to strike. Structural damage from this impact, and the searing heat of blazing aviation fuel, then combined to bring down the top 20 floors. The accumulating weight caused both buildings to collapse, floor by floor, faster and faster. If they had struck right at the top of the tower that would have had a greater effect on the foundations, but the fire would have been more contained. However, they struck in both cases at an extremely vulnerable point. :: Marc Mimram says very tall buildings are prey to a phenomenon called moment tensor inversion - a shock from a seismic wave or impact that ripples through the building but has a whiplash, leveraged effect on its higher floors. [Source]

collapse could be avoided

December 1st, 2003 :: Abdolhassan Astaneh-Asl in "World Trade Center Collapse, Field Investigations and Analyses" opines that the highly redundant exterior tube of the World Trade Center with many closely spaced columns was able to tolerate the loss of many columns and support the gravity while almost all occupants who could use a stairway escaped to safety. The collapse of the towers was most likely due to the intense fire initiated by the jet fuel of the planes and continued due to burning of the building contents. It is also the opinion of the author that had there been better fireproofing installed to delay the steel structure, specially the light weight truss joists and exterior columns from reaching high temperature until the content of the buildings burned out, probably the collapse could be avoided and the victims above the impact area rescued. [Source]

9/11 Report: hollow steel shaft

July 22nd, 2004 :: The National Commission on Terrorist Attacks Upon the United States, an independent, bipartisan commission created by congressional legislation and the signature of President George W. Bush, issue their final report which states that the interior core of the buildings was a hollow steel shaft. [Source]

comes straight down

October 21st, 2004 :: According to Boulder Weekly, construction manager for the World Trade Center and University of Colorado civil engineering professor Hyman Brown explains that the airplane fuel and the fire-suppression system caused the building to collapse. The fire-suppression system blocks off five-floor blocks so the fire can’t go up or down. The steel in that five-floor area melts. All the tonnage above the five-floor area comes straight down when the steel melts. That broke all the connections, and that caused the building to collapse. [Source]

NIST Draft comments

June 2005 :: After a six-week period for public review of their 10000-page epos, NIST publishes the "Public Comments Received by NIST on DRAFT Reports". John Lyle, Arup, objects that results from 5 FEM indicate that the natural frequency of the structure is not significantly altered by a large hole in the outer tube and missing parts of the core (p. 197); that the global model ignored that the impact scenario was shorter than the natural period of the towers; misses clarification whether P-Δ effects were taken into account as result from swaying (p. 183) :: Barbara Lane, Arup, does not agree with NIST's conclusion that only impact induced fire proofing damage caused the collapse. Loss of fire proofing may have reduced the time to collapse, but the analysis presented does not prove that it would have prevented collapse, particularly when so much emphasis is placed, in this regard, on the tests carried out. She believes that the repeated statements about fire protection are a little misleading. (pp. 308, 322) :: Dave Johnston, BOMA International, asks NIST exactly how many building collapses have there been.....ever? (p. 354) :: Professor James Quintiere does not believe that NIST has presented a convincing argument for their collapse hypotheses for WTC 1 and 2. NIST fails to document the rationale for the speedy elimination of the steel in this incident. NIST has not presented clear and sufficient evidence that the aircraft impacts caused the elimination of insulation, especially from the core columns. The NIST report reads like a scientific enterprise using computer simulations that have never been used (or validated) in this way before. The October surprise in the NIST investigation was the assertion that all of the core column insulation was knocked off by the airplane impacts. NIST has relied on state-of-the-art computer models that are at the forefront of their technologies. However, these models have not been proven comprehensively for less complex incidents than the WTC. Can an engine possibly hit a core column without hitting anything on the floor occupancy and structure? That does not seem possible, so how can an engine damage a core colum? The global collapse mechanism of the buildings must be made as clear as possible. A vague answer expressed by the current NIST working hypothesis is not sufficient. If the core of the answers are really revealed and understood, NIST should be able to explain them in simple fundamental physics, and not shroud them in computer graphics. (pp. 374 ff) :: Jon Magnusson, PE, SE, Hon. AIA, says NIST has provided absolutely no data showing there has been even a single death as a result of progressive collapse anywhere in the U.S. The consequences of the WTC attack did NOT constitute a case of progressive collapse. The WTC design MET AND BETTERED all currently proposed progressive collapse standards and code provisions (p. 554) [Source]

NIST: does not actually include

September 2005 :: With the specific objective (p. xxxix) to determine why and how WTC 1 and WTC 2 collapsed following the initial impacts of the aircraft, NIST in the "Federal Building and Fire Safety Investigation of the World Trade Center Disaster: Final Report of the National Construction Safety Team on the Collapses of the World Trade Center Tower" examine more than 170 areas on 16 perimeter column panels, but only three columns have evidence that the steel reached temperatures above 250ºC. Only two core column specimens have sufficient paint remaining to make such an analysis, and their temperatures did not reach 250 ºC. Using metallographic analysis, NIST determine that there was no evidence that any of the samples had reached temperatures above 600 ºC (pp. 176-177). In state-of-the-art computer simulations, none of the columns with intact insulation reached temperatures over 300 °C. Only a few isolated truss members with intact insulation were heated to temperatures over 400 °C in the WTC 1 simulations and to temperatures over 500 °C in the WTC 2 simulations. In WTC 1, if the fires had been allowed to continue past the time of building collapse, complete burnout would likely have occurred within a short time since the fires had already traversed around the entire floor, and most of the combustibles would already have been consumed. In WTC 2, if the fire simulation were extended for 2 hours past the time of building collapse with all windows broken, the temperatures in the truss steel on the west side of the building (where the insulation was undamaged) would likely have increased for about 40 min before falling off rapidly as the combustibles were consumed. Temperatures of 700 °C to 760 °C were reached over approximately 15 percent of the west floor area for less than 10 min. Approximately 60 percent of the floor steel had temperatures between 600 °C and 700 °C for about 15 min. Approximately 70 percent of the floor steel had temperatures that exceeded 500 °C for about 45 min. At these temperatures, the floors would be expected to sag and then recover a portion of the sag as the steel began to cool. The temperatures of the insulated exterior and core columns would not have increased to the point where they would have experienced significant loss of strength or stiffness. (p. 184) NIST finds the towers withstood the impacts and would have remained standing were it not for the dislodged insulation and the subsequent multi-floor fires. (p. 174). The focus of the investigation however is on the sequence of events from the instant of aircraft impact to the initiation of collapse for each tower. For brevity in the report, this sequence is referred to as the ‘probable collapse sequence,’ although it (includes little analysis of | does not actually include) the structural behaviour of the tower after the conditions for collapse initiation were reached and collapse became inevitable. (footnotes on pp. xxxvii & 82) [Source]

mechanics of progressive collapse: W[g] > W[p]

June 26th, 2006 :: Professor Z. P. Bažant and Mathieu Verdure, at the U.S. National Congress of Theoretical
and Applied Mechanics, Boulder, Colorado, present "Mechanics of Progressive Collapse: Learning from World Trade Center and Building Demolitions". The destruction of the Towers was not only the largest mass murder in U.S. history, but also a big surprise for the structural engineering profession. No experienced structural engineer watching the attack expected the WTC towers to collapse. No skyscraper has ever before collapsed due to fire. The fact that the WTC towers did, beckons deep examination. Further calculations beyond the 2002 "Simple Analysis" are found superfluous, but a theory describing the progressive collapse dynamics beyond the initial trigger, with the WTC as a paradigm, could still be very useful for other purposes, especially for learning from demolitions. They go on to show that progressive collapse will be triggered if the total (internal) energy loss during the crushing of one story (equal to the energy dissipated by the complete crushing and compaction of one story, minus the loss of gravity potential during the crushing of that story) exceeds the kinetic energy impacted to that story. Regardless of the load capacity of the columns, there is no way to deny the inevitability of progressive collapse driven by gravity alone. Also, some critics are addressed, who have been under the mistaken impression that collapse cannot occur if (because of safety factors used in design) the weight mg of the upper part is less than the load capacity F[0] of the floor, which led them to postulate various strange ideas (such as “fracture wave” and planted explosives). However, since the energy loss up to the point at which the load-deflection diagram intersects the line F=mg(z) is smaller than the kinetic energy of the impacting mass m(z) (Fig. 3), it is clear that this impression is erroneous. What matters is energy, not strength, nor stiffness. :: March 2007 the paper will be published in the Journal of Engineering Mechanics. [Source]

super-heated jet fuel

August 21st, 2006 :: CBC interviews Lee Hamilton, 9/11 Commission co-chair. He says "the engineers and the architects have studied this thing in extraordinary detail, and they can tell you precisely what caused the collapse of those buildings. What caused the collapse of the buildings [...] was that the super-heated jet fuel melted the steel super-structure of these buildings and caused their collapse. There’s a powerful lot of evidence to sustain that point of view, including the pictures of the airplanes flying into the building." [Source]

what did and did not: gravity alone

March 31st, 2008 :: To check whether the recent allegations of controlled demolition, which rest on the free fall hypothesis, have any scientific merit and to prove that they do not, Z. P. Bažant, Jia-Liang Le, Frank R. Greening and David B. Benson present "What Did and Did Not Cause Collapse of WTC Twin Towers in New York". It is shown that the observed size range (0.01 mm—0.1 mm) of the dust particles of pulverized concrete is consistent with the theory of comminution caused by impact, and that less than 10% of the total gravitational energy, converted to kinetic energy, sufficed to produce this dust — whereas more than 150 tons of TNT per tower would have to be installed to produce the same pulverization. Also, initial speculation that very high temperatures were necessary to explain collapse are revised based on NISTs findings, since steel temperature ≥150°C sufficed to trigger the viscoplastic buckling of columns. To get convinced of the inevitability of gravity driven progressive collapse, further analysis is, for a structural engineer, superfluous, but nevertheless needed to dispel false myths. It is no surprise at all that the towers collapsed essentially on their
footprint. Gravity alone must have caused just that. [Source 1, Source 2]

NIST FAQ: multiple factors

September 19, 2011 :: The NIST FAQ state that the WTC towers collapsed because: (1) the impact of the planes severed and damaged support columns, dislodged fireproofing insulation coating the steel floor trusses and steel columns, and widely dispersed jet fuel over multiple floors; and (2) the subsequent unusually large number of jet-fuel ignited multi-floor fires (which reached temperatures as high as 1,000 degrees Celsius) significantly weakened the floors and columns with dislodged fireproofing to the point where floors sagged and 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. Both photographic and video evidence — as well as accounts from the New York City Police Department aviation unit during a half-hour period prior to collapse — support this sequence for each tower. NIST’s findings do not support the “pancake theory” of collapse, which is premised on a progressive failure of the floor systems in the WTC towers. Instead, the NIST investigation showed conclusively that the failure of the inwardly bowed perimeter columns initiated collapse and that the occurrence of this inward bowing required the sagging floors to remain connected to the columns and pull the columns inwards. Thus, the floors did not fail progressively to cause a pancaking phenomenon. [Source] The NIST FAQ for WTC 7 clarify that progressive collapse did NOT occur in the WTC towers. First, the collapse of each tower was not triggered by local damage or a single initiating event. Second, the structures were able to redistribute loads from the impact and fire-damaged structural components and subsystems to undamaged components and to keep the building standing until a sudden, global collapse occurred. Had a hat truss that connected the core columns to the exterior frame not been installed to support a TV antenna atop each WTC tower after the structure had been fully designed, it is likely that the core of the WTC towers would have collapsed sooner, triggering a global collapse. Such a collapse would have some features similar to that of a progressive collapse. The Twins were brought on by multiple factors, including structural damage caused by the aircraft impact, extensive dislodgement of the sprayed fire-resistive materials or fireproofing in the impacted region, and a weakening of the steel structures created by the fires. [Source]

Ten years later...

all factors

September 21st, 2011 :: According to a theory advanced by materials scientist Christian J. Simensen of SINTEF Materials and Chemistry, Norway, a mixture of water from sprinkler systems and molten aluminum from melted aircraft hulls created explosions that led to the collapses of the Twin Towers: both scientific experiments and 250 reported disasters suffered by the aluminium industry have shown that the combination of molten aluminium and water releases enormous explosions. [Source] Professor Bažant thinks that the official explanation suffices, as he had explained it in six papers in leading journals. In his opinion, all factors related to the collapse have been accounted for. [Source]

above free fall

March 12th, 2015 :: Professor of theoretical physics Per Hedegård from the University of Copenhagen implies that the speed of the building's collapse could theoretically be above free fall due to the complex nature of the energy waves. [Source]

erroneous

April 27th, 2015 :: A /r/AskEngineers moderator explains that progressive collapse can't be stopped because, even though the lower undamaged structure was designed to be able to handle the upper damaged 18 stories, this is only true in a static sense. Once the loads became dynamic (due to gravity), the dynamic loads had already far exceeded the structural design limit and the failure had to propagate until all the energy is dissipated. This is true for nearly every structure in existence, and certainly true for every skyscraper: once progressive collapse begins, it can't be stopped because the supporting structure below cannot handle the many multiplications of the load it was designed to support. It's erroneous to think of a skyscraper as a solid column of material that's able to stop falling objects. In reality a skyscraper is more like a skeletal structure with thousands of interconnected supporting members. The loss of too many members/supports will cause failure. A giant mass falling through the center of the skeleton will only knock more of the structure away, further propagating the collapse. [Source]

stoichiometry

August 2015 :: reddit hivemind thinks "Jet fuel can't melt steel beams" is a dank meme.