Claim One:
“The Twin Towers collapsed at free-fall accelerations through the path of greatest resistance.”Perhaps the most bizarre aspect of September 11 was the rapid destruction of both 110-story Twin Towers: after the collapses began due to cascading structural failures at the airplane impact locations, each tower fell completely in just fifteen to twenty seconds. Mainstream scientific analyses, including years of work by the National Institute of Standards and Technology (NIST), generally looked at the cause of each collapse: the intense fires (started by jet fuel and fed by office contents and high winds) eventually caused floor trusses to sag, pulling the perimeter walls inward until they finally snapped. At this instant, the entire upper section of each tower fell the height of one floor, initiating an inevitable, progressive, and utterly catastrophic collapse of each of the structures.
While the mainstream explanation (dismissed as the “official story” by 9/11 Truthers) usually ends with the initiation of these unstoppable collapses, the 9/11 Truth movement’s attacks begin there. Gage of AE911 Truth says on that group’s website, “Destruction [of the Twin Towers] proceeds through the path of greatest resistance at nearly free-fall acceleration” (Gage 2011; emphasis added). Many 9/11 Truther pundits drop the “nearly” and say simply that the collapses were at free fall. Truthers then insist that free fall acceleration indicates a complete lack of resistance, proving that the structures were demolished with explosives. We are also told that the sheer mass of the towers, “80,000 tons of structural steel,” would simply resist collapse.
{View full-size chart on the linked page}
How could the buildings fall so quickly? It’s been explained very well in the technical literature by Northwestern’s Zdenek Bazant, PhD, and others (see, for example, Bazant 2008). I’ve developed a simpler physics model of the progressive collapses that agrees quite well with the main points of Bazant’s more rigorous results (Thomas 2010b). Here are some of my findings:
Each floor of the towers contained over two million kilograms of mass. The gravitational potential energy of a standing tower with twelve-foot floors extending upward 110 stories can be calculated straightforwardly; it comes to over 420 billion joules of energy, or the equivalent of 100 tons of TNT per tower. This energy, which was released completely during the collapses, is more than the energy of some of the smaller nuclear weapons in the U.S. arsenal, such as the W-48 (72 tons TNT) (Sublette 2006). This is where the energy required to break columns, pulverize concrete, and expel debris through windows came from. (Truthers often compare such expulsions of air and debris, visible several floors below the collapse fronts, to “squibs,” explosive devices often used in demolitions. However, they are readily explained by pressure changes as the towers, acting like a gigantic bicycle pump being compressed, collapsed.)
The Twin Towers used a “tube within a tube” architectural design, which provided considerable open office space in the interiors of the Towers. Much of the structural support was provided by a dense grouping of thick central core columns in the interior and the perimeter walls on the outside. When the towers began to collapse, large parts of the inner cores (called “the Spires” in 9/11 Truth circles) were actually left standing, briefly, before they, too, toppled over. The perimeter walls were largely forced to peel outward in large sections, producing the iconic images of Ground Zero with which we’re all familiar. Between the outer perimeter and the inner core, the weight of the upper sections plowed through one floor after another, breaking the floor connection brackets and support columns, pulverizing concrete decks, and gaining momentum and mass with each additional floor failure. Had the buildings been constructed differently (the Port Authority was allowed to circumvent some existing New York buildings requirements for the Towers), the collapses might not have even happened (Young 2007).
Even the 9/11 Truth movement’s most eminent physicists are confused about the basic principle of the difference between static and dynamic forces. A piece of paper, taped across a jar’s opening, will support a heavy coin such as a quarter indefinitely (static load). However, if the coin is dropped from just a few inches up, it will tear right through the paper (dynamic load). Given the information at hand—for example, the mass of the upper section of the north tower (fifty-eight million kilograms), the distance it fell (3.8 meters, about twelve feet), and the stiffness/rigidity of the lower structure itself, the dynamic force imparted on the lower section can be estimated as some thirty times the upper portion’s weight. This is many times the lower structure’s safety margin, which explains why it was quickly overwhelmed.
Once progressive collapse began, there were decreasing time intervals of free fall (between floors), punctuated by very brief, incredibly violent collisions—decelerations—of the upper mass, for each floor in turn. There was resistance at every step of the collapse, as the upper section collided with and incorporated each floor below. Conservation of momentum shows that the reductions in falling speed were slight as each floor was impacted, going as the ratio of floors before to floors after (e.g. 14/15, or about 94 percent, for the first impact). Accordingly, the upper section fell from rest to about 19 mph, was slowed down to 18 mph by the first impact, continued to fall until a speed of 26 mph was reached, was then slowed down to 24 mph by another impact, and so on. While the first plunge lasted about nine-tenths of a second, the upper section took only four-tenths of a second to fall through the next floor, three-tenths of a second for the next one, and so on until the bottom floors, which were crushed at a rate of just seven-hundredths of a second each, at speeds of over 100 mph. Yes, there was resistance at every step, as many tons of structural steel was demolished; yet the entire process, like an avalanche, lasted only fifteen to twenty seconds, about 50 to 100 percent longer than true “free fall” would have lasted.
Physics teacher David Chandler’s measurements of the first seconds of the collapse of the North Tower (WTC 1) showed that it fell with increasing speed but at only two-thirds of gravitational acceleration (g) (Chandler 2010). Chandler argues that this means the bottom section exerted a constant upward force of one-third of the upper section’s weight upon its mass, and he declares that this force should have been much larger, indicating that “some sort of controlled demolition was at work.”
Second, Chandler argues that being a Newtonian action/reaction pair, the impact force of the upper section on the lower section was only a third of the upper part’s weight. However, I’ve found that his estimate of the downward impact force was too low by a factor of one hundred. In addition, I found that the actual process—a series of twelve-foot free falls punctuated by violent and brief collisions with each floor—would have resulted in an average acceleration of precisely what Chandler measured for the start of the collapse of WTC 1, namely 2/3 g. (By the end of the collapse, my calculations indicate an average acceleration of only 1/3 g, but this can’t be measured in dust-obscured videos.)
http://www.csicop.org/si/show/the_9_11_truth_movement_the_top_conspiracy_theory_a_decade_later
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