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9-11, Conspiracies, and Science of Building Collapse

Registered Architect, 40 years experience, investigative forensic specialist, engineering trained, college teacher, NCARB mentor, MBA.


About a month ago we marked the 16th anniversary of the 9-11 attacks. I wrote my first article on the event back in 2009, and a follow-up article in 2011. I will attempt not to repeat here what I wrote in those articles. Much of the misinformation fervor has thankfully faded away, however much still lingers. Just recently I was at a function of a trade association that I am a member of, and was discussing with a colleague past events in our industry. As many times has seen in the past, 9-11 became a topic of discussion, and I came face to face once again with these conspiracy theories. Reading those previous articles of mine, you will know I hold little agreement with many of these theories as I find most of those theories fail the scrutiny of science.

By my experience, I am a very different type of Architect. Not so much in the art, which is the root of Architecture, but more in the science of Architecture. I often think of myself as a very board based Architectural Engineer, going far beyond the limited structural engineering component that most of those that view themselves as an Architectural Engineer possess. My engineering background is certainly multi-discipline and that is readily revealed in my views, beliefs and approaches. This will prove out as you read the rest of this article.

I will once again I find myself addressing some of the component conspiracy arguments I have heard when it comes to the twin towers. The north tower (WTC 1) was hit first about 8:45 AM local time, hitting floors 94 – 98 (5 floors). About 18 minutes later, at 9:03 AM local time the south tower (WTC 2) was hit, taking out floors 78 – 84 (7 floors). A total of at least 12 floors in both towers damaged and now on fire. At about 10:05 AM local time, the south tower (WTC 2) collapses, 62 minutes after it was first hit. At about 10:28 AM local time, the north tower (WTC 1) collapses, 105 minutes after it was first hit. The following map shows what the World Trade Center site looked like that fateful morning in September 2001. Please note that the twin towers (WTC 1 and WTC 2) were not the only buildings lost that day. The 47-story WTC 7 building was also destroyed, allowed to collapse as fires within the building were not able to be controlled. All the other buildings on the site (WTC 3, WTC 4, WTC 5, and WTC 6) all sustained serious damage that day.

WTC Site Plan on 9-11

WTC Site Plan on 9-11

As we enter this discussion, it is not my intent to “change” people’s minds about conspiracy theories. People will choose to believe whatever they want, it is not my place tell them that they are wrong. That is what civil discourse is all about, expression of all views and exchange of ideas. I will present evidence as I have found, and let each person decide what they want to do with that information. After almost 40 years of design and building, I have concluded that there must be someone that created this universe, intelligent design if you will, as I find it intellectually dishonest to make the claim that this universe just “happened”. Compared to many other complex rudiments in this universe, and seeing how much planning it takes to create even the simplest of structures or buildings, I cannot reconcile in my mind that this all just “happened” without guidance, forethought, or planning. That is my belief. Other’s may choose to believe something else, it is not for me to tell them that their belief is wrong. That does not mean that we should not exchange ideas, data, or information. It is under this essential pretense that I submit the following data to each reader to let them decide for their self.

Conspiracy: WTC 7 was Allowed to Collapse for Some Insidious Purpose

To begin, let me take part of this “conspiracy” away immediately. WTC 7 was allowed to burn out of control and collapse! Let me say it again, WTC 7 was allowed to burn out of control and collapse! I hope I made that clear to everyone reading this. After saying that, the decision to allow WTC 7 was no more insidious than your bank account having a zero balance at the end of the month. In fact, they both attest to the exact same thing, lack of resources. I am not sure if that even constitutes anything that could be considered as insidious, although the end of the month often finds me having those sentiments as I look at the balance in my account.

Now let me illustrate why it was a resource issue. To begin, each of the twin towers (WTC 1 and WTC 2) were about an acre (43,560 square feet) footprint each. Based on the hazard classification of light (office building), these buildings would require a fire sprinkler design rate of approximately 0.1 GPM (gallons per minute) per square foot, for a total of 4,356 GPM per floor. With at least 12 impact floors on fire that would equate to 52,272 GPM for the fire sprinkler systems in twin towers alone just for the impacted floors alone.

That demand does not take into account the water loading by the Fire Department’s equipment for those towers as the Fire Department fought the fires in the towers on the other floors. These building fire sprinkler systems are automatic, so once the heads open, they most likely stay open until manually shut off. Also keep in mind that these systems also required about 1,500 PSI (pounds per square inch) pressure to operate. That is a lot of volume and an enormous amount of pressure.

As I have never designed anything in New York, I can only relate to projects I have worked on in the U.S. southwest desert. What I know is most of the fire lines I have tapped into in my career, had the largest one in the range of 36” to 48” inches in diameter. At the time of this writing I could not find max loading for pipes of that size, but I did find that a 24” diameter, schedule 40, steel pipe had a maximum capacity of 18,000 GPM. Maybe barely enough capacity for the twin towers, but what about the demands from the Fire Department fighting fires in other parts of the twin towers or other WTC buildings? How about the demands in the WTC 7 fire sprinkler systems? How many other demands were created by the other WTC buildings? Or the surrounding buildings? As one can see, the overall demand was GIGANTIC!

It would appear that lack of water resources was the basis for the decision to leave WTC 7 burn, and I pose this question, how insidious is that basis? When does lack of resources become an insidious plan? How is that different from any other time resources restrict reactions or design? Is this insidious claim based on the lack of resources? If so, how could this lack of water have been predicated? Is the claim that resources should be design with no limitations? How is that to be accomplished? Unlimited budgets? Where do those come from? I have never had a project with an unlimited budget. You see where this reasoning could lead if followed to the nth degree? This basic premise for conspiracist seems ludicrous at the very least.

Conspiracy: The Buildings had to be Controlled Demolished Because They Pancaked (1)

I will be looking at this conspiracy theory from two perspectives. This first one is from an alternative result expectation. The basis for this view appears to be that there is an expectation that something else should have happened in the collapse of these towers than what actually occurred. I pose the question, what should have been that other “expected” or anticipated result? How should the buildings have collapsed? Is the expectation from those that subscribe to this conspiracy theory saying that the buildings should have toppled over? Stop reading for a moment and ponder that question, think critically about this very question. Is that really the intent of this alternative result being expected?

Before exploring that result, I want to present data to set the framework for this alternative scenario. To begin, on that September morning in 2001, the population for Manhattan, New York was about 1,562,000 in a 22.82 square mile area. Extrapolating from current data, there were approximately 68,451 people per square mile (net) in Manhattan that September morning. The north tower (WTC 1) was 1,310 feet tall, while the south tower (WTC 2) was the taller at 1,362 feet.

The smallest constituted area would be if the tower toppled directly perpendicular to one of the sides, a length of about 209 feet long. Now assume that it falls exactly flat, the depth of the building would not have stayed exactly on top of the flat side, so let us say that half of it would fall off the pile on each side, that would make the width of the flattened area widen out another 105 feet wider, making the entire width of crushing zone 314 feet by 1,310 feet equaling an area of 411,340 square feet, or .01475 square miles. Based on the density stated previously that would have led to a death toll of about 1,010 people on the ground surrounding the building. Adding the people that would have occupied the building at that time, around 25,000, that would not have been evacuated as a result of the immediate toppling effect, would have brought the total dead to approximately 26,010 for the one tower. Using the same methodology, the second tower would have added another 26,050, for a total of 52,060 dead in the attack. That is a very different result than the 2,500 from the actual event.

My immediate question, forgetting everything else, is this the way we as a society really want our high-rise buildings designed? Is that what the conspiracist really wanted to propagate? I have no ability to relate to what these conspiracist are saying so I am unable to frame their argument in this framework. Furthermore, do you realize the problems with buildings designed to be that rigid? Imagine a high rise falling in a hurricane or high wind storm. A building falling from this type of rigidness, also has a risk of undermining the foundation of every neighboring structure in densely developed areas such as Manhattan or New York.

Nature’s Design

I know that some may not agree with what I am about to say, but I hold Frank Lloyd Wright as one of the greatest modern Architects. That is my opinion. Several years ago, a late colleague of mine introduced me to a book, “The Oral History of Modern Architecture”, in it there was a CD with a recorded interview of several great modern Architects of our time, Frank Lloyd Wright being one of those interviews. His philosophy of organic architecture goes beyond the bottles and tires we see referred to as organic architecture today. It really focused on seeing how nature creates structures and following that as basis for design. In the interview on that CD he closes his interview with the Welsh definition of what a genius is as he called it. He stated the definition was handed down from King Arthur’s round table, “A genius is a man who has an eye to see nature, a genius is a man that has a heart to feel nature, and a genius is a man that has the courage follow nature.”

Looking at nature, rigid tall slender design is not observed. Referring to the illustration below, some palm trees can grow to 25 feet, sometimes 30 feet or more in height. There is so much I could talk about the structure of the palm tree, but I will focus on just this one point at this time. This tall, slender tree has the capabilities to bend significantly in high wind, reflective of how in nature tall, slender structures survive long term in all conditions. Tall, slender buildings need to perform in a similar manner to survive. That is what nature shows us.

Palm Trees in High Winds

Palm Trees in High Winds

The very premise for this conspiracy belief seems to be unsubstantiated in the physical world, and even nature. I am not even sure that the building codes would allow that type of ridged design to occur. Yet I do not hear anything about that component in this discussion when conspiracist talk about this ideology.

Newton’s Laws of Physics

Before we begin our discussion on the next conspiracy theory, I want to take a moment to give a quick basic lesson in physics. Sir Isaac Newton (1643 – 1727), who is considered by many as the father of physics, described three principal laws of physics. When speaking with respect to building structures, two of these three laws are in the center of focus.

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His first law of physics, sometimes referred to as the law of inertia, is stated in this manner: objects in motion tend to stay in motion, while objects at rest tend to stay at rest. Simply articulated: if an object is moving, it takes very little energy to keep it moving, on the other hand an object not moving will take a significant amount of energy to start movement. An example of this is when you are driving. You stop at a red light, when the light turns green, you have to push the accelerator down a lot to start to move forward again, once moving, you have to lighten off the accelerator once you have reached speed to maintain a certain speed.

The other law of Newton’s in this focus of structures is his third law, which is stated in this manner: for every action, there is an opposite, but equal reaction. Simply articulated: for every force exerted in one direction, there is a force of equal strength in the opposite direction. An example of this in line with the driving example previously mentioned is when you approach a red light. As you apply the brake, your body and contents of the vehicle continue to move forward. When you apply the brake too fast, something sitting on the seat next to you slides forward and onto the floorboard. An example as we approach the topic of structures would be a beam. The weight of the loading acting on the beam wants to move the beam downward, but the reactions (posts / columns) along with the strength of the beam exert an upward resisting force to hold the beam in place, this is also called equilibrium. This is always the desired result, as a building in movement never has a good outcome. The topic of this paper proves that.

Conspiracy: The Buildings had to be Controlled Demolished Because They Pancaked (2)

The first thing I hear with this view is the phrase “controlled demolition”. My first question is all explosives leave some sort of chemical residue behind, yet nothing has been found in any of the debris of the collapsed towers. The number of explosives required to demolish a floor would have been immense leaving a significant amount of chemical residue behind. If controlled demolition is true even in part, where is the evidence of explosive residue? How was this residue totally wiped clean off every piece of debris? Are the conspiracist claiming that everyone that had any knowledge of this chemical residual is keeping that information suppressed? Is that even a realistic expectation?

I read once that one of the Watergate conspirators wrote that the thought of a large number of people keeping a secret until all their deaths was not only impossible, but absurd. As he put it, there were only nine people that knew about the Watergate break-in and they could not keep that secret very long. How is it possible for a secret to be kept that was known to so many people? Especially after 16 years?

Structural Basics: Vertical and Horizontal Supporting Elements

When I first began taking my ARE (Architectural Registration Exam) back in 1983, one of the study materials I had to use was ANSI A58.1-1982, “Minimum Design Loads for Buildings and Other Structures”. Yes, for those that recognize it, the last four digits represent the publication year so I am dating myself. As part of this standard it introduced me to the concept of “localized collapse”. That concept is based on the precept that a building design should include a means that will allow part of a building to collapse without jeopardizing the entire structure. I have seen this followed through my career by the use of safety margins when designing structures.

Using that as a basis of progression, I will provide a narrative with illustrations of the component parts to the catastrophic progressive collapse seen in the collapse of the twin towers, explaining in basic terms what occurred and why. In an attempt to keep the concepts understandable to the largest number of readers, will explain these concepts in very general terms. After reviewing this, I would hope to build a case that would indicated that this conspiracy theory is flawed at the very least. So, let us begin.

Every structure has two basic supporting elements, those that run horizontal (floors) and those that run vertical (walls, columns). There are many different components that form these basic elements. For instance, horizontal elements can be formed by beams, trusses, panelized systems such as slabs, and even decks (wood, concrete, and steel). The same can be said for the vertical members. A variety of materials can be used as well. The combination of these material characteristics forms the basis for how these systems react when exposed to varies conditions such as heat and moisture. Horizontal spanning members tend to deflected downward when loaded or over stressed, also referred to as bending deflection or Δ (pronounced “delta”). Vertical elements can see bending deflection as well when the element has an eccentric (off center) load; this will be manifested in a bending of the element where the top and bottom tend to come together and just like horizontal elements are caused by loading or overstressing. The illustrations below show this deflection of a horizontal member, which can be a joist, trusses, beam, or even a floor structure. The progression goes from the static (equilibrium, no movement) on the left through failure on the right.

Collapse of a Simple Beam

Collapse of a Simple Beam

Notice as the horizontal element gains in deflection until it ultimately fails (breaks). Also, notice the tops of the supporting vertical elements start to be pulled toward the center of the span causing the vertical elements to fall towards the center span.

This effect occurs when the horizontal element is overload, or in the case of catastrophic collapse, when the element is overstressed. This overstressing of the horizontal element occurs when excessive load is added, such as a floor above collapsing on it or damage that decreases its load carrying capacity. This also occurs in the case of a structural steel beam exposed to high temperatures (about 500 - 600° F) for an extended period (maybe in the range of 10 – 15 minutes). This fatigue will not occur until the entire member is heated to this level throughout and that heat transfer only occurs with a duration of exposure, not immediate exposure. That is why the building codes have always required structural steel to be coated with a fire-resistive material, at least for much longer than my 40 years of practice. So, the structural steel did not have to melt to have its strength compromised, it only need to be directly exposed to high heat for a period of time duration. Short term heat exposure would not have allowed the steel to absorb enough heat to begin this fatigue degradation.

Now imagine what would happen if this was an internal span, having adjacent spanning bays. The failing span would bring the adjacent spanning bays right along with it as it failed, causing a progressive catastrophic collapse. To help illustrate this, please look at the figure below. The progression goes from the static (equilibrium, no movement) on the top through failure on the bottom.

Collapse of a Complex Beam

Collapse of a Complex Beam

The center spanning bay acts just as in the previous figure, however notice that the adjacent horizontal elements to the failing spanning bay are being drug towards the failing spanning bay. This movement causes what is called a moment (rotation) at the top of the vertical elements, building stresses in those elements, potentially leading to a failure as indicated at the end of the movement in the illustration. This begins to show the progressive catastrophic failure as the center bay drags the two adjacent bays into the collapsing motion. Now imagine how this will impact the progression of the collapse when there are multiple floors in the center of a high-rise building, which I will be illustrating a little later.

Structural Basics: The Towers

Wright’s design of his “mile high tower” for Chicago was designed much like a tree, a center core (trunk) with cantilevering floors (branches) from that core (trunk). This typifies the culmination of much of Wright’s organic design ideology. You will find the twin towers held something in common with that concept. The main core was a grid of columns in both the x and y axes that housed the stairs, elevators, and restrooms for each floor that cantilevered out to the outside perimeter. This design allowed for maximized flexibility where the leasable space was located because it was essentially cleared spanned.

When you look at the illustration below, you may see that this column gridded core area created what could be considered a trunk of a tree, and the semi-cantilevered floors being much like the branches of the tree. Please remember this is only a schematic representation of the tower, not a detailed drawing, to illustrate just the concepts being discussed at this time. The floors are semi-cantilevered as they were support at the core side, and at the outside of the floor, as the outside tube columns were integrated into a vertical spanning system. Each line in the illustration will kind of represent either a vertical or horizontal element. As this is only a schematic representation, please keep in mind that it is neither to scale or exact number of floors. The purpose of this is just to illustrate macro concepts of the anatomy of a catastrophic progressive building collapse.

WTC Tower Schematic Drawing

WTC Tower Schematic Drawing

The Anatomy of a Collapse

Before we continue, I want to briefly re-visit Newton’s first law. This law tells us that it takes a significant amount of energy to commence movement in an object that is not moving, but once movement begins, it takes a lot less energy to keep the object moving or even accelerating movement. Conversely once this movement begins it takes an immense amount of energy to halt that movement.

Now think of the floors above the impact floors in the towers as a large weight acting on the damaged floors below. When an entire floor collapses below, this weight also collapses on it adding load on the un-collapsed entire floor below the just collapsed floor through this kinetic energy transfer in the form of an additional force on the still un-collapsed structure below. The question is how much are these weights in the two towers? I will be using only general numbers, not exact numbers as those are not available at the time of writing this paper. The construction method used for the towers weighed about 208 pounds per square foot. Each floor had an area of about an acre, that is 43,560 square feet per floor, making the weight of each floor 4,545.5 tons per floor. There were 110 floors in the tower, giving a total weight of each tower 500,000 tons. With 12 complete floors untouched above the impact floors, the north tower (WTC 1) had a weight of 54,546 tons above the highest impact floor. The south tower (WTC 2) had 26 complete floors above the impact floors weighing in at 118,183 tons.

Now going back to Newton’s third law, for every action there is an opposite but equal reaction. A force on an element is external to that element, be that a horizontal (floor) or vertical (column / post) element. The resistance to that external force is called a stress and stress is an internal reaction to the external force being placed on an element. Both the impact load and the added weight creates additional stresses within the supporting elements (both horizontal and vertical) as a result of this third law. Stresses these elements were not intended or designed to resist. If this added stress overstresses (goes beyond the element’s designed capabilities) the supporting elements (either vertical and horizontal) may be placed in failure (collapse) mode. As a result, the entire floor collapses adding to the loads of the entire floor below it through the same processes just described, creating the progressive catastrophic collapse, until the entire mass hits the ground and dissipates that energy into the soil below.

As a floor in the tower collapses, it receives energy, called kinetic energy. The two main contributors to this kinetic energy are mass and velocity. As mass and / or velocity increases so does the kinetic energy of the moving mass being transferred to the entire floor below. That mass becomes the floors above the impact floors and the collapsing floor, and then is added on its way down as the floors that receive this load collapses, adding to the kinetic energy of the mass impacting lower floors as it continues to progress. This kinetic energy transfer will have much the same effects on surrounding structures as an earthquake when the movement is arrested upon impact with the ground. These are the same effects that were reported on 9-11 in lower Manhattan and surrounding areas, even miles away.

The weight will add to the moving mass above as this mass is added to the mass of the floors below, increasing the mass above as it continues lower and lower in the tower. Adding mass will not add to the speed to the collapse, but it will certainly add to the kinetic energy hitting every floor below, which will increase the overstressing of those structural elements below that moving mass. That overstressing will cause failure (collapse) in those elements creating a cascade event that will continue to transfer in the downward motion. As the magnitude of these loads so increase, failure due to over stressing occurs faster and faster as the mass and kinetic energy increase on each subsequent floor. This is what the basic anatomy of a progressive catastrophic collapse looks like in a multi-story building.

Collapse of The Twin Towers

Once again, I want to re-state that the following illustrations are schematic representations of very complex occurrences that constitute a progressive catastrophic collapse. The illustrations are not to scale, and do not reflect all the stories in either tower or a specific tower of the twin towers. The goal of this section is to place into illustrations the discussions just completed on the anatomy of a progressive catastrophic collapse in a high-rise building.

This is the Architect in me, a picture is worth a thousand words and all. If you would like to see some more information that is substantially detailed on the collapse of the twin towers you may want to review the video from Purdue University. There is also some who contend that the aluminum from the planes actually melted and that this molten aluminum mixed with water to create some explosive events that were heard as the towers collapsed, as seen in this video. Another thought for the method of collapse is through momentum transfer as seen in this paper. I also found some great information on the engineering side of the collapse from this site. I am not saying one of these will be superior to the others, I personally think that they all address somethings that were recorded on that fateful September morning. However, each one of these specifically exclude any sort of controlled demolition. It appears that the buildings had the ability to collapse under their own weight in such extreme conditions as found on 9-11, without the assistance of any explosives in a controlled or augmented demolition scenario.

Looking at all the videos I could find from a variety of sources and perspectives, it seems almost clear to me to say that both towers collapsed in similar manner, but not in identical manners. This could be the result of more floors above the impact floors, or the amount of “fireproofing” lost on the initial impact of the planes, the specific location of the lost fireproofing, or… or… or… I have heard so many different things. The problem is this, these towers collapsed in the real world, not in a laboratory’s controlled environment so few if any variables could be captured, isolated, and identified, so we may never know exactly everything that happened that fateful day as the sheer number of variables are astronomical alone, but the issue at hand right now is not “answering” how the towers collapsed in exacting detail, but did the collapse required explosive assistance to cause the towers to collapse or “pancake”. The goal of this paper is just to show that explosive assistance WAS NOT REQUIRED for these two towers to collapse as was visually seen. I doubt if it will be ever known exactly what happened immediately prior to the collapse and exactly what was damaged. Without that knowledge, the best we can create are models that fit a solution to the observed collapse.

The video evidence I have reviewed showed that one tower might have had a slight tilt to one side above the impact floors as the collapse began, but then quickly leveled out as the “pancake” collapse continued to the ground. The other tower appeared to just drop straight down. Using the following illustrations, I will try to clear up both of these possible scenarios. Before we get into that, we need to grasp some basic principles of building structures.

Basic Structures 101: All loading forces generated upon a structure must be ultimately transferred to the ground. It is through the mechanism of transferring these forces to the ground a building is placed in equilibrium or a static condition. This means the building is not moving or in motion. In a building all forces that are created in a horizontal plane (floor or roof for instance) must be transferred to vertical elements (walls or columns) that will then distribute those forces to the ground, which is the ultimate reaction (resistance) for these forces.

To begin, look at the illustration above to see the towers as they were before any of the plane impacts. That schematic drawing shows the floors in between the column grids as solid, which is not exactly the way they appeared in real life. Remember that there were elevators going though many of those bays, but the structural design would have accounted for the transfer of the forces in the design of the diaphragm (horizontal force transfer system), so in this schematic drawing they are represented solid lines since the integrity of the transfer system, or diaphragm, is intact. Once that transfer mechanism is broken, it will be shown as a break in the system in the following illustrations, as the progression of the collapse is illustrated.

WTC Tower Immediately After Plane Impact

WTC Tower Immediately After Plane Impact

The tower illustration, above left, indicates what the tower possibly looked like immediately after the impact of the plane. The red elements would indicate those elements that possibly had sustained load capacity limiting damage by the immediate impact of the plane. That means that the elements would be incapable of sustaining their full intended design loads. Adjacent elements should have been taking some of those loads as witnessed in the fact that they did not immediately fail or collapse as seen in see picture above right.

What cannot be known exactly is what elements were damaged at this moment in either tower. The very best we have can only be modeled by computer, which can provide an idea, based on the parameters programmed for that particular model. One tower may not have had as extensive damage in the critical collapse zone of the core area as the other tower. It just can never be known with certainty. What is known for certain is that much of the fire proofing material that was sprayed on the structural steel at this point was most likely blown off by the explosion of the jet fuel. Again, the exact extent will never be known, and as previously discussed, any of the structural steel exposed to higher temperatures for an extended period would start to deform, lowering the load carrying capacity of component / element. Once this overstressed condition occurs movement of the component / element in the form of deflection is inevitable and unavoidable. Collapse is a certainty, but the timing is the only unknown. This deflection will lead to eventual catastrophic failure. Compounding of this will lead to progressive collapse as earlier discussed in this paper. It is possible that one tower had damage that went deeper into the core than the following illustrations indicate, thus explaining the slight difference in the collapsing of the towers visually seen and recorded. Those details are what will be forever unknown.

WTC Progressive Collapse

WTC Progressive Collapse

The left illustration, above, reflects the continual deflection, after the initial impact, in components / elements as a result of the fatigue caused by the elevated heat the structural steel was experiencing. This progressive deflection in these components / elements will start to move adjacent bays / elements / components as discussed above. All this leads to the beginning of motion, or movement, in the tower that would inevitably end in the tower’s total collapse.

In this illustration the tower’s core is only partially compromised. It is possible that in the other tower the damage to the core was, more than likely, much more extensive with respect to key structural components / elements.

As the deflections increased at the entry point on the right side of this illustration, it will eventually lead to the overstressing of vertical elements (in tension, an elongating type of deformation) on the left side of the tower, causing the weight from the top floors to tilt to the right, as seen in the center illustration. Once the right side starts to move and hits those lower floors which could quite possibly stop that movement, at least temporarily, the left side elements in tension probably failed giving the left side movement in the collapse sequence towards the lower floor(s) as illustrated in the right illustration. Completion of this motion, or movement, may possibly have brought the tower’s motion to a momentary halt, until the next inevitable structural failure of the floors immediately below.

This progressive failure was inevitable as the impact, due to the kinetic energy, loading onto the lower floors created a very high short-term increase in the stresses of those elements being hit by that direct impact load of the floors above. Even if those elements could with stand such an impact loading that is only a short-term type of resistance to this type of loading, the elements were never intended to support the added dead load, along with the continual heating of the structural steel from the fires below would continually weaken the structure’s overall carrying capacity capabilities and they too would eventually fail continuing the catastrophic progressive collapse.

WTC Progressive Collapse

WTC Progressive Collapse

The illustrations above depict the progressive “pancaking” collapse of the tower, until all of the mass’s movement came to rest at ground level. The distribution of that kinetic energy in the ground would have resulted in forces not too dissimilar to a mild earthquake on nearby structures. This is further borne out by the seismic readings on nearby seismic devices as noted in a paper by Columbia University / Earth Institute, as outlined above.

Final Thoughts / Wrap-up

The collapse of WTC 7 was allowed by a conscientious decision. Yes, to the conspiracists, the 47-story WTC 7 WAS ALLOWED to burn and collapse. Let me repeat that to be crystal clear to everyone. The 47-story WTC 7 WAS ALLOWED to burn and collapse, but this is no more insidious than your bank account being depleted by the end of the month. It was clearly a lack of resources available to fight the fires in all the damaged WTC buildings, and the surrounding buildings. The City of New York did not have enough resources to counter such a heinous and cowardly act by these individuals. The required demand to fight these simultaneous fires far exceeded the volume, flow, and pressure the infrastructure could provide. So as part of the decision making, it was decided to concentrate resources on certain areas and not others. If conspiracists claim this is such an insidious reason for the WTC 7 being allowed to collapse, are they going to also address the issue of wage disparity under the same reasoning? After all isn’t that also caused by a lack of resources too? Should that reasoning be excepted for intellectual consistency if nothing else? Or is there an ulterior motive that these conspiracists are not making known in their arguments?

If the towers should not have “pancaked” collapsed, what exactly is the proposed method of collapse that the conspiracists expected? Was the expected result for the buildings to topple over immediately upon impact? What do the conspiracists propose that is better than nature’s method of designing tall, slender structures? What is a clear expectation for an alternative collapse being contended by the conspiracists? Or was the expectation that no collapse should have ever occurred? How could that have been achieved? I am open to discussions.

Now back to “controlled demolition”. Where is the chemical residual that is left behind when explosives are detonated? These residuals were left at Oklahoma City, Boston, London, Manchester, Paris, and all others. Where are they at the twin towers? Are my expectations too high? Are the conspiracist claiming there is a new type of bomb that does not leave a residual? What is the science and materials that make up such a bomb? While thermite burns extremely hot, it burns so fast that it does not heat up the structural steel enough to start the fatigue processes. So, if it were not thermite, what was it? How did these demolition experts hide their work as it would have taken weeks to prepare? How would they have concealed their work on entire floors? How did they get the tons (pallets) of explosives into the building without notice by anyone, even occupants of the buildings? This type of operation would have taken a lot of people. How is it that all those people have kept quite all this time? Again, all I ask is for a little intellectual honesty and continuity.

The momentum transfer of a weight starting in the range of 50,000 tons to 118,000 tons and picking up another 4,500 tons every 10 – 12 feet, falling hundreds of feet alone indicates that the towers had enough kinetic energy to sustain catastrophic progressive collapse without assistance, once movement began. Explosives were not needed, only the removal of the key structural elements / components to begin the failure mode. This, by the way, was the exact goal of an attempted bombing, in 1993, on the North Tower (WTC 1) that was meant to send that tower crashing into the South Tower (WTC 2), but that attempt failed. It seems to be very clear, based on the overwhelming scientific evidence, that both towers had the capacity of sustaining total collapse once failure had begun.

© 2017 Dan Demland


Dan Demland (author) from Phoenix, AZ on October 16, 2017:

CJ, thanks for the comment. My hope really is to put to rest a lot that we hear from these "truthers". I am still a science geek.

CJ Kelly from the PNW on October 16, 2017:

One of the best explanations of Tower 7 I've ever read. Great job. Detailed, logical look at the issue.

Hope it does change some minds among the 9/11 "inside job" crew, but probably not.

Sharing everywhere.

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