Frank Culbertson helplessly watched from space. While miles above the Earth aboard the International Space Station, Culbertson took incredible pictures of the Twin Towers after they were struck by the hijacked jets. Scroll down for video. He was the only American off-planet at the time. As word of the terror strike reached him, the station was passing over New York, and what he saw had a profound effect on the astronaut.
First, the perimeter tube design of the towers protected them from failing upon impact. There would be a horrendous fire. As bufning pancaked, all that air—along with towerw concrete and other debris pulverized by the force of the collapse—was ejected with enormous energy. There was no evidence of such aluminum ignition, which would have been visible even through the dense soot. Others say the architectural design actually saved lives. Third, given the near free-fall collapse, there was insufficient time for portions to attain significant lateral velocity.
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September : Following the September 11, terrorist attacks against the United States, a wave of conspiracy theories swept the nation. Asked in September 11 Attacks When were the twin towers took down? Each "sharp spike of short duration," says Prisonplanet. Jose Jimenez Getty Images. Archived from the original on April 19, Type keyword s to burnign. The towers were designed as "framed tube" structures, which provided tenants with open floor plans uninterrupted by columns or walls. Plus, the lobbies of both buildings were visibly damaged before the towers collapsed. NIST developed several highly detailed structural models for specific sub-systems such as the floor trusses as well as a The twin towers burning down model of the towers as a whole which is less detailed. Each of the twin towers had columns around a central core that housed the elevators, stairwells, mechanical systems, and utilities. Collapse tsin the World Trade Center category. Also, water pressure was low, with little or no water to feed sprinklers. Tall office buildings are required to have more durable fireproofing, extra emergency exits, and many other fire safety features. In the years since the terrorist attacks in New York City, individual engineers and committees of experts have studied the crumpling of the World Trade Center twin towers. During The twin towers burning down collapse, large portions Tracy bingham nude pic 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 main factor was really the size of the fire -- the total area it covered.
- New York City 's World Trade Center was destroyed during the terrorist attacks of September 11, , after being struck by two hijacked commercial airliners.
- Read on to learn how to reveal the towers.
- The twin towers of the World Trade Centre in New York were destroyed in a terrorist suicide attack on September 11,
- Now a materials scientist has come up with a more scientific explanation for the mystery booms, and says his model of the Twin Towers collapse leaves no room for conspiracies.
- Barnett, R.
- September : Following the September 11, terrorist attacks against the United States, a wave of conspiracy theories swept the nation.
New York City 's World Trade Center was destroyed during the terrorist attacks of September 11, , after being struck by two hijacked commercial airliners. As a result of the attacks to the towers, a total of 2, people died including 2, civilians , firefighters , and 71 law enforcement officers as well as all the passengers and crew on the airplanes, including civilians and the 10 hijackers.
The scale of the destruction initially puzzled engineers, who had expected the buildings to survive airplane impacts. In September , the National Institute of Standards and Technology published the results of its investigation into the collapse. The investigators did not find anything substandard in the design of the twin towers, noting that the severity of the attacks was beyond anything experienced in buildings in the past.
They determined the fires to be the main cause of the collapses, finding that sagging floors pulled inward on the perimeter columns, causing them to bow and then to buckle. Once the upper section of the building began to move downwards, a total progressive collapse was unavoidable. The cleanup of the World Trade Center site involved round-the-clock operations, many contractors and subcontractors, and cost hundreds of millions of dollars.
The demolition of the surrounding damaged buildings continued even as new construction proceeded on the Twin Towers' replacement, One World Trade Center , which was opened in November 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 report issued by Lamont-Doherty includes various graphs showing the seismic readings produced by the planes crashing into the two towers as well as the later collapse of both buildings. CAD Digest. This helped me a lot because I like pictures more than words. September 11, December 12,
The twin towers burning down. Watch Next
The remaining rubble was only a few stories high. The twin towers were built between and No building constructed at that time would have been able to withstand the impact of the terrorist attacks in We can, however, learn from the collapse of the skyscrapers and take steps to construct safer buildings and minimize the number of casualties in future disasters.
When the twin towers were constructed, the builders were granted some exemptions from New York's building codes. The exemptions allowed the builders to use lightweight materials so the skyscrapers could achieve great heights. Others say the architectural design actually saved lives. These skyscrapers were designed with redundancies—anticipating that a small plane could accidentally penetrate the skyscraper skin and the building would not fall from that type of accident. The north tower was hit at a.
ET, between floors 94 and 98—it did not collapse until a. Even the south tower was able to stand for a remarkable 56 minutes after being hit at a. The second jet hit the south tower on lower floors, between floors 78 and 84, which structurally compromised the skyscraper earlier than the north tower.
Most of the south tower occupants, however, began evacuating when the north tower was hit. The towers could not have been designed any better or stronger. Nobody anticipated the deliberate actions of an aircraft filled with thousands of gallons of jet fuel. Conspiracy theories often accompany horrific and tragic events. Some occurrences in life are so shockingly incomprehensible that some people begin to doubt theories.
They might reinterpret evidence and offer explanations based on their prior knowledge. Passionate people fabricate what becomes alternative logical reasoning.
When the buildings collapsed, some thought it had all of the characteristics of a "controlled demolition. Some people believe that the twin towers were brought down by explosives, although others find no evidence for this belief. Writing in the Journal of Engineering Mechanics ASCE, researchers have shown "the allegations of controlled demolition to be absurd" and that the towers "failed due to gravity-driven progressive collapse triggered by the effects of fire.
Engineers examine evidence and create conclusions based on observations. On the other hand, the Movement seeks the "suppressed realities of September 11th" that will support their mission. Conspiracy theories tend to continue in spite of the evidence. While architects strive to design safe buildings, developers don't always want to pay for over-redundancies to mitigate outcomes of events that are unlikely to happen.
Tall office buildings are required to have more durable fireproofing, extra emergency exits, and many other fire safety features. Share Flipboard Email. Table of Contents Expand. Aircraft Impact. Heat From Fires.
Collapsing Floors. Why They Flattened. Strong Enough? Legacy on Building. Jackie Craven, Doctor of Arts in Writing, has over 20 years of experience writing about architecture and the arts. Our first report appeared as the cover story for the March issue. The reporting grew into a book with a forward by Sen. John McCain, which was updated with a new version in Below you'll find the section about the World Trade Center, and you can also get the facts about the planes , the attack on the Pentagon , and the crash of United Flight In the years following the report of this publication, truthers would launch of variety of attacks on PM, accusing the magazine of being a tool of the federal government and drawing tinfoil-hat diagrams to tie Popular Mechanics to the Bush Administration and the supposed big conspiracy.
If all this nonsense accomplished anything, it was to presage our current era of "alternative facts" and attacking the messenger whenever the message clashes with one's predetermined beliefs. The collapse of both World Trade Center towers—and the smaller WTC 7 a few hours later—initially surprised even some experts.
But subsequent studies have shown that the WTC's structural integrity was destroyed by intense fire as well as the severe damage inflicted by the planes. That explanation hasn't swayed conspiracy theorists, who contend that all three buildings were wired with explosives in advance and razed in a series of controlled demolitions.
Claim: The first hijacked plane crashed through the 94th to the 98th floors of the World Trade Center's story North Tower; the second jet slammed into the 78th to the 84th floors of the story South Tower. The impact and ensuing fires disrupted elevator service in both buildings. Plus, the lobbies of both buildings were visibly damaged before the towers collapsed. Department of Commerce.
NIST shared its initial findings with PM and made its lead researcher available to our team of reporters. The NIST investigation revealed that plane debris sliced through the utility shafts at the North Tower's core, creating a conduit for burning jet fuel—and fiery destruction throughout the building.
Burning fuel traveling down the elevator shafts would have disrupted the elevator systems and caused extensive damage to the lobbies. NIST heard first-person testimony that "some elevators slammed right down" to the ground floor. As Jules Naudet entered the North Tower lobby, minutes after the first aircraft struck, he saw victims on fire, a scene he found too horrific to film. No kerosene fire can burn hot enough to melt steel.
However, experts agree that for the towers to collapse, their steel frames didn't need to melt, they just had to lose some of their structural strength—and that required exposure to much less heat. What happens is that the steel tries to expand at both ends, but when it can no longer expand, it sags and the surrounding concrete cracks. But jet fuel wasn't the only thing burning, notes Forman Williams, a professor of engineering at the University of California, San Diego, and one of seven structural engineers and fire experts that PM consulted.
He says that while the jet fuel was the catalyst for the WTC fires, the resulting inferno was intensified by the combustible material inside the buildings, including rugs, curtains, furniture and paper. It was the rest of the stuff burning afterward that was responsible for the heat transfer that eventually brought them down.
Claim: As each tower collapsed, clearly visible puffs of dust and debris were ejected from the sides of the buildings. They do occur from explosions. FACT: Once each tower began to collapse, the weight of all the floors above the collapsed zone bore down with pulverizing force on the highest intact floor. Unable to absorb the massive energy, that floor would fail, transmitting the forces to the floor below, allowing the collapse to progress downward through the building in a chain reaction.
Like all office buildings, the WTC towers contained a huge volume of air. As they pancaked, all that air—along with the concrete and other debris pulverized by the force of the collapse—was ejected with enormous energy. Those clouds of dust may create the impression of a controlled demolition, Sunder adds, "but it is the floor pancaking that leads to that perception.
Demolition expert Romero regrets that his comments to the Albuquerque Journal became fodder for conspiracy theorists. Romero, who agrees with the scientific conclusion that fire triggered the collapses, demanded a retraction from the Journal. It was printed Sept. Directly or indirectly, pressure was brought to bear, forcing Romero to retract his original statement.
That is the farthest thing from the truth.
The World Trade Center Fire | HowStuffWorks
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.