In the immediate aftermath of 9/11, numerous articles were published in newspapers, with the journalists consulting structural engineers and other experts for their opinions on what caused the towers to collapse. The experts also included engineers who worked on the design and construction of the towers. Numerous engineers consulted were not surprised that the buildings ended up collapsing, "given the extraordinary circumstances."[1]
Most experts mentioned explained that the tremendous heat of the fire heated steel structural elements, with the heat causing the structural steel to loose much of its strength. Upon heating, the steel deformed (creep) at first, and eventually reached a point where it fractures. A number of experts describing how the collapse occurred, mentioned that the steel "melted", though the temperatures of the fire did not get that hot and did not need to be that hot in order to substantially reduce the strength of steel. An added factor mentioned was the damage to many of the steel columns due to the aircraft impact, causing load to be shifted to the remaining columns which became overloaded. Many also noted that the fireproofing was likely damaged or dislodged, allowing the steel structural elements to heat up more rapidly, and that the sprinkler systems probably were not functioning.
Experts hypothesized that the fires "weakened the steel supports, causing the external walls to buckle and allowing the floors above to fall almost straight down. That led to catastrophic failures of the rest of the buildings."[2]
Paul Ast of Jablonsky, Ast and Partners in Toronto, "Any building, of any construction, anywhere in the world, would go down if hit by a 767 (jet). I can guarantee you that, whether it's built of concrete or steel it doesn't matter. No one designs buildings to be hit by a 767 loaded with tens of thousands of litres of fuel. What really brings these buildings down is the tremendous heat of the fire."[3]
Hassan Astaneh - "The crash of the planes themselves must have had little, if any, impact on the internal structure of the towers at first, Astaneh explained, as the planes' weight alone -- even at a few hundred thousand pounds each -- would not have ruptured the girders. But the thousands of gallons of burning fuel and the instant explosions produced temperatures of more than 2,000 degrees, Astaneh said. That was more than enough to destroy the insulation around the steel beams and columns and weaken the steel members until they became "soft and mushy," he said. Those structural columns and beams, however strong originally, could not possibly have supported the undamaged upper floors of the two towers above the explosions. Suddenly, "as the columns collapsed, those whole upper stories of the buildings dropped like dead bodies," Astaneh said. "That impact was too much, and no building could possibly withstand such weight, so floor after floor came down in what we call progressive collapse."[4]
Aine Brazil (Thornton-Tomasetti Engineers) - "There isn't anything particularly vulnerable about it [the design of the Twin Towers]," said Aine Brazil of Thornton-Tomasetti Engineers in New York, a structural engineering firm that worked on the Petronas Towers, the world's largest buildings, in Malaysia. Buildings are simply not designed to withstand "the extreme levels of heat that would be found in the situation with the amount of jet fuel and the explosion that occurred," Ms. Brazil said.[2]
Hyman Brown - "This building would have stood had a plane smashed into it," said Hyman Brown, a University of Colorado civil engineering professor and the trade center's construction manager. "But 24,000 gallons of [burning] aviation fuel melted the steel. Nothing is designed or will be designed to withstand that kind of fire."[5]
Joseph Burns (Thornton-Thomasetti) - "The building did survive the hit but the fire weakened the building and it collapsed in a way that no one could have predicted." Burns said that firefighters are trained to battle blazes in skyscrapers, and typically know when to get out to avoid serious risk. But the magnitude of this situation could never have been anticipated. "I don't think anyone imagined that the building would collapse that quickly," he said.[6]
Jack Cermak, president of Cermak Peterka Peterson in Fort Collins, Colo., the firm that did the wind-tunnel testing for the design of the towers, agreed that the impact of the crash itself probably could not have collapsed the massively reinforced building on its own. "I presume, without knowing the details, that that collapse was caused by weakening of the structure due to the heat," Dr. Cermak said.[2]
Shelley Clark (Skilling Ward Magnusson Barkshire) - the towers' stout, column-heavy structure allowed it to absorb the initial impacts of the jets, just as it enabled one of the towers to survive a 1993 bombing at its base. This time, however, Clark said, "It just couldn't handle the heat." The towers' steel frames were designed to withstand three hours of heat experienced during a "normal" building fire, she said. But Clark and other structural engineers speculate that the heat from the plane-crash explosions exceeded 1,000 degrees -- enough to weaken steel floor beams stretching from the inner to the outer columns. The towers probably started to fall once several of their exterior columns got hot enough to buckle, Clark said. The weight above them then triggered a successive collapse. And no floor was strong enough to break the fall.[7]
Joseph Colaco (CBM Engineers) - "There appears to have been an enormous amount of jet fuel. You could see there was a big fireball that was caused by the impact when the plane hit the building. As temperatures surpassed 1,000 degrees, the steel weakened."[8] "Structural steel loses its strength very quickly past 600 degrees Fahrenheit." The steel in modern buildings is wrapped in insulating material to maintain structural integrity for three hours in the kinds of fires that are fed by typical building contents. But, Colaco observed: "In New York, with the jet fuel, you easily go past those numbers." The jet fuel might have burned as hot as 1,600 degrees. "The floor trusses would turn very quickly into pieces of spaghetti," Colaco said. "They start to drape, they deflect downward. One floor falls onto the one below, which now must carry two floors of load. There's a domino effect."[9]
Dan Cuoco (Thornton Tomasetti) - "Any building that was subjected to that type of loading and flames would have had the same result," said Dan Cuoco "The buildings did not collapse immediately because the loads that were in those damaged columns were redistributed to other adjacent columns, and the building was able to survive the initial impact, However, the steel was progressively getting weaker as the flames continued."[10]
Jim Destefano of the U.S. National Council of Structural Engineers told CNN yesterday that the vertical columns bent and twisted in the World Trade Center collapse. "Those are the critical columns or elements that clearly failed, in a buckling mode, from the high temperatures and the damage from the impact," he said, "It is very difficult, in designing a building, to take into account all possible scenarios. He said the World Trade Center was designed, for example, to take a direct hit from a Boeing 707 jet, an airliner designed in the 1950s. "Clearly planes are larger today and [the World Trade Center design] didn't consider the effects of the aftermath fires and high temperatures," he said.[11]
Paul J. Donnelly (Washington University) - The buildings were not destroyed by the lateral force of the crash into upper floors of the buildings. Rather, he described the destruction in terms of having your legs kicked out from under you. What happened, as best Donnelly sees it, was that the planes knocked out some columns at the level of impact. And although the towers of the World Trade Center were engineered to sway within a certain frequency, the impact caused extraordinary oscillation, compromising the integrity of the other columns supporting the stories at the level of impact. That, combined with the tremendous heat from the fire that erupted, caused columns to lose their strength and buckle. This resulted in the catastrophic weakening of the structure. This weakness allowed the upper stories of the towers to crash down. The colossal weight and the enormous gravitational force of their falling smashed onto and demolished the lower floors, and ultimately the entire structures. "It was gravity loading, rather than lateral loading, that brought about the collapse," Donnelly said. "Lateral load failure would overturn the building."[12]
Mahjoub Elnimeiri (Illinois Institute of Technology) - "Could it have been avoided? I don't think so," Elnimeiri said, adding that the design work on the towers, opened in 1970, was excellent, but "You couldn't have designed to safeguard against this situation," he said, adding that too many things happened at once. "The impact, the overheating, the gas--just about everything is happening in a very short time and in a very violent way."[13]
William Faschan (Leslie E. Robertson Associates) - Some of the upper floors probably sagged 2 feet or more before finally breaking loose from the steel outer frames and inner cores that supported the buildings. That dumped tons of concrete, fixtures and furniture on lower floors. A single floor could weigh as much as 3,000 tons. The falling debris started a top-down domino effect engineers call pancaking. "The momentum was too much to handle," says William Faschan, a partner at Leslie E. Robertson Associates, the towers' structural engineers. "A given floor normally could support the weight of three floors. But that assumes the weight is imposed in a gradual manner." There was nothing gradual about the process once each tower began to collapse. It took just moments for the growing weight and velocity of the wreckage to rip through lower floors as though they were tissue.[14]
According to Gregory Fenves, a professor of Civil Engineering at the University of California at Berkeley, the planes weakened the buildings' structures at key points. Fenves, working on information gleaned from preliminary TV reports, stressed that he was speculating. He said that if the planes had hit the structures higher, they could have merely damaged their tops; if they had hit lower, they would have been up against the enormous weight and resistance of the base of the buildings.[15] Fenves said the collapse "happened too quickly to be caused by the fire. The fireproofing on the steel structure probably has a rating of three hours" before failing, and the buildings came down sooner than that, about an hour after being struck.[16]
Wilem Frischmann (Pell Frischmann Group and the City University, London) - The twin towers should not have collapsed so quickly. "Prior to 11 September, I scarcely believed that this icon was vulnerable. The impact of the Boeings, puncturing the outer steel shell of the towers would not in itself have caused the towers to fall. "My current analysis of the collapse sequence [suggests that] damage caused to the outside would not have triggered collapse." Although the explosion caused by the fuel-laden aircraft would have been intense, the lack of available oxygen inside the towers would, according to Professor Frischmann, have limited the fireball's temperature to less than 1,000 Celsius. This was within the design limits that the towers were supposed to withstand. Sprayed-water fire protection should have maintained the buildings internal strength for several hours, allowing a more complete evacuation. But the steel supports in the central cores supporting the towers were protected from fire by plaster that had been sprayed on to them. This plaster could have been cracked by the impact, exposing the structural steel to the fire at an early stage.[1]
Jeff Garrett (Exponent Failure Analysis Associates) - "It takes a while for the steel [columns] to heat up. It's encased in a little coat" of fire retardant, said Jeff Garrett, a senior managing engineer in the Chicago office of Exponent Failure Analysis Associates, based in Menlo Park, Calif. "But as the fire heated up, the structure got soft, and it collapsed. The extra weight caused the whole building to pancake down on itself."[17]
Al Ghorbanpoor (University of Wisconsin-Milwaukee) - "No building in the world has been designed to withstand the impact of a fully loaded jetliner of this size" -- a 757 and a 767. Ghorbanpoor says that the best anyone can hope for is buildings designed against sudden collapse, not total collapse. And on that score, the damaged trade center towers performed, allowing an hour or so for those who were in a position to get out to do so before the building pancaked.[18]
F.H. (Bud) Griffis (Polytechnic University in Brooklyn) - "If the plane had not been full of jet fuel, the building would have not fallen down. Steel does not respond well to heat."[10]
Scott Gustafson (Demtech Inc.) - The impact of the planes themselves, and the tremendous heat generated by tons of burning jet fuel--upward of 1,000 degrees Fahrenheit--would suffice to destroy the buildings, said Scott Gustafson, owner of Demtech Inc. of Blue Springs, Mo., one of the world's leading demolition experts.[19]
Bob Halvorson, of architects Halvorson and Kaye, thinks it may prove too expensive to modify existing buildings. He said: "There is going to be a debate about whether or not the World Trade Center Towers should have collapsed in the way that they did." The post-mortem on the twin towers will not be swift and will rely on the plans of the buildings, records of its construction, the testimonies of survivors, video of the collapse and forensic examination of the wreckage. "We are operating well beyond realistic experience," said Halvorson.[1]
Ronald Hamburger - To many who saw the buildings fall on television, the collapse resembled a planned demolition, especially in the way that the twin towers imploded--tumbling in on themselves. But engineering experts discounted the notion that additional explosives had been planted around the base of the buildings to ensure that they came down. Demolition of a building the size of the ones in the World Trade Center would require "literally hundreds of charges around the building," Hamburger said. "It's inconceivable to me anyone would be able to place that many charges--even with years of planning."[19]
John Hooper (Skilling, Ward, Magnusson, Barkshire) - The buildings "withstood the impact of the airplanes, and that's not what took it down. The issue was the fire that followed" Once the fires began, the structural steel began heating. Hooper said that at a constant burning temperature of 1,500 to 1,600 degrees, steel starts to lose its strength and then melts. Every degree of temperature makes the steel less capable of bearing weight.[20] "Nothing would have stopped the World Trade Center from coming down, given the heat of the fire and the length of time it burned, Too much fire for too long a duration will bring any building down." "Until now, a fire has never caused this level of catastrophe," he said. "Who could have envisioned something like this? It was a unique and devastating attack that firefighters could not handle."[21]
Richard M. Kielar, a spokesman for Tishman Realty and Construction Company, the construction manager for the original project said, "No structure could have sustained this kind of assault."[2] He said it was too early to piece together a precise train of events, but he agreed that weakening by fire, followed by catastrophic collapse of the floors, was the most likely possibility. "As the structure warped and weakened at the top of each tower, it -- along with concrete slabs, furniture, file cabinets and other materials -- became an enormous consolidated weight that eventually, progressively crushed each tower below," he said in a statement.[2] "These were airliners scheduled for long flights, full of fuel, causing massive explosions," says Richard M. Kielar, a Tishman senior vice president. "No structure could have sustained this kind of assault," says Kielar.[22]
Ron Klemencic (president of Skilling Ward Magnusson Barkshire and chairman of the Council on Tall Buildings and Urban Habitat)- "The technical term is progressive collapse--the slang term is pancaking," said Ron Klemencic, president of Skilling Ward Magnusson Barkshire, the Seattle firm that engineered the World Trade Center. "What basically happens is that one floor falls on top of the floor below it, and with one floor falling on top of another there's no way to stop it."[19]
Ted Krauthammer (Penn State) - "The impact of the planes would have caused substantial damage," And then came the fire. That would have caused two things to happen. First, fire "would have created very severe thermal stresses on the structural frame. Steel would have begun to expand, pushing against other parts. "Second, high temperatures weaken structural steel. It starts to lose its resistance, become softer, like Play-Doh. We're talking here about very high temperatures, 500 to 800 degrees Celsius (900 to 1,500 degrees Fahrenheit). "Everything just progressively collapses until you reach the foundation," Krauthammer said. Krauthammer and others emphasized that such a structural failure after such a cataclysm was not unexpected.[23] The collapse "was not something that would happen just because the outside tubing was taken out [from the aircraft impact]," Krauthammer added. "You had localized damage from the impact and the initial explosion, so some members are going to be damaged and cannot carry the load. As a result, the building is shifting the load to adjacent members, which get overloaded, and eventually it collapses."[16] "This is what in a technical term is called progressive collapse, where the initial damage does not cause instantaneous collapse but it causes sufficient damage to gradually have the damage propagate through the building until the building becomes unstable and then it collapses." That, he said, is like knocking out the legs of a heavily laden table. "The columns were damaged, the load that was sheared from the damaged columns went into other parts of the building and overloaded them. And then the subsequent fires weakened the steel even further and caused additional stresses to come in. Gradually, the whole thing continuously collapsed upon itself."[11]
"The towers took the impact," said Jack Lebduska, professor of construction and life safety at the New Jersey Institute of Technology. But the buildings ultimately failed because the extreme heat melted the floor trusses. "At 1,000 degrees, steel loses about half its strength; at 1,500 degrees it turns to taffy," said Lebduska, consultant to a Japanese firm that designed a bank on the South Tower's 80th floor. Burning jet fuel combined with plastics in computers, desks and chairs plus the extreme height presented firefighters with an impossible challenge. Ultimately, one floor fell onto the next, overloading it, and then it was like a house of cards, one falling down on top of another, Lebduska said. However, one benefit of having the load-bearing exterior was that it contained the collapse and prevented it from falling over.[24]
Richard Liew, a lecturer at the School of Civil Engineering at the National University of Singapore, said that it was already quite remarkable that the towers stood up for as long as they did after they were hit. He added that buildings are usually designed for a fire rating of between one-and-a-half and two hours, that is, the time a structure is expected to withstand a fire. Though a combination of factors probably brought the towers down, the fires which broke out because of the fuel from the planes would have caused the most damage, he said. Such fires, he added, can burn to temperatures way over 650 deg C, the maximum temperature that a steel structure can resist before it buckles. Though the buildings would have had sprinkler systems, these would have to be fed by a pressurised water source. But this might have failed. "It is possible that the impact of the crash knocked out the sprinkler system," he said. "Inertia force" did the rest, resulting in the progressive collapse of the towers from top down.[25]
Richard Little (National Academy of Sciences) - Experts said no building could have sustained the latest assault. "It was several orders of magnitude beyond anything we'd seen before," said Dr Richard Little.[26]
Mark Loizeaux (Controlled Demolitions Inc.) - Based on what I know right now, I am not inclined to believe there were any bombs," "Gravity brought the buildings down. When the airliner impacted into the building, the fire burned, and as the fire burned the columns got hot, the metal melts and bends, and slowly the object which was at rest came into motion. Once it began to move, there was no stopping it."[27]
Jon Magnusson (Skilling Ward Magnusson Barkshire) - The enormous heat from the jet fuel fire probably caused the steel trusses holding up concrete-slab floors and vertical steel columns to bend like soft plastic, said Jon Magnusson, chairman and chief executive of Skilling Ward Magnusson Barkshire in Seattle, a structural engineering firm that worked out the original design.[2] "The subsequent fires caused the collapse," said Jon Magnuson, "What happens in a fire is steel at 1,500, 1,600 degrees Fahrenheit, steel loses its strength. Then it just collapses." He said nobody could design a building and account for the possibility of heat generated by burning jet fuel. The impact of the plane could have damaged the sprinkler system, allowing the fire to burn uncontrolled. The combination of the plane crashes and the heat from the fires on the steel columns probably began the collapse. A structural engineer said that after the upper floors gave way, each then overloaded the floor beneath it, continuously to the ground.[28] The skyscrapers had two means of defense against normal fire damage, Mr. Magnusson said. One, thick layers of insulation sprayed onto the steel beams, could have been breached by the initial crash, he said. Another, the building's sprinkler system, may have been disabled as well, or it may simply have been useless in the heat of the jet fuel fire.[2] If the sprinkler systems were broken by the impacts, the intense fires from the jets and their burning fuel would raise temperatures enough for the steel to "bend under the load" of the buildings' weight.[16]
Oliver McGee (Ohio State University) - Leakage of jet fuel through the building would have intensified the fire, contributing to the failure of structural steel, said Oliver McGee, chairman of the civil and environmental engineering and geodetic science department at Ohio State University. "Then the upper floors begin to collapse, because connections fail," said Dr. McGee. "One floor collapses on another. It's not designed to carry two floors, so it collapses on another one, and there's a domino effect." Dr. McGee said that engineers don't design buildings to withstand such an attack.[29]
Brian McIntyre (Skilling Ward Magnusson Barkshire) - "When the plane hit the corner of the building, we figure it had to take out a number of columns, but the tube system redistributed the force."[30]
Richard Miller (University of Cincinnati) - Though there was no immediate collapse, burning jet fuel overwhelmed the building's fireproofing and sprinklers, which were intended to withstand lesser blazes fed by paper or office furnishings. "In an aircraft hangar, you'd design [to resist] a jet fuel fire. Nobody would reasonably think to design the 85th floor of the World Trade Center that way."[30]
Gene Mojekwu (Matrix Engineering and adjunct professor at University of Illinois at Chicago's School of Architecture) - "The initial impact may have caused some local structural damage, but I think it was the resulting fire that weakened the steel throughout the building."[13]
Reed Mosher (U.S. Army Engineering Research and Development Center) - "My concern was the heat," Dr. Mosher said. "The burning may have caused a floor collapse, which had probably maybe 15 floors above it or more, and bringing all that weight down caused the rest of it to collapse."[11]
Masayuki Nagata (Yokohama National University) - "Jet fuel (may have) caught fire in a narrow structure and steel pillars were heated up to nearly 900 C and softened, resulting in their being unable to support the buildings."[31]
R. Shankar Nair (Chicago structural engineer, and former chairman of the Council on Tall Buildings and Urban Habitat) - The engineering investigations have only just begun, but already there is near-consensus as to the sequence of events that led to the collapse of the World Trade Center: The airplane impact and resulting fire some distance below the top of each tower caused structural failure near the site of the impact and collapse of the part of the building above that location. This collapsing mass set off progressive collapse of the rest of the building (what the press has been describing as a "pancaking" effect).[32]
Thomas J. Normile (Keast & Hood) - "A fire fueled by thousands of gallons of jet fuel is unlike any fire that a building would normally be subjected to." To some extent, being hit by a speeding jet did limit the towers' ability to withstand the high temperatures. The planes probably severed the sprinkler-system pipes, making it impossible for water to reach the blaze. "The heat built up," Normile explained. "Once that happened, the columns buckled, and the building came down on itself, progressively destroying the lower floors in the process."[33]
Kevin Parfitt (Penn State) - "When the planes come through, they cut through a number of those [perimeter] columns," Parfitt said. "At the same time, the planes are starting transcontinental flights, and they have full tanks of aviation fuel. You get a massive explosion and a fire." The initial jet fuel explosions most likely blew the insulation off the towers' girders, Parfitt suggested, incinerated easy combustibles and gave the ensuing fires free access to the unguarded steel. "Sprinklers aren't going to do too much in that situation," Parfitt said.[34]
Cesar Pelli, designer of the Petronas Towers in Kuala Lumpur, Malaysia, the world's tallest buildings, suggested that although "it will take structural engineers a long time to figure out exactly how" the towers collapsed, he agreed that "no building is prepared for this kind of stress."[34]
Housh Rahimzadeh (John Portman & Associates, structural engineer in Atlanta) - "When Housh Rahimzadeh saw the second plane crash into the World Trade Center last week, he knew exactly what would happen next. He wanted to whisper in a rescue commander's ear, and urge him to hold those firefighters back." During any fire, a floor in a high-rise building is supposed to maintain its integrity for two hours. But in the super-heated World Trade Center towers, that wouldn't happen. Once the steel framework reached a particular temperature, it would suddenly turn to licorice. And because of the towers' lack of interior supports, failure would be total as each floor pancaked onto the one below. Neither Rahimzadeh nor other engineers or architects had any criticism about the design of the World Trade Center. But in those crucial moments, Rahimzadeh said, "if someone had consulted a construction engineer, he would have told them what was going to happen." At the proper time, and with appropriate regard to the heroism shown by rescuers, the engineer said authorities will have to study the decision to send heavily laden firefighters into the second tower.[35]
Todd Rittenhouse (blast engineer in New York City) - "There was fire on 15 or 20 different floors," said Todd Rittenhouse, a blast engineer in New York City who has worked on a number of embassies and government buildings. "You never fight that many fires at once." As the temperature of steel rises, it loses its strength, causing the supporting girders to buckle. The columns do not have to break; a little bend is enough to cause problems, Rittenhouse said. Investigators are still piecing together facts such as how hot the towers got during the blaze, but Rittenhouse said the fires must have reached extreme temperatures.[6]
Masoud Sanayei (Tufts University - "The heat from fires weakened the steel structure system above the crash, and one or more floors collapsing caused a domino effect. The top floors kept falling, hitting floors below with a bigger and bigger weight," he said. "Nothing can stop it. That's why the building imploded into itself."[8] "The structure itself performed very well. After the impact, we saw the structure shook a little bit and it would have stood if there was no fire," said Masoud Sanayei, professor of civil and environmental engineering at Tufts University, "There was intense heat -- more than ordinary fires that buildings experience because of the large amount of jet fuel." He wouldn't give an estimate, but said it would be thousands of Fahrenheit degrees. "The fire was the cause of the actual collapse," he said. "Steel softens as a result of high temperatures, and then it can't carry as much load. When one of these floors collapses, then it hits the one below" with a heavy force, he said. "Then the next one (below is hit) and the next one, causing a domino effect," even in the bottom half of the building, which had suffered no damage in the initial crash or the fire. It implodes into itself. It is very difficult for anyone to survive this pancaking. There is no space between the slabs"[36]
Greg Schindler, president of the National Council of Structural Engineers Associations, said there was no way the twin towers could have been designed to withstand the double-whammy of an airplane strike and burning jet fuel.[33] "My sense was that initially, the buildings withstood the impact of the airplanes remarkably well," says Greg Schindler, president of the National Council of Structural Engineers Association. "Were it not for the ensuing tremendous fires, the buildings would have stood to be repaired." "Basically, the steel at a particular level where the fire was the worst and the crash damage was the worst could no longer support the weight of what was above it," says Schindler, a structural engineer in the Seattle office of KPFF Consulting Engineers. "So then the tremendous weight of the building above that point came crashing down - one floor falling upon the floor below it and weight accumulating on it. And then there's just simply no way of stopping it."[37]
Michael Scott (HNTB Architects Engineers Planners) - "A lot of buildings withstand a collision but, when subjected heat, it becomes a problem of structural failure. With a high-rise like that, you experience progressive collapse. If there is damage to an upper portion, it impacts on the floor below and it continues to the ground."[38]
Jim Snedegar (University of Missouri-Kansas City) - "When you load a steel beam to the point it's bent and deformed, it will continue to bend until it reaches the yield point and gives way. I think that's why it took 45 minutes for the upper half to collapse."[38]
British engineer Professor Alastair Soane, who worked on the towers before their official opening in 1973, said "They were extremely robust buildings and built to withstand a tremendous amount. But this was of course a completely abnormal situation and one which would not have been envisaged by the people who built it. The strength of the towers was enormous but they would not have been designed for aircraft strikes."[39]
Tim Wilkinson (University of Sydney) - The impact from the planes would have severely damaged the outer tube, but said it would not have been enough to cause collapse.[26]
Ronald F. Zollo (University of Miami) - the steel in the buildings likely melted under the intense heat, causing the skyscrapers to collapse under their own weight.[5]
Paul Zucchi - "What I suspect happened is that the fire, the intense heat over a period of time, weakened the columns and led to a progressive collapse," says Paul Zucchi, partner with the structural engineering firm of Yolles Partnership Inc. "In other words, the structure above came down on the structure below and it wasn't able to withstand (the load). So it may have had more to do with the fire than the ability of the building to withstand (the impact)."
The impact of the plane itself was not enough to cause the collapse, but the tremendous heat from the fire was the key factor.
No building is designed to withstand "that kind of assault".
Buildings are not designed to withstand the extreme levels of heat that would be found in the situation with the amount of jet fuel and the explosion that occurred
Fires were unlike any fire that a building would normally be subjected to.
The structure itself performed very well, and would have survived, had there been no fire.
The buildings performed well, not collapsing immediately from the impact and allowing an hour for evacuation.
The structural damage from the aircraft impact, combined with tremendous heat from the fire which weakened the remaining columns, reached a point where the columns lost their strength and buckled.
Load was shifted to adjacent members, which got overloaded, and eventually collapsed.
Heating of the steel structural members cause them to loose much of their structural strength, deforming at first, and then eventually to the point where they could not hold up the above structure, with steel fracturing.
The heating due to the fire caused the steel to melt.
Fireproofing was damaged or dislodged, allowing the steel structural elements to heat up more rapidly.
Office furnishings, computers, and other office contents were also a factor in fueling the fire and extreme heat.
The height of the fire within the building made the fires even more of a challenge.
The sprinkler system might not have functioned, allowing the fires to burn uncontrolled.
The sprinklers should have maintained the buildings internal strength for several hours.
Once some columns (or other structural elements) failed and the collapse initiated, progressive collapse was inevitable.
The planes weakened the buildings' structures at key points. Any higher, and the plane impact would have damaged just the tops, much lower and the base of the buildings were much stronger and could have withstood the damage.
Discounted the notion that controlled demolition or additional explosives were involved.
It will take structural engineers a long time to figure out exactly how the towers collapsed.
The Twin Towers should not have collapsed so quickly.
The towers should have withstood the fires, which reached up to 1,000 C., which is within the design limits for the towers.
Even among designers of skyscrapers, not many of us can honestly say that we would have anticipated this particular chain of events leading to complete structural collapse from an airplane strike near the top of a tall building.
The collapse of the World Trade Center came as a surprise to engineers.
There will be debate on whether or not the World Trade Center Towers should have collapsed in the way that they did.
The collapses well beyond realistic experience.
I don't think anyone imagined that the building would collapse that quickly.
Thought/anticipated that the towers would collapse.
Bazant published a draft paper on September 13, 2001, with a simplified analysis of the World Trade Center collapse. Bazant suggested that the cause of the World Trade Center collapse was the heating of the steel columns, with the fire reaching sustained temperatures estimated at 800°C. Bazant suggested that the aircraft impact and initial blast from the impact caused the thermal protection (fireproofing) to be dislodged, thereby accelerating the heating of the steel columns. With such high temperatures, the strength of the steel decreases substantially and creep (viscoelastic deformation) occurs. As deformation and heating occur, the columns eventually lose their ability to support the weight of the above structure, and some columns may fracture. This, then causes some floor structures to collapse onto the floors beneath, applying some amount of added kinetic energy, making the dynamic load far beyond what the underlying structure can support. This triggers further progressive collapse of the entire structure.
Ronald O. Hamburger, a structural engineer and member of the initial ASCE investigative team, explained "Steel is born of fire. As it's reheated, it expands and loses its rigidity. Above 1,000 degrees Fahrenheit, it loses a significant amount of its strength." He said the extreme heat from the fires might have caused the steel floors to expand and bow, which may have caused the support columns to bend inward and buckle. Heat also may have caused the steel flooring to separate from the columns, or the columns themselves may have heated up and buckled outward.[40]