Let's be candid. Aside from the fact that Al-Qaeda leaders personally claimed responsibility in the aftermath of the attacks, and aside from the staggering evidence of Al-Qaeda's involvement on the whole as far as converging reports concerning plans of action and personal testimonies that further validate the group's involvement, it's not as though structural engineers are convinced that there's no plausible explanation as far as the collapse of the towers. In structural engineering terms, progressive collapse occurs when the main structural element of a building fails and subsequently causes interconnecting elements to collapse, and so on and so forth. The destruction of both towers is soundly attributable to total progressive collapse, and, moreover, the working theory concerning the role the aircraft played in developing the stresses that eventually caused the collapses are well-established.
Since you'll obviously discount NIST as another extension of the supposed cover-up, we'll go with the ASCE's prior report conducted as part of a BPS investigation. As part of a conclusion reached before the NIST's more detailed report published later, the ACSE stated:
As each tower was struck, extensive structural damage, including localized collapse, occurred at the several floor levels directly impacted by the aircraft. Despite this massive localized damage, each structure remained standing. However, as each aircraft impacted a building, jet fuel on board ignited. Part of this fuel immediately burned off in the large fireballs that erupted at the impact floors. Remaining fuel flowed across the floors and down elevator utility shafts, igniting intense fires throughout upper portions of the buildings. As these fires spread, they further weakened the steel-framed structures, eventually leading to total collapse.
In essence, this is the prevailing view. Although the fires were incapable of melting the steel support frames that eventually led to the collapse of the structures, they did weaken them, inducing further strain on the structures past the initial impact damage caused by the aircraft themselves.
With that said, the fires were the principle cause of the collapses. Per the TMS, an independent organization consisting of professional scientists and engineers devoted in part to the research and application of materials:
Of equal or even greater significance during this initial impact was the explosion when 90,000 L gallons of jet fuel, comprising nearly 1/3 of the aircraft’s weight, ignited. The ensuing fire was clearly the principal cause of the collapse.
In speaking of the progressive domino effect that initiated the total progressive collapse of the structures...
The perimeter tube design of the WTC was highly redundant. It survived the loss of several exterior columns due to aircraft impact, but the ensuing fire led to other steel failures. Many structural engineers believe that the weak points—the limiting factors on design allowables—were the angle clips that held the floor joists between the columns on the perimeter wall and the core structure (see Figure 5). With a 700 Pa floor design allowable, each floor should have been able to support approximately 1,300 t beyond its own weight. The total weight of each tower was about 500,000 t.
As the joists on one or two of the most heavily burned floors gave way and the outer box columns began to bow outward, the floors above them also fell. The floor below (with its 1,300 t design capacity) could not support the roughly 45,000 t of ten floors (or more) above crashing down on these angle clips. This started the domino effect that caused the buildings to collapse within ten seconds, hitting bottom with an estimated speed of 200 km per hour. If it had been free fall, with no restraint, the collapse would have only taken eight seconds and would have impacted at 300 km/h.1 It has been suggested that it was fortunate that the WTC did not tip over onto other buildings surrounding the area. There are several points that should be made. First, the building is not solid; it is 95 percent air and, hence, can implode onto itself. Second, there is no lateral load, even the impact of a speeding aircraft, which is sufficient to move the center of gravity one hundred feet to the side such that it is not within the base footprint of the structure. Third, given the near free-fall collapse, there was insufficient time for portions to attain significant lateral velocity. To summarize all of these points, a 500,000 t structure has too much inertia to fall in any direction other than nearly straight down.
Thus, both the cause and the nature of the failures is readily explainable, and conspiracies that allege a different story have no sufficient case to make that credibly diverges from official investigations and analyses from experts. If you'd rather believe armchair engineers who've been watching too much Zeitgeist, you're free to do so, but it doesn't detract from the frankly laughable lack of professional credibility of the conspiratorially inclined.
