Demolition Theories
Theories of the Twin Towers' Destruction Rejecting Gravity-Driven Collapse
Plausible theories of the destruction of the Twin Towers recognize that the observed features of the collapses are not consistent with gravity-driven collapses, and postulate additional inputs of energy by some means. In the five years since the attack there has been a great deal of speculation about just how the demolitions were engineered. We review some of the most prominent theories, some of which are credible and some of which aren't. We divide the theories into three categories: untenable ones contradicted by evidence, exotic ones lacking proof of concept, and plausible ones consistent with evidence.
- Untenable theories
- Exotic theories
- Plausible theories
The various theories are not mutually exclusive. It is possible that a combination of distributed conventional explosives, thermobaric devices, and thermite preparations were used in combination to execute the demolitions.
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Basement Bombs
Theories that Subterranean Bombs Destroyed the Twin Towers
Theories that powerful bombs in the basements of the Twin Towers were partly or wholly responsible for their collapses have enjoyed some circulation. They have been fed by reports of molten steel in the Towers' foundations and dubious interpretations of seismic data.
The presence of high temperatures on the surface of the rubble pile is documented by remote sensing data from September 16 and 23, and the persistence of smoke-generating fires into December. There are also numerous reports of molten metal in portions of the rubble pile as late is mid-October. These features, while they may be evidence for the use of high explosives or thermite, are not evidence that such were used to attack the foundations of the Towers to begin the collapses.
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| This segment from the Palisades seismic chart shows the onset of the signal for the South Tower collapse, where the vertical lines denote 10-second intervals. The collapse probably starts about four seconds into this segment, indicated by the marked increase in amplitude. That increase is small compared to the much larger amplitudes apparently caused by the rubble crashing onto the ground, starting about twelve seconds later. For a more in-depth look at this topic, see Seismic Records of the Twin Towers' Destruction: Clarifying the Relationship Between Seismic Evidence and Controlled Demolition Theories |
Misinterpreting the Seismic Data
Proponents of basement bomb theories have adduced questionable interpretations of seismic data. They have suggested that the largest seismic spikes were generated at the beginnings of the collapse events. This ignores the marked increases in amplitude that start about ten seconds before the large spikes start. The most referenced charts are those recorded at the Lamont-Doherty station in Palisades, NY. These show a large signal lasting about eight seconds followed by a smaller one lasting more than fifteen seconds. The amplitudes of these signals make it easy to miss the signals that start about ten seconds before the large ones. Those smaller signals are still several times greater than the baseline signal seen at the beginning of the charts. Thus, contrary to indicating strong ground-coupled explosions of basement bombs at the collapses outsets, the charts suggest that the large spikes registered the ends of the collapse events, when the rubble reached the ground.
Eyewitness Reports
Theories that basement bombs were causative in the collapses should not be confused with theories that bombs damaged the basements and lobbies of the Towers at around the times of the plane crashes. An article in ChiefEngineer.org contains a detailed account of Stationary Engineer Mike Pecoraro containing descriptions of profound damage in the basement of the North Tower following the plane crash. 1 This report has been combined with observations that windows in the lobby of the North Tower were broken to support the idea of basement bombs were detonated coincident with the plane crashes. However, detailed corroboration of Pecoraro's account has not emerged, either in media reports like the ChiefEngineer.org's or in the detailed oral histories of emergency responders. Furthermore, there are other possible explanations for the damage in the lobby, such as impacts from ricocheting debris from the plane impact, and for damage in the basement, such as explosions of electrical equipment.
The individual whose accounts of explosions in the basements of the World Trade Center have received the most publicity is William Rodriguez, a janitor who helped victims escape from the Twin Towers during the attack. In an interview about his experiences on the morning of 9/11/01, Rodriguez describes hearing an loud explosion in the basement followed a few seconds later by a second explosion that sounded like it came from far above. 2 Rodriguez' account has been used to support basement bombs theories, including the idea that bombs exploded in the basement just before Flight 11 hit the North Tower. However, there are many possible explanations for Rodriguez' recollections, such as secondary explosions triggered by the crash.
Rodriguez, a lauded hero of the 9/11 attack, swears that he saw Mohald Alshehri, an alleged hijacker of Flight 175, in the World Trade Center in June of 2001. 3 Assuming Rodriguez didn't know Alshehri, this would be a remarkable feat of memory: viewing mug shots of the alleged hijackers, Rodriguez recognized one as being a someone he saw some three months earlier among the thousands of strangers he must have seen in the course of his work.
The body of oral histories of emergency responders has dozens of accounts of perceptions of explosions high in the building coinciding with or slightly preceding the onsets of each Tower's destruction. Apparently absent from the accounts are recollections of large explosions low in the Towers at the outsets of these events. That should not be confused with the perception of flashes. Stephen Gregory stated:
I thought that when I looked in the direction of the Trade Center before it came down, before No. 2 came down, that I saw low-level flashes.
...
I know about the explosion on the upper floors. This was like at eye level. I didn't have to go like this. Because I was looking this way. I'm not going to say it was on the first floor or the second floor, but somewhere in that area I saw to me what appeared to be flashes.
...
I know about the explosion on the upper floors. This was like at eye level. I didn't have to go like this. Because I was looking this way. I'm not going to say it was on the first floor or the second floor, but somewhere in that area I saw to me what appeared to be flashes.
Flashes suggest explosive devices detonating. However, there is no credible photographic, video, or eyewitness evidence supporting the notion that large explosions low in the building preceded the descents of the exploding rubble from around the crash zones down to the ground. Instead, the flashes are consistent with a theory of demolition in which thermobaric devices are detonated in a sequence from the crash zones downward. Since such devices may employ a small charge to disperse the fuel, followed by a secondary explosion to ignite the mixture and produce a large structure-shattering explosion, the flashes seen by Gregory may have been the dispersal charges preceding the main explosions by ten or more seconds.
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| This photograph from this collection shows a standing fragment of the base of the North Tower's core. This suggests that the core was destroyed from the top down, not by bombs in the basements. |
Nuclear Devices
Basement bombs theories are often found in conjunction with the nuclear attack theory. For example, Rick Siegel promotes both ideas in his film 9/11 Eyewitness and in his talks. Although the two theories dovetail with each other, both are blatantly contradicted by the evidence.
Failures of the Theory
Some investigators have suggested that basement bombs by themselves, or in combination with failure scenarios of official theories, brought down the Twin Towers. These suggestions are not tenable in light of the observed features of the collapses. Bombs that destroyed the bases of the Towers' core columns would not cause the Towers to collapse in the rapid top-down manner observed, nor to totally collapse in any fashion. The core structures were surrounded by the floor diaphragms, which were in turn surrounded by load-bearing perimeter columns. The obliteration of a few stories of core columns just above the foundation may have led to the entire core structures, from the basement to the roof, falling several stories before coming to rest on the foundations. That would have damaged most of the floor diaphragms, and may have even caused some kind of perimeter wall failures. However, such damage would have occurred nearly simultaneously from the bottom to the top of each Tower. It would not have looked anything like the observed methodical destruction starting from the crash zones and proceeding down the Towers.
The combination of basement bombs with the dubious truss failure scenario also cannot account for the destruction observed for all of the following reasons:
- Most of the concrete in the Towers was apparently pulverized to dust the consistency of talcum powder in the air. That required far more energy than floors falling a few feet or even a thousand feet.
- The mushrooming of the Towers into dust clouds three to five times the diameter of each Tower before the destruction reached the ground cannot be explained by the falling of floors and damage to the core foundations.
- The dust evolving from each Tower before it reached the ground was already several times the Tower's original volume. Only huge inputs of heat in portions of the building above the ground can account for that.
- This South Tower collapse video shows most of the block of stories above the impact zone breaking up before it started to fall. The steel columns of the perimeter wall wouldn't simply shatter due to a lack of support from the core and floors.
In addition to all of the above, all theories that basement bombs were instrumental in the destruction of the Towers are directly contradicted by the accounts of members on Ladder Company 6 who survived the destruction of both Towers trapped in the North Tower's B stairway. 4
References
2. William Rodriguez 9/11 Interview, Google Video,
3. Janitor tells 9/11 panel of brush with WTC thug, NY Daily News, 6/15/04 [cached]
4. The Miracle of Ladder Company 6, NBC News, 9/28/01 [cached]
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Distributed Explosives
Theories that Distributed High Explosives Destroyed the Twin Towers
Of the types of Twin Tower demolition theories, the most widely embraced are ones describing the detonation of numerous explosive charges hidden in locations from the top to the bottom of the Towers. The explosives would be used to shatter the steel structure, much like conventional demolitions, but the timing of detonations would be made to start near the crash zones and move downward, to simulate a progressive collapse.
Perhaps the first person to clearly articulate the distributed explosives theory was Eric Hufschmid, in his book Painful Questions. 1 He describes how packages containing high explosives such as RDX could have been hidden in the Towers. Each package would contain a detonator controlled by a radio receiver, allowing it to be individually detonated by remote control, perhaps from a control center in Building 7.
Conventional Demolitions
Conventional demolitions employ numerous explosive charges, placed adjacent to all the vertical support structures of the building and on multiple levels, starting at the ground. Careful timing of the detonations is essential to achieving vertical collapse of the building into its footprint. Typically, charges on the interior columns at ground level are detonated first, causing the central portion of the building to begin to collapse, pulling the perimeter of the building inward. Detonations then proceed outward, destroying perimeter columns after inner columns, and upward, destroying sections of columns just as they reach the ground. The net effect is a vertical collapse in which the building implodes, falling into a rubble pile almost entirely within the building's footprint.
Most conventional demolitions use detonation cords to set off the explosive charges. However, equipment currently on the market, such as the HiEx TeleBlaster, allows the detonation of charges without the use of cords or wires. 2 Such wireless equipment allows demolitions to be set up in a much less intrusive manner than was previously possible.
Twin Towers' Demolition
The Twin Towers' demolition, if achieved through distributed explosive charges, was engineered in a decidedly different manner from conventional demolitions to make it consistent with the story that the Towers collapsed as a result of the jet impacts and fires. The main differences were:
- Explosions started at the impact zone and proceeded down the intact portion of the Tower and up the overhanging portion, instead of starting at ground level.
- Much more powerful explosives were used than in a conventional demolition.
- In the South Tower demolition, the overhanging portion was allowed to tip for a few seconds before the larger explosions commenced.
The Twin Towers were also demolished at a more rapid rate than is the case in conventional demolitions. When buildings are demolished from the ground up, gravity is typically relied upon to do most of the work once several floors have been demolished, and the upper portion of the building is falling with considerable kinetic energy. The tapping of that energy to break up the structure slows the fall. In the case of the Twin Towers, it appeared that the explosive events were progressing down the Towers' intact portions at a rate only slightly slower than free-fall.
Disaster Engineering
One might ask questions about two features of the demolition, both of which would seem to detract from the realism of simulated gravity-driven collapses.
- Why would the perpetrators use much greater quantities of explosives than in a conventional demolition?
- Why would they destroy the Towers at such a rapid rate?
The answer to the first question probably has to do with economics. The use of less explosives would fail to break up the Towers' structures as thoroughly. Since the destruction started a thousand feet aloft, large falling pieces could have been deflected hundreds of feet when they hit other objects near the ground. Had not the Towers' perimeter walls been so thoroughly shredded, they might have fallen away in large sections, pivoting large distances from the Towers. The result could have been more damage to buildings outside the World Trade Center complex than occurred. While the high explosives threw pieces of the perimeter wall as much as 500 feet laterally, their distribution was more predictable and their sizes were smaller and less likely to do serious damage than might have happened with less powerful explosives.
The answer to the second question probably has to do with hiding obvious evidence of explosions. Increasingly powerful explosions occurred as the destruction progressed down the Towers, perhaps to ensure the thorough destruction of the stronger structures lower in the Towers. If the rate at which the explosives were detonated was not fast enough to stay ahead of the falling rubble, the perpetrators ran the risk that the rubble would fall away from the moving zone of destruction, revealing large obvious explosions at the top of the exposed structures.
Contrary Evidence
The distributed explosives theory can easily explain the gross features of the collapses from the top-to-bottom destruction to the pulverization of the Towers' materials. However, there are a number of more subtle features of the collapses that do not appear to be consistent with this theory, at least in its simplest form. The following collapse features suggest that the demolition of the Towers was accomplished using technologies other than just distributed conventional explosives. In contrast, many of the same features do appear to be consistent with the thermobarics theory, and the final one suggests the thermite theory.
- Absence of high blast pressures in collapse onsets
Careful study of photographs and videos of both collapses shows that the perimeter walls do not immediately blow out in the way one would expect if explosives adjacent ot the perimeter columns were used to destroy them. Rather, the walls telescope as they disappear into the burgeoning dust clouds, to partially reappear seconds later as fragments outracing the dust cloud. - Rapid degradation of structure
Features of the onset of both collapses indicate that structures around and above the crash zones lost almost all their strength. In the South Tower, the top not only tips, it bends: The outer wall exhibits a peculiar curve extending about 15 floors above the crash zone. Similarly, in the North Tower, the top begins to telescope straight down with no evidence of bucking in the perimeter columns. In both cases the structure's strength seems to disappear even before any of the explosive features appear. - Uniformity of pulverization
Photographs and reports from Ground Zero indicate that the vast majority of the estimated 90,000 tons of concrete in each Tower was turned to fine dust, not a mixture of dust and gravel or larger chunks. Since blast pressures from explosive charges fall off with the square of the distance from the source, achieving such thorough pulverization with distributed explosives would seem to have required a huge number of individual packages being placed throughout the building. - Vaporization of people
Over 1000 victims were never identified despite over a year of efforts to identify victims from even the smallest fragments using DNA. Explosive charges would be unlikely to so thoroughly degrade the remains of so many people. - Persistence of core structures
In both collapses, a large section of the core structure extending up over 600 feet remains standing for a few seconds and then collapses. The persistent remnant of the North Tower is very narrow and delicate. It is difficult to imagine how such structures could have survived the blast pressures generated by demolition waves of explosive charges, only to themselves collapse a few seconds later. - Rapid oxidation and intergranular melting of steel pieces
The limited metallurgical examination of some of the few pieces of structural steel that escaped the blast furnaces shows very peculiar features, such as rapid oxidation turning inch-thick steel into paper-thin scrolled pieces, cavitation giving steel the appearance of Swiss cheese, and intragranular melting. These suggest a more exotic process of destruction than mere explosives.
References
2. The HiEx TeleBlaster II, hiex.bc.ca, [cached]
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Thermobarics
Theories that Thermobaric Devices Destroyed the Twin Towers
A 'thermobaric' weapon is one that uses atmospheric oxygen, instead of carrying its own oxidizer, to achieve an explosion. Thus, such weapons are often called fuel air explosives (FAE). The most common type of thermobaric weapon uses a primary charge to disperse its fuel into an aerosol, and a secondary charge to ignite the aerosol. 1 The flame front rapidly propagates through the mixture producing a pressure wave and a potentially large area of intense overpressures.
Note that any of a number of fuels can be used in a thermobaric bomb, and not all of them generate visible fireballs. Hydrogen, for example, burns in air with a flame that is not visible in daylight. 2
The ability of thermobaric devices to generate high overpressures may seem be counterintuitive, since they use only air and a simple fuel such as propane, rather than a high-explosive compound such as TNT. The overpressures result from the speed at which the pre-mixed fuel-air mixture combusts, causing pressures and temperatures to build up over a large area. H. Michael Sweeney explains the process:
The chief difference in METC unit (Multiple Explosives Transitional Container) design over traditional explosive devices moves away from a densely packed explosive core towards a large volume of highly explosive but low-density mass in the form of a gaseous cloud. In the normal bomb all explosive energy comes from a tightly packed core and must drive outward against air pressure and objects it encounters. It rapidly bleeds off energy at the square of the distance as it accumulates a wall of pressure resistance and a mass of heavy debris, which it must continually regather and push along.
The new design starts as a small device but transforms itself through simple means from a dense-core technology to a much larger gaseous-cloud state. Igniting the explosive cloud at any peripheral or central point creates a chain-reaction-like and progressively growing explosive force. As the force of the explosion moves outward, it continues to ignite fresh explosive materials as encountered and gains momentum rather than loosing sic it. Further, because the gaseous cloud is efficiently mixed explosive materials combined with abundant free-air oxygen, ignition is far more complete and productive - leaving little or no chemical residue or traditional flash evidence (other than a burn signature, which any investigator would presume to be from ordinary fire) on immediately encountered objects. The net result is as if a significantly larger central core device had been detonated, with the complete and even combustion making difficult any aftermath analysis as to the true nature of the explosives used. Finally, the shape of the cloud and the ignition point within the cloud, if properly controlled, provides an extremely easy means to create shaped charge effects despite a relatively free-form original cloud shape. 3
The new design starts as a small device but transforms itself through simple means from a dense-core technology to a much larger gaseous-cloud state. Igniting the explosive cloud at any peripheral or central point creates a chain-reaction-like and progressively growing explosive force. As the force of the explosion moves outward, it continues to ignite fresh explosive materials as encountered and gains momentum rather than loosing sic it. Further, because the gaseous cloud is efficiently mixed explosive materials combined with abundant free-air oxygen, ignition is far more complete and productive - leaving little or no chemical residue or traditional flash evidence (other than a burn signature, which any investigator would presume to be from ordinary fire) on immediately encountered objects. The net result is as if a significantly larger central core device had been detonated, with the complete and even combustion making difficult any aftermath analysis as to the true nature of the explosives used. Finally, the shape of the cloud and the ignition point within the cloud, if properly controlled, provides an extremely easy means to create shaped charge effects despite a relatively free-form original cloud shape. 3
A Thermobarics Demolition Scenario
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| Were the Towers blasted apart in successive floor-wide detonations of distributed thermobaric bombs, marched down from the crash zones at quickening rates? Note the clumping in the elongated features of the rubble cloud in this photo taken mid-way through the North Tower's destruction, where the average spacing between clumps is similar to the spacing between floors. |
One can imagine a scenario in which a thermobaric devices were installed at each floor in the service core of each Tower. Each device would listen for a radio signal with a particular signature which would trigger a primary charge, dispersing the aerosol throughout its floor. Then, about five seconds later, a secondary charge would be triggered causing an explosion with overpressures sufficient to shatter the perimeter walls.
One advantage this theory has over most other explosives theories is that it avoids the need to install explosives near the Towers' perimeter columns. The thermobaric devices could have been installed entirely in discretely accessed portions of the Towers' cores. The number of devices could also be much smaller -- perhaps just one per floor. The devices could have been encased in impact- and heat-resistant containers similar to those used to protect aircraft voice and data recorders, so as to prevent accidental detonation from the aircraft impacts and fires.
Other advantages of thermobarics include an absence of conventional explosive residues, and much higher energy densities than conventional explosives. For example, whereas TNT yields 4.2 MJ/kg, hydrogen produces 120 MJ/kg (not counting the weight of the oxygen it uses to burn).4
Of the possible fuels that could be used in thermobarics, hydrogen has several unique attributes which could have been used to advantage by the planners.
- The flash produced by hydrogen combustion is not visible to the naked eye in daylight conditions. The use of hydrogen-based thermobarics is thus consistent with the absence of colorful fireworks in the destruction of the Twin Towers.
- On a weight basis, hydrogen has one of the highest energy densities of any fuel -- several times that of any hydrocarbon. The use of hydrogen would have allowed operatives to install far less material than would be required with other explosives.
- The combustion of hydrogen in air produces only water vapor, a residue that is consistent with the vast light-colored clouds produced by the Towers' destruction.
- Hydrogen has a very wide explosive range -- from 4 to 75 percent in air. That compares to 2.1 to 10.1 percent for propane and 0.7 to 5 percent for kerosene. 5 Thus it would be relatively easy to design hydrogen-based thermobarics that would function reliably in a variety of conditions.
- Hydrogen has a very high vapor pressure compared to other fuels. This would have enabled its rapid dispersal into ambient air by shattering pressure vessels containing it.
Technical Challenges
The use of thermobarics to destroy the Twin Towers would have presented some technical challenges. One would be to assure that ignitions of the aerosols on each floor not proceed downward faster than the descending rubble cloud. If the planners allowed five seconds for the mixing of aerosol on each floor, they would have to start the dispersals about 30 stories below the zones of destruction. Two potential problems would be:
- Aerosols on lower floors being prematurely ignited by stray sparks
- Combustion propagating from floors to floors below them
The shut-off of electrical power to the Towers may have largely obviated the first problem. It is interesting that dust jets are seen around the mechanical equipment floors, where sparks would have been more likely.
Engineering of the thermobarics may have addressed the second problem. The isolation of floors by fire doors and elevator-shaft fire dampers, combined with the distribution of aerosols primarily in the tenant spaces, may have been sufficient to prevent flames from propagating from one floor to the next. The floors themselves, even after being shattered by the thermobarics, would provide a barrier to the propagation of flames for at least the eighth of a second or so between the destruction of successive floors.
Both of these problems could have been avoided by designing the thermobaric charges to disperse their contents in a split second, eliminating the interval of several seconds during which sparks or flame propagation could have caused premature ignition.
References
2. Hydrogen Safety, Humboldt.edu,
3. CIA's METC Explosives, totse.com, [cached]
4. Chemical Potential Energy, HyperTextBook.com,
5. Gases - Explosive and Flammability Concentration Limits, EngineeringToolBox.com,
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Thermite
Theories that Aluminothermic Materials Were Used to Destroy the Twin Towers
The idea that thermite or similar preparations were used exclusively or in combination with other methods to destroy the Twin Towers remained unexamined for several years after the attack, despite its merits.
Aluminothermic reactions are exothermic chemical reactions in which aluminum is oxidized while an oxide of another metal is reduced. Although high temperatures are required to initiate such reactions, they are easily self-sustaining once started due to the heat they generate. The most common example of an aluminothermic reaction is thermite, in which powdered aluminum reacts with an iron oxide. Because aluminum has a greater affinity for oxygen than iron, oxygen is transferred from the iron oxide to the aluminum, releasing a great deal of energy and leaving behind molten iron and aluminum oxide.
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| The spout of orange molten metal and rising white smoke emerging form the South Tower have the appearance of a thermite reaction. |
Professor Steven Jones has noted that a number of features evident both before and after the falls of the Towers fit the theory that thermite was used. These include:
- A spout of orange molten metal seen just before the South Tower's fall in videos of the Tower's north face around the crash zone
- Reports of molten metal in the remarkably hot rubble of Ground Zero
- High levels of metals found in aluminothermic incendiaries -- such as manganese, zinc, and barium -- in samples of World Trade Center dust
- Remains of the Towers' structural steel showing severe corrosive attack involving sulfur
- Minute solidified droplets of previously molten iron in samples of World Trade Center dust
- Pieces of active nano-engineered aluminothermic chips in samples of World Trade Center dust
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| An iron-rich sphere found in dust form the Twin Towers. The solidified droplet indicates temperatures in excess of the melting point of iron (2795ºF). |
Thermite Variants
Basic thermite preparations can be modified and augmented in various ways to change their properties. The fineness of the aluminum powder determines the speed of the reaction. The use of ultra-fine aluminum powder gives the reaction an explosive quality, resulting in 'super-thermites'. The addition of sulfur in preparations called thermates enhances the ability of the reaction to cut through steel.
Findings reported in Appendix C of FEMA's World Trade Center Building Performance Study seem to fit the thermite theory remarkably well.
Evidence of a severe high temperature corrosion attack on the steel, including oxidation and sulfidation with subsequent intergranular melting, was readily visible in the near-surface microstructure. A liquid eutectic mixture containing primarily iron, oxygen, and sulfur formed during this hot corrosion attack on the steel.
...
The severe corrosion and subsequent erosion of Samples 1 and 2 are a very unusual event. No clear explanation for the source of the sulfur has been identified.
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The severe corrosion and subsequent erosion of Samples 1 and 2 are a very unusual event. No clear explanation for the source of the sulfur has been identified.
Because there are many possible types of pyrotechnics that exploit the energy-dense thermite reaction, the architects of the World Trade Center demolition might have relied entirely on such materials to attack the structures in several different ways, as is suggested by this hypothetical blasting scenario.
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Simulations
Modeling Aspects of the Twin Towers' Collapse
One might think that the unprecedented way in which the Twin Towers exploded into dust might warrant the creation of some computer models to better understand this behavior.FEMA's investigation appeared to be timed to coincide with the site cleanup. With the rapid pace of operations there apparently wasn't time for any computer models.
With much more time and money on its hands, NIST churned out simulation after simulation as part of its study of the World Trade Center catastrophe.
NIST's simulations model phenomena such as the impact fireballs and smoke plumes that rose from the Towers, and the pattern of damage caused by the impact of jetliners. Conspicuously missing from their study are simulations, models, or even calculations that attempt to describe how and why the Towers came down. With tens of millions of dollars at its disposal, NIST couldn't spend a few thousand dollars to study progressive collapse, the new-found phenomenon that accounted for the total destruction of all three skyscrapers, WTC 1, 2, and 7.
Modeling Total Collapse Time
One aspect of the destruction of the Twin Towers that suggests controlled demolition as a cause is the speed of their descent. In a passage noting the events' rapidity, an article in Scientific American quotes Eduardo Kausel as stating, "The towers' resistive systems played no role. Otherwise the elapsed time of the fall would have been extended." However, even if the crushing of the Towers' intact structures below the crash zones did not slow the rubble at all, the acceleration of the stationary mass would have. Thus, if one accepts the central assumption of the pile-driver theory -- that the tower's falling mass remained aligned over the tower's intact portion as the destruction progressed downward -- the total collapse time would have been extended considerably by the time required to accelerate each floor encountered by the falling mass.
This program, created by Jim Hoffman in 2003, computes total collapse times based on parameters describing the floor on which the collapse started. The program makes the following assumptions, all of which favor short collapse times.
- Each floor's support vanishes when touched by the falling block.
- Momentum is conserved.
- None of the kinetic energy of the falling mass is diverted to other sinks (concrete pulverization, steel bending, etc.)
- Each floor is an infinitely thin slab, and all the mass of a story is concentrated in the slab.
- The overhanging portion (eg: 14 floors in the North Tower) falls as a block, with its bottom floor accumulating pancaked slabs of the once-intact floors as it encounters them.
- The accumulation of floors is inelastic.
- Once the bottom of the block reaches the ground, the floors in it start to pancake from bottom to top, the roof of the tower falling at freefall at that point.
- Mass is uniformly distributed among the stories.
- The falling block remains perfectly centered over the intact portion.
The following table summarizes the results of running the program with parameters specifying that the collapse starts at the 80th and 95th floors:
| elapsed time in seconds | ||
|---|---|---|
| start floor | crash zone to ground (seconds) | roof to ground (seconds) |
| 80 | 9.733 | 11.613 |
| 95 | 11.604 | 12.608 |
In 2006, Hoffman created a generalized version of the program that allows the removal of the last two assumptions. In particular, it allows the specification of:
- A linear increase in the mass of stories from the roof to the ground.
- The movement of a fixed fraction of the mass falling within the Tower's profile to move outside of the profile with the collapse of each story. Once mass moves outside of the Tower's profile, it does not participate in the acceleration of mass downward.
The Twin Towers, as all large steel-framed skyscrapers, had columns that became less massive at increasing elevation. This means that the Towers' upper stories were considerably lighter than the lower ones. The assumption that the lowest stories were about 1.5 times as massive as the top stories seems like a reasonable assumption. Implementing this adjustment to the model means longer collapse times, of course, since more of the mass would initially be lower in the Tower where it would have less mass underneath it to accelerate downward. However, the simulation shows that even making the mass ratio of the bottom to the top story 2.0 has relatively little effect on total collapse times.
In contrast, the second adjustment of the new model has a pronounced effect on total collapse times. Assuming that just 6 percent of the mass above the impact zone is ejected outside of the Tower's footprint for each story crushed results in a total collapse time of nearly 20 seconds, assuming the collapse started at the 95th floor.
| input parameters | elapsed time in seconds | ||||
|---|---|---|---|---|---|
| start floor | mass of bottom story relative to top | mass dispersal per story | mass dispersed by end | crash zone to ground | roof to ground |
| 80 | 1.5 | 0 | 0 | 9.98 | 11.92 |
| 95 | 1.5 | 0 | 0 | 11.86 | 12.91 |
| 80 | 1.5 | 0.03 | 0.67 | 11.56 | 14.07 |
| 95 | 1.5 | 0.03 | 0.68 | 14.05 | 15.49 |
| 80 | 1.5 | 0.06 | 0.82 | 13.57 | 16.53 |
| 95 | 1.5 | 0.06 | 0.82 | 16.59 | 18.34 |
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