By Pamela Skillings. The two identical story "Twin Towers" of the World Trade Center officially opened in and went on to become New York City icons and key elements of Manhattan's famous skyline. Once home to almost businesses and approximately 50, employees, the World Trade Center towers were tragically destroyed in the terrorist attacks of September 11, In , the New York State Legislature authorized the development of a "world trade mart" in downtown Manhattan, a concept that was the brainchild of real estate developer David Sholtz. However, it wasn't until that Chase Manhattan Bank vice chair David Rockefeller announced plans to build a multi-million-square-foot complex on Lower Manhattan's east side.
Rivera called the television ads "not so easy to dismiss as those demonstrators were," and stated that, "If explosives were involved, that would mean the most obnoxious protesters in recent years Retrieved April 26, When the buildings collapsed, some thought it had all of the characteristics of a "controlled demolition. Retrieved July 9, Most observers construed this initially to be an accident involving a small commuter plane. In their view, the towers were uniquely vulnerable to the effects of Destroyed tower twin fires on several floors at the same time.
Reading faces for business and pleasure. A history of the Manhattan landmark destroyed on September 11, 2001
November 24, Each of the World Trade Center towers had high-strength, load-bearing perimeter steel columns which acted as Vierendeel trusses. Al Qaida has taken the blame, So, the Taliban did it. Hidden categories: CS1 errors: chapter ignored Webarchive template wayback links All articles with dead external links Articles with dead external links from Bleach image galleries Articles with permanently dead external links Wikipedia semi-protected pages Wikipedia indefinitely move-protected pages Use mdy dates from February Coordinates on Wikidata Articles containing potentially dated statements How to look for kiddie porn July All articles containing potentially dated statements Articles with specifically marked weasel-worded phrases from March Wikipedia articles needing page number citations from July All articles with unsourced Destroyed tower twin Articles with unsourced statements from November Pages using New York City Subway service templates. The Twin Towers were destroyed in a terrorist attack on September 11, Barnett was a member of the World Trade Centre Building Performance Study, one of the government groups that investigated the towers' collapse. Archived from the original on October Destroyed tower twin, It took fourteen months for the slurry wall to be completed. Dewey to begin developing plans for the project,  but the plans were put on hold in CBS News. Existing questions. What's Going On. The project was seen as being monolithic and overambitious,  with the design having had no public input. The Port Authority renovated the observatory inthen leased it to Ogden Entertainment to operate.
Evidence, he says, of the biggest cover-up in history - one too evil for most to believe, but one he has staked his academic career on exposing.
- It opened on April 4, , and was destroyed in during the September 11 attacks.
- Hijackers associated with the Islamic terrorist group Al Qaeda committed suicide by flying passenger aircraft into the two World Trade Center towers, and also into the Pentagon, on September 11,
- New York City 's World Trade Center was destroyed during the terrorist attacks of September 11, , after being struck by two hijacked commercial airliners.
- For those who do not address the physics, mechanics or forensic evidence relating to the destruction of the Twin Tower, the account of the dozen survivors from inside the North Tower — the so called "Miracle of Ladder 6" — is a very compelling story.
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World Trade Center , complex of several buildings around a central plaza in New York City that in was the site of the deadliest terrorist attack in American history. See September 11 attacks. The complex—located at the southwestern tip of Manhattan , near the shore of the Hudson River and a few blocks northwest of Wall Street —was built by the Port Authority of New York and New Jersey as a central facility for businesses and government agencies involved in international trade.
Until the attack, it was notable for its huge twin towers , each of which had stories, that formed a distinctive feature of the New York skyline. Each of the twin towers had 97 passenger elevators, 21, windows , and roughly an acre 0.
An observation deck was situated on the th floor of the south tower Two World Trade Center , and a television-broadcasting mast feet metres high was attached to the north tower One World Trade Center. Prior to the complex had sustained a major terrorist attack. On February 26, , a large bomb planted by terrorists exploded in the underground garage of Two World Trade Center, damaging the base of the building subsequently repaired , killing 6 people, and injuring some 1, The attack that occurred on September 11, , was much more destructive.
Hijackers deliberately flew two commercial airliners into the towers, the first striking the north tower at am local time and the second hitting the south tower at Badly damaged and engulfed in flames, the weakened south tower collapsed at ; the north tower fell about a half hour later.
Debris from the towers also destroyed or damaged other buildings in and around the complex. Some 2, people were killed. Workers spent nearly a year removing the debris and recovering bodies from the ruins at the World Trade Center complex site, and an intense national discussion ensued on how best to rebuild it. Two design competitions were launched, beginning in the summer of one to replace the fallen towers and the other for a memorial to the victims of both the and attacks.
A winning building design was announced in early Several other buildings were slated for the complex site, and 4 World Trade Center, with a height of feet metres , opened in The winning design for the memorial was announced in January ; the plans for it were also revised unveiled in June The memorial opened to the public on September 12, World Trade Center. Article Media. Info Print Cite. Submit Feedback. Thank you for your feedback. See Article History.
Start Your Free Trial Today. Learn More in these related Britannica articles:. September 11 attacks , series of airline hijackings and suicide attacks committed in by 19 militants associated with the Islamic extremist group al-Qaeda against targets in the United States, the deadliest terrorist attacks on American soil in U. The attacks against New York City and…. A far more devastating attack—the deadliest terrorist act in American history—occurred on September 11, , when hijackers intentionally flew two airliners into….
Most observers construed this initially to be an accident involving a small commuter plane. The second plane, United Airlines flight , also from Boston, struck the south tower 17 minutes later. At this point there was no doubt…. History at your fingertips. Sign up here to see what happened On This Day , every day in your inbox! By signing up, you agree to our Privacy Notice. Be on the lookout for your Britannica newsletter to get trusted stories delivered right to your inbox.
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The scale of the destruction initially puzzled engineers, who had expected the buildings to survive airplane impacts. Archived from the original on November 3, Retrieved March 11, Why did the terrorists choose the twin towers to destroy? The twin towers were destroyed on September 11, Wikimedia Commons has media related to World Trade Center — Those who criticise the Bush administration's "war on terror" are accused of being unpatriotic.
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Once the upper section of the building began to move downwards, a total progressive collapse was unavoidable. The cleanup of the World Trade Center site involved round-the-clock operations, many contractors and subcontractors, and cost hundreds of millions of dollars. The demolition of the surrounding damaged buildings continued even as new construction proceeded on the Twin Towers' replacement, One World Trade Center , which was opened in November As of July [update] , five new buildings had been erected on the site; the last one, Two World Trade Center , is scheduled for completion in Upon completion, the Twin Towers were briefly the tallest buildings in the world, and at the time of the terrorist attacks they were still in the top five.
Though fire studies and even an analysis of the impacts of low speed jet aircraft impacts had been undertaken prior to their completion, the full scope of those studies no longer exists.
Nevertheless, since fire had never before caused a skyscraper to collapse and aircraft impacts had been considered in their design, their destruction initially came as a surprise to many in the engineering community.
The towers were designed as "framed tube" structures, which provided tenants with open floor plans uninterrupted by columns or walls. Numerous, closely spaced perimeter columns provided much of the strength to the structure, along with gravity load shared with the steel box columns of the core.
Above the tenth floor, there were 59 perimeter columns along each face of the building, and there were 47 heavier columns in the core. All of the elevators and stairwells were located in the core, leaving a large column-free space between it and the perimeter that was bridged by prefabricated floor trusses. A grid of lightweight bridging trusses and main trusses supported the floors with shear connections to the concrete slab for composite action. The top chords of the trusses were bolted to seats welded to the spandrels on the exterior side and a channel welded to interior box columns on the interior side.
The floors were connected to the perimeter spandrel plates with viscoelastic dampers, which helped reduce the amount of sway felt by building occupants. The towers also incorporated a "hat truss" or "outrigger truss" located between the th and th floors, which consisted of six trusses along the long axis of core and four along the short axis.
This truss system allowed optimized load redistribution of floor diaphragms between the perimeter and core, with improved performance between the different materials of flexible steel and rigid concrete allowing the moment frames to transfer sway into compression on the core, which also mostly supported the transmission tower. The structural engineers working on the World Trade Center considered the possibility that an aircraft could crash into the building.
In July , a B bomber that was lost in the fog had crashed into the 79th floor of the Empire State Building. However, in an interview with the BBC , Robertson claimed that, "with the , the fuel load was not considered in the design, I don't know how it could have been considered. The original documentation of the study, which was part of the building's 1,page structural analysis, was lost when the records in the Port Authority of New York and New Jersey 's offices were destroyed in the collapse of the 1 WTC; the copy was lost in 7 WTC.
There would be a horrendous fire. A lot of people would be killed", he said. After the bombing , inspections found fireproofing to be deficient.
During the September 11th attacks , four teams of al-Qaeda terrorists hijacked four different jetliners. NIST estimated that each floor of both buildings contained around four pounds per square foot 60 tons per floor of combustibles. The light construction and hollow nature of the structures allowed the jet fuel to penetrate far inside the towers, igniting many large fires simultaneously over a wide area of the impacted floors.
The fuel from the planes burned at most for a few minutes, but the contents of the buildings burned over the next hour or hour and a half. Debris and fuel would likely have remained mostly outside the buildings or concentrated in more peripheral areas away from the building cores, which would then not have become unique failure points. In this scenario, the towers might have stood far longer, perhaps indefinitely.
The fires were hot enough to weaken the columns and cause floors to sag, pulling perimeter columns inward and reducing their ability to support the mass of the building above. Almost all the deaths in the Twin Towers occurred in the zones above the points of aircraft impact.
As the North Tower had been struck directly midway into the structure, the three stairways in the tower core were all damaged or blocked by debris preventing escape to lower floors. In the South Tower, the impact was slightly off center to the central section of the tower and stairway A in the northwest portion of the central core was only partially blocked, and 14 to 18 civilians managed to escape from the point of aircraft impact and the floors above that. The exact numbers of who perished and where in some cases is not precisely known; however the National Institute of Standards and Technology report indicated that a total of 1, civilians perished at or above the impact point in the North Tower with hundreds estimated to have been killed at the moment of impact.
In the South Tower, civilians perished at the impacted floors and the floors above that. Fewer than of the civilian fatalities occurred in the floors below the impact points but all civilian passengers and crew on the two aircraft as well as all 10 terrorists perished, along with at least 18 people on the ground and in adjacent structures.
The total death toll for civilian and non-civilians is estimated to be 2, persons. The destruction of Twin Towers has been called "the most infamous paradigm" of progressive collapse. The propagating action is the vertical impact force," the principal forces are parallel, and the primary load transfer is serial. Under these conditions, the towers collapsed symmetrically and more or less straight down, though there was some tilting of the tops of the towers and a significant amount of fallout to the sides.
In both cases, the section of the building that had been damaged by the airplanes failed, which allowed the floors above the impact zone to fall onto the undamaged structure below. As the collapse progressed, dust and debris could be seen shooting out of the windows several floors below the advancing destruction, caused by the sudden rush of air from the upper levels.
During each collapse, large portions of the perimeter columns and possibly the cores were left without any lateral support, causing them to fall laterally towards the outside, pushed by the increasing pile of rubble. The result was that the walls peeled off and separated away from the buildings by a large distance about feet in some cases , hitting other neighboring buildings, and starting fires that would later lead to the collapse of Building 7.
Some connections broke as the bolts snapped, leaving many panels randomly scattered. The lower portions of both buildings' cores 60 stories of WTC 1 and 40 stories of WTC 2 remained standing for up to 25 seconds after the start of the initial collapse before they too collapsed.
After the planes struck the buildings, but before the buildings collapsed, the cores of both towers consisted of three distinct sections. Above and below the impact floors, the cores consisted of what were essentially two rigid boxes; the steel in these sections was undamaged and had undergone no significant heating. The section between them, however, had sustained significant damage and, though they were not hot enough to melt it, the fires were weakening the structural steel. As a result, the core columns were slowly being crushed, sustaining plastic and creep deformation from the weight of floors above.
As the top section tried to move downward, however, the hat truss redistributed the load to the perimeter columns. Meanwhile, the perimeter columns and floors were also being weakened by the heat of the fires, and as the floors began to sag they pulled the exterior walls inwards. Later, the south wall of 1 WTC buckled in the same way, and with similar consequences. Structural systems respond very differently to static and dynamic loads and, while the towers were designed to support enormous weight under normal conditions, they provided little resistance to the moving mass of the section above the damaged floors.
In both cases, the collapses began with the drop of the upper section through the height of at least one story roughly three meters or ten feet , yet a fall of only half a meter about 20 inches would have released the necessary energy to begin an unstoppable collapse. From there collapse proceeded through two phases. During the crush-down phase, the upper block destroyed the structure below in a progressive series of column failures roughly one story at a time.
Each failure began with the impact of the upper block on the columns of the lower section, mediated by a growing layer of rubble consisting mainly of concrete from the floor slabs. The energy from each impact was "reintroduced into the structure in [the] subsequent impact, After the columns buckled the block was once again unsupported and fell through the distance of that story, again impacting the columns of the story below, which then buckled in the same way.
This repeated until the upper block reached the ground and the crush-up phase began. Here, too, the columns buckled one story at a time, now starting from the bottom.
As each story failed, the remaining block fell through the height of the story, onto the next one, which it also crushed, until the roof finally hit the ground. As the fires continued to burn, occupants trapped in the upper floors of the South Tower provided information about conditions to dispatchers. They escaped via Stairwell A, the only stairwell which had been left intact after the impact. Numerous police hotline operators who received calls from individuals inside the South Tower were not well informed of the situation as it rapidly unfolded.
Many operators told callers not to descend the tower on their own, even though it is now believed that Stairwell A was most likely passable at and above the point of impact.
Large pieces are hanging up there". Due to radio communications problems , firefighters inside the towers did not hear the evacuation order from their supervisors on the scene, and most were unaware that the other tower had collapsed. Also, water pressure was low, with little or no water to feed sprinklers.
They attempted to extinguish small pockets of fire, but low water pressure hindered their efforts. In August , Fiterman Hall was scheduled for dismantling. Many of the surrounding buildings were also either damaged or destroyed as the towers fell. Other buildings destroyed include St. Customs 6 WTC. One Liberty Plaza survived structurally intact but sustained surface damage including shattered windows. The Deutsche Bank Building, which was covered in a large black "shroud" after September 11 to cover the building's damage, was deconstructed because of water, mold, and other severe damage caused by the neighboring towers' collapse.
In the immediate aftermath of the attacks, numerous structural engineers and experts spoke to the media, describing what they thought caused the towers to collapse. Abolhassan Astaneh-Asl , a structural engineering professor at the University of California at Berkeley , explained that the high temperatures in the fires weakened the steel beams and columns, causing them to become "soft and mushy", and eventually they were unable to support the structure above.
Astaneh-Asl also suggested that the fireproofing became dislodged during the initial aircraft impacts. He also explained that, once the initial structural failure occurred, progressive collapse of the entire structure was inevitable.
Once more than half of the columns on a particular floor buckled, the overhead structure could no longer be supported and complete collapse of the structures occurred. Immediately following the collapses, there was some confusion about who had the authority to carry out an official investigation. While there are clear procedures for the investigation of aircraft accidents, no agency had been appointed in advance to investigate building collapses.
The investigation was criticized by some engineers and lawmakers in the U. It had little funding, no authority to demand evidence, and limited access to the WTC site. One major point of contention at the time was that the cleanup of the WTC site was resulting in the destruction of the majority of the buildings' steel components. FEMA published its report in May While NIST had already announced its intention to investigate the collapses in August of the same year, by September 11, a year after the disaster , there was growing public pressure for a more thorough investigation.
FEMA suggested that fires in conjunction with damage resulting from the aircraft impacts were the key to the collapse of the towers. This is because the fires were originally said to have "melted" the floors and columns. The large quantity of jet fuel carried by each aircraft ignited upon impact into each building. A significant portion of this fuel was consumed immediately in the ensuing fireballs. The remaining fuel is believed either to have flowed down through the buildings or to have burned off within a few minutes of the aircraft impact.
The heat produced by this burning jet fuel does not by itself appear to have been sufficient to initiate the structural collapses. The heat output from these fires is estimated to have been comparable to the power produced by a large commercial power generating station.
Over a period of many minutes, this heat induced additional stresses into the damaged structural frames while simultaneously softening and weakening these frames. This additional loading and the resulting damage were sufficient to induce the collapse of both structures. The scope of the NIST investigation was focused on identifying "the sequence of events" that triggered the collapse, and did not include detailed analysis of the collapse mechanism itself after the point at which events made the collapse inevitable.
NIST developed several highly detailed structural models for specific sub-systems such as the floor trusses as well as a global model of the towers as a whole which is less detailed. These models are static or quasi-static, including deformation but not the motion of structural elements after rupture as would dynamic models. So, the NIST models are useful for determining how the collapse was triggered, but do not shed light on events after that point.
James Quintiere, professor of fire protection engineering at the University of Maryland, called the spoliation of the steel "a gross error" that NIST should have openly criticized. Gene Corley , P. The report did not reach conclusions about the cause of the collapse and called for further investigation. The bulk of the investigation of 7 World Trade Center was delayed until after reports were completed on the collapse of the World Trade Center twin towers. However, the current study does benefit greatly from the significant technological advances achieved and lessons learned from our work on the towers.
But the lack of water to fight the fire was an important factor. The fires burned out of control during the afternoon, causing floor beams near Column 79 to expand and push a key girder off its seat, triggering the floors to fail around column 79 on Floors 8 to With a loss of lateral support across nine floors, Column 79 soon buckled — pulling the East penthouse and nearby columns down with it. With the buckling of these critical columns, the collapse then progressed east-to-west across the core, ultimately overloading the perimeter support, which buckled between Floors 7 and 17, causing the entire building above to fall downward as a single unit.
From collapse timing measurements taken from a video of the north face of the building, NIST observed that the building's exterior facade fell at free fall acceleration through a distance of approximately 8 stories 32 meters, or feet , noting "the collapse time was approximately 40 percent longer than that of free fall for the first 18 stories of descent.
The collapse of the old 7 World Trade Center is remarkable because it was the first known instance of a tall building collapsing primarily as a result of uncontrolled fires.
Recognizing that current building codes are drawn to prevent loss of life rather than building collapse, the main point of NIST's recommendations is that buildings should not collapse from fire even if sprinklers are unavailable. They provisionally concluded the fires alone, without any damage from the airplanes, could have been enough to bring down the buildings. In their view, the towers were uniquely vulnerable to the effects of large fires on several floors at the same time.
On September 22, a preliminary cleanup plan was delivered by Controlled Demolition, Inc. CDI of Phoenix, Maryland. This caused confrontations with firefighters. The collapse of the World Trade Center produced enormous clouds of dust that covered Manhattan for days. Dust from the collapse seriously reduced air quality and is likely the cause of many respiratory illnesses in lower Manhattan.
Asbestosis is such an illness, and asbestos would have been present in the dust. Health effects also extended to residents, students, and office workers of Lower Manhattan and nearby Chinatown. From Wikipedia, the free encyclopedia. See also: Construction of the World Trade Center. See also: Casualties of the September 11 attacks and Emergency workers killed in the September 11 attacks. Main article: World Trade Center — Main article: 7 World Trade Center. New York City portal.
It is unclear whether the effect of jet fuel and aircraft contents was a consideration in the original building design, but this study is in line with remarks made by John Skilling following the WTC bombing. Without original documentation for either study, NIST said any further comments would amount to speculation.
By this time, only the fireproofing of the lower 40 floors of the north tower had been completed, and more than half of this was later replaced before the building was completed. NIST noted that upon reviewing the building records, it could not determine how this thickness was arrived at. NIST further concluded that the aircraft impact removed a significant portion of the fireproofing, contributing to the buildings' collapse, though many engineers, including one of NIST's top advisers, strongly disputed that claim, instead saying the fireproofing was simply not thick enough.
CBC News. October 29, Retrieved July 18, National Institute of Standards and Technology. April 5, Retrieved November 1, September 11, The New York Times. Retrieved August 29, The collapse of WTC 7 was also unusual, he admits. However, firefighters do not usually let a fire rage unabated for seven hours as they did on the morning of September 11, because they had prioritised the rescue of victims.
Since the attacks, the US government has issued three reports into the events of the day, all of which involved hundreds of professors, scientists and government officials. The National Institute of Standards and Technology, a government agency, filed 10, pages of reports examining the towers' collapse. And the Federal Emergency Management Agency weighed in, examining the response to the attacks.
It's too huge a project and would require far too many people to keep it a secret afterwards," says Christopher Pyle, professor of constitutional law at Mt Holyoke College. It's a waste of time. But Barrett says the experts have been fooled by an "act of psychological conversion" not unlike the tactics CIA interrogators use on their victims. And then, when the voice of authority told us what happened, we just believed it.
History has revealed that governments have a tradition of misleading the public into going to war, says Barrett, and the next generation of Americans will realise the truth. However, a study of the full extent of their claims is a journey into the increasingly absurd: Flight 93 did not crash in Pennsylvania but landed safely in Cleveland; desperate phone calls received by relatives on the ground from passengers were actually computer-generated voices from a laboratory in California.
The Pentagon was not hit by American Airlines Flight 77, but by a smaller, remote-controlled A-3 Sky Warrior, which shot a missile into the building before crashing into it. He recounts a history of religious and political leaders using conspiracy theories for personal and political gain.
The common enemy is usually Jews, big government or corporations. The public laps it up, either because these theories are more exciting than the truth, or out of emotional need.
It's a breathless account. They are willing to say hearsay is a fact, and rumour is true, and accidents are never what they seem. But what is missing is: what actually happened to the plane and the people on it? Conspiracy theorists avoid discussion of those facts that don't fit. Perhaps it is no coincidence that the public's willingness to believe conspiracy theories parallels their dissatisfaction with the Bush administration. Many fear infringements on their civil liberties now the National Security Agency has gained access to phone billing records from telecommunications companies, the Bush administration has engaged in wiretapping without court warrants and there are thousands of cases of indefinite detentions of American and foreign citizens without trial.
Those who criticise the Bush administration's "war on terror" are accused of being unpatriotic. By taking their criticisms to such extremes, though, the scholars risk caricaturing the opposition. None the less, they are pushing on, and imploring Congress to reopen the investigation.
We don't want to be lumped in with those folks.
Who really blew up the twin towers? | Education | The Guardian
Barnett, R. Biederman, and R. Sisson, Jr. The collapse of the World Trade Center WTC towers on September 11, , was as sudden as it was dramatic; the complete destruction of such massive buildings shocked nearly everyone. Immediately afterward and even today, there is widespread speculation that the buildings were structurally deficient, that the steel columns melted, or that the fire suppression equipment failed to operate. In order to separate the fact from the fiction, we have attempted to quantify various details of the collapse.
The major events include the following:. Each will be discussed separately, but initially it is useful to review the overall design of the towers. The towers were designed and built in the mids through the early s.
They represented a new approach to skyscrapers in that they were to be very lightweight and involved modular construction methods in order to accelerate the schedule and to reduce the costs. To a structural engineer, a skyscraper is modeled as a large cantilever vertical column. Each tower was 64 m square, standing m above street level and 21 m below grade.
This produces a height-to-width ratio of 6. The total weight of the structure was roughly , t, but wind load, rather than the gravity load, dominated the design. This permitted windows more than one-half meter wide. It also housed the elevators, the stairwells, and the mechanical risers and utilities. Web joists 80 cm tall connected the core to the perimeter at each story. Concrete slabs were poured over these joists to form the floors. In essence, the building is an egg-crate construction that is about 95 percent air, explaining why the rubble after the collapse was only a few stories high.
The egg-crate construction made a redundant structure i. Prior to the World Trade Center with its lightweight perimeter tube design, most tall buildings contained huge columns on 5 m centers and contained massive amounts of masonry carrying some of the structural load.
The early news reports noted how well the towers withstood the initial impact of the aircraft; however, when one recognizes that the buildings had more than 1, times the mass of the aircraft and had been designed to resist steady wind loads of 30 times the weight of the aircraft, this ability to withstand the initial impact is hardly surprising.
The only individual metal component of the aircraft that is comparable in strength to the box perimeter columns of the WTC is the keel beam at the bottom of the aircraft fuselage. While the aircraft impact undoubtedly destroyed several columns in the WTC perimeter wall, the number of columns lost on the initial impact was not large and the loads were shifted to remaining columns in this highly redundant structure.
The ensuing fire was clearly the principal cause of the collapse Figure 4. The fire is the most misunderstood part of the WTC collapse. Even today, the media report and many scientists believe that the steel melted. It is argued that the jet fuel burns very hot, especially with so much fuel present. This is not true. Part of the problem is that people including engineers often confuse temperature and heat.
While they are related, they are not the same. Thermodynamically, the heat contained in a material is related to the temperature through the heat capacity and the density or mass. Temperature is defined as an intensive property, meaning that it does not vary with the quantity of material, while the heat is an extensive property, which does vary with the amount of material.
One way to distinguish the two is to note that if a second log is added to the fireplace, the temperature does not double; it stays roughly the same, but the size of the fire or the length of time the fire burns, or a combination of the two, doubles. Thus, the fact that there were 90, L of jet fuel on a few floors of the WTC does not mean that this was an unusually hot fire.
The temperature of the fire at the WTC was not unusual, and it was most definitely not capable of melting steel. In combustion science, there are three basic types of flames, namely, a jet burner, a pre-mixed flame, and a diffuse flame.
A jet burner generally involves mixing the fuel and the oxidant in nearly stoichiometric proportions and igniting the mixture in a constant-volume chamber. Since the combustion products cannot expand in the constant-volume chamber, they exit the chamber as a very high velocity, fully combusted, jet. This is what occurs in a jet engine, and this is the flame type that generates the most intense heat.
In a pre-mixed flame, the same nearly stoichiometric mixture is ignited as it exits a nozzle, under constant pressure conditions. It does not attain the flame velocities of a jet burner. An oxyacetylene torch or a Bunsen burner is a pre-mixed flame. A fireplace flame is a diffuse flame burning in air, as was the WTC fire. Diffuse flames generate the lowest heat intensities of the three flame types. If the fuel and the oxidant start at ambient temperature, a maximum flame temperature can be defined.
This maximum flame temperature is reduced by two-thirds if air is used rather than pure oxygen. The reason is that every molecule of oxygen releases the heat of formation of a molecule of carbon monoxide and a molecule of water. If pure oxygen is used, this heat only needs to heat two molecules carbon monoxide and water , while with air, these two molecules must be heated plus four molecules of nitrogen. Thus, burning hydrocarbons in air produces only one-third the temperature increase as burning in pure oxygen because three times as many molecules must be heated when air is used.
But it is very difficult to reach this maximum temperature with a diffuse flame. There is nothing to ensure that the fuel and air in a diffuse flame are mixed in the best ratio.
Typically, diffuse flames are fuel rich, meaning that the excess fuel molecules, which are unburned, must also be heated. This fuel-rich diffuse flame can drop the temperature by up to a factor of two again. Some reports suggest that the aluminum from the aircraft ignited, creating very high temperatures. While it is possible to ignite aluminum under special conditions, such conditions are not commonly attained in a hydrocarbon-based diffuse flame. In addition, the flame would be white hot, like a giant sparkler.
There was no evidence of such aluminum ignition, which would have been visible even through the dense soot. It was noted above that the wind load controlled the design allowables. The WTC, on this low-wind day, was likely not stressed more than a third of the design allowable, which is roughly one-fifth of the yield strength of the steel. The additional problem was distortion of the steel in the fire. The temperature of the fire was not uniform everywhere, and the temperature on the outside of the box columns was clearly lower than on the side facing the fire.
The temperature along the 18 m long joists was certainly not uniform. This produced distortions in the slender structural steel, which resulted in buckling failures. Thus, the failure of the steel was due to two factors: loss of strength due to the temperature of the fire, and loss of structural integrity due to distortion of the steel from the non-uniform temperatures in the fire.
Nearly every large building has a redundant design that allows for loss of one primary structural member, such as a column. However, when multiple members fail, the shifting loads eventually overstress the adjacent members and the collapse occurs like a row of dominoes falling down.
The perimeter tube design of the WTC was highly redundant. It survived the loss of several exterior columns due to aircraft impact, but the ensuing fire led to other steel failures. With a Pa floor design allowable, each floor should have been able to support approximately 1, t beyond its own weight.
The total weight of each tower was about , t. As the joists on one or two of the most heavily burned floors gave way and the outer box columns began to bow outward, the floors above them also fell.
The floor below with its 1, t design capacity could not support the roughly 45, t of ten floors or more above crashing down on these angle clips. This started the domino effect that caused the buildings to collapse within ten seconds, hitting bottom with an estimated speed of km per hour.
There are several points that should be made. First, the building is not solid; it is 95 percent air and, hence, can implode onto itself. Second, there is no lateral load, even the impact of a speeding aircraft, which is sufficient to move the center of gravity one hundred feet to the side such that it is not within the base footprint of the structure. Third, given the near free-fall collapse, there was insufficient time for portions to attain significant lateral velocity. To summarize all of these points, a , t structure has too much inertia to fall in any direction other than nearly straight down.
The World Trade Center was not defectively designed. No designer of the WTC anticipated, nor should have anticipated, a 90, L Molotov cocktail on one of the building floors. Skyscrapers are designed to support themselves for three hours in a fire even if the sprinkler system fails to operate. This time should be long enough to evacuate the occupants. No normal office fires would fill 4, square meters of floor space in the seconds in which the WTC fire developed. Usually, the fire would take up to an hour to spread so uniformly across the width and breadth of the building.
This was a very large and rapidly progressing fire very high heat but not unusually high temperature. After all, 1,, t of rubble will require 20, to 30, truckloads to haul away the material.
The asbestos fire insulation makes the task hazardous for those working nearby. Separation of the stone and concrete is a common matter for modern steel shredders. The land-filling of , t of concrete and stone rubble is more problematic.
There will undoubtedly be a number of changes in the building codes as a result of the WTC catastrophe. For example, emergency communication systems need to be upgraded to speed up the notice for evacuation and the safest paths of egress. Emergency illumination systems, separate from the normal building lighting, are already on the drawing boards as a result of lessons learned from the WTC bombing in There will certainly be better fire protection of structural members.
Protection from smoke inhalation, energy-absorbing materials, and redundant means of egress will all be considered. A basic engineering assessment of the design of the World Trade Center dispels many of the myths about its collapse. First, the perimeter tube design of the towers protected them from failing upon impact. The outer columns were engineered to stiffen the towers in heavy wind, and they protected the inner core, which held the gravity load.
Removal of some of the outer columns alone could not bring the building down.