This column had become unsupported over nine stories after initial local fire-induced damage led to a cascade of local floor failures. The buckling of this column led to a vertical progression of floor failures up to the roof and to the vertical progression of floor failures up to the roof and to the buckling of adjacent interior columns to the south of the critical column. An east-to-west horizontal progression of interior column buckling followed, due to loss of lateral support to adjacent columns, forces exerted by falling debris, and load redistribution from other buckled columns. The exterior columns then buckled as the failed building core moved downward, redistributing its loads to the exterior columns. Global collapse occurred as the entire building above the buckled region moved downward as a single unit. This was a fire-induced progressive collapse, also known as disproportionate collapse, which is defined as the spread of local damage, from an initiating event, from element to element, eventually resulting in the collapse of an entire structure or a disproportionately large part of it.
The report claims that "within seconds" of the failure of "Column 76" due to the loss of lateral support from the thermal expansion of beams leading to the collapse of several floors, "the entire core was failing." With the loss of the core columns, loads were redistributed to the exterior columns, which buckled under the extra weight. "The entire building above the buckled-column region then moved downward as a single unit, completing the global collapse sequence."
In examining the speed of the collapse, "NIST was interested in estimating how closely the time for WTC 7 took to fall compared with the descent time if the building were falling freely under the force of gravity." Using video footage of the collapse, NIST calculated the speed of the collapse based on how far the roofline of the north face could be observed falling before being obstructed by other buildings and how long it took for the building to collapse that far. The report concludes that "the actual time for the upper 18 stories to collapse, based on video evidence, was approximately 40 percent longer than the computed free fall time and was consistent with physical principles."
NIST notes that "thermal expansion" contributing to failure occurred "at temperatures hundreds of degrees below those typically considered in design practice for establishing structural fire resistance ratings." In other words, it should have required hotter fires to expand structural elements to the point where it risked failure. But the report also says the effect of thermal expansion was magnified due to the length of floor spans. Connections failed "that were designed to resist gravity loads, but not thermally induced lateral loads".
It then adds the curious statement that WTC 7 had "a structural system that was not designed to prevent fire-induced progressive collapse."
Conspiracy theorists have long argued that the damage to WTC 7 from the collapse of the twin towers was not sufficient to significantly affect the structure to lead to global collapse. NIST confirms this, noting:
Other than initiating the fires in WTC 7, the damage from the debris from WTC 1 had little effect on initiating the collapse of WTC 7. The building withstood debris impact damage that resulted in seven exterior columns being severed and subsequently withstood conventional fires on several floors for almost seven hours. The debris damaged the spray-applied fire resistive material that was applied to the steel columns, girders, and beams, only in the vicinity of the structural damage from the collapse of WTC 1. This was near the west side of the south face of the building and was far removed from the buckled column that initiated the collapse.
The FEMA report suggested the unique design of WTC 7, relying upon stress transfer elements to support the building over a substation below, might have contributed to the collapse. Conspiracy theorists have noted that FEMA fails to elaborate and explain just how such design elements might have resulted in such a collapse. And the NIST report states that "The transfer elements such as trusses, girders, and cantilever overhangs that were used to support the office building over the Con Edison substation did not play a significant role in the collapse of WTC 7."
While thus confirming many of the arguments put forth by conspiracy theorists, NIST also discounts the use of explosives to bring down the building, stating that:
Hypothetical blast events did not play a role in the collapse of WTC 7. NIST concluded that blast events did not occur, and found no evidence whose explanation required invocation of a blast event. Blast from the smallest charge capable of failing the critical column would have resulted in a sound level of 130 dB to 140 dB at a distance of at least half a mile. There were no witness reports of such a loud noise, nor was such a noise heard on the audio tracks of video recordings of the WTC 7 collapse.
"Considerable effort was expended," the report states, "to compile evidence and to determine whether intentionally set explosives might have caused the collapse of WTC 7," using both scenarios in which explosives were prepared in advance of 9/11 or placed on the day of the attacks. "Attention focused on a single hypothetical blast scenario explosive location. This involved preliminary cutting of Column 79 and the use of 4 kg (9lb) of RDX explosives in linear shaped charges." Neither the breakage of windows nor the blast sound that would have resulted are evidenced in the collapse, the report states.
This conclusion is unlikely to satisfy conspiracy theorists, who have long argued that thermite, not RDX explosives, was used to take out load-bearing columns.
In its recommendations, NIST suggests that its findings be taken with "urgency" since they are pertinent "to the collapse of a tall building that was based on a structural system design that is in widespread use." In other words, there could be a great many other high-rise buildings at risk of total collapse due to fire, suggesting that existing building codes are insufficient and conventional wisdom on engineering and architectural standards outdated.
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