Sudden Onset

WTC 2 exterior columns bowing inward across the east face between Floors 77 and 83 at 9:58:55 a.m. (NIST NCSTAR 1-6, page 178, Figure 6-21). to determining what could have happened to initiate the collapses. After that point, NIST asserted that global (total) collapse became inevitable. NIST clearly described its approach in a footnote on page 82 of its final report:

The focus of the Investigation was on the sequence of events from the instant of aircraft impact to the initiation of collapse for each tower. For brevity in this report, this sequence is referred to as the “probable collapse sequence,” although it includes little analysis of the structural behavior of the tower after the conditions for collapse initiation were reached and collapse became inevitable.

As discussed in Chapter 1, one of the features of controlled demolition is the sudden onset of collapse; whereas one of the features of fire-induced failure is that the onset of collapse is gradual, with randomly distributed, visible deformations appearing prior to the collapse. According to the authors of Multi-Storey Buildings in Steel, “A steel structure, generally speaking, does not collapse suddenly when attacked by fire. There are unmistakable warning signs, namely, large deformations.”2

By most accounts, the onset of collapse of WTC 1 and WTC 2 was sudden. As described by researchers Frank Legge and Anthony Szamboti in the paper 9/11 and the Twin Towers: Sudden Collapse Initiation was Impossible, “A slow, protracted, and sagging collapse was not observed…. As observed in the videos…the upper sections suddenly start to fall and disintegrate.”

NIST’s probable collapse sequence, however, portrays the onset as being not sudden in two ways:

1. NIST claimed that the bowing of exterior columns began several minutes before the collapses initiated.3 However, the observed bowing, which occurred only on a portion of one wall in each building, does not amount to the kind of “unmistakable warning signs” or “large deformations” that would be expected to precede fire-induced failure. If the inward bowing had been significant enough to affect the structure, it should have been much more pronounced, in which case

NIST’s hypothesis of inward bowing exterior columns would have become a leading hypothesis much earlier than it did.

2. NIST then claimed that the bowing walls buckled and that instability subsequently spread to the rest of the exterior columns. Yet there is no visual evidence of either of these phenomena occurring prior to the onset of collapse. Readers are left to assume that this process was invisible and/or that it all happened in a single instant as part of the collapse initiation. According to Kevin Ryan, a former laboratory manager at Underwriters Laboratories, “instability spread would have taken much more time and would not result in uniform free fall [of the upper section onto the lower structure for a distance of up to one story].”4 In addition, prior to any movement in the area of the 95th floor, where inward bowing was focused, the videos of WTC 1 show the collapse initiating at the 98th floor, with a large amount of smoke being ejected on all sides of the building.

Above left: The initiation of collapse of WTC 2.

Above right: The initiation of collapse of WTC 1. These video frames show the collapse initiating at the 98th floor instead of at the 95th floor where the inward bowing of exterior columns was focused. The expulsion of smoke at the 98th floor is acknowledged by NIST on page 163 of NCSTAR 1-6.