Saturday, October 1, 2022

WTC Collapse Time Studies: What They Found and Why it Matters

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By Adam Taylor

Three weeks ago I published an updated version of my paper on the WTC collapse in the International Journal of Engineering and Techniques. I'm happy to see it's just passed three-hundred views (the original version, published back in April, didn't get that many views until several months). The paper can be accessed here. Today, I'd like to summarize the paper's findings in a somewhat more colloquial way, in order to emphasize why its conclusions are so important.

First, I need to preface this discussion by pointing out what I found during the course of my research. In the two decades since the attacks, there has been a substantial amount of literature published on the collapse of the Twin Towers, both by those for and against demolition theories. And while admittedly a large portion of that literature discusses how the plane impacts and fires are what led to collapse initiation, there's actually been very little comparatively on the collapse itself. The vast majority of papers I've seen discuss the ways in which the damage and fires affected the steel and how that led to the collapse starting. But very little seems to discuss what happens after that. The actual "progressive collapse" - what happens to the lower sections of the buildings after the collapse starts - has gotten very little discussion. 

I point this out because it seems many people have erroneously not made the distinction between collapse initiation and the collapse itself. I've even run across papers which, based on the title, suggests they're going to talk about the progressive collapse, when in fact they're only talking about the conditions which led up to collapse initiation. That this distinction rarely gets made is problematic. 

Part of the problem is that this has led to an unfortunate disconnect in the discussions of what happened to the buildings. Those who reject WTC demolition often feel that as long as it can be shown that the fires are what actually started the collapses, that effectively disproves any demolition theories. Plenty of diligent researchers in the movement have, of course, shown why the plane impacts and fires were not sufficient to cause collapse. However, suppose they are all dead wrong. That still fails to account for what happens after that. And that is the main focus of my paper.

In essence, my paper is a meta-analysis of the literature published in the two decades after 9/11 that actually discusses the mechanics of the collapse itself, not just collapse initiation. Much of that discussion is on the work of Dr. Zdenek Bazant, who seems to be the primary person responsible for the wide-spread belief that collapse initiation inevitably leads to total collapse. His original paper, published only days after the attacks, planted the seed in the public's mind that regardless of how the collapse started, there was no question that it would be total once it began. The errors of Dr. Bazant are decently summarized here: and are discussed in my paper as well.

However, even if we assume a total collapse would occur, there's still the question of how long the collapse should take. This dispute, unfortunately, has led to another disconnect between so-called truthers and debunkers. A popular meme in the movement has, of course, been that the Towers collapsed either at or near "free fall speed," and that they occurred in 9, 10, or 11 seconds. None of this is correct. While it is difficult to estimate exactly how long it took the Towers to fall, given the heavy amount of dust and debris covering the collapse, watching the videos still definitively shows the collapses took longer than a free fall time, which would be approx. 9.2 seconds. Instead, a better estimate is that it took around 15 seconds for them both to collapse.

Again, the dispute over free fall or not free fall has led to truthers and debunkers talking over each other, with debunkers feeling that as long as they can show the buildings didn't fall at free fall, that effectivly proves they weren't demolitions. This completely bypasses the question of whether the collapse times we actually saw are possible from a natural gravity-driven collapse. My contention is that it is not, and that's what my paper looks at.

The various papers I looked at and discuss came to conclusions that were very suprising to me. To be clear, what these papers did was estimate what hypothetically would have been the shortest possible collapse times, based on assumed unrealistic variables that would lead to such short collapse times. When estimating the collapse times, the various relevant variables that need to be considered include:

  • The elastic response from the floors falling on to one another.
  • The energy needed to crush the building contents and the concrete of the floors.
  • Air resistance between collapsing floors, through aerodynamic drag and/or the air pressure, or "cushion" effect built up.
  • Energy needed to damage and distort the steel support columns.
  • The energy needed to disperse material outside the buildings' perimeters.

What the papers I looked at do is essentially ignore these various variables and assume that the collapse is slowed only by intertia, in order to estimate the hypothetical lowest possible collapse times. With regards to the papers looking at the overall collapse times, here's a summary of what they found:

Cherepanov, G. (2005), "Mechanics of the WTC collapse," International Journal of Fracture 141: 287-289.

Assumes collapse is driven by gravity only, that there is zero resistance (i.e., no support from the columns and inelastic collisions between floors). Finds the collapse will be slowed to about 1/3 of graviational acceleartion, which means a collapse time of approx. 15.95 seconds. Paper accessible at (pay charge required)

Ross, G. (2006), "Reply to Dr. Greening," Journal of 9/11 Studies 2.

Responds to criticisms from Frank Greening. Also estimates the lowest possible collapse time. Assumes a “momentum only analysis” and a collapse “progressing from initiation level to the ground level,” with the mass per floor emulating that of the Tower. Finds the minimum time to collapse will be 13.5 seconds. This is only for the section below the plane impact point. When the upper section is included, total collapse time will be approx. 15.44 seconds. Paper accessible at

Kuttler, K. (2007), "Collapse Time Calculations for WTC 1," Journal of 9/11 Studies (Letter).

Looks at a broad range of collapse variables. Estimates collapse time (in section 1.4.2) by assuming: The upper section remains intact as it crushes; the collisions between floors are perfectly inelastic; no support at all from the steel columns is considered; no energy is lost due to crushing concrete; no steel is thrown outside the building’s perimeter, allowing it all to impart momentum and contribute to crushing. Finds the collapse time will be 15.95 seconds. Paper accessible at

Denny, M. (2010), "How Fast Does a Building Fall?," European Journal of Physics 31(4): 943-948.

Presents itself as refuting demolition claims. However, uses a number of questionable and unrealistic assumptions in the calculations, including: simulates collapse as a simplified 1D model, removing horizontal forces or resistance during collapse; assumes collisions between floors are totally inelastic; ignores any resistance from the columns; assumes no loss of mass during collapse; omits air resistance. In essence, calculates lowest possible collapse time. Finds it will be 50% greater than free fall, which results in a collapse time of 13.8 seconds. Paper accessible at (pay charge required)

Szuladziński, G. (2012), "Temporal Considerations in Collapse of WTC Towers," International Journal of Structural Engineering 3(3): 189-207.

Simulates the collapse of the North Tower, assuming sequential collisions between floors and that the columns are perfectly frangible, i.e. reduced to only 1/10 their original strength. Finds the lowest possible collapse time will be 15.33 seconds, and concludes "the pancaking mode is not a realistic proposition, as the calculated fall time becomes much too long." Paper accessible at

Němec, I., M. Juráňová, I. Ševčík, P. Frantík, and Z. Vlk (2012), "Dynamics of Collapse of a High Building," Journal of Mechanics Engineering and Automation 2: 349-354.

Simulates the collapses through a computer simulation, uses two independent programs. Makes similar assumptions made by Kuttler, assumes no structural support from the columns and no elastic resistance from the floors. Concludes lowest possible collapse time is 15.85 seconds. Paper accessible at

Němec, I., M. Trcala, J. Vala, and A. Vaněčková (2018), "A Contribution to Analysis of Collapse of High-Rise Building Inspired by the Collapses of WTC1 and WTC2: Derivation of Simple Formulas for Collapse Upper Speed and Acceleration," Journal of Applied Mathematics and Physics 6: 2666-2680.

Updated analysis of Němec et al. (2012). Assumes no resistance from the columns, no air resistance, no loss of mass through floor collisions, and inelastic responses from the floors. When the collapse is slowed only by inertia, overall collapse will take 15.82 seconds. Paper accessible at

Mitteldorf, J. (2018), "How Did the Three WTC Towers Fall? A Minimum Time for ‘Pancaking’ Collapse."

Whitepaper, assumes the collapse is slowed only by inertia, but assumes no support from the columns and inelastic collisions between floors. Finds the lowest possible fall time to be approx. 16 seconds. Notes that neglecting continuous mass distribution in the calculations could result in the collapse being one second shorter, around 15 seconds instead. Paper accessible at

Chandler, D. (2019), "Idealized Pancake Theory Collapse Time."

Models the successive collapse of each floor in a spreadsheet. No support from the columns is assumed, and floor collisions are assumed to be totally inelastic. The result is a collapse estimated to take approximately 14.92 seconds. Paper accessible at

These results were surprising to me, as they are quite consistent. What we observe are nine different studies which give virtually the same results - that the lowest possible time it would take the buildings to fall, assuming highly unrealistic variables, would be approximately 14-15 seconds. And when nine different studies give virtually the same result, it's a safe bet that that result is most likely correct.

Why these results are so problematic for the official narrative for the WTC collapse is fairly straightforward: this is approximately how long it took the Towers to collapse. To my mind, this means one of two things: either such unrealistic variables were built into the Towers (which obviously wasn't the case), or something was happening ahead of the collapse front to remove the strength of the structures, which allowed them to fall that quickly. Several of the studies cited above also calculate more realistic collapse times, and find the Towers' collapses should have taken over 20 seconds.

Clearly, these results show that the official narrative of what actually happened to the Towers is false. As such, critical re-evalution of the the collapses is needed, which is precisely what I call for in my paper. 


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