What specific structural failures did NIST identify as initiating collapse in each World Trade Center tower?
Executive summary
NIST concluded that neither World Trade Center tower collapsed from a single blown-out column or instant failure; rather, each collapse began when airplane impact damage plus loss of fireproofing led to fire-heated floor trusses sagging, which pulled exterior columns inward on one side until those columns buckled and a localized instability propagated into global collapse (WTC 1 and WTC 2) [1] [2]. Once that initial buckling occurred, gravity-driven progressive collapse of the floors below became inevitable because the mass above created loads the intact lower structure could not arrest [3] [4].
1. WTC 1 (North Tower): impact, fireproofing loss, truss sag and exterior-column buckling
For the North Tower, NIST’s analysis found the initiating sequence began at the floors struck by the aircraft: impact damage removed some columns and dislodged sprayed fire-resistive material, exposing steel to the ensuing multi-floor fires; as the unprotected floor trusses heated they thermally softened and sagged, and that sagging pulled perimeter columns inward on the fire-affected side until one or more exterior columns buckled, creating the first structural instability that led to collapse initiation [2] [1]. NIST explicitly modeled the aircraft impact, the distribution of fires, and the thermal-mechanical response of trusses and connections to show that the combined effect of damage plus fire—rather than impact alone—was required to initiate the buckling sequence [2] [4].
2. WTC 2 (South Tower): similar mechanism with different damage and timing
NIST reached a substantively similar conclusion for the South Tower: the aircraft strike and subsequent fires severely damaged floors and dislodged fireproofing; heated floor trusses sagged and exerted lateral pull on adjacent perimeter columns on the impacted side, causing those exterior columns to buckle and trigger collapse initiation [1] [2]. The report emphasizes differences in the number of floors above the initiation level and the geometries of damage—factors that affected collapse timing and behaviour—but the proximate initiating mechanism in both towers was heat-induced truss sag pulling perimeter columns inward until buckling occurred [1] [4].
3. Why a local buckling became an unstoppable global collapse
NIST explained that once the perimeter columns buckled and the upper block began to fall, the kinetic energy and sudden gravitational load applied to the floors below exceeded the intact lower structure’s capacity to arrest the motion; there was “more than enough gravitational load” from the upper floors to cause progressive collapse once collapse initiation took place, and video and seismic timing were used to show that the structure below provided minimal resistance after initiation [3] [4]. The investigation therefore separates the “collapse initiation” mechanism (truss sag → column buckling) from the “collapse progression” phase (gravity-driven crush-down/pancaking), noting that pans of progressive floor failures then propagated downward [4] [3].
4. Alternate interpretations, NIST’s scope and accepted limits
NIST’s conclusions were careful: they reported that impact damage alone was insufficient to produce collapse initiation and that widespread removal of fireproofing plus prolonged fires were key contributors, based on extensive modeling, laboratory tests on recovered steel, and thousands of documents and interviews [2] [5]. The agency also noted that its analysis addresses structural response up to collapse initiation and that once initiation occurred, the enormous kinetic energy made full progressive collapse inevitable—an important boundary of the study that has been invoked in critiques and conspiracy theories, which NIST and engineering bodies have addressed and rejected based on evidence [4] [6]. Where sources do not provide fine-grained single-column attributions for the towers comparable to WTC 7’s Column 79 analysis, reporting remains focused on the multi-factor sequence NIST modeled [7] [8].