Finding 16:
The aircraft impact on WTC 1 resulted in extensive damage to the north wall of the tower,
which failed in the regions of the fuselage, engine, and fuel-filled wing section impacts. Damage to the
exterior wall extended to the wing tips, but the exterior columns were not completely failed in the outer
wing and vertical stabilizer impact regions. According to photographs, columns 112 to 144 along with
column 151 were completely severed, while columns 145 to 148 were heavily damaged and columns 149
to 150 were moderately damaged (for reference, columns 101 and 159 are located on the west and east
corner, respectively, of the north wall). The results of the impact analyses matched well with this damage
pattern to the north wall. Photographic evidence also indicated that an exterior panel with columns 329,
330, and 331 on the south wall between floors 94 to 96 was dislodged. Failure of the exterior columns
occurred both at the bolted connections between column ends and at various locations in the column
depending on the local severity of the impact load and the proximity of the bolted connection to the
impact. Subject to the uncertainties inherent in the models, the global impact simulations indicated that a
total of three core columns were failed and four columns were heavily damaged in the base case,
compared to six columns failed and three columns heavily damaged in the more severe case and one
columns failed and two columns heavily damaged in the less severe case. In the analyses, the floor
trusses, core beams, and floor slabs experienced significant impact-induced damage on floors 94 to 96,
particularly in the path of the fuselage. The analyses indicated that the wing structures were completely
fragmented due to the interaction with the exterior wall and as a result, aircraft fuel was dispersed on
multiple floors. In addition, aircraft debris resulted in substantial damage to the nonstructural building
contents (partitions and workstations) and also in dislodging of fireproofing. The bulk of the fuel and
aircraft debris was deposited in floors 93 through 97, with the largest concentration on floor 94.
Finding 17:
The aircraft impact on WTC 2 resulted in extensive damage to the south wall of the tower,
which failed in the regions of the fuselage, engine, and fuel-filled wing section impacts. Damage to the
exterior wall extended to the wing tips, but the exterior columns were not completely failed in the outer
wing and vertical stabilizer impact regions. According to photographs, columns 410 to 436 and columns
438 to 439 were completely severed, while column 437 was heavily damaged (for reference, columns 401
and 459 are located on the east and west corner, respectively, of the south wall). The results of the impact
analyses matched well with this damage pattern to the south wall. In addition, columns 407 to 409 were
obscured by smoke, but the analysis results indicated that these columns were moderately damaged.
Photographic evidence also indicated that columns 253, 254, 257, and 258 on the north wall were failed.
Failure of the exterior columns occurred both at the bolted connections between column ends and at
various locations in the column depending on the local severity of the impact load and the proximity of
the bolted connection to the impact. Subject to the uncertainties inherent in the models, the global impact
simulations indicated that a total of five core columns were failed and four columns were heavily
damaged in the base case, compared to ten columns failed and one column heavily damaged in the more
severe case and three columns failed and two columns heavily damaged in the less severe case. In some
cases, failure of the column splices located on floors 77, 80, and 83 contributed significantly to the failure
of the core columns. In the analyses, the floor trusses, core beams, and floor slabs experienced significant
impact-induced damage on floors 79 to 81, particularly in the path of the fuselage. The analyses indicated
that the wing structures were completely fragmented due to the interaction with the exterior wall, and as a
result, aircraft fuel was dispersed on multiple floors. In addition, aircraft debris resulted in substantial
damage to the nonstructural building contents (partitions and workstations) and also in dislodging of
fireproofing. The bulk of the fuel was concentrated on floors 79, 81, and 82, while the bulk of the aircraft
debris was deposited in floors 78 through 80, with the largest concentration on floor 80.
The towers sustained significant structural damage to the exterior walls, core columns, and
floor systems due to aircraft impact. This structural damage contributed to the weakening of the tower
structures, but did not, by itself, initiate building collapse. However, the aircraft impact damage
contributed greatly to the subsequent fires and the thermal response of the tower structures that led
ultimately to the collapse of the towers by: (1) dispersing jet fuel and igniting building contents over large
areas, (2) creating large accumulations of combustible materials containing aircraft and building contents,
and (3) increasing the air supply into the damaged buildings that permitted significantly higher energy
release rates than would normally be seen in ventilation building fires, allowing the fires to spread rapidly
on multiple floors.
http://wtc.nist.gov/pubs/NCSTAR1-2ExecutiveSummary.pdf
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